No intra-patient dose-escalation will be allowed. Each treatment cycle consists of 21 days. All patients will receive a single IV dose of 2B3-101 on day 1 of each cycle. In order to minimize the risk of infusion reactions 5% of the total dose of 2B3-101 (in mg) should be infused slowly over the first 30 minutes. As long as 2B3-101 is well tolerated, the remaining 95% of the infusion could thereafter be administered over the next 60 min, resulting in a total infusion time of 90 minutes. The infusion of trastuzumab will then follow 30 minutes after the completion of the 2B3-101 infusion. Blood samples will be taken on day 1, 2, 3, 5, 8 and 11 of cycle 1, on day 1, 8 and 15 of cycle 2 and if applicable on day 1 of cycle 3 and day 1 of cycle 4 to assess the PK profile of 2B3-101 during the first 4 cycles. The dose limiting toxicity (DLT) observation period will be cycle 1 (day 1 to day 21) at each individual dose level. Patients who do not complete the DLT observation period (cycle 1) for other reasons than a DLT will be replaced.