Cancer Immunotherapy: Present Status, Future Perspective, and a New Paradigm of Peptide Immunotherapeutics
Abstract: A promising new era of cancer therapeutics with agents that inhibit specific growth stimulatory pathways is finding a new niche in our armamentarium in the war against cancer. Targeted cancer therapeutics, including humanized monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs), are amongst the major treatment options for cancer today together with cytotoxic chemotherapies. Targeted therapies are more selective for cancer cells and improve the quality of life for cancer patients undergoing treatment. Many of these drugs have been approved by the FDA, and several more are being studied in clinical trials. Although development of targeted therapeutics has improved cancer treatment significantly, the harsh reality is that the "War on Cancer" still exists. Major challenges still exist with the currently marketed inhibitors, including limitations associated with mAbs and TKIs drug types, acquired mechanisms of drug resistance that cause patient relapse, and tumor heterogeneity. Today, there is an urgent need for the development of novel anti-tumor agents that are cheaper, stable, can selectively target cancer dependent pathways without affecting normal cells, and most importantly, avoid development of resistance mechanisms. Peptide mimics have the potential benefits of being highly selective, stable, cheap, and non-toxic. The focus of this review is to discuss the disadvantages associated with the use of monoclonal antibodies and tyrosine kinase inhibitors. A special emphasis will be placed on efforts taken in our laboratory to 1) design peptide vaccines and therapeutics that target cancer dependent pathways and 2) use a combination approach that will shut down alternative mechanisms that lead to resistance.
Over a century ago, Paul Ehrlich used histological cell staining techniques and postulated that “magic bullets” could be developed to selectively target human disease. This “magic bullet” concept referred to drugs that selectively destroy pathogens or tumor cells without eliciting harmful effects in normal cells and tissues. Ehrlich’s achievements in the laboratory immediately led to the development of chemotherapy and, more recently, have inspired scientists to develop “personalized and tailored” drugs to target cancer (Strebhardt and Ullrich, 2008). Traditionally, chemotherapies were identified by screening for compounds that killed rapidly dividing cells, and these compounds have been restricted to cytotoxic agents that mainly target DNA synthesis and events that regulate cell division (Galmarini et al., 2012; Vanneman and Dranoff, 2012). Unfortunately, these processes are not specific to cancer, and these medications cause a broad range of toxic side effects. As a result, the prototype for cancer treatment has evolved over the past few decades from cytotoxic drugs to more selective, targeted therapeutics (Vanneman and Dranoff, 2012). Targeted therapies are tailored to exploit genetic abnormalities and signal transduction pathways that are specific to rapidly dividing tumor cells (Stegmeier et al., 2010). Tamoxifen, an anti-estrogen inhibitor introduced into the clinical setting in the early 1970s, was one of the first rationally designed targeted anti-tumor drugs. Since then, scientists have uncovered numerous other pathways that are deregulated in human cancers, and several novel targets for anticancer therapy have been identified. Figure 1 illustrates the timeline for different targeted therapies against cancer (Chin and Gray, 2008). Targeted cancer therapies approved for use in specific cancers include drugs that interfere with cell signaling, inhibit tumor blood vessel development, promote apoptosis of cancer cells, stimulate the immune system to destroy cancer cells, and deliver toxic agents to cancer cells. At present, there are two main tumor targeted therapies available for use in clinical practice, and they include monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs). These therapies have significantly improved treatment and clinical outcomes for millions of cancer patients around the world. Despite the improvements, however, the reality is that the 40-year old “War on Cancer” is still ongoing. Conventional chemotherapeutic drugs remain the backbone of current treatment, and major limitations, such as acquisition of drug resistance and patient relapse, still exist with currently marketed mAbs and TKIs. The future success of targeted cancer therapeutics will be dependent on overcoming these obstacles. In this review, the advantages and drawbacks of targeted therapeutics will be discussed. A special emphasis will be placed on two alternative strategies that our lab has taken over the last couple of decades to overcome current cancer drug challenges: development of peptide therapeutics and combination therapy.
