Determination of the immunosuppressive drug tacrolimus in its dosage forms by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) method has been developed and validated for the determination of FK506, a new immunosuppressant, in bulk drug samples and dosage forms. Equilibration between FK506 (I) and its tautomeric compounds (II and III) was accomplished “artificially” in water-dehydrated alcohol (1∶1) employed as an extraction solvent. After reaching equilibrium, separation of I, treated as the representetive of equilibrated FK506, from its related substances was achieved by reversed-phase HPLC on a C18 column with water-isopropyl alcohol-tetrahydrofuran (5∶2∶2, v/v) as the mobile phase, and detection at 220 nm. Component I in dosage forms could be recovered satisfactorily and determined with good precision. The calibration graph was linear over the range 2–6 μg for I.