Male breast cancer (BC) accounts for fewer than 1% of all BC diagnoses, with 80% of cases being hormone-receptor positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-). Given the rarity of the disease, it is difficult to conduct phase III clinical trials to assess treatment regimens. CDK4/6 inhibitors (i.e., palbociclib, abemaciclib, and ribociclib) have been shown to improve progression-free survival (PFS) in women with metastatic HR+ HER2- metastatic BC.

A retrospective study was conducted to help assess the role of CDK4/6 inhibitors in male BC patients. A multicenter study involving 14 tertiary care centers was conducted in Turkey between 2019 and 2022. In total, 25 male patients with HR+ and HER2- BC were studied.

The primary endpoint of this study was PFS, which was defined as time between the onset of CDK 4-6 inhibitors and date of progression or death. Secondary endpoints were response rate and safety.

The mean age of the male BC patients who were started on CDK4/6 inhibitors was 57.25 years, and all had a normal to good (0-1) Eastern Cooperative Oncology Group (ECOG) performance status. The majority of patients (64%) were receiving an aromatase inhibitor, and over one-third (36%) were on a gonadotropin release hormone and fulvestrant. No patients received abemaciclib, 64% received Palbociclib, and 36% received ribociclib. Almost one-half (48%) had been given the CDK4/6 inhibitor as first-line therapy. All patients had had either metastatic disease (48%) or recurrent disease (52%).

By the median follow-up period of 19.5 months, almost one-quarter (24%) of patients had died. The median PFS was 20.6 months for the whole group. PFS was 20.6 months for palbociclib and 11.9 months for ribociclib, but this difference was not statistically significant (P = .085). For nine of the 16 patients who received palbociclib as first-line therapy, PFS was not achieved at the time of censoring but was 20.76 months for the ribociclib group; this difference was not statistically significant (P = .617). Patients with liver metastases had the shortest PFS. There were no differences in PFS based on whether the disease involved de novo metastases or recurrent disease or if patients were receiving an aromatase inhibitor or fulvestrant. Factors that affected PFS included the presence of liver metastases and order of treatment.

The median overall survival was not reached at the time of censor, but the survival rate was 68.8% when a CDK4/6 inhibitor was used first line and 63.9% when one of these agents was used later in the course of therapy.

Adverse events were similar as seen in female cohorts, with the most common being neutropenia (88%), fatigue (40%), anemia (28%), thrombocytopenia (20%), increased alanine aminotransferase (12%), asthenia (15%), and increased serum creatinine levels (12%). Serious adverse events that were grade 3 or higher were neutropenia (28%), anemia (4%), and increased alanine aminotransferase (4%). Over one-third of patients (36%) required a dose reduction, but this did not adversely affect survival. No drug-related adverse effects were thought to contribute to mortality.

Pharmacists can use this study to help define the place in therapy of CDK4/6 inhibitors in male patients with BC as they have been shown to have a similar efficacy and safety profile as in female patients with BC.

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