Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer-related mortality in the US for both men and women [
1]. In 2015 alone, it is estimated that 48,960 new cases of pancreatic cancer will be diagnosed in the US and 40,560 patients will die of this disease. As such, PDAC remains one of the most challenging malignancies with a dismal prognosis and limited therapeutic options. The 5-year survival rate for pancreatic cancer (all stages combined) is around 7%, which is the lowest among all different cancer sites. At the time of initial pancreatic cancer diagnosis, approximately 9% of patients present with localized disease, 28% have regional spread, and the remaining 53% of patients already have distant spread of their disease. There has been a very limited clinically meaningful improvement in survival rates for this disease during the past two decades. The poor prognosis of PDAC is largely attributed to delayed diagnosis due to nonspecific symptoms in the early stages of the disease, biological aggressiveness leading to rapid metastases, lack of effective screening methods, and resistance to radiation and chemotherapies.
Risk factors
The known risk factors that increase the likelihood of developing PDAC include cigarette smoking [
3], alcohol abuse [
4], high fat diet [
5], and certain trace elements [
6]. It is estimated that cigarette smoking doubles the risk of developing PDAC and accounts for approximately 20%–25% of the cases [
3]. Chronic pancreatitis is also associated with an increased risk of PDAC, especially among smokers [
7]. It has also been noted that the majority of patients with PDAC develop diabetes mellitus which is usually diagnosed in the preceding 1–2 years or concomitant with the new cancer diagnosis [
7]. It is not entirely clear whether diabetes is a predisposing factor or a manifestation of PDAC itself. Obesity, which predisposes to insulin resistance, might be a common link between the two.
Approximately 5%–10% of patients with PDAC report a history of pancreatic cancer diagnosis in their family member [
8]. The genetic syndromes such as familial breast cancer (
BRCA2,
BRCA1, and
PALB2), the Peutz-Jeghers syndrome (
LKB1/
STK11), the familial atypical multiple mole melanoma (FAMMM) syndrome (p16/
CDKN2A), hereditary pancreatitis (
PRSS1), and the lynch syndrome (
MLH1,
MSH2,
MSH6,
PMS2) are also associated with an increased risk of developing PDAC [
8,
9]. Thus, patients with a family history of pancreatic cancer or these mutation carriers should undergo appropriate screening, as per the guidelines provided by the International Cancer of the Pancreas Screening Consortium [
10].
Genetics and molecular pathogenesis
Pancreatic cancer most commonly originates in the exocrine cells of the pancreas [
11]. Among the exocrine tumors, ‘ductal adenocarcinoma’ is the most frequently encountered pathological subtype and accounts for more than 90% of the cases. Initiation and development of PDAC involve a series of specific genetic alterations which promote growth and survival of aberrant precursors, initiation of a desmoplastic reaction in the stroma, and ultimately tissue invasion and metastases [
12]. This oncogenic process begins with transformation of normal pancreatic duct epithelium into infiltrating cancer through a series of histologically defined precursors called pancreatic intraepithelial neoplasia (PanIN)-1, -2, and -3 [
11]. These morphological changes occur in conjunction with several genetic alterations. Disease progression often involves development of distant metastases, which occurs late during the genetic evolution of pancreatic cancer [
13]. Using genome sequencing, it has been determined that after the initiation of tumorigenesis, an average of 11.7 years is required for the birth of parental, non-metastatic founder pancreatic cancer clone, additional 6.8 years for the development of cancer cell subclones with metastatic potential, and an average of 2.7 years from then until the patient’s death [
13].
It is now well established that pancreatic cancer cells contain one or more of the four primary genetic mutations that drive pancreatic cancer tumorigenesis [
14]. These include
KRAS, p16/
CDKN2A,
TP53, and
SMAD4 mutations.
KRAS plays a critical role in regulating important cellular functions including cell survival, cell differentiation, and proliferation [
15]. Single point mutations in codon 12, 13, 59, or 61 of exon 2 and exon 3 of the
KRAS oncogene lead to uncontrolled downstream signaling of RAF/MEK/ERK, leading to enhanced tumor cell proliferation and survival. It has been shown that these activating mutations in the
KRAS are a necessary event for the initiation of pancreatic cancer and are therefore commonly found in the early precursor lesions (PanIN-1) [
16,
17]. With disease progression, the prevalence of oncogenic
KRAS mutation increases and is present in over 90% of the tumors [
17,
18]. Inactivating mutation in the
CDKN2A tumor suppressor gene results in the loss of p16 protein and thereby loss of regulation of the G1/S transition of the cell cycle. It is also thought to be a relatively early event in PDAC progression (PanIN-2 lesions) and is associated with larger tumors and early metastasis [
17,
19].
TP53 is a DNA checkpoint regulator in response to mutations from reactive oxygen species as well as telomere shortening. Abnormal
TP53 gene allows cells to avoid DNA damage control checkpoints and subsequently apoptotic signals [
20].
SMAD4 is a key component of the transforming growth factor-β (TGF-β) receptor signaling pathway and plays a role in activating transcription of cell cycle inhibitory factors. Inactivation of
TP53 and
SMAD4 occur at a later stage (PanIN-3) in pancreatic carcinogenesis [
17].
