Further examination of 9-alkyl- and sugar-modified anthracyclines in the circumvention of multidrug resistance.

Anthracyclines possessing either a 9-alkyl modification in the A-ring of the tetracyclic aglycone and/or specific changes to the amino sugar moiety retain effective cytotoxic activity against multidrug resistant (MDR) cell lines. To obtain a better understanding of the structural features responsible for this potentially valuable behaviour, we used the MTT tetrazolium dye reduction assay to calculate resistance factors (RF = the ratio of ID50 for the drug-resistant line to that for the parental line) for the EMT6/P mouse mammary tumour and its MDR variant EMT6/AR1.0, and the H69/P human small cell lung cancer line and its MDR counterpart H69/LX4. Both MDR lines exhibit marked resistance to doxorubicin, MDR 1 gene amplification, hyperexpression of the membrane P-glycoprotein and reduced drug accumulation. RF values for doxorubicin were 34 and 131 in the EMT6 and H69 cell line pairs, respectively. The 9-alkyl-substituted anthracyclines were confirmed as having RF values 9- to 15-fold lower than those for doxorubicin. The 9-ethyl analogues Ro 31-1966 (RF for EMT6 2.2, RF for H69 4.7) and Ro 31-1749 (RF for EMT6 3.9, RF for H69 9.5) were superior to the previously studied 9-methyl analogue Ro 31-1215 (RF for EMT6 8.1 RF for H69 12.4). A clear trend for RF values to decrease with increasing 9-alkyl chain length was also noted in the structurally more complex aclacinomycin series. For example, 13-methyl-aclacinomycin (RF for EMT6 1.0, RF for H69 2.2) featuring a 9-isopropyl moiety was superior to the 9-alkyl-containing aclacinomycin A (RF for EMT6 4.7, RF for H69 5.8), and this was in turn more effective than the 9-methyl analogue sulfurmycin A (RF for EMT6 6.4, RF for H69 14.2). The trisaccharide moiety was not an essential feature for activity against MDR lines in the aclacinomycins, as shown by the low RF value with aklavine (RF for EMT6 2.1, RF for H69 2.5). However, a small change in one of the sugar moieties of aclacinomycin A, as in marcellomycin, resulted in a considerable increase in RF values (RF for EMT6 18.5, RF for H69 25.3). The complex anthracyclines AD 32 (RF for EMT6 6.5, RF for H69 11.7) and particularly tetrahydropyranyl-doxorubicin (RF for EMT6 1.4, RF for H69 3.2) were effective against MDR lines.(ABSTRACT TRUNCATED AT 400 WORDS)