The dose-escalation will be conducted in steps of 10 mg/m2 up to the MTD level determined for 2B3-101 as single agent. The trastuzumab dose will remain fixed to a loading dose of 8 mg/kg at day 1 and 6 mg/kg every 3 weeks at the subsequent cycles throughout the determination of the MTD. Enrolment of HER2+ patients breast cancer patients in the "2B3-101 in combination with trastuzumab dose-escalation" phase of the study will be allowed in parallel with the determination of the MTD of 2B3-101 as single agent. A "3+3" dose-escalation design will be used. Thus, the study will investigate sequential cohorts of 3-6 patients, who will be enrolled and treated at the applicable dose levels.