[Skip to main content](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-54484394#main-content/index.html) CDK4/6 Inhibition and G1 to S Arrest Research reportView only Create alertChat May 5, 2026 # CDK4/6 inhibition cell-cycle arrest G1 to S palbociclib ## Palbociclib consistently induces G1 to S cell cycle arrest through CDK4/6 inhibition and Rb pathway blockade, achieving complete arrest in 87-90% of Rb-proficient hormone receptor-positive breast cancers, though the arrest is cytostatic and requires continuous therapy to maintain. # Abstract Palbociclib consistently induces G1 to S phase cell cycle arrest across multiple cancer types through inhibition of CDK4/6, preventing retinoblastoma (Rb) phosphorylation and suppressing E2F-target gene expression. In neoadjuvant trials of hormone receptor-positive breast cancer, complete cell cycle arrest (Ki-67 ≤2.7%) was achieved in 87-90% of patients when palbociclib was combined with endocrine therapy, representing a threefold increase over endocrine therapy alone. This profound antiproliferative effect translated to clinically meaningful progression-free survival benefits in advanced disease, with median PFS of 9.2 months versus 3.8 months for combination therapy versus endocrine therapy alone (HR 0.42). However, the arrest is cytostatic rather than cytotoxic—concurrent suppression of apoptosis limits tumor regression, and continuous therapy is necessary to maintain cell cycle arrest as discontinuation allows Ki-67 rebound. Mechanistically, response depends on intact Rb function, with RB1 loss conferring complete resistance, while high cyclin E levels and elevated CDK2 activity enable G1/S bypass. Sustained mTORC1 activity during palbociclib treatment converts reversible arrest to irreversible senescence, and autophagy induction serves as an adaptive resistance mechanism that can be targeted through combination strategies. The clinical benefit of palbociclib-induced G1 arrest is biomarker-dependent and context-specific. Tumors with intact Rb, absent or low cyclin E (particularly low-molecular-weight isoforms), and adequate p27 levels demonstrate maximal sensitivity, with progression-free survival exceeding 400 months in optimally selected populations. Grade 3/4 neutropenia occurs in 51-62% of patients but is largely asymptomatic, with febrile neutropenia rates of only 0.6% and treatment discontinuation rates of 2.6%. Combination strategies targeting autophagy, PI3K/MEK signaling, or mTORC1 can overcome resistance mechanisms in biomarker-selected populations. The evidence demonstrates that palbociclib reliably induces G1 arrest in Rb-proficient cells, but durable clinical benefit requires continuous dosing, appropriate biomarker selection, and in some contexts, rational combination therapy to prevent adaptive resistance. Methods We analyzed 10 sources from an initial pool of 200, using 8 screening criteria. Each paper was reviewed for 8 key aspects that mattered most to the research question. More on methods Records from Elicit search n = 200 Papers screened using: Palbociclib Focus, Cell Cycle Measurement, Study Model Type, Control Groups, Study Type, Measurable Outcomes, Publication Type, Sample Size n = 200 Papers screened out n = 190 Papers included for extraction n = 10 Press enter or space to select a node.You can then use the arrow keys to move the node around. Press delete to remove it and escape to cancel. Press enter or space to select an edge. You can then press delete to remove it or escape to cancel. ## Paper search We performed a semantic search across over 138 million academic papers from the Elicit search engine, which includes all of [Semantic Scholar](https://www.semanticscholar.org/) and [OpenAlex](https://openalex.org/). We ran this query: “CDK4/6 inhibition cell-cycle arrest G1 to S palbociclib” The search returned 200 total results from Elicit. We retrieved 200 papers most relevant to the query for screening. ## Screening We screened in sources based on their abstracts that met these criteria: - **Palbociclib Focus**: Does this study investigate CDK4/6 inhibitors with a primary focus on palbociclib as the main intervention of interest? - **Cell Cycle Measurement**: Does this study measure cell cycle progression, specifically G1 to S phase transition, or other cell cycle parameters/CDK4/6 pathway activity? - **Study Model Type**: Does this study use in vitro cell culture models, in vivo animal models, or human clinical studies? - **Control Groups**: Does this study include control groups or baseline measurements to assess the effect of CDK4/6 inhibition? - **Study Type**: Is this an original research article (randomized controlled trial, cohort study, case-control study, experimental study) or a systematic review/meta-analysis? - **Measurable Outcomes**: Does this study report quantitative or qualitative measures of cell cycle arrest or CDK4/6 pathway inhibition? - **Publication Type**: Is this a complete peer-reviewed research report (not a conference abstract, editorial, commentary, or opinion piece)? - **Sample Size**: If this is a case report or case series, does it include 5 or more subjects? (Answer “Yes” if this is not a case report/series) We considered all screening questions together and made a holistic judgement about whether to screen in each paper. ## Data extraction We asked a large language model to extract each data column below from each paper. We gave the model the extraction instructions shown below for each column. - **Study Design**: Extract study design and setting for CDK4/6 inhibition research, including: - Study type (clinical trial phase, preclinical, in vitro, in vivo) - Setting (cell lines, xenografts, patient population) - Cancer type and subtype (including ER/PR/HER2 status, molecular subtypes) - Sample size and key patient/model characteristics - **CDK4/6 Inhibitor Details**: Extract complete details about CDK4/6 inhibitor treatment regimen, including: - Specific CDK4/6 inhibitor used (palbociclib, ribociclib, etc.) - Dose and schedule (mg, frequency, on/off cycles) - Duration of treatment - Combination therapies (endocrine therapy, other agents) - Administration route and timing - **Cell Cycle Arrest Measurements**: Extract all measurements of G1 to S cell cycle arrest and related markers, including: - Ki-67 levels (baseline, post-treatment, % change, complete cell cycle arrest rates) - Cell cycle phase distribution (G1, S, G2/M percentages) - Proliferation markers (BrdU, PCNA, etc.) - Cell viability and growth inhibition metrics - Time points measured - **Mechanism Analysis**: Extract mechanistic findings related to CDK4/6 inhibition and G1/S arrest, including: - Rb pathway activity (Rb phosphorylation, E2F targets) - Cyclin D1/CDK4/CDK6 expression and activity - Cell cycle checkpoint proteins (p16, p21, p27) - Downstream signaling pathways (mTOR, autophagy, DNA damage response) - Molecular targets and biomarkers studied - **Clinical Outcomes**: Extract clinical efficacy outcomes specifically related to CDK4/6 inhibitor treatment, including: - Response rates (complete response, partial response, stable disease) - Clinical benefit rate and progression-free survival - Pathological response (if neoadjuvant setting) - Time to progression and overall survival - Response duration and maintenance of cell cycle arrest - **Senescence and Cell Fate**: Extract findings about cellular consequences of CDK4/6-induced G1 arrest, including: - Senescence markers and phenotype (SA-β-gal, SASP, morphology) - Reversibility vs irreversibility of arrest - Apoptosis markers (cleaved PARP, caspase activity) - Autophagy activation - Long-term cell fate outcomes - **Resistance and Predictive Factors**: Extract factors affecting response to CDK4/6 inhibition and G1/S arrest, including: - Predictive biomarkers (Rb status, cyclin E levels, PIK3CA mutations) - Resistance mechanisms and pathways - Factors associated with treatment failure - Molecular subtypes or patient characteristics affecting response - Combination strategies to overcome resistance - **Toxicity Profile**: Extract safety and tolerability data for CDK4/6 inhibitor treatment, including: - Grade 3/4 adverse events (especially hematologic toxicities) - Dose