Design and evaluation of self-microemulsifying drug delivery system (SMEDDS) of tacrolimus

The objective of present investigation was to formulate self-microemulsifying drug delivery systems (SMEDDS) of tacrolimus (FK 506), a poorly water soluble immunosuppressant that exhibits low and erratic bioavailability. Solubility of FK 506 in various oils, surfactants cosurfactants and buffers was determined. Phase diagrams were constructed at different ratios of surfactant/cosurfactant (K(m)) to determine microemulsion existence region. The effect of oil content, pH of aqueous phase, dilution, and incorporation of drug on mean globule size of resulting microemulsions was studied. The optimized SMEDDS formulation was evaluated for in vitro dissolution profile in comparison to pure drug and marketed formulation (Pangraf capsules). The in vivo immunosuppressant activity of FK 506 SMEDDS was evaluated in comparison to Pangraf capsules. Area of o/w microemulsion region in phase diagram was increased with increase in K(m). The SMEDDS yielded microemulsion with globule size less than 25 nm which was not affected by the pH of dilution medium. The SMEDDS was robust to dilution and did not show any phase separation and drug precipitation even after 24 h. Optimized SMEDDS exhibited superior in vitro dissolution profile as compared to pure drug and Pangraf capsules. Furthermore, FK 506 SMEDDS exhibited significantly higher immunosuppressant activity in mice as compared to Pangraf capsules.