In 1975, Kohler and Milstein (1975) published their manuscript on hybridoma technology and production of mouse mAbs, which was described more recently by others (Hansel et al., 2010; Weiner et al., 2010). Their initial discovery motivated scientists in the field to develop therapeutic mAbs, and over time, these molecules became widely accepted as ideal reagents for imaging and therapy; similar to the “magic bullets” imagined by Paul Ehrlich at the beginning of the 20th century (Chames et al., 2009). Antibody-based therapy for cancer has become established over the last couple of decades, and it is now a relatively successful and important strategy for treating patients with various hematological malignancies and solid tumors. Since 1997, the FDA has approved 12 antibodies for use in oncology (Scott et al., 2012). Table 1 illustrates most of the monoclonal antibodies approved for treatment of various cancer types. The first therapeutic anti-cancer antibody to be approved by the FDA was rituximab, a humanized anti-cluster of differentiation (CD) 20 antibody used for treatment against B-cell lymphoma. Approval of rituximab was quickly followed by trastuzumab, a humanized anti-HER2 antibody used for treatment of HER2 positive breast carcinoma, and cetuximab, a chimeric anti-HER1 antibody for treatment of colorectal cancer (Mach, 2012). Other therapeutic antibodies include alemtuzumab, a humanized anti-CD52 mAb approved in 2001 to treat chronic lymphocytic leukemia, and bevacizumab, a humanized anti-VEGF antibody used for treating colorectal and lung cancer (Oldham and Dillman, 2008).
Antibodies have been selected as anti-cancer therapeutics due to their long serum half-lives and capacity to bind with high specificity and affinity to a wide variety of molecules (Chames et al., 2009). Their ability to mediate target-specific inhibition and immune-mediated tumor suppression has enabled antibodies to demonstrate improved efficacy over standard chemotherapy regimens. MAbs exert an anti-tumor effect through several different mechanisms, including: induction of apoptosis, inhibition of tumor cell signaling, and activation of complement dependent cytotoxicity or antibody dependent cellular cytotoxicity (Kubota et al., 2009; Scott et al., 2012; Shuptrine et al., 2012; von Mehren et al., 2003; Weiner et al., 2012; 2010). Unfortunately, current therapeutic mAbs are associated with several drawbacks that hinder widespread use of these molecules. Antibody limitations include: repeated treatments, high costs, inadequate pharmacokinetics and tissue accessibility, limited duration of action, and undesired immunogenicity and toxicity (Chames et al., 2009). Therapeutic mAb production involves relatively complex processes in mammalian cells with huge monetary costs. Antibodies are large proteins (around 150 kDa) that are generally intravenously administered at high dosages to achieve clinical efficacy. The recommended dosage of rituximab for patients with non-Hodgkin’s lymphoma is 375 mg/m2, and if using the body surface area dosing, for the average 1.73 m2 person, a dose of 650 mg would be required, which is expensive comparing to non-monoclonal antibody drugs (Pescovitz, 2006). Antibodies are widely considered as molecules that are unable to pass through the cellular membrane, therefore, these drugs can only act on molecules that are either expressed on the cell surface or secreted. Their large size, shape, affinity, and valency also limit favorable pharmacokinetics, antibody uptake by the tumor, and efficient penetration into the targeted tissue. In a recent study with murine xenograft models, mAbs directed against tumor-specific antigens remained largely in the blood, and no more than 20% of the administered dose interacted with the tumor (Beckman et al., 2007; Butowski and Chang, 2005). Administration of mAbs carries the risk of undesirable immune reactions, such as acute anaphylaxis, serum sickness, and production of neutralizing antibodies. Although researchers have taken strides to overcome these problems, with the advent of chimerization and humanization techniques, these problems still exist. Completely humanized monoclonal antibodies likely still cause undesired immunogenicity. There are also several adverse effects that are related to mAb targets, including infections and cancer, autoimmune disease, and organ-specific adverse events such as cardiotoxicity (Hansel et al., 2010).
