ERBB2 and breast carcinoma: Only 5% administered dose of Trastuzumab, which targets HER2+ breast cancer, is found in brain lesions in preclinical mouse models.34 Clinically, the ratio of trastuzumab in serum to cerebrospinal fluid is 420:1.53 Often a factor limiting drug delivery to brain metastases is the high degree of expression of ABC efflux transporters at the BBB and BTB.