Effect of etretinate on cyclosporin metabolism in vitro.

Cyclosporin (CyA) is an effective treatment for psoriasis, including cases unresponsive to other therapies. The major side-effect of CyA treatment is dose-related nephrotoxicity. Combinations of CyA and etretinate (Et) have been tested with a view to reducing CyA dose requirements, and therefore minimizing adverse effects. We have studied the effect of Et on the cytochrome P-450-mediated metabolism of CyA. Microsomes prepared from histologically normal human (obtained from four cadaver kidney transplant donors; all male; age range 21-56) were incubated with CyA and various concentrations of Et. Metabolism was quantified by high-performance liquid chromatography with radiometric detection, and metabolites tentatively identified from the retention times of authentic standards. After 30 min incubation of CyA and microsomal protein at 37 degrees C, 10.1 +/- 3.0% (mean +/- SD) 3H-CyA was converted to the monohydroxylated metabolites M1 and M17, and 3.3 +/- 0.8% to the N-demethylated metabolite M21. At an Et concentration of 100 microM inhibition of CyA hydroxylase and N-demethylase was < 20%. This study indicates that there is no metabolic interaction between CyA and Et in vitro; it is likely that the two drugs are metabolized by different P-450 isoenzymes.

Cyclosporin metabolism by human gastrointestinal mucosal microsomes.

The in vitro metabolism of the immunosuppressant cyclosporin (CsA) by human gastrointestinal mucosal microsomes has been studied. Macroscopically normal intestinal (n = 4) and liver (n = 2) tissue was obtained from kidney transplant donors, and microsomes prepared. Intestinal metabolism was most extensive with duodenal protein (15% conversion to metabolites M1/M17 after 2 h incubation at 37 degrees C; metabolite measurement by h.p.l.c). Western blotting confirmed the presence of P-4503A (enzyme subfamily responsible for CsA metabolism) in duodenum and ileum tissue, but not in colon tissue. The results of this study indicate that the gut wall may play a role in the first-pass metabolism of CsA, and could therefore be a contributory factor to the highly variable oral bioavailability of CsA.

Cyclosporin metabolism by the gastrointestinal mucosa.

The intestinal mucosal metabolism of the immunosuppressant cyclosporin (CsA) has been studied in vitro using the Ussing chamber technique. Histologically normal colon was obtained from six patients undergoing resections. The mucosal sheets were mounted between two perspex chambers. Three hours after addition of [3H]-CsA (0.2 microCi; 10 microM) to the mucosal chamber, more than 90% of the radioactivity was present in that chamber. Metabolite analysis, by high performance liquid chromatography, indicated that 77.6 +/- 9.2% (mean +/- s.d.) of the drug present was CsA, 9.9 +/- 4.4% and 8.7 +/- 4.7% were the oxidative metabolites M17 and M21 respectively (metabolites identified by co-chromatography with authentic standards). Total metabolite production in tissues from the six individuals was variable (10.1-30.6% at 3 h) and increased over the time period of the study. A different pattern of metabolism was obtained from a single sample of gastric mucosa. More than 20% of CsA was metabolised although neither M17 nor M21 were detected. The results of this study suggest that the gut wall is involved in the first pass metabolism of CsA in vivo and that this could be a contributory factor to the poor systemic availability of CsA seen in some patients.