P. Arvela

Pharmacokinetics of Oxcarbazepine and Carbamazepine in Human Placenta

Summary: Purpose: To study the transfer and metabolism of oxcarbazepine (OCBZ) and 10‐hydroxy‐10,11‐dihydrocarbamazepine (10‐OH‐CBZ) and carbamazepine (CBZ) metabolism and its possible induction in human placenta.

Carbamazepine and its metabolites in human perfused placenta and in maternal and cord blood

Summary: Placental transfer and metabolism of carbamazepine (CBZ) was studied in a dual recirculating placental cotyledon perfusion system and was also evaluated in 16 pairs of maternal venous and cord blood samples. Among the parameters studied as possible indicators of a successful perfusion, volume changes in perfusate divided the perfusions into two groups, whereas no significant differences between perfusions were noted in blood gas analysis or in antipyrine transfer. CBZ added into the maternal circulation crosses the placenta in the beginning quicker than antipyrine which is in agreement with the different lipid solubilities of these compounds. Because the transfer rates of antipyrine and CBZ were about the same, the mechanism of transfer of CBZ is probably similar to that of antipyrine (passive diffusion). No metabolites of CBZ could be detected in the perfusate by high‐performance liquid chromatography (HPLC) or gas chromatographyhass spectrometry. With the improved HPLC methodology for CBZ metabolites, six metabolites were detected in clinical samples, including 10‐hydroxy‐10, 11‐dihydro‐CBZ (10‐OH‐CBZ), which has been described earlier in only 1 uremic patient. Relative levels of metabolites showed significant individual differences. CBZ crosses perfused placenta rapidly, but this does not contribute to CBZ metabolites detected in maternal and fetal circulation.

Effect of five structurally diverse H2-receptor antagonists on drug metabolism

Some H2-receptor antagonists can interact with the biotransformation of other drugs. This is due to their binding to cytochrome P-450. We tested the in vitro effects of 5 different H2-receptor antagonists cimetidine (C), oxmetidine (O), ranitidine (R), famotidine (F) and nizatidine (N) on arylhydrocarbon-hydroxylase, 7-ethoxycoumarin-O-deethylase and 7-ethoxy-resorufin-O-deethylase activity using liver microsomes from man as well as from untreated, phenobarbital and 3-methylcholanthrene treated rats. In addition their binding to human microsomal cytochrome P-450 was evaluated. The in vivo effects of these antagonists were investigated on the hepatic elimination of diazepam in healthy volunteers. In vitro O was found to be the most effective inhibitor of the enzyme activities studied. C showed a clear inhibitory effect only with rat liver microsomes whereas the remaining drugs were more than 10 times less potent. The binding affinities of these antagonists showed a similar tendency: the Ks-values for O, C and R were 0.2, 0.9 and 5.1 mM, respectively; for F and N no binding up to 4 mM could be observed. However, in man, only C inhibited the hepatic elimination of diazepam by about 45% while R, O, N and F did not affect the pharmacokinetics of diazepam. Thus, it could be concluded from our studies that one cannot extrapolate in vitro data of the inhibitory potency of H2-receptor antagonists in every case to human in vivo drug metabolism.

Cerium-induced strain-dependent increase in Cyp2a-45 (cytochrome P4502a-45) expression in the liver and kidneys of inbred mice

The murine Cyp2a-4 and Cyp2a-5 genes encode P450 isoforms catalysing testosterone 15 alpha-hydroxylase and coumarin 7-hydroxylase (COH) activities, respectively. Two days after the administration of a hepatotoxic dose of cerium chloride (2 mg/kg i.v.), COH activity was increased 3.2-fold in the liver of DBA/2 mice. Three and 4 days after the cerium treatment, coinciding with the occurrence of overt liver damage, there was a dramatic decrease in COH activity. The activities of testosterone 15 alpha-hydroxylase and the Cyp1a-1-mediated 7-ethoxyresorufin O-deethylase (EROD) were decreased in response to cerium. Much less pronounced changes in the enzyme activities occurred in the C57BL/6 mouse liver. Northern blot analysis showed a 21-fold increase in the hepatic Cyp2a-4/5 mRNA in the DBA/2 mice at day 2, whereas no increase occurred in the C57BL/6 mice. Also in the kidneys the increase in COH activity and in Cyp2a-4/5 mRNA was marked only in the DBA/2 mice. A polymerase chain reaction-mediated analysis method utilizing a unique PstI restriction site in the Cyp2a-5 cDNA was used to differentiate between the highly homologous Cyp2a-4 and Cyp2a-5 mRNAs. Cerium was found to increase the amount of hepatic and renal Cyp2a-4 and Cyp2a-5 mRNA only in the DBA/2 mice. These data indicate that the Cyp2a-4/5 complex is regulated in a different way in DBA/2 and C57BL/6 mice and that some association exists between the development of liver damage and COH induction.

Comparison between cobalt and pyrazole in the increased expression of coumarin 7-hydroxylase in mouse liver.

The data in this report show that administration of both cobalt and pyrazole results in an elevation in the amount of hepatic mRNA encoding for microsomal P45015 alpha/P450Coh, an increase in the amount of P450Coh protein, and an activation of COH and to a lesser extent testosterone 15 alpha-hydroxylase in two inbred strains of mice. Considerable quantitative differences between the two compounds and the two mouse strains in the response suggest that the effects of cobalt and pyrazole are mediated, at least partly, through different mechanisms. It is of interest that human hepatic COH resembles very closely that in the mouse liver.

