55P - The SOS inhibitor BAY293 contributes to amplified vertical inhibition of the MAP kinase pathway in human melanoma cells

Background

The driver mutation Braf^V600E in human melanoma has been successfully targeted by vemuarfenib and successors. Since then vertical inhibition of the mitogen-activated protein (MAP)-kinase pathway by dual inhibition of B-Raf and MAP-kinase kinase (MEK)1/2 has been introduced in the therapy of advanced melanoma. Here we investigated in vitro inhibition of the guanine nucleotide exchange factor Son of Sevenless (SOS) by BAY293 in order to identify possible synergism with Braf or/and MEK1/2 inhibitors in human melanoma cell lines.

Methods

The 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) assays and wound healing assays were used to monitor viability, proliferation and migration in human melanoma cell lines from the early (WM35, WM278, and WM793b) and the metastatic growth phase (A375 and 518a2), all harbouring the driver mutation Braf^V600E, but wild type NRAS. Apoptosis and kinase signalling were investigated by caspase assays and Western blot analyses.

Results

The SOS inhibitor BAY293 inhibited basal ERK1/2 phosphorylation in a dose dependent manner in human metastatic A375 and 518a2 melanoma cells. Importantly, viability was not affected in these cells following BAY293 exposure times below 24h. Longer incubation times enhanced cell death and reduced gap closure in a wound healing assay. Co-application of BAY293 with inhibitors of B-Raf and/or MEK1/2 augmented these effects significantly. Concomitantly, ERK1/2 phosphorylation was significantly reduced by these drug combinations. Similar effects were seen in early stage melanoma cells so that no stage dependent differences were observed.

Conclusions

While the SOS inhibitor BAY293 is not capable to substantially inhibit viability of melanoma cell lines, co-administration with vemuarfenib and trametinib augment cytotoxicity in an additive manner. Taken together these results are indicative for additivity in vertical inhibition of the MAP kinse pathway using BAY293 also in wild type RAS melanoma.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.