Dog breed specific differences in clinical pharmacology (Proceedings)

The field of veterinary clinical pharmacology is growing tremendously with new veterinary drugs introduced and with the use of human labeled drugs in veterinary patients. Veterinary approved drugs undergo extensive efficacy and safety testing in dogs, whereas many human approved drugs undergo minimal safety and efficacy testing in animals. Most often studies in dog studies are conducted in healthy Beagles with the assumption that Beagles provide a good estimate of drug disposition and effects for the general population of dogs, however this is a big assumption. It is well recognized that subpopulations of humans have distinct patterns of drug disposition and may be more susceptible to drug toxicity or lack of efficacy.

The primary differences that occur within subpopulations are pharmacokinetic differences, that is altered drug absorption, distribution, metabolism, or elimination. The end effect is often drug toxicity, due to increased drug concentrations at the site of toxicity from increased drug penetration or decreased drug metabolism and elimination. However it is also important to realize that the adverse effect may be lack of efficacy in subpopulations. For example, tramadol is metabolized to an active metabolite which is primarily responsible for its analgesic effects. A subpopulation of humans is deficient in the cytochrome P450, CYP2D6, responsible for metabolism to the active metabolite, and as a result requires significantly higher doses of tramadol to provide analgesia.

P-glycoprotein mutation

The p-glycoprotein (p-gp) efflux pump is an active transport system present in the central nervous system (a component of the blood-brain barrier), gastrointestinal tract, biliary system, and renal tubules, among other locations. Some drugs are substrates for p-gp, whereas others are not, therefore it is important to realize not all drugs are affected by changes in the expression of p-gp. The primary effect of p-gp in the blood-brain barrier is to efflux absorbed drug from the CNS, decreasing exposure. Variable effects of p-gp in the intestines have been observed. Digoxin, a substrate for p-gp, is effluxed back into the lumen of the intestine by p-gp with a net effect of decreased drug absorption and bioavailability. In contrast, phenytoin, a substrate for p-gp, is actively transported from the intestinal lumen with a net effect of increased drug absorption and bioavailability. P-glycoprotein effluxes drug into the biliary tract and urinary tract resulting in increased drug elimination.

P-glycoprotein expression is related to a mutation on the allele that encodes p-gp. A "normal" animal, homozygous for the functional allele (+/+), has a normal expression of p-gp (toxic dose of ivermectin > 2 mg/kg). An animal which is heterozygous for the nonfunctional mutation (±) has decreased expression of functional p-gp and exhibits an increased sensitivity to p-gp substrates (toxic dose of ivermectin 0.3 mg/kg). An animal homozygous for the mutation (-/-) is the most susceptible to toxicity from p-gp substrates (toxic dose of ivermectin 0.12 mg/kg). Therefore it is important to know if an animal is homozygous normal, heterozygous for the mutation, or homozygous for the mutation.

The breeds affected by the p-gp mutation are primarily herding dogs (see table below). However the rate of the mutations in dogs is variable with Collies most frequently affected by the mutation. It is important to realize not all Collies are affected, with only ~ 31% being homozygous for the mutation. Some p-gp substrates may show toxicity in heterozygous mutants.

There have been some drugs well documented in veterinary medicine as producing increased toxicity in dogs affected by p-gp mutations. Ivermectin is the most well recognized drug that can result in toxicity in affected breeds. Ivermectin toxicity is probably do mostly to increased drug penetration in to the CNS. However it is important to note that heartworm prevention doses do not produce ivermectin toxicity as it is such a low dose. Loperamide toxicity has also been described in dogs deficient in p-gp when administered anti-diarrheal doses. Loperamide is typically excluded from the CNS, due to p-gp efflux, but signs of opioid toxicity (i.e. profound sedation, disorientation, ataxia) have been described in dogs with homozygous mutations. The clinical signs of loperamide toxicity can persist for greater than 24 hours. Digoxin toxicity has also been described in p-gp deficient animals, with toxicity probably due to increased oral bioavailability. Digoxin can still be administered to these breed, but a lower dose must be administered and more frequent therapeutic drug monitoring is recommended. Profound sedation and prolonged effects of acepromazine have been reported in dogs with p-gp mutations, with the current recommendation to reduce the dose by 25% in heterozygous dogs and up to 50% in dogs with homozygous mutations. The increased effect is probably due to increased drug penetration into the CNS. Butorphanol is also a p-gp substrate with p-gp dogs experiencing enhanced effects. Increased butorphanol effects are probably due to increased penetration into the CNS. Butorphanol has a large safety margin (LD50 10 mg/kg IV in dogs) and currently recommended doses in dogs are probably too low anyway. However, it would be most conservative to use another opioid, such as morphine or hydromorphone, which are not affected by p-gp mutations. Finally, several chemotherapeutic agents have demonstrated increased toxicity in dogs with p-gp mutations. Vincristine, vinblastine, and doxorubicin appear to have increased toxicity in dogs with p-gp mutations, probably as a result of decreased drug elimination.

