Somatostatin analogs for cancer treatment and diagnosis: an overview.

Due to the limited efficacy and considerable toxicity of conventional chemotherapy, novel cytotoxic agents and innovative noncytotoxic approaches to cancer treatment are being developed. Amongst the various hormonal agents, increasing attention is being directed to somatostatin analogs. This is largely due to the demonstration of antineoplastic activity of these compounds in a variety of experimental models in vitro and in vivo and to the elucidation of some aspects of the molecular mechanisms underlying their antineoplastic activity. On the other hand, clinical experience with somatostatin analogs in the treatment of conditions like acromegaly and GEP tumors has shown that they are well tolerated compared to other antineoplastic therapies currently in use. As a consequence, there is much ongoing clinical research to determine whether or not results from experimental studies will translate into clinically useful antineoplastic activity. Besides being used in cancer treatment and palliation, radiolabelled somatostatin analogs are employed for the localization of primary and metastatic tumors expressing somatostatin receptors. The so-called 'somatostatin receptor scintigraphy' is indeed the most important clinical diagnostic investigation for patients with suspected neuroendocrine tumors. Targeted radiotherapy, which is being evaluated in clinical trials, represents an obvious extension of somatostatin scintigraphy. Since the short half-life of native somatostatin makes continuous intravenous infusion mandatory, several long-acting analogs have been synthesized. Amongst the hundreds of peptides synthesized, octreotide (which binds mainly to SSTR-2 and SSTR-5 receptor subtypes) has been the most extensively investigated. A thorough analysis of the pharmacological activities and therapeutic efficacy of the native somatostatin and the synthetic analogs (octreotide, lanreotide and vapreotide) reveals that the biological actions of these peptides are not always identical. These differences appear to be related to the different affinities of the natural hormone and synthetic derivatives for the different receptor subtypes. For all the three peptides long-lasting formulations have been developed to provide patients with the convenience of once or twice a month administration and to ensure stable drug serum concentrations between injections. Radiolabelled derivatives of octreotide, lanreotide and vapreotide have been synthesized and used as radiopharmaceuticals for somatostatin receptor scintigraphy and somatostatin receptor-targeted radiotherapy. The safety profile of synthetic somatostatin analogs is well established. Most adverse reactions to these peptides are merely a consequence of their pharmacological activity and consist mainly of gastrointestinal complaints, cholelithiasis and effects on glucose metabolism. They are often of little clinical relevance, thus making somatostatin analogs safe drugs for long-term use. While immediate release preparations are the drugs of choice in the short term, long-acting formulations are better indicated, on an outpatient basis, for the long-term management of chronic conditions. New 'receptor-selective' and 'universal' somatostatin analogs are being developed and combinations of currently available derivatives with other (cytotoxic and/or hormonal) agents are being explored in the search for an efficacious and well-tolerated treatment of the various malignancies. Somatostatin receptor-targeted chemotherapy (with conjugates of somatostatin peptides with cytotoxic drugs) and gene therapy (e.g. transferring the SSTR-2 gene into neoplastic cells), which have been successfully tested in experimental studies, should be applied to human beings in a not too distant future.