Article Published in the Author Account of

Megan Jo Miller

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.



Introduction

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.

Monoclonal Antibodies

Figure 1.

Figure 1. History of targeted therapeutics against cancer.

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).

Peptide Immunotherapy

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.

Combination Immunotherapy

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.

Future Perspectives

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.

Conclusions

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.

Disclosure

The authors report no conflicts of interest.

Corresponding Author

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.

References

Ahrens VM, Bellmann-Sickert K, Beck-Sickinger AG. Peptides and peptide conjugates: therapeutics on the upward path. Future Med Chem 4(12):1567-1586, 2012.

Allen SD, Garrett JT, Rawale SV, Jones AL, Phillips G, Forni G, Morris JC, Oshima RG, Kaumaya PT. Peptide vaccines of the HER-2/neu dimerization loop are effective in inhibiting mammary tumor growth in vivo. J Immunol 179(1):472-482, 2007.

Allen SD, Rawale SV, Whitacre CC, Kaumaya PT. Therapeutic peptidomimetic strategies for autoimmune diseases: costimulation blockade. J Pept Res 65(6):591-604, 2005.

Arora A, Scholar EM. Role of tyrosine kinase inhibitors in cancer therapy. J Pharmacol Exp Ther 315(3):971-979, 2005.

Arteaga CL, Baselga J. Impact of genomics on personalized cancer medicine. Clin Cancer Res 18(3):612-618, 2012.

Beckman RA, Weiner LM, Davis HM. Antibody constructs in cancer therapy: protein engineering strategies to improve exposure in solid tumors. Cancer 109(2):170-179, 2007.

Berchuck A, Rodriguez G, Kinney RB, Soper JT, Dodge RK, Clarke-Pearson DL, Bast RC, Jr. Overexpression of HER-2/neu in endometrial cancer is associated with advanced stage disease. Am J Obstet Gynecol 164(1 Pt 1):15-21, 1991.

Blume-Jensen P, Hunter T. Oncogenic kinase signalling. Nature 411(6835):355-365, 2001.

Boohaker RJ, Lee MW, Vishnubhotla P, Perez JM, Khaled AR. The use of therapeutic peptides to target and to kill cancer cells. Curr Med Chem 19(22):3794-3804, 2012.

Brown D, Superti-Furga G. Rediscovering the sweet spot in drug discovery. Drug Discov Today 8(23):1067-1077, 2003.

Butowski N, Chang SM. Small molecule and monoclonal antibody therapies in neurooncology. Cancer Control 12(2):116-124, 2005.

Chames P, Van Regenmortel M, Weiss E, Baty D. Therapeutic antibodies: successes, limitations and hopes for the future. Br J Pharmacol 157(2):220-233, 2009.

Cheng AC, Coleman RG, Smyth KT, Cao Q, Soulard P, Caffrey DR, Salzberg AC, Huang ES. Structure-based maximal affinity model predicts small-molecule druggability. Nat Biotechnol 25(1):71-75, 2007.

Cheng H, Force T. Why do kinase inhibitors cause cardiotoxicity and what can be done about it? Prog Cardiovasc Dis 53(2):114-120, 2010.

Chin L, Gray JW. Translating insights from the cancer genome into clinical practice. Nature 452(7187):553-563, 2008.

Cho HS, Mason K, Ramyar KX, Stanley AM, Gabelli SB, Denney DW, Jr, Leahy DJ. Structure of the extracellular region of HER2 alone and in complex with the Herceptin Fab. Nature 421(6924):756-760, 2003.

Cirisano FD, Karlan BY. The role of the HER-2/neu oncogene in gynecologic cancers. J Soc Gynecol Investig 3(3):99-105, 1996.

Clement S, Still JG, Kosutic G, Mcallister RG. Oral insulin product hexyl-insulin monoconjugate 2 (HIM2) in type 1 diabetes mellitus: the glucose stabilization effects of HIM2. Diabetes Technol Ther 4(4):459-466, 2002.

