The small animal practitioner must considered the following questions when developing an antimicrobial treatment regimen:

1) Does the diagnosis warrant antimicrobial therapy?

Much of the use of antimicrobials in clinical practice is irrational. Antimicrobial therapy for superficial wounds and single doses of penicillin after performing spays or neuters in normal animals encourages antimicrobial resistance without any real therapeutic benefit to the patient.

2) What organisms are likely to be involved?

For many infections, it is possible to predict what microorganisms are involved. Traumatic wounds are likely to be contaminated with skin and fecal flora. Urinary tract infections in dogs routinely involve E coli or Staphylococcus intermedius. Upper respiratory tract infections are often caused by Chlamydia and Mycoplasma spp.

3) What is the in vitro antimicrobial susceptibility of the pathogen?

For many pathogens, the antimicrobial susceptibility of the organism is very predictable. For example, Streptococcus spp. and most anerobic species are routinely susceptible to penicillin. Some pathogens are characterized by their unpredictability. Gram negative enteric bacteria are very efficient at transmitting and acquiring resistance genes, therefore their susceptibility patterns are unpredictable and susceptibility testing should always be done on cultured samples in order to choose effective therapy.

4) In what part of the body or tissue is the infection located? Will the antimicrobial penetrate to the infection?

Practitioners must consider the effects of physiology and pathology on distribution of drugs in order to adjust dosages to achieve effective therapy. Many infections occur in sequestered areas of the body. Abscesses can be walled off with a thick fibrous capsule. Because of specialized blood and tissue barriers, it is difficult for many drugs to reach therapeutic concentrations in the central nervous system, mammary gland and accessory sex glands.

Volume of Distribution

Volume of distribution (Vd) is the pharmacokinetic parameter of a drug that indicates its distribution within the body. The physical characteristics of the drug molecule, including ionization, lipid solubility, molecular size, protein binding, determine its ability to cross membranes. Volume of distribution is a mathematical term describing the apparent volume of the body in which a drug is dissolved. The relative value of this parameter indicates to the clinician how a drug is going to distribute to the tissues. Values for Vd are reported in most drug reference texts, usually in units of liters/kilogram (L/kg). The numerical value of Vd relates to the dose of drug administered and the amount of drug measured in a blood sample by the equation:

Vd = dose/concentration measured in plasma

To understand the concept of Vd, think of the body as a beaker filled with fluid (Figure 1). The fluid represents the plasma and other extracellular fluid (ECF). If a drug is administered intravenously, it rapidly distributes in the ECF, as represented by the stars in the beaker on the left. If the drug does not readily cross lipid membranes, it will be confined mainly to the ECF. A sample taken from this beaker will have a high drug concentration. The higher the measured concentration in relation to the original dose, the lower the numerical value for Vd.

Some drugs readily cross lipid membranes and distribute into tissues. This is represented by the beaker on the right, where the stars at the bottom of the beaker represent drug molecules that have been taken up by tissues. A sample taken from the fluid in this beaker will have a low drug concentration in proportion to the original dose, therefore will have a high numerical value for Vd. Using the formula given above, highly lipid soluble drugs can have plasma drug concentrations low enough to result in a value of Vd that is greater than 1 L/kg. In the pharmacokinetics, there are several different terms for volume of distribution (volume of distribution of the central compartment, volume of distribution at steady-state, volume of distribution by area). Their numerical values will differ slightly, but they all indicate the drug's ability to cross membranes. Most references will use the value for volume of distribution at steady-state, when the rate of drug entry into the tissues from the vascular system is equal to its exit rate from the tissues back into the vascular system.

It is useful to compare a value for Vd for a drug to the distribution of total body water. Adult animals are considered to be approximately 60% water, therefore total body water has a Vd of 0.6 L/kg. The extracellular fluid compartment is approximately 30% of body weight, therefore has a Vd of 0.3 L/kg. So an antimicrobial with a value for Vd of 0.3 L/kg will be distributed primarily to extracellular fluid, while another antimicrobial with a value for Vd of 3.4 L/kg will be distributed beyond body water compartments and will achieve high concentrations in tissues, yet relatively low concentrations in plasma. The antimicrobials can be categorized as low (Vd < 0.3 L/kg), medium (Vd of 0.3 to 1 L/kg) and high (Vd > 1 L/kg) volume of distribution drugs (Table 1).

Volume of distribution is constant for any drug, and will only change with physiological or pathological conditions that change the distribution of the drug. Unfortunately, drug doses are often determined in normal, healthy, adult animals. Yet any condition that changes the ECF volume will dramatically affect the plasma concentrations of low Vd drugs. Drugs with high Vd's normally distribute throughout fluid and tissue compartments, so are not significantly affected by changes in body water status. There are many clinical conditions where there are major changes in the ECF volume. Neonates are considered to be closer to 80% total body water, and the additional 20% is primarily found in the ECF compartment. Geriatric animals tend to have reduced total body water, this reduction is primarily in ECF volume. There are many clinical conditions that cause volume contraction or dehydration of the ECF: shock, colic, enteritis, etc. Parasites, heart failure, and vasculitis can all cause edema and an increase in the ECF volume. Local changes in acid/base status can alter the ionization state of drugs and effect movement across membrane barriers. Conditions that alter the amount or affinity of plasma proteins will change the volume of distribution of highly proteing bound drugs. Physiological or pathological changes in volume of distribution are very important in determining doses of drugs that are predominantly confined to the ECF compartment. For example, a puppy is 80% total body water, compared to the adult dog at 60% total body water. So for any given dose of a drug with a low Vd like gentamicin, the puppy will have lower plasma concentrations than the adult dog. Therefore, a higher dose must be given to a puppy to achieve the same effective plasma drug concentration. This is not intuitive, as it is common to think that a neonate should be given a lower dose than the adult because of concern of gentamicin nephrotoxicity. Unfortunately, this thinking leads to underdosing and ineffective therapy.

Drug Ionization

Changes in acid/base balance are very common in disease states. The antimicrobial drugs exist as weak acids or weak bases. Their lipid solubility depends a great deal on their degree of ionization (charged state). An ionized drug is hydrophillic and poorly lipid soluble. A nonionized drug is lipophilic and can cross biological membranes. The degree of ionization for a weak acid or weak base depends on the pKa of the drug and the pH of the surrounding fluid. When the local pH is equal to the pKa of the drug, then the drug will be 50% ionized and 50% nonionized (log 1 = 0). The proportions of drug in each state are classically calculated from the Henderson-Hasselbach equation:

For a weak acid: pH = pKa + log(ionized drug/nonionized drug)

For a weak base: pH = pKa + log (nonionized drug/ionized drug)

Clinically, it is sufficient to remember that only nonionized drug crosses membranes readily and that "like is nonionized in like", meaning a weak acid will be most nonionized in a fluid whose pH is acidic and a weak base will be most nonionized in a fluid whose pH is basic. This concept effects the distribution of antimicrobials into sequestered infections. For example, cases of mastitis are treated parenterally with antimicrobials that are weak bases (Figure 3). Why? Milk is more acidic than plasma. Weak bases in the plasma are highly nonionized, and the nonionized drug fraction readily cross into the mammary gland, then become "ion-trapped" in the more acidic milk. A new equilibrium is established between ionized and nonionized drug. Although smaller than the fraction in plasma, the nonionized drug fraction in the milk is available to cross the cell walls of the pathogenic bacteria and convey the desired antimicrobial action. Weak acids on the other hand, are highly ionized in plasma, therefore do not penetrate into the mammary gland very well. Successful mastitis therapy with a weak acid requires local therapy with an intramammary infusion, as is routinely done in cattle, where the high local drug concentrations overcome any effect of drug ionization. Sequestered infections such as abscesses, metritis, and meningitis, are typically acidic environments compared to plasma, and effective parenteral therapy requires a basic antimicrobial.

The antimicrobial drugs can be classified according to their status as weak acids or weak bases (Table 2). Note that the acidic antimicrobials are also the drugs with low values for Vd and how most of the weak bases have high values for Vd. The exceptions are the aminoglycosides. Despite being weak bases, the aminoglycosides are very large, hydrophillic molecules that are highly ionized at physiological pHs and do not readily cross lipid membranes. Therefore, parenterally administered aminoglycosides do not achieve therapeutic concentrations in milk, abscesses or cerebrospinal fluid. The amphoteric drugs like the fluoroquinolones and tetracyclines have acidic and basic groups on their chemical structures. These drugs have a pH range where they are maximally nonionized. For example, enrofloxacin is most lipid soluble in the pH range of 6-8, so it is lipid soluble in most physiological situations. In acidic urine, significant ionization occurs, which reduces enrofloxacin antibacterial activity. But enrofloxacin is primarily eliminated in urine, so this reduction in activity is offset by the extremely high concentration of enrofloxacin, so it is of no clinical importance. The fluoroquinolones and tetracyclines are highly nonionized at most physiological pHs, so are similar to bases with high Vd values.

