Emerging Therapeutic Targets for Pancreatic Cancer
Pancreatic cancer remains a leading cause of cancer mortality in the United States, however a number of novel molecular targets have emerged that offer promising new therapeutic options for this deadly disease. For instance, inhibition of various transcription factors has demonstrated therapeutic potential in preclinical studies. The transcription factors discussed include nuclear factor kappa B, transforming growth factor β, specificity protein 1, and Gli1. Another important area of research involves preventing the phenotypic switch known as the epithelial-to-mesenchymal transition (EMT) that affects the invasive potential of cancer cells. Targeting the hepatocyte growth factor and its receptor c-Met may reverse EMT and also results in decreased pancreatic cancer growth and may provide a means to reverse chemoresistance to gemcitabine. Furthermore, the DNA repair pathway involves various genes including BRCA2 that are involved in homologous recombination, and CHEK-1/2, a cell cycle checkpoint kinase, both of which provide opportunities for individualizing pancreatic cancer treatment in patients with alterations in these pathways. Preclinical data have shown that BRCA2 mutants are more sensitive to traditional cytotoxic agents, including cisplatin, while specific polymorphisms in CHEK-1, as an example, strengthen gemcitabine’s utility as a radiosensitizer in patients with locally-advanced pancreatic cancer. Also of interest, the proteasome inhibitor bortezomib causes endoplasmic reticulum stress in vitro and in vivo, thereby activating apoptosis in pancreatic cancer cells. Finally, advances in gene therapy will be discussed as this provides a mechanism for targeting pancreatic cancer cells with potentially few side effects.
KeywordsVascular Endothelial Growth Factor Pancreatic Cancer Endoplasmic Reticulum Stress Hepatocyte Growth Factor Pancreatic Cancer Cell
54.1 1 Introduction
Pancreatic cancer remains a leading cause of cancer mortality in the United States with gemcitabine chemotherapy being the mainstay of treatment for over a decade. Since its approval in 1996, a number of gemcitabine-based cytotoxic combinations have been compared to gemcitabine alone without a significant improvement in survival. Targeted therapy has since emerged as a potential therapeutic option with erlotinib, an inhibitor of the epidermal growth factor receptor (EGFR) demonstrating a modest survival benefit when added to gemcitabine . Since that time, a number of other agents aimed at molecular targets have been studied without a significant improvement in outcome. Some of these targets include vascular endothelial growth factor (VEGF) inhibitors, Ras-farnesyltransferase inhibitors, matrix metalloproteinase inhibitors, and other EGFR inhibitors [2, 3, 4, 5].
More recently, novel targets involving transcription factors, DNA repair pathways, and the microenvironment, among others, have drawn attention because they represent new frontiers for targeted therapy. This chapter will highlight some of the newer concepts in molecularly targeted agents that move beyond the previously studied therapies and could represent a new era of personalized medicine in pancreatic cancer.
54.2 2 Novel Signal Transduction Pathways and Transcription Factors
54.2.1 2.1 Nuclear Factor-Kappa B
Early on, interest in NFκB’s role in pancreatic cancer rose when it became clear that this pathway was one mechanism for chemoresistance in pancreatic cancer. Studies demonstrated that gemcitabine-resistant pancreatic cancer cell lines have a higher level of basal NFκB activity . Furthermore, inhibition of NFκB with sulfasalazine sensitized these cell lines to gemcitabine therapy. Another inhibitor of NFκB, soy isoflavone genistein has demonstrated antitumor activity in vitro and in vivo in part through its inactivation of NFκB. Pretreatment of pancreatic cancer cell lines with genistein followed by administration of docetaxel and cisplatin led to inhibition of cell growth that was more pronounced than with the use of either chemotherapeutic alone, further supporting the role of NFκB in chemoresistance .
Aspirin inhibits NFκB activation and thus has been investigated in pancreatic cancer as well. Sclabas and colleagues created an orthotopic mouse model using human pancreatic cancer cell lines and demonstrated dose-dependent abrogation of NFκB with aspirin administration . Additionally, aspirin was able to prevent tumorigenesis in a prophylactic setting rather than in a therapeutic one, suggesting that aspirin may have a chemopreventive role in pancreatic cancer through NFκB inhibition. Another inhibitor, FUT-175, a synthetic serine protease that has anti-inflammatory properties, was shown to inhibit constitutive NFκB activation and subsequently enhance caspase-8-mediated apoptosis in pancreatic cancer cells .
