FORMULATION DEVELOPMENT – Peptide-Based Cancer Therapeutics
Over the years, peptides have been evolved as promising therapeutic agents in the treatment of cancer, diabetes, and cardiovascular diseases – and application of peptides in a variety of other therapeutic areas are growing rapidly. Currently, there are about 60 approved peptide drugs in the market generating an annual sale of more than $13 billion. Out of four peptides drugs in the market that have reached global sales over $1 billion, three peptides are used in treating cancer directly or in the treatment of episodes associated with certain tumors (leuprolide, goserelin, and octreotide).The number of peptide drugs entering clinical trials is increasing steadily; it was 1.2 per year in the 1970s, 4.6 per year in the 1980s, 9.7 per year in the 1990s and 16.8 per in 2000s. There are several hundred peptide candidates in the clinic and pre-clinic development. From 2000 onward, peptides entering clinical study were most frequently for indications of cancer (18%) and metabolic disorders (17%).
In conventional chemotherapy, the cancer cell-specific delivery of cytotoxic agents is difficult without affecting normal cells, which leads to systemic toxicity, causing undesirable severe side effects. “Molecularly targeted cancer therapies” using proteins, peptides, and related biomolecules are gaining momentum due to the possibility of improved drug potency and efficiency and minimal side effects. Peptides can be used as: direct anti-cancer drugs, cytotoxic drug carriers, vaccines, hormones, radio-nuclide carriers, and drug targets. Though shorter in vivo half-life of peptides is a concern, recent advances in drug delivery systems and peptide modification are expected to override those difficulties.
Emergence of Biologics in Cancer Treatment
Mortality from cancer is about to surpass that from cardiovascular diseases in the near future. About 7 million people die from cancer-related cases per year, and it is estimated there will be more than 16 million new cancer cases every year by 2020. Cancer is characterized by uncontrolled division of cells and the ability of these cells to invade other tissues leading to the formation of tumor mass, vascularization, and metastasis (spread of cancer to other parts of the body). Though angiogenesis (growth of new blood vessels from pre-existing vessels) is a normal and vital process in growth and development, it is also a fundamental step in the transition of tumors from a dormant state to a malignant one. So, angiogenesis inhibitors have been used to suppress tumor cell growth. Chemotherapy is one of the major approaches to treat cancer by delivering a cytotoxic agent to the cancer cells. The main problem with the conventional chemotherapy is the inability to deliver the correct amount of drug directly to cancer cells without affecting normal cells. Drug resistance, altered biodistribution, biotransformation, and drug clearance are also common problems. Targeted chemotherapy and drug delivery techniques are emerging as a powerful method to circumvent such problems. This will allow the selective and effective localization of drugs at predefined targets (eg, overexpressed receptors in cancer) while restricting its access to normal cells, thus maximizing therapeutic index and reducing toxicity. Discovery of several protein/peptide receptors and tumor-related peptides and proteins is expected to create a new wave of more effective and selective anti-cancer drugs in the future, capturing the large share of the cancer therapeutic market. The “biologics” treatment option against cancer includes the use of proteins, monoclonal antibodies, and peptides. The monoclonal antibodies (mAbs) and large protein ligands have two major limitations compared to peptides; poor delivery to tumors due to their large size and doselimiting toxicity to the liver and bone marrow due to nonspecific uptake into the reticulo-endothelial system. The use of such macromolecules has therefore been restricted to either vascular targets present on the luminal side of tumor vessel endothelium or hematological malignancies. Peptides possess many advantages, such as small size, ease of synthesis and modification, tumor penetrating ability, and good biocompatibility. Peptide degradation by proteolysis can be prevented by chemical modifications, such as incorporation of Damino acids or cyclization.
LHRH Agonists & Antagonists
The best classical example of the application of peptides in cancer treatment is the use of LHRH (luteinising hormonereleasing hormone) agonists introduced by Schally et al as a therapy for prostate cancer. Since then, depot formulations of LHRH agonists such as buserelin, leuprolide, goserelin, and triptorelin have been developed for more efficacious and more convenient treatment of patients with prostate cancer. Administration of these peptides causes down-regulation of LHRH receptors in the pituitary, leading to an inhibition of follicle-stimulating hormone (FSH) and LH release, and a concomitant decrease in testosterone production. This offered a new method for androgen deprivation therapy in prostate cancer patients. Discovery of LHRH antagonists resulted in therapeutic improvement over agonists as they cause an immediate and dose-related inhibition of LH and FSH by competitive blockade of the LHRH receptors. To date, many potent LHRH antagonists are available for the clinical use in patients. Cetrorelix was the first LHRH antagonist given marketing approval and, thus, became the first LHRH antagonist available clinically. Subsequently, newgeneration LHRH antagonists, such as abarelix and degarelix, have been approved for human use. A list of LHRH agonists and antagonists available in the market are shown in Table 1.