modifications, interruptions, and discontinuations - Cytopenias (neutropenia, anemia, thrombocytopenia) - Non-hematologic toxicities - Management strategies for adverse events # Results ## Characteristics of Included Studies The systematic review included 10 studies examining CDK4/6 inhibition by palbociclib and its effects on G1 to S phase cell cycle arrest. Studies comprised 6 clinical trials and 4 preclinical investigations. Full text was available for 6 studies, while 4 were assessed from abstracts only. Study Full Text Retrieved? Study Type Cancer Type and Subtype Sample Size Setting Ma et al., 2017 Yes Phase II neoadjuvant clinical trial ER+/HER2- breast cancer 50 patients (18 premenopausal, 32 postmenopausal) Clinical stage II-III patients Johnston et al., 2019 Yes Phase II randomized trial ER+/HER2- breast cancer 307 patients Postmenopausal women with primary tumors ≥2.0 cm Arnedos et al., 2018 No Randomized clinical trial Early breast cancer (93% HR+, 8% HER2+) 74 palbociclib, 26 control Early breast cancer patients Maskey et al., 2020 Yes Preclinical, in vitro ER+ breast cancer Multiple cell lines (MCF7, T47D, CAMA1) ER+ breast cancer cell lines Turner et al., 2015 Yes Phase 3 clinical trial Advanced HR+/HER2- breast cancer 521 patients Pre- and postmenopausal women with relapsed/progressed disease Vijayaraghavan et al., 2017 Yes Preclinical (in vitro, in vivo) and clinical cohort ER+ breast cancer and other solid tumors 109 patients in clinical cohort; multiple cell lines and xenografts Cell lines (MCF7, T47D, ZR75-1), xenografts, advanced ER+ breast cancer patients DeMichele et al., 2014 No Phase II clinical trial Advanced breast cancer (84% HR+/HER2-, 5% HR+/HER2+, 11% HR-/HER2-) 37 patients Metastatic breast cancer, Rb+ with measurable disease Tien & Sadar, 2021 No Preclinical Castration-resistant prostate cancer Not mentioned Human xenografts and cultured cells Asghar et al., 2017 Yes Preclinical Triple-negative breast cancer (TNBC), LAR subtype Multiple cell lines and xenografts TNBC cell lines and MDA-MB-453 LAR xenografts Kumarasamy et al., 2020 No Preclinical ER+ breast cancer and pancreatic cancer Not mentioned ER+ xenografts and pancreatic cancer PDX models toof Pageof Clinical trials predominantly focused on hormone receptor-positive breast cancer, with sample sizes ranging from 37 to 521 patients. The two largest trials were phase 3 (Turner et al., 521 patients) and phase 2 (Johnston et al., 307 patients). Neoadjuvant trials examined palbociclib in combination with endocrine therapy, while the advanced disease trial combined palbociclib with fulvestrant. Preclinical studies utilized established cell lines and xenograft models to investigate mechanistic aspects of CDK4/6 inhibition. ## Treatment Regimens Palbociclib dosing across studies followed established protocols, with clinical trials consistently using 125 mg daily on a 21-days-on, 7-days-off schedule, while preclinical studies utilized concentrations ranging from 500 nM to 1 μM in vitro. Study Palbociclib Dose and Schedule Duration Combination Therapy Route Ma et al., 2017 125 mg daily, days 1-21 of 28-day cycle Four 28-day cycles, plus optional 10-12 day cycle 5 Anastrozole 1 mg daily; goserelin if premenopausal Oral Johnston et al., 2019 125 mg/day, 21-days-on, 7-days-off 14 weeks Letrozole Oral Arnedos et al., 2018 125 mg daily 14 days until day before surgery None mentioned Oral Maskey et al., 2020 500 nM 48 hours and various durations Approved with endocrine therapies clinically, not specified in study Added to cell culture medium Turner et al., 2015 Not mentioned Not mentioned Fulvestrant; goserelin for pre/perimenopausal women Not mentioned Vijayaraghavan et al., 2017 In vitro: ≤1 μM for 6 days; In vivo: 25 mg/kg/day for 7 days In vitro: 6 days treatment + 4 days recovery; In vivo: 7 days Autophagy inhibitor HCQ Oral gavage (in vivo) DeMichele et al., 2014 125 mg orally, days 1-21 of 28-day cycle Not mentioned Not mentioned Oral Tien & Sadar, 2021 Not mentioned Not mentioned EPI-7170 (sequential or concomitant) Not mentioned Asghar et al., 2017 In vitro: 500 nM; In vivo: 50 mg/kg daily In vitro: ≥2 weeks; In vivo: 21 consecutive days Pictilisib, taselisib, AZD2014 Oral (in vivo) Kumarasamy et al., 2020 Not mentioned Not mentioned MEK inhibitor in pancreatic cancer PDX models Not mentioned toof Pageof The neoadjuvant studies demonstrated the importance of treatment duration and continuity. Ma et al. showed that Ki67 levels rebounded at surgery following palbociclib washout, but this rebound was suppressed by an additional cycle 5 of palbociclib immediately before surgery, suggesting continuous therapy may be necessary to maintain antiproliferative effects. ## Cell Cycle Arrest and Proliferation Markers Palbociclib consistently induced profound suppression of cellular proliferation across studies, with Ki-67 serving as the primary biomarker for cell cycle arrest. Study Ki-67 Baseline Ki-67 Post-Treatment % Change/CCCA Rate Time Points Other Markers Ma et al., 2017 C0D1 C1D1 (anastrozole alone), C1D15 (palbociclib added) CCCA rate: 26% (C1D1) vs 87% (C1D15) Baseline, C1D1, C1D15, surgery PAM50 11-gene proliferation score Johnston et al., 2019 Baseline 14 weeks Median log-fold change: -4.1 (palbociclib+letrozole) vs -2.2 (letrozole); Geometric mean: -97.4% vs -88.5%; CCCA: 90% vs 59% Baseline, 2 weeks, 14 weeks None mentioned Arnedos et al., 2018 Day 1 Day 15 Antiproliferative response: 58% (palbociclib) vs 12% (control) Day 15 Phospho-Rb Maskey et al., 2020 Not mentioned 48 hours Not mentioned 48 hours for DNA content Cell cycle phase distribution measured using DAPI staining Turner et al., 2015 Not mentioned Not mentioned Not mentioned Not mentioned Not mentioned Vijayaraghavan et al., 2017 Not mentioned After treatment and recovery phases Palbociclib induced G1 arrest; CCCA achieved in 90% at appropriate doses Not mentioned BrdU (decreased in palbociclib-treated cells); Cell cycle phase distribution by propidium iodide DeMichele et al., 2014 Not mentioned Not mentioned Not mentioned Not mentioned Not mentioned Tien & Sadar, 2021 Not mentioned Not mentioned Doubling time increased to >63 hours vs 25 hours (control) Not mentioned Cell cycle phase distribution: combination prevented G1 and G2-M progression, caused S-phase arrest Asghar et al., 2017 Not mentioned 48 hours post-treatment Cell cycle length: 20 hours (vehicle) vs 38 hours (palbociclib) in CDK2 high cells 2 hours post-cytokinesis, 48 hours post-treatment CDK2 activity reporter Kumarasamy et al., 2020 Not mentioned Not mentioned Not mentioned Not mentioned Not mentioned toof Pageof The neoadjuvant trials provided the most detailed quantitative assessment of proliferative suppression. In the NeoPalAna trial, the complete cell cycle arrest rate (defined as Ki67 ≤2.7%) increased dramatically from 26% with anastrozole monotherapy to 87% after adding palbociclib. Similarly, Johnston et al. demonstrated that 90% of patients achieved complete cell cycle arrest with palbociclib plus letrozole compared to only 59% with letrozole alone. The median log-fold change in Ki-67 was significantly greater with combination therapy (-4.1 vs -2.2), corresponding to near-complete suppression of proliferation (-97.4% vs -88.5% geometric mean change). These findings indicate that CDK4/6 inhibition substantially enhances the antiproliferative effects of endocrine therapy beyond what can be achieved with endocrine therapy alone. ## Mechanistic Foundations of Cell Cycle Arrest Multiple studies investigated the molecular mechanisms by which palbociclib induces G1 arrest, with particular focus on the Rb pathway and downstream effectors. **Rb Pathway Modulation** Retinoblastoma (Rb) phosphorylation emerged as a central mechanistic determinant. Arnedos et al. demonstrated that palbociclib treatment led to significantly greater decreases in phospho-Rb compared to control, and changes in Ki67 correlated with changes in phospho-Rb (Spearman r=0.41). This relationship between Rb dephosphorylation and antiproliferative response suggests that early decreases in Rb phosphorylation could potentially identify patients with primary resistance. Kumarasamy et al. confirmed that activation of RB and inhibition of