Activation of sigma1 (σ1) receptors contributes to the behavioral and toxic effects of (−)-cocaine. We studied a key step, the ability of (−)-cocaine to occupy σ1 receptors in vivo, using CD-1® mice and the novel radioligand [125I]E-N-1-(3′-iodoallyl)-N′-4-(3″,4″-dimethoxyphenethyl)-piperazine ([125I]E-IA-DM-PE-PIPZE). (−)-Cocaine displayed an ED50 of 68 μmol/kg for inhibition of specific radioligand binding in whole brain, with values between 73 – 80 μmol/kg for heart, lung and spleen. For comparison, an ED50 of 26 μmol/kg for (−)-cocaine occupancy of striatal dopamine transporters (DAT) was determined by inhibition of [125I]3β-(4-iodophenyl)tropan-2β-carboxylic acid isopropyl ester ([125I]RTI-121) binding. A chief finding is the relatively small potency difference between (−)-cocaine occupancy of σ1 receptors and the DAT, although the DAT occupancy is likely underestimated. Interactions of (−)-cocaine with σ1 receptors were assessed further using [125I]E-IA-DM-PE-PIPZE for regional cerebral biodistribution studies and quantitative ex vivo autoradiography of brain sections. (−)-Cocaine binding to cerebral σ1 receptors proved directly proportional to the relative site densities known for the brain regions. Non-radioactive E-IA-DM-PE-PIPZE gave an ED50 of 0.23 μmol/kg for occupancy of cerebral σ1 receptors, and a 3.16 μmol/kg (i.p.) dose attenuated (−)-cocaine induced locomotor hyperactivity by 30%. This effect did not reach statistical significance, but suggests that E-IA-DM-PE-PIPZE is a probable σ1 receptor antagonist. As groundwork for the in vivo studies, we used standard techniques in vitro to determine ligand affinities, site densities and pharmacological profiles for the σ1 and σ2 receptors expressed in CD-1® mouse brain.

Two series of novel ether analogs of the sigma (σ) receptor ligand 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine (SA4503) have been prepared. In one series, the alkyl portion of the 4-methoxy group was replaced with allyl, propyl, bromoethyl, benzyl, phenethyl, and phenylpropyl moieties. In the second series, the 3,4-dimethoxy was replaced with cyclic methylenedioxy, ethylenedioxy and propylenedioxy groups. These ligands, along with 4-O-des-methyl SA4503, were evaluated for σ1 and σ2 receptor affinity, and compared to SA4503 and several known ether analogs. SA4503 and a subset of ether analogs were also evaluated for dopamine transporter (DAT) and serotonin transporter (SERT) affinity. The highest σ1 receptor affinities, Ki values of 1.75 nM – 4.63 nM, were observed for 4-O-des-methyl SA4503, SA4503 and the methylenedioxy analog. As steric bulk increased, σ1 receptor affinity decreased, but only to a point. Allyl, propyl and bromoethyl substitutions gave σ1 receptor Ki values in the 20 nM – 30 nM range, while bulkier analogs having phenylalkyl, and Z- and E-iodoallyl, ether substitutions showed higher σ1 affinities, with Ki values in the 13 nM – 21 nM range. Most ligands studied exhibited comparable σ1 and σ2 affinities, resulting in little to no subtype selectivity. SA4503, the fluoroethyl analog and the methylenedioxy congener showed modest six- to fourteen-fold selectivity for σ1 sites. DAT and SERT interactions proved much more sensitive than σ receptor interactions to these structural modifications. For example, the benzyl congener (σ1 Ki = 20.8 nM; σ2 Ki = 16.4 nM) showed over 100-fold higher DAT affinity (Ki = 121 nM) and 6-fold higher SERT affinity (Ki = 128 nM) than the parent SA4503 (DAT Ki = 12650 nM; SERT Ki = 760 nM). Thus, ether modifications to the SA4503 scaffold can provide polyfunctional ligands having a broader spectrum of possible pharmacological actions.