Signaling pathways in pancreatic ductile adenocarcinoma and potential therapeutic targets

INTRODUCTION

Despite extensive research efforts, pancreatic cancer remains highly resistant to targeted as well as traditional therapies. As of 2018, some epidemiological studies have ranked pancreatic cancer as the 14^th most common, while the GLOBICON estimates are as high as 11^th with 458,918 new cases and 432,232 deaths worldwide.^[1,2] With the incidence continuing to rise, pancreatic cancer is projected to become the second leading cause of cancer-related death by the year 2020.^[3] At 90% of all pancreatic cancers reported, pancreatic ductile adenocarcinoma (PDAC) accounts for the overwhelming majority.^[4] PDAC has a dismal prognosis, with 24% of people living 1 year and 5-year survival of <10%.^[1,5] Unfortunately, successful screening methods are lacking, and most people with PDAC are diagnosed at more advanced stages of the disease. Although not guaranteed to be a curative measure, surgical resection remains a mainstay of PDAC treatment. However, only 15–20% of patients are eligible for immediate surgical resection, and tumor relapse rates are extremely high post-surgery with a median survival rate of only 8–10 months.^[6] A significant effort over the past few decades has been put forth to develop adjuvant therapies which extend the overall survival for PDAC patients. The current status of adjuvant therapy for PDAC involves systemic chemotherapies. In patients deemed physically capable, the current standard of care is a combination of modified folinic acid, 5-fluorouracil, irinotecan, and oxaliplatin, which has been given the name modified FOLFIRINOX (mFOLFIRINOX).^[7] Patients who are unable to tolerate the adjuvant mFOLFIRINOX regimen are treated using the pyrimidine analog gemcitabine in combination with capecitabine.^[6,7]

Extensive efforts have been made to understand the genetic driver mutations and signaling pathways involved in PDAC. Among the gene mutations studied, the four most prominent are Kirsten Ras (KRAS), CDKN2A, SMAD4, and TP53. Of the four most prominent mutations, KRAS is mutated in 90% of PDACs.^[8] Mutations in KRAS result in the transcription of a constitutively activated GTPase which, when locked in a GTP-bound conformation, results in uncontrolled activation of downstream signaling pathways which ultimately promote oncogenesis, metastasis, and tumor progression.^[4,8,9] Because KRAS mutations are represented in the majority of PDACs, a significant amount of work has been done to find clinically relevant targeted therapies. Unfortunately, attempts at designing direct KRAS inhibitors have been disappointing as RAS proteins lack significant druggable sites which would allow small-molecule inhibitors to be efficacious.^[9] Therefore, most of the research effort has been spent developing upstream and downstream targets in the KRAS pathway. Of note, there has been some relative preclinical and clinical success with the EGFR small-molecule tyrosine kinase inhibitor erlotinib used in combination with gemcitabine against PDAC. The erlotinib and gemcitabine combination is currently Food and Drug Administration (FDA) approved in the treatment of PDAC.^[8,10,11] Other aspects of KRAS signaling pathways have also been studied as potential druggable targets, some of which already have FDA approved drugs on the market. Examples explored in this article have been studied with the purpose of potentially repurposing drugs, which are already FDA approved for other pathologies that could act synergistically with erlotinib to inhibit KRAS-related signaling pathways in PDAC. Metformin, a biguanide drug used in the first-line treatment of type II diabetes mellitus, has been heavily researched for the drug’s role in mammalian target of rapamycin complex 1 (mTORC1) inhibition through AMPK activation.^[12-15] Several studies have demonstrated metformin’s efficacy in prohibiting PDAC cell growth and proliferation, while other studies have demonstrated synergism between metformin and erlotinib in other cancer cell types.^[12-15] Lithium, a divalent cation FDA approved in the clinical treatment of bipolar disorder, has also been studied as a potential repurposed drug for PDAC due to the inhibition of glycogen synthase (GSK)-3β, an important regulator of many signaling pathways.^[16-19]