Cools J, Maertens C, Marynen P. Resistance to tyrosine kinase inhibitors: calling on extra forces. Drug Resist Updat 8(3):119-129, 2005.

Dakappagari NK, Douglas DB, Triozzi PL, Stevens VC, Kaumaya PT. Prevention of mammary tumors with a chimeric HER-2 B-cell epitope peptide vaccine. Cancer Res 60(14):3782-3789, 2000.

Dakappagari NK, Lute KD, Rawale S, Steele JT, Allen SD, Phillips G, Reilly RT, Kaumaya PT. Conformational HER-2/neu B-cell epitope peptide vaccine designed to incorporate two native disulfide bonds enhances tumor cell binding and antitumor activities. J Biol Chem 280(1):54-63, 2005a.

Dakappagari NK, Pyles J, Parihar R, Carson WE, Young DC, Kaumaya PT. A chimeric multi-human epidermal growth factor receptor-2 B cell epitope peptide vaccine mediates superior antitumor responses. J Immunol 170(8):4242-4253, 2003.

Dakappagari NK, Sundaram R, Rawale S, Liner A, Galloway DR, Kaumaya PT. Intracellular delivery of a novel multiepitope peptide vaccine by an amphipathic peptide carrier enhances cytotoxic T-cell responses in HLA-A*201 mice. J Pept Res 65(2):189-199, 2005b.

Dancey J, Sausville EA. Issues and progress with protein kinase inhibitors for cancer treatment. Nat Rev Drug Discov 2(4):296-313, 2003.

De Kogel CE, Schellens JH. Imatinib. Oncologist 12(12):1390-1394, 2007.

Dennis MS, Zhang M, Meng YG, Kadkhodayan M, Kirchhofer D, Combs D, Damico LA. Albumin binding as a general strategy for improving the pharmacokinetics of proteins. J Biol Chem 277(38):35035-35043, 2002.

Druker BJ. STI571 (Gleevec) as a paradigm for cancer therapy. Trends Mol Med 8(4 Suppl):S14-S18, 2002.

Druker BJ, Tamura S, Buchdunger E, Ohno S, Segal GM, Fanning S, Zimmermann J, Lydon NB. Effects of a selective inhibitor of the Abl tyrosine kinase on the growth of Bcr-Abl positive cells. Nat Med 2(5):561-566, 1996.

Engelman JA, Settleman J. Acquired resistance to tyrosine kinase inhibitors during cancer therapy. Curr Opin Genet Dev 18(1):73-79, 2008.

Faderl S, Talpaz M, Estrov Z, O’brien S, Kurzrock R, Kantarjian HM. The biology of chronic myeloid leukemia. N Engl J Med 341(3):164-172, 1999.

Foy KC, Liu Z, Phillips G, Miller M, Kaumaya PT. Combination treatment with HER-2 and VEGF peptide mimics induces potent anti-tumor and anti-angiogenic responses in vitro and in vivo. J Biol Chem 286(15):13626-13637, 2011.

Foy KC, Miller MJ, Moldovan N, Bozanovic T, Carson WE III, Kaumaya PT. Immunotherapy with HER-2 and VEGF peptide mimics plus metronomic paclitaxel causes superior antineoplastic effects in transplantable and transgenic mouse models of human breast cancer. Oncoimmunology 1(7):1004-1016, 2012a.

Foy KC, Miller MJ, Moldovan N, Carson Iii WE, Kaumaya PT. Combined vaccination with HER-2 peptide followed by therapy with VEGF peptide mimics exerts effective anti-tumor and anti-angiogenic effects in vitro and in vivo. Oncoimmunology 1(7):1048-1060, 2012b.

Franklin MC, Carey KD, Vajdos FF, Leahy DJ, De Vos AM, Sliwkowski MX. Insights into ErbB signaling from the structure of the ErbB2-pertuzumab complex. Cancer Cell 5(4):317-328, 2004.

