Efficacy of MEK inhibition in patients with histiocytic neoplasms

Abstract

Histiocytic neoplasms are a heterogeneous group of clonal haematopoietic disorders that are marked by diverse mutations in the mitogen-activated protein kinase (MAPK) pathway^1,2. For the 50% of patients with histiocytosis who have BRAF^V600 mutations^3-5, RAF inhibition is highly efficacious and has markedly altered the natural history of the disease^6,7. However, no standard therapy exists for the remaining 50% of patients who lack BRAF^V600 mutations. Although ERK dependence has been hypothesized to be a consistent feature across histiocytic neoplasms, this remains clinically unproven and many of the kinase mutations that are found in patients who lack BRAF^V600 mutations have not previously been biologically characterized. Here we show ERK dependency in histiocytoses through a proof-of-concept clinical trial of cobimetinib, an oral inhibitor of MEK1 and MEK2, in patients with histiocytoses. Patients were enrolled regardless of their tumour genotype. In parallel, MAPK alterations that were identified in treated patients were characterized for their ability to activate ERK. In the 18 patients that we treated, the overall response rate was 89% (90% confidence interval of 73-100). Responses were durable, with no acquired resistance to date. At one year, 100% of responses were ongoing and 94% of patients remained progression-free. Cobimetinib treatment was efficacious regardless of genotype, and responses were observed in patients with ARAF, BRAF, RAF1, NRAS, KRAS, MEK1 (also known as MAP2K1) and MEK2 (also known as MAP2K2) mutations. Consistent with the observed responses, the characterization of the mutations that we identified in these patients confirmed that the MAPK-pathway mutations were activating. Collectively, these data demonstrate that histiocytic neoplasms are characterized by a notable dependence on MAPK signalling-and that they are consequently responsive to MEK inhibition. These results extend the benefits of molecularly targeted therapy to the entire spectrum of patients with histiocytosis.

Extended Data Figure 1.. Waterfall plot of maximum change in tumor size by RECIST following cobimetinib treatment in histiocytosis patients (n=14).

The upper and lower dotted lines represent cut-offs for progressive disease and partial response, respectively. Colors of bars indicate genomic alteration present. Notations above bars indicate specific mutation. One patient (asterisk) had prior BRAF inhibitor therapy that was discontinued due to intolerance. One patient (dagger) died due to underlying disease.

Extended Data Figure 2.. PET-Defined Duration of Response (N=16).

Depicts the duration of response according to PET criteria in the 16 responding patients, beginning with date of initial response.

Extended Data Figure 3.. Histopathology of histiocytoses with novel activating mutations in MEK2, RAF1, and BRAF treated on study.

(a) Protein diagram (top) and histological images (middle) demonstrating Erdheim-Chester disease (ECD) with a MAP2K2 Y134H mutation. (b) Protein diagram (top) and histological images (middle) demonstrating non-Langerhans cell histiocytosis with a RAF1 K106N mutation. (c) Protein diagram (top) and histological images (middle) demonstrating ECD with a BRAF N486_T491delinsK mutation.

Extended Data Figure 4.. Study CONSORT diagram.

Shows the flow of patients through all phases of study participation from enrollment, follow-up, and data analysis.

Figure 1:. Efficacy of MEK1/2 inhibition with cobimetinib across molecular subtypes of histiocytoses.

(a) Waterfall plot of the maximum change in tumor metabolism according to standardized uptake values (SUVs) measured by positron emission tomography (PET). Colors of bars indicate genomic alteration present. Notations above bars indicate specific mutation. One patient (dagger) died due to underlying disease prior to first response evaluation. The lower dotted lines represent cut-off for partial response. (b) Swimmer plot of outcomes in all 18 patients. (c) PET-Defined progression-free survival (n=18).

Figure 2:. Characterization of novel activating mutations in MEK2, RAF1, and BRAF and their dependence on ERK signaling in histiocytoses.

(a) Coronal PET and fused PET/CT imaging of femurs showing characteristic femoral lesions of ECD from a MEK2 Y134H mutant ECD patient pre- and during cobimetinib treatment. (b) Western blot (left) and number of viable cells (right) following IL-3 withdrawal of Ba/F3 cells stably expressing an empty vector, wild-type (WT) MEK2, or MEK2 Y134H mutant (the average of n=3 biological replicates ± standard deviation (SD) is plotted). Calculation of p-values was performed using two-way ANOVA; ****p<0.0001. (c) Axial fused PET/CT imaging showing skull lesions (arrow) pre- and during cobimetinib treatment in a patient with BRAF N486_T491delinsK mutant LCH. (d) Western blot (left) and number of viable cells (right) following IL-3 withdrawal of Ba/F3 cells stably expressing an empty vector, WT BRAF, or BRAF N486_T491delinsK mutant (the average of n=3 biological replicates ± SD is plotted). Calculation of p-values was performed using two-way ANOVA; ****p<0.0001. (e) Axial fused PET/CT imaging showing sacral lesions (arrow) pre-and during cobimetinib treatment in a patient with mixed histiocytosis and a RAF1 K106N mutation. (f) Western blot (left) and number of viable cells (right) following IL-3 withdrawal of Ba/F3 cells stably expressing an empty vector, WT RAF1, or RAF1 K106N mutant (the average of n=3 biological replicates ± SD is plotted). Calculation of p-values was performed using two-way ANOVA; ****p<0.0001. (g) IC₅₀ of cells from (b), (d), and (e) to 72 hours of cobimetinib. Each experiment was performed with n=3 biological replicates and average ± SD is plotted. The calculation of p-values utilized the Ordinary one-way ANOVA; **p <0.01.