In vitro studies on the metabolism of cimetidine by rat liver microsomes--identification of a new N-desmethylcimetidine metabolite

Biomimetic models for monooxygenases

The microsomal mixed function oxidase system contains the cytochrome P-450 oxidative drug metabolizing family of enzymes. The catalytic cycle of cytochrome P-450 is believed to involve the formation of an active iron-oxygen species which is responsible for oxygen transfer to the substrate. This assumption is supported by the fact that a number of peroxidative agents can replace NADPH, the reductase, and oxygen as co-reactants in most oxidative reactions of microsomal cytochrome P-450. We have found that a mixture of either ferrous or ferric ions with hydrogen peroxide (Fenton and Ruff reagents) can serve as biomimetic models for cytochrome P-450 in hydroxylation, exposidation, sulfoxidation, and N-demethylation of various drugs. The existance of an iron-oxo active species in both Fenton and Ruff type reactions has been postulated and provides reaction cycles similar to those of cytochrome p-450. Other model systems for the hepatic hydroxylation and epoxidation using transition metal complexes with porphyrin are also discussed. The present paper reviews the various biomimetic models of the heme cytochrome P-450 and emphasizes their simulation of hepatic drug metabolism and their potential medical and industrial applications.

The fate of cimetidine sulphoxide in the guinea pig

1. An oral dose of cimetidine given to guinea pigs was totally excreted in 3 days, divided equally between urine and faeces. 2. Only 40% of an oral dose of cimetidine sulphoxide was voided in 3 days, the remainder being found in the mesentery and omental tissues. 3. In contrast, 90% of an i.v. dose of cimetidine sulphoxide was excreted in urine in 24 h. 4. Approx. 20% of an oral dose of both cimetidine (17.2%) and cimetidine sulphoxide (24.9%) was secreted in bile during the first day. 5. The major metabolic routes encountered were sulphoxidation and sulphoxide reduction.

Decreased estrogen hydroxylation in male rat liver following cimetidine treatment

Administration of cimetidine (600 mumol/kg x 5) to adult male rats resulted in 55 and 25% decreases, respectively, in estradiol 2- and 16 alpha-hydroxylation. The same treatment also decreased the activities of ethylmorphine demethylase, aryl hydrocarbon hydroxylase, aniline hydroxylase and heme oxygenase but did not inhibit the activities of 7-ethoxycoumarin de-ethylase and delta-aminolevulinic acid synthase or decrease cytochrome P-450 content. In vitro addition of cimetidine (10-300 microM) also inhibited estradiol hydroxylations, and the effect was additive in rats pretreated with cimetidine in vivo; the other enzymic activities studied were completely unaffected by in vitro addition of cimetidine. In contrast, there was no effect of cimetidine either in vivo or in vitro on any of these activities in female rats. The results point to a wide variation in the susceptibilities of different isozymes of cytochrome P-450 to inhibition by cimetidine and suggest that such differential susceptibilities are also highly dependent on the sex of the animal.

The role of iron chelates on the selectivity of Fenton reagent in hydroxylation, N-demethylation, and sulfoxidation of cimetidine: a novel biomimetic model for the regioselectivity of cytochrome P-450

The effect of iron chelates on the reaction of cimetidine with Fenton reagent [Fe(II)/H2O2] has been investigated. Iron chelates with high affinity to ferrous ions inhibited this reaction. However, iron chelates with high affinity to ferric ions selectively promote either hydroxylation, N-demethylation, or sulfoxidation of cimetidine. These results indicate that the oxidation of cimetidine with hydrogen peroxide activated by various chelated ferrous ions serves as a biomimetic model for the regioselectivity of multiple forms of cytochrome P-450 in the metabolism of cimetidine.

Dose-dependent pharmacokinetics of cimetidine in the rat

The pharmacokinetics of cimetidine were studied in the rat after intraperitoneal administration of 10, 40 and 100 mg/kg. The area under the plasma concentration-time curve increased more than proportionately with dose. The total plasma clearance of cimetidine decreased as the dose increased (4.11 to 2.21 l/h per kg) with a consequent increase in half-life (24.0 to 37.9 min) but no change in volume of distribution (mean 2.31 l/kg). The fraction of dose excreted unchanged increased slightly with dose (0.37 to 0.45), whereas the fraction excreted as the sulphoxide metabolite decreased significantly with increased dose (0.35 to 0.14). Both the renal clearance (1.52 to 0.99 l/h per kg) and the formation clearance of the sulphoxide metabolite (1.45 to 0.30 l/h per kg) decreased with increasing dose. Residual clearance, calculated as the difference between total clearance and the sum of renal and metabolic clearance, did not change with dose (mean 1.08 l/h per kg). The formation clearance of the sulphoxide metabolite became saturated at a lower cimetidine concentration than the renal clearance.

Reaction of cimetidine with Fenton reagent. A biomimetic model for the mixed-function oxidase drug metabolism

Cimetidine sulfoxide, N-desmethylcimetidine, N-desmethylcimetidine sulfoxide, cimetidine guanylurea, and the 5-hydroxymethylimidazole derivative of cimetidine sulfoxide were isolated from the reaction of cimetidine with Fenton reagent. The product distribution from the reaction of cimetidine with Fenton reagent clearly simulates the metabolism of cimetidine by the mixed-function oxidase.