Trial reportESMO open2026
TBCRC 035: randomized phase II pharmacodynamic study of standard and reduced-dose palbociclib with endocrine therapy in hormone receptor (HR)-positive previously treated metastatic breast cancer.
Trial report in ESMO open, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Authors and funding
22 authors.
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No grant is acknowledged in the PubMed record.
Abstract
backgroundThe cyclin-dependent kinase 4/6 inhibitor palbociclib induces neutropenia, resulting in dose delays and reductions. PATIENTS AND
methodsThis multicenter phase II trial randomly assigned patients with metastatic hormone receptor-positive, human epidermal growth factor receptor 2-negative breast cancer to receive palbociclib 100 mg versus 125 mg with fulvestrant or tamoxifen; cross-over was allowed. Baseline and on-treatment skin and tumor biopsies were analyzed for expression of phosphorylated retinoblastoma protein (pRb), total Rb, and Ki-67. Baseline circulating tumor DNA was collected. The primary endpoint was grade ≥3 neutropenia.
resultsSeventy patients with a median of 3 prior treatment lines (range 0-6) were enrolled; 36 received 100 mg and 34 received 125 mg. Grade ≥3 neutropenia occurred in 12 (33%) and 19 (56%) patients in the 100 versus 125 mg groups, respectively (P = 0.04). Median progression-free survival (PFS) was 6.28 versus 9.28 months for 100 versus 125 mg, respectively (hazard ratio 1.697, 95% confidence interval 0.973-2.96, P = 0.0585). Five patients crossed over to 125 mg; two patients were treated for >12 months. Tumor and skin pRb and Ki-67 decreased on treatment, with similar percent changes across palbociclib dose (tumor pRb: -2.16 and -4.64, P = 0.897; tumor Ki-67: -10.27 and -6.7, P = 0.437 for 100 mg and 125 mg, respectively; skin pRb and Ki-67 P < 0.02 at both doses). Baseline mutations in PIK3CA and TP53, and higher baseline Ki-67 were associated with shorter PFS. Changes in pRb and Rb were not associated with PFS.
conclusionsPalbociclib 100 mg was associated with a reduced incidence of ≥grade 3 neutropenia. While PFS was numerically lower at 100 mg, the difference was not statistically significant and was limited by sample size and complicated by varying degrees of pre-treatment. Analysis of skin and tumor pRb or Ki-67 demonstrated robust molecular response at both doses. PIK3CA and TP53 mutations and higher baseline Ki-67 were associated with inferior clinical outcome.
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