Studies on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and analogs

The cyclic allylamine I-methyl-4-phenyl-l ,2,3,6-tetrahydropyridine (MPTP) is a potent and specific neurotoxin that causes a parkinsonian like syndrome in humans and subhuman primates. Research has revealed that MPTP is bioactivated in a reaction catalyzed by flavin containing monoamine oxidase B (MAO B) to yield the dihydropyridinium species MPDP+ which undergoes further oxidation to the ultimate toxin, the I-methyl-4-phenylpyridinium species MPP+. The research summarized in this thesis describes a potential model reaction for the MAO catalyzed conversion of MPTP to MPDP+ and the synthesis and biological evaluation of MPTP analogs bearing a heteroatom at C-4 of the tetrahydropyridine ring.

The model for the enzyme catalyzed oxidation of MPTP to MPDP+ is based on the anhydride mediated conversion of MPTP Noxide to MPDP+. This reaction pathway was visualized to mimic a reaction sequence in which an FAD-MPTP adduct cleaves to yield MPDP+ and FADH2. Attempts were made to assess the isotope effect associated with this reaction and to compare that value with the corresponding values for the MAO-B catalyzed reaction [D(V max/Km) = 7-9], the cytochrome P-450 catalyzed reaction [D(V max/Km) = 1.04] and the electrochemical oxidation (D k = 1.35). Unfortunately experimental difficulties prevented a complete analysis of the problem. Specialized equipment will be required to obtain accurate isotope effect measurements.

The second study concerns the preparation of the MPTP analogs 4-chloro-, 4-cyano-, and 4-( 4-fluorophenoxy)-1,2,3,6-tetrahydropyridine as potential MAO B substrates that could generate neurotoxic pyridinium metabolites. Results obtained with MAO B have revealed that the dihydropyridinium intermediate formed from 4-( 4-fluorophenoxy)-1 ,2,3, 6-tetrahydropyridine undergoes spontaneous hydrolysis to generate 4-fluorophenol and 1- methyl-4-pyridone. The significance of this finding with respect to neurotoxic mechanisms and design are discussed.