The prognosis for patients with pancreatic cancer is poor (1
). Chemotherapeutic options are limited, and the standard of care, GEM, improves survival only minimally (21
). Resistance to apoptosis is a critical event in tumorigenesis as it allows for the development of mutations, survival in harsh conditions, and proliferation in an anchorage-independent fashion (22
). Efforts to increase therapy-induced apoptosis are appropriate and attractive for cancer chemotherapy (5
). We demonstrate that JP1201, a novel Smac mimetic, robustly improves the efficacy of standard chemotherapy in pancreatic cancer xenografts, syngenic tumors, and spontaneous transgenic tumors in mice. In vitro
mechanistic studies demonstrated that inhibition of XIAP is critical for sensitization of MIA PaCa-2 and PANC-1 cells. While in vivo
treatment with JP1201 resulted in induction of TNFα, these data suggest that JP1201 promotes apoptosis via two potential mechanisms that result in enhanced activity of GEM.
TRAIL is known to induce apoptosis in a variety of cell lines and is currently being pursued as a potential cancer therapy. We show here a moderate effect of TRAIL monotherapy in vivo
and potent anti-tumor activity in animals that were treated with the combination of JP1201 and TRAIL. This mirrors the in vitro
data with MIA PaCa-2 cells. Other studies in glioma (23
) and pancreatic cancer (10
) models show TRAIL alone is ineffective but TRAIL combined with inhibition of XIAP can induce apoptosis and inhibit tumor growth. These studies showed a reduction in tumor burden and/or an increase in survival, consistent with the results presented here. Thus, our data support that Smac mimetics in combination with TRAIL is a promising anticancer strategy.
Since GEM is the standard of care for pancreatic cancer, we determined if inhibition of XIAP would sensitize tumors to GEM therapy. We found that knockdown of XIAP but not related IAPs using siRNA increases sensitivity to GEM in pancreatic cancer cell lines. This data expands on work in other pancreatic cancer cell lines that shows resistance to chemotherapy is mediated in part by XIAP (8
). We also show that the sensitization to GEM with XIAP knockdown is strikingly similar to the sensitization seen with JP1201 in vitro
; supporting the concept that JP1201 sensitizes pancreatic cancer to GEM through inhibition of XIAP.
We also explored whether JP1201 would sensitize tumors to GEM treatment. We demonstrated a significant decrease in tumor weights in animals treated with the combination of JP1201 and GEM compared to saline in a late intervention model in both immune-compromised and immune-competent mice. The JP1201 sensitization was also evident at lower doses of GEM. In a separate experiment, this effect translated into increased survival in tumor-bearing animals. These data alone are very suggestive that Smac mimetics combined with standard therapy could greatly improve pancreatic cancer patient response to therapy. This study adds to the growing body of literature that demonstrate the potency of Smac mimetics in combination with various standards of care, such as a study done in a glioblastoma model where Smac mimetics were shown to enhance tumor cell death after irradiation treatment (24
). We further explored the effect of combination therapy in an aggressive transgenic model of pancreatic cancer and again saw increased survival.
We also found that treatment with JP1201 induced TNFα in vivo
. Previous reports (12
) have shown that Smac mimetic sensitive cell lines produce TNFα, can be induced to produce more TNFα after Smac mimetic treatment, and are dependent on TNFα for Smac mimetic-induced cell death. However, there is currently no published data indicating Smac mimetics induce TNFα in resistant cell lines, such as MIA PaCa-2. Possible explanations include prolonged treatment with JP1201 may induce TNFα production. Alternatively, the in vivo
tumor microenvironment may be required to facilitate TNFα production after treatment with JP1201 or other Smac mimetics. In that regard, it is worth noting that while the pancreatic cell lines showed in vitro
resistance to JP1201, xenografts did show reduction of tumor burden with JP1201 as a monotherapy, which might be linked to the production of TNFα. These observations suggest that alternative pathways may be engaged that promote cell death. TNFα expression has been linked to non-canonical activation of NFκB as a result of cIAP1/2 degradation (25
). NFκB can in turn stimulate autocrine expression of TNFα (25
), which activates receptor interacting protein kinase 1 (RIPK1) via the TNF receptor and subsequently induces caspase activation. In support of this, preliminary studies suggest that NFκB actively particpates in JP1201-mediated sensitization of pancreatic tumor cells, as treatment of cells with GEM, JP1201 and an IKK inhibitor (SC-514) increased the IC50
for GEM 3–13 fold compared to JP1201 + GEM (data not shown). Thus JP1201 could promote apoptosis by inducing the degradation of cIAP1/2, resulting in NFκB activation, and inhibiting the activity of XIAP both of which could potentiate the efficacy of GEM (30
In summary resistance to therapy-induced apoptosis is a major road block in the treatment of patients with pancreatic cancer. Another significant challenge is the lack of predictive power of pre-clinical models of pancreatic cancer. In an effort to address both of these challenges, we have demonstrated potent anti-tumor activity of a unique Smac mimetic in animal models of pancreatic cancer that are robust and clinically relevant. We believe our data support the development of agents designed to sensitize pancreatic tumors to apoptosis and suggest JP1201 deserves serious consideration for clinical investigation.