Targeting Cytotoxic Conjugates of Somatostatin, Luteinizing Hormone- Releasing Hormone and Bombesin to Cancers Expressing Their Receptors: A “Smarter” Chemotherapy

Chemotherapy is one of the main modalities in the therapy of cancer. However, an improvement in the efficacy and a reduction in the toxicity of chemotherapeutic agents remains a great challenge to oncologists. A specific delivery of cytotoxic drugs to cancerous cells may help improving both aspects. Peptide hormones, for which receptors have been found in various human cancers, can serve as carriers for a local delivery of cytotoxic agents or radiopharmaceuticals to the tumors, as demonstrated by the successful clinical use of radiolabeled somatostatin analog Octreoscan for the detection and treatment of some somatostatin receptor-positive tumors. Thus, in recent years we developed a series of cytotoxic peptide hormone conjugates based on derivatives of hypothalamic hormones such as somatostatin and luteinizing hormone-releasing hormone (LHRH), and the brain-gut hormone bombesin. To create targeted conjugates with high cytotoxic activity, a derivative of doxorubicin (DOX), 2-pyrrolino-DOX (AN-201), which is 500-1, 000 times more active than its parent compound, was developed. This agent was coupled to somatostatin octapeptide RC-121 to form cytotoxic conjugate AN-238, and to [D-Lys6]LHRH carrier to produce analog AN-207. Cytotoxic bombesin hybrid AN- 215 also contains AN-201. DOX was likewise linked to [D-Lys6]LHRH to form AN-152. A comprehensive testing of these cytotoxic conjugates in experimental models of various human and rodent cancers led to their selection as candidates for clinical trials.