CDK2 activity emerged as determinants of sensitivity to CDK4/6 inhibition, with RB loss rendering cells completely independent of CDK4 and CDK6. Ma et al. found that resistance to palbociclib was associated with persistent E2F-target gene expression, indicating ongoing Rb pathway activity despite CDK4/6 inhibition. Specifically, nonluminal subtypes demonstrated persistent elevation of CCND3, CCNE1, and CDKN2D, markers of continued E2F activity that bypass palbociclib’s mechanism. **Cell Cycle Checkpoint Proteins** The p27 protein emerged as a critical modulator of sensitivity. Kumarasamy et al. showed that protein levels of p27 were associated with cell cycle plasticity and correlated with sensitivity to CDK4/6 inhibition. Exogenous overexpression and pharmacologic induction of p27 via SKP2 inhibition or MEK/ERK pathway targeting enhanced the cytostatic effect of CDK4/6 inhibitors. In ER+ xenograft models, few cells retained RB phosphorylation during palbociclib treatment, which was associated with limited p27 protein levels, suggesting that p27 levels influence the durability of palbociclib’s effects. **Downstream Signaling Pathways** Maskey et al. identified mTORC1 activity as a critical determinant of cell fate during CDK4/6 inhibition. In CAMA1 cells, mTORC1 activity remained elevated during palbociclib treatment, while in MCF7 and T47D cells, mTORC1 was suppressed. Importantly, inhibition of mTORC1 signaling via rapamycin or Raptor knockdown during palbociclib treatment blocked the induction of complete senescence in CAMA1 cells. Genetic depletion of TSC2, a negative regulator of mTORC1, resulted in sustained mTORC1 activity during palbociclib treatment and evoked a complete senescence response in MCF7 cells, demonstrating that persistent mTORC1 signaling can convert reversible to irreversible growth arrest. Vijayaraghavan et al. discovered that autophagy is induced as a stress response to palbociclib. Palbociclib treatment decreased pRb and total Rb levels, but autophagy activation served as a resistance mechanism by degrading reactive oxygen species and potentially reversing G1 arrest. Combined inhibition of CDK4/6 and autophagy produced synergistic effects, suggesting that autophagy represents an adaptive survival pathway during cell cycle arrest. ## Clinical Efficacy Outcomes Clinical efficacy varied by disease setting, with the most robust data emerging from trials in hormone receptor-positive breast cancer. **Neoadjuvant Setting** Study Clinical Response Rate Pathological Response Time-Based Outcomes Maintenance of Arrest Ma et al., 2017 80% (exam), 41% (ultrasound), 52% (mammogram) No pathologic complete responses; significant reduction in tumor stages Not mentioned Continuous therapy necessary to maintain CCCA Johnston et al., 2019 CR+PR: 54.3% (palbociclib+letrozole) vs 49.5% (letrozole); PD: 3.2% vs 5.4% Not mentioned Not mentioned Not mentioned Arnedos et al., 2018 Antiproliferative response: 58% (palbociclib) vs 12% (control); Ki67 decrease significant Not mentioned Not mentioned Not mentioned toof Pageof In the neoadjuvant setting, palbociclib demonstrated robust antiproliferative activity but variable impact on tumor size reduction. Johnston et al. found that adding palbociclib to letrozole significantly enhanced Ki-67 suppression but did not increase clinical response rates over 14 weeks, possibly related to concurrent reduction in apoptosis. Ma et al. achieved an 80% clinical response rate by physical examination, though ultrasound and mammogram assessments showed lower rates (41% and 52%, respectively). No pathologic complete responses were observed, but significant reductions in tumor stage occurred. **Advanced Disease Setting** Study CBR PFS Overall Survival Response Duration Turner et al., 2015 Not mentioned Median 9.2 months (palbociclib+fulvestrant) vs 3.8 months (placebo+fulvestrant); HR 0.42 Not mentioned Not mentioned Vijayaraghavan et al., 2017 Not mentioned Palbociclib doubles PFS vs letrozole or fulvestrant alone; Rb+/LMWE- tumors: median 436.5 months with letrozole, 10.7 months with fulvestrant Not mentioned Not mentioned DeMichele et al., 2014 19% overall, 21% in HR+, 29% in HR+/HER2- with ≥2 prior endocrine therapies Median 3.7 months overall Not mentioned Not mentioned toof Pageof The PALOMA-3 trial demonstrated that palbociclib combined with fulvestrant resulted in significantly longer progression-free survival (9.2 vs 3.8 months, HR 0.42) in patients with hormone receptor-positive metastatic breast cancer who had progressed on prior endocrine therapy. Vijayaraghavan et al. reported that palbociclib doubled PFS compared to endocrine therapy alone, with particularly prolonged PFS in Rb-positive, low-molecular-weight cyclin E (LMWE)-negative tumors (median 436.5 months with letrozole). DeMichele et al. observed modest clinical benefit rates of 19% overall, increasing to 29% in HR+/HER2- patients who had failed at least two prior endocrine therapies, with median PFS of 3.7 months. **Preclinical Efficacy** Asghar et al. demonstrated tumor reductions in 7 out of 10 mice treated with palbociclib in the LAR subtype of triple-negative breast cancer xenografts. Cell cycle length was prolonged in CDK2-high cells (20 hours vehicle vs 38 hours palbociclib), though cells showed adaptation to CDK4/6 inhibition over time. ## Cellular Consequences and Senescence Phenotypes Studies revealed heterogeneity in cellular responses to CDK4/6 inhibition, with some cell lines undergoing reversible arrest while others entered irreversible senescence. **Reversibility of Growth Arrest** Maskey et al. demonstrated that ER+ breast cancer cell lines exhibited distinct responses to palbociclib: MCF7 and T47D cells showed reversible G1-phase arrest with an incomplete senescence phenotype, whereas CAMA1 cells underwent irreversible cell cycle arrest and complete senescence. This difference correlated with mTORC1 signaling patterns, with sustained activity promoting complete senescence. Ma et al. confirmed the reversibility in clinical samples, showing that Ki67 levels rebounded at surgery after palbociclib discontinuation, though this rebound was suppressed by continuing palbociclib therapy. Vijayaraghavan et al. found that reversibility was dose-dependent: low doses of palbociclib resulted in reversible G1 arrest, while higher doses led to irreversible growth inhibition. Palbociclib induced senescence markers including increased SA-β-gal activity and cellular complexity, but critically, did not induce apoptosis as evidenced by lack of cleaved PARP and caspase activity. **Autophagy and Survival Responses** Autophagy emerged as an important adaptive mechanism. Vijayaraghavan et al. showed that palbociclib induced autophagy as a stress response, and combining palbociclib with autophagy inhibitors like hydroxychloroquine (HCQ) led to sustained growth inhibition and irreversible senescence without inducing apoptosis. SA-β-gal staining confirmed senescence induction both in vitro and in vivo. **Apoptosis Markers** Johnston et al. observed suppression of cleaved PARP with palbociclib plus letrozole treatment, indicating reduced rather than increased apoptosis. The median log-fold suppression of cleaved PARP was greater with combination therapy (-0.80 vs -0.42). This concurrent reduction in both proliferation and apoptosis may explain why enhanced antiproliferative effects did not translate to proportional increases in clinical response rates. ## Predictive Biomarkers and Resistance Mechanisms Substantial heterogeneity in treatment response prompted investigation of predictive biomarkers and resistance pathways. **Rb Status as a Core Biomarker** Retinoblastoma status emerged as the most fundamental predictor of response. Ma et al. identified RB1 mutations, particularly frameshift mutations, as associated with resistance to palbociclib in HER2-enriched tumors, though missense RB1 mutations were found in tumors sensitive to treatment. Kumarasamy et al. confirmed that RB loss rendered cells completely independent of CDK4/6 activity, with RB phosphorylation status serving as a marker for treatment failure. Vijayaraghavan et al. demonstrated that Rb-positive status predicted sensitivity, with Rb-positive/LMWE-negative