Resistance to BRAF-inhibitors remains a major obstacle in the treatment of BRAF-mutant melanoma. This resistance frequently involves reactivation of the MAPK pathway and evasion of the immune system. Combination therapy with MEK inhibitors provides additional benefit, but resistance still occurs in a majority of patients, indicating new combination therapies are required. The CDK4 pathway is activated in a majority of melanomas, and this pathway converges with the MAPK pathway downstream of ERK, making it a promising target for combination therapy.
We have shown that a CDK4i/BRAFi combination has a synergistic effect in vitro and provides more effective inhibition of proliferation and prevention of resistance than either drug alone or a BRAFi/MEKi combination. The CDK4i/BRAFi combination causes a senescent-like phenotype in a majority of cells, and a significant increase in cell death. In vivo, the CDK4i/BRAFi combination has a synergistic effect on melanoma xenografts in nude mice, with robust tumour regression occurring in the first 14 days of combination treatment. This tumour regression is associated with an increase in tumour-infiltrating CD45.2+ immune cells, including a significant increase in natural killer cells. A375 melanoma cells treated in vitro demonstrate changes in expression of cytokines, including an increase in NK-promoting IL-2, and a decrease in the anti-inflammatory IL-10, indicating that the CDK4i/BRAFi combination may affect the microenviroment. Combination therapy also prevents the increase in PDL-1 expression that is associated with BRAF-inhibitor resistance.
To better understand how the innate immune system may contribute to tumour regression in response to targeted therapy, we are currently investigating the effect that CDK4i and the CDK4i/BRAFi combination has on innate immune cell proliferation, migration and activity.