Protein Binding

Many drugs in circulation are bound to plasma proteins (mainly albumin). Bound drug is too large to pass through biological membranes, so only free drug is available for delivery to the tissues and to produce the desired pharmacologic action. There is an equilibrium between free and bound drug however, just like the relationship of ionized and non-ionized drug. Degree of protein binding is only clinically significant with those drugs that are more than 90% protein bound. For these drugs, conditions such as liver disease or kidney disease that decreases plasma protein concentrations will cause significant increases in the amount of free drug available for pharmacologic action. Protein binding may also be involved in drug interactions, when a second highly protein bound drug is administered, if it uses the same binding site as the first drug, it can displace the first drug and increase the amount of the first drug available for pharmacologic action. The classical example is of the interaction of phenylbutazone and warfarin, as phenylbutazone displaces warfarin from its protein binding sites. Reducing the amount of warfarin bound from 99% to 98% effectively doubles the plasma concentration of free warfarin available for anti-coagulant activity and can lead to bleeding problems. The only antimicrobial that is significantly protein bound is ceftiofur. The efficacy of ceftiofur is attributed to binding to acute phase proteins, such as a1-alpha-trypsin, which act as reservoirs for active drug and carry the bound drug to the site of inflammation, where a new equilibrium is established between free and bound drug. Because of the degree of protein binding, ceftiofur does not readily cross into the milk of lactating animals when administered parenterally, hence the "zero" milk withdrawal time when administered to lactating dairy cattle.

5) Will the antimicrobial be effective in the local environment of the organism?

Even if the pharmacokinetic parameters of a drug are such that it reaches the site of the infection, local factors can influence the antimicrobial activity. Aminoglycosides are ineffective in hyperosmolar, anaerobic, acidic environments such as the purulent environment of an abscess. Sulphonamides act by substituting for p-aminobenzoic acid (PABA) in the folic acid pathway of bacteria, so they are also ineffective in purulent material and necrotic tissue which provide alternative sources of PABA to the bacteria.

6) What drug formulation and treatment regimen will maintain the appropriate antimicrobial concentration for the proper duration of time?

Drug formulation influences drug availability to the systemic circulation. Intravenous administration achieves the most rapid onset of drug action. With intramuscular or subcutaneous injections, absorption is delayed as drug moves from injection site to vascular system. Absorption rate will vary depending on the site of injection (absorption is usually more rapid from the neck muscles than from the hindquarter muscles) and the drug formulation. Some formulations are designed to have slow release from the injection site to make the antimicrobial "long-acting". In herbivores like the horse, oral preparations may have reduced or erratic systemic availability due to adsorption to feedstuffs and incomplete absorption. Some drugs that are easily absorbed may become inactivated by the liver as they pass through the portal circulation ("first pass effect"). Drugs with large molecular weights may not be well absorbed, unless attached to a "carrier" that allows for absorption through the lymphatic system.

Elimination Half-Life

Infectious diseases are typically treated with multiple doses of an antimicrobial. Timing of repetitive doses is determined by the elimination half-life (t½) of the drug. The elimination half-life is the time required for drug concentration to decrease by one half. For most drugs, the value for t½ remains constant for the duration of the drug dose in the body. The t½ of a drug is defined as:

t½ = 0.693/elimination rate of the drug,

where 0.693 = ln2 (the natural logarithm of 2)

Mean Residence Time (MRT) is the equivalent of t½ when pharmacokinetics are calculated using statistical moment theory. Some pharmacokinetic studies report MRT instead of t½.MRT is actually the time it takes for drug concentration to decrease by 63.2%, so a MRT's value is somewhat greater than t½.

Clinically, the t½ determines (a) the drug dosing interval; (b) how long a toxic or pharmacologic effect will persist; and (c) drug withdrawal times for food animals or performance horses. Table 3 demonstrates the relationship between t½ and the amount of drug in the body. With each half-life, the amount of drug remaining reduces by 50%. Note it takes 10 t½'s to eliminate 99.9% of drug from the body. Also recognize that doubling a drug dose (so that the table would start with 200%) does not double the withdrawal time. It merely adds one half-life to reach the same concentration endpoint.

7) What adverse drug reactions or toxicities might be expected? Do the benefits of antimicrobial therapy outweigh the risks?

Antimicrobial drugs frequently cause adverse reactions in animals. Antimicrobials with anaerobic activity, combinations of broad spectrum antimicrobials and those antimicrobials that undergo enterohepatic recirculation are often incriminated in disturbing the normal gut flora and allowing the proliferation of pathogens such as Clostridium spp. and Salmonella spp. The use of antimicrobials for relatively trivial infections encourages development of antimicrobial resistant organisms. Without evidence of a susceptible pathogen, antimicrobial use is irrational and exposes the patient to unnecessary risks.

8) Can you choose a product approved for use in dogs or cats? If using an antimicrobial in a performance animal, can you determine appropriate withdrawal times?

Whenever possible, veterinarians should use approved products. If there is no suitable approved product, then drugs may be used in an extralabel manner as long as there is a valid veterinarian-client-patient relationship. The veterinarian must be available in the case of treatment failure or adverse reactions and must be able to provide withdrawal information for slaughter of a food animal.

Document the Infection

A diagnosis must be established before any therapy can be administered. It is not always necessary to culture samples from all patients with infectious diseases in order to identify the organism involved. Often, the practitioner can base a diagnosis on clinical experience from similar cases. The signs of some infectious diseases are so obvious that the need for microbiological identification is minimal; but for those infectious diseases of unknown cause or for those attributable to organisms with unpredictable antimicrobial susceptibility, there is no substitute for isolation and identification of the causative agent. For these organisms, initial therapy while waiting for culture results must include an antimicrobial with a broad spectrum of activity. However, broad spectrum antimicrobials are usually more toxic and more expensive.

Obtain representative samples of infected material from clinical patients. Beware of sampling grossly contaminated sites, such as wounds and purulent nasal discharges. Specific samples may improve odds of culturing pathogens. Septic arthritis is best diagnosed from synovial fluid rather than a synovial membrane biopsy, salmonellosis from rectal biopsy, and septicemias from blood cultures. An immediate Gram stain can be performed from a direct smear and will direct initial therapy until laboratory results are obtained. Submit samples for appropriate culture and identification and specify type of culture: aerobic, anaerobic, mycoplasma. In some clinical cases, identification of the pathogen may be made by serologic demonstration of antibodies (e.g. leptospirosis, brucellosis, ehrlichiosis).

Dosage Regimen Design

Successful antimicrobial therapy relies on administering sufficient doses so that pathogens at the site of infection are killed or sufficiently suppressed that they can be eliminated by the host's immune system. The relationship between the host, the bacteria and the drug may be very complex. High plasma antimicrobial concentrations are assumed to be advantageous in that a large concentration of drug will diffuse into various tissues and body fluids. Drug concentration at the infection site is assumed to be of major importance in determining drug efficacy. Remember, drug diffusibility from the plasma to extravascular tissues depends on molecular size, lipid solubility, drug pKa, local pH, specific cellular transport mechanisms and degree of protein binding.

Minimum Inhibitory Concentration and Minimum Bactericidal Concentration

In the laboratory, the relationship between an antimicrobial drug and a pathogen is described by the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC). The MIC is the lowest drug concentration that inhibits bacterial growth. The MBC is the lowest drug concentration that kills 99.9% of the bacteria. Minimum inhibitory concentrations are used to determine drug dose, in an attempt to achieve blood and tissue concentrations that exceed the in vitro MIC for the pathogen. Antimicrobial susceptibility is based on these assumptions:

MIC > local drug concentration = no effect = resistant ("R")

MIC = local drug concentration = doubtful = intermediate ("I")

MIC < local drug concentration = successful therapy = susceptible ("S ")

The "S", "I", "R" designations are assigned by the laboratory based on safely achievable plasma concentrations. When a practitioner receives a culture and susceptibility report, it contains the name of the cultured organism and a list of common antimicrobials designated as susceptible, intermediate or resistant. This information is intended to guide antimicrobial selection, however the report must be carefully interpreted. There are important factors that are not accounted for by antimicrobial susceptibility data:

1) Host defences.

