4-Aminosalicylic acid

Toxicity

LD50=4 gm/kg (orally in mice); LD50=3650 mg/kg (orally in rabbits)

Background

An antitubercular agent often administered in association with isoniazid. The sodium salt of the drug is better tolerated than the free acid.

Biological Activity

4-Aminosalicylic acid is an antimetabolite of p-aminobenzoic acid (PABA) that has antibacterial activity.It is active against streptomycin-sensitive and -resistant strains of M. tuberculosis (MICs = 0.78 and 0.39 μg/ml, respectively), an effect that can be reversed by PABA. 4-Aminosalicylic acid is an alternative substrate for mycobacterial dihydropteroate synthase (FolP1) and misincorporation into the folate pathway leads to accumulation of several folate-dependent metabolites including serine, homocysteine, dUMP, and AICAR, markers of folate pathway inhibition, in a concentration-dependent manner. It reverses manganese-induced increases in rat hippocampal levels of NOD-like receptor protein 3 (NLRP3), cleaved caspase-1, and phosphorylated p65, markers of NLRP3 inflammasome-dependent pyroptosis, when administered at a dose of 300 mg/kg. 4-Aminosalycilic acid is also a building block that has been used in the synthesis of luminescent lanthanide complexes. Formulations containing 4-aminosalicylic acid have been used in the treatment of tuberculosis.

Mechanism of action

p-aminosalicylic acid is thought to act as an antimetabolite interfering with the incorporation of p-aminobenzoic acid into folic acid. When coadministered with INH, PAS is found to reduce the acetylation of INH, itself being the substrate for acetylation, thus increasing the plasma levels of INH. This action may be especially valuable in patients who are rapid acetylators.

Pharmacokinetics

Aminosalicylic acid is an anti-mycobacterial agent used with other anti-tuberculosis drugs (most often isoniazid) for the treatment of all forms of active tuberculosis due to susceptible strains of tubercle bacilli. The two major considerations in the clinical pharmacology of aminosalicylic acid are the prompt production of a toxic inactive metabolite under acid conditions and the short serum half life of one hour for the free drug. Aminosalicylic acid is bacteriostatic against Mycobacterium tuberculosis (prevents the multiplying of bacteria without destroying them). It also inhibits the onset of bacterial resistance to streptomycin and isoniazid.

Synthesis

p-Aminosalicylic acid, 5-amino-2-hydroxybenzoic acid (34.1.22), is synthesized in a Kolbe reaction, which consists of direct interaction of m-aminophenol with potassium bicarbonate and carbon dioxide while heating at a moderate pressure of 5¨C10 atm.