Affinities of methylphenidate derivatives for dopamine,norepinephrine and serotonin transporters

We have synthesized several derivatives of dl-threo-methylphenidate (Ritalin) bearing substituents on the phenyl ring. IC₅₀ values for binding of these compounds to rat brain monoamine transporters were assessed using [³H]WIN 35,428 (striatal membranes, dopamine transporters, DAT), [³H]nisoxetine (frontal cortex membranes, norepinephrine transporters, NET) and [³H]paroxetine (brain stem membranes, 5HT transporters, 5HTT). Affinities (1/Ki) decreased in the order: DAT > NET ⪢ 5HTT. Substitution at the para position of dl-threo-methylphenidate generally led to retained or increased affinity for the dopamine transporter (bromo > iodo > methoxy > hydroxy). Substitution at the meta position also increased affinity for the DAT (m-bromo > methylphenidate; m-iodo-p-hydroxy > p-hydroxy). Substitution at the ortho position with bromine considerably decreased affinity. Similar IC₅₀ values for binding of o-bromomethylphenidate to the dopamine transporter were measured at 0, 22 and 37 degrees. N-Methylation of the piperidine ring of methylphenidate also considerably reduced affinity. The dl-erythro isomer of obromomethylphenidate did not bind to the DAT (IC₅₀ > 50,000 nM). Affinities at the dopamine and norepinephrine transporters for substituted methylphenidate derivatives were well correlated (r² = 0.90). Abilities of several methylphenidate derivatives to inhibit [³H]dopamine uptake in striatal synaptosomes corresponded well with inhibition of [³H]WIN 35, 428 binding. None of the compounds examined exhibited significant affinity to dopamine D1 or D2 receptors (IC₅₀ > 500 or 5,000 nM, respectively), as assessed by inhibition of binding of [³H]SCH 23390 or [¹²³I]epidepride, respectively, to striatal membranes.