The interaction between antimicrobial and pathogen in the laboratory does not take into account normal host defence systems. The humoral and cell mediated immune systems play a major role in pathogen eradication; their contribution is underestimated by the susceptibility report. Antimicrobials act in concert with endogenous microbial inhibitors, such as immunoglobulins, T-lymphocytes, phagocytes, complement components, lactoferrin, lactoperoxidase and lysozymes.

2) Drug distribution in the body.

The susceptibility designations are based on achievable plasma concentrations and do not take into account preferential drug accumulation in specific sites. Most antimicrobials are eliminated by the kidney, achieving urine concentrations hundreds of times the concentrations found in plasma. Tetracyclines accumulate in pneumonic lung tissue, resulting in successful therapy that would not be predicted by the susceptibility report. Direct application of an antimicrobial, such as a topical or ophthalmic formulation, also achieves such high concentrations that the susceptibility report is not applicable.

3) Growth rates and size of inoculum at the infection site.

The MIC is measured in broth with a standardized inoculum. In clinical cases, there may be sites of infection with only a few bacteria and other sites with many bacteria. Some bacteria grow and multiply very slowly at the site of infection, while the laboratory incubator encourages rapid growth and multiplication. Rapidly multiplying bacteria in test media are very sensitive to antimicrobials.

4) Mixed infections.

In the laboratory, cultured organisms are separated prior to susceptibility testing. This prevents observation of the pathological synergism between organisms. For example, Pasteurella and anaerobes demonstrate an in vivo synergistic pathogenicity that can not be seen when they are cultured separately.

5) Infection environment.

The culture plate of the laboratory is an ideal environment for the drug-organism interaction. In clinical cases, the infection environment has a large effect on antimicrobial action. In diseases characterized by abscesses, treatment failure may occur when the antimicrobial chosen is ineffective in the acidic, anaerobic and hyperosmolar environment. Antimicrobial action of many drugs decreases in the presence of milk.

6) Unless specifically requested, topically administered antimicrobials are not tested. Systemically toxic antimicrobials such as polymixinB, bacitracin and mupirocin are often not included in a susceptibility profile, yet they have great value in veterinary medicine. If they are not listed on the report, they are often not considered for therapy.

So the true relevance of any in vitro MIC predicting the in vivo results of antimicrobial therapy is questionable. But by convention, drug dosage regimens use a target plasma drug concentration that is based on some multiple (usually 2 to 10) of the in vitro MIC. Proposed treatment regimens are then evaluated in clinical patients and modified as needed to maximize efficacy.

Bacteriostatic Versus Bactericidal

It is common to classify antimicrobials as bactericidal or bacteriostatic (Table 4). If the ratio of the MBC to MIC is small (< 4 to 6), a drug is considered bactericidal and it is possible to obtain drug concentrations that will kill 99.9% of the organisms exposed. If the ratio of MBC to MIC is large, it may not be possible to safely administer dosages of the drug to achieve kill 99.9% of the bacteria and the drug is considered bacteriostatic. For many drugs, the distinction between bactericidal and bacteriostatic is not exact, and may depend on the drug concentration attained in the target tissue and the pathogen involved. For example, florfenicol is considered bactericidal for the very susceptible bovine respiratory tract pathogens but bacteriostatic for enteric pathogens. Specific situations in which a bactericidal drug may be preferred over a bacteriostatic drug include immunosuppressed patients such as septic neonates, life-threatening conditions such as bacterial endocarditis and meningitis, and for surgical prophylaxis.

For some bacteria-drug interactions, bacterial growth remains suppressed for a period after drug concentration has decreased below the MIC. This Post Antibiotic Effect (PAE) may be the reason that dosage regimens that fail to maintain drug concentration above the MIC are still efficacious. The PAE depends on the antimicrobial and the specific bacterial pathogen (Table 5).

Concentration of the Antimicrobial

Bacterial kill-curve studies show that antimicrobials can be categorized as concentration-dependent bacterial killers or time-dependent bacterial killers (Table 6). For concentration-dependent killers, high plasma concentration levels relative to the MIC of the pathogen are the major determinant of clinical efficacy. These drugs also have prolonged PAEs, thereby allowing long dosing intervals with maximum clinical efficacy. For time-dependent bacterial killers, the time during which the antimicrobial concentration exceeds the MIC of the pathogen determines clinical efficacy. The bactericidal activity of time-dependent killers such as the penicillins and cephalosporins does not increase with increasing plasma concentrations once the MIC of the bacteria is exceeded.

Calculating the Drug Dosage Regimen

Utilizing the previous information, antimicrobial dosage regimens are designed in one of two ways; either to maximize plasma concentration or to provide a plasma concentration above the bacterial MIC for most of the dosage interval.

For concentration-dependent killers with a prolonged PAE it is suggested that the peak plasma drug concentration be 8- to 10-fold higher than the MIC of the pathogen. If the Vd of the antimicrobial is known, a precise drug dosage regimen for the pathogen can be calculated from the following equation:

Dose = (Vd)(desired plasma concentration)

Example: You are treating a dog with a Klebsiella pneumonia with gentamicin. The MIC of gentamicin for Klebsiella is 2 g/ml. The desired plasma concentration would be ten times the MIC at 20 g/ml. The Vd of gentamicin in the dog is 0.3 L/kg. The dose calculated is:

Dose = (300 ml/kg)(0.02 mg/ml) = 6 mg/kg

Given once daily, this dosage would provide effective concentration-dependent bacterial killing, while limiting renal accumulation of gentamicin associated with nephrotoxicity.

For time-dependent killers , the objective is to keep the average plasma drug concentration above the pathogen's MIC for the duration of the dosage interval. Again, utilizing Vd and t½ information, you can precisely calculate a dosing regimen.

Example: You want to treat a cat with a Mycoplasma pneumonia with doxycycline. The MIC for Mycoplasma is 3 g/ml. In cats, the Vd is 2 L/kg and the elimination half-life is12 hr. You pick a dosage interval of 6 hours and an average desired plasma concentration of 10 times the MIC because of the pathology.

Dose = (desired ave plasma conc)(Vd)(dosage interval)/1.44(t1/2)

Dose = (0.03 mg/ml)(200 ml/kg)(12 hr)/1.44(12 hr)

Dose = 4.2 mg/kg

The recommended dose from Plumb's Handbook is 5 mg/kg every 12 hours.

Site of the Infection

The pathophysiology of the infection influences the distribution and activity of antimicrobials. Abscess formation is a significant therapeutic problem for antimicrobial activity. The abscess wall limits penetration by non-lipid soluble drugs. The acidic environment encourages weak bases to accumulate but the low pH and the presence of cellular debris within the abscess interferes with the activity of some antimicrobials. Rifampin, chloramphenicol, florfenicol, the tetracyclines and metronidazole all achieve high concentrations in abscesses and retain their antimicrobial efficacy in a purulent enviroment. The macrolides and fluoroquinolones achieve high concentrations, but their activity is lessened by the acidic environment. Aminoglycoides do not penetrate abscesses well and are inactivated in the acidic, anaerobic, hyperosmolar environment. Penicillins and cephalosporins do not penetrate abscesses well. Trimethoprim/sulphonamide combinations achieve adequate concentrations, but the competitive mechanism of action of the sulphonamide is overwhelmed by the abundance of free PABA from lysed phagocytes.

Combination Antimicrobial Therapy

Combination antimicrobial therapy is commonplace in practice, but combination therapy has not been demonstrated as superior to single drug therapy in controlled clinical trials. Use of multiple antimicrobial drugs should be limited to:

1) Combinations with known synergism against specific organisms.

Synergism occurs when the antimicrobial effect of a combination of drugs is greater than the sum of their independent effects. For example, penicillins or cephalosporins are synergistic with aminoglycosides in the treatment of enterococcal endocarditis. Disruption of the bacterial cell wall by the ß-lactam antibiotic allows for greater uptake of the aminoglycoside, for a synergistic killing effect.

2) To prevent the rapid development of bacterial resistance.

Erythromycin and rifampin are used in combination in foals with Rhodococcus equi. Each drug has a completely different mechanism of antimicrobial action; their combination reduces the chance of chromosomal mutations conferring bacterial resistance.

3) To extend the antimicrobial spectrum of initial therapy of life-threatening conditions.

In emergency situations, such as septicemia or meningitis, where the causative organism is unknown, combination therapy may be initiated to provide antimicrobial activity against Gram positive, Gram negative and anaerobic bacteria.