54.2.2 2.2 Specificity Protein 1
Angiogenesis plays an important role in pancreatic cancer as VEGF is overexpressed in most pancreatic tumors. This overexpression directly correlates with blood vessel density and disease progression . While direct targeting of VEGF with bevacizumab has not demonstrated an improvement in outcome, it is now clear that regulation of proangiogenic factors occurs at the gene expression level via transcriptional, translational, and posttranslational modifications. Specificity protein 1 (Sp1) is a zinc finger transcription factor and is part of a multigene family that to appears to play a pivotal role in the expression of VEGF and other angiogenic molecules. Sp1 is a DNA binding protein that binds to a number of genes containing GC boxes in their promoters, including VEGF, EGFR and insulin-like growth factor-1 receptor (IGF-1R). Its role in angiogenesis was surmised when it was noted to be constitutively overexpressed in pancreatic cancer along with VEGF . Additionally, tumor suppressors including p53 bind to Sp1 and prevent the physical interaction between Sp1 and the promoter of VEGF, thereby inhibiting expression .
It was previously shown that the COX-2 inhibitor celecoxib degrades Sp1 in pancreatic cancer cells and thus decreases VEGF expression . Subsequently, another nonsteroidal anti-inflammatory drug, tolfenamic acid was shown to interfere with VEGF expression in pancreatic tumors through induction of Sp protein degradation. The same study showed that tolfenamic acid abrogated pancreatic tumor growth and decreased liver metastases in an orthotopic mouse model . These effects on pancreatic cancer growth and metastasis were modulated via proteasome-mediated degradation of Sp proteins and subsequent loss of VEGF expression.
The precise role of Sp1 in angiogenesis and pancreatic cancer was further delineated when human pancreatic cells demonstrated decreased tumor growth in vivo when Sp1 expression was knocked down . Importantly, the cells transfected with Sp1 small interfering RNA (siRNA) were less vascularized than controls, correlating angiogenesis with Sp1 expression. Moreover, mithramycin A, a potent inhibitor of Sp1, demonstrated suppression of pancreatic tumor growth in xenograft models via decreased expression of Sp1’s downstream angiogenic mediators, illustrating a potential role for targeted inhibition of Sp1 in pancreatic cancer.
The role of Sp1 in angiogenesis has become more clearly defined, however the interplay of angiogenesis and pancreatic tumorigenesis is quite complex, with multiple levels of involvement. For instance, VEGF receptor 1 (VEGFR1) is one of three VEGF receptors that is directly involved in angiogenesis. In pancreatic cancer cells, VEGFR1 activations leads to the phenotypic switch known as the epithelial to mesenchymal transition (EMT) that is associated with increased invasiveness of pancreatic cancer. The VEGFR1 promoter contains multiple GC boxes for Sp1 binding and it has been demonstrated that tolfenamic acid not only caused Sp protein degradation, but it also inhibited VEGFR1 expression . Similarly, tolfenamic acid prevented pancreatic cell migration by likely affecting the epithelial to mesenchymal transition in these VEGFR1-negative cells.
54.2.3 2.3 Transforming Growth Factor Beta (TGF-β)
Specifically in pancreatic cancer, increased levels of TGF-β have been associated with venous invasion, advanced stage, progressive disease, poor survival, and liver metastases [23, 24, 25, 26]. In addition, TGF-β mediates the progression of fibroblasts into myofibroblasts, which are thought to play a role in the desmoplastic reaction that is typically seen in pancreatic cancer. These myofibroblasts promote tumorigenesis via elaboration of growth factors that increase the metastatic potential of cells [27, 28].
