Morphological and functional changes in p-glycoprotein during dexamethasone-induced hepatomegaly.

1. The effect of dexamethasone on hepatic and renal P-glycoprotein (P-gp) expression, localization and activity was investigated in rats after 4 days oral administration of two dose regimens (1 or 25 mg/kg per day). Simultaneous increases in liver weight were evaluated by quantitative histological examination. 2. In the liver, dexamethasone pretreatment produced hepatomegaly as a consequence of extensive periportal fat accumulation, which was quantified by densitometry of oil red O-stained liver sections. Quantitative immunohistochemical analysis revealed preferential periportal zonation of P-gp in control animals. Dexamethasone pretreatment resulted in spatially disproportional induction of P-gp protein expression within the liver acinus characterized by preferential increase in pericentral areas, with consequent uniform panlobular distribution. Western blot analysis confirmed these results, showing increases in P-gp protein. Quantitative reverse transcription-polymerase chain reaction analysis revealed no statistically significant change in liver mdr1b mRNA expression after either dexamethasone treatment regimen. The expression of mdr1a mRNA was significantly decreased by 85-87%. 3. In the kidney, dexamethasone reduced mdr1a mRNA expression by 69-89%, whereas mdr1b mRNA expression was increased in a dose-dependent manner. However, despite tendencies, no significant increases in P-gp expression were observed at the protein level. 4. The in vivo function of P-gp was evaluated by measuring renal and biliary secretion of rhodamine-123 (Rho123) under a steady state plasma concentration. The biliary, renal and tubular secretory clearance of Rho123 was significantly increased only after high-dose dexamethasone. 5. In conclusion, the present study suggests that drug interactions observed during corticosteroid therapy may be mediated, at least in part, through increased biliary, and also renal, excretion of P-gp substrates. Expression of P-gp in the liver showed primary periportal zonation with differential changes during induction. Accompanying hepatomegaly may be explained by severe microvesicular steatosis selectively localized to the periportal areas.

Inhibitory effects of memantine on human cytochrome P450 activities: prediction of in vivo drug interactions.

OBJECTIVE: The aim of the present study was to predict the drug interaction potential of memantine by elucidation of its inhibitory effects on cytochrome P450 enzymes using pooled human liver microsomes (HLM) and recombinant P450s. METHODS: The inhibitory potency of memantine on CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 activities was examined with specific probe drugs in HLM and recombinant P450s. The in vivo drug interactions of memantine were predicted in vitro using the [ I]/([ I] + KI) values. RESULTS: In HLM, memantine inhibited CYP2B6 and CYP2D6 activities, with KI (IC50) values of 76.7 (279.7) and 94.9 (368.7) microM, respectively. Both inhibitions were competitive. In addition, cDNA-expressed P450s were used to confirm these results. Memantine strongly inhibited recombinant CYP2B6 activity with IC50 ( KI) value of 1.12 (0.51) microM and activity of recombinant CYP2D6 with IC50 (KI) value of 242.4 (84.4) microM. With concentrations up to 1,000 microM, memantine showed no appreciable effect on CYP1A2, CYP2E1, CYP2C9, or CYP3A4 activities and a slight decrease of CYP2A6 and CYP2C19 activities. Based on [ I]/([ I] + KI) values calculated using peak total plasma concentration (or enzyme-available concentration in the liver) of memantine and the KI obtained in HLM, 1.3 (13.5), and 1.0% (11.2%), inhibition of the clearance of CYP2B6 and CYP2D6 substrates could be expected, respectively. Nevertheless, when considering KI values obtained from cDNA-expressed CYP2B6, as generally recommended, even 66.2% (95.9%) decrease in metabolism of coadministered CYP2B6 substrates could be anticipated. CONCLUSION: Memantine exerts selective inhibition of CYP2B6 activity at clinically relevant concentrations, suggesting the potential for clinically significant drug interactions. Inhibition of other CYPs during memantine therapy is unlikely. Moreover, memantine represents a new, potent, selective inhibitor of recombinant CYP2B6, which may prove useful for screening purposes during early phases of in vitro drug metabolism studies with new chemical entities.