Background: Breast cancer is the most common cancer diagnosed in women. HER2 gene amplification/protein overexpression is seen in about 20% of breast cancer cases. Pertuzumab (P), an EGFR inhibitor, is a monoclonal antibody administered with Trastuzumab in the treatment of HER2-positive breast cancer. Breast oncologists across multiple institutions have noted hypomagnesemia in patients receiving P-containing regimens. Retrospective data from Memorial Sloan Kettering Cancer Center and University of Florida Health have reported the incidence of hypomagnesemia in patients treated with P neoadjuvantly (8/90 and 7/25, respectively). Cetuximab and Panitumumab, also EGFR inhibitors, are well known to cause hypomagnesemia. Hypomagnesemia can be clinically associated with convulsions, atrial tachycardias, hypocalcemia and hypokalemia. This prospective study details the incidence, severity, and timeline of hypomagnesemia in HER2-positive breast cancer patients treated with P. Methods: 27 eligible patients were identified at two institutions through medical oncology clinic visits and weekly tumor board screenings. Inclusion criteria was HER2+ breast cancer patients initiating treatment with P. Patients receiving platinum agents were excluded (8/27). Following informed consent, magnesium levels were obtained along with standard of care comprehensive metabolic panels at the first four P infusions (19/27). Results: A one-sample binomial test was used to compare the proportion of hypomagnesemia at 3 months among subjects who receive P with 2% (historical control). Results demonstrated that only one patient developed a magnesium level below normal range, resulting in no significant difference with historical control (P = 0.291) This patient’s magnesium level was low at baseline and did not significantly improve or worsen throughout treatment with P. Conclusions: There was no significant incidence of hypomagnesemia noted in HER2+ breast cancer patients treated with P. This emphasizes the safety of P in patient populations prone to developing hypomagnesemia such as those receiving concurrent Carboplatin. Our results do not indicate clinical monitoring of hypomagnesemia is necessary for patients receiving P.

* The values were converted into categorical variables (Y/N), since both institutions had different lab reference values.

**4^th trial date unavailable for 2 patients. One patient received only 3 cycles of P; one patient transferred care elsewhere after 3 cycles of P.