Deuterium Isotope Effect Measurements on the Interactions of the Neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine with Monoamine Oxidase B

Deuterium Kinetic Isotope Effect Studies of a Potential in Vivo Metabolic Trapping Agent for Monoamine Oxidase B

“…Iodocoumarin (4) was prepared via Pechmann condensation by treating 3-iodophenol with ethyl acetoacetate in an acidic solution. Reaction of 4 with p-toluenesulfonic acid and mchloroperbenzoic acid (mCPBA) in dichloromethane gave iodonium salt 5, which was converted to the 4-methoxyphenyl-substituted iodonium salt (6). A subsequent substitution reaction with 4-phenol-d 5 yielded the pyridine Cou-d 7 precursor (7).…”

“…It was therefore surprising that our in vitro assays of MAO-B activity showed no isotope effect for deuterium substitution of Cou (Table 1); in comparison, the KIE for [ 11 C]deprenyl-d 2 was estimated to be 3.8 (using k 3 ′E), 5 and values for MAO oxidation of MPTP-6,6-d 2 to the corresponding 1-methyl-4-phenyl-2,3-dihydropyridinium species were 3.55 (V max ) and 8.01 (V max /K m ). 6 The in vivo studies in primate brain for [ 11 C]Cou-d 3 and [ 11 C]Cou-d 7 were supportive of the lack of a deuterium substitution effect as the pharmacokinetic curves for radiotracer trapping were similar for all three substrates, showing a rapid uptake to a plateau with very little washout of radioactivity, consistent with irreversible trapping. This is markedly different than the effect seen for [ 11 C]deprenyld 2 , where incorporation of deuterium resulted in a significant egress of radioactivity after peak uptake and a lower plateau level of trapped radioactivity, and thus very different shapes for the brain time−radioactivity curves.…”

“…Kinetic isotope effects have been clearly demonstrated for a number of amine substrates of the monoamine oxidases, leading to the conclusion that the abstraction of an allylic hydrogen at the position α to the nitrogen is the rate-limiting step. − This has been exploited in the design of PET imaging agents for monoamine oxidases, where incorporation of deuterium has been used to slow the irreversible trapping of [ 11 C]­deprenyl, resulting in improved in vivo pharmacokinetics in animals and humans. , In vitro kinetic isotope effects have also been demonstrated for MAO oxidation of MPTP and related analogues, , prompting our expectation that deuterium substitution could be employed to modify the in vivo pharmacokinetics of our newly developed MAO substrate, [ 11 C]­Cou. It was therefore surprising that our in vitro assays of MAO-B activity showed no isotope effect for deuterium substitution of Cou (Table ); in comparison, the KIE for [ 11 C]­deprenyl- d 2 was estimated to be 3.8 (using k 3 ′E), and values for MAO oxidation of MPTP-6,6- d 2 to the corresponding 1-methyl-4-phenyl-2,3-dihydropyridinium species were 3.55 ( V max ) and 8.01 ( V max / K m ) . The in vivo studies in primate brain for [ 11 C]­Cou- d 3 and [ 11 C]­Cou- d 7 were supportive of the lack of a deuterium substitution effect as the pharmacokinetic curves for radiotracer trapping were similar for all three substrates, showing a rapid uptake to a plateau with very little washout of radioactivity, consistent with irreversible trapping.…”

“…KIE experiments using deuterium substitutions on the tetrahydropyridine ring have demonstrated that the hydrogen abstraction step is also rate-limiting in the MAO-B-mediated oxidation of MPTP. The KIE value for MAO oxidation of MPTP-6,6- d 2 to the corresponding 1-methyl-4-phenyl-2,3-dihydropyridinium species was 3.55 (using V max ) . Given this strong literature precedent, it was reasonable to expect a similar KIE for [ 11 C]­Cou, and we attempted to apply this concept to further optimize the in vivo pharmacokinetics of this radiotracer.…”

“…The KIE value for MAO oxidation of MPTP-6,6-d 2 to the corresponding 1-methyl-4-phenyl-2,3-dihydropyridinium species was 3.55 (using V max ). 6 Given this strong literature precedent, it was…”