Novel betulin derivatives as antileishmanial agents with mode of action targeting type IB DNA topoisomerase.

Toward developing antileishmanial agents with mode of action targeted to DNA topoisomerases of Leishmania donovani, we have synthesized a large number of derivatives of betulin. The compound, a natural triterpene isolated from the cork layer of Betula spp. plants exhibits several pharmacological properties. Three compounds (disuccinyl betulin, diglutaryl dihydrobetulin, and disuccinyl dihydrobetulin) inhibit growth of the parasite as well as relaxation activity of the enzyme type IB topoisomerase [Leishmania donovani topoisomerase I (LdTOP1LS)] of the parasite. Mechanistic studies suggest that these compounds interact with the enzyme in a reversible manner. The stoichiometry of these compounds binding to LdTOP1LS is 1:1 (mole/mole) with a dissociation constant on the order of ∼10(-6) M. Unlike CPT, these compounds do not stabilize the cleavage complex; rather, they abrogate the covalent complex formation. In processive mode of relaxation assay condition, these compounds slow down the strand rotation event, which ultimately affects the relaxation of supercoiled DNA. It is noteworthy that these compounds reduce the intracellular parasite burden in macrophages infected with wild-type L. donovani as well as with sodium antimony gluconate resistant parasite (GE1). Taken together, our data suggest that these betulin derivatives can be exploited as potential drug candidates against threatening drug resistant leishmaniasis.