Background: Bireociclib (XZP-3287) is a novel selective cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor, with a favorable safety profile demonstrated in preclinical and phase I studies. BRIGHT-1 aimed to further explore the efficacy and safety of bireociclib monotherapy in patients with locally advanced, recurrent or metastatic, hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR⁺/HER2⁻) breast cancer who had progressed on or after prior chemotherapy and endocrine therapy in advanced settings, without previous exposure to CDK4/6 inhibitors.

Methods: In this open-label phase II trial, eligible patients received bireociclib 480 mg twice daily (BID) until disease progression or intolerable toxicities. The primary endpoint was the confirmed objective response rate (ORR) assessed by an independent review committee (IRC). The secondary endpoints included progression-free survival (PFS), investigator-assessed ORR, disease control rate (DCR), clinical benefit rate (CBR), duration of response (DoR), overall survival (OS), safety and the pharmacokinetic properties of bireociclib.

Results: A total of 131 patients were enrolled. At data cutoff (July 31, 2023), the IRC-assessed ORR was 29.8% (95% confidence interval [CI], 22.1% to 38.4%), with a DCR of 73.3% (95% CI, 64.8% to 80.6%), CBR of 42.0% (95% CI, 33.4% to 50.9%) and a median DoR of 15.2 months (95% CI, 9.5 months to not reached). The median PFS was 11.0 months (95% CI, 7.3 months to 12.9 months) assessed by the IRC, and the median OS was 29.0 months (95% CI, 24.9 months to not reached). The most frequently reported treatment-emergent adverse events (TEAEs) of any grade were diarrhea (93.1%), neutrophil count decreased (87.0%), white blood cell decreased (86.3%), vomiting (78.6%), anemia (72.5%), and platelet count decreased (72.5%). The grade ≥3 TEAEs occurred in 109 (83.2%) patients. The most common grade ≥3 TEAEs were neutrophil count decreased (43.5%), white blood cell decreased (32.8%), hypokalemia (20.6%), and diarrhea (19.1%).

Conclusions: Bireociclib monotherapy at 480 mg BID exhibited promising and sustained clinical activity, with no unexpected and acceptable toxicity in patients with recurrent or metastatic HR⁺/HER2⁻ breast cancer who had progressed on or after previous therapy.

Trial registration: Clinicaltrials.gov ID, NCT04539496.

Background: Glioblastoma (GBM) is largely refractory to antibodies against programmed cell death 1 (anti-PD-1) therapy. Fully understanding the cellular heterogeneity and immune adaptations in response to anti-PD-1 therapy is necessary to design more effective immunotherapies for GBM. This study aimed to dissect the molecular mechanisms of specific immunosuppressive subpopulations to drive anti-PD-1 resistance in GBM.

Methods: We systematically analysed single-cell RNA sequencing and spatial transcriptomics data from GBM tissues receiving anti-PD-1 therapy to characterize the microenvironment alterations. The biological functions of a novel circular RNA (circRNA) were validated both in vitro and in vivo. Mechanically, co-immunoprecipitation, RNA immunoprecipitation and pull-down assays were conducted.

Results: Mesenchymal GBM (MES-GBM) cells, which were associated with a poor prognosis, and secreted phosphoprotein 1 (SPP1)⁺ myeloid-derived macrophages (SPP1⁺ MDMs), a unique subpopulation of MDMs with complex functions, preferentially accumulated in non-responders to anti-PD-1 therapy, indicating that MES-GBM cells and SPP1⁺ MDMs were the main anti-PD-1-resistant cell subpopulations. Functionally, we determined that circular RNA succinate dehydrogenase complex assembly factor 2 (circSDHAF2), which was positively associated with the abundance of these two anti-PD-1-resistant cell subpopulations, facilitated the formation of a regional MES-GBM and SPP1⁺ MDM cell interaction loop, resulting in a spatially specific adaptive immunosuppressive microenvironment. Mechanically, we found that circSDHAF2 promoted MES-GBM cell formation by stabilizing the integrin alpha 5 (ITGA5) protein through N-glycosylation. Meanwhile, the N-glycosylation of the ITGA5 protein facilitated its translocation into exosomes and subsequent delivery to MDMs to induce the formation of SPP1⁺ MDMs, which in turn maintained the MES-GBM cell status and induced T-cell dysfunction via the SPP1-ITGA5 pathway, ultimately promoting GBM immune escape. Importantly, our findings demonstrated that antibody-mediated ITGA5 blockade enhanced anti-PD-1-mediated antitumor immunity.

Conclusions: This work elucidated the potential tissue adaptation mechanism of intratumoral dynamic interactions between MES-GBM cells, MDMs and T cells in anti-PD-1 non-responders and identified the therapeutic potential of targeting ITGA5 to reduce anti-PD-1 resistance in GBM.