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

Introduction: Sigma-1 (σ₁) 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-ND́-4-phenethylpiperazine, from known ligands TPCNE and SA4503 for use as a scaffold for development of radioiodinated σ₁ receptor ligands. Methods: E-and Z-N-1-(3'-iodoallyl)-ND́-4-(3",4"-dimethoxyphenethyl)-piperazine (E-1 and Z-1), N-1-allyl-ND́-4-(3',4'-dimethoxyphenethyl)-piperazine (2) and E-N-1-(3'-iodoallyl)-ND́-4-(3"-methoxy-4'D́-hydroxyphenethyl)-piperazine (3) were synthesized. Affinities for σ₁ and σ₂ receptors were determined. [¹²⁵I]E-1 and [¹²⁵I]Z-1 were prepared and evaluated in vivo in mice. [¹²⁵I]E-1 was further evaluated in σ₁ receptor binding assays in vitro. Results: E-1 displayed moderately high apparent affinity (15 nM) for σ₁ sites and 84-fold selectivity against σ₂ sites. Z-1 showed similar σ₁ 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. [¹²⁵I]E-1 and [¹²⁵I]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 σ₁ receptors in mouse brain and peripheral organs in vivo. [¹²⁵I]E-1 showed a higher level of specific binding than [¹²⁵I]Z-1 and displayed good metabolic stability. Further, [¹²⁵I]E-1 selectively labeled σ₁ receptors in mouse brain homogenates (K_d 3.79 nM; B_max=599 fmol/mg protein). Conclusions: [¹²⁵I]E-1 is a selective σ₁ 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.