A comprehensive genome analysis of 24 different human pancreatic cancers revealed an average of 63 genetic alterations per cancer, the majority of which were point mutations [
2]. These mutations occur in several primary oncogenes and tumor suppressor genes and contribute to the genetic diversity of pancreatic cancer. This, in turn, leads to tumor heterogeneity, instability, and early metastasis. The genetic alterations associated with pancreatic cancer can be classified into a set of 12 core cellular signaling pathways: apoptosis, control of G1/S phase transition, sonic hedgehog (SHH) signaling, KRAS signaling, TGF-β signaling, Wnt/Notch signaling, DNA damage control, homophilic cell adhesion, integrin signaling, JNK signaling, invasion, and small GTPase signaling [
2]. These pathways are responsible for some of the key cellular functions such as intracellular signaling, cell cycle regulation, metabolism, and DNA repair. Targeting these pathways has now become the main focus of drug development in pancreatic cancer.
A prominent histologic hallmark of PDAC is the presence of a desmoplastic reaction which consists of extracellular proliferation of leukocytes, fibroblasts, endothelial cells, neuronal cells, and collagen. This is mediated by paracrine signals from the pancreatic cancer cells which results in the formation of a dense stroma in the tumor microenvironment [
21]. It is now established that the signals that promote this stromal reaction originate from the
KRAS-mutant oncogene in the epithelium of pancreatic cancer cells. SHH signaling also acts in a paracrine fashion on the extracellular fibroblasts, resulting in their growth and differentiation [
22]. The desmoplastic reaction not only acts as a mechanical barrier to the effective delivery of chemotherapeutic agents, it also provides an antiangiogenic and hypoxic microenvironment in which the pancreatic cancer cells like to grow and flourish.
Thus, it is now established beyond doubt that the wide range of genetic alterations and the stromal reaction play an important role in the initiation, progression, chemotherapeutic resistance, and recurrence of pancreatic cancer.
Emerging novel therapeutic targets and treatment strategies
Currently, a multitude of innovative therapeutic approaches are being developed to target the known molecular pathways involved in pancreatic tumorigenesis. Table
1 provides a list of the selected clinical trials that are currently recruiting patients to evaluate the safety and efficacy of novel agents in PDAC.
Table 1
Summary of selected novel agents that are under evaluation in currently actively recruiting clinical trials
MEK inhibitor + Bcl-2 inhibitor | Trametinib, navitoclax | NCT02079740 | Ib/II | LA, metastatic |
MEK inhibitor + ErbB inhibitor | PD-0325901, dacomitinib | NCT02039336 | I/II | Metastatic |
| Trametinib, lapatinib | NCT02230553 | I/II | Metastatic |
EGFR TKI | Afatinib | NCT01728818 | II | Metastatic |
PI3K inhibitor | BKM120 | NCT01571024 | I | Metastatic |
| BYL719 | NCT02155088 | I | LA, metastatic |
PI3K inhibitor + MEK inhibitor | BYL719, MEK162 | NCT01449058 | Ib/II | Metastatic |
AKT inhibitor | MK2206 | NCT01783171 | I | LA, metastatic |
PTEN inducer | AXP107-11 | NCT01182246 | Ib/II | LA, metastatic |
Wnt signaling inhibitor | OMP-54 F28 | NCT02050178 | Ib | Metastatic |
| OMP-18R5 | NCT02005315 | Ib | Metastatic |
| PRI-724 | NCT01764477 | I | LA, metastatic |
| LGK974 | NCT01351103 | I | LA, metastatic |
Glycogen synthase kinase-3 inhibitor | LY2090314 | NCT01632306 | I, II | Metastatic |
Notch signaling inhibitor | MK0752 | NCT01098344 | I | LA, metastatic |
| PF-03084014 | NCT02109445 | Ib/II | Metastatic |
| OMP-59R5 | NCT01647828 | Ib/II | Metastatic |
| OMP-21 M18 | NCT01189929 | Ib | LA, metastatic |
Anti-connective tissue growth factor mAb | FG-3019 | NCT02210559 | II | LA unresectable |
Heparan sulfate mimetic | M402 | NCT01621243 | I/II | Metastatic |
Hyaluronidase | PEGPH20 | NCT01839487 | II | Metastatic |
| | NCT01959139 | I/II | Metastatic |
Hyaluronidase + anti-EGFR mAb | PEGPH20, cetuximab | NCT02241187 | NP | Resectable |
Oncolytic adenovirus encoding hyaluronidase | VCN-01 | NCT02045589 | I | LA, metastatic |
Hedgehog inhibitor | IPI-926 | NCT01383538 | I | LA, metastatic |
| GDC-0449 | NCT01088815 | II | Metastatic |
| LDE-225 | NCT01431794 | I/II | LA |
Hypoxia targeting agent | TH-302 | NCT02047500 | I | LA, metastatic |
TGF-β receptor I inhibitor | LY2157299 | NCT01373164 | Ib/II | LA, metastatic |
Hypomethylating agent | Azacitidine | NCT01845805 | II | Resected |
AMP-activated protein kinase (AMPK) activator | Metformin | NCT01954732 | I | Localized |
