Divalent cations are dispensable for binding to DNA of a novel positively charged olivomycin A derivative

The current model of binding of the antitumor antibiotic olivomycin A (1) to GC-rich DNA regions presumes that coordination of the magnesium divalent cation with drug dimers is necessary for binding of 1 into the minor groove of the DNA duplex. Previously we have synthesized the derivatives of 1 termed ‘short acid’ (2) and its N,N-dimethylaminoethylamide (3). The latter compound demonstrated an improved tolerance in vivo compared to 1 and good therapeutic potency in animal models. We herein report that compound 3 is able to form stable complexes with DNA in the absence of Mg(2+), in striking contrast to 1 whose binding to the DNA absolutely requires Mg(2+). The mode of binding of 3 to DNA is similar in the presence or absence of Mg(2+) as determined by circular dichroism. The affinity to DNA of 3 in Mg(2+)-free solution was similar to that of 1 or 3 in the presence of Mg(2+) at low ionic strength. Non-electrostatic contributions to total free energy of binding of 1 and 3 to DNA were comparable for Mg(2+)-free complexes. Our data strongly suggest that electrostatic interaction of the positively charged 3 can compensate for the absence of divalent ions in complexes with DNA. This new property of the olivomycin A derivative expands the mechanistic knowledge of the modes of interaction with DNA of small molecular weight drug candidates.