Comparative effects of H2-receptor antagonists on drug metabolism in vitro and in vivo

Abstract Some H2-receptor antagonists can interact with the biotransformation of other drugs. This is due to their selective binding to the various forms of cytochrome P-450. We tested the in vitro effects (IC50) of 5 different H2-receptor antagonists (Cimetidine (C), Oxmetidine (O), Ranitidine (R), Famotidine (F) and Nizatidine (N)) on aryl hydrocarbon hydroxylase (AHH), 7-ethoxycoumarin-O-deethylase (ECDE) and 7-ethoxyresorufin-O-deethylase (ERDE) activities using microsomes from rats. In addition, their binding to human microsomal cytochrome P-450 was evaluated. The in vivo effects of these antagonists were investigated on the hepatic elimination of diazepam in healthy human volunteers. O was found to be the most effective inhibitor (IC50=0.1 to 0.3mM) of the enzyme activities studied. C also showed a clear inhibitory effect (IC50=0.4 to 0.7 mM) whereas the remaining drugs were more than 10 times less potent. Likewise, when diazepam was used as a substrate, microsomal conversion to desmethyldiazepam and temazepam was inhibited by oxmetidine (IC50=0.02 mM). The binding effects of these antagonists showed the same tendency. However, only C had a clear inhibitory effect on the elimination of diazepam while R,O,N and F did not affect the pharmacokinetics of diazepam. Thus, it could be concluded from our structure/activity relationship studies that one cannot extrapolate, in every case, in vitro data to the inhibitory potency of H2-receptor antagonists on human drug metabolism.

Lipoperoxidation rates and drug-oxidizing enzyme activities in the liver and placenta of some mammal species during the perinatal period.

Lipoperoxidation and drug-metabolizing enzymes were measured in livers and placentas of different mammal species during the perinatal period. In placentas and fetal livers of rat, rabbit and guinea-pig, cofactor-supported lipoperoxidation was negligible, as were the activities of drug-oxidizing enzymes. Human fetal liver contained an intact drug-oxidizing electron transport chain, and lipoperoxidation activity was accordingly observed. It is suggested that lesions mediated by lipoperoxidation may be possible in human fetus, but they are less probable in animal fetuses.

Metabolites and DNA-binding of carbamazepine and oxcarbazepine in vitro by rat liver microsomes

DNA-binding of carbamazepine (CBZ) and oxcarbazepine (OCBZ) catalysed by non-induced, phenobarbital-induced or methylcholanthrene-induced rat liver microsomes in vitro was studied. 14C-CBZ 200 nmol incubated with DNA, liver microsomes and cofactors led to the formation of a significant amount of CBZ-epoxide, which has been suspected as the cause of teratogenesis and other side- effects of CBZ,1,2 but has not been reactive in any test systems for genotoxicity, including the Ames test.3 No enzyme-dependent DNA-binding of CBZ was found. Using the same conditions, however, OCBZ was bound to DNA. This binding was dependent on the presence of NADPH. 10-hydroxy-10,11-dihydro-carbamazepine, which is known to be the major metabolite of OCBZ, and an unknown peak were demonstrated by HPLC. These results are the first indication of a higher level of covalent DNA binding of OCBZ than of CBZ. The nature of the unknown metabolite and the pathway leading to covalent binding remain to be studied.

Debrisoquine Hydroxylation Phenotypes of: Patients with High Versus Low to Normal

Debrisoquine hydroxylation phenotype was determined in 22 psychiatric patients who had previously developed exceptionally high serum antidepressant (AD) concentrations, and in 22 sex-, age-, and dose-matched counterparts who had low to normal serum AD levels. The patients were recruited from 641 subjects in whom serum AD levels were monitored. In each AD level group, 16 patients had been treated with tricyclic antidepressants (amitriptyline, doxepin, trimipramine, imipramine, clomipramine) and 6 with mianserin. Eight poor metabolizer (PM) phenotypes (debrisoquine/hydroxydebrisoquine ratio in 6-hour urine greater than or equal to 41.5) were identified in the high AD level group, but only two in the group with low to normal AD level (p = 0.03, Fishers test). Comedications in the two study groups did not differ markedly from ach other and could not, therefore, explain the greater frequency of PMs among the patients with high serum AD levels. Three of 6 mianserin patients, who had developed high serum AD levels, were PMs. This high proportion of PMs raises the question of a possible involvement of the same metabolic pathway (cytochrome P-450IID6 isoenzyme) also in mianserin hydroxylation. The results suggest further that during AD therapy with standard dosage, PM phenotypes are at special risk for high serum AD concentrations and, consequently, for clinical symptoms of toxicity.

Antipyrine, coumarin and glipizide affect n-acetylation measured by caffeine test

The effect of various treatments on acetylation status measured by caffeine metabolites was investigated in 17 subjects with non-insulin dependent diabetes mellitus (NIDDM). The test drugs, caffeine (200 mg), antipyrine (20 mg/kg) and coumarin (5 mg), were given simultaneously, and urinary 5-acetylamino-6-formyl-amino-3-methyluracil/1-methylxanthine (AFMU/1X) molar ratio was measured before and after 8 weeks of therapy. The urinary AFMU/1X molar ratio decreased (p < 0.05) after 8 weeks of therapy with glipizide (2.5 mg), but remained unaltered in those treated with placebo or those who self-monitored blood glucose (SMBG) by glucometer. Antipyrine and coumarin decreased (p < 0.05) the AFMU/1X molar ratio both in diabetics and healthy volunteers. Our data demonstrate that glipizide, antipyrine and coumarin may interfere with the classification of acetylator status measured by caffeine metabolites.