| Metformin | NCT02005419 | II | Localized |
| Metformin | NCT01666730 | II | Metastatic |
AMPK activator + mTOR inhibitor | Metformin, Rapamycin | NCT02048384 | Ib/II | Metastatic |
poly (ADP-ribose) polymerase (PARP) inhibitor | Veliparib | NCT01908478 | I | LA |
| Veliparib | NCT01489865 | I/II | Metastatic |
| Veliparib | NCT01585805 | II | LA, metastatic |
| Rucaparib (AG-14699) | NCT02042378 | II | LA, metastatic (BRCA mutant) |
| Olaparib (AZD2281) | NCT02184195 | III | Metastatic (BRCA mutant) |
Vascular targeting agent | ADH-1 | NCT01825603 | I | LA, metastatic |
Antiangiogenic combination | Tl-118 | NCT01509911 | II | Metastatic |
Arginine degrading enzyme | ADI-PEG 20 | NCT02101580 | Ib | LA, metastatic |
Aurora A kinase inhibitor | Alisertib (MLN8237) | NCT01924260, NCT01677559 | I | LA, metastatic |
CDK inhibitor + AKT inhibitor | Dinaciclib, MK2206 | NCT01783171 | I | LA, metastatic |
α-ketoglutarate dehydrogenase (KGDH) inhibitor | CPI-613 | NCT01835041 | I | Metastatic |
| CPI-613 | NCT01839981 | I | LA, metastatic |
c-Met inhibitor | Cabozantinib (XL184) | NCT01663272 | I | LA, metastatic |
CRM-1 inhibitor | Selinexor (KPT-330) | NCT02178436 | Ib/II | Metastatic |
DNA minor groove binder | Lurbinectedin | NCT02210364 | I | LA unresectable |
Src inhibitor | Dasatinib | NCT01652976 | II | Metastatic |
Trk A, B, C inhibitors | PLX7486 | NCT01804530 | I | LA, metastatic |
IDO inhibitor | Indoximod | NCT02077881 | I/II | Metastatic |
| NLG919 | NCT02048709 | I | Refractory |
Chemokine receptor 2 (CCR2) antagonist | PF-04136309 | NCT01413022 | I | LA |
Anti-tissue factor mAb | MORAb-066 | NCT01761240 | I | LA, metastatic |
Wee 1 inhibitor | MK1775 | NCT02037230 | I/II | LA unresectable |
BET bromodomain inhibitor | OTX015 | NCT02259114 | Ib | LA, metastatic |
SMAC mimetic | LCL161 | NCT01934634 | I | Metastatic |
Cancer stemness inhibitor | BBI608 | NCT02231723 | Ib | Metastatic |
Janus Kinase (JAK) inhibitor | Ruxolitinib | NCT01822756 | I | Metastatic |
| Ruxolitinib | NCT02117479, NCT02119663 | III | Metastatic |
| INCB039110 | NCT01858883 | Ib | Metastatic |
| Momelotinib | NCT02101021 | II | Metastatic |
Autophagy inhibitor | Hydroxychloroquine | NCT01506973 | I/II | Metastatic |
| Hydroxychloroquine | NCT01978184 | II | Resectable |
| Hydroxychloroquine | NCT01494155 | II | Resectable |
Cancer vaccine | GVAX | NCT01088789 | II | Localized |
| Poly ICLC and dendritic cells | NCT01677962 | 0 | LA unresectable |
| Autologous tumor-derived HSP gp96 | NCT02133079 | I/II | Resected |
| GVAX/CRS-207 | NCT02004262 | II | Metastatic |
| Algenpantucel-L | NCT01836432 | III | LA |
hTERT DNA cancer vaccine | INO-1400 | NCT02327468 | I | Non-metastatic |
CTLA-4 inhibitor | Ipilimumab | NCT01473940 | Ib | LA, metastatic |
Anti-PD-1 mAb | CT-011 | NCT01313416 | II | Resected |
Vaccine + CTLA-4 inhibitor | GVAX, ipilimumab | NCT01896869 | II | Metastatic |
Vaccine + anti-PD-1 mAb | GVAX/CRS-207, nivolumab | NCT02243371 | II | Metastatic |
CTLA-4 inhibitor + anti-PD-1 mAb | Ipilimumab, nivolumab | NCT01928394 | I/II | LA, metastatic |
Anti-CPAA mAb | NPC-1C | NCT01834235 | I/II | LA, metastatic |
| NPC-1C | NCT01040000 | II | LA, metastatic |
Anti-MUC1 mAb | BTH1704 | NCT02132403 | I | LA, metastatic |
Anti-CEA BiTE mAb | MEDI-565 | NCT01284231 | I | Refractory |
Anti-CA-125 mAb | Oregovomab | NCT01959672 | II | Non-metastatic |
Pegylated recombinant human IL-10 | AM0010 | NCT02009449 | I | Metastatic |
IL-1 receptor antagonist | Anakinra | NCT02021422 | I | Metastatic |
RAS specific immunotherapy | TG01 | NCT02261714 | I/II | Resected |
Radioimmunotherapy | 90Y-clivatuzumab tetraxetan (IMMU-107) | NCT01956812 | III | Metastatic |
Activated T-cells | EGFRBi armed ATC infusions | NCT01420874 | I | Metastatic |
Dendritic cell/cytokine-induced killer cells | DC-CIK | NCT01781520 | I/II | LA, metastatic |
siRNA-transfected PBMC | APN401 | NCT02166255 | I | LA, metastatic |
Autologous CAR T-cells | RNA mesothelin re-directed CAR T-cells | NCT01897415 | I | Metastatic |
| Anti-mesothelin gene engineered lymphocytes | NCT01583686 | I/II | Metastatic |
Autologous natural killer T-cells | NKT cells | NCT01801852 | I | Refractory |
Activated dendritic cells | DCVax-Direct | NCT01882946 | I/II | LA, metastatic |
Autologous tumor infiltrating lymphocytes + interleukin | TIL, IL-2 | NCT01174121 | II | Metastatic |
Antiguanylyl cyclase C antibody-drug conjugate (ADC) | MLN0264 | NCT02202785 | II | LA, metastatic |
Micellar nanoparticle-encapsulated cisplatin | NC-6004 | NCT02043288 | III | LA or metastatic |
Alkylating agent | Glufosfamide | NCT01954992 | III | Metastatic |
Source:
http://www.clinicaltrials.gov; Accessed on January 01, 2015.
