Mechanism of selectivity reveals novel antifolate drug interactions

Antimicrobial agents that target a specific pathogen of interest is the gold standard in drug design. para-Aminosalicylic acid (PAS), remains a cornerstone therapy, in the treatment against Mycobacterium tuberculosis, owing to its high level of selectivity. Despite its high level of selectivity, PAS has been reassigned to treat drug-resistant strains of M. tuberculosis because it causes severe gastrointestinal (GI) distress that results in poor patient compliance. We have previously shown PAS inhibits the folate biosynthetic pathway specifically inhibiting dihydrofolate reductase1,2. In this study, we sought to determine the mechanistic basis of PAS selectivity and determined that PAS can be utilized in folate biosynthesis by other bacterial pathogens. The utilization of PAS ultimately led to the antagonism of key antibiotics, specifically the sulfonamides, used to prophylactically treat individuals with HIV-AIDS3. In addition, we found many bacteria in the GI tract could also utilize PAS to make a hydroxy-folate species which resulted in GI toxicity. Using sulfonamides as a tool to prevent PAS associated toxicity in the GI tract, we discovered that the sulfonamides antagonized the antimycobacterial activity of PAS. These findings indicate a new need for understanding the mechanisms of selective therapies and more important, that HIV-AIDS/M. tuberculosis co-infected individuals should avoid co-treatment of PAS and sulfonamides.