Binding orientation of amphetamine and norfenfluramine analogues in the benzonorbornene and benzobicyclo[3.2.1]octane ring systems at the active site of phenylethanolamine N-methyltransferase (PNMT).

In a continuation of studies directed at characterizing the conformational basis of binding beta-phenylethylamines at the active site of phenylethanolamine N-methyltransferase (PNMT), anti-10-amino- (12) and syn-10-amino-5,6,7,8-tetrahydro-5,8-methano-9H-benzocycloheptene (13) were prepared and evaluated as substrates and inhibitors for PNMT. These conformationally defined amphetamine analogues mimic a low energy half-chair form of 2-aminotetralin (2AT). Further, in order to determine the active site binding orientation of beta-phenylethylamines bearing aryl lipophilic substituents, the aryl trifluoromethyl-substituted derivatives of 12 and 13 (20-27), as well as anti-9-amino-5-(trifluoromethyl)-(18) and anti-9-amino-6-(trifluoromethyl) benzonorbornene (19), were prepared and evaluated. The competitive inhibition displayed by the fully extended analogue 12 coupled with the uncompetitive kinetics exhibited by the folded isomer 13 supports previous findings that a fully extended side chain conformation is optimal for binding to the active site of PNMT. In addition, the fact that 12 displayed enhanced affinity as an inhibitor over its beta-phenylethylamine counterparts in the benzonorbornene and 1,4-ethanonaphthalene ring systems suggests that a half-chair conformation is preferred when 2AT analogues interact at the active site of the enzyme. This would be consistent with previous results that PNMT preferentially binds molecules with a more coplanar relationship between the aromatic ring and the amino nitrogen. The lack of activity as a substrate in 12 indicates that the negative steric interactions of the ethano bridging unit prohibits it from binding in a manner consistent with the known PNMT substrates exo-2-amino- (6) and anti-9-aminobenzonorbornene (8). Given the emergence of activity as a substrate in 20 and 21 (the 1-trifluoromethyl- and the 2-trifluoromethyl-substituted derivatives of 12), it appears that the positive interaction of the trifluoromethyl group orients these analogues in a manner in which the ethano bridge lies in regions of steric bulk tolerance. This would suggest that the region of steric intolerance has a degree of directionality. Finally, although the aromatic ring binding region of the active site of PNMT contains a large degree of lipophilic character, only specific spatial orientations between the trifluoromethyl group and the amino nitrogen of aryl trifluoromethyl-substituted beta-phenylethylamines allow both to interact simultaneously in a manner that allows the amine to bind in a region of the active site in which methylation can occur.