The TGF-β pathway has been implicated in tumorigenesis through its ability to activate EMT and increasing tumor metastatic potential . Because pancreatic cancer is known to be a highly aggressive and invasive disease, it is hypothesized that inhibiting the TGF-β cascade may prevent tumor growth and metastases. In fact, this has been shown through in vitro and in vivo studies using a TβRI/II inhibitor. A dual TGF-β receptor I/II (TβRI/II) kinase selective inhibitor LY2109761 was recently investigated to determine the effect of a TGF-β inhibitor (TβI) on the growth, motility, and invasiveness of pancreatic cancer cells in vitro and metastasis development in vivo . This oral TβI inhibitor, LY2109761, significantly inhibited the growth of a human pancreatic cancer cell line while also reducing cell migration and invasion. In vivo, when combined with gemcitabine, LY2109761 also significantly decreased tumor volume, prolonged survival, and decreased abdominal and liver metastases. Importantly, the results seen were superior when gemcitabine was combined with TGF-β blockade than with either agent alone. Based on these findings, targeting TβRI/II kinase activity on pancreatic cancer cells or the cells of the liver microenvironment could provide a novel therapeutic approach to prevent metastatic disease in pancreatic cancer.
54.2.4 2.4 Hedgehog and GLI Signaling
An oral, small molecule inhibitor of Hedgehog signaling, IPI-269609 was tested in vitro on twenty-one different pancreatic cancer cell lines with a spectrum of growth inhibition seen . Further studies on two specific cell lines demonstrated decreased GLI mRNA expression, diminished cell migration, and reduced colony formation in vitro. The clonogenic potential of one cell line was decreased after incubation with this small molecule inhibitor as well. In addition, in vivo studies in xenograft models exhibited some inhibition of tumor growth after treatment with the Hedgehog inhibitor alone, but a statistically significant decrease in tumor volume with the combination of the inhibitor and gemcitabine. Importantly, distant metastases were not seen in any of the xenograft models treated with IPI-269609 and pretreatment ex vivo with this inhibitor eliminated a tumorigenic subpopulation of cells such that xenograft engraftment was abrogated.
54.2.5 2.5 The Epithelial-Mesenchymal Transition (EMT)
EMT is an integral part of embryological development, but it is also known to be involved in tumor progression and metastasis . Specifically, EMT involves a phenotypic switch from epithelial cells to mesenchymal cells resembling fibroblasts. This switch results in a loss of cell-cell adhesions, decreased polarity and increased cell motility. In tumors undergoing EMT, a loss of E-cadherin expression is noted, a protein integral to maintaining sheets of epithelial cells via adherens junctions. Decreased E-cadherin expression has been demonstrated in pancreatic cancer .
Transcriptional regulation of E-cadherin expression is mediated by a number of proteins, including Snail, Slug, ZEB-1, ZEB-2, and Twist . Immunohistochemical staining of pancreatic cancer tissue has demonstrated Snail and Slug staining with low E-cadherin expression in poorly differentiated adenocarcinoma, suggesting activation of the EMT phenotype . This pattern of expression was confirmed in human pancreatic cancer cell lines as well. Furthermore, the metastatic potential of these cells was directly correlated with Snail expression and inversely correlated with E-cadherin expression.
Activators of EMT include TGF-β, tumor necrosis factor α (TNF α), and hepatocyte growth factor, all of which increase expression of the transcriptional repressors of E-cadherin. ZEB-1 expression is upregulated by both TGF-β and TNF α and its abrogation prevents EMT. MicroRNAs are small silencing RNAs with a number of functions, including promotion of epithelial differentiation. ZEB-1 can directly suppress transcription of specific microRNA families thus promoting EMT . These findings all suggest ZEB-1 as a putative target for anticancer therapy.
Aberrant activation of the Hedgehog (Hh) signaling pathway is seen in pancreatic cancer, with loss of E-cadherin expression promoting EMT . Inhibition of tumor growth in human pancreatic cancer cell lines was seen with the use of cyclopamine, a known Hh inhibitor . Concordantly, these cell lines demonstrated decreased expression of Snail mRNA and increased expression of E-cadherin, implying stabilization of the epithelial phenotype. Cyclopamine also decreased the migratory potential and number of metastases in vitro and in vivo.
