Objective: The serum profiles of isoniazid and its hydrazine metabolite were investigated in patients with tuberculosis in steady-state conditions in the hope of identifying a pharmacokinetic approach that could be useful in the clinical assessment of other patients with the same disease state. Patients and Study Design: Isoniazid was coadministered with the other drugs included in the antitubercular regimen (rifampicin, ethambutol, streptomycin, morinamide). Concentrations of isoniazid and its hydrazine metabolite were measured in the collected serum samples by high performance liquid chromato-graphy. The pharmacokinetic parameters of isoniazid were estimated by WINNONLIN. Since an isoniazid serum concentration of 1.5 mg/L 3 hours after (C3) the administration of the dose (D) was demonstrated to be therapeutically efficacious with minimal neurotoxic adverse effects, the theoretical dosage adjustment (D(a)) required to achieve this optimal concentration in all our patients was calculated according to the inactivator index (I3) method proposed by Vivien et al. [I3 = (C) + 0.6)/D]; [D(a) = 2.1/I3]. Results: A large interindividual pharmacokinetic variability was observed [especially for maximum concentration (C(max)), trough levels, area under the curve (AUC), and hydrazine metabolite production] not only according to the acetylator status as expected, but inside each group as well. The mean D(a) was significantly lower than the mean D in the slow acetylators (2.32 ± 0.78 vs 4.75 ± 0.47 mg/kg; p < 0.00001), while no statistically significant difference was found in the rapid acetylators (4.16 ± 3.07 vs 4.68 ± 0.81 mg/kg; p = 0.53). Conclusion: Our findings suggest that slow acetylators show a mean daily isoniazid exposure (AUC) two-fold higher than rapid acetylators (36.42 ± 11.53 vs 16.50 ± 7.02 mg/L·h; p < 0.0001) when nearly equal isoniazid daily doses are administered (4.75 ± 0.47 vs 4.68 ± 0.81 mg/kg; p = 0.81) and no peculiar pathophysiological conditions affecting isoniazid disposition are present. It might be speculated that slow acetylators may often benefit from reduced doses (< 5 mg/kg), since this strategy could either guarantee efficacy and reach the desired C(max) and C3 levels, or minimise potential toxicity risks lowering isoniazid daily exposure and fluctuations of hydrazine metabolite serum concentrations. However, care should be exercised and therapeutic drug monitoring needs to be performed in all patients, especially when peculiar pathophysiological conditions such its malabsorption - could affect the drug's pharmacokinetics.
