Forced degradation of tacrolimus and the development of a UHPLC method for impurities determination.

An ultra-high performance liquid chromatography method for simultaneous determination of tacrolimus impurities in pharmaceutical dosage forms has been developed. Appropriate chromatographic separation was achieved on a BEH C18 column using gradient elution with a total run time of 14 min. The method was applied to analyses of commercial samples and was validated in terms of linearity, precision, accuracy, sensitivity and specificity. It was found to be linear, precise and accurate in the range of 0.05 to 0.6 % of the impurities level in pharmaceutical dosage forms. Stability indicating power of the method was demonstrated by the results of forced degradation studies. The forced degradation study in solution revealed tacrolimus instability under stress alkaline, thermal, light and photolytic conditions and in the presence of a radical initiator or metal ions. The drug was stable at pH 3-5. Solid-state degradation studies conducted on amorphous tacrolimus demonstrated its sensitivity to light, elevated temperature, humidity and oxidation.

Solid state compatibility study and characterization of a novel degradation product of tacrolimus in formulation.

Tacrolimus is macrolide drug that is widely used as a potent immunosuppressant. In the present work compatibility testing was conducted on physical mixtures of tacrolimus with excipients and on compatibility mixtures prepared by the simulation of manufacturing process used for the final drug product preparation. Increase in one major degradation product was detected in the presence of magnesium stearate based upon UHPLC analysis. The degradation product was isolated by preparative HPLC and its structure was elucidated by NMR and MS studies. Mechanism of the formation of this degradation product is proposed based on complementary degradation studies in a solution and structural elucidation data. The structure was proven to be alpha-hydroxy acid which is formed from the parent tacrolimus molecule through a benzilic acid type rearrangement reaction in the presence of divalent metallic cations. Degradation is facilitated at higher pH values.