CDK4/6 Targeting May Be Effective in Acquired BRAF Plus MEK Inhibitor–Resistant Melanoma

Combination CDK4/6 and MEK inhibitor treatment reduced tumor growth in mouse models of melanoma with acquired resistance to BRAF and MEK inhibition.

A preclinical study published in Molecular Cancer Therapy identified the combination of CDK4/6 and MEK inhibitors as a potential avenue to treat melanoma with acquired resistance to BRAF/MEK inhibitors.

The BRAF inhibitor (BRAFi) dabrafenib in combination with trametinib, an MEK inhibitor (MEKi), is known to improve progression-free survival (PFS) in metastatic melanoma with BRAF-V600 alterations compared with BRAFi monotherapy. But long-term responses are rare, with a 5-year PFS rate of 19%. A majority of patients appear to acquire resistance and experience tumor regrowth, and a known mechanism of resistance is reactivation of the MAPK pathway.

This preclinical study aimed to identify genetic resistance mechanisms and validate potential combination therapies to treat BRAF-V600E mutant melanoma refractory to BRAFi plus MEKi combination therapy, utilizing computational and experimental methods.

Tumor tissue and peripheral blood samples were collected from 2 patients with melanoma whose disease was resistant to MRAFi plus MEKi treatment at the University of Colorado Cancer Center between 2010 and 2014. The samples then underwent whole exome sequencing.

At the time of disease progression, patient 1 showed BRAF-V600E and NRAS-G12V mutation and a CDKN2A copy number loss. The sample from patient 2 was analyzed at baseline before MEKi treatment, at disease progression after BRAFi treatment, and after disease progression on BRAFi plus MEKi. This patient developed an NRAS-Q61K mutation and lost 1 copy of CDKN2A after BRAFi and MEKi therapy. All of the treatment-resistant tumor samples from both patients harbored BRAF-V600E mutations.

“These results suggest that the potential resistance mechanisms were through reactivation of the MAPK pathway and activation of the cell cycle pathway,” the authors wrote. They hypothesized that inhibiting both the MAPK pathway and the cell cycle pathway could overcome acquired resistance to treatment with BRAFi plus MEKi. They focused on CDK4/6 inhibition with palbociclib combined with either a MEKi (trametinib) or an ERK inhibitor (ulixertinib) as potential treatment options.

The samples were used to develop xenograph (PDX) models. Once tumors reached 100 to 300 mm3, mice were randomized to receive 30 days of treatment with one of the following 6 regimens:

  • Vehicle
  • Palbociclib (75 mg/kg oral gavage daily)
  • Ulixertinib (75 mg/kg oral gavage twice daily)
  • Trametinib (0.5 mg/kg oral gavage daily)
  • Palbociclib and ulixertinib
  • Palbociclib and trametinib

Tumor size was measured twice per week, and mice were monitored daily for any signs of toxicity. For the analysis, the mean for each group was calculated using data from at least 3 independent experiments.

In vivo, palbociclib and ulixertinib regimen reduced tumor size by 64.8% on average in the first model, and palbociclib plus trametinib reduced tumor size by an average of 80.4% compared with the vehicle-treated control tumors. In the second model, palbociclib and trametinib reduced tumor size by 42% on average compared with the control, but palbociclib plus ulixertinib showed an average 32.7% reduction, which researchers did not consider to be significant.

“Going forward, our studies will focus on the combination of CDK4/6i and MEKi, given that MEK inhibitors are currently FDA approved for melanoma treatment,” the authors wrote, “and our observation that this specific combination more significantly reduced PDX tumor growth.”

For in vitro BRAFi-resistant cell line models, combination palbociclib and trametinib reduced cellular growth, inhibited the cell cycle, and inhibited downstream MAPK and other cell cycle pathway signaling. Loss of CDKN2A also increased sensitivity to CDK4/6 inhibition both as monotherapy and in combination with an MEKi.

“A key implication of our study is that the combination of CDK4/6 and MEK inhibitors overcomes acquired resistance to BRAF/MEK inhibitors, and loss of CDKN2A may represent a biomarker of response to the combination,” the authors conclude. “Inhibition of the cell-cycle and MAPK pathway represents a promising strategy for patients with metastatic melanoma who are refractory to BRAF/MEK inhibitor therapy.”


Nassar K, Hintzsche J, Bagby S, et al. Targeting CDK4/6 represents a therapeutic vulnerability in acquired BRAF/MEK inhibitor-resistant melanoma. Mol Cancer Ther. Published online August 10, 2021. doi:10.1158/1535-7163.MCT-20-1126