Despite the rapid advances in many fronts, PDA remains one of the most difficult human malignancies to treat, indicating the need for novel therapeutic strategies to improve the poor prognosis of patients with this disease. The existence of CSCs probably influences the intractable nature of PDA, explaining why conventional cancer therapy fails in the vast majority of patients. CSCs have been shown to be highly resistant to chemotherapy and radiotherapy. Thus, from a clinical standpoint, targeting cancer-sustaining pancreatic CSCs will be of paramount significance because there are few effective therapies for PDA and most of the patients die within the first year of diagnosis.
In this report, we provide evidence that pancreatic CSCs are relatively enriched with DR5 compared with the non–stem cell bulk tumor populations. A combination therapy using DR5 agonist mAb, tigatuzumab, and GEM produced robust antitumor activity in freshly generated PDA xenografts. More importantly, combination therapy resulted in the reduction of pancreatic CSCs, tumor remission, prevention of tumor recurrence, and significant prolongation of the time to tumor progression. The combination therapy tested here has a wide potential therapeutic role, which may possibly kill cancer-sustaining stem cell pool to prevent tumor recurrence and as an adjuvant therapy in clearing CSC populations following primary tumor resection and radiation therapy.
DR5 agonists represent a new class of therapeutics that selectively target apoptosis. Significant effort has been devoted to the discovery and development of mAbs against DR5, based on the impressive tumor growth inhibition and superior survival advantage obtained in a broad range of human tumor xenografts treated with DR5 agonist mAbs in combination with chemotherapy or radiation therapy (34
). Activation of the DR5 pathway triggers programmed cell death through the extrinsic apoptotic pathway, independent of p53, which involves the direct recruitment and activation of caspase-8 through FADD (38
). The extrinsic apoptosis pathway is an intriguing target for cancer therapy because it can circumvent a common apoptosis resistance mechanism associated with mutations in the p53
tumor suppressor gene, which account for 50% of human cancers (39
). On the other hand, many conventional chemotherapeutic agents generally require the function of p53 to initiate apoptosis by engaging the intrinsic apoptotic pathway (41
). Combinations of the two approaches may facilitate tumor cell death that resists death induction through either one of the pathways as well as reduces the probability of tumor cells to develop resistance to either therapy.
Mounting data over the recent years have indicated the robust preclinical activity of TRAIL receptor–targeted human agonistic mAbs across a broad range of tumor types. Based on this, multiple clinical trials of either single agents or in combination with chemotherapeutics are currently at various stages (42
). Early-phase clinical trials using agonistic anti-TRAIL receptor antibodies indicate that these agents can be delivered safely, are generally well tolerated, and seem to mediate some clinical benefit in terms of disease stabilization or objective responses (43
). Death receptor agonists were safely combined with standard doses of cancer chemotherapeutics in small cohorts of patients. These combinations include single-agent cytotoxics, cytotoxic combinations, targeted agents, and cytotoxic-targeted agent combinations (25
). Our results clearly show that GEM treatment initially results in rapid tumor shrinkage in human primary PDA xenografts. GEM treatment did not deplete the CSC population and the tumors progressed after an interim in all xenografts. However, tumors in 50% (4 of 8) of the xenografts treated with GEM and tigatuzumab not only did not relapse but also did not double its size until sacrificed on day 120. Specifically, this notable cooperativity between tigatuzumab and GEM in tumor growth inhibition as well as CSC reduction suggests that this therapeutically attractive combination therapy could be efficacious in the clinical setting.
Notwithstanding our ability to sequence cancer genome and to create personalized targeted therapies, it is apparent that combination therapies, which target the CSC subpopulation as well as the bulk of the tumor cells, will be required to effectively manage cancer treatment (44
). Emerging studies show that CSCs are indeed more resistant to therapy than other cancer cells and injection of a small number of CSCs is able to reproduce an entire tumor (46
). It is thought that CSCs are relatively drug and radiation resistant by virtue of quiescence, expression of drug resistance mechanisms, and possibly their location in tissue niches with restricted drug access. Therefore, development of novel therapeutic strategy to combat CSCs is an attractive strategy for more effective cancer treatment. Although the true relevance of the CSC is yet to be revealed, there are tantalizing reports that the CSC can be selectively targeted without ablating normal stem cells (47
). As the concept of CSC is becoming scientifically accepted, there is increasing interest in evaluating potential therapeutic targets in these cells. Initial studies have focused on the evaluation of developmental pathways such as the Wnt and the Sonic Hedgehog pathways (48
). Indeed, a recent report from our group showed that the Sonic HH inhibitor cyclopamine induced tumor regression in combination with GEM in one patient PDA xenograft (16
). Another study showed that a combination of cyclopamine, rapamycin, and GEM was capable of eliminating pancreatic CSCs (49
). The present results indicate that there may be other targets in CSCs, and target-focused as well as systematic approaches are needed to investigate this aspect. As novel CSC directed therapies emerge, studies such as the one presented here can be of great value when trying to prioritize drugs with anti-CSC activity for preclinical testing and validation. This may allow us to better select targeted agents directed to CSCs, which may ultimately improve the chances that the chosen therapy successfully completes the clinical development.
The quest for potent targeted therapeutic approaches to combat CSCs is an imperative clinical issue. A recent report from our group suggests that pancreatic CSCs play a key role in the development of metastatic disease that negatively affects the overall survival of patients with PDA (30
). An important question is how we move forward and translate these findings to the clinic. By the time tumors are diagnosed, the disease is usually advanced and does not respond to treatment. Adding CSC-directed therapeutics to conventional agents in this advanced stage may unlikely be effective. A more appropriate scenario can be patients with resectable disease, who, despite treatment with GEM, uniformly develop disease progression. Here, the most important clinical goal is the elimination of micrometastatic disease, in which CSCs probably have a critical role. Combination of CSC-directed treatment with chemotherapy can be tested with the primary objective of preventing disease progression, relapse, and metastasis. Importantly, agents that eliminate the CSCs within a tumor may bring little or no immediate reduction in tumor size. However, tumor growth is not sustainable without CSCs to replenish the bulk population and the tumor will eventually degenerate as bulk tumor cells are depleted.
Our results agree with a previous report (16
) that ALDH and CD44 mRNA levels were elevated in the GEM-treated tumors as compared with the saline-treated tumors (). However, increased average mRNA levels did not correspond to an increase in the number of ALDH+
tumor cells as revealed by fluorescence-activated cell sorting (FACS) data (). At present, we do not know the reason behind this discrepancy. Additional studies are needed to investigate whether posttranscriptional modification or trafficking of antigens between cell membrane/cytoplasm and nucleus plays a role on this aspect. There is now abundant evidence that stem cell properties are highly relevant to the biology of several human cancers. An obvious question is whether CSCs in other tumor types have DR5 expression. Studies are needed to investigate whether DR5 is also a CSC target in other tumor types besides pancreas cancer.
Taken together, our results provide strong evidence that pancreatic CSCs are enriched with DR5. Combination therapy using GEM plus tigatuzumab results in long-term cures in an otherwise incurable direct PDA model. Furthermore, the novel combination proved to be more efficacious than either single agent alone by providing a double hit to kill both CSCs and non–stem cell bulk tumor populations. Considering that combination therapy produced remarkable reduction in PDA stem cells, tumor remission, and significant improvement in time to tumor progression, this innovative approach may represent a valuable treatment option to improve the current standard of care for PDA patients. Further investigation of this promising approach in PDA is warranted.