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CDK4/6 inhibitors combined with endocrine therapy have demonstrated higher antitumor activity than endocrine therapy alone for the treatment of advanced estrogen receptor-positive breast cancer. Some of these tumors are de novo resistant to CDK4/6 inhibitors and others develop acquired resistance. Here, we show that p16 overexpression is associated with reduced antitumor activity of CDK4/6 inhibitors in patient-derived xenografts (n = 37) and estrogen receptor-positive breast cancer cell lines, as well as reduced response of early and advanced breast cancer patients to CDK4/6 inhibitors (n = 89). We also identified heterozygous RB1 loss as biomarker of acquired resistance and poor clinical outcome. Combination of the CDK4/6 inhibitor ribociclib with the PI3K inhibitor alpelisib showed antitumor activity in estrogen receptor-positive non-basal-like breast cancer patient-derived xenografts, independently of PIK3CA, ESR1 or RB1 mutation, also in drug de-escalation experiments or omitting endocrine therapy. Our results offer insights into predicting primary/acquired resistance to CDK4/6 inhibitors and post-progression therapeutic strategies. This study has been supported by the Susan G. Komen Foundation (CCR15330331) and by the Catalan Agency AGAUR (2017 SGR 540) [to V.S.]. V.S. received funds from the Instituto de Salud Carlos III: grants PI13/01714, CP14/00228, MV15/00041, CPII19/00033 and PI20/00892. M.P. received a Juan de la Cierva Grant from the Ministerio de Economía y Competitividad (FJCI-2015-25412), L.Mo. a grant from FI-AGAUR (2019 FI_B 01199), F.B-M. a grant from the Fundación Científica Asociación Española Contra el Cáncer (AECC_Postdoctoral17-1062) and M.S-G, a Marie Slodowska-Curie Innovative Training Networks PhD fellowship (H2020-MSCA-ITN-2015_675392). This work was supported by Breast Cancer Research Foundation (BCRF-19-08), Instituto de Salud Carlos III Project Reference number AC15/00062 and the EC under the framework of the ERA-NET TRANSCAN-2 initiative co-financed by FEDER, Instituto de Salud Carlos III (CB16/12/00449 and PI19/01181) and Asociación Española Contra el Cáncer (to J.A.). R.B.C. laboratory is supported by Breast Cancer Now (grant numbers: MAN-Q1 and MAN-Q2), NIHR Manchester Biomedical Research Centre (IS-BRC-1215-20007) and EdiREX Horizon 2020 grant No.731105. The xenograft program in the C.C. laboratory is supported by Cancer Research UK and also received funding from an EU H2020 Network of Excellence (EuroCAN). This work has been supported by NIH grants P30 CA008748 and RO1CA190642-01, the CDMRP grant BC171535P1, and the Breast Cancer Research Foundation [to M.S.]. A.P. received funds from Instituto de Salud Carlos III—PI16/00904 and PI19/01846, Breast Cancer Now—2018NOVPCC1294, Breast Cancer Research Foundation-AACR Career Development Awards for Translational Breast Cancer Research 19-20-26-PRAT, Fundació La Marató TV3 201935-30, the European Union’s Horizon 2020 research and innovation program H2020-SC1-BHC-2018-2020. IRB Barcelona is a recipient of a Severo Ochoa Centre of Excellence Award from the Spanish Ministry of Economy and Competitiveness (MINECO; Government of Spain) and is supported by CERCA (Generalitat de Catalunya). C. S. Verma reports grants from MSD International and grants from Ipsen outside the submitted work.

Phase II immuno-oncology clinical trials screen for efficacy an increasing number of treatments. In rare cancers, using historical control data is a pragmatic approach for speeding up clinical trials. The drop-the-losers design allows dropping off ineffective arms at interim analyses. We extended the original drop-the-losers design for a time-to-event outcome using a historical control through the one-sample log-rank statistic. Simulated trials featured three arms at the first stage, one at the second stage, nine scenarios, eight sample sizes with 5%- and 10%- nominal family-wise error rate (FWER). A numerical algorithm is provided to solve power calculations at the design stage. Our design was compared with a group of three independent single-arm trials (fixed design) with and without correction for multiplicity. Our design allowed strict control of the FWER at nominal levels while the misspecification of survival distribution and fixed design inflated the FWER up to three times the nominal level. The empirical power of our design increased with the sample size, the treatment effect and the number of effective treatments and dropped when more patients were recruited at the second stage. The fixed design with correction showed comparable power, while our design advantageously included more patients to the most promising arm. Recommendations for future applications are given. By taking advantage of the use of historical control data and a time-to-event outcome, the drop-the-losers design is a promising tool to meet the challenge of improving phase II clinical trials in immuno-oncology.