Targeted BRAF inhibitors: Immunological effects and combination with immunotherapy

<p>Metastatic melanoma is the most aggressive form of skin cancer, associated with a poor prognosis, and the incidence worldwide is increasing. Recently, selective mutant BRAF inhibitors and checkpoint blockade immunotherapy have advanced clinical treatment of metastatic melanoma. However, efficacy of these therapies individually is limited. Combining treatments may allow BRAF inhibition to augment immunotherapy by increasing tumour antigen availability and improving immune system targeting of tumours. The success of this approach depends upon fully elucidating immunological interactions of BRAF inhibitors, and optimizing combination strategies. To study the immunological effects of BRAF inhibitors and their combination with immunotherapy, novel murine BrafV600E Pten-/- Cdkn2a cell lines were characterized. These were found to be moderately sensitive to BRAF inhibition compared with the widely used human BRAFV600E cell lines A375 and SK-mel-5. In vitro targeted BRAF inhibition was shown to induce cell death through apoptosis, and partially reverse melanoma-mediated immunosuppression by human melanoma cell lines. Utilising subcutaneously injected syngeneic, murine BRAFV600E cell lines, the BRAF inhibitor PLX4720 was shown to decrease tumour growth in vivo. Host immune involvement in BRAF inhibitor efficacy was determined by comparing PLX4720 treatment in NOD/Scid and C57BL/6 mice. PLX4720 control of tumour growth was significantly less effective in immunocompromised mice, resulting in reduced survival advantage. These findings demonstrate that the anti-tumour effects of mutant BRAF inhibitors are partially immune dependent, although the nature of this immune involvement remains to be defined. It was further shown that BRAFV600E inhibition directly affected immune responses. In vitro, both human and murine T cell activation were boosted by low concentrations of mutant BRAF inhibitors. This was confirmed in vivo, with antigen-specific T cell proliferation significantly increased by PLX4720 treatment. The final chapters of this thesis explored the combination of active immunotherapy with targeted BRAF inhibition. A vaccine was devised that consisted of irradiated, autologous tumour cells loaded with the adjuvant α-galactosylceramide. This vaccine was shown to be effective in both prophylactic and therapeutic settings in a BRAFV600E melanoma model. Mechanistically, vaccine increased effector T cell responses and decreased frequencies of Tregs. Vaccine efficacy was CD4⁺ T cell-dependent, and did not require CD8⁺ T cells. Combination of vaccine with targeted BRAF inhibition was investigated in different settings. A combination therapy strategy was developed that achieved additive, but not synergistic benefit. Additionally, targeting specific aspects of the tumour microenvironment that may confer tumour resistance to BRAF inhibitor-mediated cell death was investigated. Both depletion of Tregs and inhibition of TNFα were explored, but did not result in a significant improvement in therapy. In summary, the studies undertaken in this thesis demonstrate that BRAF inhibitors can augment vaccine-induced T cell responses. Moreover, this research revealed the anti-tumour efficacy of BRAFV600E inhibition is partially immune dependent and can be improved by combination with active immunotherapy. These discoveries generated a combination therapy strategy with improved efficacy over single agent treatment. Further studies are needed to realise the full potential of this combination therapy approach, and achieve a synergistic benefit.</p>

Targeted BRAF inhibitors: Immunological effects and combination with immunotherapy

<p>Metastatic melanoma is the most aggressive form of skin cancer, associated with a poor prognosis, and the incidence worldwide is increasing. Recently, selective mutant BRAF inhibitors and checkpoint blockade immunotherapy have advanced clinical treatment of metastatic melanoma. However, efficacy of these therapies individually is limited. Combining treatments may allow BRAF inhibition to augment immunotherapy by increasing tumour antigen availability and improving immune system targeting of tumours. The success of this approach depends upon fully elucidating immunological interactions of BRAF inhibitors, and optimizing combination strategies. To study the immunological effects of BRAF inhibitors and their combination with immunotherapy, novel murine BrafV600E Pten-/- Cdkn2a cell lines were characterized. These were found to be moderately sensitive to BRAF inhibition compared with the widely used human BRAFV600E cell lines A375 and SK-mel-5. In vitro targeted BRAF inhibition was shown to induce cell death through apoptosis, and partially reverse melanoma-mediated immunosuppression by human melanoma cell lines. Utilising subcutaneously injected syngeneic, murine BRAFV600E cell lines, the BRAF inhibitor PLX4720 was shown to decrease tumour growth in vivo. Host immune involvement in BRAF inhibitor efficacy was determined by comparing PLX4720 treatment in NOD/Scid and C57BL/6 mice. PLX4720 control of tumour growth was significantly less effective in immunocompromised mice, resulting in reduced survival advantage. These findings demonstrate that the anti-tumour effects of mutant BRAF inhibitors are partially immune dependent, although the nature of this immune involvement remains to be defined. It was further shown that BRAFV600E inhibition directly affected immune responses. In vitro, both human and murine T cell activation were boosted by low concentrations of mutant BRAF inhibitors. This was confirmed in vivo, with antigen-specific T cell proliferation significantly increased by PLX4720 treatment. The final chapters of this thesis explored the combination of active immunotherapy with targeted BRAF inhibition. A vaccine was devised that consisted of irradiated, autologous tumour cells loaded with the adjuvant α-galactosylceramide. This vaccine was shown to be effective in both prophylactic and therapeutic settings in a BRAFV600E melanoma model. Mechanistically, vaccine increased effector T cell responses and decreased frequencies of Tregs. Vaccine efficacy was CD4⁺ T cell-dependent, and did not require CD8⁺ T cells. Combination of vaccine with targeted BRAF inhibition was investigated in different settings. A combination therapy strategy was developed that achieved additive, but not synergistic benefit. Additionally, targeting specific aspects of the tumour microenvironment that may confer tumour resistance to BRAF inhibitor-mediated cell death was investigated. Both depletion of Tregs and inhibition of TNFα were explored, but did not result in a significant improvement in therapy. In summary, the studies undertaken in this thesis demonstrate that BRAF inhibitors can augment vaccine-induced T cell responses. Moreover, this research revealed the anti-tumour efficacy of BRAFV600E inhibition is partially immune dependent and can be improved by combination with active immunotherapy. These discoveries generated a combination therapy strategy with improved efficacy over single agent treatment. Further studies are needed to realise the full potential of this combination therapy approach, and achieve a synergistic benefit.</p>

