Semisynthetic Antimycobacterial C-3 Silicate and C-3/C-21 Ester Derivatives of Fusidic Acid: Pharmacological Evaluation and Stability Studies in Liver Microsomes, Rat Plasma, and Mycobacterium tuberculosis culture

Abstract

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.

Strategies to Combat Multi-Drug Resistance in Tuberculosis

“… 33 , 34 The rapid biotransformation of FA to its inactive epimer (3-epifusidic acid) via 3-keto fusidic acid in rodents complicated proof-of-concept studies in this model. 34 A prodrug approach involving masking the metabolically labile C-3 position through esterification was found to improve absorption and tissue distribution of FA. 35 To assess the molecular target of FA, our current efforts are focused on deconvoluting its MoA.…”

Recent Highlights in Anti‐infective Medicinal Chemistry from South Africa

“…Furthermore, fusidic acid has shown in vitro activity against Mtb, in the absence of cross‐resistance to first‐line Tb therapies, and is considered by some to be a potential option for Tb repositioning . In two related studies, Chibale and co‐workers evaluated a series of fusidic acid analogues modified at 3‐OH ( 24 ) and C‐21 acid ( 25 ) . These studies indicated that in general, compounds featuring functionalisation at these two positions were stable to hydrolysis.…”

“…These studies indicated that in general, compounds featuring functionalisation at these two positions were stable to hydrolysis. While C‐21 functionalisation reduced biological activity and increasing plasma albumin binding, the improved metabolic stability of C‐3 silicate esters such as 24 , which maintained biological activity, suggested a possible avenue for further optimisation…”

Crystal Structure of Mycobacterium tuberculosis Elongation Factor G1

“…Structural characterization of Mtb EF-G1 has been limited by the lack of a Mtb EF-G1 crystal, and this has hampered structurebased inhibitor design. Although FA is ineffective against Mtb in vivo owning to pharmacokinetic limitations, as demonstrated using a rodent model of Mtb characterized by rapid clearance and poor exposure (Kigondu et al, 2014;Njoroge et al, 2019), hence repurposing the idea that FA and its derivatives could represent a potential novel strategy for TB drug discovery (Njoroge et al, 2019). Indeed, various FA derivatives have been proposed as anti-TB agents (Kigondu et al, 2014;Dziwornu et al, 2019;Njoroge et al, 2019;Strydom et al, 2020).…”

“…Although FA is ineffective against Mtb in vivo owning to pharmacokinetic limitations, as demonstrated using a rodent model of Mtb characterized by rapid clearance and poor exposure (Kigondu et al, 2014;Njoroge et al, 2019), hence repurposing the idea that FA and its derivatives could represent a potential novel strategy for TB drug discovery (Njoroge et al, 2019). Indeed, various FA derivatives have been proposed as anti-TB agents (Kigondu et al, 2014;Dziwornu et al, 2019;Njoroge et al, 2019;Strydom et al, 2020). The structure determination of Mtb EF-G1 represents the key step for the structure-based design of inhibitors, in future activities, which will be investigated to assess their potential in the context of anti-TB drug development.…”