CONCURRENT USE OF ABEMACICLIB AND RADIOTHERAPY IN METASTATIC BREAST CANCER PATIENTS: A SINGLE CENTRE EXPERIENCE.

There is little evidence regarding the safety and efficacy of the combination of abemaciclib plus radiotherapy (RT). The majority of studies investigated the combination of RT with Palbociclib or Ribociclib reporting that hematological toxicity is common. Given the unique toxicity profile of Abemaciclib with greater gastrointestinal toxicity compared to hematological toxicity, we wanted to evaluate the toxicity of the combination with RT in metastatic breast cancer (BC) patients. Patients with histologically proven metastatic or locally advanced BC treated with RT and concurrent Abemaciclib were selected. Toxicity was assessed according to the NCI-CTCAE V4.0. 32 metastatic sites were treated in nineteen patients and analyzed. All patients received Abemaciclib during the RT course. 68% of patients received a full dose of Abemaciclib during RT. 71.9% of patients received a palliative intent (median dose= 30Gy, range= 8-30Gy), 26.3% were treated to 9 oligo-metastatic or oligo-progressive sites of disease with stereotactic body radiotherapy (SBRT) (median dose = 30Gy, range 21-30Gy given in 3-5 fractions Overall the rate of G3 toxicity was 15.7%. The rate of G3 hematological toxicity was 10.6 % (2/19 patients, one G3 neutropenia and one G3 anemia). No patient presented diarrhea, including those treated to RT sites close to the bowel. One patient developed G3 skin toxicity. Pain significantly improved after RT (mean value NRS pre-RT=3.9, SD=3.07; mean value NRS after-RT=0.9, SD= 0.46; p=<0.0001) Conclusion: Abemaciclib and Concomitant RT seem well tolerated showing acceptable toxicity.

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