The history of somatostatin analogs.

In the middle of the last century, there was a spectacular progress in the discovery, characterization and synthesis of neuropeptides. This was only possible because increasingly sophisticated analytical and isolation technology was becoming available. The pituitary lobes have become a real treasure house for the detection of different peptides, but also other glands and organs in the gastrointestinal (GI) and central nervous system (CNS) tracts have contained an ever growing list of regulatory peptides with sometimes unknown functionality. The main burning issues were to elucidate their role in physiology and, case by case and based on their structure, whether it was possible to design useful drugs for human therapy. Both issues were and are still being dealt with, and the history of somatostatin and somatostatin analogs is a good example of how such issues are being tackled successfully. In 1973, Brazeau and Guillemin's search at the Salk Institute for a GHRH in extracts of thousands of sheep hypothalami was crowned by a surprise, the discovery of a GHRH antagonist, a 14-amino acid Cystin bridge-containing peptide which they called somatostatin. This neuropetide appeared to be widely distributed in animal and human organs in the periphery and CNS, suggesting its potential regulatory functions, yet a thorough characterization of its properties due to its extremely short half-life was not possible. More insight could only be feasible with the synthesis of stable and potent analogs, a program that soon started in different research centers around the world. After having elucidated the 3-dimensional structure, the enzymatic degradation pattern and minimal chain length for biological activity of the natural hormone, the synthesis of a large number of analogs was started as early as 1974. The approach of the Sandoz team was to start with a hexapeptide lead structure Cys-Phe-DTrp-Lys-Thr-Cys and, by systematic elongation of the N and C terminals, in 1980 they managed to characterize the most stable and active analog with the following structure: H-DPhe-Cys-Phe-DTrp-Lys-Thr-Cys-Thr-OI-Octreotide. It was more potent in inhibiting GH in vivo compared to the native hormone. It demonstrated sufficient stability in vivo and, therefore, it was selected for clinical studies. In 1988, the first registration was obtained for treating acromegaly and carcinoid tumors. Since then, different depot preparations have been made available. Other analogs with similar structures have been also synthesized and are commercially available. The so-called targeting approach takes advantage of the presence of somatostatin receptors on different tumors. By coupling octreotide structural elements to so-called cage molecules complexing B or Y emitting isotopes, also the detection of somatostatin receptor containing tumors could be visualized and treated. The use of different somatostatin derivatives found its way since then both in basic research and in human therapy, and it is still opening new and exciting prospects.