A new study published in PNAS has shown that a dual kinase inhibitor targeting platelet-derived growth factor receptor-β (PDGFRβ) and B-RAF produces potent anti-angiogenic effects, which are not seen with the inhibition of either target alone. Compared with currently used multi-target kinase inhibitors, specific dual inhibitors might offer more efficacious anticancer activity with minimal side effects.
Cheresh and colleagues proposed that a compound that inhibited B-RAF and PDGFRβ would have a potent anti-angiogenic effect by targeting endothelial cells and pericytes, the two cell types involved in angiogenesis and vascular remodelling, respectively.
The authors used computational docking studies to design an amino-triazole scaffold that fitted into the allosteric site in the inactive conformation of PDGFRβ and B-RAF, as well as of other kinases while avoiding the ATP-binding pocket. The docking studies were based on the binding of the multi-target inhibitors sorafenib (Nexavar; Bayer/Onyx) and imatinib (Glivec/Gleevec; Novartis), which act, in part, to stabilize the inactive conformation of these kinases and have some allosteric activity against them. Nine amino-triazole-based compounds were designed in silico, then chemically synthesized and screened for activity against PDGFRβ in cell-based assays. The compounds were also screened for their capacity to inhibit the phosphorylation of endothelial cell mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated kinase (ERK), which are downstream of B-RAF signalling. Active compounds were then analysed for anti-angiogenic activity in developing zebrafish.
One compound — known as compound 6 — that inhibited both PDGFRβ activity and B-RAF heterodimer formation, and had cytotoxic activity was chosen for further study. Compared with imatinib and sorafenib, compound 6 had improved selectivity against 70 diverse kinases. In zebrafish embryos treated with compound 6, endothelial cells failed to form mature blood vessels. Moreover, the compound disrupted a late step in the formation of blood vessel lumens, which was followed by the induction of apoptosis. To produce the anti-angiogenic effect, dual inhibition of PDGFRβ and B-RAF was required, as both a B-RAF inhibitor and a PDGFRβ inhibitor were needed to produce a similar response to compound 6; either inhibitor alone did not produce this effect.
The authors next examined the anti-angiogenic properties of compound 6 in mice. Injection of the compound to mice with induced neovascularization blocked angiogenesis and inhibited signalling mediated by B-RAF and PDGFRβ. In an orthotopic pancreatic carcinoma model, tumour growth was suppressed in mice systemically treated with the compound — with treated tumours showing less vascularization. In addition, oral administration of compound 6 suppressed tumour growth in an orthotopic renal cell carcinoma model.
A unique feature of compound 6 is that it is the first strictly allosteric inhibitor of B-RAF that avoids the ATP-binding pocket of the kinase. Indeed, three recent papers (see page 271) have highlighted that B-RAF inhibitors that target the ATP-binding pocket cause B-RAF dimerization and increased activation of MEK and ERK, which leads to tumour cell proliferation. So, allosteric inhibitors — such as compound 6, which disrupts B-RAF dimerization — could offer the anticancer benefits seen with other B-RAF inhibitors while avoiding the problems associated with ATP mimetics of this target.
ORIGINAL RESEARCH PAPER
Murphy, E. A. et al. Disruption of angiogenesis and tumor growth with an orally active drug that stabilzes the inactive state of PDGFRβ/B-RAF. Proc. Natl Acad. Sci. USA 107, 4299–4304 (2010)
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Harrison, C. A double strike at kinases. Nat Rev Drug Discov 9, 271 (2010). https://doi.org/10.1038/nrd3152
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DOI: https://doi.org/10.1038/nrd3152