CTNI-32. CASE SERIES OF BRAF INHIBITORS FOR THE TREATMENT OF BRAFV600E GLIOBLASTOMA

Glioblastoma is the most common and aggressive primary brain tumor. Beyond upfront therapy with radiation and temozolomide chemotherapy there is no standard therapy that has been effective. Inhibitors of BRAF and MEK, a downstream protein immediately following BRAF, have been shown to have survival benefit for patients with other BRAF V600E mutant neoplasm including advanced melanoma. We describe our experience using this combined target therapy for two patients with BRAF V600E mutant glioblastoma. Two adult patients with pathologically diagnosed glioblastoma presented with radiographic evidence of tumor progression after prior treatment with chemotherapy or immunotherapy. Neither had received radiation therapy within 3 months of starting treatment. Molecular characterization was performed though Caris which showed evidence of BRAF V600E mutation. BRAF inhibitors were initiated in combination with standard therapy options. MRI imaging was obtained to monitor for disease progression. BRAF inhibitors were tolerated well without any side effects not previously reported. Partial objective response was seen in both patients on subsequent MRI imaging within 8 weeks of starting treatment. Progression free survival and overall survival have not been reached in either case. BRAF inhibition may have therapeutic benefit in BRAF mutated glioblastoma. Partial response was seen in this case series. The molecular profile of glioblastoma may suggest treatment options beyond standard chemotherapy options. This series supports the use of BRAF inhibitors for the treatment of BRAFV600E glioblastoma A controlled trial should be supported.

CSIG-04. UNCOUPLING OF ETS1 FROM MAPK PATHWAY SIGNALS AS RESISTANCE MECHANISM TOWARDS BRAF INHIBITORS IN BRAF-MUTATED CHILDHOOD GLIOMA

BACKGROUND High-grade gliomas are among the most aggressive brain tumors across all age groups. BRAF is within the most frequently altered genes in pediatric glioma, sometimes connected with telomerase reverse transcriptase (TERT) promoter mutations, predicting a particularly aggressive course of disease. Precision medicine approaches targeting the MAPK pathway have shown promising results in patients with BRAF-mutated glioma. Although acquired insensitivity to BRAF inhibitors appears as major issue for therapy failure, underlying molecular mechanisms are still poorly understood. METHODS Cell models from an anaplastic pleomorphic xanthoastrocytoma with BRAF V600E and TERT promoter mutations and the recurrent tumor, operated following MAPK pathway-targeting therapy, were established. Furthermore, a dabrafenib-resistant subline of the recurrent tumor was generated. The patient-derived cell models were genetically characterized using array-based genomic hybridization (aCGH). Sensitivity of the cells towards different MAPK pathway inhibitors was tested. Basal expression and activation of MAPK pathway and downstream signals were analyzed by qRT-PCR and Western blots. RESULTS Screening a panel of both primary and immortalized glioma cell models with different BRAF and TERT promoter status revealed significantly induced MAPK pathway activation and enhanced TERT levels in BRAF V600E and TERT promoter double-mutant gliomas. Furthermore, cells with both mutations were hyper-sensitive towards BRAF-targeting agents and BRAF inhibition resulted in reduced TERT levels. ETS1 expression was strongly increased in the recurrent tumor and identified as important player in telomerase re-activation. aCGH revealed gains of chromosomal regions encoding for different ETS-factors in the dabrafenib-resistant subline. Western blot analyses suggested a BRAF/ERK-independent survival mechanism in the dabrafenib-resistant subline. Accordingly, dabrafenib insensitivity triggered cross-resistance towards the MEK inhibitor trametinib. Uncoupled from the MAPK pathway, ETS1 expression was further upregulated in the dabrafenib-resistant subline. CONSLUSION: Taken together, our data demonstrate that MAPK-independent ETS transcription factor upregulation is a central mechanism of BRAF inhibitor therapy failure in BRAF-mutated pediatric glioma patients.

