Decrease in oral bioavailability of ciclosporin by intravenous pulse of methylprednisolone succinate in rats.

We examined the effects of high-dose methylprednisolone on the bioavailability of orally administered ciclosporin in rats. To emulate the clinical protocol of methylprednisolone pulse therapy, methylprednisolone sodium succinate (MPS), a prodrug of methylprednisolone, was intravenously administered as repeated doses (66.3 mg kg(-1)) for 3 days. The area under the blood ciclosporin concentration versus time curve after oral administration was significantly reduced by 60% by pulse treatment with MPS. Based on our previous finding that the total body clearance of ciclosporin was reduced by about 20% by the same methylprednisolone pulse protocol, the extent of reduction in the oral bioavailability of ciclosporin was estimated to be approximately 50%, indicating a drug interaction between high-dose methylprednisolone and orally administered ciclosporin, which affected the absorption process. In rats treated with MPS, an in-situ efflux experiment using rhodamine-123 demonstrated that the reverse transport function of P-glycoprotein (P-gp) in the small intestine was significantly enhanced, although there was no significant increase in the intestinal microsomal activity of triazolam alpha- and 4-hydroxylation, metabolic probes for CYP3A. In addition, a significant decrease was observed in the amount of secreted bile acids serving as an enhancer of gastrointestinal absorption of ciclosporin in MPS treatment. To directly estimate the absorptive capacity, an in-situ absorption test was conducted using a closed-loop of small intestine in control and MPS-treated rats. Intestinal absorption of ciclosporin was significantly decreased, not only in the absence of bile flow but also by treatment with MPS, which well reflected the change in the in-vivo pharmacokinetic behaviour of ciclosporin after methylprednisolone pulsing. These results demonstrate that bioavailability of ciclosporin is markedly reduced by MPS pulse treatment, and the mechanism of this interaction was confirmed to involve enhancement of small-intestinal P-gp function and decrease in bile secretion.