Tyrosine Kinase Inhibitors (TKIs)
Over the last few decades, tremendous progress has been made in oncology with the development of new molecular and genomic technologies. Among these targets are protein tyrosine kinases (PTKs), an important family of signaling proteins involved in many protein-protein interactions that form the basis of many cellular processes. Examples of PTKs include: members of the human epidermal growth factor receptor family (HER1, HER2, HER3, and HER4), vascular endothelial growth factor receptor (VEGFR), platelet-derived growth factor receptor (PDGFR), insulin growth factor receptor (IGFR), and mast/stem cell growth factor receptor (SCFR/c-KIT). PTKs are involved in cellular growth, proliferation, migration, differentiation, and apoptosis (Schlessinger, 2000). Most PTKs are membrane receptors that are activated upon ligand binding, while others are cytosolic proteins that become activated downstream of transmembrane receptors or other signaling proteins. Activation of PTKs results in phosphorylation of intracellular signaling proteins that link PTKs with multiple signaling pathways, such as phosphoinositide 3 kinase (PI3K), Rat sarcoma/mitogen-activated protein kinase (Ras/MAPK), signal transducers and activators of transcription (STATs), phospholipase C (PLCγ), etc. (Arora and Scholar, 2005; Cools et al., 2005; London, 2013). Deregulation of kinase signaling pathways is extremely common in cancer and can occur via gain-of-function mutations, gene amplification, overexpression, and/or chromosomal translocation (Blume-Jensen and Hunter, 2001; Cheng and Force, 2010; Gossage and Eisen, 2010). The human genome encodes 90 genes with putative tyrosine kinase activity, and there are approximately 60 receptor tyrosine kinases that have been identified. Of these, 20 have already been implicated in cancer (Manning et al., 2002; Pawson, 2002). PTKs are now widely recognized as attractive targets for anticancer drugs, and there is an intense research effort to design inhibitors that target deregulated kinase signaling pathways. Today, several inhibitors of PTKs have been approved by the FDA for treatment of specific types of cancer, and others are being studied in clinical trials and preclinical testing.
Small molecule tyrosine kinase inhibitors (TKIs) now represent a major class of cancer targeted therapeutics. These agents are small molecule compounds that compete with adenosine triphosphate (ATP) binding and inhibit kinase activity. In 2000, imatinib was introduced into clinical settings, and 13 other TKIs followed (see Table 2). Imatinib binds to the ATP pocket of the Abelson kinase (ABL), as well as c-KIT and PDGFR (de Kogel and Schellens, 2007; Druker, 2002; Druker et al., 1996). Breakpoint-cluster region-ABL (BCR-ABL) fusion proteins are present in over 90% of human patients with chronic myeloid leukemia (CML), and approximately 15-30% of adult patients with acute lymphoblastic leukemia (ALL) (Faderl et al., 1999; Melo et al., 2003; Shawver et al., 2002; Van Etten, 2004). Other FDA approved TKIs include gefitinib, erlotinib, and sunitinib. Table 2 illustrates current FDA approved TKIs used to treat various cancer types. Gefitinib and erlotinib are anti-EGFR drugs used to treat EGFR-expressing non-small cell lung carcinomas (NSCLC) and head and neck squamous cell carcinomas (HNSCC). Sunitinib is a new type of small molecule agent that exerts its effects on various PTKs, such as VEGFR, EGFR, and PDGFR, and shows broad-spectrum antitumor activity by inhibiting both tumor proliferation and angiogenesis. All of these agents are small molecule compounds that are inexpensive to synthesize, they are usually orally available and easily administered (Huang et al., 2004). Their small size (usually <500 Da) also allows them to translocate through the plasma membrane and interact with the cytoplasmic domain of cell-surface receptors and intracellular signaling molecules. As a result, TKIs can theoretically be developed to target any portion of a molecule, regardless of the cellular location (Dancey and Sausville, 2003).