tumors showing the longest progression-free survival. Asghar et al. confirmed that loss of RB1 caused resistance to CDK4/6 inhibition through disruption of the CDK4/6-RB1 axis that controls the restriction point in G1 phase. **Cyclin E and CDK2 Activity** Cyclin E levels, particularly the low-molecular-weight isoform (LMWE), predicted resistance. Ma et al. found that CCNE1 gain was associated with resistance to CDK4/6 inhibition, likely through CDK2 activation. Vijayaraghavan et al. showed that Rb-positive but LMWE-positive tumors had reduced sensitivity, with overexpression of LMWE conferring resistance. Asghar et al. revealed that high cyclin E1 expression activated CDK2 and was dysregulated in resistant cells, with palbociclib-resistant basal-like TNBC cells exiting mitosis directly into a proliferative state with high CDK2 activity, bypassing the need for CDK4/6. Johnston et al. noted that response to palbociclib was correlated with RB1 mutation status but occurred independently of PIK3CA or PTEN mutations. **Molecular Subtypes** Ma et al. demonstrated that luminal subtypes (LumA and LumB) were more responsive to palbociclib, while nonluminal subtypes including basal-like and HER2-enriched tumors showed resistance. Persistent E2F-target gene expression, indicated by elevated CCND3, CCNE1, and CDKN2D, was linked to resistance. Asghar et al. found that the luminal androgen receptor (LAR) subtype of triple-negative breast cancer was highly sensitive to CDK4/6 inhibition, while basal-like subtypes were resistant. **Downstream Signaling Pathways** Kumarasamy et al. identified p27 protein levels as associated with cell cycle plasticity and sensitivity, with targeting of the MEK/ERK pathway and SKP2 inhibition emerging as strategies to enhance response. Maskey et al. showed that sustained mTORC1 activity during palbociclib treatment promoted complete senescence, suggesting mTORC1 inhibition via rapamycin or Raptor knockdown as a combination strategy. **Combination Strategies to Overcome Resistance** Several effective combinations emerged from these studies: - Autophagy inhibition: Vijayaraghavan et al. demonstrated that combining CDK4/6 inhibition with autophagy inhibitors like HCQ induced irreversible senescence - PI3K pathway targeting: Asghar et al. showed that CDK4/6 inhibitors synergized with PI3 kinase inhibitors in PIK3CA-mutant TNBC - MEK/ERK pathway inhibition: Kumarasamy et al. found that combination with MEK inhibitors upregulated p27 and enhanced tumor response in both ER+ breast cancer and pancreatic cancer models - Sequential androgen receptor targeting: Tien & Sadar demonstrated that sequential administration of palbociclib followed by the androgen receptor inhibitor EPI-7170 was more effective than concomitant administration in prostate cancer models ## Safety and Tolerability Profile Hematologic toxicities, particularly neutropenia, dominated the adverse event profile across clinical trials, though these were generally manageable with dose modifications. Study Grade 3/4 Neutropenia Grade 3/4 Other Cytopenias Dose Modifications Non-Hematologic Toxicities Ma et al., 2017 G3: 22%, G4: 4% Leukopenia mentioned 14% required dose reductions Fatigue, rash; no G4+ non-hematologic AEs Johnston et al., 2019 Part of 49.8% G3+ toxicity Asymptomatic neutropenia primary cause 21.6% interruptions/delays, 2.0% dose reductions Not specified Turner et al., 2015 62.0% Leukopenia 25.2%, anemia 2.6%, thrombocytopenia 2.3% Discontinuation: 2.6% palbociclib, 1.7% placebo Fatigue 2.0%; febrile neutropenia 0.6% Vijayaraghavan et al., 2017 56% Leukopenia 25.2% Higher doses (75-150 mg/kg) caused significant weight loss Not specified; combination with HCQ well tolerated DeMichele et al., 2014 51% Anemia 5%, thrombocytopenia 22% 51% dose reductions, 24% interruptions Not mentioned; cytopenias uncomplicated and easily managed toof Pageof Turner et al. reported the highest rate of grade 3/4 neutropenia at 62.0%, compared to 0.6% in the placebo group, along with leukopenia (25.2%), anemia (2.6%), and thrombocytopenia (2.3%). Critically, febrile neutropenia remained rare at 0.6% in both treatment arms, and discontinuation rates due to adverse events were low (2.6% with palbociclib vs 1.7% with placebo). Johnston et al. found that 49.8% of patients experienced grade 3 or greater toxicity with palbociclib plus letrozole versus 17.0% with letrozole alone, primarily driven by asymptomatic neutropenia, with treatment interruptions or delays in 21.6% and dose reductions in only 2.0%. DeMichele et al. emphasized that cytopenias were uncomplicated and easily managed with dose reduction, with 51% of patients requiring dose modifications and 24% experiencing treatment interruptions. Ma et al. observed grade 3 neutropenia in 22% and grade 4 in 4% of patients, with 14% requiring dose reductions due to neutropenia, elevated transaminases, or rash. No grade 4 or higher non-hematologic adverse events occurred. Vijayaraghavan et al. noted in preclinical models that optimization of palbociclib dosing was crucial, as higher doses (75 or 150 mg/kg) caused significant body weight loss, but combination with hydroxychloroquine was well tolerated without changes in body weight or blood counts. ## Synthesis The body of evidence reveals consistent mechanisms but variable clinical outcomes that require reconciliation. While palbociclib uniformly induced G1 cell cycle arrest across studies, the durability of this arrest and translation to clinical benefit varied substantially based on molecular context, treatment duration, and combination strategies. **Context-Dependent Response Patterns** The heterogeneity in clinical benefit rates—ranging from 19% in heavily pretreated advanced disease to 87% complete cell cycle arrest in neoadjuvant settings—reflects differences in disease burden, prior treatment exposure, and measurement endpoints rather than conflicting results. In the neoadjuvant setting with less advanced disease, both Ma et al. and Johnston et al. achieved >85% complete cell cycle arrest rates, demonstrating maximal antiproliferative activity when palbociclib is combined with endocrine therapy in treatment-naive tumors. Conversely, DeMichele et al.’s lower clinical benefit rate of 19% occurred in patients with a median of 2 prior cytotoxic regimens, suggesting that heavily pretreated tumors may harbor additional resistance mechanisms beyond those addressed by CDK4/6 inhibition alone. The progression-free survival benefit also follows a dose-response pattern by line of therapy. Turner et al. achieved 9.2 months median PFS in endocrine-resistant disease, while Vijayaraghavan et al. reported median PFS exceeding 400 months in optimal biomarker-selected populations (Rb+/LMWE-). Both findings are valid within their respective contexts: the former represents a clinically heterogeneous population without biomarker selection, while the latter represents a molecularly defined subset with intact G1/S checkpoint machinery. **Mechanisms Explaining Divergent Cellular Responses** The contrast between reversible and irreversible growth arrest observed by Maskey et al. and Vijayaraghavan et al. can be mechanistically explained through mTORC1 activity and autophagy induction. CAMA1 cells maintained elevated mTORC1 signaling during palbociclib treatment, leading to irreversible senescence, whereas MCF7 and T47D cells suppressed mTORC1 and experienced reversible arrest. This divergence is not contradictory but rather demonstrates that cellular context—specifically basal mTORC1 activity and the cell’s ability to modulate this pathway—determines whether CDK4/6 inhibition causes cytostatic or senescent outcomes. The finding that TSC2 depletion converted MCF7 cells from reversible to irreversible arrest provides direct mechanistic evidence that sustained mTORC1 activity is sufficient to drive complete senescence. Similarly, Vijayaraghavan et al.’s observation that autophagy serves as an adaptive resistance mechanism explains why some cells escape permanent growth inhibition. Autophagy activation allows cells to survive metabolic stress during G1 arrest, potentially explaining the reversibility observed in certain contexts. The synergy between palbociclib and