The R Enantiomer of the Antitubercular Drug PA-824 as a Potential Oral Treatment for Visceral Leishmaniasis

Abstract

The novel nitroimidazopyran agent (S)-PA-824 has potent antibacterial activity against Mycobacterium tuberculosis in vitro and in vivo and is currently in phase II clinical trials for tuberculosis (TB). In contrast to M. tuberculosis, where (R)-PA-824 is inactive, we report here that both enantiomers of PA-824 show potent parasiticidal activity against Leishmania donovani, the causative agent of visceral leishmaniasis (VL). In leishmania-infected macrophages, (R)-PA-824 is 6-fold more active than (S)-PA-824. Both des-nitro analogues are inactive, underlining the importance of the nitro group in the mechanism of action. Although the in vitro and in vivo pharmacological profiles of the two enantiomers are similar, (R)-PA-824 is more efficacious in the murine model of VL, with >99% suppression of parasite burden when administered orally at 100 mg kg of body weight−1, twice daily for 5 days. In M. tuberculosis, (S)-PA-824 is a prodrug that is activated by a deazaflavin-dependent nitroreductase (Ddn), an enzyme which is absent in Leishmania spp. Unlike the case with nifurtimox and fexinidazole, transgenic parasites overexpressing the leishmania nitroreductase are not hypersensitive to either (R)-PA-824 or (S)-PA-824, indicating that this enzyme is not the primary target of these compounds. Drug combination studies in vitro indicate that fexinidazole and (R)-PA-824 are additive whereas (S)-PA-824 and (R)-PA-824 show mild antagonistic behavior. Thus, (R)-PA-824 is a promising candidate for late lead optimization for VL and may have potential for future use in combination therapy with fexinidazole, currently in phase II clinical trials against VL.

Snapshot Profiling of the Antileishmanial Potency of Lead Compounds and Drug Candidates against Intracellular Leishmania donovani Amastigotes, with a Focus on Human-Derived Host Cells

“…Hence, potencies against L. donovani amastigotes in PEMs are reported in Table 2 without direct comparisons to the human-derived cells. Potencies were in agreement with values reported in the literature (14)(15)(16). Cell lines are often used over primary cells due to ease of culture and an argument of homogeneity.…”

“…VL-2098 (15), and the (R) and (S) enantiomers of PA-824 (16). Notably fexinidazole has entered clinical trials for VL (www.dndi.org).…”

“…Structurally related compounds were tested in parallel in the same experiment, and miltefosine (Zentaris GmbH, Germany) was included as a standard drug in selected assays. Nitroheterocyclic drugs were synthesized at the University of Dundee as described previously (14,16). VL-2098 was prepared in a single step from 4-(Trifluoromethoxy)phenol and (R)-2-bromo-1-((2-methyloxiran-2-yl)methyl)-4-nitro-1H-imidazole using a modification of the published synthesis of delamanid (OPC-67683) (18).…”

Evaluating Aziridinyl Nitrobenzamide Compounds as Leishmanicidal Prodrugs

“…It is now apparent that several nitro-based compounds are not as toxic as initially thought (13)(14)(15). Such observations have stimulated renewed interest in this group of agents, with calls for the reinstatement of nitrofurantoin as a treatment for uncomplicated urinary tract infections, while several others have emerged as lead structures to treat various microbial infections and different forms of cancer, with fexinidazole and PA-824 undergoing evaluation to treat visceral leishmaniasis (16)(17)(18)(19)(20)(21)(22)(23)(24).…”

“…For the treatment of hypoxic cancers, the FMN-type I NTR activity must be introduced into mammalian cells using gene-or antibody-based approaches before addition of the nitrobenzamide whereas in trypanosomes an essential endogenous enzyme can be exploited to catalyze the nitro reduction described above (18,21,55). As Leishmania parasites also express a FMN-type I NTR, it is postulated that nitroaromatic compounds may have potential for the treatment of various forms of leishmaniasis (24).…”