Significant hurdles do exist for the development of tyrosine kinase inhibitors. In general, TKIs are thought to be less specific than mAbs, their half-lives are much shorter, and there is a high level of variation in toxic side effects. Unfortunately, the first TKI developed, imatinib, remains the most successful in the clinical setting, and all others have only had modest activity on the cancers they target. Despite their large investment, commercial enterprises are becoming reluctant to continue developing small molecule inhibitors against kinases that have not already been subject to intense biological investigation (Haber et al., 2011). Developers of small molecule inhibitors are also limited to the number of potential drug candidates they can target, and novel drug development is reaching a plateau. Many proteins implicated in cancer are considered “undruggable,” because they lack a cavity for the small organic inhibitors to bind and interact with protein partners through extensive and flat surfaces. New drugs targeting protein-protein interactions often require larger interaction sites than small molecules can offer, and screening efforts have consequently been limited to a small fraction of the proteome (ion channels, nuclear receptors, GPCRs, or enzymes) (Ahrens et al., 2012; Overington et al., 2006; Verdine and Walensky, 2007). In a recent study, an estimated 60% of small molecule drug discovery projects fail due to the biological target being “undruggable” (Brown and Superti-Furga, 2003; Cheng et al., 2007). In addition, the advent of mAb chimerization and humanization techniques has ultimately given mAbs an unprecedented advantage over TKIs. Many biotechnology and pharmaceutical firms are moving toward development of therapeutic mAbs, and newer small-molecule agents have relatively low approval rates when compared to mAbs (5% and 18-24%, respectively) (Imai and Takaoka, 2006). Unfortunately, the overall low success rate of TKIs may indicate that the shortcomings of these drugs outweigh their potential benefit and subsequent use in the clinical setting.
Peptides: A New Paradigm
A newer class of targeted therapeutic drugs are emerging and revolutionizing cancer drug development. These drugs are called peptide therapeutics. Peptides are short sequences of amino acids (usually >5,000 Da) that can be produced biosynthetically, via natural or recombinant microbial fermentations, or chemically, through mechanisms such as solution-based or solid-phase peptide synthesis. Regrettably, the concept of using peptides as therapeutics has been historically disregarded, mainly due to peptide stability, susceptibility to degradation, size (>5,000 Da), and consequent limitations in methods of delivery (low oral bioavailability). In recent years, however, the development of peptides as drugs is attracting increasing attention from the pharmaceutical industry; especially as the technologies for peptide development and manufacturing continue to mature. New synthetic strategies to improve productivity and reduce metabolism of peptides, along with alternative routes of administration have been developed in recent years, and a large number of peptide-based drugs are now being marketed (Vlieghe et al., 2010). Development of peptide therapeutics has the potential of overcoming some of the obstacles observed with mAbs and TKIs. Currently, one of the greatest strengths of peptides as potential therapies lies in the powerful new approaches for discovering and screening new drug candidates. Other advantages include: low cost, higher specificity and potency due to their superior compatibility with targeted proteins, ability to penetrate the cell membrane, reduced immunogenicity, and improved safety.
In theory, peptide-based drugs have given oncologists an opportunity to expand the repertoire of “druggable proteins.” As mentioned before, mAbs and TKIs are limited to the number of candidates they can effectively target; MAbs are too large and complex for entry into the cell, while small molecule inhibitors are limited to proteins with well-defined small binding pockets. Peptides are small enough to penetrate the cell membrane, and they can be rationally designed to target almost any protein of interest. The advantage of the smaller size of peptides in penetrating tumor tissue has been clearly demonstrated recently, where an antibody-mimicking peptide (~3 kDa) showed much greater capacity to target and penetrate tumors than its parent antibody (Qiu et al., 2007). Peptides now have a broad range of potential clinical benefits against numerous protein targets, with applications in some of the most prevalent disease categories (Latham, 1999). Peptide therapeutics also take advantage of the highly specific and selective interactions between proteins. These molecules are rationally designed to mimic the binding region between two or more proteins involved in various disease states. A perfect peptide agent is designed rationally and has the ability to reach, bind, and antagonize the function of a target protein for the required amount of time and efficacy. A major benefit of peptide therapeutics lies in their high selectivity and affinity, which are frequently in the nanomolar range (Ahrens et al., 2012). Peptide-therapeutics may consequently aide in the reduction of current therapeutic problems, such as off-target side effects and the requirement of high dosage treatment. Ultimately, their small size and superior specific binding properties make peptides ideal candidates for physiologically disrupting on-target functional protein-protein complexes.