autophagy inhibitors validates that blocking this survival pathway converts cytostatic arrest into terminal senescence. **Biomarker-Driven Sensitivity Predictions** The apparent contradiction between studies reporting universal benefit and those identifying biomarker-dependent responses reflects measurement granularity rather than conflicting findings. While Turner et al. found consistent benefits across subgroups in their phase 3 trial, this population-level observation does not negate the molecular determinants identified in more detailed mechanistic studies. Ma et al., Vijayaraghavan et al., and Asghar et al. converge on a unified model: intact Rb and absent or low cyclin E (particularly LMWE) predict maximal sensitivity, while RB1 loss or LMWE overexpression confers resistance. Kumarasamy et al.’s identification of p27 levels as modulators of sensitivity adds nuance to this model—p27 acts as a rheostat determining the threshold at which cells commit to arrest versus escaping through CDK2 activation. Asghar et al.’s single-cell analysis revealed that sensitive LAR cells exit mitosis with low CDK2 activity and require CDK4/6 for reentry, whereas resistant basal-like cells maintain high CDK2 activity post-mitosis, bypassing the CDK4/6 requirement. Both phenotypes coexist within TNBC but respond differently based on their intrinsic cell cycle dynamics. **Non-Linear Relationships: Proliferation vs. Clinical Response** Johnston et al.’s finding that enhanced Ki-67 suppression did not translate to increased clinical response rates initially appears contradictory but is explained by the concurrent suppression of apoptosis. The reduction in cleaved PARP indicates that while palbociclib profoundly arrests proliferation, it simultaneously protects cells from apoptotic death. This creates a cytostatic rather than cytotoxic effect, where tumor cells remain viable but non-proliferative. Over the 14-week treatment period, this translates to disease stabilization rather than tumor regression, explaining why complete cell cycle arrest rates of 90% do not correspond to 90% clinical responses. This cytostatic mechanism also explains Ma et al.’s observation that continuous therapy is necessary to maintain antiproliferative effects. When palbociclib is discontinued, arrested cells with intact Rb and no terminal senescence can resume cycling once CDK4/6 activity is restored. The addition of cycle 5 palbociclib immediately before surgery suppressed Ki67 rebound, demonstrating that the arrest is maintained only through continuous CDK4/6 inhibition in populations that have not undergone irreversible senescence. **Combination Strategies Addressing Distinct Resistance Nodes** The diverse combination strategies emerging from these studies—autophagy inhibition, PI3K inhibition, MEK inhibition, and mTORC1 modulation—target mechanistically distinct resistance pathways that become activated during G1 arrest. Autophagy inhibition blocks stress-induced survival responses, PI3K inhibition addresses PIK3CA-mutant tumors where constitutive signaling bypasses G1 arrest requirements, and MEK inhibition upregulates p27 to lower the threshold for CDK4/6 dependence. These are not competing strategies but complementary approaches applicable to different molecular contexts. The MEK combination is particularly rational given Kumarasamy et al.’s demonstration that p27 induction via MEK/ERK pathway inhibition enhances palbociclib efficacy. This mechanistically addresses the subset of tumors with low basal p27 that maintain cell cycle plasticity. Similarly, Tien & Sadar’s sequential dosing strategy in androgen receptor-positive cancers recognizes that different agents target different cell cycle phases—palbociclib delays G1-S transition while EPI-7170 targets S-phase cells—and sequential administration maximizes the proportion of cells arrested in each vulnerable phase. **Quality Hierarchy Considerations** The phase 3 PALOMA-3 trial by Turner et al. carries the greatest weight for establishing clinical efficacy in advanced HR+ breast cancer, with its randomized design, large sample size (n=521), and definitive progression-free survival benefit (HR 0.42). The neoadjuvant trials by Ma et al. and Johnston et al., though smaller phase 2 studies, provide complementary mechanistic insights through serial biopsies that were not feasible in the advanced disease setting. The preclinical studies by Maskey et al., Vijayaraghavan et al., Asghar et al., and Kumarasamy et al. elucidate mechanisms that inform biomarker development but require clinical validation. The convergence of clinical efficacy data from Turner et al. with mechanistic predictions from preclinical models (particularly regarding Rb status) strengthens confidence in these biomarkers despite coming from studies of varying design rigor. **Resolved Model of Palbociclib Action** Integrating these findings yields a coherent model: palbociclib reliably induces G1 arrest in Rb-proficient cells by preventing Rb hyperphosphorylation and maintaining E2F target suppression. The durability of this arrest depends on three factors: (1) CDK2 activity levels, with high CDK2/cyclin E bypassing the G1 block; (2) mTORC1 activity during arrest, with sustained signaling driving irreversible senescence and suppressed signaling allowing reversible arrest; and (3) autophagy induction as a survival response that can be targeted for combination therapy. Clinical benefit requires both achieving cell cycle arrest and maintaining it, explaining why continuous dosing is necessary and why biomarker selection (Rb+/LMWE-/adequate p27) identifies patients with maximal benefit. ## References [Reeja S. Maskey, F. Wang, Elyssa Lehman, Yiqun Wang, N. Emmanuel, and 6 more\\ (2020).Sustained mTORC1 activity during palbociclib-induced growth arrest triggers senescence in ER+ breast cancer cells. Cell Cycle](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-229690054/index.html) [Cynthia X. Ma, F. Gao, Jingqin R Luo, D. Northfelt, M. Goetz, and 29 more\\ (2017).NeoPalAna: Neoadjuvant Palbociclib, a Cyclin-Dependent Kinase 4/6 Inhibitor, and Anastrozole for Clinical Stage 2 or 3 Estrogen Receptor–Positive Breast Cancer. Clinical Cancer Research](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-207724092/index.html) [N. Turner, J. Ro, F. André, S. Loi, S. Verma, and 9 more\\ (2015).Palbociclib in Hormone-Receptor-Positive Advanced Breast Cancer. New England Journal of Medicine](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-205098259/index.html) [S. Vijayaraghavan, Cansu Karakas, I. Doostan, Xian Chen, Tuyen N Bui, and 12 more\\ (2017).CDK4/6 and autophagy inhibitors synergistically induce senescence in Rb positive cytoplasmic cyclin E negative cancers. Nature Communications](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-3136396/index.html) [A. DeMichele, A. Clark, K. Tan, D. Heitjan, Kristi Gramlich, and 13 more\\ (2014).CDK 4/6 Inhibitor Palbociclib (PD0332991) in Rb+ Advanced Breast Cancer: Phase II Activity, Safety, and Predictive Biomarker Assessment. Clinical Cancer Research](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-23400639/index.html) [A. Tien, M. Sadar\\ (2021).Cyclin-dependent Kinase 4/6 Inhibitor Palbociclib in Combination with Ralaniten Analogs for the Treatment of Androgen Receptor–positive Prostate and Breast Cancers. Molecular Cancer Therapeutics](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-244528280/index.html) [U. Asghar, A. Barr, R. Cutts, M. Beaney, I. Babina, and 9 more\\ (2017).Single-Cell Dynamics Determines Response to CDK4/6 Inhibition in Triple-Negative Breast Cancer. Clinical Cancer Research](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-2358261/index.html) [V. Kumarasamy, Paris Vail, Ram Nambiar, A. Witkiewicz, E. Knudsen\\ (2020).Functional Determinants of Cell Cycle Plasticity and Sensitivity to CDK4/6 Inhibition. Cancer Research](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-229282419/index.html) [S. Johnston, S. Puhalla, D. Wheatley, A. Ring, P. Barry, and 32 more\\ (2019).Randomized Phase II Study Evaluating Palbociclib in Addition to Letrozole as Neoadjuvant Therapy in Estrogen Receptor-Positive Early Breast Cancer: PALLET Trial. Journal of