An increasing amount of interest has been placed on the unique microenvironment of PDAC. Pancreatic stellate cells (PSCs) have been shown to be a key regulator in orchestrating the accumulation of collagens, fibronectin, and laminin to form a dense stroma in a process termed desmoplasia.^[15,20] The thick stroma created by desmoplasia creates a hypovascular environment, which has been implicated in PDAC resistance to current treatments. The PDAC tumor microenvironment displays immunosuppressive qualities and several studies have linked the regulatory T-cells role in the release of transforming growth factor-beta (TGF-β) via the hedgehog (HH) signaling pathway.^[15,21,22] Heat shock proteins (HSPs) have been implicated in many cancers. HSP is a family of chaperone proteins, which are activated in times of cellular stress and act to prevent cellular damage and protect function.^[23] Several studies have demonstrated how HSP may play a role in PDAC resistance to treatment with gemcitabine.^[24,25] Other HSPs play a role in cellular survival signaling, including NF-κβ. It has been hypothesized how PDAC, among other cancers, can hijack these protective mechanisms to avoid apoptosis.^[26,27] In this article, we review the brief history of adjuvant therapy in PDAC as well as the current standards, and KRAS signaling pathways with a specific focus on potential candidates for targeted therapies. We will also review the PDAC tumor microenvironment, the process of desmoplasia, the connection to HH signaling, and the role of HSP in PDAC.

CURRENT STANDARDS

As discussed by Klaiber et al., clinical trials for the use of adjuvant therapy in PDAC were performed as early as the 1970s. From 1974 until 1982, the first randomized trial to compare chemoradiotherapy and observation took place. The results showed prolonged survival in the treatment cohort (21.0 vs. 10.9 months), but the trial was terminated prematurely due to poor accrual^[6,28] [Table 1]. Although several subsequent trials were performed to analyze the use of chemotherapy as an adjuvant treatment, the first high impact study which compared gemcitabine against 5-fluorouracil was published in 1997.^[7] In this study, 126 patients were randomized to either receive weekly gemcitabine at a dosage of 1000 mg/m², followed by a week of rest, then subsequent gemcitabine treatments every 3 out of 4 weeks or 5-fluorouracil at a weekly dose of 600 mg/m².^[29] As described in the study, the results demonstrated a clinical benefit of 23.8% in the patients treated with gemcitabine compared to 4.8% in those treated with 5-fluorouracil (5.65–4.41 months of median overall survival)^[7,29] [Table 1].

The 1997 trial marked the beginning of increased interest in gemcitabine as adjuvant therapy in PDAC. However, it was not until the second version of the ESPAC-3 clinical trial, which occurred during the recruitment period of 2000 through 2007, when gemcitabine became the treatment of choice for PDAC following surgical resection.^[6] At the time of the trial, ESPAC-3 (v.2) was the largest adjuvant therapy study for PDAC ever conducted, in which 1088 patients from 16 different countries were randomized into two different treatment groups with either 5-fluorouracil/folinic acid or gemcitabine.^[30,31] Interestingly, the results of the ESPAC-3 trial showed no significant difference in survival between patients treated with gemcitabine compared with 5-fluorouracil/folinic acid (23.0 months for 5-FU/FA vs. 23.6 months for gemcitabine) [Table 1]. However, the grade ¾ toxicities were significantly higher in the 5-fluorouracil/ folinic acid treatment group.^[6,30] The ESPAC-4 trial was conducted from 2008 until 2014. In this trial, 730 post-PDAC resection patients were treated with either a combination of gemcitabine plus oral capecitabine or gemcitabine alone.^[32] The median survival was significantly improved in the patients taking the combination therapy with a median overall survival of 28.0 months compared with gemcitabine only treatment^[6,32] [Table 1]. In 2013, a nanoparticle-bound taxane in combination with gemcitabine was shown to enhance overall survival by 2 months when compared to gemcitabine monotherapy.^[33] The results of the clinical trials, further, justified the use of gemcitabine as adjuvant therapy for PDAC.