Galmarini D, Galmarini CM, Galmarini FC. Cancer chemotherapy: a critical analysis of its 60 years of history. Crit Rev Oncol Hematol 84(2):181-199, 2012.

Garrett JT, Rawale S, Allen SD, Phillips G, Forni G, Morris JC, Kaumaya PT. Novel engineered trastuzumab conformational epitopes demonstrate in vitro and in vivo antitumor properties against HER-2/neu. J Immunol 178(11):7120-7131, 2007.

Gossage L, Eisen T. Targeting multiple kinase pathways: a change in paradigm. Clin Cancer Res 16(7):1973-1978, 2010.

Haber DA, Gray NS, Baselga J. The evolving war on cancer. Cell 145(1):19-24, 2011.

Haluska P, Carboni JM, Teneyck C, Attar RM, Hou X, Yu C, Sagar M, Wong TW, Gottardis MM, Erlichman C. HER receptor signaling confers resistance to the insulin-like growth factor-I receptor inhibitor, BMS-536924. Mol Cancer Ther 7(9):2589-2598, 2008.

Hansel TT, Kropshofer H, Singer T, Mitchell JA, George AJ. The safety and side effects of monoclonal antibodies. Nat Rev Drug Discov 9(4):325-338, 2010.

Haubner R, Wester HJ, Burkhart F, Senekowitsch-Schmidtke R, Weber W, Goodman SL, Kessler H, Schwaiger M. Glycosylated RGD-containing peptides: tracer for tumor targeting and angiogenesis imaging with improved biokinetics. J Nucl Med 42(2):326-336, 2001.

Holmes DL, Thibaudeau K, L’archeveque B, Milner PG, Ezrin AM, Bridon DP. Site specific 1:1 opioid:albumin conjugate with in vitro activity and long in vivo duration. Bioconjug Chem 11(4):439-444, 2000.

Huang S, Armstrong EA, Benavente S, Chinnaiyan P, Harari PM. Dual-agent molecular targeting of the epidermal growth factor receptor (EGFR): combining anti-EGFR antibody with tyrosine kinase inhibitor. Cancer Res 64(15):5355-5362, 2004.

Imai K, Takaoka A. Comparing antibody and small-molecule therapies for cancer. Nat Rev Cancer 6(9):714-727, 2006.

Jain RK. Normalization of tumor vasculature: an emerging concept in antiangiogenic therapy. Science 307(5706):58-62, 2005.

Kaumaya P. “De novo” engineering of peptide immunogenic and antigenic determinants as potential vaccines. Peptides: Design, Synthesis, and Biological Activity. pp133-164. Birkhauser, Boston, Massachusetts, USA, 1994.

Kaumaya PT, Foy KC, Garrett J, Rawale SV, Vicari D, Thurmond JM, Lamb T, Mani A, Kane Y, Balint CR, Chalupa D, Otterson GA, Shapiro CL, Fowler JM, Grever MR, Bekaii-Saab TS, Carson WE, 3rd. Phase I active immunotherapy with combination of two chimeric, human epidermal growth factor receptor 2, B-cell epitopes fused to a promiscuous T-cell epitope in patients with metastatic and/or recurrent solid tumors. J Clin Oncol 27(31):5270-5277, 2009.

Kern JA, Schwartz DA, Nordberg JE, Weiner DB, Greene MI, Torney L, Robinson RA. p185neu expression in human lung adenocarcinomas predicts shortened survival. Cancer Res 50(16):5184-5187, 1990.

Koehler MF, Zobel K, Beresini MH, Caris LD, Combs D, Paasch BD, Lazarus RA. Albumin affinity tags increase peptide half-life in vivo. Bioorg Med Chem Lett 12(20):2883-2886, 2002.

Kohler G, Milstein C. Continuous cultures of fused cells secreting antibody of predefined specificity. Nature 256(5517):495-497, 1975.

