CELLULAR-RESISTANCE AGAINST THE NOVEL HYBRID ANTHRACYCLINE N-(2-CHLOROETHYL)-N-NITROSOUREIDODAUNORUBICIN (AD-312) IS MEDIATED BY COMBINED ALTERED TOPOISOMERASE-II AND O-6-METHYLGUANINE-DNA METHYLTRANSFERASE ACTIVITIES

N-(2-Chloroethyl)-N-nitrosoureidodaunorubicin (AD 312), a novel semisy nthetic compound with combined anthracycline and nitrosourea alkylatin g functionalities, circumvents resistance conferred by either reduced DNA topoisomerase II (topo II) or increased P-glycoprotein expression with less myelosuppression and cardiotoxicity than adriamycin (doxorub icin; ADR). Cellular resistance to AD 312 could arise from a novel mec hanism that confers resistance to both functions simultaneously, or on e or more mechanisms common to anthracyclines and/or alkylating agents . The mechanism contributing to AD 312 resistance was investigated fol lowing selection of AD 312-resistant murine J774.2 macrophage-like cel ls and human NCI-H460 non-small-cell lung carcinoma cells. Murine J/31 2-400 (> 4.7-fold) and human H/312-40 cells (6.3-fold) were cross-resi stant to topo II inhibitors (ADR, teniposide, etoposide) and nitrosour eas (carmustine, lomustine) but remained sensitive to vinblastine, col chicine, and camptothecin. There was approximately a twofold decrease in topo II decatenation activity and protein. Decreased net intracellu lar drug accumulation was not observed. There were no increases in glu tathione content or glutathione-S-transferase activity. Increased O-6- methylguanine-DNA methyltransferase (MGMT) activity (2.3-fold) was det ected in J/312-400, and AD 312 resistance was partially reversed by O- 6-benzylguanine, a potent inhibitor of MGMT activity. The results sugg est that AD 312 resistance arose through selective pressure by both cy totoxic functions in a serial manner.