Doctor of Philosophy
School of Chemistry
Ashford, Mark, The development and SAR of selective sigma-2 receptor ligands for the diagnosis of cancer, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3634
Three series of compounds formed the basis for sigma-1 and sigma-2 receptor structure activity studies. In total, thirty four compounds based on the lead benzamide compound  were synthesised, incorporating a variety of isoquinoline and carboxylic acid moieties, linked together with either a linear or cyclic amine spacer. Furthermore, the incorporation of a halogen on selected carboxylic acid moieties provided a convenient strategy for the introduction of a radiohalogen for applications in pharmacological and imaging studies.
Thirteen compounds with modifications to the carboxylic acid region (region 3), bearing substituted benzoic and benzofuran carboxylic acids were synthesised in five steps. Each of these carboxylic acid fragments were linked to a 6,7-dimethoxytetrahydroisoquinoline ring through a rigid piperidine spacer. In vitro evaluation of these compounds to determine their sigma-1 and sigma-2 receptor affinities gave IC50 values for the sigma-2 receptor ranging from 9.8 – 955 nM. In vitro structure activity relationship (SAR) studies indicated that halogen substitution yielded compounds with greater affinity than their unsubstituted analogues and that most brominated compounds displayed higher binding affinities than their iodinated analogues. The optimum benzamide compounds contained a 5-bromo or 5- iodobenzofuran-2-carboxylic acid fragment. The highest affinity iodinated ligand,  with a sigma-2 affinity of 12.2 nM, was radiolabelled with 123I in 51-82% radiochemical yield and was consequently evaluated in vivo in Sprague-Dawley rats. Biodistribution studies indicated uptake in organs known to contain sigma-2 receptors including the pancreas and lungs.
Fourteen compounds with spacer modifications (region 2) were synthesised. Nine of these compounds contained a flexible carbon spacer, and five contained a semi-rigid (cyclic) spacer. The highest affinity ligand  bearing an iodobenzofuran fragment and displaying an IC50 of 1.0 nM linked to the isoquinoline via a six carbon flexible spacer was radiolabelled with 123I in 78% radiochemical yield and also evaluated in cell studies and in vivo in rats. This compound showed high uptake in breast tumour cells known to express sigma-2 receptors. Although this compound displayed high uptake in organs known to contain sigma-2 receptors including the pancreas and lungs, initial drug competition studies with the non-specific sigma-2 ligand “haloperidol” failed to displace the radioactivity corresponding to this compound. In the absence of a complete receptor screen, it suggests that this compound may have a higher affinity for other receptor types in vivo and that the uptake of radioactivity may not be due to its affinity to the sigma-2 receptor in vivo. However, metabolite studies showed that the radioactivity extracted from the cortex, lungs and spleen after 60 min was > 95% intact, whilst indicating only between 55-65% unchanged tracer in the plasma at this time point. Three novel nitrosulfonamides were also synthesised in this series, however all exhibited only moderate to weak sigma-2 receptor IC50 values of 31-800 nM.
Seven compounds containing modified isoquinolines (region 1) were synthesised and evaluated. Compound  bearing an iodobenzofuran fragment linked to the 6- methoxytetrahydroisoquinoline via a six carbon flexible spacer displayed an IC50 value of 5.0 nM for the sigma-2 receptor; however, it also displayed comparable high affinity for the sigma-1 receptor (IC50 14.0 nM). All other compounds displayed moderate sigma-2 IC50 values ranging from 35-150 nM.