Koren E, Apte A, Jani A, Torchilin VP. Multifunctional PEGylated 2C5-immunoliposomes containing pH-sensitive bonds and TAT peptide for enhanced tumor cell internalization and cytotoxicity. J Control Release 160(2):264-273, 2012.

Kubota T, Niwa R, Satoh M, Akinaga S, Shitara K, Hanai N. Engineered therapeutic antibodies with improved effector functions. Cancer Sci 100(9):1566-1572, 2009.

Latham PW. Therapeutic peptides revisited. Nat Biotechnol 17(8):755-757, 1999.

Lien S, Lowman HB. Therapeutic peptides. Trends Biotechnol 21(12):556-562, 2003.

London CA. Kinase dysfunction and kinase inhibitors. Vet Dermatol 24(1):181-e140, 2013.

Lu Y, Zi X, Pollak M. Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells. Int J Cancer 108(3):334-341, 2004.

Mach JP. Introduction to monoclonal antibodies. Cancer Immun 12:11, 2012.

Manning G, Whyte DB, Martinez R, Hunter T, Sudarsanam S. The protein kinase complement of the human genome. Science 298(5600):1912-1934, 2002.

Melo JV, Hughes TP, Apperley JF. Chronic myeloid leukemia. Hematology Am Soc Hematol Educ Program 2003:132-152, 2003.

Morrison C, Zanagnolo V, Ramirez N, Cohn DE, Kelbick N, Copeland L, Maxwell GL, Fowler JM. HER-2 is an independent prognostic factor in endometrial cancer: association with outcome in a large cohort of surgically staged patients. J Clin Oncol 24(15):2376-2385, 2006.

Oldham RK, Dillman RO. Monoclonal antibodies in cancer therapy: 25 years of progress. J Clin Oncol 26(11):1774-1777, 2008.

Overington JP, Al-Lazikani B, Hopkins AL. How many drug targets are there? Nat Rev Drug Discov 5(12):993-996, 2006.

Pawson T. Regulation and targets of receptor tyrosine kinases. Eur J Cancer 38(Suppl 5):S3-S10, 2002.

Pescovitz MD. Rituximab, an anti-cd20 monoclonal antibody: history and mechanism of action. Am J Transplant 6(5 Pt 1):859-866, 2006.

Qiu XQ, Wang H, Cai B, Wang LL, Yue ST. Small antibody mimetics comprising two complementarity-determining regions and a framework region for tumor targeting. Nat Biotechnol 25(8):921-929, 2007.

Reichert J. Development Trends for Peptide Therapeutics. Tufts Center for the Study of Drug Development, Tufts University, Medford, Massachusetts, USA, 2008.

Sandmann T, Boutros M. Screens, maps & networks: from genome sequences to personalized medicine. Curr Opin Genet Dev 22(1):36-44, 2012.

Schlessinger J. Cell signaling by receptor tyrosine kinases. Cell 103(2):211-225, 2000.

Scott AM, Wolchok JD, Old LJ. Antibody therapy of cancer. Nat Rev Cancer 12(4):278-287, 2012.

Shawver LK, Slamon D, Ullrich A. Smart drugs: tyrosine kinase inhibitors in cancer therapy. Cancer Cell 1(2):117-123, 2002.

Shuptrine CW, Surana R, Weiner LM. Monoclonal antibodies for the treatment of cancer. Semin Cancer Biol 22(1):3-13, 2012.

Siegel R, Desantis C, Virgo K, Stein K, Mariotto A, Smith T, Cooper D, Gansler T, Lerro C, Fedewa S, Lin C, Leach C, Cannady RS, Cho H, Scoppa S, Hachey M, Kirch R, Jemal A, Ward E. Cancer treatment and survivorship statistics, 2012. CA Cancer J Clin 62(4):220-241, 2012a.

Siegel R, Naishadham D, Jemal A. Cancer statistics, 2012. CA Cancer J Clin 62(1):10-29, 2012b.

