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S636 Biochemical Society Transactions (1997) 25 100 Marine organisms in somatostatin receptor scintillation proximity assays KIRSTEN MILLS*, PAUL STEAD’ and MARTYN BANKS* *Lead Discovery Unit “Compound Diversity Unit Glaxo Wellcome R&D, Gunnel’s Wood Road, Stevenage, SG12NY, England. Somatostatin is a small endogenous peptide first discovered for its ability to inhibit growth hormone secretion (1). It has been described as the “universal endocrine off switch” (2), and is now known to be widely distributed in the body, and involved in many disease states, such as cancer and diabetes mellitis. The actions of somatostatin are complex, involving two types of peptide ligand which bind to 5 receptor types, coupled through various G proteins to several signal transduction pathways. Cellular effects of somatostatin are mediated by 7transmembrane receptors. The majority of these metabotropic receptors are negatively coupled to adenylyl cyclase vial Gi or Go; somatostatin binds to reduce CAMPproduction and thus decrease cellular activation. Five receptor subtypes have been described, known as SST,., (3). SST,, and are classified as SRIF-1 receptors, and SST, and are classified as SRIF-2 receptors. Within each class, there is 70 - 80% sequence homology and a similar pharmacological profile (3), although SRIF-I receptors bind the truncated somatostatin analogue, octreotide, with high affinity, whereas SRIF-2 receptors do not (4). At least one of the receptors exists in 2 forms: SST,, and SSTzB are splice variants of the human SST, receptor which differ only in the C-terminal amino acid sequence (5). Overall, SSTs share 4550% homology, only slightly higher than their 40% homology with the opioid receptor family (6). The families show similar pharmacology, with many compounds shown to display affinity for both receptor types (7). The effects of somatostatin are decreased in many disease states including cancer, CNS disorders and the increased perception of pain, although conditions, such as diabetes rnellitus, show an increased concentration of somatostatin. Characterisation of the pathophysiological role of somatostatin in these diseases makes it a valuable pharmacologicaltarget. Most of the peptides known to act as agonists at SSTs in vivo also cause down-regulation of the receptors following repeated administration, an effect commonly seen with G protein coupled receptors (8). Automated laboratory analysis systems now routinely perform receptor-ligand binding assays such as scintillation proximity assays for high throughput screening, where large numbers of samples such as compounds and combinatorial libraries can be screened within a short time period. This mechanistic, in vitro approach also allows the screening of large numbers of crude natural product extracts, regardless of cytotoxicity. Nonkelective extracts can be quickly discarded, allowing functional cell assays to be carried out only on those extracts which have selective high affinity for the receptor of interest. Natural products are the source, or inspiration, for the majority of the world’s medicines, and marine species are responsible for approximately half of total global biodiversity (9). This vast resource is being exploited by pharmaceutical companies in the search for novel therapies, yet to date, no marine-derived drugs are on the market. Some marine invertebrates have been reported to show somatostatin-like activity, and there are published reports describing active somatostatin receptor antagonists which have ., been isolated from sponges (1 0). This literature precedence for marine-derived active compounds led the Lead Discovery Unit at Glaxo Wellcome to test a variety of crude marine extracts from Harbor Branch Oceanographic Institute in the SST, and assays. The majority of these samples were sponges from the phylum Porifera and coelenterates from the phylum Cnidaria. Samples were tested initially at single concentration, and active samples were retested in duplicate at a single concentration. Any confirmed positives were tested in a dose response, and progressed to functional whole cell assays. For crude marine extracts, samples were submitted for extraction after active ICso values were obtained in the receptor binding assay. Fractionated samples were tested in the SPA. This identified a SST, selective genus of soft corals which had no previously reported biological activity against SSTs. , References 1. Krulich, L, Dhariwal, A P S and McCann, SM. 1968. Endocrinology 83,783-790. 2. Evers, BM, Parekh, D, Townsend, CM and Thompson, JC. 1991. Ann Surg 213, 190-198. 3. Hoyer, D, Bell, GI, Berelowitz, M, Epelbaum, J, Feniuk, W, Humphrey, PPA, O’Carroll, A-M, Patel, YC, S c h o n b m , A, Taylor, JE and Reisine, T. 1995. Trends‘Pharmacol Sci 16, 8688. 4. Reubi, JC and Laissue, JA. 1995. Trends Pharmacol Sci 16, 110-115. 5. Patel, YC, Greenwood, MT, Panetta, R, Demchyshyn, L, Nimik, H and Srikant, CB. 1995 Life Sci 57 (13), 1249-1265 6. Reisine, T. 1995. Am J Physiol 269 (l), G813-G820. 7. Mulder, AH, Wardeh, G, Hogenboom, F, Kazrnierski, W, Hruby, VJ and Schoffelmeer, ANM. 1991. European Journal of Pharmacology 205, 1-6 8. Raynor, K and Reisine, T. 1992. Crit Rev Neurobiol 16, 273289. 9. Vries, DJ and Beart, PM. 1995 Trends Pharmacol Sci 16 275279. 10. Vassas, A, Bourdy, G, Paillard, JJ, Lavayre, .J, Pais, M, Quirion, JC and Debitus, C. 1995. Planta Medica 62,28-30