In recent years, considerable progress has been made to convert peptides, which are typically unstable and rapidly degraded, into therapeutically useful molecules. Researchers in the field have addressed the major limitations of peptide therapeutics, and new synthetic technologies have improved productivity and reduced the metabolism of peptides. In addition to their high specificity, peptides are now easily synthesized and very amenable to site-specific modification (Boohaker et al., 2012). Currently, chemical synthesis has enabled easy conjugation of small molecules and incorporation of non-natural amino acids by design, and peptides can now be easily modified to enhance their pharmacokinetic and pharmacodynamic profiles (Lien and Lowman, 2003). The most common modifications found in marketed peptides include: glycosylation, pegylation, lipid incorporation, antibody incorporation, and radiolabeling. Modifications such as cyclization, glycosylation, pegylation, and incorporation of D-amino acids all help to extend the half-life of a peptide by rendering it unrecognizable to proteases (Clement et al., 2002; Dennis et al., 2002; Haubner et al., 2001; Holmes et al., 2000; Koehler et al., 2002; Lien and Lowman, 2003). Peptide modifications can also overcome current therapeutic limitations, such as inefficient drug delivery to the target site and adverse off-target effects. Today, peptides can be developed to act as homing devices for specific tissue types or organs, and cell-penetrating peptides provide a promising solution to drug delivery (Koren et al., 2012; Svensen et al., 2012).
Novel antitumor peptide drugs are increasingly making their way into clinical application, and they demonstrate several advantages over the currently available mAbs and TKIs. Table 3 displays some examples of currently marketed peptide therapeutics. As of 2010, there were approximately 60 therapeutic peptides on the market, 150 in clinical phases, and 400 in advance pre-clinical stages. Peptide drugs are now being universally marketed, and more than 100 pharmaceutical and biotech companies are now active in the peptide field. Of the products now approved, about 30 peptide drugs are currently marketed in the U.S., and 17% are used in the cancer clinical setting (Reichert, 2008). The peptide therapeutics foundation anticipates that pharmaceutical and biotechnology industries will continue to focus on these versatile molecules because of the increased acceptance of injected drugs on the market, the availability of new formulation and delivery technologies, and the relatively high approval success rates. Between 1984 and 2000, peptide candidates that entered clinical study demonstrated approval success rates in the range of 23-26% (Reichert, 2008). As mentioned before, the approval success rates for both mAbs (18-24%) and small molecule inhibitors (5%) have been reported as significantly lower (Sierra et al., 2010).
