Intensely cytotoxic anthracycline prodrugs: galactosides

We have reported the synthesis of a series of anthracycline analog prodrugs that give rise to intensely cytotoxic metabolites in the presence of carboxylate esterases and beta-glucuronidases. We now report structurally related prodrugs that are converted to similar potent metabolites in the presence of beta-galactosidases. The prototypical compound, N-[(4"RS)-4"-ethoxy-4"(1'"-O-beta-D-galactopyranosyl)butyl]daunorubicin, 8a, was prepared by reductive condensation of daunomycin with 1-O-[(1'RS)-1'-ethoxy-4'-oxobutyl]-2, 3, 4, 6-tetra-O-acetyl-beta-D-galactopyranoside in the presence of sodium cyanoborohydride, followed by deacetylation of the galactoside moiety with sodium methoxide. A related prodrug (8b) with enhanced lipophilicity (the 4'-hexoxy analog of 8a) and 8c (the propyldaunomycin analog of 8a) were prepared for comparative studies. 8a and 8b were isolated after chromatography on silica as a mixture of 4'R and 4'S diastereomers; 8c, on the other hand, was resolved into its component 3' diastereomers, 8c(R) and 8c(S). 8a, 8c(R) and 8c(S) showed no evidence of decomposition when incubated at 37 degrees C in 0.05 M phosphate buffer, pH 7.4, for 2 weeks; 8b, under the same conditions, was degraded with a half-life of 49 h. In the presence of two units of Escherichia coli beta-galactosidase per pmol of substrate, the half-lives of 8a, 8b, 8c(R) and 8c(S) were 1.98, 1.06, 3.5 and 2.4 h, respectively. HPLC analysis of the incubation mixtures showed that 8a and 8b gave rise to a single, chromatographically identical metabolite. 8c(R) and 8c(S) also gave rise to a single, identical metabolite. 8a and 8b were nearly one million-fold more toxic to human A375 melanoma cells in culture in the presence of E. coli beta-galactosidase than in the absence of the enzyme. The activation products of 8c(R) and 8c(S) were approximately 1000-fold less potent. These beta-galactoside prodrugs have chemotherapeutic potential for use in conjunction with tissue-targeting strategies such as antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT).