Abstract
This paper demonstrates that the stable isotope tracer technique using NMR spectroscopy and the selective 13C labeling of protonated carbons can provide a relatively sensitive method to investigate pharmacokinetic problems in man. The urinary excreted [1, 3, 5-13C3] hippuric acid ([13C] HA) formed from orally administered [1, 3, 5-13C3] benzoic acid ([13C] BA) as a model substrate was successfully quantitated without any separation procedures by proton-decoupled 13C-NMR spectroscopy of 10-fold concentrated urine in a 10 min accumulation time. In spite of the low dosage (10mg BA), the C3, 5 resonances of [13C] HA were detected with favorable signal-to-noise ratios to quantitate [13C] HA concentration. The administered [13C] BA was found to be quantitatively biotransformed to HA and excreted in urine within 4h. The lower limit of detection was estimated to be 50 nmol in an NMR tube, which was improved about one order of magnitude over that of BA labeled in the quaternary carbon (C7). The potential of an inverse detection experiment using heteronuclear multiple quantum coherence was also investigated in order to detect [13C] HA in urine with a higher sensitivity. The inverse experiment improved the sensitivity by a factor of 2-3 over 13C{1H}-NMR, although the specificity of detection was relatively poor.
