Synthesis and Structural Activity Relationship Study of Ursolic Acid Derivatives as Antitubercular Agent.

Objective: The chemical transformation of ursolic acid (UA) into novel C-3 aryl ester derivatives and in vitro and silico assessment of their antitubercular potential.

Background: UA is a natural pentacyclic triterpenoid with many pharmacological properties. Semisynthetic UA analogs have demonstrated enhanced anticancer, antimalarial, and antifilarial properties in our previous studies.

Methods: The C-30 carboxylic group of previously isolated UA was protected, and various C-3 aryl ester derivatives were semi-synthesized. The agar dilution method was used to evaluate the in vitro antitubercular efficacy of Mycobacterium tuberculosis (Mtb) H₃₇Ra. In silico docking studies of the active derivative were carried out against Mtb targets, catalase peroxidase (PDB: 1SJ2), dihydrofolate reductase (PDB: 4M2X), enoyl-ACP reductase (PDB: 4TRO), and cytochrome bc1 oxidase (PDB: 7E1V).

Results: The derivative 3-O-(2-amino,3-methyl benzoic acid)-ethyl ursolate (UA-1H) was the most active among the eight derivatives (MIC1 2.5 μg/mL) against Mtb H₃₇Ra. Also, UA-1H demonstrated significant binding affinity in the range of 10.8-11.4 kcal/mol against the antiTb target proteins, which was far better than the positive control Isoniazid, Ethambutol, and co-crystallized ligand (HEM). Moreover, the predicted hit UA-1H showed no inhibition of Cytochrome P450 2D6 (CYP2D6), suggesting its potential for favorable metabolism in Phase I clinical studies.

Conclusion: The ursolic acid derivative UA-1H possesses significant in vitro antitubercular potential with favorable in silico pharmacokinetics. Hence, further in vivo assessments are suggested for UA-1H for its possible development into a secure and efficient antitubercular drug.