Synthesis, radioiodination and in vitro and in vivo sigma receptor studies of N-1-allyl-N´-4-phenethylpiperazine analogs

Abstract Introduction Sigma-1 (σ 1 ) receptor radioligands are useful for basic pharmacology studies and for imaging studies in neurology, psychiatry and oncology. We derived a hybrid structure, N- 1-allyl- N ´-4-phenethylpiperazine, from known ligands TPCNE and SA4503 for use as a scaffold for development of radioiodinated σ 1 receptor ligands. Methods E -and Z - N -1-(3′-iodoallyl)- N ´-4-(3″,4″-dimethoxyphenethyl)-piperazine ( E -1 and Z -1 ), N -1-allyl- N ´-4-(3′,4′-dimethoxyphenethyl)-piperazine ( 2 ) and E - N -1-(3′-iodoallyl)- N´ -4-(3″-methoxy-4′´-hydroxyphenethyl)-piperazine ( 3 ) were synthesized. Affinities for σ 1 and σ 2 receptors were determined. [ 125 I] E -1 and [ 125 I] Z -1 were prepared and evaluated in vivo in mice. [ 125 I] E -1 was further evaluated in σ 1 receptor binding assays in vitro. Results E -1 displayed moderately high apparent affinity (15 nM) for σ 1 sites and 84-fold selectivity against σ 2 sites. Z -1 showed similar σ 1 affinity, but only 23-fold selectivity. In contrast, 2 exhibited poor binding to both subtypes, while 3 had good affinities but poor selectivity. E- 1 profiled as a probable antagonist in the phenytoin shift assay. [ 125 I] E -1 and [ 125 I] Z -1 were prepared in good yields and with high specific radioactivities. Log D 7.4 values (2.25 and 2.27) fall within the optimal range for in vivo studies. Both radioligands selectively labeled σ 1 receptors in mouse brain and peripheral organs in vivo. [ 125 I] E -1 showed a higher level of specific binding than [ 125 I] Z -1 and displayed good metabolic stability. Further, [ 125 I] E -1 selectively labeled σ 1 receptors in mouse brain homogenates ( K d 3.79 nM; B max =599 fmol/mg protein). Conclusions [ 125 I] E -1 is a selective σ 1 receptor radioligand that exhibits properties amenable to in vitro and in vivo studies, with possible extension to single photon emission computed tomography using iodine-123.