Over the past few decades, our laboratory has focused on designing peptide-based vaccines and therapeutics to target protein-protein interactions that are specific to aggressive forms of cancer. Our approach has relied on computer-aided analysis reviewed by Kaumaya (1994) and published crystal structural analysis to identify potential B cell epitopes in target proteins. Previous efforts in the laboratory led to the development of novel peptides that block the interaction and subsequent signaling of the CD28 receptor with its B7 ligand (Allen et al., 2005; Srinivasan et al., 2002; 2001). We have also made significant strides in developing novel peptide vaccines that target the HER2 receptor, which is overexpressed in several types of solid tumors, including: breast, ovarian, colon, pancreatic, endometrial, and lung (Berchuck et al., 1991; Cirisano and Karlan, 1996; Kern et al., 1990; Morrison et al., 2006; Yano et al., 2006). The innovation of our approach lies in the fact that we incorporate a chimeric peptide vaccine that stimulates both B and T cells and engages the immune system to elicit high affinity antibodies and establish memory. Two HER-2 B cell epitope vaccine candidates established their anti-tumor effects in preclinical studies, and we were able to translate our novel findings into a phase I clinical trial in which patients were vaccinated with a combination of the two vaccine epitopes (Dakappagari et al., 2000; 2005a; 2003; 2005b; Kaumaya et al., 2009). All patients had heavily been treated for their advanced solid tumors and were unresponsive to all prior drug regimens. The vaccine was immunogenic in most of the patients, no serious effects of toxicity were reported and 25% showed clinical benefits (Kaumaya et al., 2009). Clinical findings from this study validated our hypothesis that the antibodies to the vaccine were able to disrupt signaling pathways in cancer.
After the publication of the crystal structures of HER-2 in complex with trastuzumab and pertuzumab (Cho et al., 2003; Franklin et al., 2004), we gained significant insights into the key HER2 amino acids involved in binding to the antibodies. As a result, we developed a second generation of HER2 vaccine epitopes that mimic the trastuzumab and pertuzumab binding sites of HER2. In pre-clinical studies, we demonstrated that the vaccines were able to prevent tumor growth in animal models, and that antibodies raised against the vaccines were able to disrupt HER-2 dependent pathways in HER-2 expressing human cancer cells (Allen et al., 2007; Garrett et al., 2007). Currently, we are conducting a new FDA approved NCI funded phase 1/11b trial (NCT01376505) with the second generation vaccines. In addition, the established role of angiogenesis in cancer growth and metastasis has led us to expand our approach towards targeting VEGF and its main receptor VEGFR2. We have developed effective inhibitors of VEGF:VEGFR2 and have shown their anti-tumor effects in several different models of cancer and angiogenesis (Foy et al., 2011; Vicari et al., 2011). We are also advancing our stride by targeting other proteins involved in cancer, such as HER1, HER-3, and IGF-IR. The future goal of our laboratory is to develop an arsenal of therapeutic agents that target different receptors involved in cancer. Most importantly, we plan on using these agents in a combination approach that will prevent emergence of resistance to treatment.
Although development of mAbs and TKIs has improved cancer treatment significantly, the reality is that the “War on Cancer” is still ongoing. Emergence of resistance to targeted therapies is a problem frequently faced in the clinic, and cancer death rates in the U.S. still remain alarmingly high; especially in patients diagnosed with advanced tumor stages III and IV (Siegel et al., 2012a; 2012b). Several molecular mechanisms of acquired resistance to these agents have now been documented and suggest that most tumors have multiple molecular drivers. The known mechanisms of resistance include: acquisition of secondary mutations or amplification of the drug target itself, expression of cell membrane transporters that alter drug efflux and activation of alternative or complementary signaling pathways, often via molecular feedback loops and cross-talk (Engelman and Settleman, 2008; Turner and Reis-Filho, 2012). In addition, recent tumor genome sequencing studies have revealed that cancer is characterized by genetic mutation profiles that differ between cancer types, tumors of the same cancer lineage, and even cancer cells in the same tumor. This suggests that the heterogeneous nature of cancer renders it unlikely to be amenable to universal cures with single agent therapy alone (Arteaga and Baselga, 2012; Sandmann and Boutros, 2012; Ziogas et al., 2012). Overall, the future success of targeted therapeutics will ultimately be dependent on overcoming these molecular mechanisms of drug resistance and tumor heterogenicity. Promising and new alternative strategies taken to accomplish this include combination therapy and development of multi-target inhibitors. Increasingly, therapies are being designed to target multiple kinase pathways. This can be achieved by using a single agent that inhibits multiple signaling pathways (multi-target inhibitors) or a combination of highly selective agents. Advantages of a multi-targeted approach include: the potential for increased efficacy and reduced resistance by simultaneous inhibition of multiple pathways and common escape pathways (Gossage and Eisen, 2010).
