Differential influence of vemurafenib and dabrafenib on patients' lymphocytes despite similar clinical efficacy in melanoma

Abstract

Background: Since the majority of melanomas eventually become resistant and progress, combining selective BRAF inhibitors (BRAFi) with immunotherapies has been proposed to achieve more durable treatment responses. Here, we explored the impact of selective BRAFi on the hosts' immune system.

Patients and methods: Clinical data, whole blood counts (WBC) and serum lactate dehydrogenase (LDH) of 277 vemurafenib- and 65 dabrafenib-treated melanoma patients were evaluated. The frequency and phenotype of lymphocyte subpopulations were determined by flow cytometry while T cell cytokine secretion was measured by multiplex assays.

Results: Progression-free survival (PFS) as well as overall survival (OS) were similar in patients treated with either BRAFi. High pretreatment LDH was associated with shorter PFS and OS in both groups. During therapy, peripheral lymphocytes decreased by 24.3% (median, P < 0.0001) in vemurafenib-treated patients but remained unchanged in dabrafenib-treated patients (+1.2%, P = 0.717). Differentiation of peripheral lymphocytes of vemurafenib-treated patients showed a significant decrease in CD4(+) T cells (P < 0.05). Within CD4(+) T cells obtained during treatment, an increase in CCR7(+)CD45RA(+) (naïve) and a decrease in CCR7(+)CD45RA(-) (central memory) populations were found (P < 0.01 for both). Furthermore, secretion of interferon-γ and interleukin-9 by CD4(+) T cells was significantly lower in samples obtained during vemurafenib treatment compared with baseline samples.

Conclusion: While both compounds have comparable clinical efficacy, vemurafenib but not dabrafenib decreases patients peripheral lymphocyte counts and alters CD4(+) T cell phenotype and function. Thus, selective BRAFi can significantly affect patients' peripheral lymphocyte populations. Fully understanding these effects could be critical for successfully implementing combinatorial therapies of BRAFi with immunomodulatory agents.

Keywords: T cells; dabrafenib; lymphocytes; melanoma; treatment; vemurafenib.

Figure 1.

Specific and common characteristics of selective BRAFi: vemurafenib (A), but not dabrafenib (B), causes a loss of absolute numbers of patients’ lymphocytes. Bars indicate medians, whiskers showing interquartile range. However, the Kaplan–Meier analyses of patients' OS and PFS grouped by patients below the first quartile, the median, the third quartile, and patients above the third quartile showed no association of changes in lymphocyte count and survival for both selective BRAFi (C–F). Absolute numbers of peripheral lymphocytes per nanoliter are shown (A and B). Each dot represents one individual patient. Cumulative survival expressed in weeks shown. Patients without an event were censored at last follow-up (C–F).

Figure 2.

Vemurafenib selectively diminishes circulating CD4⁺ T cells: (A) vemurafenib decreases the number of peripheral CD4⁺ T cells but increases NK cell numbers significantly while B cells and CD8⁺ T cells are unaffected. Absolute numbers of cells per nanoliter from 18 independent donors are shown. Each dot represents one individual patient. *P < 0.05. (B) The frequency of dead cells within the CD4⁺ gate is not affected by vemurafenib as determined in 10 independent donors before and during treatment. (C) However, an increase in the frequency of naïve CD4⁺ T cells and a decrease in CD4⁺ T cells with the phenotype of central memory (CM) T cells was observed. The frequency of CCR7⁺CD45RA⁺ (naïve) and CCR7⁺CD45RA⁻ (CM) T cells within the CD4⁺ gate from 10 different donors is shown. **P < 0.01. (D) Representative density plots showing Th cell phenotype before therapy (top row) and during treatment (bottom row) of one melanoma patient. The frequency of lymphocytes as defined by their forward and side scatter (FS/SS) characteristics as well as the frequency of CD4⁺ cells within the lymphocyte gate decreases during treatment (left panel). In addition, the frequency of naïve CD4⁺ T cells increases while CM CD4⁺ T cells are reduced within peripheral lymphocytes during treatment (middle panel). However, no increase in the percentage of dead CD4⁺ T cells can be observed (right panel, MFI of aqua viability dye shown).