Transfer of trastuzumab and pertuzumab into human breast milk: a case report

Purpose: Lactation could offer benefit to the mother and child. Information on the safety of lactation during anti-cancer treatment, however, is scarce. Therefore, clinical data about drug exposure through breast milk is needed. To contribute to this, we present a case of a 34-year old woman who was treated with intravenous trastuzumab 600 mg and pertuzumab 1200 mg in the first cycle. Over the first 17 days of treatment, she collected breast milk samples. Methods: Trastuzumab and pertuzumab concentrations in breast milk were determined by validated ELISAs. Subsequently, infant exposure parameters such as infant daily dose (IDD), relative infant dose (RID), and cumulative RID were calculated. Results: The cumulative RID was 16.8% for trastuzumab and 24.3% for pertuzumab. Based on our results, we can conclude that intravenously administered trastuzumab and pertuzumab transfer into breast milk exceeding the safety threshold of an RID of 10%. Conclusion: Although the extent of systemic exposure in infants after ingestion of breast milk containing these monoclonal antibodies is unknown, systemic exposure cannot be excluded. Therefore, breastfeeding during treatment with trastuzumab and pertuzumab should be discouraged.