Determination of ethambutol in plasma and urine by GC-CI-MS using a deuterated internal standard

In order to study the pharmacokinetics of ethambutol (a tuberculostatic drug) in patients with renal insufficiency, we have developed a GC-CI-MS analytical procedure which overcomes the problems inherent in the existing colorimetric, microbiological and GC methods, i.e., that of specificity, sensitivity and reproducibility. The method involves the use of a tetradeuterated ethambutol (ethambutol-d4) standard. (+)-2,2'-(Ethylene-d4-diimino)-di-l-butanol is added to each biological sample prior to extraction with chloroform. The dried residue was allowed to react with trimethylsilyl imidazole prior to injection onto a 3% SP-2250 DB column in a Finnigan Model 3200E GC-MS, interfaced with a Finnigan Model 6100 data system. Mass fragmentography was monitored at m/e 349 and 353, the M + 1 ions for the di-TMS derivatives of ethambutol and ethambutol-d4, respectively, and at m/e 333 and 337, corresponding to the M-CH3 ions for ethambutol and ethambutol-d4. The pharmacokinetics of ethambutol in anephric patients can be fitted by a two-compartment open model which has a zero asymptote when the samples are analyzed by the GC-CI-MS method, as compared to measurement using 14C-ethambutol which gives a two-compartment closed model with a positive non-zero asymptote, indicating the buildup of metabolic by-products