Background:Cyclin dependent kinase 4/6 inhibitors (CDK 4/6i) are standard of care for hormone receptor positive, HER2 negative metastatic breast cancer (MBC), alone or in combination with endocrine therapy. Combination with paclitaxel (P) is possible with an intermittent, alternating dosing schedule in order to synchronize (sync) the cell cycle (cc) with a short burst of CDKi. To optimize this strategy, we measured cc biomarkers before, during and after a 4 day run in of ribociclib (R) in patients (pts) receiving R with P in a phase I clinical trial (NCT02599363). Methods:Eligible pts with Rb-positive MBC of any subtype and measurable disease were treated with 200, 400 or 600 mg of R on Days (D) 2-5, 9-12 and 16-19, and P 80mg/m2 on D1, 8, 15 and 22 in 28-day cycles (C). Skin biopsies, blood samples and FLT-PET CT were assessed at baseline (pre- R burst), D-3 (post R burst) and C1D1 P (after cc re-entry). Skin biopsies were analyzed for Ki-67 (Dako Santa Clara, CA), phospho-Rb (Ser807/811; Cell Signaling, Danvers, MA) and Rb (clone 1F8, Thermo Scientific, Waltham, MA) by IHC. Serum and plasma were analyzed for thymidine kinase (TK, Biovica, Sweden) and R pharmacokinetics (PK, WuXi, China) respectively.P-Rb and Rb results were binned for intensity and pattern of staining.Intensity was defined as None (0), Weak (W, 1+), Moderate (M, 2+), Strong (S, 3+). Pattern was defined as: Rare (R, <10%), Focal (F, 10-50%) and Diffuse (D, >50%). SUV max was averaged over the 5 brightest lesions.Results:13 pts enrolled to the Phase I trial. Nine pts had at least 1 skin biopsy; 8 had ≥2. Five pts had at least one FLT-PET. Biomarker results are summarized in table. Ki-67 (skin) and TK and FLT (tumor) drop after 4 days of R treatment, when R PK levels are highest. Decreases in P-Rb, were less consistent, observed in 4/8 (50%) pts. While R PK levels drop after 2 days off treatment, only 2/8 (25%) subjects had higher cell cycle measurements on C1D1 compared to baseline.Conclusions:CC synchronization with intermittent dosing is possible, and allows safe administration of P. More time off R is needed for cc re-entry. Future trials should give more time off of R before P delivery.

Pt ID (Dose(mg))TKi-67 (%)P-RbIntensity- PatternRbIntensity- PatternR PKR PKTK (Du/L)FLT(SUV*)
     LEE011(ng/mL) LEQ803(ng/mL)   
002 (200) 22 M-F M-D 2045 n/a 
004 (200) M-R M-D 1903 11.6 
 D-3 M-D 69.6 12.0 1459.5 5.2 
 C1D1 M-R M-D 16.9 6.5 1653 10 
005 (200) 11 M-R W-F 832 12.4 
 D-3 M-R W-D 83.0 13.8 122.5 5.1 
 C1D1 61 M-F M-F 17.5 6.18 234.5 14.8 
006 (200) n/a n/a n/a 9030.5 12.3 
 D-3 n/a n/a n/a 154 12.3 7429.5 9.6 
013 (200) 19 M-F M-D 856 6.0 
 D-3 M-F W-F 158 15.8 767.5 5.2 
007 (400) M-F M-D 3070.5 n/a 
 D-3 M-D 332 30.8 2009.5 n/a 
010 (400) 23 M-F M-D 196 4.7 
 D-3 0.5 M-F 359 29.3 22.7 4.1 
 C1D1 M-R M-D 78.5 16.4 <20 4.0 
016 (400) 30 M-R W-D n/a n/a 75.5 6.4 
 D-3 12 W-F W-D n/a n/a n/a 4.1 
 C1D1 10 M-F W-D n/a n/a 21.6 5.1 
017 (400) 17 M-R W-F n/a n/a 46.6 n/a 
 D-3 14 M-R W-D n/a n/a 23 n/a 
 C1D1 16 M-R W-F n/a n/a <20 n/a 
014 (600) 15 M-R W-D n/a n/a n/a 
 D-3 12 M-R W-D n/a 106 <20 n/a 
 C1D1 26 M-F W-D n/a n/a <20 n/a 
Pt ID (Dose(mg))TKi-67 (%)P-RbIntensity- PatternRbIntensity- PatternR PKR PKTK (Du/L)FLT(SUV*)
     LEE011(ng/mL) LEQ803(ng/mL)   
002 (200) 22 M-F M-D 2045 n/a 
004 (200) M-R M-D 1903 11.6 
 D-3 M-D 69.6 12.0 1459.5 5.2 
 C1D1 M-R M-D 16.9 6.5 1653 10 
005 (200) 11 M-R W-F 832 12.4 
 D-3 M-R W-D 83.0 13.8 122.5 5.1 
 C1D1 61 M-F M-F 17.5 6.18 234.5 14.8 
006 (200) n/a n/a n/a 9030.5 12.3 
 D-3 n/a n/a n/a 154 12.3 7429.5 9.6 
013 (200) 19 M-F M-D 856 6.0 
 D-3 M-F W-F 158 15.8 767.5 5.2 
007 (400) M-F M-D 3070.5 n/a 
 D-3 M-D 332 30.8 2009.5 n/a 
010 (400) 23 M-F M-D 196 4.7 
 D-3 0.5 M-F 359 29.3 22.7 4.1 
 C1D1 M-R M-D 78.5 16.4 <20 4.0 
016 (400) 30 M-R W-D n/a n/a 75.5 6.4 
 D-3 12 W-F W-D n/a n/a n/a 4.1 
 C1D1 10 M-F W-D n/a n/a 21.6 5.1 
017 (400) 17 M-R W-F n/a n/a 46.6 n/a 
 D-3 14 M-R W-D n/a n/a 23 n/a 
 C1D1 16 M-R W-F n/a n/a <20 n/a 
014 (600) 15 M-R W-D n/a n/a n/a 
 D-3 12 M-R W-D n/a 106 <20 n/a 
 C1D1 26 M-F W-D n/a n/a <20 n/a 

Citation Format: Amy Clark, Azadeh Elmi, Nicholas P. McAndrew, Paul Wileyto, Natalie Shih, Michael Feldman, Mark Rosen, Jessica Savage, Robin Holmes, Nancy Dinubile, Theresa Berger, Erin Schubert, Alice Matthai, Melissa Volpe, Payal Shah, Susan Domchek, David Mankoff, Angela DeMichele. Cell cycle synchronization: Biomarker analysis in a phase I trial of alternating ribociclib and paclitaxel in advanced breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB052.