(+/-)-
Mexiletine is a class Ib
antiarrhythmic drug useful in the treatment of
premature ventricular contractions. It is predominantly metabolized by the liver with less than 15% being excreted in urine as unchanged
drug.
p-Hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM) are the two major mammalian metabolites. The purpose of our study was to develop a stereospecific high-performance liquid chromatographic (HPLC) method to determine whether the fungus, Cunninghamella echinulata (UAMH 4145), was able to biosynthesize these same two metabolites from the substrate (+/-)-
mexiletine. Furthermore, it was desirable to ascertain whether metabolism of
mexiletine was stereoselective. The method requires pre-column derivatization of the
drug and metabolites with S-(+)-1-(1-naphthyl)
ethyl isocyanate (NEIC) followed by normal-phase HPLC.
Mexiletine, PHM, HMM and (+/-)-1-(4-hydroxyphenoxy)-3-isopropylaminopropan-2-ol (internal standard) were extracted from microbial broth using two volumes of
diethyl ether after basifying with
sodium carbonate. The combined
ether extracts were evaporated to dryness, using a gentle stream of
nitrogen, and reconstituted in 0.3 ml of
chloroform to which was added 0.075 ml of NEIC (0.1%, v/v, in
chloroform). This
solution was immediately evaporated to dryness under a
nitrogen stream. The residue was reconstituted with 0.220 ml of
chloroform and 0.030 ml of
n-butylamine (0.33%, v/v, in
chloroform) and injected into the HPLC system.