In 2011, Conroy et al. published the results of a landmark clinical trial, in which 342 patients were either treated with a combination therapy of irinotecan (180 mg/m²), oxaliplatin (85 mg/m²), leucovorin (400 mg/m²), and 5-fluorouracil as an initial 400 mg/m² bolus, followed by a continuous infusion of 2400 mg/m² or monotherapy of gemcitabine at 1000 mg/m² weekly for 7 of 8 weeks and then weekly for 3 of 4 weeks.^[7,34] The survival benefit from the combination therapy, termed FOLFIRINOX, was significant with overall survival of 11.1 months compared to 6.8 months in the gemcitabine treatment group^[7,34] [Table 1]. Due to the significant hematologic toxicities and diarrhea associated with FOLFIRINOX, Conroy et al. followed up ± previously described original ACCORD11/PRODIGE4 study with another clinical trial, in which the bolus of 5-fluorouracil was removed. This mFOLFIRINOX regimen resulted in an overall survival of 54.4 months compared with gemcitabine monotherapy at 35.0 months^[6,35] [Table 1]. Several groups have also reported reducing the toxicity of the original FOLFIRINOX regimen by reducing the dosage of irinotecan.^[7] The mFOLFIRINOX treatment regimen is currently the standard of care for PDAC adjuvant therapy in patients deemed fit enough, whereas gemcitabine therapies are being used when mFOLFIRINOX is considered too toxic.^[6,7]

TUMOR MICROENVIRONMENT

The microenvironment of PDAC is relatively unique among neoplasms due to the hypovascular nature created by a dense stroma which consequently limits the ability of therapeutic agents to reach specified targets.^[20] The cellular components of PDAC that is responsible for the microenvironment include myeloid-derived suppressor cells, tumor-associated macrophages, regulatory T-cells (Tregs), and PSCs.^[21] As mentioned in the introduction, studies have indicated how PSCs are largely responsible for the accumulation of extracellular matrix (ECM) constituents.^[21,22] As reported by Murakami et al. PSCs are one of the types of cancer-related fibroblasts (CAF) represented in the PDAC microenvironment, in which the fibroblastic population can be as high as 90%. The role of CAF, such as PSC, has been shown to contribute to epithelial-mesenchymal transition and metastasis by the secretion of proteoglycans, collagen types I and III, fibronectin, and glycosaminoglycans which are believed to add a mechanical force in the ECM for cancer cellular migration.^[22] The process of ECM growth and hypovascularization is described in the literature as desmoplasia.^[15,20,22]

Immunosuppression is also a hallmark of the PDAC microenvironment. As described by Murakami et al., the normal immune response against tumors consists of the antigen-presenting cells’ antigen containing MHC I molecule interacting with cytotoxic T-cells (CD8⁺ T-cells). In PDAC, there is a decrease in MHC I expression, which blunts the response of CD8⁺ T-cells, causing disruption to the induction of apoptosis through the extrinsic cascade through the expression of Fas ligand and subsequent excretion of perforin and granzyme, as well as the CD8⁺ T-cell release of cell checkpoint inhibitors.^[22] The immunosuppressive cytokine TGF-β is also implicated in the lack of normal immune responses in PDAC.^[21] TGF-β is secreted by Tregs and in the normal immune response is responsible for preventing autoimmune reactions through the process of self-tolerance.^[21] In the PDAC microenvironment, the accumulation of TGF-β secreting Tregs is associated with tumor aggressiveness and a poor prognosis that attributed to the immunosuppressive effects of TGF-β as well as the stimulation of CAF by TGF-β.^[21,22]

The HH signaling pathway plays a role in the desmoplastic reaction of PDAC. Originally discovered in Drosophila, the HH pathway has been shown to be an important regulator of development in animals by controlling segmental pattern formation.^[16,19] As eloquently described by McCubrey et al., HH signaling begins with the binding of an HH ligand to 12-pass transmembrane receptor patched which stimulates the internalization and destruction of the G-protein coupled receptor Smoothened (SMO). The degradation of SMO, as also described by McCubrey et al., promotes the dissociation of glioma-associated oncogene homologue (GLI) from suppressor of fused (SUFU). Once free from SUFU, GLI can be phosphorylated and subsequently activated by PKA, GSK-3, and CK-1. Once activated, GLI can stimulate the transcription of downstream HH genes which promote cellular growth and survival^[16] [Figure 1].

