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BRAF(V600E) Kinase Domain Duplication Identified in Therapy-Refractory Melanoma Patient-Derived Xenografts

The therapeutic landscape of melanoma is improving rapidly. Targeted inhibitors show promising results, but drug resistance often limits durable clinical responses. There is a need for in vivo systems that allow for mechanistic drug resistance studies and (combinatorial) treatment optimization. Therefore, we established a large collection of patient-derived xenografts (PDXs), derived from BRAF(V600E), NRAS(Q61), or BRAF(WT)/NRAS(WT) melanoma metastases prior to treatment with BRAF inhibitor and after resistance had occurred. Taking advantage of PDXs as a limitless source, we screened tumor lysates for resistance mechanisms. We identified a BRAF(V600E) protein harboring a kinase domain duplication (BRAF(V600E/DK)) in ∼10% of the cases, both in PDXs and in an independent patient cohort. While BRAF(V600E/DK) depletion restored sensitivity to BRAF inhibition, a pan-RAF dimerization inhibitor effectively eliminated BRAF(V600E/DK)-expressing cells. These results illustrate the utility of this PDX platform and warrant clinical validation of BRAF dimerization inhibitors for this group of melanoma patients.

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S7842 LY3009120 LY03009120 is a potent pan-Raf inhibitor with IC50 of 44 nM, 31-47 nM, and 42 nM for A-raf, B-Raf, and C-Raf in A375 cells, respectively. Phase 1. (21) (5)

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Raf