4) To treat mixed bacterial infections.

For example, it is rational to combine antimicrobials to treat hospital-acquired pneumonias, which predictably involve mixed infections of Streptococcus zooepidemicus, Gram negative enterics and anaerobic bacteria.

5) Non-synergistic or antagonistic combinations should be avoided.

Classically, penicillins are not administered concurrently with tetracyclines. The penicillins require actively dividing bacterial cells to carry out their action on cell wall formation, while the tetracyclines' bacteriostatic action inhibits bacterial replication. Procaine penicillin G plus a trimethoprim/sulphonamide are minimally additive in antimicrobial activity against pathogens but additive against normal anaerobic flora, so combination therapy increases the risk of colitis.

Prophylactic Use of Antimicrobial Drugs

The principles upon which drugs are used prophylactically to prevent surgical infections are based on human studies, as there are few veterinary studies that evaluate these recommendations.

1) The relative risk of infection must warrant the use of prophylactic antimicrobials. Typically, they are used in surgical procedures associated with an infection rate that exceeds 5%. The risks of the prophylactic antimicrobial must be less than the risk of infection and its consequences.

2) The organisms that are likely to cause the infection and their antimicrobial susceptibility should be known or accurately predicted. Routine monitoring in surgical hospitals provides information on normal flora and nosocomial pathogens that are involved in infections.

3) The drug must be administered and must distribute to the site of potential infection before the onset of infection. To achieve high concentrations rapidly, prophylactic antimicrobials are administered intravenously.

4) Drugs used prophylactically should not be those that are routinely used therapeutically, to avoid the chance of bacterial resistance from previous exposure to the antimicrobial.

5) The duration of antimicrobial prophylaxis should be as abbreviated as possible. Most studies demonstrate that prolonging treatment for longer than 24 hours after the procedure is of no additional benefit.

7) The selected dosage regimen should be bactericidal rather that bacteriostatic. It is assumed that surgical patients are immunocompromised to some degree.

8) The selected protocol should be cost effective.

Inflammation and pain are very common clinical problems in veterinary medicine. It is a tremendously expanding area in human medicine (all the baby boomers are getting older and suffering from their physical activities like bungee jumping and mountain biking!). Many of the human anti-inflammatory drugs are being explored for use in animals. Practitioners need a basic understanding of the action of these drugs in order to appreciate clinical differences between them.

Inflammation is one of the body's primary responses to an insult or injury initiated by infectious organisms, or chemical and physical agents. The eicosanoids are the arachidonic acid derivatives that play a direct role in the inflammatory process as well as acting synergistically with other mediators such as the vasoactive amines and bradykinin. In addition, they are chemotactic, enhancing leukocyte accumulation at the site of injury. ·

Chirality and the Anti-inflammatory Drugs

An important concept in understanding the pharmacokinetics and pharmacodynamics of drugs (especially the new NSAIDs) is that some drugs exist as stereoisomers (enantiomers) . Stereoisomers are compounds with the same molecular formula, but because of asymmetrically oriented chemical groups in space, they produce nonsuperimposable mirror images and are known as "chiral" compounds. This means that they are like your hands - superimposable palm to palm, but not palm to back.

There are several (confusing) ways of referring to the configuration of asymmetric molecules. For this course, we will use the "S" (sinister) and "R" (rectus) designation for each of a pair of enantiomers. Although each member of a pair of enantiomers differs in three dimensional orientation, their physical properties (melting and boiling points, refractive index, solubility, etc) are identical. But it is very important to realize that biological systems are highly chiral environments. The pharmacokinetics and pharmacodynamic effects of each of a pair of enantiomers may be very different. Therapeutic efficacy and/or toxic effects may be related specifically to one enantiomer. However, many drugs are formulated as racemic mixtures, containing equal (50:50) amounts of each enantiomer, because chemical synthesis of pure enantiomers is very expensive. It is estimated that 25% of the drugs used clinically are chiral compounds. This includes many of the plant alkaloids and glycosides (morphine, digoxin) and the new NSAIDs. Barbiturates are used in veterinary medicine as sedatives, anaesthetics, and anticonvulsants. The "S" enantiomer of a barbiturate causes excitation, while the "R" enantiomer is responsible for CNS depression. Clearly, there is potential for improving the quality of barbiturate anaesthesia and the effects of other chiral drugs by using single enantiomer formulations.

Stereospecificity may occur in the pharmacokinetic processes of absorption, distribution, metabolism and excretion, especially if the process involves a carrier protein. If the fit of a drug molecule into the binding site on a protein, enzyme or receptor involves the chiral center, then the affinity for attachment will be different for each of a pair of enantiomers.

To further confuse the issue of sorting out the different pharmacokinetics for each enantiomer, some enantiomers can undergo "chiral inversion", as hepatic enzymes convert one form of the enantiomer to the other form. The degree of

chiral inversion for any drug varies between species, and can not be extrapolated from one species to another.

All of the propionic acid NSAIDs (ketoprofen, carprofen, etc) are chiral compounds and except for naproxen they are formulated as racemic mixtures.

Nonsteroidal Anti-inflammatory Drugs

The nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most widely used drugs in veterinary medicine. Some NSAIDs have valuable therapeutic properties, and some have a great potential for toxicity. Due to their potential for misuse, a thorough knowledge of their clinical pharmacology is important for effective use.

A. Physical Features:

The NSAIDs can be grouped according to their chemical structure: the enolic acids (phenylbutazone and dipyrone), and the carboxylic acids (aspirin, naproxen, ketoprofen, carprofen, flunixin meglumine, and meclofenamic acid). Almost all NSAIDs are weak acids and highly bound to plasma proteins such as albumin. Therefore they are well absorbed from the stomach, and most of the drug in the plasma is protein bound. Because of protein binding, most of the drug is distributed in the extracellular fluid and only low levels of NSAIDs are found in normal tissues and joint fluid. In damaged tissues and joints however, NSAID levels increase to therapeutic levels because of increased blood flow, vascular permeability, and protein penetration into sites of inflammation. Most NSAIDs undergo hepatic metabolism either through oxidation or glucuronide conjugation, prior to elimination in urine.

B. Mechanism of Action: 1) Cyclooxygenase inhibition: NSAIDs interrupt formation of thromboxane, prostacyclin and the prostaglandins from arachidonic acid. This results in antipyretic action, mild analgesia, anti-platelet effects, and some anti-inflammatory effects. Recently, it has been shown that there are different, distinct forms of cyclooxygenase. The constitutively expressed form (normal for homeostasis) is referred to as COX-1, and the inducible form (in response to injury) is referred to as COX-2. COX-1 is found in platelets, the kidneys and the gastrointestinal tract. COX-2 has been identified in fibroblasts, chondrocytes, endothelial cells, macrophages, and mesangial cells. COX-2 is induced by exposure to various cytokines, mitogens and endotoxin, and it is up-regulated at inflammation sites.

The prostaglandins produced in the gastrointestinal tract and the kidney that maintain mucosal integrity in the GI tract and renal perfusion appear to be derived from COX-1. Therefore, suppressing COX-1 activity by NSAIDs is believed to be critical to the development of toxicity. It is suggested that COX-2 selective NSAIDs would suppress prostaglandin synthesis at sites of inflammation but would spare constitutive prostaglandin synthesis in the GI tract and kidney. The currently available NSAIDs vary in their potency as inhibitors of COX-2, but virtually all are far more potent inhibitors of COX-1 than COX-2. The pharmaceutical companies are racing to develop COX-2 selective NSAIDs, but this may not be the perfect solution. If COX-2 is primarily responsible for the prostaglandins that mediate pain, inflammation and fever, it is unlikely that COX-2 selective drugs will be more therapeutically effective, because the available NSAIDs are already very effective inhibitors of COX-2. It is still possible that COX-1 prostaglandins contribute to pain, inflammation and fever, so COX-2 selective NSAIDs could be less effective. Also, COX-2 may produce beneficial prostaglandins; therefore highly selective COX-2 inhibitors may produce adverse reactions not seen with existing NSAIDs. Also, most GI ulceration is associated with significant mucosal inflammation. In these circumstances, it is likely that COX-2 is being expressed, and that the derived prostaglandins are responsible for promoting healing (it is well known that NSAIDs retard the healing of ulcers).

The following table compares the specificity of the available NSAIDs for COX-1 and COX-2. Note that the numbers are from different studies and different animal models - the numbers are meant for comparison only. The smaller the ratio, the more specific the drug is for inhibiting COX-2.

