Removal of the basic center from doxorubicin partially overcomes multidrug resistance and decreases cardiotoxicity

Hydroxyrubicin, a synthetic doxorubicin analog in which the basic amino group at C-3' is replaced by a hydroxyl group, was used as a prototype compound to study the effects of basicity of the sugar moiety on the toxicity and antitumor activity of anthracycline antibiotics. Compared with doxorubicin, hydroxyrubicin showed similar or superior in vitro cytotoxicity against P388, L1210, and M5076 cells, as determined by an MTT assay, and against 8226 and CEM cells, as determined by a growth inhibition assay. Hydroxyrubicin was 5 and 13 times more effective than doxorubicin in inhibiting the growth of multidrug-resistant CEM (CEM(vbl)) and 8226 (8226R) cells, respectively. Hydroxyrubicin was not cross-resistant with doxorubicinin a cytotoxicity assay against KB 3-1 and KB V1 cells (resistance index 1.1 for hydroxyrubicin versus > 15.6 for doxorubicin). Cellular uptake and retention of hydroxyrubicin were studied by flow cytometry in parent and multidrug-resistant 8226 cells, and compared with those of doxorubicin. In 8226 sensitive cells, 2 h uptake and retention of doxorubicin were similar or higher than those of hydroxyrubicin. In 8226R cells, uptake and retention of hydroxyrubicin were about 3-fold higher than those of doxorubicin. In mice, the acute LD₅₀ of hydroxyrubicin was about 3-fold higher than that of doxorubicin (79.1 versus 25.7 mg/kg). At equitoxic doses, hydroxyrubicin was as myelosuppressive as doxorubicin but less cardiotoxic, as assessed by the Bertazzoli test. In contrast to doxorubicin, hydroxyrubicin, due to the lack of basic amine function, showed no selective interaction with negatively-charged cardiolipin (CL). The observed decrease of affinity to CL might be responsible for the reduced cardiotoxicity of hydroxyrubicin. In in vivo antitumor activity studies, hydroxyrubicin at the optimal dose (37.5 mg/kg, i.p., on day 1) had significant activity against intraperitoneal P388 leukemia resistant to doxorubicin, whereas doxorubicin (10 mg/kg, i.p., on day 1) was inactive (%T/C 163-200 versus 118-120). These studies indicate that: (i) the amino group at position 3' is not essential for doxorubicin to exert its biological activity, (ii) removal of the basic center (deamination at the C-3 ) results in an increased cellular uptake and retention, (iii) the increased cellular uptake and retention of hydroxyrubicin in multidrug-resistant cells correlate with a partial or total lack of cross-resistance of this analog with the parent compound, doxorubicin, and (iv) deamination at position 3' confers a reduced cardiotoxicity and diminished affinity for CL.