Efficacy of intermittent combined RAF and MEK inhibition in a patient with concurrent BRAF- and NRAS-mutant malignancies

Abstract

Vemurafenib, a RAF inhibitor, extends survival in patients with BRAF(V600)-mutant melanoma but activates extracellular signal-regulated kinase (ERK) signaling in RAS-mutant cells. In a patient with a BRAF(V600K)-mutant melanoma responding to vemurafenib, we observed accelerated progression of a previously unrecognized NRAS-mutant leukemia. We hypothesized that combining vemurafenib with a MAP-ERK kinase (MEK) inhibitor would inhibit ERK activation in the melanoma and prevent ERK activation by vemurafenib in the leukemia, and thus suppress both malignancies. We demonstrate that intermittent administration of vemurafenib led to a near-complete remission of the melanoma, and the addition of the MEK inhibitor cobimetinib (GDC-0973) caused suppression of vemurafenib-induced leukemic proliferation and ERK activation. Antimelanoma and antileukemia responses have been maintained for nearly 20 months, as documented by serial measurements of tumor-derived DNA in plasma in addition to conventional radiographic and clinical assessments of response. These data support testing of intermittent ERK pathway inhibition in the therapy for both RAS-mutant leukemia and BRAF-mutant melanoma.

Figure 1

Treatment and clinical course of a patient with BRAF^V600K-mutant melanoma and NRAS^G12R-mutant CMML treated with vemurafenib and cobimetinib. A, the peripheral WBC (blue lines) and monocyte counts (red lines) throughout the treatment. Gray bars, vemurafenib therapy; open green bars, cobimetinib therapy. The width of the bars indicates duration of treatment. The relative heights of the bars reflect dose level adjustments as indicated on the right Y-axes [960, 720, and 480 mg twice a day (bid) for vemurafenib; 60, 40, and 20 mg every day for cobimetinib]. Arrows indicate times of bone marrow examinations. The antimelanoma response based on radiographic tumor measurements using Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria is shown in B. C, the quantification of circulating CMML-derived (red line) and melanoma-derived (black line) DNA in the plasma measured by digital PCR. Y-axes indicate the ratio of mutated DNA to wild-type DNA circulating in the plasma. No samples were available for analysis between weeks 30 and 69 (indicated by the dotted line).

Figure 2

Viability curves and immunoblots revealing viability and activation of mitogen–activated protein kinase (MAPK) signaling intermediates in NRAS^WT and NRAS^G12D AML cell lines (indicated with *) treated with various concentrations of cobimetinib, PLX4720, or both. A, IC ₅₀ values of BRAF WT/ NRAS^G12D and BRAF^WT/NRAS^WT AML cell lines with exposure to cobimetinib or PLX4720. B, the cell viability of BRAF^WT/NRAS^G12D AML cell lines with exposure to varying concentrations of cobimetinib, PLX4720, or both. DMSO, dimethyl sulfoxide. In A and B, data were derived from 72 hours of exposure to cells followed by CellTiter-Glo Luminescence assessment. C, immunoblots of pERK1/2 and total ERK1/2 (top) in NRAS^G12D-mutant and NRAS^WT human leukemia cell lines exposed to vehicle, cobimetinib, PLX4720, or PLX4720 + cobimetinib. Quantification of pERK1/2 relative to total ERK1/2 is displayed at the bottom. Cells were treated with 500 nmol/L of each drug for 24 hours.