Successful rechallenge in two patients with BRAF-V600-mutant melanoma who experienced previous progression during treatment with a selective BRAF inhibitor.

The v-raf murine sarcoma viral oncogene homolog B1 (BRAF) gene is mutated at position 600 in about 50% of melanoma. Mutant BRAF activates the downstream effectors of the RAS-RAF-MEK-MAPK pathways and is a driver oncogene in these melanoma cells. Selective BRAF-V600 inhibitors (vemurafenib, dabrafenib) have high antitumor activity against BRAF-V600-mutant melanoma with objective tumor response rates. Resistance, however, develops within less than a year in the majority of patients. Several different mechanisms have been found to mediate acquired resistance, but these do not involve the occurrence of secondary mutations in the BRAF gene. Two patients with BRAF-V600E mutant melanoma who had documented progression during treatment with dabrafenib/GSK1120212 and dabrafenib, respectively, were rechallenged with dabrafenib and vemurafenib after a treatment-free interval of 8 and 4 months during which further progression was documented. Both patients showed a marked clinical response and, in both, objective tumor regression (qualifying as a mixed and a partial response according to RECIST) was documented. These two case observations indicate that resistance to BRAF-selective inhibitors can be reversible following treatment interruption.