New clinical trials report the efficacy of two mechanism-based therapies for treating human pancreatic neuroendocrine tumours. Studies in mouse models have contributed to these success stories, and continue to do so.
Advances in cancer medicine have reset our clinical and social expectations: the aim now is to effectively combat formidable tumours — an effort that was previously deemed improbable. Writing in The New England Journal of Medicine, Raymond et al.1 and Yao et al.2 report phase III clinical trials of two drugs that target distinctive cancer-associated signalling pathways. The results suggest an impressive efficacy of both drugs (sunitinib and everolimus) for treating pancreatic neuroendocrine tumours. It is therefore likely that these drugs, which are already standard treatments for other cancers, will become the first new approvals in 25 years by the US Food and Drug Administration (FDA) for treating these cancers, a remarkable milestone.
Pancreatic neuroendocrine tumours (PNET) are uncommon, but difficult to diagnose and treat. These cancers, which originate from the hormone-producing pancreatic islet cells, stand in stark contrast to another type of pancreatic cancer, pancreatic ductal adenocarcinoma, which is much more prevalent and deadly: a larger proportion of patients with PNET undergo surgical excision, and the clinical course of the disease is highly variable. Nonetheless, patients with advanced PNET who are not candidates for surgery have a terminal illness, and their tumours are difficult to manage; the FDA-approved chemotherapeutic agent streptozotocin shows only modest activity in these patients.
A vast number of potential anticancer drugs are currently in the pipelines of biopharmaceutical companies. Indeed, the scope of mechanism-based targeting is broad, often with several potential drugs affecting the same target. Consequently, it is challenging to decide which targets and candidate drugs might be of value in particular forms of human cancer, especially those that are rare but deadly like PNET. There is growing optimism that genetically engineered mouse models, which can mimic the progression of specific types of human cancer at the genomic and tissue levels, can contribute to this prioritization3. The hope is that preclinical trials of candidate drugs in representative mouse models could help to motivate and guide clinical trials of targeted therapies in the related human tumours (Fig. 1). The two new papers1,2 reflect proof of this concept.
The mouse model of PNET, called RIP-Tag2, shows similar tissue-level features to the human tumours4. However, the cancer in the animal does not follow the same — currently obscure5 — initiating events that lead to human PNET; it is instead driven by a viral oncogene that abrogates the function of two generic tumour-suppressor pathways commonly lost in human tumours.
Preclinical trials in this model had predicted that both sunitinib, a pan-specific inhibitor of tyrosine-kinase enzymes, and everolimus, which inhibits another kinase, mTOR, would be effective in treating human PNET. Several studies6,7,8 showed that sunitinib, and other kinase inhibitors that target signalling associated with angiogenesis through receptors for the growth factors VEGF and PDGF (thus inhibiting angiogenesis), cause tumour shrinkage. Sunitinib also produced increased survival in the animal studies. These results motivated Raymond and colleagues to perform two phase II trials9,10 and now the phase III trial1 of sunitinib in patients with PNET (Box 1). Similarly, a separate study11 reported the efficacy of another mTOR inhibitor, rapamycin, in treating PNET in the mouse model, presaging the clinical success of everolimus, a refined mTOR inhibitor, which Yao et al.2 now describe.
Although drug efficacies seen in the preclinical trials were encouraging, the trials also revealed limitations — in tumour shrinkage and long-term survival of the mice — that may well influence how these drugs are most effectively used to treat human PNET. Yao and colleagues also find that, whereas everolimus delays time to progression of the disease (progression-free survival), it seemingly does not increase overall survival rates. This trial is still ongoing, however, so the lack of effect on overall survival is not yet conclusive.
Pertinent to this clinical observation is an intriguing result with translational potential from preclinical trials of rapamycin in the mouse model of PNET. Rapamycin on its own produced only a modest overall survival benefit, and the animals also showed evidence of rapamycin resistance following treatment, in the form of regrowth of the previously responding tumours9. But when rapamycin was given in combination with another approved drug — erlotinib, which inhibits the growth factor receptor EGFR — the animals' overall survival rate improved significantly and there was a decrease in relapse during treatment11.
These outcomes in the PNET mouse model are consistent with the possibly limited overall survival of patients with PNET following treatment with everolimus only2. The preclinical results therefore encourage clinical trials on everolimus in combination with erlotinib (or with other drugs that target downstream effectors in the same signalling pathway). A small clinical trial12 combining the two drugs to treat PNET is already under way.
For sunitinib, the tumour shrinkage and increased overall survival seen in preclinical trials8 are recapitulated in the human trial: Raymond et al.1 report improved both progression-free survival and overall survival after administration of this drug. But, as with everolimus, preclinical trials revealed limitations to the effectiveness of sunitinib in the form of adaptive resistance in PNET. In other words, faced with sunitinib's potent effect in blocking angiogenesis, the tumours not only adapt after a period of shrinkage, but also survive the treatment better, inducing alternative pro-angiogenic signalling circuits13 and becoming more invasive and metastatic8; this reflects a phenomenon seen in other preclinical models as well as in clinical trials14,15,16.
The mouse data therefore predict eventual failure of therapy with sunitinib alone, and should motivate preclinical and clinical trials to circumvent the evasive resistance — an iterative and bidirectional process of translational therapeutic oncology.
The clinical results with everolimus and sunitinib1,2 are landmarks for treating PNET. The approach that led to this — aligned preclinical trials in a representative mouse model and human clinical trials — could also be used to test the efficacy of other anticancer drugs and may well replicate this success story. Indeed, this approach heralds a future in which preclinical trials in genetically engineered mouse models, and in other representative animal models, could guide the development of more effective therapies for human cancers, revealing efficacy, beneficial drug combinations and (potentially surmountable) mechanisms of resistance.
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Scientific Data (2018)