NCT National Clinical Trial,
EGFR epidermal growth factor receptor,
TKI tyrosine kinase inhibitor,
PTEN Phosphatase and tensin homolog,
TGF transforming growth factor receptor,
mAb monoclonal antibody,
CRM1 chromosome region maintenance 1,
DNA deoxyribonucleic acid,
BET bromodomain and extra-terminal,
SMAC second mitochondrial-derived activator of caspases,
hTERT telomerase reverse transcriptase,
CTLA-4 cytotoxic T-lymphocyte-associated protein 4,
PBMC peripheral blood mononuclear cell,
siRNA small interfering RNA,
LA locally advanced.
RAS/RAF/MEK/ERK signaling pathway
Despite being the most common mutation associated with PDAC, attempts to target
KRAS by inhibiting its post-translational modification have been unsuccessful so far. Tipifarnib (R115777) is an inhibitor of farnesyltransferase (FTase) which is a dominant enzyme involved in post-translational modification of RAS [
60]. So far, it has not demonstrated any significant antitumor activity both as a single agent and in combination with gemcitabine [
44,
61,
62].
Attempts are being made to identify downstream targets (mitogen-activated protein kinase [MAPK], phosphatidylinositide 3-kinase [PI3K]) to block KRAS-dependent signaling pathways. Towards this goal, a number of MEK inhibitors are currently being evaluated in clinical trials [
63]. Selumetenib (AZD6244) is a selective MEK inhibitor that was found to have similar efficacy as capecitabine in a phase II clinical trial that enrolled APC patients after failing first-line gemcitabine therapy [
64]. It is also being tested in combination with erlotinib in APC patients resistant to gemcitabine (NCT01222689). Based on the encouraging results of a phase I clinical trial, trametinib (another MEK inhibitor) was combined with gemcitabine in a phase II clinical trial of metastatic pancreatic cancer patients but failed to demonstrate a clinical benefit [
65]. Combinations of other novel MEK inhibitors (pimasertib [MSC1936369B], refametenib [BAY86-9766]) with gemcitabine are currently under evaluation in clinical trials.
It is now known that expression of RAS oncogene up-regulates basal autophagy, which is required for cancer cell survival in starvation and in tumorigenesis [
66]. Autophagy is therefore believed to be a significant mechanism for pancreatic cancer cell survival. Hydroxychloroquine, an anti-malarial drug is being evaluated as an autophagy inhibitor for the treatment of these aggressive cancers.
Epidermal growth factor receptor pathway
ErbB-1 (EGFR) and ErbB-2 (HER2/neu) expression is found in 90% and 21% of pancreatic cancers, respectively [
67,
68]. Therapies targeted against EGFR (both TKIs and monoclonal antibody [mAb]) in pancreatic cancer have yielded overall disappointing results so far. As discussed previously, the phase III PA.3 trial evaluated gemcitabine in combination with erlotinib in the first-line treatment of APC and was associated with a clinically insignificant improvement in median OS when compared with gemcitabine alone [
54]. In another phase II study of gemcitabine-refractory APC patients, a combination of erlotinib and capecitabine was associated with only 10% radiological response and a median OS of 6.5 months [
69]. The combination of cetuximab and gemcitabine has also been evaluated in the treatment of APC patients [
70,
71]. The initial phase II study demonstrated stable disease (SD) in 63% and partial response (PR) in 12% of the EGFR-expressing APC patients that were treated with cetuximab plus gemcitabine combination [
70]. In a subsequent phase III study (Southwest Oncology Group [SWOG]-directed intergroup trial S0205), this combination was not associated with any survival benefit when compared with the single agent gemcitabine (6.3 months vs. 5.9 months; HR 1.06; 95% CI 0.91 to 1.23;
P = 0.23) [
71]. A randomized phase II study of panitumumab, erlotinib, and gemcitabine combination suggested a trend towards OS benefit when compared with erlotinib plus gemcitabine [
72]. However, this three-drug combination with dual inhibition of the EGFR pathway was associated with significant toxicities leading to early termination of the study. Anti-HER2 agent trastuzumab has been combined with gemcitabine in a phase II study that included metastatic pancreatic cancer patients with 2+ (88% patients) or 3+ (12% patients) HER2/neu overexpression by immunohistochemistry [
73]. The response rate of this combination was very similar to gemcitabine alone.
One of the probable explanations for the lack of a meaningful benefit from anti-EGFR TKIs in pancreatic cancer could be the development of acquired resistance to these agents, which is a mechanism well studied in lung cancer [
74]. Clinical trials evaluating newer EGFR TKIs such as afatinib (NCT01728818) and dacomitinib (NCT02039336) in pancreatic cancer are currently underway.
