Design of an i.v. formulation of an unstable prodrug candidate for prostate cancer treatment: solution chemistry of N-(glutaryl-hyp-ala-ser-cyclohexylglycyl-gln-ser-leu)-doxorubicin.

The chemical degradation of N-(glutaryl-hyp-ala-ser-cyclohexylglycyl-gln-ser-leu)-doxorubicin (henceforth referred to as doxorubicin peptide conjugate 1) was studied in buffered aqueous solution. The pH-rate profile of degradation shows that the doxorubicin conjugate is most stable between pH 5 and 6. The dependence of log k(obsd) on pH in acidic medium is characteristic of specific acid-catalysis of the sugar hemiaminal of 1 (as in the case of doxorubicin). Isolation of degradates and structural determination shows that the degradation at lower pH values yields the water-insoluble aglycone doxorubicinone, supporting the mechanism of acid-catalyzed loss of the amino sugar. At pH higher than 5, a more complicated degradation pattern is observed, including the loss of the amino sugar and the aromatization of the saturated ring to give 7,8-dehydro-9,10-desacetyldoxorubicinone as one of the major products. Around the pH of maximum stability in solution, the rate of degradation of 1 is significantly greater than that for doxorubicin, which rules out the formulation of a room temperature solution product with a sufficiently long shelflife for market use. Design of a stable lyophilized formulation for sterile reconstitution based on the physicochemical properties of 1 is described.