NSAID	Ratio COX-2/COX-1
aspirin	166
ibuprofen (Advil®, Motrin®)	10
ketoprofen (Anafen®)	5
naproxen (Naprosyn®)	0.6
tolfenamic acid (Tolfedine®)	16
meloxicam (Metacam®)	0.8
piroxicam (Feldene®)	300
carprofen (Rimadyl®)	1
meclofenamic acid (Arquel®)	6
ketorolac (Toradol®)	2
phenylbutazone	>5

2) Anti-inflammatory effects: NSAIDs primarily are anti-inflammatory due to their inhibition of prostaglandin production. Therefore, NSAIDs do not resolve inflammation, but prevent its on-going occurrence. So while prostaglandin production will rapidly diminish, any previously present prostaglandin must be removed before inflammation will subside. From tissue cage work, it has been shown that phenylbutazone, flunixin, meloxicam and carprofen have delayed peak concentrations at the site of inflammation and persist in inflammatory exudates for long periods of time after plasma concentrations are negligible. This explains the delayed onset and prolonged duration of anti-inflammatory action that does not correlate with plasma pharmacokinetics.

Also, cyclooxygenase inhibition does not explain all of the anti-inflammatory activity of NSAIDs. Some anti-inflammatory action appears to be related to their ability to insert into the lipid bilayer of cell and disrupt normal signals and protein-protein interactions in cell membranes. NSAIDs are more lipophilic at a low pH, such is found in inflamed tissues. In the cell membrane of neutrophils, NSAIDs inhibit neutrophil aggregation, decrease enzyme release and superoxide generation,and inhibit lipoxygenase.

3) Analgesic effects: NSAIDs act as analgesics by inhibiting COX and preventing the production of prostaglandins that sensitize the afferent nociceptors at peripheral sites of inflammation. However, there is increasing evidence that some NSAIDs have a central mechanism of action for analgesia and act synergistically with opioids. Recent work has shown that the analgesic effect of flunixin in a sheep foot rot model of pain is reversed by the administration of an opiate antagonist, naloxone. To further complicate our understanding of their analgesic action, work with the specific enantiomers of some NSAIDs have shown the "S" enantiomers to have good cyclooxygenase inhibitory effects, while the "R" forms can have weak activity against cyclooxygenase yet still produce analgesia. So for clinical use of NSAIDs of analgesics, there are a few practical principles to consider:

a) NSAIDs are likely to be more effective as analgesics when inflammation is a part of the pain process.

b) NSAIDs are more effective as analgesics when given prior to the onset of the inflammatory processes or insult.

c) The time to onset and duration of analgesic properties of NSAIDs does not correlate well with their anti-inflammatory properties. The analgesic effect is likely to have a more rapid onset and shorter duration of action than the anti-inflammatory action.

d) Dosage regimens for effective analgesia may need to be different than for anti-inflammatory properties.

4) Inhibition of bradykinin-induced edema: ketoprofen, flunixin, and tolfenamic acid inhibit bradykinin-induced edema in animal models. Since bradykinin is a mediator associated with inflammatory pain and edema, this effect contributes to the anti-inflammatory and analgesic effects of these drugs.

5) Platelet aggregation: is classically inhibited by NSAIDs by preventing thromboxane production via the COX-1 pathway. Recovery of platelet function is dependent on the pharmacokinetics of the NSAID and the mechanism of COX inhibition. Aspirin permanently modifies COX, so platelet function is only restored by the production of new platelets.

6) Anticancer effects: for some tumours, some NSAIDs appear to have anti-proliferative effects related to the inhibition of prostaglandin.

7) Anti-Alzheimer's Disease effects: chronic NSAID use is epidemiologically linked to a reduced risk of Alzheimer's Diease.

C. Drug Interactions

The occurrence and potential hazards of drug interactions must be considered with therapeutic use of the NSAIDs. In general, any two NSAIDs administered together will be additive in their effect. Since they act by the same mechanism of cyclooxygenase inhibition, higher dose of a single NSAID should produce the same response. Because all of the NSAID drugs are highly bound to plasma proteins, caution must be used when other highly protein bound drugs are administered. Competition for protein binding sites can result in dramatic increases in free drug available for pharmacological action.

1) Furosemide (Lasix®): the early preload reducing effect of furosemide (independent of diuretic effect) is mediated by prostaglandins. A decrease in prostaglandin synthesis by NSAIDs may diminish the cardiovascular effects of furosemide.

2) ACE-inhibitors (captopril, enalapril): some of the vasodilating effect of these drugs is related to prostaglandin synthesis and may be decreased by administration of NSAIDs. This interaction occurs the most with naproxen and least with aspirin, ibuprofen and piroxicam.

3) Fluoroquinolones: in humans, some fluoroquinolones given in combination with NSAIDs reduce the efficacy of GABA, the inhibitory neurotransmitter, leading to seizures.

4) Antacids, mucoprotective agents and adsorbent antidiarrheal drugs can interfere with the absorption of NSAIDs.

D. Adverse Effects of NSAIDs

The adverse effects of the NSAIDs are related to cyclooxygenase inhibition in tissues where prostaglandins are beneficial and protective. Reduction in protective prostaglandins results in blood vessels constriction and tissue necrosis in the kidney and reduction in blood flow and protective mucus production in the gastrointestinal tract results in ulcers. NSAIDs have a higher incidence of toxicity in neonates because kidney function is not fully developed. When indicated in neonates, should be administered at the lowest possible doses. NSAIDs should be administered very cautiously to dehydrated animals. As they predominately distribute in extracellular water (low Vd), plasma concentrations will be greater than normal in the dehydrated animal and more likely to cause toxicity. Many of the NSAIDs are hepatically metabolized by glucuronidation. As the cat is deficient in the glucuronidation enzyme system, drugs like aspirin and acetaminophen, that are eliminated by this route have prolonged elimination half-lives and high potential for toxicity. NSAIDs such as ketoprofen, that are cleared by alternate pathways, can be safely used in cats. Glucuronidation also results in significant enterohepatic recirculation. This tends to be greater in dogs than in other species, such as humans. This is why dogs are more sensitive to the intestinal toxicity of NSAIDs such as naproxen than humans, despite its low COX-2/COX-1 ratio.

1) GI Toxicity: Gastrointestinal side effects are common to all NSAIDs and are the dose-limiting factor for these drugs. Some NSAIDs, including aspirin, are topically irritating to the gastric mucosa. This topical irritation is mainly seen with acidic NSAIDs due to "ion trapping" and accumulation in gastric epithelial cells. GI prostaglandins are natural inhibitors of gastric acid secretion and support mucosal blood flow. NSAID inhibition of prostaglandin biosynthesis results in increased acidity and decreases mucosal blood flow and mucous production, leading to ulcer formation. NSAIDs should not be used in conjunction with glucocorticoids as they can potentiate gastrointestinal toxicity.

Treatment of NSAID gastrointestinal toxicity is intensive and mainly symptomatic. The hypoproteinemia that results from loss of plasma proteins into the ulcerated gastrointestinal tract can be corrected with intravenous infusions of plasma. The fluid and electrolyte losses that accompany the diarrhea are managed with commercially available intravenous fluids. Broad-spectrum antibiotics are indicated when there is evidence of bacterial septicemia. Pain must be managed with narcotic analgesics. Anti-ulcer medications may be beneficial and speed recovery. Surgical removal of damaged sections of intestine may be necessary in some cases. Recovery is usually slow and in severe cases, the prognosis is always guarded.

2) Hematological Toxicity: NSAIDs should not be used in animals with concurrent haematological disorders or potential bleeding disorders, including thrombocytopenias and vonWillebrand's disease. Their use should be avoided during or near surgeries in noncompressible sites where hemorrhage may be a problem.

3) Nephrotoxicity: The renal toxicity of NSAIDs is a major concern, particularly in the peri-operative period. NSAIDs typically have little effect on renal function in normal animals. However, they decrease renal blood flow and glomerular filtration rate in patients with congestive heart failure, that are hypotensive or hypovolemic (esp during surgery) or have chronic renal disease. Under these circumstances, acute renal failure may be precipitated as NSAIDs block the ability of renal prostaglandins to mitigate the vasoconstrictive effects of norepinephrine and angiotensin II on glomerular arteries. Currently, it is thought that COX-1 is responsible for renal protection, so COX-2 selective drugs may avoid this problem. A more severe dose-dependent toxicity associated with NSAIDs is renal papillary necrosis. Although attributed to impaired renal blood flow, other mechanisms such as direct nephrotoxicity of the drug or its metabolites, may also be involved. Acetaminophen and phenylbutazone both produce nephrotoxic metabolites.

