AbstractsBiology & Animal Science

Hepatic biotransformation and drug transport mechanisms vary significantly between species. While these processes that determine largely the kinetic behavior of drugs have been studied abundantly in dogs, corresponding investigations in cats are hardly available, despite the increasing role of cats in veterinary practice, following the increasing popularity of cats in The Netherlands. Drug intolerance, toxic side effects or a lack of efficacy have been observed when treating feline patients with drugs licensed for other species. In this thesis, we designed a series of experimental in vitro approaches that address the major phases of hepatic drug metabolism and excretion. Main findings: Phase I - Cytochrome P450 (CYP) activities and substrate specificities differ between feline and canine liver microsomes, as expected. Also gender differences are observed. However, fluorometric assays intended for rapid analysis of CYP activity of patient-derived liver biopsies, was found to be non-realistic for a clinical routine as its sensitivity is too low and in turn the amount of liver tissue needed for quantitative results is too high to allow the application in clinical diagnostics. - Diazepam, a drug that is regularly associated with undesirable hepatic side effects in cats, is converted in feline hepatic microsomes mainly into temazepam, while in dog microsomes nordiazepam appears to be the principle metabolite. The lack of quantifiable formation of nordiazepam and oxazepam in feline liver microsomes suggests a feline CYP2B11 ortholog that significantly differs from the corresponding enzyme in dogs. Phase II - A deficient UDP-glucuronosyltransferase (UGT) 1A6 activity was confirmed in feline liver microsomes, and also functional UGT2B homologs are apparently absent in the feline liver. However a functional UGT1A1 and probably other UGT1A homologs in cats were identified, albeit with a lower capacity than dogs. - The overall very low glucuronidation capacity in cats and its small substrate-specificity remain of clinical importance in pharmacotherapy and clinical toxicology. Phase III - Structure and functional characteristics of the feline bile salt export pump (BSEP) was described for the first time and appeared to be very similar between cats, dogs and humans. - BSEP is essential for the transport of bile acids out of the hepatocyte, and inhibition of BSEP, as demonstrated for diazepam and its metabolites, seems to contribute to the observed hepatotoxicity in feline patients after repeated dosing. Everted membrane vesicles transfected with feline BSEP are a valuable screening tool to assess the transport capacity of feline BSEP . - Feline lymphoma cells express the efflux transporter P-glycoprotein (P-gp) at a high rate and hence constitute an easy accessible tool to study P-gp inhibition and drug-drug interactions at the level of P-gp transport. In conclusion, our investigations identified some major differences between hepatic drug metabolism between humans, dogs and cats, explaining various undesirable drug side effects…