Anti-angiogenesis
Targeting vascular endothelial growth factor (VEGF) pathway has shown promising results in the treatment of many solid cancers. However, anti-VEGF therapies have been ineffective clinically in treating patients with PDAC. The phase III Cancer and Leukemia Group B (CALGB) 80303 trial randomized 602 patients with APC to receive gemcitabine with or without bevacizumab in the first-line setting [
75]. The addition of bevacizumab to gemcitabine was associated with increased toxicity and without any improvement in survival (5.8 months vs. 5.9 months;
P = 0.95). In another large, randomized phase III trial (AVITA), 607 metastatic pancreatic cancer patients were randomized to receive gemcitabine plus erlotinib with either bevacizumab or placebo [
76]. The bevacizumab arm was associated with statistically significant PFS advantage (4.6 months vs. 3.6 months; HR 0.73;
P = 0.0002) but a non-significant improvement in median OS (7.1 months vs. 6 months; HR 0.89;
P = 0.2087). Axitinib is a selective oral inhibitor of VEGF receptor-1, -2, and -3 that has been combined with gemcitabine in a phase II clinical trial of APC patients and showed a statistically non-significant gain in OS [
77]. A subsequent phase III study that randomized 632 APC patients to receive gemcitabine plus axitinib or placebo was terminated early due to the lack of survival benefit (8.5 months vs. 8.3 months; HR 1.014;
P = 0.5436) at the time of planned interim analysis [
78]. Ziv-Aflibercept is an anti-VEGF recombinant fusion protein that has also been combined with gemcitabine in a phase III trial for the treatment of metastatic pancreatic cancer patients [
79]. However, this trial was terminated early as well due to the lack of efficacy at the time of planned interim analysis. Sorafenib and masitinib are oral multikinase inhibitors with antiangiogenic properties. In the phase III BAYPAN trial, addition of sorafenib to gemcitabine did not improve PFS in APC patients [
80]. The phase III study of gemcitabine plus masitinib also did not result in improvement of OS in patients with unresectable pancreatic cancer [
81].
Insulin-like growth factor-1 receptor
Insulin-like growth factor (IGF)-1 receptor is highly expressed in PDAC and participates in downstream signaling pathways that are involved in cancer cell survival and proliferation. Several mAbs against IGF-1 receptor (cixutumumab, ganitumab, dalotuzumab) are currently being tested in clinical trials. Ganitumab (AMG 479) was studied in combination with gemcitabine in a phase II trial of metastatic pancreatic cancer patients and showed a slight improvement in 6-month survival rate when compared with gemcitabine plus placebo (57% vs. 50%) [
82]. However, the phase III GAMMA trial of ganitumab plus gemcitabine combination was terminated early due to lack of efficacy at the preplanned interim analysis. In another phase II trial that evaluated cixutumumab plus gemcitabine and erlotinib in metastatic pancreatic cancer patients, the three-drug combination did not improve the PFS or OS when compared with gemcitabine plus erlotinib [
83].
PI3K/AKT/mTOR pathway
This is one of the major downstream effector pathways of
KRAS gene that is being evaluated as a potential target for pancreatic cancer treatment [
84]. Rigosertib is a small molecular inhibitor of PI3K that was combined with gemcitabine in a phase II/III clinical trial (ONTRAC trial). The study was terminated early due to lack of demonstration of benefit at the time of interim analysis. Buparlisib (BKM120) is another PI3K inhibitor being evaluated in combination with mFOLFOX6 regimen in a study of advanced stage solid tumors including pancreatic cancer (NCT01571024). MK2206 is an AKT inhibitor currently under clinical evaluation in patients with pancreatic cancer (NCT01783171, NCT01658943). Archexin (RX-0201) is another AKT inhibitor that was evaluated in combination with gemcitabine in a phase II study (NCT01028495). BEZ235 is a combined inhibitor of PI3K and mTOR. A phase I study evaluating the activity of BEZ235 plus a MEK inhibitor (MEK162) in advanced solid tumor patients (including pancreatic cancer) with
KRAS,
NRAS and/or
BRAF mutations has recently completed (NCT01337765). Everolimus (RAD001) is a mTOR inhibitor that was associated with a PFS of 1.8 months and OS of 4.5 months in a phase II study consisting of 33 gemcitabine-refractory, metastatic pancreatic cancer patients [
85]. It is also being evaluated as a part of combination regimens with other agents in ongoing clinical trials.
Wnt/β-catenin pathway
Wnt signals are transduced through the frizzled receptor and lipoprotein-related protein to the β-catenin signaling cascade. There is evidence to suggest that Wnt pathway plays a role in pancreatic cancer formation via involvement in pancreatic cancer stem cells (CSCs) [
86,
87]. Phase I trials using mAbs (OMP-54 F28, OMP-18R5) against frizzled receptors to inhibit Wnt signaling in PDAC are currently ongoing (NCT02050178, NCT02005315).
Notch signaling pathway
Notch signaling has been shown to be upregulated in many human cancers including PDACs [
88,
89]. It mediates pancreatic CSC function and contributes to chemotherapy resistance, tumor recurrence, and metastases. Gamma secretase is an enzyme that causes proteolytic cleavage and release of the intracellular domain of the Notch, leading to activation of the Notch signaling pathway. In preclinical models, inhibition of Notch pathways with a gamma-secretase inhibitor (GSI) in combination with gemcitabine showed enhanced antitumor activity [
90]. A phase II study evaluating an oral GSI (RO4929097) in pretreated metastatic pancreatic cancer patients was recently completed (NCT01232829). MK0752 is another GSI being tested in combination with gemcitabine for first-line treatment of stage III and IV PDAC patients (NCT01098344). Tarextumab (anti-Notch2/3 mAb, OMP-59R5) and demcizumab (anti-DLL4 mAb, OMP-21 M18) also inhibit Notch signaling and are being evaluated in clinical trials. ALPINE trial is studying the combination of tarextumab with nab-paclitaxel and gemcitabine (NCT01647828).