Greyhounds

Studies comparing Greyhounds to non-sighthound dogs have documented Greyhounds have a significantly larger proportion of muscle mass and significantly smaller proportion of fat. Recent studies have documented that dose adjustments may be necessary for some drugs in Greyhounds due to altered drug distribution. Amikacin, an aminoglycoside antimicrobial, has a significantly smaller volume of distribution (Vd) in Greyhounds. The result of the smaller Vd is proportionally higher plasma concentrations despite administration of a similar dose, compared to non-sighthound dogs. Toxicity of aminoglycosides have been well correlated to total drug administered (as determined by the area under the plasma curve) and persistent drug concentrations (failure to maintain a drug-free interval) The current dose recommendation is to decrease the dose of amikacin from 15 mg/kg IV & 20 mg/kg SC/IM q 24 hours to 10 mg/kg IV and 15 mg/kg SC q 24 hours in Greyhounds. A similar dose reduction for gentamicin is also expected, for Greyhounds 6 mg/kg IV, 9 mg/kg SC/IM q 24 hours. A similar difference in the disposition of propofol was noted with Greyhounds exhibiting a significantly smaller Vd and higher plasma concentrations. However, since propofol is typically administered "to effect," differences are typically not noted clinically. Recovery (time to standing) was also significantly longer in Greyhounds, 22 ± 3 minutes as compared to 15 ± 4 minutes, but the difference is clinically irrelevant.

Greyhounds also appear to have decreased metabolism of certain drugs, although this has not been well defined. Drugs are primarily metabolized by 2 major pathways, Phase I (metabolic reactions catalyzed primarily by cytochrome P450) and Phase II metabolism (primarily conjugation reactions). Greyhounds appear to have decreased CYP activity compared to other dog breeds; however the specifics have not been fully described. The activity of cytochrome P450 2B11 (CYP2B11), as determined by propofol metabolism, was significantly slower in Greyhounds as compared to Beagles, but was not different than mixed breed dogs. Early studies on thiopental indicated Greyhounds eliminated the drug slower than mixed breed dogs and is the major reason for the increased duration of effect. Pretreatment of Greyhounds with phenobarbital, which induced CYP activity, resulted in similar anesthetic effects as reported for mixed breed dogs indicating decreased metabolism is the primary reason for the prolonged anesthetic effects. Antipyrine is a marker for general CYP activity and was eliminated significantly slower in Greyhounds as compared to mixed breed dogs which also indicates decreased CYP activity occurs in Greyhounds. Preliminary data indicate the CYP3A activity in Greyhounds is similar to other dog breeds. CYP3A is the primary CYP isoform expressed in the human liver. Greyhounds also exhibited a similar plasma profile as poor metabolizer Beagles for celecoxib (see below). Celecoxib is primarily eliminated by CYP2D in dogs, but other isoforms contribute.

Beagles

A polymorphism has been described in Beagles for the elimination of celecoxib. Studies indicated celecoxib is a CYP 2D substrate. Approximately 50% of the Beagles were poor metabolizers, while 50% of the Beagles were extensive metabolizers. Celecoxib administered to Greyhounds exhibited similar pharmacokinetics as the poor metabolizer Beagles. It is unclear the relevance of this polymorphism and how or if it applies to other drugs.

Giant schnauzers and Alaskan Malamutes

Azathioprine is an immunosuppressant drug used for a variety of conditions in dogs including autoimmune mediated hemolytic anemia. Studies have correlated toxicity (primarily bone marrow suppression) with the activity of the enzyme thiopurine methyltransferase (TPMT). Azathioprine is metabolized to 6-mercaptopurine (6MP), an active metabolite, than TPMT inactivates 6-MP. Giant Schnauzers have been identified as a breed with decreased activity of TPMT, which is expected to correlate with increased toxicity if dosage adjustments are not made. In contrast, Alaskan Malamutes have markedly increased activity of TPMT, which presumably correlates with decreased drug efficacy if dose adjustments are not implemented.