In PDAC, the HH pathway stimulates CAF and is considered to play a large role in desmoplasia.^[20,22] There has been increasing interest in targeting the HH pathway, and the pathway’s inhibition was one of the first successful preclinical approaches to decrease the CAF population in the PDAC microenvironment which decreased the stromal volume and allowed for better delivery of gemcitabine.^[22] A popular targets of the HH pathway are through the regulation of GSK-3-beta, which phosphorylates SUFU after HH pathway stimulation and promotes a disassociation from GLI^[16] [Figure 1]. Lithium is a divalent cation which was originally FDA approved for the treatment of mania, bipolar, and other depressive disorders since 1970 and was one of the first direct GSK-3 inhibitors described.^[16] In several recent studies, lithium has been shown to decrease tumorigenesis in PDAC through GSK-3’s role in HH signaling.^[17,19] These results show the promise of repurposing drugs such as lithium through the exploitation of the anti-tumorigenic effects in the HH pathway. Unfortunately, after promising preclinical results, the results of several phase II clinical trials, where specific HH inhibitors were employed, were disappointing as tumor progression was not halted, even though desmoplasia was shown to be reduced.^[22] The HH pathway is induced by the KRAS pathway through the activation of nuclear factor-kappa light chain enhancer of activated B-cells (NF-κβ)^[21] which establish a link between the two pathways. Perhaps, future experiments can explore this connection by finding drugs, which will act synergistically to inhibit PDAC proliferation as well as address desmoplasia and the microenvironment.

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Figure 1:

HH signaling and the role of GSK-3. When the HH ligand binds the PTCH1 receptor in the HH pathway, SMO is internalized which leads to the release of GLI from SUFU. Once released, PKA, GSK-3, and CK-1 can activate GLI through phosphorylation. Once phosphorylated, GLI can translocate into the nucleus and act as a transcription factor for many downstream targets. Lithium is an inhibitor of GSK-3. HH: Hedgehog, GSK-3: Glycogensynthase 3, PTCH1: Pass transmembrane receptor patched, SMO: Smoothened, GLI: Gliomaassociated oncogene homologue, and SUFU: Suppressor of fused.

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KRAS SIGNALING PATHWAYS

Kirsten rat sarcoma viral oncogene homolog, known as KRAS, a member of the Ras GTPase family, is mutated in PDAC to be constitutively activated in a GTP bound configuration.^[8,33,36] The KRAS gene is reported in the literature as being mutated from 90% to 95% of all PDACs.^[8,36] Early in PDAC tumorigenesis, as described by Waters and Der, abnormal morphological changes occur in pancreatic epithelial cells, where a transition from a flat to more cuboidal appearance is observed, and mucin is produced. These Early changes in PDAC morphology are called pancreatic intraepithelial neoplasms (PanINs). KRAS mutations are found in the majority of the early PanINs and are believed to be the driving mutation of most PDACs.^[10,33] The downstream effects of KRAS signal transduction dictate cellular growth and survival. RAF-MEK-ERK and phosphatidylinositol-3-kinase (PI3K)/Akt are some of the most well-studied downstream signaling pathways of KRAS^[8,9] [Figure 2]. Due to the difficulty in creating a direct KRAS inhibitor, much effort has been put forth to find upstream or downstream targets.^[33] In this article, we discuss the EGFR pathway as well as the downstream mTORC1 pathway through AMPK-TSC1 and the potential drugs, which target them.

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Figure 2:

KRAS downstream signaling to EGFR. Aberrant KRAS signaling in PDAC can activate downstream targets which stimulate cellular proliferation, growth, and survival. Shown is the well characterized RAS/RAF/MEK/ERk and PI3K/Akt/mTOR pathway. EGFR is a tyrosine kinase transmembrane receptor that has been explored as an upstream target for dysfunctional KRAS signaling using the small-molecule inhibitor erlotinib. KRAS: Kirsten Ras, EGFR: Epidermal growth factor receptor, and PDAC: Pancreatic ductile adenocarcinoma.