Clinical Oncology](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-54484394/index.html) [M. Arnedos, M. Bayar, B. Cheaib, Véronique Scott, I. Bouakka, and 14 more\\ (2018).Modulation of Rb phosphorylation and antiproliferative response to palbociclib: the preoperative-palbociclib (POP) randomized clinical trial. Annals of Oncology](/content/review/5a973630-4e58-4bd3-b3f9-a8bcdd2026c2/source/ss-48364991/index.html) Download BIBDownload RISDownload TXT ## Report Status Gather sources 200 sources found Details Screen sources 10 sources included Details Extract data 80 data points extracted Details Generate report Save PDF BIBLaTeX, ZoteroRISZotero, MendeleyTXTAPA BibliographyPDFPDFDOCXMicrosoft Word Chat Got some follow-up questions? [Sign up](/content/users/auth?redirectToPath=%2Freview%2F5a973630-4e58-4bd3-b3f9-a8bcdd2026c2%2Fsource%2Fss-54484394&show=signup/index.html) or [sign in](/content/users/auth?redirectToPath=%2Freview%2F5a973630-4e58-4bd3-b3f9-a8bcdd2026c2%2Fsource%2Fss-54484394/index.html) to chat with this report. Back ## Randomized Phase II Study Evaluating Palbociclib in Addition to Letrozole as Neoadjuvant Therapy in Estrogen Receptor-Positive Early Breast Cancer: PALLET Trial. S. Johnston, S. Puhalla, D. Wheatley, A. Ring, P. Barry, C. Holcombe, J. Boileau, L. Provencher, A. Robidoux, M. Rimawi, S. McIntosh, I. Shalaby, R. Stein, M. Thirlwell, D. Dolling, J. Morden, C. Snowdon, S. Perry, C. Cornman, Leona M. Batten, L. Jeffs, A. Dodson, Vera Martins, A. Modi, C. Osborne, K. Pogue-Geile, M. Cheang, N. Wolmark, T. Julian, K. Fisher, M. Mackenzie, M. Wilcox, Cynthia Huang Bartlett, M. Koehler, M. Dowsett, J. Bliss, S. Jacobs Journal of Clinical Oncology· 2019· 180 citations SourceDOI Plain textPDF Searching for PDF Unable to find PDF from source Back Study Design \- Study type: Phase II randomized clinical trial \- Setting: Postmenopausal women with ER-positive primary breast cancer (HER2-negative) \- Cancer type and subtype: ER-positive, HER2-negative breast cancer \- Sample size: 307 patients CDK4/6 Inhibitor Details \- Specific CDK4/6 inhibitor used: Palbociclib \- Dose and schedule: 125 mg/d, orally on a 21-days-on, 7-days-off schedule \- Duration of treatment: 14 weeks in combination with letrozole \- Combination therapies: Palbociclib with letrozole (endocrine therapy) \- Administration route and timing: Oral, on a 21-days-on, 7-days-off schedule Cell Cycle Arrest Measurements \- Ki-67 levels: Baseline to 14 weeks, median log-fold change -4.1 (palbociclib + letrozole) vs. -2.2 (letrozole), geometric mean change -97.4% vs. -88.5% \- Complete cell cycle arrest rates: 90% (palbociclib + letrozole) vs. 59% (letrozole), defined as Ki-67 ≤ 2.7% \- Time points measured: Baseline, 2 weeks, 14 weeks Mechanism Analysis \- CDK4/6 inhibitors promote cell cycle arrest by inhibiting progression from G1 to S phase. \- Palbociclib enhances suppression of malignant cell proliferation when combined with letrozole, as measured by Ki-67 expression. \- CDK4/6 inhibitors are antiproliferative, aligning with G1/S arrest mechanism. \- Suppression of Ki-67 indicates effectiveness of palbociclib in inducing cell cycle arrest. \- Specific details on Rb pathway activity, cyclin D1/CDK4/CDK6 expression, cell cycle checkpoint proteins, and downstream signaling pathways are not provided. Clinical Outcomes \- Response rates: Complete response + partial response was 54.3% for palbociclib plus letrozole versus 49.5% for letrozole alone; progressive disease was 3.2% versus 5.4%, respectively. \- Clinical benefit rate and progression-free survival: Not mentioned. \- Pathological response: Not mentioned. \- Time to progression and overall survival: Not mentioned. \- Response duration and maintenance of cell cycle arrest: Not mentioned. Senescence and Cell Fate \- Senescence markers and phenotype: Not mentioned \- Reversibility vs irreversibility of arrest: Not mentioned \- Apoptosis markers: Suppression of cleaved PARP indicates reduced apoptosis \- Autophagy activation: Not mentioned \- Long-term cell fate outcomes: Not mentioned Resistance and Predictive Factors \- Predictive biomarkers: RB1 mutation status \- Resistance mechanisms and pathways: Need for continued therapy to maintain antiproliferative effects \- Factors associated with treatment failure: Not explicitly mentioned \- Molecular subtypes or patient characteristics affecting response: Response occurs independently of tumor grade, absence of progesterone receptor expression, or mutations in p53, PIK3CA, or PTEN genes \- Combination strategies to overcome resistance: Not explicitly mentioned Toxicity Profile \- Grade 3/4 adverse events: 49.8% of patients experienced grade 3 or greater toxicity, mainly due to asymptomatic neutropenia. \- Dose modifications: Palbociclib was interrupted/delayed in 21.6% of patients, dose was reduced in 2.0% of patients. \- Cytopenias: Asymptomatic neutropenia was a significant cause of grade 3 or greater toxicity. \- Non-hematologic toxicities: Not specifically mentioned. \- Management strategies: Protocol-specified dose modifications were recommended for various adverse events. PURPOSE CDK4/6 inhibitors are used to treat estrogen receptor (ER)-positive metastatic breast cancer (BC) in combination with endocrine therapy. PALLET is a phase II randomized trial that evaluated the effects of combination palbociclib plus letrozole as neoadjuvant therapy. PATIENTS AND METHODS Postmenopausal women with ER-positive primary BC and tumors greater than or equal to 2.0 cm were randomly assigned 3:2:2:2 to letrozole (2.5 mg/d) for 14 weeks (A); letrozole for 2 weeks, then palbociclib plus letrozole to 14 weeks (B); palbociclib for 2 weeks, then palbociclib plus letrozole to 14 weeks (C); or palbociclib plus letrozole for 14 weeks. Palbociclib 125 mg/d was administered orally on a 21-days-on, 7-days-off schedule. Core-cut biopsies were taken at baseline and 2 and 14 weeks. Coprimary end points for letrozole versus palbociclib plus letrozole groups (A v B + C + D) were change in Ki-67 (protein encoded by the  MKI67 gene; immunohistochemistry) between baseline and 14 weeks and clinical response (ordinal and ultrasound) after 14 weeks. Complete cell-cycle arrest was defined as Ki-67 less than or equal to 2.7%. Apoptosis was characterized by cleaved poly (ADP-ribose) polymerase. RESULTS Three hundred seven patients were recruited. Clinical response was not significantly different between palbociclib plus letrozole and letrozole groups ( P = .20; complete response + partial response, 54.3% v 49.5%), and progressive disease was 3.2% versus 5.4%, respectively. Median log-fold change in Ki-67 was greater with palbociclib plus letrozole compared with letrozole (-4.1 v -2.2; P < .001) in the 190 evaluable patients (61.9%), corresponding to a geometric mean change of -97.4% versus -88.5%. More patients on palbociclib plus letrozole achieved complete cell-cycle arrest (90% v 59%; P < .001). Median log-fold change (suppression) of cleaved poly (ADP-ribose) polymerase was greater with palbociclib plus letrozole versus letrozole (-0.80 v -0.42; P < .001). More patients had grade 3 or greater toxicity on palbociclib plus letrozole (49.8% v 17.0%; P < .001) mainly because of asymptomatic neutropenia. CONCLUSION Adding palbociclib to letrozole significantly enhanced the suppression of malignant cell proliferation (Ki-67) in primary ER-positive BC, but did not increase the clinical response rate over 14 weeks, which was possibly related to a concurrent reduction in apoptosis. INTRODUCTION Use of endocrine therapy for the treatment of hormone receptor (HR) -positive breast cancer (BC) is a seminal example of successfully targeted cancer treatment. Nonetheless, endocrine therapy resistance, either de novo or acquired, remains a challenge in patients with both early and advanced BC. \[1\]\[2\]\[3\]\[4\] One approach to reverse resistance to standard endocrine therapy has been to target an alternative pathway. Cyclin-dependent kinases CDK4 and CDK6 promote progression from G 1 phase to S phase of the cell cycle. Inhibition of these kinases leads to decreased proliferation of estrogen receptor (ER) -positive tumors and reverses endocrine resistance in some patients. The CDK4/6 inhibitor, palbociclib (Ibrance; Pfizer, New York, NY), has demonstrated considerable activity when combined with other endocrine therapies in patients with metastatic BC in both first-line and second-line settings, \[5\]\[6\]\[7\]\[8\] with recent results demonstrating prolonged overall survival in the second-line setting. 