Toward a Stable and Potent Coenzyme A-Targeting Antiplasmodial Agent: Structure-Activity Relationship Studies of -Phenethyl-α-methyl-pantothenamide.

Pantothenamides (PanAms) are potent antiplasmodials with low human toxicity currently being investigated as antimalarials with a novel mode of action. These structural analogues of pantothenate, the vitamin precursor of the essential cofactor coenzyme A, are susceptible to degradation by pantetheinase enzymes present in serum. We previously discovered that α-methylation of the β-alanine moiety of PanAms increases their stability in serum and identified -phenethyl-α-methyl-pantothenamide as a pantetheinase-resistant PanAm with potent, on-target, and selective antiplasmodial activity. In this study, we performed structure-activity relationship investigations to establish whether stability and potency can be improved further through alternative modification of the scissile amide bond and through substitution/modification of the phenyl ring. Additionally, for the first time, the importance of the stereochemistry of the α-methyl group was evaluated in terms of stability versus potency. Our results demonstrate that α-methylation remains the superior choice for amide modification, and that while monofluoro-substitution of the phenyl ring (that often improves ADME properties) was tolerated, -phenethyl-α-methyl-pantothenamide remains the most potent analogue. We show that the 2,2'-diastereomer is far more potent than the 2,2'-diastereomer and that this cannot be attributed to preferential metabolic activation by pantothenate kinase, the first enzyme of the coenzyme A biosynthesis pathway. Unexpectedly, the more potent 2,2'-diastereomer is also more prone to pantetheinase-mediated degradation. Finally, the results of studies to assess permeability and metabolic stability of the 2,2'-diastereomer suggested species-dependent degradation via amide hydrolysis. Our study provides important information for the continued development of PanAm-based antimalarials.