Kinase inhibitor library screening identifies synergistic drug combinations effective in sensitive and resistant melanoma cells

Background: Melanoma is the most aggressive and deadly form of skin cancer with increasing case numbers
worldwide. The development of inhibitors targeting mutated BRAF (found in around 60% of melanoma patients)
has markedly improved overall survival of patients with late-stage tumors, even more so when combined with MEK
inhibitors targeting the same signaling pathway. However, invariably patients become resistant to this targeted
therapy resulting in rapid progression with treatment-refractory disease. The purpose of this study was the
identification of new kinase inhibitors that do not lead to the development of resistance in combination with BRAF
inhibitors (BRAFi), or that could be of clinical benefit as a 2nd line treatment for late-stage melanoma patients that
have already developed resistance.
Methods: We have screened a 274-compound kinase inhibitor library in 3 BRAF mutant melanoma cell lines (each
one sensitive or made resistant to 2 distinct BRAFi). The screening results were validated by dose-response studies
and confirmed the killing efficacies of many kinase inhibitors. Two different tools were applied to investigate and
quantify potential synergistic effects of drug combinations: the Chou-Talalay method and the Synergyfinder
application. In order to exclude that resistance to the new treatments might occur at later time points, synergistic
combinations were administered to fluorescently labelled parental and resistant cells over a period of > 10 weeks.
Results: Eight inhibitors targeting Wee1, Checkpoint kinase 1/2, Aurora kinase, MEK, Polo-like kinase, PI3K and Focal
adhesion kinase killed melanoma cells synergistically when combined with a BRAFi. Additionally, combination of a
Wee1 and Chk inhibitor showed synergistic killing effects not only on sensitive cell lines, but also on intrinsically
BRAFi- and treatment induced-resistant melanoma cells. First in vivo studies confirmed these observations.
Interestingly, continuous treatment with several of these drugs, alone or in combination, did not lead to emergence
of resistance.
Conclusions: Here, we have identified new, previously unexplored (in the framework of BRAFi resistance) inhibitors
that have an effect not only on sensitive but also on BRAFi-resistant cells. These promising combinations together
with the new immunotherapies could be an important step towards improved 1st and 2nd line treatments for
late-stage melanoma patients.