54.3 3 Hepatocyte Growth Factor (HGF) and c-Met
The knowledge of c-Met overexpression in pancreatic cancer has led to the development of inhibitors of this pathway as potential therapeutic options. Early research centered on NK4, an HGF antagonist. This inhibitor blocked cell migration and invasion in human pancreatic cell lines . Furthermore, combining NK4 with gemcitabine prevented pancreatic tumor growth and metastasis in xenograft models . Another potential target in the HGF/c-Met pathway that has been identified is HGF activator (HGFA), a serine protease produced mostly in the liver . This enzyme plays a pivotal role in the transformation from pro-HGF to the active form of HGF. Studies have demonstrated that HGFA expression and c-Met phosphorylation is increased in hypoxic conditions in pancreatic cancer cell lines . Additionally, HGFA siRNA significantly decreased the invasiveness of these cells. Most recently, a monovalent monoclonal antibody directed against c-Met (MetMAb) has been developed and had shown promising activity in a glioblastoma tumor model . This antibody blocked proliferation of two human pancreatic cancer cell lines with an associated decrease in c-Met phosphorylation . Tumor xenograft growth was also suppressed with the use of this antibody and orthotopic modeling demonstrated the efficacy of MetMAb in pancreatic cancer as well. Importantly, the suppression of tumor growth in these orthotopic models translated into a significant survival benefit, further illustrating the potential role for anti-c-Met directed therapy.
Beyond its direct effects on pancreatic tumor growth, HGF is thought to induce EMT by causing separation and scatter of cancer cells. Moreover, EMT may contribute to the continued problem of chemoresistance in cancer cells, including pancreatic cancer . Gemcitabine resistant pancreatic cancer cell lines have been established that demonstrate morphologic changes consistent with an EMT phenotype. These cells have increased migratory and invasive potential and express markers typical of EMT. Interestingly, c-Met was constitutively activated in these cells, indicating a role for HGF/c-Met in EMT and gemcitabine resistance . Targeting this signaling pathway could provide a mechanism for sidestepping chemoresistance, a common problem in pancreatic cancer.
54.4 4 DNA Repair Targets
54.4.1 4.1 BRCA2
Pancreatic cancer is often characterized by a number of genetic alterations including K-ras mutations in as many as 90% of tumors. Approximately 7–10% of “sporadic” pancreatic tumors have an inherited intragenic, inactivating mutation of the breast cancer susceptibility gene, BRCA2 located on chromosome 13 . BRCA2 mediates multiple biologic functions including transcriptional regulation, cell cycle progression, maintaining genomic integrity and mediating error-free repair of DNA double-strand breaks (DSBs) via homologous recombination, a process integral to cell proliferation and viability . Typically, patients with intact BRCA2 are able to repair DSBs caused by cytotoxic agents and prevent the stalling or collapse of replication forks.
Patients with BRCA2 mutations have decreased homologous recombination leading to the activation of alternative repair pathways to compensate for the primary repair defect. These error-prone pathways include non-homologous end joining and single strand annealing and lead to the accumulation of more mutations and promote tumor growth and survival. Because of defective homologous recombination, it would stand to reason that patients with BRCA2-driven tumors are more susceptible to chemotherapy causing DSBs .
Human pancreatic adenocarcinoma cell lines expressing a defective or truncated BRCA2 protein have been shown to be hypersensitive to DNA damaging agents, including ionizing radiation and cisplatin, and exhibit a slower rate of DSB repair when compared to cells with wild-type BRCA2 [55, 56, 57]. The rationale behind this sensitivity to cisplatin stems from BRCA2 mutants having defective homologous recombination with subsequent stalling of replication forks and mis-repaired DSBs leading to cell death.
Mitomycin-C also causes interstrand DNA crosslinks with subsequent DSBs from blocked replication forks. When mitomycin-C was given to an embryonic stem cell line with truncated BRCA2, an increased frequency of chromosome aberrations were noted including chromatid breaks . Another potential treatment for patients with BRCA2 mutations are inhibitors of poly (ADP-ribose) polymerase (PARP). PARP is integral to maintaining genomic integrity as cells deficient in this enzyme acquire more chromosomal aberrations . An increase in chromosomal abnormalities would require intact homologous recombination for repair, however in patients with BRCA2 mutations, this repair pathway is lacking. It has been previously demonstrated that PARP knockout mice are exquisitely sensitive to ionizing radiation and methylating agents . Subsequent to this, embryonic stem cells that were BRCA2 deficient were transfected with siRNA targeting PARP and were then treated with small molecule inhibitors of PARP. The BRCA-deficient cells were very sensitive to PARP inhibition . Furthermore, xenograft mouse models that were BRCA2-deficient showed significantly less tumor formation when treated with small molecule PARP inhibitors. These results were replicated in human breast cancer cell lines with BRCA2 defects . Based on these findings, PARP inhibitors may specifically target tumors that lack effective homologous recombination, such as cancers caused by mutations in BRCA2. There are currently ongoing trials looking at patients with BRCA2-positive pancreatic cancers who are receiving treatment with mitomycin-C or PARP inhibitors. The results of these studies will be important as we determine how best to treat patients whose tumors harbor specific DNA-repair defects.