4) Hepatotoxicity: NSAIDs are well documented to produce idiosyncratic hepatotoxicity in humans. Recent reports of carprofen-induced hepatotoxicity in dogs may reflect an idiosyncratic hepatotoxicity with this drug. Hepatitis has been reported in horses receiving very large doses of phenylbutazone. In dogs and cats, acetaminophen hepatotoxicity may have time to develop if the primary haematological toxicity is appropriately treated.

Salicylates

Salicylates include aspirin (acetylsalicylic acid, ASA), magnesium salicylate, sodium salicylate, and nonacetylated salicylates such as diflunisal (Dolobid).

A. Pharmacokinetics

1) Absorption: ASA is only available in oral form. Because it is a weak acid, it is best absorbed in the acidic environment of the upper GI tract. In dogs, enteric-coated aspirin reduces GI irritation, but absorption may be erratic and often incomplete. Absorption is slow in cattle, but bioavailability is about 70%.

2) Distribution: during absorption, aspirin is partially hydrolysed to salicylic acid and distributed throughout the body. Highest concentrations are attained in the liver, heart, lungs, renal cortex and plasma. Extent of protein binding is moderate (about 60%) and depends on species, and drug and albumin concentrations.

3) Metabolism: occurs in the liver by glycine and glucuronide conjugation. Because cats are deficient in the glucuronide enzyme system, they have prolonged drug elimination half-lives and significant drug accumulation occurs.

4) Excretion: salicylates and their metabolites are excreted in urine via glomerular filtration and active tubular excretion. In the horse, salicylic acid is the primary salicyl compound found in urine while in other domesticated species varying quantities of metabolites are excreted. Significant tubular reabsorption occurs which is highly pH dependent. There are tremendous species differences in the elimination rate.

Species	t ½ (hr)
Cats	38
Cattle	0.5
Dogs	8.6
Goats	0.8
Horses	1.0
Swine	5.9

B. Clinical Use: Anti-inflammatory, Analgesic and Anti-pyretic Effects

Dogs and Cats: aspirin is used to treat mild to moderate inflammatory conditions. Dose is 10-25 mg/kg every 8-12 hours for dogs and every 48-72 hours for cats.

C. Clinical Use: Antiplatelet Therapy

Aspirin is the most effective NSAID for antiplatelet therapy. In animals, antiplatelet therapy may be beneficial in the management of heartworm disease, feline thromboembolism, pulmonary thrombosis, equine laminitis, disseminated intravascular coagulation, and equine verminous arteritis. Aspirin acetylates the cyclooxygenase present in platelets. This inhibits the formation of thromboxane A2, which is responsible for vasoconstriction and platelet aggregation. Since platelets can not produce additional cyclooxygenase, the effect is irreversible. Since the platelets are very sensitive to aspirin, low doses at long intervals (2-3 days) are effective. A precise antiplatelet dose has not been established. 10-25 mg/kg twice a week is suggested for cats, 10 mg/kg every 24 to 48 hr for dogs, and 30 mg/kg BID for horses.

E. Adverse Effects:

1) The most common adverse effect is gastric or intestinal irritation with varying degrees of gastrointestinal blood loss.

2) Aspirin therapy should be discontinued one week prior to surgery.

3) If used in pregnant animals, aspirin may delay parturition.

4) Acute aspirin overdose causes a severe metabolic acidosis. Administer sodium bicarbonate IV to treat acidosis and alkalinize urine to reduce reabsorption and administer a diuretic like mannitol to increase elimination.

5) An aspirin dose of 50 mg/kg increases serum digoxin levels up to 130% of normal. ·

Phenylbutazone (bute, PBZ)

Phenylbutazone has analgesic, anti-inflammatory, and antipyretic activity from inhibition of cyclooxygenase.

A. Pharmacokinetics

1) Absorption: PBZ is available in intravenous and oral formulations. Following oral administration, it is well absorbed, but time to peak concentration may be delayed by feeding. The drug is distributed throughout the body, with highest concentrations in the liver, heart, kidney, lungs and plasma.

2) Metabolism: PBZ is metabolized in the liver to oxyphenbutazone, an active metabolite that is eliminated slower from the body than PBZ. The capacity of the liver to metabolize PBZ becomes overwhelmed at relatively low drug doses. Therefore, increasing doses of PBZ result in disproportionately increasing plasma concentrations which can easily result in toxicity.

3) Elimination: Like the salicylates, PBZ has tremendous species differences in regards to elimination.

Species	t½ (hr)
Cats	18
Cattle	32-60
Dogs	2.5-6
Goats	14-19
Horses	3-8
Pigs	2-6

B. Clinical Use

Dogs and Cats: PBZ is used in dogs for anti-inflammatory and analgesic effects, but rarely used in cats. Dose: 14 mg/kg PO TID initially, titrate to lowest effective dose. Do not exceed 800 mg/day regardless of body weight.

C. Adverse Effects:

1) Gastrointestinal: are the most important adverse effects of PBZ in horses and ruminants. Clinical diseases include hypoproteinemia, protein losing enteropathy, ulcerations (oral, esophageal, gastric [abomasal], cecal and right dorsal colon).

2) In dogs, PBZ may produce the same GI effects as well as blood dyscrasias.

3) Renal effects: renal papillary necrosis (renal medullary crest necrosis) occurs due to inhibition of prostaglandins that maintain renal blood flow and/or direct toxicity of drug and metabolites.

4) PBZ may interact with other highly protein bound drugs: phenytoin, warfarin, etc. ·

Flunixin Meglumine

Flunixin meglumine (Banamine®, generics) is a very potent inhibitor of cyclooxygenase that is approved for use in horses, but is used extralabel in other species as well. It is available in injectable and oral granule formulations.

A. Pharmacokinetics

1) Absorption: Flunixin is rapidly absorbed following oral administration, with a bioavailability of 80% and peak serum levels within 30 minutes. The onset of action is within 2 hours and duration of action is 36 hours. Its volume of distribution is 0.2 L/kg in horses and it is highly protein bound.

2) Elimination: Flunixin appears to be renally eliminated, and can be measured in urine for 48 hours after a single dose. The plasma half-life of flunixin does not correlate directly with its clinical effect (probably due to recently documented central effects).

Species	t½ (hr)
Horse	1.6-2.5
Cow	8
Dog	3.7

B. Clinical Use

Dogs: Flunixin can be used in the therapy of disk disease and other musculoskeletal disorders, endotoxic shock, ophthalmic diseases and surgery, general surgery and viral diarrhea. Dose is usually 1.1 mg/kg SID. Therapy for longer than 3 days often results in toxicity. No recommendations are available for use in cats (Flunixin has less anti-platelet activity than ASA). Currently, flunixin is not recommended in small animals as there are safer NSAIDs labelled for small animals. ·

Ketoprofen

Ketoprofen (Anafen®) is a propionic acid derivative labelled in Canada for use in dogs, cats and horses. Anafen is available as 5, 10 and 20 mg tablets and a 10 mg/ml solution. Thereis a 100 mg/ml solution for horses. There is a 50 mg/tablet human generic available in Canada. In the US, there is a veterinary injectable for horses (100 mg/ml), and OTC human formulations that contain 12.5 mg/tablet. Initial work suggested that ketoprofen had an inhibitory action on lipoxygenase in addition to cyclooxygenase inhibition. This was claimed as a therapeutic advantage, giving it glucocorticoid-like activity. However, clinical work in horses and other species has shown that ketoprofen only blocks the production of cyclooxygenase derived mediators.

A. Pharmacokinetics

1) Ketoprofen is highly protein bound, weakly acidic, and has low lipid solubility (Vd 0.16 L/kg).

2) Ketoprofen is metabolized by the liver by conjugation reactions. Like carprofen, ketoprofen exists as two enantiomers, which possess different elimination half-lives, but is formulated as a racemic mixture. The "S" enantiomer is associated with anti-prostaglandin activity and toxicity, while the "R" enantiomer is associated with analgesia and does not produce GI ulceration. Because of chiral inversion, the "S" isomer predominates in horses, dogs and cats, where the "R" enantiomer predominates in sheep.

3) There are marked species differences in ketoprofen plasma elimination half-life and clearance of the enantiomers. Typically, the plasma elimination t½ is short - approximately 1 hour in horses, 1.6 hr in cats and 5 hr in dogs. Ketoprofen accumulates in inflammatory exudates in the horse, where the t½ of "S" is 22.6 hr and "R" is 19.7 hr.

