A combined dual inhibition of vascular endothelial growth factor (VEGF) and c-MET is showing promise in preventing tumor invasion and metastasis. The data thus far are in a laboratory model of pancreatic neuroendocrine tumors (PNET). The results are published in the journal Cancer Discovery.
According to the lead author of the study, Donald M. McDonald, MD, PhD, Comprehensive Cancer Center, University of California, San Francisco, this study was designed to demonstrate a proof of principle. The concept is likely to extend to other tumor types.
McDonald explained that the inhibition of both VEGF and c-MET signaling has a synergistic effect on tumors that leads to slowing down of the tumor growth and decreased metastasis.
"The translational significance of the work is not necessarily to demonstrate a promising approach in PNET, but rather to show in general that targeting MET simultaneously with VEGFR is a better general approach than targeting the VEGF pathway alone without inhibiting MET," says Dana T. Aftab, PhD, part of the translational research team at Exelixis, and coauthor of the Cancer Discovery paper.
Many cancers show increased c-MET activity and mutations, including breast, liver, lung, kidney, thyroid, and ovarian cancers. Deregulated c-MET signaling has been shown to be associated with a more aggressive tumor phenotype.
McDonald and his team assessed the role of c-MET signaling when VEGF signaling is inhibited using a pancreatic neuroendocrine tumor mouse model and a second model that exhibited late-onset aggressive and metastatic tumors when the mice are in old age. The range of timing of tumor onset, including the very late-timing in the one model uniquely allows the assessment of whether the aggressive tumor characteristic can be advanced or reversed.
"Because tumors in [one of our mouse models] naturally become more invasive and metastatic with age, but generally do so near the time the mice succumb, this model provided the opportunity to determine whether aggressiveness could be advanced, delayed, or reversed by treatment with a c-MET inhibitor," explained McDonald.
Previous laboratory research showed that while inhibition of VEGF decreased tumor size, there is also a downside. According to Dana T. Aftab, "increased invasiveness after anti-VEGF therapy has been studied in the clinic most extensively in glioblastoma, where frequent imaging with high resolution techniques is standard."
The researchers now show that c-MET expression and activity increase upon VEGF inhibition and tumor hypoxia is elevated. When both the VEGF and c-MET pathways were inhibited, invasion and metastasis went down. This effect was also observed in mouse models of two more types of pancreatic cancers.
The team used a murine anti-VEGF antibody. The c-MET inhibitors used were crizotinib (Xalkori) and PF-04217903. Crizotinib was approved last year for metastatic non–small-cell lung cancer that harbors the EML4-ALK fusion gene. Crizotinib works by inhibiting both the anaplastic lymphoma kinase (ALK), as well as the tyrosine kinase c-MET. PF-04217903 is currently in phase I trials as a monotherapy for a range of advanced solid tumor types.
Cabozantinib, an oral, small-molecule inhibitor, blocks both c-MET and VEGF signaling. "A trial for cabozantinib in patients with pancreatic neuroendocrine tumors will open for patient recruitment soon," said Aftab. "However, the translational significance of the [currently published] work is not necessarily to demonstrate a promising approach in PNET," he added.
The drug is showing promising results in early-stage clinical trials. There was promising activity in metastatic medullary thyroid cancer in a phase I trial published last year in the Journal of Clinical Oncology. Of 35 patients, 10 showed a partial response. A phase III medullary thyroid clinical trial of cabozantinib further showed an almost three-fold improvement in progression-free survival (PFS) compared to placebo in results announced last October. The median PFS in the cabozantinib arm was 11.2 months compared to 4 months in the placebo arm (P < .0001). The full results will be presented at a clinical meeting.
"We have evaluated cabozantinib in many preclinical models with generally good results, but to my knowledge the only careful examination of VEGF plus MET inhibition, using combinations of selective inhibitors, is [this Cancer Discovery paper]," said Dana T. Aftab.
Activity with cabozantinib was also seen for many solid tumors. In a phase II trial presented at the 2011 American Society of Clinical Oncology annual meeting, cabozantinib showed an ability to shrink bone metastases in prostate cancer, breast, and melanoma patients. It also showed activity in ovarian, and prostate cancers, and preliminary data from a phase II liver cancer trial shows an increased progression-free survival for both treatment-naïve and previously treated patients.
McDonald and his team are already working further along these lines of research. "We are continuing to study the effects of c-MET inhibitors on tumor invasiveness and metastasis," he says.