The objective of this study was to evaluate the potential for a pharmacokinetic (PK) drug-drug interaction (DDI) between trastuzumab and carboplatin and to evaluate the potential effect of trastuzumab on the electrocardiogram QT interval. Here, we report the results of the PK DDI assessment and an interim safety analysis. Patients with metastatic or locally advanced, inoperable, human epidermal growth factor receptor 2-positive cancer received docetaxel and carboplatin on cycle 1, day 1 and then on day 1 of each subsequent 3-weekly treatment cycle. Trastuzumab was administered by intravenous infusion, with an accelerated loading dose on cycle 1, day 2 and cycle 1, day 8, and then a maintenance dose on day 1 of each subsequent 3-weekly treatment cycle. Blood was collected at various time points to assess free (unbound) plasma carboplatin and serum trastuzumab PK. The study enrolled 59 patients. Carboplatin concentrations in the presence and absence of trastuzumab were similar, as demonstrated by the geometric mean ratios for PK parameters, which were close to 1.0 (no effect). The observed trastuzumab concentrations were similar to the values predicted by population PK modelling on the basis of a prediction-corrected visual predictive check, computed using the actual sampling time. In this interim safety analysis, 84.5% of patients had experienced adverse events of grade three or higher, the most common of which were hematologic and as expected. The results suggest that there is no clinically relevant PK DDI between carboplatin and trastuzumab. The safety profile of trastuzumab plus carboplatin and docetaxel was consistent with the known safety profile of this combination.