In view of the known problems that diabetic recipients have with drug absorption and of the potential advantages of the microemulsion formulation of cyclosporine (Neoral), 17 previously diabetic recipients of simultaneous pancreas and kidney allografts were studied before and after conversion from Sandimmune (SIM) to Neoral (NEO) formulations of cyclosporine a mean of 4.75 +/- 2.0 years after transplantation. The aims of this study were to determine whether there were changes in the absorption profiles using the 2 formulations and to analyze the safety of conversion from SIM to NEO on a 1:1 ratio. Pharmacokinetic, clinical, and renal function parameters were measured 2 weeks before, and 2 and 4 weeks after conversion. Pharmacokinetics demonstrated that, when taking SIM, only 5 patients had "good absorption" (maximal concentration >500 ng/ml and time to maximal concentration < / = 2 hr), but after conversion to NEO, 14 at week +2, and 13 at week +4 were good absorbers (P<0.02 and <0.01, respectively). There was a 50% reduction in the time to maximal blood concentration (from 3.2 +/- 1.9 hr with SIM, to 1.8 +/- 1.3 hr on NEO [P<0.007]) and a 36% increase in trough and a 143% increase (P<0.01) in maximal blood concentrations, although the 9-hr cyclosporine AUC showed an 89% increase (P<0.01). There was an 18% mean reduction in dose per kilogram prescribed 4 weeks after conversion. Renal function, measured by serum creatinine and technecium-99m diethylenetriamine pentaacetic acid glomerular filtration rate, was not altered and no adverse events were attributed to NEO. This study confirmed that previously diabetic recipients of simultaneous renal pancreas allografts had variable and poor absorption of cyclosporine using conventional Sandimmune and that this absorption was improved by conversion to Neoral.