Peptide Combination Approaches
Recent studies have shown that extensive cross-talk exists between the HER family and other receptors, like IGF-IR and VEGFR. We hypothesize that targeting more than one of these receptors at the same time will prevent cross-talk and decrease the possibility of drug resistance due to activation of an alternative signaling pathway. Recent studies have shown that resistance to trastuzumab is mediated by increased signaling through IGF-IR, while cetuximab resistance is attributed to HER-3 up-regulation in the presence of EGFR inhibition (Haluska et al., 2008; Lu et al., 2004; Wheeler et al., 2008). VEGF is also well known to exist as a downstream mediator in HER family, and combining EGFR and VEGF inhibitors has been shown to produce maximal effects (Jain, 2005). We have made significant efforts in validating our combination approach by targeting HER2 and VEGF in transgenic and transplantable breast cancer mouse models. Studies from our laboratory have shown that a combined approach produces superior antitumor effects in vitro and in vivo when compared to single treatments (Foy et al., 2011). Immunization with our novel HER2 epitope vaccine, followed by VEGFR peptide mimic treatment, increased survival and significantly delayed tumor growth in mouse models of cancer (Foy et al., 2012b). We also evaluated a multi-modality approach in which low doses of chemotherapy (paclitaxel) combined with HER2 or VEGF peptide mimics improved response outcomes and minimized toxicity (Foy et al., 2012a). Taken together, our results show that shutting down two or more cancer dependent pathways at the same time is a better approach than mono-specific treatment alone.
The present cancer situation clearly indicates that better treatment options that selectively target cancer dependent pathways with little or no toxicity to normal tissues are urgently warranted. Novel and cheaper alternatives to current cancer therapeutics, including vaccine candidates that can train the human immune system to fight deadly forms of cancer, offer hope for cancer patients in winning the war against cancer. Also, a combination approach that will completely shut down cancer dependent pathways and other alternative pathways in cancer will prevent acquired resistance and prevent patient relapse. Efforts in our laboratory put these key aspects in the forefront with the use of peptides as therapeutic and vaccine candidates. We intend to establish a multi-targeted approach that will avoid cross-talk between HER1, HER2, HER3, HER4, IGF-IR, and VEGF. As we learn more about the complexities of new signaling pathways, we can incorporate additional peptide therapeutics into our combination strategy. The approach will depend on the type of cancer and the receptors that are involved in tumor progression. For example, a HER2 positive breast cancer patient with moderate to low levels of HER3 can be treated with both HER2 and HER3 targeted peptides, as opposed to targeting HER2 alone. Another strategy used is to co-immunize the patient with HER2 and HER3 vaccine epitopes to enable the patient to produce HER2 and HER3 specific antibodies that can completely neutralize the cancer cells. Both of these strategies are personalized combination approaches that will target HER2 and shut down the HER3 pathway that may lead to cancer relapse.
Today, the ultimate goal of cancer therapy is to effectively destroy tumor cells without eliciting harmful effects in normal cells or tissues. Targeted therapies against cancer have been maturing over the last several decades, and they have ultimately demonstrated improved efficacy over conventional chemotherapeutics. Unfortunately, there are still many problems associated with current therapeutics, and the “War on Cancer” is far from over. Major challenges still exist with the currently marketed inhibitors, and patient relapse due to acquired drug resistance occurs frequently. Today, there is a dire need for novel cancer therapeutics and combination strategies. The future success of cancer treatment will rely on whether or not these alternative strategies can efficiently overcome current treatment limitations.
The authors report no conflicts of interest.
Pravin T.P. Kaumaya, Ph.D., Department of Obstetrics & Gynecology, OSU Wexner Medical Center, James Cancer Hospital & Solove Research Institute & the Comprehensive Cancer Center, The Ohio State University, Columbus, Ohio 43210, USA.
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