Exploring the Importance of Piperazine N-Atoms for σ2Receptor Affinity and Activity in a Series of Analogs of 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28)

sigma(2)-Agonist 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (7, PB 28), which proved to revert doxorubicin resistance in breast cancer cells, was taken as a template to prepare new analogs. One of the two basic N-atoms was alternatively replaced by a methine or converted into an amide or ammonium function, with the aim of finding out which of them was essential for sigma(2) receptor affinity and activity. Some simply 4-substituted 1-cyclohexylpiperazines were also investigated. None of the compounds was as high-affinity as 7 (sigma(2)K(i) = 0.68, sigma(1)K(i) = 0.38 nM), proving that both basic N-atoms ensure better sigma(2) receptor binding. Amide 36 emerged as high-affinity (K(i) = 0.11 nM) and noteworthy() selective (1627-fold) sigma(1) ligand. Small N-cyclohexylpiperazine 59 displayed the highest sigma(2) affinity (K(i) = 4.70 nM). The sigma(2)/sigma(1) selectivities were generally low. Antiproliferative assay in SK-N-SH cells revealed piperidines 24 and 15 as putative sigma(2) agonists (EC(50)s 1.40 and 3.64 muM respectively) more potent than 7.