Interaction of H2-receptor antagonists with drug-metabolizing enzymes.

Abstract

The Hireceptor antagonist cimetidine inhibits aminopyrineN-demethylation activity of rat liver microsomes and prolongs sleeping times in hexobarbital treated rats (Puurunen & Pelkonen, 1979). In man, cimetidine interacts with warfarin (Flind et al., 1978; Serlin el al., 1979), antipyrine (Serlin et al., 1979) and diazepam (Klotz et al., 1979). The results may be explained by the interaction of cimetidine with the microsomal cytochrome P-450 linked mono-oxygenase system (Puurunen & Pelkonen, 1979). Rend% et al. (1979) have shown that cimetidine interacts with oxidized liver microsomes to produce a ‘type 11’ spectral change characteristic of lipophilic nitrogenous bases. Ranitidine and AH 18801 are novel H,-receptor antagonists which do not contain the imidazole nucleus regarded essential for H,-receptor blockade (Ganellin et al., 1976). Ranitidine is effective in the treatment of duodenal ulcer in man (Saunders et al., 1980). We have compared the interactions of cimetidine, AH 18801 and ranitidine with drug-metabolizing enzymes in vitro and in uiuo. Liver microsomal fractions from male Wistar (A and H strain) albino rats (350-400g) were prepared by differential centrifugation. Difference spectra were obtained for cimetidine, ranitidine and AH1 8801 by using a Pye Unicam SP8-100 UV/VIS spectrophotometer. Each cuvette contained 2.5 ml of microsomal suspension (2.5 mg of protein/ml). Methanolic solutions of each compound were added to the sample cuvette; an equal volume of methanol was added to the reference cuvette. The final concentrations of each compound were 0.05-2.93 mM.. The difference in absorption (AA) between the peak and trough in each spectrum divided by the concentration of compound was plotted against AA. The spectral dissociation constant ( K 3 was obtained from the reciprocal of the slope of this line. Each compound was given orally at doses of 15,45, 115 and 135 mg/kg body wt. to male Wistar (A and H strain) albino rats (90-1 30 g) 1 h before intraperitoneal pentobarbitone sodium (40mg/kg body wt.). The sleeping times were measured as the time between loss and subsequent recovery of righting reflex. Significant differences between vehicle control and test groups were assesed by the Mann-Whitney U test (Siegel, 1956). The binding spectra of each compound showed a minimum absorbance at 390nm and a maximum at 420nm, indicating ligand interaction between these compounds and ferrihaemoprotein (Schenkman et al., 1967). The structural formulae, lipophilicity (log P), spectral dissociation constants and doses which produced significant increases in sleeping time are shown in Table 1. Two spectral dissociation constants (K,I and K,2) for each compound were obtained indicating high and low affinity sites. The K,I values indicate that the affinity for cytochrome P-450 increases in the order ranitidine, AH 18801 then cimetidine. Cimetidine e 4 5 m g / k g body wt.) and AH 18801 (2 1 15 mg/kg body wt.) significantly increased sleeping times; ranitidine had no significant effect over the dose range studied. From e.p.r. studies, Rendic et al. (1979) were unable to determine unambiguously which hetero-atom of cimetidine was co-ordinated to the iron of cytochrome P-450. The absence of a cyanoguanidine group in ranitidine may be the reason for ranitidine not being as strongly bound as cimetidine and AH1 8801 to cytochrome P-450. The affinity of ligand binding is affected by the hydrophobic interaction of the aliphatic or aromatic groups in a molecule (Ullrich er al., 1975). The lower binding affinity of ranitidine may, therefore, be a function of its lower lipophilicity (logP = 0.20) compared with that of cimetidine (log P=0.45) or AH 18801 (log P= 1.19). The results of these studies suggest that, in man, ranitidine will not inhibit microsomal drug metabolism.