Drug Discovery Opinion

Protein kinases play important roles in regulating most cellular processes and are commonly activated in cancer cells. A number of kinase inhibitors – including antibodies and small molecules –have already been approved for the treatment of cancer and many others are currently being tested. The majority of kinase inhibitors developed so far are ATP mimetics identified by high-throughput screening of catalytic kinase domains at low ATP concentration. Such compounds – so-called type I inhibitors – may lack specificity for individual kinases and/or be less effective when ATP concentrations are high. Crystal structures have revealed that some compounds – the type II inhibitors – occupy an allosteric site accessible only in the inactive conformation of the kinase and researchers at the Moores Cancer Center at the University of California have now designed selective type II inhibitors of PDGFRβ (important for pericyte recruitment) and B-RAF (important for endothelial cell survival).

Using the X-ray crystallographic structure of the type II inhibitor, sorafenib, bound to B-RAF, the team designed a small library of compounds based on a constrained amino-triazole scaffold predicted to stabilise kinases in the inactive state. The compounds were then tested for antivascular activity in both cell-based models and a zebrafish embryogenesis model. Compound 6 was found to inhibit both PDGFRβ and B-RAF cellular signalling – which produces a synergistic effect on tumour growth – but to have no effect on a variety of other cellular targets. The compound showed antiangiogenic activity in both zebrafish and murine models of angiogenesis and was also shown to suppress murine orthotopic tumors in both the kidney and pancreas.

The study is published in the Proceedings of the National Academy of Sciences.

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