Rate of inactivation of isoniazid in South Indian patients with pulmonary tuberculosis. 3. Serum concentrations of isoniazid produced by three regimens of isoniazid alone and one of isoniazid plus PAS.

A series of studies on the rate of inactivation of isoniazid in Indian patients with pulmonary tuberculosis in a 1-year comparison of four domiciliary chemotherapeutic regimens-three of isoniazid alone, either in moderate (HI-1 and HI-2 regimens) or in low (H regimen) dosage, and one of isoniazid in low dosage plus p-aminosalicylic acid (PAS) (PH regimen) -has recently been undertaken by the Tuberculosis Chemotherapy Centre, Madras. This paper-the third of the series-presents information on the length of time during which inhibitory concentrations of isoniazid (0.2 mug/ml or more) were maintained in the serum and on the estimated peak serum concentrations of isoniazid produced by the four regimens and relates these factors to the therapeutic effectiveness of the different regimens.THE RESULTS SUGGESTED THAT: (a) the therapeutic superiority of the PH regimen over the isoniazid-alone regimens was only in part due to the effect of PAS in enhancing the serum levels of isoniazid; (b) the therapeutic superiority of the HI-1 regimen (moderate dosage, given in one dose a day) over the HI-2 and H regimens (moderate and low dosage, respectively, given in two doses a day) was due to the higher peak serum concentration of isoniazid produced rather than to the period minimal inhibitory concentrations of isoniazid were maintained in the serum; and (c) there was a possibility that raising the peak serum concentration of isoniazid above a critical concentration of about 3 mug/ml had a greater therapeutic effect than similar proportional increases below this concentration. It also appeared that the slightly better therapeutic response observed in the slow inactivators of isoniazid than in the rapid inactivators was due to the higher peak serum concentrations of isoniazid produced; the enhanced therapeutic effect was approximately the same as would be expected from a 50% increase in isoniazid dosage.