"Radiochemically pure [1-14C]valproic acid"--a mixture of labeled structural isomers

Ongoing studies of the disposition of valproic acid (VPA) and its glucuronide conjugate required the radiolabeled drug for greater sensitivity and tracing of oxidation metabolites. [1-14C]VPA hereinafter called LABEL (radiochemical purity greater than 98% as determined by paper and thin layer chromatography) was purchased from Amersham International, U.K. Quantitative analysis of VPA and VPA-glucuronide in bile and urine samples from rats given VPA and tracer LABEL by our standard gas chromatographic assay showed gross discrepancies with the results obtained by liquid scintillation counting of the same extracts. Examination of the purity of LABEL was therefore undertaken. Equilibration of LABEL between various organic-aqueous solvent pairs was identical to that of authentic VPA. However, gas chromatographic-mass spectrometric analysis of the trimethylsilyl derivative of LABEL revealed it to be a mixture of labeled 2-methylheptanoic acid (approximately 60%), 2-ethylhexanoic acid (approximately 30%), and 2-propylpentanoic acid (i.e., VPA, 5-10%). The origin of the isomers of VPA in LABEL was logically traced to the synthetic procedure--coupling of the Grignard reagent of (an isomeric mixture of 2-, 3-, and 4-) chloroheptane(s) with [14C]carbon dioxide. This result highlights the inadequacy of the quality control procedures used and reinforces the necessity for caution in accepting the quoted purity of radiolabeled drugs.