7-Substituted-1,2,3,4-tetrahydroisoquinolines (7-substituted-THIQs) are potent inhibitors of phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28), the enzyme involved in the biosynthesis of epinephrine. Unfortunately, most of these compounds also exhibit strong affinity for the alpha2-adrenoceptor. To design a selective (PNMT vs alpha2-adrenoceptor affinity) inhibitor of PNMT, the steric and electrostatic factors responsible for PNMT inhibitory activity and alpha2-adrenoceptor affinity were investigated by evaluating a number of 7-substituted-THIQs. A classical quantitative structure-activity relationship (QSAR) study resulted in a three-parameter equation for PNMT (PNMT pKi = 0.599pi - 0.0725MR + 1. 55sigmam + 5.80; n = 27, r = 0.885, s = 0.573) and a three-parameter equation for the alpha2-adrenoceptor (alpha2 pKi = 0.599pi - 0. 0542MR - 0.951sigmam + 6.45; n = 27, r = 0.917, s = 0.397). These equations indicated that steric effects and lipophilicity play a similar role at either active site but that electronic effects play opposite roles at either active site. Two binding orientations for the THIQs were postulated such that lipophilic and hydrophilic 7-substituents would not occupy the same region of space at either binding site. Using these two binding orientations, based on the lipophilicity of the 7-substituent, comparative molecular field analysis (CoMFA) models were developed that showed that the steric and electrostatic interactions at both sites were similar to those previously elaborated in the QSAR analyses. Both the QSAR and the CoMFA analyses showed that the steric interactions are similar at the PNMT active site and at the alpha2-adrenoceptor and that the electrostatic interactions were different at the two sites. This difference in electrostatic interactions might be responsible for the selectivity of THIQs bearing a nonlipophilic electron-withdrawing group at the 7-position. These QSAR and CoMFA results will be useful in the design of potent and selective (PNMT vs alpha2-adrenoceptor affinity) inhibitors of PNMT.

A series of 3-fluoromethyl-1,2,3,4-tetrahydroisoquinolines (3-fluoromethyl-THIQs) was proposed, and their phenylethanolamine N-methyltransferase (PNMT) and alpha(2)-adrenoceptor affinities were predicted through the use of comparative molecular field analysis (CoMFA) models. These compounds were synthesized and evaluated for affinity at PNMT and the alpha(2)-adrenoceptor. It was discovered that these compounds are some of the most selective inhibitors of PNMT versus the alpha(2)-adrenoceptor known. To determine the ability of these compounds to penetrate the blood-brain barrier (BBB), a series of THIQs possessing a variety of calculated partition coefficients (Clog P) were assayed using an in vitro BBB model. This study found a good correlation between lipophilicity (Clog P) and BBB permeability, which indicated that THIQs possessing Clog P values of at least 0.13-0.57 should have some penetration into the brain. Two compounds [3-fluoromethyl-7-N-(4-chlorophenyl)aminosulfonyl-THIQ (18) and 3-fluoromethyl-7-cyano-THIQ (20)] possess calculated partition coefficients greater than 0.57 and display selectivities (alpha(2)-adrenoceptor K(i)/PNMT K(i)) greater than 200 and thus represent promising leads in the development of highly selective inhibitors of PNMT with the ability to penetrate the BBB.

In the search for potent and selective inhibitors of the enzyme phenylethanolamine N-methyltransferase (PNMT; EC 2.1.1.28), we examined the effect of ring size or an additional heteroatom in the conformationally-restricted benzylamine-type PNMT inhibitors. Based on semiempirical calculations (MNDO) and molecular modeling studies, PNMT-inhibitory activity of these compounds seemed to be dependent on (a) the torsion angle between the plane of the aromatic ring and the endo N atom lone pair ( tau 2 angle), with the optimal value of tau 2 being about - 75 degrees, and (b) the amount of steric bulk about the 3-position of 1,2,3,4-tetrahydroisoquinoline (5, THIQ). 2,3,4,5-Tetrahydro-1H-2-benzazepine (6) was found to have the highest selectivity (PNMT Ki = 3.34 microM, alpha 2 Ki = 11 microM, selectivity = 3.2) as compared to other homologues of THIQ (PNMT Ki = 9.67 microM, alpha 2 Ki = 0.35 microM, selectivity = 0.036). The higher PNMT-inhibitory activity of 6 was attributed to favorable steric interactions of the puckered methylene groups in the putative bioactive conformation of 6 at the PNMT active site, whereas unfavorable interactions of these puckered methylene groups at the alpha 2-adrenoceptor were thought to be the cause of reduced alpha 2 affinity of 6. No further enhancement of the selectivity of the benzazepine ring system could be obtained via introduction of a second heteroatom (N, O, S) at the 5-position in this ring system.

(+/-)-7-Aminosulfonyl-3-fluoromethyl-1,2,3,4-tetrahydroisoquinoline (7) is one of the most potent and selective inhibitors of phenylethanolamine N-methyltransferase (PNMT) reported to date, but a blood-brain barrier (BBB) model indicates that it cannot penetrate the BBB. To increase the lipophilicity of 7 by addition of a nonpolar substituent to the sulfonamide nitrogen, a small library of (+/-)-3-fluoromethyl-7-(N-substituted aminosulfonyl)-1,2,3,4-tetrahydroisoquinolines was synthesized and evaluated as inhibitors of PNMT and for affinity at the alpha2-adrenoceptor. In addition, this library probed the PNMT active site surrounding the sulfonamide nitrogen of 7. Bulky substituents on the sulfonamide nitrogen are disfavored at the PNMT active site more so than at the alpha2-adrenoceptor (thus reducing selectivity). On the other hand, alkyl chains on the sulfonamide nitrogen that contain an electron dense atom, such as a fluorine, are favored in the PNMT active site and possess little alpha2-adrenoceptor affinity, thereby conferring good selectivity (>500). Several members of the library (8, 14, 17, and 18) have excellent PNMT inhibitory potency and selectivity and are predicted, on the basis of their ClogP value (>0.5), to penetrate the BBB to a significant extent. Compounds 17 and 18 are the most potent and selective PNMT inhibitors reported to date.