Synthesis and pharmacology of potential cocaine antagonists. 2. Structure-activity relationship studies of aromatic ring-substituted methylphenidate analogs.
As part of a program to develop medications which can block the binding of cocaine to the dopamine transporter, yet spare dopamine uptake, a series of aromatic ring-substituted methylphenidate derivatives was synthesized and tested for inhibitory potency in [3H]WIN 35,428 binding and [3H]dopamine uptake assays using rat striatal tissue. Synthesis was accomplished by alkylation of 2-bromopyridine with anions derived from various substituted phenylacetonitriles. In most cases, erythro compounds were markedly less potent than the corresponding (+/-)-threo-methylphenidate (TMP; Ritalin) derivatives. The ortho-substituted compounds were much less potent than the corresponding meta- and/or para-substituted derivatives. The most potent compound against [3H]WIN 35,428 binding, m-bromo-TMP, was 20-fold more potent than the parent compound, whereas the most potent compound against [3H]dopamine uptake, m,p-dichloro-TMP, was 32-fold more potent. Threo derivatives with m- or p-halo substituents were more potent than TMP, while electron-donating substituents caused little change or small loss of potency. All of the derivatives had Hill coefficients approaching unity, except m,p-dichloro-TMP, which had an nH of 2.0. Although the potency of the (+/-)-methylphenidate derivatives in the two assays was highly correlated (R2 = 0.986), the compounds m-chloro-,m-methyl-, and p-iodo-TMP were 4-5-fold more potent at inhibiting [3H]-WIN 35,428 binding than [3H]dopamine uptake (cocaine has a ratio of 2.3). These and other compounds may be promising candidates for further testing as potential partial agonists or antagonists of cocaine.