To the best of our knowledge, there have been no reports from clinical settings regarding safety information on a fixed‑dose combination of pertuzumab and trastuzumab for subcutaneous injection (PHESGO®) when switched from the intravenous formulation of pertuzumab and trastuzumab in Japan. The lack of information on switching from the intravenous formulation to PHESGO in clinical settings may contribute to hesitation in making the switch. The present study analyzed the safety of 51 patients with breast or colorectal cancer treated with PHESGO. The focus was on evaluating infusion reactions (IRs), and skin and subcutaneous tissue disorders. The study included patients who received PHESGO at the Saitama Cancer Center between January 1, 2024 and March 31, 2024. The group using it as initial induction therapy was compared with the group that switched from the intravenous formulation. IRs were assessed using the Common Terminology Criteria for Adverse Events version 5.0. Patients with grade 1 or higher symptoms on the day of administration or the following day were considered to have IRs. IRs occurred in 4 of 16 patients (25%) in the initial induction group and none in the switching group (P=0.0073). It was suggested that IRs with PHESGO were more likely to occur at the time of first administration, similar to existing intravenous formulations. The study also examined patients who switched to an intravenous formulation after receiving PHESGO; skin and subcutaneous tissue disorders occurred in five patients, three of whom continued on a slower dose rate, and two of whom discontinued PHESGO and switched to pertuzumab and trastuzumab for intravenous infusion. With PHESGO, IRs are also more likely to occur the first time, and when skin and subcutaneous tissue disorders occur, there is a tendency to switch formulations in groups with a history of intravenous formulation use.

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