Characterization of phenformin and metabolites in plasma

A colorimetric assay of biguanides was adapted for small volumes of plasma and its specificity was improved. This method is based on the reaction of guanidine groups with alpha-naphtol-diacetyl. Interference of endogenous guanidine derivatives and of the water-soluble metabolites of phenformin can be excluded by the extraction procedure. Counting of plasma fractions from 14C-phenformin-injected rats and thin-layer chromatography, before and after treatment with beta-glucuronidase, were also performed: the results suggest that after adequate extraction of plasma, the colorimetric assay measures specifically the biologically active phenformin. Results of this assay in plasma from biguanide-induced lactic acidotic patients and rats are given and compared with controls : results are consistent with the hypothesis of an accumulation of biologically active biguanide in such cases.