Targeting desmoplastic tumor microenvironment
The desmoplastic stroma in the tumor microenvironment is now regarded as a key component of pancreatic cancer biology which not only acts as a physical barrier to effective drug delivery inside the tumor but also facilitates tumor growth and promotes metastases. Strategies aimed at targeting the stromal compartment may enhance the delivery of chemotherapeutic agents to the tumor cells leading to improved efficacy. The most promising targets include the SHH signaling pathway, hyaluronic acid, and SPARC.
SHH pathway is an important signaling system that can activate the characteristic desmoplastic reaction present in the microenvironment of pancreatic tumors [
22]. Sustained activation of this pathway enhances tumor growth during pancreatic oncogenesis [
91]. Several clinical trials have been initiated to investigate the activity of SHH inhibitors in patients with PDAC. Vismodegib (GDC-0449) is a SHH inhibitor under clinical evaluation in combination with gemcitabine and nab-paclitaxel (NCT01088815). Saridegib (IPI-926) is another agent targeting the SHH pathway that was combined with gemcitabine in a phase II study of APC, but the trial was prematurely terminated since the combination was associated with a shorter survival than gemcitabine alone (NCT01130142). A study combining the SHH inhibitor sonidegib (LDE225) with FOLFIRINOX in untreated APC patients is ongoing (NCT01485744).
Hyaluronan is a glycosaminoglycan present in the extracellular matrix of PDAC, and high levels within the tumor are usually associated with a poor prognosis. Pegylated human recombinant PH20 hyaluronidase (PEGPH20) degrades hyaluronan and has been shown to decrease the hyaluronic acid content in a genetically-engineered PDAC mouse model, allowing for re-expansion of the PDAC blood vessels and enhanced intratumoral delivery of chemotherapeutic agents which leads to decreased tumor growth [
92]. Clinically, the combination of PEGPH20 plus gemcitabine has shown promising activity in a phase Ib study of metastatic pancreatic cancer patients [
93], and a phase II study of this combination is ongoing (NCT01453153). Another phase Ib/II study of PEGPH20 plus modified FOLFIRINOX combination in metastatic pancreatic cancer patients is currently under clinical evaluation (NCT01959139).
SPARC (osteonectin) is an extracellular matrix protein that plays a role in collagen turnover in the dense stroma. It is associated with invasion and metastasis in PDAC, and elevated levels are associated with poor prognosis. Nab-paclitaxel is albumin-bound paclitaxel that increases tumor accumulation of paclitaxel through binding of albumin to the stroma rich in overexpression of SPARC. As described previously, the efficacy of nab-paclitaxel plus gemcitabine combination in the first-line treatment of metastatic pancreatic cancer was demonstrated in the phase III MPACT trial which ultimately led to its FDA approval [
59].
Some of the other novel strategies aimed at targeting the desmoplastic stroma within the pancreatic tumor include the use of matrix metalloproteinase (MMP) inhibitors, heparin derivatives, and hypoxia targeting agents. MMP inhibitors have been tried for the treatment of PDAC without much success to date. Marimastat (BB-2516) is a broad spectrum MMP inhibitor that was combined with gemcitabine in the treatment of APC but did not show any demonstrable clinical benefit [
94]. Tanomastat (BAY12-9566) is another biphenyl MMP inhibitor with antiangiogenic and antimetastatic properties that was compared with gemcitabine for the treatment of APC patients and was found to be inferior to gemcitabine [
95]. Heparin-derivative agents such as 2-0, 3-0 desulfated heparin (ODSH) and necuparanib (M402) are currently being studied in combination with gemcitabine and nab-paclitaxel for treatment of patients with metastatic pancreatic cancer (NCT01461915, NCT01621243). It is now well established the pancreatic tumor microenvironment is characterized by hypoxia. Consequently, hypoxia-targeting agents are being developed to evaluate this novel therapeutic strategy. TH-302 is a hypoxia-activated prodrug that is activated into a potent DNA-alkylating agent, bromo-isophosphoramide mustard selectively under hypoxic conditions. A recent phase II study of TH-302 plus gemcitabine showed a significant improvement in primary end point of PFS when compared with gemcitabine alone (5.6 months vs. 3.6 months; HR 0.61; 95% CI 0.43 to 0.87;
P = 0.005) [
96]. A phase III trial of this combination is currently in progress (MAESTRO study; NCT01746979).
TGF-β signaling pathway
TGF-β participates in stimulating stromal reaction, invasion, metastases, and angiogenesis in PDAC [
97]. Examples of novel agents that target TGF-β signaling include trabedersen (AP-12009) and galunisertib (LY2157299). Trabedersen is a specific inhibitor of TGF-β2 that has demonstrated good safety and encouraging survival results in the phase I/II clinical study [
98]. Galunisertib is being evaluated in combination with gemcitabine for the treatment of patients with APC in an ongoing phase Ib/II clinical trial (NCT01373164).
