Arylamides hybrids of two high-affinity σ2 receptor ligands as tools for the development of PET radiotracers

1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine 1 (PB28) and 2-Methoxy-5-methyl-N-[4-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)butyl]benzamide 2 (RHM-1) represent leads for tumor diagnosis, given their high affinity at σ(2) receptors. With the purpose of obtaining good candidates for σ(2) PET tracers development, hybrid structures between 1 and 2 were designed. Excellent σ(1)/σ(2) selectivities were reached when 6,7-dimethoxytetrahydroisoquinoline was linked to an o-methoxy substituted arylamide (11a, 12a, 15a), and for these benzamides an intramolecular H-bond in the active conformation at the σ sites, was hypothesized. However these excellent σ(2) ligands were accompanied by interaction with P-gp, which may limit their use as σ(2) receptor PET agents when tumors overexpress P-gp. Compound 15a whose P-gp interaction was just moderate represents an interesting tool for the development of σ(2) PET tracers useful in tumors overexpressing P-gp.