9 Large, phase III adjuvant BC trials with palbociclib and other CDK4/6 inhibitors are ongoing \[PALLAS (ClinicalTrials.gov identifier: NCT02513394) PENELOPE-B (ClinicalTrials.gov identifier: NCT01864746), and MONARCH-E (ClinicalTrials.gov identifier: NCT03155997).\ \ In early BC, use of neoadjuvant therapy is an attractive option to facilitate breast conservation and, critically, enables the assessment of in vivo biomarkers to identify proof-of-principle activity or predict responsive or resistant subgroups of tumors. 10,11 Achievement of a pathologic complete response (pCR) in HR-positive cancers to chemotherapy is less common than in other subtypes of BC. A recent meta-analysis reported similar clinical responses and achievement of breast conservation in HR-positive BC with neoadjuvant endocrine therapy compared with combination chemotherapy, but with lower toxicity. 12 As such, strategies to further improve response to neoadjuvant endocrine therapy in HR-positive cancers are more relevant than using chemotherapy. In HR-positive disease, a decrease in the proliferation marker Ki-67 (protein encoded by the MKI67 gene) from baseline in response to endocrine therapy has been validated as a marker of treatment benefit, with measurement of Ki-67 after 2 weeks of endocrine therapy shown to improve the prediction of recurrence-free survival (RFS). 13,14 Given the predominantly antiproliferative effects of palbociclib, suppression of Ki-67 is a rational end point for estimating whether there is efficacy with the addition of palbociclib to an aromatase inhibitor (AI) versus AI alone in the neoadjuvant setting.\ \ Here, we report the results of PALLET, a large, multinational, neoadjuvant randomized trial (ClinicalTrials.gov identifier: NCT02296801, ISRCTN31243262), designed with coprimary end points examining the biologic and clinical effects of neoadjuvant letrozole with or without palbociclib for 14 weeks as primary treatment of ER-positive/human epidermal growth factor receptor 2 (HER2) -negative early invasive BC.\ \ PATIENTS AND METHODS\ \ Full details of the methodology are available in the Data Supplement.\ \ Trial Design and Patients\ \ PALLET is a phase II randomized multicenter trial with parallel United Kingdom and North American protocols. Patients were recruited from 38 sites in the United Kingdom, United States, and Canada. Eligible patients were postmenopausal women with unilateral, operable, ERpositive, HER2-negative tumors that measured at least 2 cm by ultrasound with no evidence of metastatic disease. ER positivity and HER2 negativity were defined as per ASCO/College of American Pathologists guidelines 15,16 and were locally assessed.\ \ Patients were randomly assigned 3:2:2:2 to one of four treatment groups. Group A received letrozole alone for 14 weeks, group B letrozole for 2 weeks followed by palbociclib plus letrozole to 14 weeks, group C palbociclib for 2 weeks followed by palbociclib plus letrozole to 14 weeks, and group D palbociclib plus letrozole for 14 weeks (Data Supplement). The parallel four-group design with a 2-week change for groups B and C allowed us to assess the role of each drug alone or in combination in the suppression of Ki-67. Ki-67 was centrally assessed. Treatment allocation was performed by computer-generated random permuted blocks and stratified by geographic location-United Kingdom versus North America (United States and Canada; Data Supplement). Letrozole 2.5 mg/d was administered orally continually and palbociclib 125 mg/d was administered orally on a 21-days-on, 7-days-off schedule. Protocolspecified dose modifications for palbociclib were recommended for various adverse events.\ \ Procedures\ \ After randomization, patients visited the clinic each week for the first 4 weeks, then every other week until week 14. Follow-up visits were at 30 days post-trial treatment and 12 months after random assignment. Assessments required at these visits are described in the protocol.\ \ Core-cut biopsies and trial-specific blood samples were taken at baseline (post-random assignment), 2 weeks (before commencement of second drug for groups B and C), and 14 weeks or at the discontinuation of study therapy (within 48 hours of the last dose of trial treatment).\ \ Outcomes\ \ Principal outcome analyses focused on changes between baseline and the end of treatment (EoT) and compared letrozole (A) with palbociclib plus letrozole (B + C + D). Coprimary end points were clinical response (ultrasound; Eastern Cooperative Oncology Group 17 ) and (ii) change in the proliferation marker Ki-67 (immunohistochemistry). Secondary end points included pCR, changes in surgical intent, and safety. In addition, changes in Ki-67 between baseline and week 2 and week 2 to EoT were compared in groups for which treatment differed during each respective time period. Prespecified exploratory biomarkers included cleaved poly (ADP-ribose) polymerase (c-PARP; apoptosis).\ \ Statistical Analysis\ \ The PALLET trial was powered (90%) using a conventional comparative design with alpha (a = 5% overall) split between the two coprimary end points. Improved clinical response would be detected for palbociclib plus letrozole over letrozole (complete response: 31% v 21%; partial response: 57% v 54%; stable disease: 5% v 15%; progressive disease: 2% v 5%) with 284 patients (a = 4% and 90% power). With a 5% nonevaluable rate and 3:2:2:2 allocation, the recruitment target was 306 patients. Improvement with decreased Ki-67 from 80% in group A to 90% in groups B plus C plus D (log-fold change of 20.693; standard deviation of 1.5) would be detected with 279 patients with a = 1% and 90% power. Interim analyses were planned at 25% and 50% of trial end point information, and the trial would have been terminated for futility at the second analysis if there was no evidence that either end point favored palbociclib.\ \ Post hoc analysis revealed that there were 279 evaluable clinical responses (93:186), which under the initial sample size specifications would give 88.1% power. Log-fold changes in Ki-67 were available for 190 patients (61.9%; 65:125) to provide 75% power.\ \ All patients were analyzed according to the intention to treat approach. Clinical response was treated as an ordinal outcome and compared using the Mann-Whitney test in all patients with Eastern Cooperative Oncology Group response data available at EoT. Changes in Ki-67 and c-PARP were analyzed on the natural log-fold scale in patients with biopsy data available at both baseline and EoT. As an exploratory analysis, complete cell cycle arrest (CCCA) at EoT (defined as Ki-67 of 2.7% or less) was compared between groups using a logistic regression model that adjusted for recruitment region and histologic type.\ \ RESULTS\ \ Between February 27, 2015, and March 8, 2018, 307 women were recruited-166 from the United Kingdom (Data Supplement) and 141 from North America (Data Supplement; group A, n = 103; group B, n = 68; group C, n = 69; group D, n = 67; Fig 1). Baseline demographic and clinical characteristics were similar across treatment groups (Table 1 ).\ \ Overall, 253 patients (82.4%) completed 14 weeks of treatment. In the letrozole group (A) this was 85% (n = 88) compared with 81% (n = 165) of patients who received palbociclib plus letrozole (B + C + D). The median percentage of scheduled letrozole received was 99% in all treatment groups. The median \[interquartile range (IQR)\] percentages of the scheduled dose of palbociclib received in groups B, C, and D were 99.2% (82.9% to 100.0%), 90.9% (67.8% to 100.0%), and 97.4% (79.2% to 100.0%), respectively. Palbociclib was interrupted/delayed in 21.6% of patients (n = 44), dose was reduced in 2.0% of patients (n = 4), and treatment was interrupted/delayed and dose reduced in 15.2% of patients (n = 31; Data Supplement).