4) The maximum anti-inflammatory effects of ketoprofen occur at 12 hours after a dose and last for 24 hours, illustrating that the anti-inflammatory effects are not related to plasma concentrations.

5) The manufacturer claims a proteoglycan stimulatory effect from ketoprofen, but this has only been demonstrated in juvenile cartilage in culture and has not been demonstrated in vivo.

B. Clinical Use

Dogs and Cats: ketoprofen was the first NSAID approved in Canada for cats. In a recent study, dogs given ketoprofen alone after orthopaedic surgery had a greater level of analgesia that lasted longer than dogs that were treated with oxymorphone or butorphanol. Cats and dogs can be given a dose of 2 mg/kg by injection (IV, IM or SC) the first treatment, followed by 1 mg/kg PO (tablets) SID. The label dose is for 5 days of therapy, but for chronic therapy the lowest possible dose and the longest possible dosing interval should be used. Ketoprofen appears to be very useful in the management of pain and inflammation of cats with feline lower urinary tract disease. Clinically, two Vietnamese Pot Bellied Pigs with hip dysplasia are doing well on chronic ketoprofen therapy twice weekly.

C. Adverse Effects

Ketoprofen-induce adverse effects are rare but typical for an NSAID. There are reports of animals developing acute renal failure following anesthesia and surgery when ketoprofen was administered in the perioperative period. There is an association with bleeding if used in the peroperative period. ·

Carprofen

Carprofen (Rimadyl®) is a propionic acid NSAID that contains an asymmetrical carbon atom and exists as "S" and "R" enantiomers. It is available in the US and Canada as a tablet for dogs. In Europe, it is also available in an injectable form that will be available in North America in the near future.

A. Pharmacokinetics

1) Carprofen has greater activity against COX-2 than COX-1, but its overall cyclooxygenase inhibition is weak. In inflammatory models, neither cyclooxygenase or lipoxygenase pathway products are inhibited by carprofen, so the anti-inflammatory and analgesic activity may be due to central effects.

2) After intravenous injection in the horse, the "S" enantiomer of carprofen undergoes rapid chiral inversion in the liver, so that by 12 hours 15% of the plasma concentration is "S" and 85% is "R". This is significant, as most of the COX inhibition anti-inflammatory action is attributed to the "S" enantiomer. Analgesia is attributed to the "R" enantiomer. In a dog study, chiral inversion of carprofen did not occur. In the cat, the "R" enantiomer predominates in plasma, but is suggested to be due to a difference in Vd and rate of clearance for each enantiomer rather than chiral inversion. In dogs, carprofen has a 90% bioavailability, a small Vd, and an elimination t½ of 8 hours. In cats, the t½ is extremely variable at 9-49 hr. It is 99% bound to plasma proteins. In the dog, carprofen is eliminated by biotransformation in the liver, followed by rapid excretion of the metabolites into the feces and urine. Some enterohepatic recycling occurs.

B. Clinical Use

Carprofen is currently approved for chronic use in dogs with osteoarthritis in the United States and Canada. The oral caplets are dosed at 2.2 mg/kg BID. Single doses have been used in cats, but multiple doses are not recommended due to toxicity.

C. Adverse Effects

When dosed at 2.2 mg/kg BID in dogs, gastrointestinal side effects were minimal and the reported adverse drug rate for carprofen is only 0.2%. However, there have been recent reports of idiosyncratic hepatotoxicity in dogs receiving carprofen. In a report of 21 affected dogs, 13 were Labrador retrievers. The remaining dogs were of various breeds. The diagnosis was based on clinical signs of hepatopathy and pathological demonstration of hepatic necrosis consistent with a drug reaction. Eight of the affected dogs were given higher than recommended doses. The onset of clinical signs ranged from 5-30 days. Seven dogs had evidence of renal tubular damage as well as hepatopathy. Survival was 100% in affected Labrador retrievers, but only 50% in other breeds (some of whom had potential predisposing conditions). Currently, the manufacturer recommends baseline and repeat serum chemistry monitoring with carprofen administration, especially in geriatric patients. Some dogs will have evidence of increased liver enzymes post-carprofen administration, without clinical signs of hepatitis, that resolves with discontinuation of the drug. Continued use of carprofen should be carefully monitored in these patients. ·

Ketorolac

Ketorolac (Toradol®) is a human labelled NSAID, that in human and laboratory animals greater analgesic potency than other NSAIDs. It is available in injectable and oral formulations. Ketorolac provided better postoperative analgesia in dogs than flunixin, butorphanol and oxymorphone. The duration of analgesia is approximately 8-12 hours. The pharmacokinetics have recently been investigated in dogs: Vd 0.42 L/kg, bioavailability (oral) = 75%, and elimination half-life 8 hours (with considerable individual variation). It can be dosed at 0.5 mg/kg IV TID, or 0.3 mg/kg PO BID, but repeated doses have considerable potential for causing GI and renal toxicity. Treated dogs should receive misoprostol (synthetic prostaglandin E). ·

Tolfenamic Acid ·

Tolfenamic acid (Tolfedine®) is available for use in dogs and cats in Canada as oral tablets (6, 20 and 60 mg) and an injectable solution (40 mg/ml).

A. Pharmacokinetics

1) Tolfenamic acid is a fenamate NSAID, like meclofenamic acid. It is similar to other NSAIDs in inhibiting cyclooxygenase, and plus it has a direct antagonistic action on prostaglandin receptors. In an experimental renal failure model in dogs, elimination of tolfenamic acid was actually increased, indicating a shift to hepatic metabolism.

2) In dogs, it has a relative large Vd for an NSAID of 1.2 L/kg and an elimination half-life of 6.5 hours with enterohepatic recycling. Peak plasma concentrations are reached in 2-4 hours in non-fasted dogs. When administered with food, the enterohepatic recirculation is intensified, resulting in greater, but more variable bioavailability than in fasted dogs. Its anti-inflammatory action lasts for 24-36 hours.

B. Clinical Use

Tolfenamic acid was the first NSAID with a chronic use label in Canada. The dose in dogs and cats is 4 mg/kg SID for 3-5 days for acute pain and (for dogs only) 4 mg/kg SID for 3 days out of 7 for chronic pain management. Chronic therapy in cats has not been addressed by the manufacturer. Tolfenamic acid has been shown to reduce the intensity of miosis and corneal edema when given SC at 4 mg/kg two hours prior to ophthalmic surgery. The injectable solution is recommended for a single SC or IM dose of 4 mg/kg on the first day of therapy followed by the oral tablets for 2 to 4 more days. It is commonly given post-operatively for pain control.

C. Adverse Effects

Despite a fairly large COX-2/COX-1 ratio (16), tolfenamic acid has a good safety profile in dogs and cats. In experimental studies, gastrointestinal ulceration and nephrotoxicity were only seen with doses > 10 times the therapeutic dose. In Europe, where the tolfenamic acid is used in dogs, cats and cattle, reports of toxicity (typical for NSAIDs - GI and renal) are infrequent. It has sufficient anti-thromboxane activity that tolfenamic acid is not safe to administer pre-surgically. ·

Meloxicam

Meloxicam (Metacam®) has recently been introduced for use in dogs in Canada. It is a widely used human NSAID (Mobic®) in Europe and has recently been approved for human use in North America. It is available as a 1.5 mg/ml syrup with a syringe dispenser. The syringe is calibrated by weight of the dog or it can be given one drop at a time (0.1 mg per drop). A loading dose of 0.2 mg/kg is given the first day followed by 0.1 mg/kg SID chronically (personally, I don't use the loading dose and can often use 0.05 mg/kg chronically). The injectable solution contains 5 mg/ml and is administered SC once at 0.2 mg/kg, and followed by oral therapy. The company is seeking a label for a single dose injection in cats. Some practitioners use the oral form at 1 drop/cat/day.

A. Pharmacokinetics

1) Meloxicam is an oxicam, related to piroxicam, but with a much greater safety record. It is an oral syrup with a pleasant taste, intended specifically for chronic therapy of osteoarthritis in dogs. Meloxicam has high activity against COX-2; in clinical studies, it had no affect on platelet aggregation or renal prostaglandin synthesis, showing a sparing of COX-1.

2) Elimination t½'s for meloxicam are species-specific.

Species	t½ (hr)
Horse	2.7
Cow	13
Pig	4
Dog	12-36
Human	20-50

B. Clinical Use

Meloxicam is the second NSAID to be approved for chronic treatment of osteoarthritis in dogs in Canada. Meloxicam was highly efficacious in dogs in two studies of experimentally induced synovitis.