Epigenetic modification
Epigenetic changes such as histone deacetylation (HDAC) and DNA methylation (cytosine methylation within CG dinucleotides) can result in inactivation of the tumor suppressor genes leading to tumor growth and progression. Vorinostat is a HDAC inhibitor being tested in a phase I/II study of LAPC patients in combination with capecitabine and radiotherapy (NCT00948688). 5-Azacytidine is a cytosine analog that inhibits DNA methyltransferase, and a phase I study of its combination with gemcitabine in APC patients was recently terminated (NCT01167816).
Adenosine monophosphate-activated protein kinase pathway
The oral anti-diabetic drug metformin is an activator of adenosine monophosphate-activated protein kinase (AMPK) and disrupts the crosstalk between insulin receptor and G protein-coupled receptors (GPCR) signaling in pancreatic cancer cells, via inhibition of mTOR and suppression of its downstream effectors [
99]. In xenograft mice models, metformin has been shown to inhibit pancreatic cancer growth [
99]. Clinically, the available data surrounding the benefit of metformin in pancreatic cancer is conflicting and is mostly derived from retrospective studies. In a hospital-based case-control study, metformin was shown to decrease the risk of developing pancreatic cancer among diabetics [
100]. In another retrospective analysis, metformin use was associated with an improvement in survival for the PDAC patients with diabetes [
101]. In contrast, a more recent study from the UK failed to demonstrate a survival benefit from metformin use in PDAC patients [
102]. There are multiple ongoing phase I and phase II trials that are evaluating the efficacy of metformin in PDAC. A phase I study of metformin plus erlotinib and gemcitabine in patients with APC has recently completed accrual (NCT01210911).
Synthetic lethality
The DNA double-strand breaks are repaired by a process of homologous recombination that is mediated via BRCA1 and BRCA2 proteins. Mutations in BRCA render this repair mechanism dysfunctional and are known to occur in both sporadic and familial cases of pancreatic cancer. Poly (ADP-ribose) polymerase (PARP) is another critical enzyme that mediates repair of DNA single-strand breaks. PARP pathway assumes the major role for DNA repair when BRCA mutation occurs. Consequently, inhibition of the PARP pathway results in ‘synthetic lethality’ via inhibition of DNA repair in BRCA-deficient tumor cells. A phase II study of veliparib alone or in combination with gemcitabine plus cisplatin for locally advanced and metastatic, BRCA 1–2, and PALB2-mutated pancreatic cancer patients is ongoing (NCT01585805). Other PARP inhibitors that are being evaluated in pancreatic cancer clinical trials include rucaparib (AG-14699; NCT02042378) and olaparib (AZD2281; NCT00515866).
Immunotherapy-based approaches
Despite significant efforts, no immunotherapeutic strategy against pancreatic cancer has demonstrated clinical benefit in a randomized phase III trial till date. This has been attributed to the immunologically quiescent microenvironment of pancreatic cancer. Currently, several approaches aimed at stimulating the host immune system against the pancreatic cancer tumor cells are under evaluation.
GI-4000, a form of RAS-specific immunotherapy, is heat-killed recombinant
Saccharomyces cerevisiae yeast that expresses mutant RAS peptides [
103]. A phase II trial of GI-4000 plus adjuvant gemcitabine is ongoing (NCT00300950). Reovirus is a tumor-targeted replication-competent virus with specificity for RAS-activated cells [
104]. It is being combined with chemotherapy for the treatment of APC patients in two phase II clinical trials (NCT00998322, NCT01280058).
Developing vaccines against tumor antigens is another potential immunotherapeutic strategy to treat pancreatic cancer. Several antigens have been explored as potential targets for vaccine-based treatment in pancreatic cancer including carcinoembryonic antigen (CEA) [
105], MUC1 [
106,
107], and heat shock proteins (HSP) [
108]. Algenpantucel-L immunotherapy is a whole-cell allogeneic pancreatic cancer vaccine composed of two irradiated human pancreatic cell lines that have been genetically modified to overexpress murine alpha(1,3)-galactosyltransferase, resulting in expression of alpha-galactosyl (Alpha-Gal) epitopes on membrane glycoproteins and glycolipids. Since human cells do not express these epitopes, an immediate hyperacute rejection response ensues leading to the development of strong T-cell mediated antitumor immunity. This immunotherapeutic agent has demonstrated encouraging activity when combined with radiation and 5-FU plus gemcitabine in a phase II adjuvant trial of resected PDAC patients [
109]. The phase III adjuvant trial that compares gemcitabine with or without algenpantucel-L, followed by chemoradiation has also completed recently (NCT01072981). Another phase III neoadjuvant trial is evaluating FOLFIRINOX with or without algenpantucel-L, followed by chemoradiation in borderline-resectable and unresectable LAPC patients (NCT01836432). GV1001 is a telomerase peptide vaccine shown to prolong survival when combined with granulocyte-macrophage colony-stimulating factor (GM-CSF) in a phase I/II study of unresectable LAPC patients [
110]. However, the phase III study comparing GV1001 and gemcitabine in sequential combination, vs. gemcitabine monotherapy in advanced unresectable pancreatic cancer was terminated early due to lack of survival benefit in the GV1001 arm (NCT00358566). G17D (Gastrimmune) is an antigastrin-17 immunogen that was evaluated in a randomized, multicenter, placebo-controlled study of APC patients and showed a non-significant improvement in OS when compared to placebo [
111]. Another potential therapeutic strategy that has been explored is combining two vaccines (GVAX plus CRS-207) with the hope to achieve an enhanced efficacy. GVAX is composed of pancreatic cancer cells that have been genetically modified to secrete GM-CSF and can induce T-cell responses. CRS-207 is a live-attenuated Listeria-based vaccine that can induce listeriolysin O and mesothelin-specific T-cell responses. The combination of GVAX plus CRS-207 is being evaluated in a phase IIb clinical trial (ECLIPSE) that consists of previously treated metastatic pancreatic cancer patients (NCT02004262).
