Real-world incidence and clinical outcomes of trastuzumab/pertuzumab-induced thrombocytopenia in adjuvant HER2-positive breast cancer: 8-year retrospective multicenter study.

Abstract

e23379 Background: Trastuzumab, alone or with pertuzumab, is standard adjuvant therapy for HER2-positive breast cancer. Real-world evidence describing the incidence, clinical course, and management impact of trastuzumab-induced thrombocytopenia (TIT) is limited. Methods: Multicenter retrospective cohort study across three UAE infusion centers (2016–2024). Electronic records of 87 women receiving adjuvant trastuzumab (IV/SC; originator or biosimilar) with or without pertuzumab were reviewed. All patients had platelet recovery after (neo)adjuvant chemotherapy (baseline > 150×10^9/L). TIT was defined as any platelet count < 150×10^9/L after HER2-targeted therapy initiation and graded by CTCAE v4.0. We assessed onset cycle, platelet nadir, recurrence (multi-cycle drops), transfusion requirement, and management actions (delay, discontinuation, regimen change). Predictors were explored using univariable and multivariable logistic regression. Results: TIT occurred in 35.6% and was typically early (median onset cycle 3). Events were predominantly mild (Grade 1: 29), with rare higher-grade events (Grade 2: 1; Grade 4: 1; no Grade 3). Mean platelet nadir was 140.1×10^9/L (SD 28.5; range 17–276). Recurrent/multi-cycle platelet drops occurred in 16/87 (18.4%). Platelet transfusion was required in 8% of TIT cases. Treatment impact was limited: > 90% continued HER2-targeted therapy without interruption; cycle postponement occurred in 2.5%, pertuzumab discontinuation in 2.5%, and regimen change in 1.3%. TIT incidence was similar with trastuzumab+pertuzumab vs trastuzumab alone (36.8% vs 31.6%; p = 0.68). No baseline demographic, stage, comorbidity, formulation, or regimen variables were associated with TIT risk. Conclusions: In this real-world multicenter UAE cohort, adjuvant trastuzumab ± pertuzumab was associated with a moderate rate of laboratory-defined thrombocytopenia that was usually early-onset and low-grade, with infrequent transfusion or treatment modification, although recurrence occurred in a meaningful minority. Routine platelet monitoring is appropriate; severe or treatment-limiting events appear rare. Logistic regression predicting thrombocytopenia. Predictor Adjusted OR 95% CI p Stage I (vs. II) 0.00 0.00 – ∞ .949 Stage III (vs. II) 1.09 0.32 – 3.72 .890 Regimen: Phesgo SC (vs. Trastuzumab alone) 0.65 0.10 – 4.17 .646 Regimen: T+P IV (vs. Trastuzumab alone) 0.74 0.17 – 3.17 .686 Age (per year) 1.01 0.94 – 1.07 .837 Weight (per kg) 0.99 0.95 – 1.03 .540 Number of cycles 1.02 0.93 – 1.12 .711 Chronic disease (yes vs. no) 0.87 0.24 – 3.10 .828 Baseline platelets <150 3.94 0.29 – 54.35 .305 Note. OR = odds ratio; CI = confidence interval. Reference groups: Stage II, Trastuzumab alone.