C. Adverse Effects

In clinical trials in Canada, meloxicam was rated highly efficacious and well tolerated, with only a few reports of mild GI intolerance. In dogs with renal failure, no further deterioration was seen when given meloxicam. Adverse drug reports have been rare with use in Canada. ·

Piroxicam

Piroxicam (Feldene®) is a relatively new NSAID for treating arthritis in people, with similar efficacy to aspirin and other NSAIDs. Its major advantage is a long t½ in humans (45 hrs) and dogs (40 hrs), which permits infrequent dosing. Piroxicam has greater activity against COX-1 than COX-2, therefore it is associated with a high incidence of gastrointestinal ulceration. It is probably a good idea to treat patients on piroxicam with misoprostol as well. Piroxicam has antitumour activity, and has recently been shown to be useful in the treatment of transitional cell carcinomas of the bladder in dogs. Recommended dose is 0.3 mg/kg PO SID. There are anecdotal reports of its efficacy in patients with squamous cell carcinoma, malignant melanoma and metastatic osteosarcoma. ·

Naproxen and Ibuprofen

Naproxen (Naprosyn®, Equiproxen®) and ibuprofen (Advil®, Motrin®) are propionic acid derivatives with potent anti-inflammatory and analgesic effects by cyclooxygenase inhibition. These drugs are available over the counter - both in the US and only ibuprofen in Canada. Most use is from owners administering to their own animals without veterinary instruction.

A. Pharmacokinetics

Naproxen is highly protein bound, and has very species dependent pharmacokinetics.

Species	t½ (hr)
Horse	4.5
Pig	5
Dog	74 (breed dependent)
Human	15

Ibuprofen is occasionally administered to small animals for conditions such as osteoarthritis. In dogs, it has a t½ of 4 hrs and a Vd of 0.164 L/kg. Dogs are more predisposed than humans to ibuprofen ulcerogenic effects because of higher gastrointestinal absorption and longer elimination half life which leads to slower drug elimination and higher blood concentrations.

B. Clinical Use

Dogs: naproxen is sometimes used in the treatment of musculoskeletal conditions in dogs. Because of the long elimination half-life, it is difficult to find a consistently safe dosage regimen for dogs, but 1.2-2.8 mg/kg PO SID has been suggested. Use is not recommended as there are safer and more effective NSAIDs approved for use in dogs. ·

Meclofenamic Acid

Meclofenamic acid (Arquel®) is an oral granule used in horses and dogs for the treatment of musculoskeletal conditions. It has pharmacological action and pharmacokinetics similar to aspirin. The plasma half-life in horses ranges from 1-8 hours. Therapeutic efficacy does not correlate well with plasma concentration, as the onset of clinical action is 36 to 96 hours after administration and significant efficacy can be seen for days following a dose. Dose for dogs: 1.1 mg/kg PO SID, for horses: 2.2 mg/kg PO SID. Meclofenamic acid is a very palatable oral granule used in horses for the treatment of musculoskeletal conditions. This drug has not been extensively researched in veterinary medicine. Feeding prior to dosing may delay absorption of meclofenamic acid. Repeated daily dosing does not result in drug accumulation, therefore this is a useful drug for chronic inflammatory conditions. Many horses can be maintained comfortably with twice weekly dosing without side effects. In clinical studies, researchers found clinical improvement in the lameness of 2/3 of treated horses, but found it difficult to predict which horses would respond to meclofenamic acid. At normal doses, some decrease in plasma protein concentration may be seen. Doses of 6-8 times the label dose result in toxicity, including mouth ulcers, anorexia, depression, edema and weight loss. Chronic administration at the label dose to stallions and pregnant mares caused no toxic effects. ·

Acetaminophen

Acetaminophen (Tylenol®) is a phencacitin derivative that has poor-anti-inflammatory activity (it only acts on centrally produced cyclooxygenase, but not peripheral), but good analgesic activity. It is not particularly useful in veterinary medicine, as cats are prone to toxicity from the metabolites of acetaminophen and develop methemoglobinemia and Heinz body anemia from low doses and studies have not shown it to be useful in musculoskeletal disorders in dogs and it may cause GI and hepatic toxicity in dogs (toxic dose is 100 mg/kg). Acetaminophen is rapidly absorbed from the gastrointestinal tract and is conjugated in most species by sulfation or glucuronidation. Because cats lack the specific glucuronyl transferase enzyme to detoxify the reactive biotransformation metabolites of acetaminophen, these reactive metabolites bind to cellular macromolecules and cause RBC lysis and hepatic necrosis. ·

Etodolac

Etodolac (EtoGesic®) is available as 150 or 300 mg tablets in the US for chronic use in dogs with osteoarthritis. Etodolac is fairly COX-2 specific, and like ketoprofen and carprofen is a racemic mixture of "S" and "R" enantiomers. Etodolac shows tremendous stereospecificity in protein binding and microsomal metabolism. It has a high volume of distribution, predominantly due to low protein binding of the "S" enantiomer. Etodolac is primarily eliminated by hepatic metabolism and fecal excretion, and undergoes enterohepatic recirculation. It is dosed at 10-15 mg/kg PO SID. There is some association with hemorrhage during orthopedic surgery in dogs receiving 15 mg/kg. In toxicity studies, high doses produced typical NSAID gastrointestinal ulceration. In an experimental model of renal failure, etodolac had no effect on renal function. ·

COX-2 Selective Drugs

Celecoxib (Celebrex) is a new nonsteroidal anti-inflammatory drug (NSAID). Incontrast to other NSAIDs that inhibit both isoforms, celecoxib specifically inhibits COX-2, with a 375-fold greater specificity for COX-2 than COX-1. Although data are limited, the adverse effect profile of celecoxib may be more favourable than that of existing NSAIDs. The most common adverse effects in studies were headache, dyspepsia, and upper respiratory tract infections, which occurred at a similar or less than placebo. Since celecoxib's release on the market, 10 deaths of patients taking celecoxib and 11 cases of GI hemorrhage have been reported. Of the 10 deaths, two patients died of acute GI hemorrhage. It is unknown what effect, if any, celecoxib has on the kidneys. In light of this information and the lack of safety data beyond six months, caution should used with celecoxib until additional safety data is available.

Epidemiologic studies have shown that people who regularly NSAIDS to treat conditions such as arthritis have lower rates of colorectal polyps, colorectal cancer, and colorectal cancer deaths. Based on these promising epidemiologic data as well animal models treated with COX inhibitors and human pathologic samples showing high levels of COX-2 expression in cancerous tissues, the National Cancer Institute is supporting studies of the use of several COX inhibitors.

The pharmacokinetics of celecoxib, a cyclooxygenase-2 inhibitor, was characterized in beagle dogs. Celecoxib is extensively metabolized by dogs to a hydroxymethyl metabolite with subsequent oxidization to the carboxylic acid analog. There are at least two populations of dogs, distinguished by their capacity to eliminate celecoxib from plasma at either a fast or a slow rate after i.v. administration. Within a population of 242 animals, 45.0% were of the EM phenotype, 53.5% were of the PM phenotype, and 1.65% could not be adequately characterized. The mean (+/-S.D.) plasma elimination half-life of celecoxib were 1.72 +/- 0.79 h for EM dogs and 5.18 +/- 1.29 h for PM dogs. Hepatic microsomes from EM dogs metabolized celecoxib at a higher rate than microsomes from PM dogs. Celecoxib is 98.5% protein bound in dogs. This drug should not be administered to dogs without further research.

There is no information available regarding rofecoxib (Vioxx) in dogs.

Anti-leukotriene Drugs

In the US and recently in Canada, leukotriene receptor antagonist drugs have been released for use in humans with asthma, and some people are trying them in dogs, cats and horses. They are not bronchodilators, so they are not used for acute bronchoconstriction. They inhibit 5-lipoxygense, the enzyme that catalyses the formation of leukotrienes from arachidonic acid. Leukotrienes produce numerous biological effects, including augmentation of neutrophil and eosinophil migration, neutrophil and monocyte aggregation, leukocyte adhesion, increased capillary permeability, and smooth muscle contraction. These effects contribute to inflammation, edema, mucus secretion, and bronchoconstriction in the airways of asthmatics. These drugs have a lot of drug interactions, so be sure to read the label information and extrapolate carefully!

Zafirlukast (Accolate®) is being used in cats for feline asthma despite little evidence that leukotrienes play a major role in this disease. Anecdotally, it is dosed at 1/4-1/2 tablet PO BID and up to 1 tablet for large dogs.