Synthesis and pharmacokinetics of a novel macromolecular prodrug of Tacrolimus (FK506), FK506–dextran conjugate

A novel macromolecular prodrug of Tacrolimus (FK506), FK506–dextran conjugate, was developed and its physicochemical, biological and pharmacokinetic characteristics were studied. The conjugate was estimated to contain 0.45% of FK506 and the coupling molar ratio was approximately 1:1 (dextran–FK506). Adsorption experiments using ion exchangers indicated that FK506–dextran conjugate acted as a weakly negatively charged macromolecule. Low molecular weight radioactive compound(s), which was eluted in the same fractions as [3H]FK506, was released from [3H]FK506–dextran conjugate by chemical hydrolysis with a half-life of 150 h in phosphate buffer. In vitro immunosuppressive activity of the conjugate, as assessed by the rat lymphocyte stimulation test, was almost comparable to that of free FK506, suggesting that biologically active FK506 could be liberated from the conjugate. In vivo biodistribution studies demonstrated that conjugation with the dextran derivative dramatically changed the pharmacokinetic properties of FK506 after intravenous injection in rats. AUC of the FK506–dextran conjugate was almost 2000 times higher than that of free FK506 and organ uptake clearances of the conjugate were significantly smaller than those of the free drug. Thus, the present study has demonstrated that the FK506–dextran conjugate behaves as a prodrug of FK506 with an extended blood circulating time and can be expected to have an improved therapeutic potency.