Prevalence of BRAF V600 in primary gliomas: a systematic review

Aims Glioma is a fatal disease that causes significant years of life lost to an individual. Mutations in the driver gene BRAF, such as the V600 alteration, may contribute to gliomagenesis in adults and children through abnormal signaling causing uncontrolled cell proliferation. The use of BRAF-inhibitor drugs including Vemurafenib and Dabrafenib have shown a favorable response in 48% and 50% of melanoma patients with BRAF V600 mutations respectively. BRAF inhibitors and MEK inhibitors have shown efficacy in certain paediatric gliomas in the recurrent setting. Despite the potential benefit of BRAF inhibitors, the prevalence of BRAF V600 within primary gliomas is not fully discovered. Some studies identify the prevalence to be over 50%, while others find the prevalence to be around 1%. We performed a comprehensive systematic review to determine the prevalence of BRAF V600 within the adult and paediatric glioma population in different diagnostic groups. Method A systematic literature search was performed using Ovid MEDLINE and Embase from genesis to the 22nd October 2020. Studies were not restricted by language. Studies were eligible if patients were histologically diagnosed according to WHO guidelines as a primary glioma evaluating the prevalence of BRAF V600 and included ≥ 10 primary glioma patients. The review protocol was registered in PROSPERO (CRD42019127704). Search results were managed using Endnote. Two independent reviewers assessed the eligibility of the publications using Rayyan, conflicts were evaluated by a third reviewer. Included articles were extracted by one reviewer and confirmed by a second reviewer. Risk of bias assessments were conducted using Hoy et al’s risk of bias tool. Results were synthesized using “metaprop” in R. The meta-analysis was carried out in R which produced forest plots. Results Our cohort included 182 studies with a total of 13669 adult and paediatric glioma patients classified diagnostically according to WHO guidelines. Among 48 glioma entities, BRAF V600 was identified most commonly in epithelioid glioblastoma with a prevalence of 69% (95% confidence interval (CI): 45-89%), followed by pleomorphic xanthoastrocytoma with a prevalence of 56% (95% CI: 48-64%), anaplastic pleomorphic xanthoastrocytoma with a prevalence of 38% (95% CI: 23-54%), ganglioglioma with a prevalence of 40% (95% CI: 33-46%), and anaplastic ganglioglioma with a prevalence of 46% (95% CI: 18-76%). Other glioma entities were found to have a prevalence of BRAF V600, these include astroblastoma (24%), desmoplastic infantile astrocytoma (16%), subependymal giant cell astrocytoma (8%), dysembryoplastic neuroepithelial tumour (3%), diffuse astrocytoma (3%), and pilocytic astrocytoma (3%). Conclusion To our knowledge, this is the largest systematic review examining the prevalence of BRAF V600 in adult and paediatric glioma classified according to diagnostic WHO criteria. However, there were some limitations in this review. The sample sizes of some studies were very small, and the method of mutational analysis for BRAF V600 varied between papers. We found BRAF V600 in a significant prevalence of epithelioid glioblastoma, pleomorphic xanthoastrocytoma, anaplastic pleomorphic xanthoastrocytoma, ganglioglioma, and anaplastic ganglioglioma. Of interest, BRAF V600 mutation was found in a lower prevalence of astroblastoma, desmoplastic infantile astrocytoma, subependymal giant cell astrocytoma, dysembryoplastic neuroepithelial tumour, diffuse astrocytoma, and pilocytic astrocytoma. Consideration of assessment of BRAF V600 mutation may enable further treatment options with BRAF and/or MEK inhibitors in these particular diagnostic entities.

Allosteric MEK inhibitors act on BRAF/MEK complexes to block MEK activation

The RAF/MEK/ERK pathway is central to the control of cell physiology, and its dysregulation is associated with many cancers. Accordingly, the proteins constituting this pathway, including MEK1/2 (MEK), have been subject to intense drug discovery and development efforts. Allosteric MEK inhibitors (MEKi) exert complex effects on RAF/MEK/ERK pathway signaling and are employed clinically in combination with BRAF inhibitors in malignant melanoma. Although mechanisms and structures of MEKi bound to MEK have been described for many of these compounds, recent studies suggest that RAF/MEK complexes, rather than free MEK, should be evaluated as the target of MEKi. Here, we describe structural and biochemical studies of eight structurally diverse, clinical-stage MEKi to better understand their mechanism of action on BRAF/MEK complexes. We find that all of these agents bind in the MEK allosteric site in BRAF/MEK complexes, in which they stabilize the MEK activation loop in a conformation that is resistant to BRAF-mediated dual phosphorylation required for full activation of MEK. We also show that allosteric MEK inhibitors act most potently on BRAF/MEK complexes rather than on free active MEK, further supporting the notion that a BRAF/MEK complex is the physiologically relevant pharmacologic target for this class of compounds. Our findings provide a conceptual and structural framework for rational development of RAF-selective MEK inhibitors as an avenue to more effective and better-tolerated agents targeting this pathway.