Phenytoin metabolism in the cat after long-term oral administration

The bioavailability and disposition kinetics of genistein in cats

The absorption and disposition kinetics of the soy isoflavone genistein were determined in cats (n = 6). An oral dose of 100 mg/kg was administered, which has previously been demonstrated to be the minimum oral estrogenic dose, and was administered intravenously at a dose of 20 mg/kg, being the largest practical dose that could be safely administered. Plasma free, and total (conjugated + free) genistein concentrations were determined by HPLC following organic extraction. Noncompartmental analysis revealed a half-life of 21.67 +/- 7.9 h (free) and 9.95 +/- 2.7 h (conjugated), volume of distribution 31.94 +/- 10.38 L/kg (free) and 11.82 +/- 3.96 L/kg (conjugated) following intravenous administration. Following oral administration the half-lives were determined to be 17 +/- 4.8 h (free) and 8.56 +/- 4.65 h (conjugated), with tmax = 4.4 +/- 0.6 h (free) and 4.42 +/- 0.99 h (conjugated), and Cmax = 0.276 +/- 0.1 microg/mL (free) and 6.24 +/- 6.58 microg/mL (conjugated). Oral bioavailabilities were 1.379 +/- 0.9% (free) and 29.85 +/- 22.61% (conjugated). The ratio of total:free genistein ranged from 25.9 to 5.5. Poor oral absorption and efficient conjugation explain the low bioavailability of free genistein. Accumulation of genistein in peripheral lipophilic compartments may occur.

Phenytoin metabolism to 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) in man, cat and rat in vitro and in vivo, and susceptibility to phenytoin-induced gingival overgrowth

Interspecies differences in phenytoin (PHT) metabolism to 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) were examined in human, cat and rat hepatic microsomes in vitro. Rat liver microsomes were 25 and 650 times more efficient at the conversion of PHT to HPPH than human and cat liver microsomes, respectively. Sulphaphenazole (83%) and tolbutamide (TOL) (64%) were the most potent inhibitors of HPPH formation in human liver microsomes, while ciprofloxacin (27%), enoxacin (27%) and TOL (26%) produced the greatest inhibition in cat liver microsomes. TOL was tested for its effect on HPPH formation and gingival overgrowth in cats in vivo. Eight cats received PHT sodium (4 mg/kg/d) and another 8 cats received PHT sodium together with TOL (20 mg/kg/d) for 10 wk. Six cats (75%) in the PHT group and 4 cats (50%) in the PHT & TOL group developed significant gingival overgrowth by the end of the study. However, the extent and incidence of the overgrowth were similar in the 2 groups. There were no significant differences in mean AUC 0-10 weeks for plasma PHT (552.90 +/- 29.6 micrograms.d/mL [PHT alone] vs. 582.41 +/- 24.49 micrograms.d/mL [PHT & TOL]) and unconjugated HPPH (1016.4 +/- 295.5 ng.d/mL [PHT alone] vs. 1174.5 +/- 397.2 ng.d/mL [PHT & TOL]) concentrations between the 2 groups of cats. Neither PHT nor HPPH were detectable in the plasma of 8 rats which received PHT (4 mg/kg/d) over a 10-wk period. The rats showed no sign of gingival inflammation (mean gingival index = 0) or gingival overgrowth (mean gingival overgrowth index = 0). Thirty-six adult epileptic patients on chronic PHT therapy were examined; 17 (47%) of the patients demonstrated clinically significant overgrowth. The mean steady-state plasma PHT concentration was comparable to, and the mean plasma unconjugated HPPH concentration 5-fold greater than, that observed in the cats. The results suggest that the rapid metabolism and elimination of PHT and HPPH in the rat may enable it to become more resistant towards developing gingival overgrowth, compared to the cat and man.

Phenytoin covalent binding and embryopathy in mouse embryos co-cultured with maternal hepatocytes from mouse, rat, and rabbit

The anticonvulsant drug phenytoin is teratogenic in a variety of species including humans. Traditional embryo culture studies have employed the addition of 9000 g supernatant (S-9) or microsomal fractions from induced rat or mouse liver as an exogenous bioactivating system to approximate a maternal contribution. However, cellular fractions, unlike cultured intact hepatocytes, may themselves be embryotoxic, and do not reflect the in vivo balance of bioactivation and detoxification. To evaluate in vitro the known in vivo differential species susceptibility to phenytoin teratogenesis, day 9.5 (day of plug = day 1) mouse embryos either were cultured alone for 24 hr or were co-cultured with hepatocytes from maternal mice, rats or male rabbits, thereby exposing the embryos to the effects of potential species-specific phenytoin metabolism. In the absence of hepatocytes, phenytoin embryotoxicity was concentration dependent (0, 10, 20 and 60 micrograms/mL), with decreases in embryonic growth, reflected by reduced yolk sac diameter and crown rump length, apparent within the maternal therapeutic range (20 micrograms/mL). Covalent binding of the radiolabeled drug to live embryonic tissue was significantly higher than in control embryos previously killed by fixation, suggesting that the embryo can bioactivate phenytoin to a toxic reactive intermediate. Mouse embryos grew equally well with hepatocytes from all three species, indicating interspecies tissue compatibility. The addition of rat and rabbit hepatocytes, but not mouse hepatocytes, significantly enhanced the phenytoin-induced impairment of mouse embryonic development, as demonstrated by reductions in somite number. The phenytoin-induced impairment of mouse embryonic growth was not enhanced by the addition of rat or rabbit hepatocytes, while mouse hepatocytes conferred protection. The covalent binding of phenytoin to extracellular proteins in the culture medium was not enhanced by the addition of mouse hepatocytes. These results suggest that mouse embryos intrinsically can bioactivate phenytoin to a toxic reactive intermediate, with embryopathic consequences. The protection conferred by maternal mouse hepatocytes suggests a species-specific maternal biochemical balance favouring detoxification that is not shared by rat and rabbit hepatocytes, which enhanced phenytoin embryopathy. Thus, while phenytoin teratogenicity likely involves embryonic bioactivation, maternal determinants may contribute variably to teratologic susceptibility in a species-specific manner.

Medical management of recurrent seizures in dogs and cats

The problem of recurrent seizures is a common and challenging one in veterinary medical practice. The pathophysiology and pharmacologic suppression of focal seizure activity have been studied extensively in basic research settings, yet little is known of the genesis, modulation, and termination of generalized seizures, the most common form of seizures noted to occur in companion animals. Knowledge concerning the pharmacokinetic fate of anticonvulsant drugs currently used in veterinary medicine is adequate, though prospective clinical studies of the efficacy of these drugs in the treatment of various types of seizures are lacking. This study will review the available literature regarding the pharmacology, use, and side effects of anticonvulsant drugs currently available for control of recurrent seizures in companion animals. Alternative anticonvulsant drugs will also be described.