Prostate stem cell antigen (PSCA) is a glycosylphosphatidylinositol-linked cell surface antigen expressed in pancreatic cancers. AGS-1C4D4 is a fully human IgG1 mAb against PSCA that has been combined with gemcitabine for the treatment of metastatic pancreatic cancer patients and has shown encouraging results [
112].
It is now established that CD40 activation can reverse immune suppression and drive antitumor T-cell responses. Utilizing this concept, agonist CD40 antibody has been used in combination with gemcitabine in a phase I study to shrink PDAC by stimulating tumor macrophages against pancreatic cancer stroma [
113].
Immunoinhibitory checkpoint pathways (cytotoxic T lymphocyte-associated protein-4 [CTLA-4]/B7, programmed cell death-1 [PD-1]/programmed cell death ligand-1 [PD-L1]) are emerging as interesting immunotherapeutic targets for the treatment of cancer. Single agent ipilimumab was evaluated in a phase II trial of APC and failed to demonstrate an appreciable antitumor activity [
114]. The combination of ipilimumab with gemcitabine is currently under phase I evaluation (NCT01473940).
Radioimmunotherapy with
90Y-clivatuzumab tetraxetan (radioimmunoconjugate comprised of the humanized mAb HuPAM4 that is radiolabeled with yttrium-90) is another potential therapeutic strategy that is being evaluated in clinical trials. The combination of
90Y-clivatuzumab tetraxetan with low-dose gemcitabine demonstrated a median OS of 7.7 months in a phase I study of untreated APC patients [
115]. The phase III study (PANCRIT-1) of this combination in pretreated metastatic pancreatic cancer patients is ongoing (NCT01956812).
An additional therapeutic strategy is adoptive cell transfer (ACT) approach which utilizes introduction of engineered T-cells with chimeric antigen receptors (CARs) to specifically recognize a tumor antigen of interest. This personalized immunotherapy approach is still under preclinical stages of development in the field of pancreatic cancer [
116].
Novel cytotoxic agents
PEP02 (MM-398) is a novel nanoparticle liposomal formulation of irinotecan. In a phase II study of gemcitabine-refractory metastatic PDAC patients, treatment with single agent PEP02 was associated with a median PFS and OS of 9 weeks and 21.6 weeks, respectively [
117]. A phase III trial (NAPOLI 1) is evaluating the combination of PEP02 with 5-FU in metastatic pancreatic cancer patients who have failed prior gemcitabine-based therapy (NCT01494506).
S-1 is a fourth-generation oral fluoropyrimidine that contains tegafur (FT, a prodrug of 5-FU), 5-chloro-2,4-dihydropyrimidine (CHDP), and potassium oxonate (Oxo). It has been evaluated in the treatment of both resectable and advanced pancreatic cancer with encouraging results. In a phase II Japanese study (PC-01), 116 patients with unresectable APC were randomized to receive gemcitabine plus S-1 vs. gemcitabine alone [
118]. There was significant improvement in the ORR (28.3% vs. 6.8%;
P = 0.005) and median OS (13.7 months vs. 8.0 months;
P = 0.035) in the S-1 arm. Japan Adjuvant Study Group of Pancreatic Cancer (JASPAC-01) is a phase III non-inferiority trial that compared S-1 with gemcitabine as adjuvant chemotherapy for patients with curatively resected pancreatic cancer [
119]. The interim analysis showed that S-1 was non-inferior to gemcitabine (OS at 2 years was 70% vs. 53%; HR 0.56; 95% CI 0.42 to 0.74;
P < 0.0001 for non-inferiority) [
120]. Based on the results of this study, the authors proposed that S-1 should be considered as a new standard treatment for patient with resected pancreatic cancer.
Identification of biomarkers
Development of more efficacious approaches for pancreatic cancer treatment would require identification of biomarkers that can predict the response and toxicity to various therapeutic agents. Research efforts geared towards this objective are underway.
The human equilibrative nucleoside transporter-1 (hENT1) plays an important role in the uptake of gemcitabine in cells and has been evaluated as a potential predictive biomarker of gemcitabine response. In a study that evaluated the expression pattern of genes involved in gemcitabine activity in 102 pancreatic tumor specimens, it was found that low hENT-1 expression levels were associated with a poorer prognosis [
121]. However, in the pivotal phase II Low hENT1 and Adenocarcinoma of the Pancreas (LEAP) study, the hENT1 status was shown to have no clinical utility for predicting gemcitabine sensitivity [
122]. Some additional potential biomarkers that have been evaluated in pancreatic cancer treatment include deoxycytidine kinase (dCK), ribonucleoside reductase-M1 (RRM1), and -M2 (RRM2) [
123,
124],
KRAS status, SPARC staining [
58], IGF-1R expression, and rs9582036 single nucleotide polymorphism (SNP) in the VEGF receptor-1 region [
125]. Recently, pharmacogenomic profiling of circulating tumor and invasive cells (CTICs) isolated from patients with PDAC was evaluated as a predictor of tumor response, progression, and resistance [
126].