MECHANISM OF COMPETITIVE INHIBITION OF p -AMINOBENZOIC ACID OXIDATION BY p -AMINOSALICYLIC ACID

Cross-resistance of isoniazid, para-aminosalicylic acid and pasiniazid against isoniazid-resistant Mycobacterium tuberculosis isolates in China

OBJECTIVES

Pasiniazid is a chemical complex of isoniazid (INH) and para-aminosalicylic acid (PAS). The aim of this study was to explore the cross-resistance of INH, PAS and pasiniazid against INH-resistant Mycobacterium tuberculosis isolates in China.

METHODS

A Microplate alamarBlue® Assay was performed to determine the minimum inhibitory concentrations (MICs) of INH, PAS and pasiniazid against 109 INH-resistant M. tuberculosis isolates. A statistical analysis of the relationship between different genotypes, gene mutations, and INH, PAS or pasiniazid susceptibility was then performed.

RESULTS

Among the 109 INH-resistant isolates, 13 (11.9%) and 21 (19.3%) showed resistance to PAS and pasiniazid, respectively. Among the 13 PAS-resistant M. tuberculosis isolates, 11 remained susceptible to pasiniazid. Of 63 INH-resistant isolates harbouring mutations in katG, the inhA promoter or the oxyR-ahpC intergenic region, 52 remained susceptible to pasiniazid. Moreover, 11 of 13 pasiniazid-resistant isolates carried mutations in katG, the inhA promoter or the oxyR-ahpC intergenic region.

CONCLUSION

Taken together, these results demonstrate that PAS resistance and mutations in thekatG gene, inhA promoter or oxyR-ahpC intergenic region in INH-resistant M. tuberculosis have little effect on pasiniazid susceptibility.

Safety and tolerability profile of second-line anti-tuberculosis medications

Tuberculosis (TB) remains a major public health problem, representing the second leading cause of death from infectious diseases globally, despite being nearly 100 % curable. Multidrug-resistant (MDR)-TB, a form of TB resistant to isoniazid and rifampicin (rifampin), two of the key first-line TB drugs, is becoming increasingly common. MDR-TB is treated with a combination of drugs that are less effective but more toxic than isoniazid and rifampicin. These drugs include fluoroquinolones, aminoglycosides, ethionamide, cycloserine, aminosalicyclic acid, linezolid and clofazimine among others. Minor adverse effects are quite common and they can be easily managed with symptomatic treatment. However, some adverse effects can be life-threatening, e.g. nephrotoxicity due to aminoglycosides, cardiotoxicity due to fluoroquinolones, gastrointestinal toxicity due to ethionamide or para-aminosalicylic acid, central nervous system toxicity due to cycloserine, etc. Baseline evaluation may help to identify patients who are at increased risk for adverse effects. Regular clinical and laboratory evaluation during treatment is very important to prevent adverse effects from becoming serious. Timely and intensive monitoring for, and management of adverse effects caused by, second-line drugs are essential components of drug-resistant TB control programmes; poor management of adverse effects increases the risk of non-adherence or irregular adherence to treatment, and may result in death or permanent morbidity. Treating physicians should have a thorough knowledge of the adverse effects associated with the use of second-line anti-TB drugs, and routinely monitor the occurrence of adverse drug reactions. In this review, we have compiled safety and tolerability information regarding second-line anti-TB drugs in both adults and children.

Competitive relationship between protocatechuic acid and p-aminosalicylic acid for a cellular transport mechanism

Hubbard, Jerry S. (Oklahoma State University, Stillwater), and Norman N. Durham. Competitive relationship between protocatechuic acid and p-aminosalicylic acid for a cellular transport mechanism. J. Bacteriol. 82:361-369. 1961.-The oxidation of protocatechuic acid by a Flavobacterium is inhibited by p-aminosalicyclic acid regardless of whether the organism is grown on protocatechuic acid or sequentially induced to protocatechuic acid by growth on p-aminobenzoic acid. Depletion of the substrate from the medium by the cell suspension is dependent, within defined limits, on the inhibitor to substrate ratio, and the inhibition can be overcome by addition of excess substrate. However, this competitive effect is not observed in high inhibitor to substrate ratios. p-Aminosalicylic acid did not affect the rate or extent of oxidation, carbon dioxide evolution, or formation of beta-ketoadipic acid during degradation of protocatechuic acid by cell extracts. The results suggest that p-aminosalicylic acid antagonizes the oxidation of protocatechuic acid by the cell suspension by competing with the substrate for a specific transport mechanism, thereby regulating the entry and internal accumulation of the substrate. The lack of a competitive effect in high inhibitor to substrate ratios could be interpreted as an indication that the mechanism for accumulating the substrate may consist of more than one active transport system.