By 1996, it was known that a number of cancer cells, including breast and prostate cancer cells, express chorionic gonadotropin (CG or LH) receptors on their surfaces. Other reports indicated that these same cancer cells also expressed luteinizing hormone releasing hormone (LHRH) receptors. We hypothesized that it would be possible to construct drugs consisting of a lytic peptide conjugated to either CG or LHRH that would target and destroy breast or prostate cancer cells and their metastases. Accordingly, we constructed four lytic peptide-CG conjugates and one LHRH-lytic peptide. These compounds, Hecate-βCG, Phor14-βCG, Phor14-βCG(ala), Phor21-βCG and LHRH-Hecate were tested in vitro and in vivo for their abilities to kill cancer cells and to regress primary tumors and to seek out and kill metastatic cells in lymph nodes, lungs, bones, and other organs.
The lytic peptides used in these studies (see Fig. 2), are linear, alpha helical, amphipathic, membrane-disrupting compounds containing 25 (Hecate), 14 (Phor14), and 21 (Phor21) amino acids. All are positively charged in a hydrophobic environment. The lytic peptides serve as defense molecules in a wide variety of species including bacteria, insects, invertebrates and vertebrates. They serve as a defense mechanism against pathogens, including bacteria, fungi and viruses in those species that lack an immune system. The βCG component in each compound consists of a 15 amino acid segment (acids 81-95) of the β chain of HCG; this segment has been reported to contain 70% of the binding capacity of the entire molecule. Phor14 consists of two identical sequences of 7 amino acids and Phor21 contains three of these same heptads. In Phor21-βCG(ala), the most potent of these three constructs in destroying breast and prostate cancer cells, the cysteines in the 15 amino acid CG segment have been replaced by alanines. Toxicity and pharmacological data on Phor21-βCG(ala) have been produced by the National Cancer Institute under their RAID (Rapid Access to Intervention Development) Program.
These lytic peptide-CG constructs are rapidly metabolized and are not antigenic. They disrupt cancer cell membranes within minutes. Since their activity is not dependent on rapid cell proliferation, they do not have the undesirable side effects of other chemotherapeutic agents. They act at the level of the cell membrane, and are not subject to the multiple drug resistance, a phenomenon described above. Although the exact mechanism by which lytic peptides destroy cell membranes is not well understood, it is known that the outer leaf of the tumor cell membrane has a high content of phosphatidyl serine compared to normal cells and is negatively charged. Once the bi-layer is disturbed the trans-membrane electrochemical potential collapses, phospholipids are released and cell death occurs. Significantly, cell death occurs by necrosis; no evidence of apoptosis has been found.
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