Background: Addition of the cyclin dependent kinase 4/6 inhibitor (CDK4/6i) palbociclib to endocrine therapy significantly improves progression free survival (PFS) in patients with HR+ MBC. The primary toxicity is neutropenia (ntp). TBCRC035 explored rates of neutropenia in patients with prior chemotherapy for MBC with 2 dose levels of palbociclib, and correlated changes in retinoblastoma protein phosphorylation (pRB) and Ki67 expression in proliferating keratinocytes and tumor, as well as mutations in cell-free DNA (cfDNA) with response.

Methods: TBCRC035 is a 1:1 randomized multicenter Phase II study evaluating palbociclib at either 125 or 100 mg in combination with physician choice fulvestrant or tamoxifen. Eligible patients (pts) with HR+ MBC had received 1-3 lines of chemotherapy for MBC, any number of hormone therapies, and were naïve to CDK4/6i. The primary endpoint was grade 3/4 ntp; secondary endpoints included PFS, clinical benefit rate (CBR), safety/tolerability, inhibition of RB phosphorylation (pRB) and change in Ki67 expression in skin and tumor (FFPE sections of skin and tumor biopsies) at day 14-21 of treatment compared to baseline, and correlation of response with mutations in cell free DNA (cfDNA). FFPE sections of skin punch and tumor biopsies were stained using antibodies to Ki67, total RB, and phospho-RB-S780 using BOND polymer red detection. Stained slides were scanned into the Aperio image analysis platform; the percentage of marker positive cells was determined. Whole blood was collected at baseline & processed for plasma; cfDNA was extracted. Using a combination of digital PCR and ultradeep next generation sequencing, cfDNA was analyzed for ESR1 and PIK3CA mutations.

Results: 70 pts were enrolled; 35 were randomized to 100 vs 125 mg of palbociclib respectively. 12 pts (100 mg), and 19 pts (125 mg) had >1 episode of grade 3/4 ntp (p=0.091). Pts on 100 mg had fewer total episodes of grade 3/4 ntp (p=0.036). Dose reductions were more frequent in patients starting with 125 mg compared to those starting with 100 mg (12 vs6). CBR and PFS were similar (100 vs 125 mg; CBR: 67 vs 74%; PFS: 6.5 vs 10 mo,p=0.18). In skin and tumor, the % of Ki67-positive nuclei was significantly lower in post-treatment biopsies (p<0.0001 for both) and was similarly reduced for pRb in skin, (tumor data pending); there was no significant difference in % change in pRB and Ki67 by palbociclib dose, or by CBR or PFS. Higher baseline tumor Ki67 was associated with worse PFS (p=0.006), regardless of dose. Presence of PIK3CA mutations in cfDNA correlated with worse PFS (p=0.008), but ESR1 mutations did not.

Conclusion: In pts with prior chemotherapy for HR+ MBC, treatment with 100 mg of palbociclib significantly reduced episodes of ntp and dose reductions; efficacy was comparable to patients treated with 125mg. Reductions in expression of Ki67 in tumor and keratinocytes and pRB in keratinocytes were comparable between dose levels indicating robust inhibition of CDK4/6 at both doses. The presence of PIK3CA mutations correlated with reduced PFS. These data suggest that dose reduction of palbociclib to mitigate toxicity should not compromise efficacy and provide additional prognostic information for use in treatment selection.

Citation Format: Hope S. Rugo, Erica Mayer, Anna Maria Storniolo, Claudine Isaacs, Ingrid Mayer, Vered Stearns, Rita Nanda, Julie Nangia, Michelle Melisko, Chiara Wabl, Alona Muzikansky, Bose Kochupurakkal, Ben H. Park, Antonio Wolff, Geoffrey Shapiro. Palbociclib in combination with fulvestrant or tamoxifen as treatment for hormone receptor positive metastatic breast cancer with prior chemotherapy for advanced disease (TBCRC 035): A Phase II study with pharmacodynamic markers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT128.