Somatostatin Analogues in Cancer Therapy
Apart from the use of peptide-based LHRH agonists and antagonists for treating cancer, somatostatin analogues are the only approved cancer therapeutic peptides in the market. Potent analogues of somatostatin (peptide hormone consisting of 14 amino acids, found in delta cells of the pancreas as well as in hypothalamic and other gastrointestinal cells), including octreotide (sandostatin), has been developed for the treatment of acromegaly, gigantism, thyrotropinoma, diarrhea, and flushing episodes associated with carcinoid syndrome, and diarrhea in patients with vasoactive intestinal peptide-secreting tumors (VIPomas). Similarly, another longacting analogue of somatostatin, lanreotide (somatuline), is used in the management of acromegaly and symptoms caused by neuroendocrine tumors, most notably carcinoid syndrome and VIPomas.
Most neuroendocrine tumors (NETs) feature a strong overexpression of somatostatin receptors, mainly of subtype 2 (sst2). Currently, five somatostatin receptor subtypes (sst) are known (sst1-5). The density of these receptors is vastly higher than on non-tumor tissues. Therefore, sst are attractive targets for delivery of radioactivity via radiolabeled somatostatin analogs. The sst2 has been shown to internalize into the cell in a fast, efficient, and reversible manner after specific binding of a receptor agonist. This molecular process is likely to be responsible for the high and long-lasting uptake of radioactivity in the target cell after binding of the radiolabeled somatostatin analog. Introduced in the late 1980s, [111In-DTPA0]-octreotide (Octreoscan), the first available radiolabeled somatostatin analog, rapidly became the gold standard for diagnosis of sst-positive NETs. Numerous peptide-based tracers targeting sst have been developed over the past decade. Octreoscan and Neotect (tc-99m depreotide) are the only radiopeptide tracers on the market approved by the Food and Drug Administration. An octreotide scan or octreoscan is a type of scintigraphy used to find carcinoid and other types of tumors and to localize sarcoidosis. Octreotide, a drug similar to somatostatin, is radiolabeled with indium- 111 and is injected into a vein and travels through the bloodstream. The radioactive octreotide attaches to tumor cells that have receptors for somatostatin. A radiationmeasuring device detects the radioactive octreotide, and makes pictures showing where the tumor cells are in the body.
Current Status & Future of Peptide Based Anti-Cancer Agents
The application of peptides as a direct therapeutic agent, in targeted drug delivery and as a diagnostic tool in cancer biology, is growing. Drug targeting exploits differences in the nature of normal and cancer cells and their microenvironment. To establish efficient and reliable therapeutic delivery into cancer cells, a number of delivery agents and concepts have been investigated in the recent years. Among many improvements in targeted and controlled delivery of therapeutics, celltargeting peptides have emerged as the most valuable non-immunogenic approach to target cancer cells. Peptides can be incorporated into multicomponent genedelivery complexes for cell-specific targeting. In contrast to larger molecules, such as monoclonal antibodies, peptides have excellent tumor penetration, which make them ideal carriers of therapeutics to the site of primary tumor and the distant metastatic sites. Different possible cancer treatment options using peptides are summarized in Figure 1.
A recently identified peptide called iRGD is able to specifically recognize and penetrate cancerous tumors but not normal tissues. Discovery of such peptides having extraordinary tumor-penetrating properties will definitely make substantial improvements in cancer treatment in the future. Chlorotoxin (a 36 amino acid peptide derived from scorpion venom) binds preferentially to glioma cells compared with non-neoplastic cells or normal brain has allowed the development of new methods for the treatment and diagnosis of cancer.
BN/GRP (bombesin/gastrin-releasing peptide) peptides were shown to bind selectively to the G-protein-coupled receptors on the cell surface, stimulating the growth of various malignancies in murine and human cancer models. Thus, it has been proposed that the secretion of BN/GRP by neuroendocrine cells might be responsible for the development and progression of prostate cancer to androgen independence. GRP is widely distributed in lung and gastrointestinal tracts. It is produced in small cell lung cancer (SCLC), breast, prostatic, and pancreatic cancer, and functions as a growth factor. The involvement of bombesin-like peptides in the pathogenesis of a wide range of human tumors, their function as autocrine/paracrine tumoural growth factors, and the high incidence of BN/GRP receptors in various human cancers prompted the design and synthesis of BN/GRP receptor (GRPR) antagonists, such as RC-3095, RC-3940-II, and RC- 3950.