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The EGFR pathway has been well characterized in the literature regarding implications in many solid tumors. EGFR is a membrane-spanning glycoprotein with intrinsic tyrosine kinase activity in the HER (erbB). On binding the ligand extracellularly, EGFR’s intracellular domain can undergo homodimerization that results in autophosphorylation and subsequent activation of downstream transduction pathways.^[8,11] Some of the most well-studied downstream pathways affected by EGFR include the PI3K-Akt and RasRaf-MAPK pathways, which play a large role in cell cycle progression, cellular growth, and survival^[8,11] [Figure 2]. As described by Chu et al. EGFR has been shown to enhance downstream KRAS signaling. However, inhibition of EGFR is not believed to affect the downstream signaling pathways of KRAS due to constitutive activity, but, interestingly, there has been growing evidence to demonstrate how KRAS driven PDAC tumorigenesis is dependent on EGFR signaling (Ardito et al., 2012). A study, by Ardito et al., demonstrated how EGFR inhibition effectively eliminated KRAS-dependent PDAC tumorigenesis. The investigators hypothesized how the KRAS dependence on EGFR signaling lies in the control of oncogenic precursors as well as the maintenance of extracellular-signal-regulated kinase (ERK) signaling after tumor initiation.^[37] Although not found to be mutated in PDAC, EGFR activity is overexpressed in approximately 90% of all PDAC tumors.^[4,9] The relevance of EGFR signaling has led to an increased interest in the possibility of using EGFR inhibitors against PDAC to gain a therapeutic advantage.

Erlotinib is a small-molecule inhibitor of the EGFR intracellular tyrosine kinase domain. Mechanistically, Erlotinib competitively inhibits the intracellular ATP binding sites which prevent autophosphorylation and subsequently disrupt downstream signaling.^[11] In 2007, after successful preclinical testing and moderate success with clinical trials, Erlotinib was approved for the treatment of PDAC when coupled with gemcitabine.^[11] The PA.3 phase III clinical trial which was largely responsible for the FDA approval of erlotinib against PDAC was conducted by the National Cancer Institute of Canada Clinical Trials Group (NCIC-CTG) and it compared the combination erlotinib plus gemcitabine against gemcitabine monotherapy.^[11,38] Although statistically significant with 1-year survival increases from 17% to 23% with erlotinib combination therapy, the overall survival of 6.24 months for the erlotinib combined with gemcitabine was relatively moderate compared with 5.91 months for gemcitabine alone.^[38] The more recent CONKO-005 stage III clinical trial, conducted in PDAC patients exclusively after R0 resection, did not show a statistically significant benefit to the use of erlotinib combined gemcitabine against PDAC, although the recurrence of metastatic disease was shown to occur in 76% of the combination therapy group compared to 82% in the group treated with gemcitabine monotherapy.^[39] After evaluating findings from both preclinical and clinical studies, future work may be beneficial to explore the possible synergistic effects of erlotinib and other inhibitors of KRAS downstream signaling.

The mechanistic target of rapamycin, or mTOR, is a downstream target of KRAS through the PI3K-Akt pathway. As a conserved member of the PI3K family, mTOR acts as a serine/threonine kinase which, when activated by upstream signals, promotes many physiological functions including cell growth and survival.^[36,40,41] The mTORC1, a multi-protein complex consisting of mTOR and a scaffolding protein named Raptor, serves a vital role in nutrient sensing and regulation of downstream anabolic processes.^[42] The upstream regulation of mTORC1 is attributed to the integration of signals by the tuberous sclerosis complex (TSC), a regulator of the mTORC1 activating GTPase Ras homolog enriched in brain (Rheb), through upstream pathways such as PI3K-Akt, RAS-ERK, and the stress response kinase AMPK.^[40,41] As discussed by Lu et al., targeting of the PI3K/Akt/ mTOR pathway using mTOR inhibitors, such as Everolimus, showed efficacy against PDAC tumors in vivo. However, a phase II clinical trial showed negligible benefit for patients with gemcitabine resistant PDAC.^[36]

