PO-332 Sequence-dependent cross-resistance of combined radiotherapy plus BRAFV600E-inhibition in melanoma

Introduction Melanoma progression is often associated with the growth of anatomically critical metastases that require immediate therapy response, e.g. in the brain. In such situations, systemic drugs are often sequentially combined with radiotherapy. However, there is a lack of clinical and preclinical studies that systematically examine the potential effects of therapy-timing on treatment efficacy and radio-cross-resistance in melanoma. Material and methods We established an experimental platform comprising cell culture and mouse xenograft models that allows predicting the outcome of different BRAF V600E inhibitor-radiotherapy sequences. Retrospectively collected clinical follow up data of 65 stage IV melanoma patients with brain metastases were analysed for their intracranial response after radiotherapy depending on the sequence of co-administered MAPK inhibition. Results and discussions Our models indicated a higher rate of tumour relapse when melanoma cells were first treated with BRAF V600E inhibition followed by radiotherapy as compared to the reverse sequence. We identified the H3K4 demethylase JARID1B/KDM5B as a cellular marker for cross-resistance between BRAF V600E inhibition and radiotherapy. JARID1B high cells appeared more frequently in vitro and in vivo under upfront BRAF V600E inhibition as compared to upfront radiation. Accordingly, retrospective follow-up data from irradiated melanoma brain metastases confirmed a shortened duration of response of MAPK inhibitor-pretreated compared to MAPK inhibitor-naive metastases. Mechanistically, JARID1B favours cell survival by transcriptional regulation of genes controlling cell cycle, DNA repair, and cell death. Conclusion Our preclinical results and the clinical follow up of patients support a higher rate of tumour relapse when melanomas are treated with upfront MAPK inhibition as compared to upfront radiation. JARID1B may represent a novel therapy-overarching resistance marker that could guide future therapy sequencing.