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Sigma Receptor Ligands: Possible Applications as Anti-Tumor Agents and as Radiopharmaceuticals
Keywords: cell death cell cycle multidrug resistance
Researchers: dr. A. Rybczynska A. van Waarde Prof. dr. P.H. Elsinga dr. W. Helfrich prof. dr. R.A. Dierckx
Type of project: Stage Wetenschap / Researchproject
Nature of the research: Sigma ligands are cytotoxic for tumor cells but much less toxic for healthy tissue. They can potentiate the effects of antitumor therapy. Underlying mechanisms will be studied
Fields of study: oncology nuclear medicine

Background / introduction

Sigma receptors are unique proteins integrated in plasma and endoplasmatic reticulum membranes of many tissues (1). The two confirmed subtypes are strongly overexpressed in rapidly proliferating cells (2-4). Many sigma ligands are effective antineoplastic agents. For example, moderate doses of haloperidol and rimcazole, antipsychotic drugs with high affinities to sigma receptors, inhibit growth both of cultured cancer cells and in vivo tumors, whereas they do not affect proliferation or survival of the noncancer tissue (5-10). Thus, sigma ligands have the potential to kill cancer cells with minimal side effects. Sigma ligands appear to inhibit tumor growth by (i) suspending progression of tumor cells through the cell cycle, and by (ii) simultaneously activating pro-apoptotic pathways. Sigma ligands can act as chemosensitizers and can potentiate the effect of other forms of anti-tumor therapy, e.g. vincristin and scFv:sTRAIL fusion proteins. Interestingly, sigma ligands act both on caspase-dependent and caspase-deficient cancer types and can inhibit the P-glycoprotein pump implicated in multidrug resistance (7,9). Thus, sigma ligands may overcome various forms of drug resistance and may potentiate the actions of other chemotherapeutic agents.

Research question / problem definition

This project aims at further characterization of sigma receptor function and of the cytotoxic effects of sigma ligands in rapidly proliferating cells. Sigma receptor density in cell lines can be measured using the radioligand 11C-SA4503. P-glycoprotein function can be assessed with the PET tracer 11C-verapamil. Drug effects on cellular metabolism may be studied with 18F-FDG (glycolysis), 11C-methionine (protein synthesis), 18F-FLT (nucleotide synthesis) and 11C-choline (phospholipid synthesis), respectively. Cell death can be quantified by cell counting and by standard assays for necrosis and apoptosis.

Workplan

Subprojects:
1. Examination of changes of sigma receptor density during the cell cycle. Tumor cells can be arrested in various phases of the cycle and the effect of cell cycle arrest on the binding of 11C-SA4503 can be examined. Since sigma receptor density has been shown to be related to cellular proliferation, increases of receptor density can be expected during the S-, G2- and M-phases of the cell cycle.
2. Characterization of the cytotoxic effects of registered drugs (e.g., the SSRI fluvoxamine, the antipsychotics haloperidol and rimcazole) or experimental compounds (PB28) in tumor cell lines: dose-dependent inhibition of growth, cell death via apoptotic and necrotic pathways, alterations of cellular metabolism.
3. Examination of the chemosensitizing effects of sigma ligands: comparison of no treatment, treatment with sigma ligand only, treatment with other cytostatic agent only, treatment with a combination of sigma ligand and another cytostatic agent. Here also, inhibition of growth, cell death via various mechanisms, cell cycle arrest and uptake of metabolic tracers can be studied.
4. Examination of P-glycoprotein modulation by sigma ligands (reversal of multidrug resistance). P-glycoprotein-positive and -negative variants of a single cell line can be used for this purpose, and function of the pump can be assessed by measuring cellular uptake of 11C-verapamil.
5. In a later phase of the project, subproject 3 and 4 may be carried out in tumor-bearing rodents rather than in cultured cells. Imaging techniques like micro-PET and micro-SPECT can be used to noninvasively assess tumor biochemistry in vivo.

References

1. Su TP. Sigma receptors: putative links between nervous, endocrine and immune systems. Eur J Biochem 1991; 200:633-642.
2. Aydar E et al. The expression and functional characterization of sigma-1 receptors in breast cancer cell lines. Cancer Lett 2006; 242:245-257.
3. Vilner BJ et al. Sigma-1 and sigma-2 receptors are expressed in a wide variety of human and rodent tumor cell lines. Cancer Res 1995; 55:408-411.
4. Thomas GE et al. Sigma and opioid receptors in human brain tumors. Life Sci 1990; 46:1279-1286.
5. Vilner BJ et al. Cytotoxic effects of sigma ligands: sigma-receptor-mediated alterations in cellular morphology and viability. J Neurosci 1995; 15:117-134.
6. Brent PJ et al. The sigma receptor ligand, reduced haloperidol, induces apoptosis and increases intracellular free calcium levels in colon and mammary adenocarcinoma cells. Biochem Biophys Res Commun 1996; 219:219-226.
7. Colabufo NA et al. Antiproliferative and cytotoxic effects of some sigma-2 agonists and sigma-1 antagonists in tumour cell lines. Naunyn-Schmiedebergs Arch Pharmacol 2004; 370:106-113.
8. Nordenberg J et al. Anti-proliferative activity of haloperidol in B16 mouse and human SK-MEL-28 melanoma cell lines. Int J Oncol 2005; 27:1097-1103.
9. Spruce BA et al. Small molecule antagonists of the sigma-1 receptor cause selective release of the death program in tumor and self-reliant cells and inhibit tumor growth in vitro and in vivo. Cancer Res 2004; 64:4875-4886.
10. Rybczynska AA et al. Cytotoxicity of sigma-receptor ligands is associated with major changes of cellular metabolism and complete occupancy of the sigma-2 subpopulation. J Nucl Med 2008 (in press).

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