54.4.2 4.2 CHEK-1/2
DNA damaging agents lead to replication arrest by activating two distinct signaling pathways. DSBs activate the ataxia-telangiectasia mutated kinase (ATM) and checkpoint kinase 2 (CHEK-2) pathways that control cell cycle arrest, replication, and apoptosis. Other forms of replication stress activate ataxia-telangiectasia mutated and Rad3-related kinase (ATR) and checkpoint kinase 1 (CHEK-1) which are integral to cell cycle progression and cause stabilization of replication forks . These checkpoint kinases are key regulators of progression through the cell cycle and thus are interesting targets for pancreatic cancer, perhaps when combined with a cytotoxic, DNA damaging agent like gemcitabine.
Previous studies have demonstrated that gemcitabine activates the ATR/CHEK-1 pathway and that inhibition of CHEK-1 eliminates the S-phase cell cycle checkpoint . Further work not only confirmed gemcitabine-induced activation of both the ATR/CHEK-1 and ATM/CHEK-2 pathways, but also that cell survival was markedly diminished after gemcitabine treatment in ATR, ATM, and CHEK-1-deficient cells . This effect was not seen in CHEK-2-deficient cells however. These data imply that there may be a greater cytotoxic effect with gemcitabine in tumors with mutations in specific cell cycle regulators, such as ATR for instance.
Gemcitabine also has a role as a radiosensitizer which is hypothesized to be mostly through its ability to cause cell cycle arrest in the S-phase. Phosphorylation of CHEK-1 and CHEK-2 was seen in a number of cell lines treated with radiation-sensitizing doses of gemcitabine, indicating association of these cell cycle checkpoints with S-phase arrest and radiosensitization . Furthermore, CHEK-1 prevented entry into mitosis in gemcitabine-treated cells suggesting that gemcitabine initiates a G2-M cell cycle checkpoint by means of CHEK-1. Taken together, it appears that CHEK-1 balances progression through the cell cycle with DNA synthesis thereby ensuring no entry into mitosis after treatment with gemcitabine. These preclinical studies demonstrate another potential role for CHEK-1 inhibitors as improving gemcitabine-mediated radiosensitization in earlier stages of pancreatic cancer.
The utility of CHEK-1 and CHEK-2 inhibitors has been tested in vivo and in vitro using EXEL-9844, an orally available selective inhibitor of these two kinases . In a pancreatic cancer cell line, EXEL-9844 in combination with gemcitabine was found to significantly increase the number of DNA DSBs, a marker for DNA damage. This effect was more pronounced than when either agent was given alone. The CHEK-1 inhibitor also caused an increase in premature mitotic entry, which is thought to be due to increased DNA damage from gemcitabine with error-prone replication and an ineffective G2-M checkpoint due to CHEK-1 inhibition. Importantly, EXEL-9844 significantly potentiated the cell killing effects of gemcitabine on several different cell lines of varying tumor types. In a pancreatic tumor xenograft model, tumor growth inhibition was significantly higher in the group treated with gemcitabine and EXEL-9844 together when compared to gemcitabine alone. In fact, some reduction in tumor size was also noted in this group and there was a prolonged antitumor effect with the combination treatment as demonstrated by a delay in tumor regrowth.
