Strictosidine Synthase Triggered Enantioselective Synthesis of N-substituted (S)-3,14,18,19-Tetrahydroangustines as novel Topoisomerase I Inhibitors.

Monoterpenoid indole alkaloids (MIAs) comprise an important class of molecules for drug discovery, and these have variant carbon skeletons with prominent bioactivities. For instance, in spite of limitations to their use, camptothecins are the only clinically approved Topoisomerase I (Top1) inhibitors. The enzyme STR1, which is key for MIA biosynthesis, was applied to the enantioselective preparation of three N-substituted (S)-3,14,18,19-tetrahydroangustine (THA) derivatives. These non-camptothecin MIAs were shown to have moderate in vitro HepG2 cytotoxicity and Top1 inhibition activities. The (S)-configured MIAs had stronger cytotoxicity and Top1 inhibition than their chemically synthesized (R)-enantiomers, which aligned with the results of molecular dynamics simulations. A series of N-substituted (S)-THAs were then chemo-enzymatically synthesized to investigate structure-activity relationships. The most active analogue observed was the N-(2-Cl benzoyl) substituted derivative (7i). Insight into the binding mode of 7i and topoisomerase I-DNA covalent complex was investigated by molecular dynamics simulations, which will facilitate future efforts to optimize the Top1 inhibitory activities of non-camptothecin MIAs.