Mechanism of hydrolysis of a novel indolocarbazole topoisomerase I inhibitor.

The degradation kinetics and reaction product profile of the antitumor agent 1 in aqueous solution was studied. Hydrolysis of the pendant imide ring of 1 is the primary mode of thermal degradation in aqueous solution, and the pH rate profile of 1 has a V-shape indicating that hydrolysis of the imide ring can be catalyzed by either acid or base. Hydrolysis of 1 to the anhydride derivative 3 or the dicarboxylic acid derivative 4 is stepwise and the intermediates 2a and 2b formed by initial hydrolytic attack have been observed under alkaline conditions. An overall mechanism for the hydrolysis of 1 in aqueous solution has been proposed. Extrapolating Arrhenius behavior to the hydrolysis reaction of 1 in aqueous solution maintained at a pH value of 4 suggests an aqueous buffered formulation has sufficient thermal stability to be considered a robust room temperature drug product.

Parenteral formulation and thermal degradation pathways of a potent rebeccamycin based indolocarbazole topoisomerase I inhibitor.

The development of a practical and pharmaceutically acceptable parenteral dosage form of 1 is described. A cosolvent formulation strategy was selected to achieve the necessary human dose of 1 for administration via intravenous infusion. The final market formulation of 1 chosen for commercial development and Phase II clinical supplies was the topoisomerase inhibitor dissolved in a 50% aqueous propylene glycol solution vehicle with 50mM citrate buffered to pH 4. The thermal degradation pathways of 1 in this aqueous propylene glycol vehicle in the pH range of 3-5 were determined by relative kinetics and degradation product identification using LC/MS, LC/MS/MS, and NMR analysis. The primary mode of degradation of 1 in this aqueous cosolvent formulation is hydrolysis affording the anhydride 2 (in equilibrium with the dicarboxylic acid 3) and release of the hydrazine diol side chain 11. Subsequent oxidative degradation of 11 occurs in several chemical steps which yield a complicated mixture of secondary reaction products that have been structurally identified.