Given gemcitabine’s effect on the ATR/CHEK-1 and ATM/CHEK-2 pathways, it was surmised that single nucleotide polymorphisms (SNPs), or individual genetic variations, within these pathways may affect patients’ response to gemcitabine and their overall survival. One-hundred nineteen untreated patients with potentially resectable pancreatic cancer were genotyped for six SNPs in the ATR, ATM, CHEK-1, and CHEK-2 genes. A significant decrease in overall survival was noted in patients with the heterozygous variant alleles of ATM G60A and homozygous variant alleles of CHEK-1 G35A. These genotypes were significant independent predictors of survival after adjusting for multiple variables . An improvement in tumor resection rate after preoperative therapy was also seen in patients with the ATR C340T genotype. Interestingly, a worse overall survival was seen when these negative genotypes were combined. The clinical implications of these findings are that SNP analysis may allow us to determine individual genetic profiles of patients with pancreatic cancer without a need for tissue, since this can be difficult in this disease. This information could be used to ascertain which therapy may work best in each patient. For instance, a patient with a genotype that has high CHEK-1 activity may benefit from the combination of a CHEK-1 inhibitor and gemcitabine as opposed to treatment with gemcitabine alone.
54.5 5 Bortezomib and Endoplasmic Reticulum Stress
The proteasome is integral to protein degradation and plays a pivotal role in a number of cellular processes including apoptosis and cell cycle regulation. Because of these various functions, proteasome inhibitors, such as bortezomib, have become an interesting potential therapeutic option. Aberrant protein regulation, such as the accumulation of misfolded proteins, can overwhelm the endoplasmic reticulum-Golgi complex and lead to endoplasmic reticulum stress (ER-stress), triggering an unfolded protein response . However, if the unfolded protein response is overwhelmed, apoptosis can be triggered. Earlier studies demonstrated that bortezomib caused growth arrest in specific human pancreatic cancer cell lines by inhibiting cell cycle progression and activating apoptosis . Interestingly certain cell lines were bortezomib-resistant. These results were correlated in orthotopic models of metastatic human pancreatic cancer.
Pancreatic epithelial cells are thought to be highly sensitive to ER-stress due to their important functions in insulin regulation and digestion. Supporting this theory, it was shown that bortezomib interferes with the unfolded protein response and causes ER-stress-mediated apoptosis in pancreatic cancer cells . This effect was not enhanced by combining gemcitabine with bortezomib; however the addition of cisplatin to bortezomib did trigger ER-stress and augment apoptosis in this pancreatic cancer cell line. In vivo studies demonstrated similar findings with a significant decrease in tumor burden after treatment with either bortezomib or cisplatin alone. Importantly, a greater degree of tumor growth inhibition was seen when these agents were combined.
The combination of bortezomib and docetaxel has also been investigated, with the combination causing a greater inhibition of pancreatic cancer cell growth than either agent alone . Moreover, human pancreatic tumor xenografts in vivo demonstrated a significant decrease in tumor volume with an associated increase in apoptotic activity in the combination arm. Also of interest, a decrease in microvessel density was noted in the tumor cells after combination treatment with an expected decrease in VEGF expression, indicating a potential role for the antiangiogenic properties of bortezomib and docetaxel.
Another interesting function of bortezomib, through proteasome inhibition, is the sequestering of proteins tagged for degradation into aggresomes in human pancreatic cancer cells, an effect not seen in normal pancreatic epithelial cells . Aggresome formation prevented ER-stress, thereby reversing bortezomib’s effect on apoptosis. Importantly, aggresome formation is dependent on the expression of a specific histone deacetylase since silencing this protein using siRNA blocked aggresome formation. Further treatment with the histone deacetylase inhibitor SAHA blocked aggresome formation, led to ER-stress, and rendered the cells sensitive to bortezomib-mediated apoptosis. In orthotopic pancreatic tumors, the combination of SAHA and bortezomib produced similar results with a disruption in aggresome formation and an increase in cell death.
