Improved oral bioavailability of cyclosporin A in male Wistar rats. Comparison of a Solutol HS 15 containing self-dispersing formulation and a microsuspension.

Oral bioavailability of the highly lipophilic and poorly water-soluble immunosuppressive agent cyclosporin A (CyA) in two different formulations was investigated in male Wistar rats. An aqueous microsuspension and a self-dispersing formulation composed of the surface-active ingredients Solutol HS 15:Labrafil M2125CS:oleic acid=7:2:1 (v/v/v) were administered to the animals at a dose level of 20 mg/kg. In order to calculate the absolute oral bioavailability, CyA was additionally administered intravenously at 10 mg/kg as microsuspension. It was found that the oral bioavailability of CyA in the Solutol HS 15-based formulation was twofold higher as compared to the microsuspension (69.9+/-2.8 vs. 35.7+/-3.3%, P=0.001). By contrast, the time to reach maximum plasma concentration (t(max)) and the terminal half-life (t(1/2)) did not differ significantly with the different formulations (t(max): 7.0+/-1.0 vs. 6.3+/-1.7 h; t(1/2): 20.5+/-2.9 vs. 16.7+/-4.7 h). In vitro solubility experiments demonstrated a marked increase in the aqueous solubility of CyA in the presence of the self-dispersing formulation as compared to the micronized powder alone (solubility after 120 min at 37 degrees C: 136 vs. 23.2 microg/ml in human gastric juice; 133 vs. 10.8 microg/ml in simulated intestinal juice). Most likely, the enhanced systemic exposure of CyA in the self-dispersing formulation was caused by improved solubility of CyA in the gastrointestinal fluids in the presence of the surface-active ingredients. Additional factors that may have contributed to increased oral bioavailability are inhibition of metabolism and/or transport processes as well as permeability enhancement by the co-administered excipients.