Mechanism of Action
The mechanism of action is based on competitive antagonism with p-aminobenzoic acid (PABA). PABA is an essential growth factor for Mycobacterium tuberculosis. By substituting for PABA in biochemical reactions, sodium p-aminosalicylate blocks the pathogen's enzyme systems, ultimately inhibiting folate synthesis.
Important note: The drug acts selectively. It affects exclusively mycobacteria that are in the active replication phase. It has no effect on dormant bacteria.
Pharmacological Effects
The drug exerts a tuberculostatic effect—it does not kill bacteria directly, but halts their growth and reproduction. In the classification of antitubercular agents, it belongs to the group of synthetic drugs (derivatives of p-aminosalicylic acid).
Its antitubercular activity is considered moderate: it is significantly lower than that of isoniazid or aminoglycoside antibiotics (e.g., streptomycin). For this reason, sodium p-aminosalicylate is never used as monotherapy. Its primary pharmacological role in a treatment regimen is to enhance the efficacy of more potent antitubercular drugs and, most importantly, to delay the development of resistance in mycobacteria to the core therapy.
Pharmacokinetics
When administered per os (orally), the drug demonstrates good absorption. However, its use is associated with a pronounced irritant effect on the gastrointestinal mucosa.
Metabolism of sodium p-aminosalicylate occurs predominantly in the liver, as well as partially within the gastrointestinal tract itself. Elimination of metabolites and unchanged drug occurs via the kidneys with urine.
Indications
The drug is indicated for all forms of tuberculosis.
Key prescribing rule: it must be used strictly as part of combination therapy (along with isoniazid, streptomycin, and other agents).
Interesting fact: The p-aminosalicylic acid (PAS) moiety structurally forms part of the molecule of capreomycin, a reserve natural peptide antibiotic used for drug-resistant forms of tuberculosis.
Adverse Effects
The spectrum of adverse reactions is largely due to the pharmacokinetic features of the drug:
- Gastrointestinal tract: due to mucosal irritation, dyspeptic symptoms (nausea, vomiting, diarrhea) develop most frequently.
- Immune system: various allergic reactions may occur.
Formulations and Administration
According to standard medical references, sodium p-aminosalicylate is available in the following dosage forms:
- Powder.
- 0.5 g tablets.
- 0.5 g enteric-coated tablets (specifically designed to minimize gastric mucosal irritation by releasing the drug only in the intestine).
- Vials of 3% solution, 250 and 500 mL.
Routes of administration:
- Oral: 3–4 g per single dose.
- Parenteral: 250 mL of 3% solution administered as an intravenous infusion.