Immune modulation in solid tumors: a phase 1b study of RO6870810 (BET inhibitor) and atezolizumab (PD-L1 inhibitor)

Abstract

Purpose: Bromodomain and extra-terminal domain (BET) inhibitors (BETi) have demonstrated epigenetic modulation capabilities, specifically in transcriptional repression of oncogenic pathways. Preclinical assays suggest that BETi potentially attenuates the PD1/PD-L1 immune checkpoint axis, supporting its combination with immunomodulatory agents.

Patients and methods: A Phase 1b clinical trial was conducted to elucidate the pharmacokinetic and pharmacodynamic profiles of the BET inhibitor RO6870810 as monotherapy and in combination with the PD-L1 antagonist atezolizumab in patients with advanced ovarian carcinomas and triple-negative breast cancer (TNBC). Endpoints included maximum tolerated dosages, adverse event profiling, pharmacokinetic evaluations, and antitumor activity. Pharmacodynamic and immunomodulatory effects were assessed in tumor tissue (by immunohistochemistry and RNA-seq) and in peripheral blood (by flow cytometry and cytokine analysis).

Results: The study was terminated prematurely due to a pronounced incidence of immune-related adverse effects in patients receiving combination of RO6870810 and atezolizumab. Antitumor activity was limited to 2 patients (5.6%) showing partial response. Although target engagement was confirmed by established BETi pharmacodynamic markers in both blood and tumor samples, BETi failed to markedly decrease tumor PD-L1 expression and had a suppressive effect on antitumor immunity. Immune effector activation in tumor tissue was solely observed with the atezolizumab combination, aligning with this checkpoint inhibitor's recognized biological effects.

Conclusions: The combination of BET inhibitor RO6870810 with the checkpoint inhibitor atezolizumab presents an unfavorable risk-benefit profile for ovarian cancer and TNBC (triple-negative breast cancer) patients due to the increased risk of augmented or exaggerated immune reactions, without evidence for synergistic antitumor effects.

Trial registration: ClinicalTrials.gov ID NCT03292172; Registration Date: 2017-09-25.

Keywords: BET inhibitor; Bromodomain; Immunotherapy; Ovarian cancer; Phase Ib solid tumors; TNBC.

Fig. 1

Schematic Overview of Study Treatment Regimens and Pharmacodynamic Biomarker Collection: RO6870810 administered at doses of 0.30 mg/kg, 0.45 mg/kg, and 0.65 mg/kg daily for 14 days, and atezolizumab given at 1200 mg intravenously on Day 1 of each 21-day cycle. A. The concomitant regimen involved patients receiving a combination of RO6870810 and atezolizumab from initiation. Tumor biopsies for RNA-sequencing and immunohistochemistry (IHC) were taken at baseline (Cycle 1 Day 1 [C1 D1]) and post-first cycle (Cycle 1 Day 21 [C1 D21]), indicated by purple arrows. Peripheral blood samples for flow cytometry and cytokine profiling, shown by red arrows, were collected on days 1, 8, 15, and 21. This regimen was applied to patients in the dose escalation and both expansion cohorts. B. To evaluate the impact of RO6870810 as a single agent, an alternative group followed a sequential regimen, starting with RO6870810 alone in a run-in cycle before transitioning to combined treatment with atezolizumab. Tumor biopsies were performed at the run-in start (Run-in Day 1 [RI D1]), post-run-in cycle (Run-In Day 21 [RI D21]), and after the initial cycle of combination therapy (C1 D21). Peripheral blood sampling occurred on the same days during the run-in and the first combination treatment cycle, facilitating a comprehensive analysis of treatment-induced changes

Fig. 2

Changes in Target Lesion Size and Best Overall Response. Each bar represents the response of an individual patient, measured according to RECIST overall response criteria. The y-axis corresponds to the maximum percentage change from baseline in sum of longest diameters (SLD) in target lesions. Colors indicate the best overall response. Out of 36 patients, 31 were evaluable for clinical response. Two patients who exhibited a decrease in target lesion size were still classified as having progressive disease due to progression in non-target lesions or the appearance of new lesions

Fig. 3

Pharmacodynamic Responses of BET Inhibitor Biomarkers in Peripheral Blood and Tumor Tissue: A. Quantification of CD14+/CD11b + monocyte populations in peripheral blood, illustrating changes from baseline (expressed as log2 fold-change from cycle onset) for individual patients (denoted as points), with longitudinal data from the same individual linked. Patients lacking baseline or sequential samples are excluded. Color highlights patients with partial response (purple), immune-mediated adverse events (orange), or systemic immune activation (red). Refer to (Fig. 1 for time point definitions. Boxplots depict median (center line), quartiles (box limits), and variability (whiskers extend to 1.5x interquartile range). B. Tumor expression levels of established BETi target genes, as determined by RNA-seq, indicating gene expression modifications (log2 fold-change) from the screening (pre-treatment) sample. Exclusions apply for participants without screening or on-treatment samples. The same color coding as in Panel A is used. C. Gene signature enrichment analysis reflecting BETi downstream effects, with heatmaps showcasing signature scores and gene expression alterations. Green and purple denote significantly up- or down-regulated signatures, respectively, with red and blue highlighting individual gene expression shifts within significant signatures. Asterisks indicate statistical significance. Time points align with those in Panel B

Fig. 4

Assessment of Immune Modulation by Flow Cytometry and Cytokine Analyses: A. The variation in immune cell populations within peripheral blood, as determined by flow cytometry. Color depicts the log2 fold-change from baseline at each defined time point (refer to Fig. 1 for time points). Red indicates an increase, blue a decrease in cell population frequency, with significant alterations marked by an ‘X’ (FDR corrected p-value < 0.05). B. Change from baseline in the CD4+/CD8 + cell ratio in peripheral blood, indicating shifts towards either T helper cells (positive values) or cytotoxic cells (negative values). Continuous lines connect sequential time point samples from individual patients, highlighting specific cases of interest in color. Boxplots aggregate data at each time point. C, D. Changes in soluble CD25 (sCD25) and TNFα levels from baseline in peripheral blood. The visualization follows the format of Panel B

Fig. 5

Differential Impact of BET inhibitor Monotherapy and Atezolizumab Combination Therapy on Immune Effector Pathways: Heatmaps illustrate the contrasting effects of atezolizumab combination therapy and BET inhibitor monotherapy on immune effector pathways within tumor tissues, based on RNA sequencing data. A. Enrichment scores for key immune pathways [34]. Green indicates significant upregulation in combination therapy, suggesting enhanced immune activity. Purple marks downregulation in BETi monotherapy, implying reduced immune response. B. Gene expression changes related to CD8 T effector, immune checkpoint, and antigen processing machinery pathways are highlighted. Red represents upregulated genes, reflecting pathway activation, while blue indicates downregulated genes, signifying pathway suppression. Significant changes are marked with asterisk.