Peptide receptor radionuclide therapy (PRRT) combines octreotide with a radionuclide (a radioactive substance) to form highly specialized molecules called radiolabeled somatostatin analogues or radiopeptides. These radiopeptides can be injected into a patient and will travel throughout the body binding to carcinoid tumor cells that have receptors for them. Once bound, these radiopeptides emit radiation and kill the tumor cells they are bound to.
Recently, many researchers are focusing on the development of GHRH (growth hormone releasing hormone – a hypothalamic polypeptide) antagonists as potential anti-cancer therapeutics because the GHRH is produced by various human tumors, including prostate cancer, and seems to exert an autocrine/paracrine stimulatory effect on tumors. Another promising and emerging approach for the therapy of prostate cancer consists of the use of cytotoxic analogues of LHRH, bombesin, and somatostatin, which can be targeted to receptors for these peptides in prostate cancers and their metastases. For example, a potential drug candidate, AEZS-108 couples a peptide, LHRH, with the chemotherapeutic agent doxorubicin to directly target cells that express LH-RH receptors-specifically, prostate cancer cells.
There is a tremendous effort to discover angiogenesis inhibitors, based on polypeptides as the safest and least toxic therapy for diseases associated with abnormal angiogenesis. A number of ongoing clinical trials in this area focus on peptides derived from extracellular matrix proteins, growth factors and growth factor receptors, coagulation cascade proteins, chemokines, Type I Thrombospondin domain-containing proteins, and serpins. Recently, it was found that angiotensin-(1- 7) can stop lung cancer tumor growth in mice. Also, peptides that can inhibit cell growth in drug-resistant ovarian cancer have been identified. Stapled peptides are showing promise for treating colon cancer and other forms of cancerous growth.
Active immunization seems to be the most promising strategy to treat cancer, though many approaches based on the employment of immune cells or immune molecules have been studied. Researchers have studied and debated the possibility of vaccinating against cancer for decades. Only in recent years has the debate changed from being focused on preclinical proof-of- principle to discussions on what defines a tumor antigen and how best to optimally deliver vaccines based on defined antigens to induce anti-cancer immunity. This new method of treating cancerous cells relies on vaccines consisting of peptides derived from the protein sequence of candidate tumorassociated or specific antigens. Tumor cells express antigens known as tumorassociated antigens (TAAs) that can be recognized by the host’s immune system (T-cells). Many TAAs have recently been identified and molecularly characterized. These TAAs can be injected into cancer patients in an attempt to induce a systemic immune response that may result in the destruction of the cancer growing in different body tissues. This procedure is defined as active immunotherapy or vaccination as the host’s immune system is either activated de novo or re-stimulated to mount an effective, tumor-specific immune reaction that may ultimately lead to tumor regression. Any protein/peptide produced in a tumor cell that has an abnormal structure due to mutation can act as a tumor antigen. Such abnormal proteins are produced due to mutation of the concerned gene. Clinical studies have therefore been initiated to assess the therapeutic potential of active immunization or vaccination with TAA peptides in patients with metastatic cancer. So far, only a limited number of TAA peptides, mostly those recognized by CD8 (+) T cells in melanoma patients, have been clinically tested. Several melanoma TAAs have been identified and are being evaluated as peptide-based cancer vaccines in clinical trials around the world. Recent advances in the field of molecular biology have enabled the rapid identification of dozens of candidate TAAs for several important human cancers. The challenges for future studies are to determine the most efficient means of administering the vaccine and to develop methods to determine if the vaccine is effective.
In summary, peptides are poised to make a significant impact in the near future in the area of cancer treatment and diagnosis. A number of peptide-based therapies, such as cancer vaccines, tumor targeting with cytotoxic drugs and radioisotopes, anti-angiogenic peptides, etc. are currently in clinical trials and expected to yield positive results. Stimuvax (palmitoylated peptide vaccine against non-small lung cancer, Merck), Primovax (peptide cancer vaccine, Pharmexa), Melanotan (pre-cancerous actinic keratosis-Clinuvel) and Cilengitide (Glioblastoma-Merck) are some examples of potential peptides in late clinical trials. Due to the tremendous advancement in the large-scale synthesis of peptides, it will be possible to cut down the manufacturing costs, thereby making peptide-based anti-cancer drugs more affordable.
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Dr. Jyothi Thundimadathil,
Technical Marketing Associate,
American Peptide Company, Inc.
Dr. Jyothi Thundimadathil is a Technical Marketing Associate at American Peptide Company Inc. He has more than 10 years of experience in peptide chemistry, previously serving as Chemistry Director and R&D Group Leader in peptide industry. After earning his PhD in Applied Chemistry from CUSAT (India) he worked as post-doctoral fellow at Ben Gurion University (Israel), IUPUI (USA) and Purdue University (USA). He has published more than 40 papers, including book chapters and patents.
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