Cancer chemotherapy based on targeting of cytotoxic peptide conjugates to their receptors on tumors

Section of Experimental Medicine, Department of Medicine, Tulane University School of Medicine, New Orleans, Louisiana 70112--2699, USA.

In view of non-specific toxicity of most chemotherapeutic agents against normal cells, the development of targeted chemotherapy is warranted. Efficient targeting of chemotherapeutic drugs to the cancerous area could be of great benefit for patients with advanced or metastatic tumors. Targeted cytotoxic peptide conjugates are hybrid molecules composed of a peptide carrier which binds to receptors on tumors and a cytotoxic moiety. New cytotoxic analogs of LHRH, AN-152 in which doxorubicin (DOX) is linked to [d-Lys(6)]LHRH, and AN-207 which consists of 2-pyrrolino-DOX (AN-201) coupled to the same carrier, show high-affinity binding and are much less toxic and more effective in vivo than their respective radicals in inhibiting tumor growth in LHRH receptor-positive models of human ovarian, mammary, or prostatic cancer. These results suggest that targeted cytotoxic LHRH analogs such as AN-207 could be considered for treatment of these cancers. The presence of receptors for bombesin-like peptides on a wide variety of tumors prompted us to use some of our bombesin/gastrin-releasing peptide antagonists as carrier molecules. Cytotoxic bombesin analogs, such as AN-215 containing AN-201, might find application in the treatment of small cell lung carcinoma (SCLC), and colorectal, gastric, pancreatic, mammary, and prostatic cancers. Since somatostatin receptors are found in various human neoplasms and the receptor subtypes to which octapeptide analogs bind with high affinity have been identified, we synthesized several cytotoxic somatostatin analogs including AN-162 and AN-238 containing DOX and 2-pyrrolino-DOX respectively, linked to octapeptide RC-121. Cytotoxic somatostatin analog AN-238 efficaciously inhibits growth of human breast or prostate cancers expressing somatostatin receptors-2 and -5 and can be used for receptor-targeted chemotherapy. Cytotoxic somatostatin analogs might also find applications for the therapy of human pancreatic, colorectal, and gastric cancer as well as brain tumors and non-SCLC. Cytotoxic compounds linked to analogs of hormonal peptides like LHRH, bombesin, and somatostatin that can be targeted to certain tumors possessing receptors for those peptides could be an important addition to oncological armamentarium.