Sigma receptors (subtypes sigma-1 and sigma-2) are a unique class of binding sites expressed throughout the mammalian body. The endogenous ligand for these sites has not been identified, but steroid hormones (particularly progesterone), sphingolipid-derived amines and N,N-dimethyltryptamine can bind with fairly high affinity. Sigma receptors are overexpressed in rapidly proliferating cells, like cancer cells. Particularly the sigma-2 subtype is upregulate when cells divide and down regulated when they become quiescent. Sigma ligands, especially sigma-2 agonists, can inhibit proliferation and induce apoptosis by a mechanism involving changes in cytosolic Ca(2+), ceramide and sphingolipid levels. Tumor cells are much more sensitive to such treatment than cells from their tissue of origin. Sigma ligands induce apoptosis not only in drug-sensitive but also in drug-resistant cancer cells (e.g., cells with p53 mutations, or caspase dysfunction). Moreover, sigma ligands may abrogate P-glycoprotein-mediated drug resistance and at subtoxic doses, they can potentiate the effect of conventional cytostatics. Thus, sigma-2 agonists may be developed as antineoplastic agents for the treatment of drug-resistant tumors. A large number of radiolabeled sigma ligands has been prepared for SPECT (single-photon emission computed tomography) and PET (positron emission tomography) imaging. Such radiopharmaceuticals can be used for tumor detection, tumor staging, and evaluation of anti-tumor therapy. There is still a need for the development of ligands with (1) high selectivity for the sigma-2 subtype, (2) defined action (agonist or antagonist) and (3) optimal pharmacokinetics (low affinity for P-glycoprotein, high and specific tumor uptake, and rapid washout from non-target tissues).

Sigma receptors (sigma-1 and sigma-2) represent two independent classes of proteins. Their endogenous ligands may include the hallucinogen N,N-dimethyltryptamine (DMT) and sphingolipid-derived amines which interact with sigma-1 receptors, besides steroid hormones (e.g., progesterone) which bind to both sigma receptor subpopulations. The sigma-1 receptor is a ligand-regulated molecular chaperone with various ion channels and G-protein-coupled membrane receptors as clients. The sigma-2 receptor was identified as the progesterone receptor membrane component 1 (PGRMC1). Although sigma receptors are over-expressed in tumors and up-regulated in rapidly dividing normal tissue, their ligands induce significant cell death only in tumor tissue. Sigma ligands may therefore be used to selectively eradicate tumors. Multiple mechanisms appear to underlie cell killing after administration of sigma ligands, and the signaling pathways are dependent both on the type of ligand and the type of tumor cell. Recent evidence suggests that the sigma-2 receptor is a potential tumor and serum biomarker for human lung cancer and an important target for inhibiting tumor invasion and cancer progression. Current radiochemical efforts are focused on the development of subtype-selective radioligands for positron emission tomography (PET) imaging. Right now, the mostpromising tracers are [18F]fluspidine and [18F]FTC-146 for sigma-1 receptors and [11C]RHM-1 and [18F]ISO-1 for the sigma-2 subtype. Nanoparticles coupled to sigma ligands have shown considerable potential for targeted delivery of antitumor drugs in animal models of cancer, but clinical studies exploring this strategy in cancer patients have not yet been reported. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

(11)C-7 is a novel tracer of P-gp function with higher baseline uptake than (11)C-verapamil. Upregulation of P-gp function in response to treatment (which is hard to detect with (11)C-verapamil) may be detectable using (11)C-7 and PET. Because (11)C-6 shows specific binding in target organs, this compound is the first PET tracer allowing measurement of P-gp expression.