USE OF NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY AND SELECTIVE C-13-LABELING FOR PHARMACOKINETIC RESEARCH IN MAN - DETECTION OF BENZOIC-ACID CONVERSION TO HIPPURIC-ACID

This paper demonstrates that the stable isotope tracer technique using NMR spectroscopy and the selective C-13 labeling of protonated carbon s can provide a relatively sensitive method to investigate pharmacokin etic problems in man. The urinary excreted [1,3,5-C-13(3)]hippuric aci d ([C-13]HA) formed from orally administered [1,3,5-C-13(3)]benzoic ac id ([C-13]BA) as a model substrate was successfully quantitated withou t any separation procedures by proton-decoupled C-13-NMR spectroscopy of 10-fold concentrated urine in a 10 min accumulation time. In spite of the low dosage (10 mg BA), the C3,5 resonances of [C-13]HA were det ected with favorable signal-to-noise ratios to quantitate [C-13]HA con centration. The administered [C-13]BA was found to be quantitatively b iotransformed to HA and excreted in urine within 4h. The lower limit o f detection was estimated to be 50 nmol in an NMR tube, which was impr oved about one order of magnitude over that of BA labeled in the quate rnary carbon (C7). The potential of an inverse detection experiment us ing heteronuclear multiple quantum coherence was also investigated in order to detect [C-13]HA in urine with a higher sensitivity. The inver se experiment improved the sensitivity by a factor of 2-3 over C-13{H- 1}-NMR, although the specificity of detection was relatively poor.