Antiplasmodial activity of pentyloxyamide-based histone deacetylase inhibitors against Plasmodium falciparum parasites

Malaria is caused by Plasmodium parasites and remains a significant health concern for almost half the world's population. There are estimated to be > 240 million malaria cases and approximately 600,000 malaria-related deaths annually, mainly due to infection with P. falciparum parasites. Parasite drug resistance is impacting malaria prevention and control efforts, and as part of the malaria eradication agenda, new drugs with novel mechanisms of action are needed. Histone/lysine deacetylase (HDAC) enzymes play essential roles in Plasmodium biology and are potential targets for the development of new antiplasmodial agents. In this study, a panel of 24 HDAC inhibitors with hydroxamic acid zinc binding group, a pentyloxyamide connecting unit linker region and substituted 4-phenyl and 4(pyridinyl)thiazole cap groups were investigated for in vitro activity against asexual intraerythrocytic stage P. falciparum parasites, the life cycle stage responsible for the clinical symptoms of malaria. The most potent compound (4o) had a P. falciparum IC₅₀ of 20 nM and >250-fold greater selectivity for P. falciparum versus human cells. Compound 4o was also active against exoerythrocytic stage parasites (IC₅₀ 24 nM), which are a target for malaria prevention. In contrast, 4o lacked potent activity against late-stage gametocytes (IC₅₀ > 2 μM), which are a target for malaria transmission-blocking drugs. Compound 4o and analogues caused in situ hyperacetylation of P. falciparum histone H4, indicating deacetylase inhibition. Furthermore, 4o was found to stabilise PfHDAC1 in P. falciparum protein lysates using solvent-induced protein stability Western blot assays with anti-PfHDAC1 antibody. Together, these data provide new structure-activity relationship and mechanistic insights on pentyloxyamide-based HDAC inhibitors as potential therapeutic starting points for malaria.