Sierra JR, Cepero V, Giordano S. Molecular mechanisms of acquired resistance to tyrosine kinase targeted therapy. Mol Cancer 9:75, 2010.

Srinivasan M, Gienapp IE, Stuckman SS, Rogers CJ, Jewell SD, Kaumaya PT, Whitacre CC. Suppression of experimental autoimmune encephalomyelitis using peptide mimics of CD28. J Immunol 169(4):2180-2188, 2002.

Srinivasan M, Wardrop RM, Gienapp IE, Stuckman SS, Whitacre CC, Kaumaya PT. A retro-inverso peptide mimic of CD28 encompassing the MYPPPY motif adopts a polyproline type II helix and inhibits encephalitogenic T cells in vitro. J Immunol 167(1):578-585, 2001.

Stegmeier F, Warmuth M, Sellers WR, Dorsch M. Targeted cancer therapies in the twenty-first century: lessons from imatinib. Clin Pharmacol Ther 87(5):543-552, 2010.

Strebhardt K, Ullrich A. Paul Ehrlich’s magic bullet concept: 100 years of progress. Nat Rev Cancer 8(6):473-480, 2008.

Svensen N, Walton JG, Bradley M. Peptides for cell-selective drug delivery. Trends Pharmacol Sci 33(4):186-192, 2012.

Turner NC, Reis-Filho JS. Genetic heterogeneity and cancer drug resistance. Lancet Oncol 13(4):e178-185, 2012.

Van Etten RA. Mechanisms of transformation by the BCR-ABL oncogene: new perspectives in the post-imatinib era. Leuk Res 28(Suppl 1):S21-S28, 2004.

Vanneman M, Dranoff G. Combining immunotherapy and targeted therapies in cancer treatment. Nat Rev Cancer 12(4):237-251, 2012.

Verdine GL, Walensky LD. The challenge of drugging undruggable targets in cancer: lessons learned from targeting BCL-2 family members. Clin Cancer Res 13(24):7264-7270, 2007.

Vicari D, Foy KC, Liotta EM, Kaumaya PT. Engineered conformation-dependent VEGF peptide mimics are effective in inhibiting VEGF signaling pathways. J Biol Chem 286(15):13612-13625, 2011.

Vlieghe P, Lisowski V, Martinez J, Khrestchatisky M. Synthetic therapeutic peptides: science and market. Drug Discov Today 15(1-2):40-56, 2010.

Von Mehren M, Adams GP, Weiner LM. Monoclonal antibody therapy for cancer. Annu Rev Med 54:343-369, 2003.

Weiner LM, Murray JC, Shuptrine CW. Antibody-based immunotherapy of cancer. Cell 148(6):1081-1084, 2012.

Weiner LM, Surana R, Wang S. Monoclonal antibodies: versatile platforms for cancer immunotherapy. Nat Rev Immunol 10(5):317-327, 2010.

Wheeler DL, Huang S, Kruser TJ, Nechrebecki MM, Armstrong EA, Benavente S, Gondi V, Hsu KT, Harari PM. Mechanisms of acquired resistance to cetuximab: role of HER (ErbB) family members. Oncogene 27(28):3944-3956, 2008.

Yano T, Doi T, Ohtsu A, Boku N, Hashizume K, Nakanishi M, Ochiai A. Comparison of HER2 gene amplification assessed by fluorescence in situ hybridization and HER2 protein expression assessed by immunohistochemistry in gastric cancer. Oncol Rep 15(1):65-71, 2006.

Ziogas DE, Katsios CS, Tzaphlidou M, Roukos DH. Targeted therapy: overcoming drug resistance with clinical cancer genome. Expert Rev Anticancer Ther 12(7):861-864, 2012.

[Discovery Medicine; ISSN: 1539-6509; Discov Med 15(82):166-176, March 2013. Copyright © Discovery Medicine. All rights reserved.]

Access This PDF as a Subscriber |
Close
Close
E-mail It
Close