Interconversion and stability of duocarmycins, a new family of antitumor antibiotics: correlation to their cytotoxic and antimicrobial activities in vitro.

Stability and interconversion of duocarmycins were studied in relation to their cytotoxicities and antimicrobial activities. The compounds studied included duocarmycin A and SA, which have a spirocyclopropylhexadienone moiety, and four halogenated seco-compounds of duocarmycin A: duocarmycin B1, B2, C1 and C2, from which the cyclopropane ring structure is absent. Duocarmycins were potent cytotoxic compounds to cells. The cytotoxic activity seen on Balb 3T3/H-ras cells after 72 h drug exposure was in the following order (IC50 (nM): concentration for 50% growth inhibition); SA (0.05) > A (0.3) > B2 (1.5) > B1 (3.0) > C2 (20) > C1 (40). Average minimum inhibitory concentrations (MICs) of duocarmycins against microorganisms showed essentially the same ranking order as that of cytotoxicity. There was a large difference between SA and A in their stability in aqueous solvents. For halogenated seco-compounds, a good correlation was found between their cytotoxicities in vitro and their conversion rate to duocarmycin A, suggesting that halogenated seco-compounds undergo closure to the spirocyclopropylhexadienone structure, the pertinent active form, in cells.