Metformin, a biguanide, is the most important drug for treating type II diabetes mellitus worldwide and it has been shown to indirectly stimulate the AMPK pathway through a proposed mechanism of increasing the AMP/ATP ratio through inhibition of complex I of the mitochondrial electron transport chain.^[43] Metformin could inhibit mTORC1 through the stimulation of AMPK, which phosphorylates the TSC complex resulting in the inhibition of the mTORC1 activator Rheb^[13,43] [Figure 3]. This unique mechanism has led to an increasing epidemiological, preclinical, and clinical support for the use of metformin as an efficacious cancer therapeutic.^[43] Metformin has also been shown to inhibit mTORC1 through an AMPK independent mechanism.^[12] A retrospective study, by Sadeghi et al., reported how PDAC patients with pre-existing diabetes treated with metformin had a statistically significant overall survival benefit of 4.1 months compared to patients not treated with metformin.^[44] Another study showed inhibition of DNA synthesis and cell proliferation of PDAC cell lines incubated in a physiological concentration of glucose (5 mM) when treated with metformin, demonstrating the anti-PDAC when treated with therapeutic dosages of metformin.^[43] An in vivo xenograft study also showed synergism against PDAC when treated with metformin and erlotinib, which resulted in drug target inhibition, tumor regression, and induction of apoptosis.^[13] Some studies have even indicated that metformin can suppress desmoplasia in the PDAC microenvironment.^[8] Interestingly, a preclinical study demonstrated how metformin-induced AMPK activation could have inhibitory effects on desmoplasia in the PDAC microenvironment by decreasing PDAC cytokine production and subsequent paracrine activation of PSCs.^[15]

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Figure 3:

Indirect inhibition of mTORC1 by metformin through AMPK. Metformin has a unique mechanism of action through its indirect inhibition of the mTORC1. The stimulation of AMPK leads to the phosphorylation of TSC which inhibits Rheb from activating mTORC1. The disruption of the mTORC1 pathway has been demonstrated to promote tumor regression and induce apoptosis in PDAC cell lines. mTORC1: Mammalian target of rapamycin complex 1, Rheb: Ras homolog enriched in brain, and PDAC: Pancreatic ductile adenocarcinoma.

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HSP IN PANCREATIC CANCER

HSP was discovered in 1962. While working with Drosophila in his laboratory, Italian scientist Ferruccio Ritossa noticed how the salivary glands of the Drosophilia, which was mistakenly incubated at a high temperature, displayed a characteristic puffing which identified the possibility of higher transcriptional activity.^[45,46] Originally, HSP was only thought to function when cells experience stress from high temperatures, but, now, HSP is known to respond to many physiological stresses including hypoxia, ischemia, acidosis, and UV exposure.^[45] Recently, an additional role has been ascribed to HSP as danger signals produced and released when cells are under stress and as activators of the immune system. This dual role of HSP as both chaperone and cytokines is now termed chaperokine (read refs below). Once such chaperokine, HSP72, has been demonstrated to enhance the production of cytokines and chemokines as well as to activate and promote maturity of immune cell lines.^[47-49] HSP72 binds to receptors with high affinity in natural killer cells, dendritic cells, macrophages, and B-cells which stimulate downstream signal transduction cascades with an increase in intracellular calcium ultimately culminating in the activation of NF-κβ.^[49]

HSP is highly evolutionarily conserved molecular chaperones and is classified into different families based on molecular weight.^[50] The HSP families which have been identified to have a large physiological role in mammals are HSP90, HSP70, HSP60, HSP40, and other smaller HSP with HSP70 and HSP90 being implicated in PDAC.^{[27,45,50,51]} In neoplastic cells, the expression of HSP70 is elevated and studies have demonstrated how the elevated levels of HSP70 indicate a poorer prognosis in several cancers that could be due to the ability of HSP70 to inhibit apoptosis by affecting the expression of transcription factors involved with the apoptotic BCL2 family.^[26] Interestingly, the cellular effects of HSP70 have been demonstrated to be protective of pancreatic acinar cells in the setting of acute pancreatitis by counteracting the NF-κβ- and trypsin-induced inflammatory response, which could lead to cellular necrotic death.^[26] The same protective function of HSP70 prevents apoptosis and promotes tumorigenesis in PDAC.^[27,45] In a study performed by Aghdassi et al., monoclonal antibodies were used to determine HSP70 expression in PDAC cell lines as well as utilize siRNAs against HSP70 to confirm the protective effects against apoptosis. The results of this study demonstrated the activation of the intrinsic apoptotic cascade when PDAC cells were depleted of HSP70.^[50] The higher molecular weight HSP90 is also implicated in PDAC and, as a molecular chaperone, is responsible for the folding of many of the important PDAC proteins already mentioned earlier in this article, including EGFR, PI3K, Akt, and GSK-3β.^[51,52] A study, conducted by Belalcazar et al., used the HSP 90 inhibitor ganetespib and proteasome inhibitor carfilzomib to demonstrate limited tumor growth in KRAS driven PDAC cell lines by inducing autophagy and reduction in PI3K and Akt signaling, leading to the observation of apoptotic biomarkers in the PDAC cell lines studied. The results of Belalcazar’s study correlated with the previous research, which demonstrated how HSP90 inhibition leads to the suppression of survival pathways such as PI3K/Akt/mTOR in other cancers.^[52,53]

