In the search for novel chemotypes with high sp3 character and activity against the human malaria parasite Plasmodium falciparum, a spiroindoline series was identified from a phenotypic high-throughput screening campaign. The spiroindoline hit 2 displayed good activity against both drug-sensitive and multidrug-resistant Plasmodium strains, making it an attractive starting point for hit-to-lead progression. Structure-activity relationship studies led to the identification of a novel pyridylspiroindoline frontrunner (50) with improved antiplasmodial activity, aqueous solubility, and microsomal metabolic stability. Data from additional parasitological profiling suggested that 50 likely has a mode of action differing from that of the original spiroindoline hit. Compound 50 showed excellent in vivo pharmacokinetics with efficacy being achieved in a humanized immunodeficient NSG mouse P. falciparum infection model. This provided a pharmacological proof-of-concept for this series, making it a valuable starting point for further optimization in the quest for novel antimalarial therapeutics.

A novel diazaspiro[3.4]octane series was identified from a Plasmodium falciparum whole-cell high-throughput screening campaign. Hits displayed activity against multiple stages of the parasite lifecycle, which together with a novel sp3-rich scaffold provided an attractive starting point for a hit-to-lead medicinal chemistry optimization and biological profiling program. Structure-activity-relationship studies led to the identification of compounds that showed low nanomolar asexual blood-stage activity (<50 nM) together with strong gametocyte sterilizing properties that translated to transmission-blocking activity in the standard membrane feeding assay. Mechanistic studies through resistance selection with one of the analogues followed by whole-genome sequencing implicated the P. falciparum cyclic amine resistance locus in the mode of resistance.

Fusidic acid (FA) has previously been shown to be rapidly metabolized in rodents to its C-3 epimer, which has significantly lower antimycobacterial activity relative to FA. This was in part hypothesized to account for FA's lack of in vivo efficacy in a mouse model of tuberculosis despite potent in vitro antimycobacterial activity. In the current work, we hypothesized that C-3 alkyl ester prodrugs of FA would deliver higher levels of the drug and prevent the rapid metabolism observed upon administration of FA in its original form. Pharmacokinetic analysis of FA and its 3-ketofusidic acid metabolite as well as novel C-3 alkyl ester prodrugs of FA revealed that FA has low exposure in mice due to rapid metabolism to a species-specific metabolite, 3-epifusidic acid. The C-3 alkyl ester prodrugs showed improved absorption and tissue distribution in pharmacokinetic and organ distribution experiments. These results support the original objective of the FA C-3 ester prodrugs to improve drug concentrations and tissue distribution.

Fusidic acid (FA) is a potent congener of the fusidane triterpenoid class of antibiotics. Structure-activity relationship (SAR) studies suggest the chemical structure of FA is optimal for its antibacterial activity. SAR studies from our group within the context of a drug repositioning approach in tuberculosis (TB) suggest that, as with its antibacterial activity, the C-21 carboxylic acid group is indispensable for its anti-mycobacterial activity. Further studies have led to the identification of 16-deacetoxy-16β-ethoxyfusidic acid (58), an analog which exhibited comparable activity to FA with an in vitro MIC99 value of 0.8 µM. Preliminary SAR studies around the FA scaffold suggested that the hydrophobic side chain at C-20, like the C-11 OH group, was required for activity. The C-3 OH group, however, can be functionalized to obtain more potent compounds.

Fusidic acid (FA), a natural product fusidane triterpene-based antibiotic with unique structural features, is active in vitro against Mycobacterium tuberculosis, the causative agent of tuberculosis (TB). While possessing good pharmacokinetics in man, FA is rapidly metabolized in rodents, thus complicating proof-of-concept studies in this model. Toward the repositioning of FA as an anti-TB agent, we herein describe the synthesis, activity, and metabolism of FA and semisynthesized ester derivatives in rat liver microsomes, rat plasma, and mycobacterial cell culture. FA and derivative molecules with a free C-3 OH underwent species-specific metabolism to the corresponding 3-OH epimer, 3-epifusidic acid (3-epiFA). FA was also metabolized in rat plasma to form FA lactone. These additional routes of metabolism may contribute to the more rapid clearance of FA observed in rodents. C-3 alkyl and aryl esters functioned as classic prodrugs of FA, being hydrolyzed to FA in microsomes, plasma, and Mycobacterium tuberculosis culture. In contrast, C-3 silicate esters and C-21 esters were inert to hydrolysis and so did not act as prodrugs. The antimycobacterial activity of the C-3 silicate esters was comparable to that of FA, and these compounds were stable in microsomes and plasma, identifying them as potential candidates for evaluation in a rodent model of tuberculosis.

Wide spread Plasmodium falciparum (P. falciparum) resistance has compromised existing antimalarial therapies to varying degrees. Novel agents, able to circumvent antimalarial drug resistance, are therefore needed. Fusidic acid is a unique antibiotic with a unique mode of action, which has shown weak in vitro antiplasmodial activity. Toward identifying new fusidic acid derivatives with superior antiplasmodial activity, a 3D-QSAR model was developed based on the antiplasmodial activity of previously synthesized fusidic acid derivatives. The validated Hypo 2 model was used as the 3D-structural search query to screen a fusidic acid-based combinatorial library. On the basis of the predicted activity and pharmacophore fit value, eight virtual hit compounds were selected and synthesized, including C-21 amide and C-3 ether derivatives. All synthesized hit compounds showed superior antiplasmodial activity compared to fusidic acid. Two C-21 amide derivatives displayed significant activity against the drug-sensitive NF54 strain with IC50 values of 0.3 μM and 0.7 μM, respectively. These two derivatives also displayed activity against the multidrug-resistant K1 strain, with an IC50 value of 0.2 μM and were found to be relatively noncytotoxic.

The emergence of drug resistance in Plasmodium falciparum to available antimalarial drugs has challenged current antimalarial treatments. New antimalarials, particularly those with novel mechanisms of action and no cross resistance to current drugs, are therefore urgently needed. To identify new growth inhibitors of Plasmodium falciparum, 2D and 3D similarity-based virtual screening methods were employed in parallel with an in-house database of steroid-type natural products using fusidic acid as a search query. The resulting hit compounds were further filtered based on the predicted partition coefficient, log P. The virtual screening strategy resulted in the identification of nine new compounds that inhibited parasite growth with IC50 values of <20 μM. Four compounds exhibited IC50 values in the range of 1.39-3.45 μM and three of which showed a promising selectivity index. Further, the predicted ADME properties of the four most active compounds were found to be comparable to fusidic acid. These compounds can be further explored using structural modifications in the identification and development of more potent parasite growth inhibitors with improved selectivity.

A series of novel fusidic acid (FA) derivatives was synthesized by replacing the carboxylic acid group with various ester and amide groups and evaluated in vitro for their antiplasmodial activity against the chloroquine-sensitive NF54 and multidrug-resistant K1 strains of the malarial parasite Plasmodium falciparum. Most of these derivatives showed a 4–49 and 5–17-fold increase in activity against NF54 and KI strains, respectively, as compared to FA and had a good selectivity index. These derivatives are stable over the incubation period and do not appear to be prodrugs of fusidic acid.

Structural modifications through bioisosteric approach yielded fusidic acid analogues with 2–35 folds increase in antiplasmodial activity as compared to fusidic acid.

A series of compounds based on metergoline were synthesized and evaluated in vitro for their antiplasmodial activity against the chloroquine-sensitive NF54 strain of the malaria parasite Plasmodium falciparum. These compounds were also screened for their cytotoxicity towards a mammalian cell line. Some of the compounds exhibited superior antiplasmodial activity with a good selectivity index relative to metergoline.