On the basis of a structure-activity study of a new series of anthracycline disaccharides, we recently identified a doxorubicin analogue (MEN 10755) with a promising antitumor activity. In the present study, to better support the pharmacological interest of MEN 10755, we extended the preclinical evaluation of antitumor efficacy to a large panel of 16 human tumor xenografts, which originated from different clinicopathological types. Tumors with typical multidrug-resistant phenotype were excluded because MEN 10755 was found unable to overcome resistance mediated by transport systems. In the doxorubicin-responsive series, MEN 10755 exhibited a higher activity in three of five tumors, as documented by a more marked tumor growth inhibition and an increased value of log-cell kill. In the series of doxorubicin-resistant tumors, MEN 10755 was found effective in 6 of 11 tumors (1 breast, 3 lung, and 2 prostate carcinomas). The overall response rates were 31% and 69% for doxorubicin and MEN 10755, respectively. The improvement in drug efficacy was also supported by a substantial increase in the long-term survivor rate of animals implanted with responsive tumors. Most of the tumors refractory to doxorubicin and responsive to MEN 10755 were characterized by overexpression of the antiapoptotic protein Bcl-2. In one of these tumors (MX-1 breast carcinoma), we examined the ability of MEN 10755 to induce phosphorylation of Bcl-2 after a single treatment with therapeutic doses. The results indicated that, unlike doxorubicin, MEN 10755 induced protein phosphorylation. A similar modification was produced by Taxol, which is known to be very effective against the tumor. The correlation between drug efficacy and Bcl-2 phosphorylation may underly a peculiar feature related to improvement of efficacy of the disaccharide analogue. In conclusion, the present study supports some favorable features of the novel doxorubicin analogue in terms of both efficacy and tolerability with comparison to doxorubicin, although the improvement is somewhat tumor- and schedule-dependent.