54.6 6 Gene Therapy
Gene therapy targeting pancreatic cancer has become an area of interest since it can allow for targeting of pancreatic cancer cells while minimizing toxicity. As an example, p202 is an IFN-inducible protein that suppresses cancer cell growth in vitro and in vivo. Specifically, it is thought that p202 sensitizes cells to apoptosis via TNF-α-mediated NFkB inactivation . This understanding makes p202-based gene therapy an interesting option in tumors that display constitutive NFkB activation like pancreatic cancer. This was tested in five human pancreatic cell lines all with constitutive NFkB activation. Cell growth was inhibited with p202 in vitro and an antitumor effect was noted in vivo in orthotopic models. Also of interest, p202 prevented liver metastases in these orthotopic models and caused a decrease in VEGF expression, implying both an antimetastatic and antiangiogenic effect. Further, p202 gene therapy was tested by creating a CMV-p202/SN2 complex where SN2 is a lipid formulation that improves transfection efficiency. Pancreatic cancer xenografts that were injected with this complex exhibited slower tumor growth when compared to controls.
Another method for identifying a rational target for gene therapy is finding a pancreatic cancer specific promoter that is overexpressed and transactivated in pancreatic cancer cells. One such promoter is the cholecystokinin type A receptor (CCKAR) that, when modified, is quite active in pancreatic cancer cells . It was previously shown that modified CCKAR, CCK/Mpdx, could enhance the expression of Bik, a pro-apoptotic protein and inhibit pancreatic cell growth in vitro . The activity of CCKAR as a pancreatic cancer specific promoter was replicated in a study where scientists were able to modify the promoter such that it had improved activity in pancreatic cancer cells with minimal activity in normal ones . This promoter was then attached to a Bik mutant, Bik-DD with enhanced apopototic activity. This complex, CCK/Mpdx-Bik-DD killed pancreatic cancer cells in vitro and slowed tumor growth in an ectopic pancreatic cancer xenograft model. Subsequently, a versatile expression vector was developed “VISA” (VP16-GAL4-WPRE integrated systemic amplifier) with the pancreatic cancer–specific promoter derived from the CCKAR gene and used the CCKAR-VISA composite to target transgene expression in pancreatic tumors in vivo. Targeted expression of BikDD, driven by CCKAR-VISA, exhibited significant antitumor effects on pancreatic cancer and prolonged survival in multiple xenograft and syngeneic orthotopic mouse models of pancreatic tumors with virtually no toxicity . These data further demonstrate the feasibility of using gene therapy to target pancreatic cancer, though this would need to be tested in a clinical setting.
54.7 7 Conclusion
The treatment of pancreatic cancer has been firmly entwined with gemcitabine cytotoxic therapy for some time now, but with an improved understanding of the molecular mechanisms underlying this disease, the era of targeted therapy is approaching rapidly. There are a number of recent discoveries that, in the preclinical setting, demonstrate promise as potential therapeutic options. However, these need to be examined in well-designed clinical trials before their efficacy can truly be determined. The identification of rational targets is a current focus of cancer research, and with time, it is very likely that the options for pancreatic cancer treatment will expand beyond conventional chemotherapy. As more is understood about the heterogeneity within individuals’ tumors, the ability to personalize treatment for each patient becomes more and more of a reality.
54.8 Key Research Points
Novel targets demonstrate exciting anticancer potential in the preclinical setting.
Transcription factors that play a variety of roles in cellular proliferation and apoptosis are promising targets for pancreatic cancer therapy.
Preventing the epithelial to mesenchymal transition may decrease the invasive potential of pancreatic cancer cells.
Reversing chemoresistance to gemcitabine can be accomplished by inhibiting the hepatocyte growth factor and its receptor, c-Met.
The DNA repair pathway provides insights into ways to personalize treatment for pancreatic cancer patients.
Increasing endoplasmic reticulum stress activates apoptosis in pancreatic cancer cells.
Gene therapy has the potential to target pancreatic cancer cells with minimal toxicity.
54.9 Future Research Directions
Recent discoveries have highlighted the extraordinary genetic and molecular complexity of pancreatic cancer, extending even to the individual patient level. Many of these genetic changes can be integrated into specific pathways that are abnormal in pancreatic cancer cells. Our challenge for the future will be to understand the hierarchical importance of these pathways and subsequently target them in order to effectively manage patients with pancreatic cancer.
54.10 Clinical Implications
Developing molecular agents that target various aspects of pancreatic cancer cellular signaling and survival is a promising option for therapy.
New discoveries about the individual profile of each pancreatic cancer will allow for therapy to be personalized to each patient.
Well designed clinical trials are needed to test the efficacy of new molecularly targeted agents in patients with this challenging disease.
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