Influence of Trishomocubanes on Sigma Receptor Binding of N-(1-Benzylpiperidin- 4-yl)-4-[123I]iodobenzamide In Vivo in the Rat Brain

Three new trishomocubane analogues based on the 4-azahexacyclo[5.4.1.02,6.03,10.05,9.08,11] dodecane-3-ol skeleton have been synthesised and assessed for their affinities at both sigma-1 and sigma-2 receptors. The effect of various N-substitution on the polycyclic moiety was examined. All synthesised compounds displayed high affinity for sigma-1 receptors (9-10 nM) and good affinity for sigma-2 receptors (230-310 nM), suggesting that substitution at the nitrogen moiety of the trishomocubane is well tolerated and represents a platform for the development of improved higher affinity sigma receptor ligands. The interaction of these functionalised trishomocubanes on the binding of the known sigma receptor radioligand, 4-[123I]IBP, was evaluated in the rat brain. Although 4-[123I]IBP had been used for imaging sigma receptors in tumours, this is the first examination of sigma receptor binding in the rat brain and therefore the potential of 4-[123I]IBP for imaging the brain was also evaluated. In vivo specificity and selectivity of 4-[123I]IBP binding was examined by studying the effects of preadministration of sigma receptor binding drugs (+)-pentazocine and unlabelled 4-IBP. This resulted in a blockade of only 42% of 4-[123I]IBP uptake in the brain indicating high degree of non-specific binding suggesting that it may not be suitable for imaging sigma receptors in the brain. The inhibition of 4-[123I]IBP uptake using the two of the three trishomocubanes displayed a consistent blockade of 48-30% in all brain structures. This demonstrates for the first time the ability of functionalised trishomocubanes to interact with sigma receptors in vivo.