Antimalarial Dibenzannulated Medium-Ring Keto Lactams

We discovered dibenzannulated medium-ring keto lactams (11,12-dihydro-5H-dibenzo[b,g]azonine-6,13-diones) as a new antimalarial chemotype. Most of these had chromatographic LogD(7.4) values ranging from <0 to 3 and good kinetic solubilities (12.5 to >100 mu g/mL at pH 6.5). The more polar compounds in the series (LogD(7.4) values of <2) had the best metabolic stability (CLint values of <50 mu L/ min/mg protein in human liver microsomes). Most of the compounds had relatively low cytotoxicity, with IC50 values >30 mu M, and there was no correlation between antiplasmodial activity and cytotoxicity. The four most potent compounds had Plasmodium falciparum IC50 values of 4.2 to 9.4 nM and in vitro selectivity indices of 670 to >12,000. They were more than 4 orders-of-magnitude less potent against three other protozoal pathogens (Trypanosoma brucei rhodesiense, Trypanosoma cruzi, and Leishmania donovani) but did have relatively high potency against Toxoplasma gondii, with IC50 values ranging from 80 to 200 nM. These keto lactams are converted into their poorly soluble 4(1H)-quinolone transannular condensation products in vitro in culture medium and in vivo in mouse blood. The similar antiplasmodial potencies of three keto lactam-quinolone pairs suggest that the quinolones likely contribute to the antimalarial activity of the lactams.

Automated summary

We discovered a new type of antimalarial compound - dibenzannulated medium-ring keto lactams. These compounds exhibited good kinetic solubilities and metabolic stability, with relatively low cytotoxicity and high antimalarial activity against Plasmodium falciparum. These keto lactams were converted into poorly soluble quinolone condensation products in vivo.