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AIMSTrastuzumab, an antibody binding to epidermal growth factor receptor-2 (HER2), has been approved to treat HER2-positive breast cancer in different settings. This study aimed at evaluating the influence of tumour size on trastuzumab pharmacokinetics (PK) in non-metastatic breast cancer patients treated with short term pre-operative trastuzumab. METHODSTrastuzumab PK data were obtained from a multicentre, randomized and comparative study. This antibody was administered pre-operatively to patients with localized HER2-positive breast cancer as a single 4 mg kg À1 loading dose followed by 5 weekly 2 mg kg À1 doses. Trastuzumab concentrations were measured repeatedly using an ELISA technique. Tumour size was evaluated at baseline using breast echography. Trastuzumab pharmacokinetics were studied using a population approach and a two compartment model. The influence of tumour burden on trastuzumab pharmacokinetics was quantified as a covariate. RESULTSA total of 784 trastuzumab concentrations were available from the 79 eligible patients. Estimated parameters (interindiviual standard deviation) were central volume of distribution =2.1 l (23%), peripheral volume of distribution =1.3 l (38%), intercompartment clearance =0.36 l day À1 , with an elimination half-life of 11.8 days. Typical clearance was 0.22 l day À1 (19%) and its value was increased with tumour size. In patients with the highest tumour size, trastuzumab clearance was 50% [18%-92%] higher than in patients with the lowest tumour size. CONCLUSIONSIn non-metastatic breast cancer patients, trastuzumab clearance increases with tumour size. The elimination half-life of trastuzumab was shorter in the present population of patients than in metastatic breast cancer patients previously studied. British Journal of Clinical PharmacologyBr J Clin Pharmacol (2016) 81 941-948 941