DISCUSSION

PDAC currently has a poor prognosis, with <10% of patients living up to 5 years after diagnosis.^[1,5] With 458,918 new cases and 432,232 deaths worldwide, PDAC is ranked as high as the 11^th most common cancer.^[1,2] mFOLFIRINOX is the current standard of adjuvant care for treating PDAC in patients fit enough to withstand the associated toxicities and gemcitabine therapies for those who are not.^[6,7] Clinical trials have demonstrated how mFOLFIRINOX has an overall survival benefit of 54.4 months compared with 35.0 with gemcitabine alone.^[6,34] Despite the improved overall survival benefit, current standards for adjuvant therapy are associated with toxic side effects and are not curative which reinforces the importance of further research into potential targets of the KRAS signaling pathways.

Erlotinib is a small-molecule inhibitor which competes with the intracellular tyrosine kinase domain of EGFR.^[11] EGFR has been shown to be over-expressed in 90% of PDAC.^[4] Although currently FDA approved for adjuvant PDAC therapy, and despite success in earlier preclinical studies, clinical trials have demonstrated very moderate to no significant difference in overall survival when using erlotinib in conjunction with gemcitabine versus gemcitabine monotherapy.^[11,38,39]

Metformin is an important medication for treating type II diabetes mellitus. Through both an AMPK-dependent and AMPK-independent mechanism, metformin has been demonstrated to have inhibitory effects on the mTORC1 pathway.^[12,13,43] Preclinical and epidemiological studies have indicated potential for metformin to be re-purposed in the use against PDAC.^[43,44] In addition, some studies have demonstrated how metformin can influence the PDAC microenvironment and desmoplasia.^[8,15] A study by Lau et al. demonstrated synergistic effects when erlotinib and the mTORC1 inhibitor metformin were used together against PDAC. Further, research could be done to explore the synergistic effects of metformin and erlotinib in combination against PDAC.

Lithium, a divalent cation used in the treatment of bipolar disorder, is an inhibitor of GSK-3 which has promise as a potential target in cancer therapy.^[16] Several preclinical studies using lithium have demonstrated an effect on PDAC tumorigenesis as well as desmoplasia, which is linked mechanistically to the inhibition of GSK-3 and HH signaling.^[17,19] Unfortunately, several phase II clinical trials using HH inhibitors did not show an effect on tumor progression, even though desmoplasia was reduced.^[22]

HSP is molecular chaperones responsible for preventing protein misfolding in times of cellular stress.^[45,46] The protective effects of HSP70 have been demonstrated to be hijacked by PDAC to enhance tumorigenesis by prevention of apoptosis.^[27,45,50] HSP90 has also been demonstrated to influence PDAC tumorigenesis, and inhibition of HSP90 resulted in a decreased signaling expression of the KRAS-related PI3K/Akt/mTOR pathway.^[51-53]

CONCLUSION

This article has discussed the current clinical standards in PDAC, important signaling pathways with special attention on KRAS-related signaling, HH pathway, potential drug targets and the role of HSP in PDAC. Future studies could be designed to target KRAS-related signal transduction pathways utilizing combination therapies to gain insight into potential synergistic responses against PDAC proliferation and survival as well as the PSC induced desmoplasia observed in the PDAC microenvironment.