Among the series of PB28 derivatives, PB221 (4-cyclohexyl-1-[3- (5-methoxy-1, 2, 3, 4-tetrahydronaphthalen-1-yl) propyl]peperidine) stood out for its sigma-2 vs. sigma-1 selectivity (sigma-1 Ki = 143 nM; sigma-2 Ki = 18.8 nM) and, more importantly, for its potent cytotoxic activity against neuroblastoma SK-N-SH cells (IC50 = 3.64 μM) [19]. Here, AP2S1 is linked to neuroblastoma.