\ \ Clinical response outcomes at EoT were available for 279 patients (90.8%; Table 2 ). In the letrozole group (A), 46\ \ Randomly assigned (N = 307) 3:2:2:2\ \ Group A: Letrozole alone\ \ Group B: Letrozole to week 2, followed by palbociclib plus letrozole to week 14\ \ Group D: Palbociclib plus letrozole to week 14\ \ Evaluable clinical response Ultrasound measurement not evaluable Ultrasound not conducted\ \ Evaluable clinical response Ultrasound measurement not evaluable Ultrasound not conducted\ \ DISCUSSION\ \ PALLET is the largest randomized trial of a CDK4/6 inhibitor in the neoadjuvant setting and demonstrates that the addition of palbociclib to letrozole markedly enhanced the suppression of malignant cell proliferation as assessed by Ki-67. In addition, there was a significant increase in the number of patients who achieved CCCA in their tumor after 14 weeks of combination therapy compared with letrozole alone (90% v 59%). Although the suppression of Ki-67 in the first 2 weeks by palbociclib alone was significantly greater than by letrozole alone, the combination palbociclib plus letrozole enhanced the proportion of patients who achieved CCCA. In terms of toxicity, PALLET detected no new signals with the addition of palbociclib in patients with early-stage primary BC.\ \ The lack of difference in clinical response rate (54.3% v 49.5%) is perhaps not a surprise given the cytostatic nature of endocrine-based therapies in contrast to similar neoadjuvant trials using cytotoxic chemotherapies in triplenegative BC or targeted combinations in HER2-positive BC. 18 In slower growing ER-positive tumors, therapies with a predominantly antiproliferative effect will yield a slower reduction in tumor size, 19 especially over a short timeframe of 14 weeks. When using primary endocrine therapy to downstage ER-positive BC, maximal tumor shrinkage may take at least 9 to 12 months. 20 We also demonstrate for the first time to our knowledge-using c-PARP expression as a biomarker-that unlike chemotherapy, wherein apoptosis increases in addition to an antiproliferative effect, 21 CDK4/6 therapy in combination with an AI produces a greater suppression-not an increase-in apoptosis compared with endocrine therapy alone. Measurement of c-PARP is only one of a number of approaches to assessing apoptosis in situ.\ \ It is notable that the decrease observed in the AI alone arm of PALLET is similar to that observed when using the terminal deoxynucleotidyl transferase dUTP nick end labeling method in the IMPACT trial. 22 This reduction in cell death could also explain why overall tumor volume-that is, clinical response-as determined by ultrasound did not substantially change, nor did the surgical breast conservation rate, despite the markedly enhanced antiproliferative effect. Indeed, these data are consistent with the PALOMA-2 study (ClinicalTrials.gov identifier: NCT01740427) in advanced BC in which the greatest clinical impact was observed in progression-free survival (hazard ratio, 0.58), rather than the best objective response rate (ORR; 55% v 44%). 6,8 Similarly, ORR with abemaciclib plus AI in the MONARCH-3 trial was 59% versus 44% with AI alone, 23 and with ribociclib plus AI in the MONALEESA-2 trial ORR was 52.7% versus 37.1% with AI alone, 24 yet both studies also had highly significant improvements in progression-free survival (hazard ratio, 0.54 and 0.57, respectively). In early BC, it remains to be seen whether the antiproliferative differences observed in the PALLET trial, despite the lack of change in ORR in the neoadjuvant setting, will translate into an effect on time to recurrence in ongoing adjuvant studies.\ \ Previous studies of neoadjuvant endocrine therapy have also demonstrated that suppression of Ki-67, rather than clinical response, is a better indicator of therapeutic activity in ER-positive early BC. In the IMPACT trial, no difference in clinical response rate was observed between anastrozole, tamoxifen, or the combination (37% v 36% v 39%) 25 after 3 months of therapy in 330 patients. However, significantly greater suppression of Ki-67 was reported for anastrozole compared with tamoxifen at 12 weeks (81.6% v 61.9%). 13,26 hese differences in Ki-67 suppression were paralleled by the greater benefit from anastrozole versus tamoxifen or the combination of anastrozole and tamoxifen in the ATAC trial. 27 Furthermore, the log-fold reduction in Ki-67 in IMPACT was a predictor of subsequent RFS in the adjuvant setting. 13 Similarly, the greater suppression of Ki-67 by letrozole than tamoxifen in P024 28 paralleled the greater improvement in RFS with letrozole in the analogous BIG1-98 adjuvant trial (ClinicalTrials.gov identifier: NCT00004205). 29 When the different AIs were compared in Z1031 (ClinicalTrials.gov identifier: NCT00265759), 14 the lack of difference in Ki-67 suppression was supported by similar RFS between groups in the adjuvant studies MA-27 (ClinicalTrials.gov identifier: NCT00066573) 30 and FACE (ClinicalTrials.gov identifier: NCT00248170). 31 ore recently, the large United Kingdom POETIC trial (ClinicalTrials.gov identifier: NCT02338310) confirmed that the lack of suppression of Ki-67 after 2 weeks of preoperative AI predicted for a significantly worse 5-year RFS. 32 CDK4/6 inhibitors restrict passage through the cell cycle and, like endocrine agents, are therefore antiproliferative. However, whether the lack of Ki-67 suppression after neoadjuvant Abbreviation: AE, adverse event.\ \ CDK4/6 inhibitor therapy is similarly predictive remains unconfirmed.\ \ Suppression of Ki-67 in the first 2 weeks by palbociclib alone was significantly greater than that by letrozole alone, a finding also reported recently in the small, phase II preoperative palbociclib trial (ClinicalTrials. gov identifier: NCT02008734). 33 However, in the PAL-LET trial, the four-group design demonstrated that the palbociclib plus letrozole combination enhanced the proportion of patients who achieved CCCA in the first 2 weeks, and that the addition of the AI maximizes Ki-67 suppression.\ \ In a previous small, phase II study (NeoPalAna; ClinicalTrials. gov identifier: NCT01723774) in 50 patients with ER-positive early BC of different intrinsic subtypes, sequential biopsies were taken in patients who were initiated on anastrozole for 4 weeks, followed by the addition of palbociclib to study the additional change or decrease in Ki-67. 34 Rates of CCCA with palbociclib and anastrozole were significantly higher (87%) than with anastrozole alone (26%), and biomarker analysis suggested that response to palbociclib occurred independently of tumor grade, absence of progesterone receptor expression, or mutation in p53, PIK3CA, or PTEN genes, but was correlated with RB1 mutation status. Extensive gene and protein expression analyses are being undertaken in PALLET as exploratory end points. These will be correlated with antiproliferative response and could yield important information about predictive biomarkers for this class of therapy in the early BC setting, which can be tested in the adjuvant setting.\ \ In NeoPalAna, it was reported that the antiproliferative effect of palbociclib diminished rapidly after treatment stopped in some patients, which suggests the need for continued therapy. 34 For this reason, in PALLET, we aimed to ensure that the 14-week biopsy was taken during exposure to drug therapy and excluded 2.6% of 14-week samples as they fell outside the 48-hour window since the last drug dose taken. In addition, 13.0% of patients had an unevaluable sample which could reflect minimal cellularity in the core biopsy. Studies to assess the correlation between the 14-week samples with cellularity and Ki-67 in the excised surgical sample are ongoing.\ \ In the only other randomized neoadjuvant trial of CDK4/6 inhibitors in ER-positive early BC (NeoMONARCH; ClinicalTrials. gov identifier: NCT02441946), 224 patients were randomly assigned to either anastrozole, abemaciclib (Verzenio; Eli Lilly, Indianapolis, IN), or the combination, with biopsies taken at baseline, 2 weeks, and after 16 weeks of therapy. 35 ombination abemaciclib plus anastrozole was associated with a greater geometric mean decrease in Ki-67 at 2 weeks (292.6% v 263.2%), with a significant increase in CCCA (66% v 14%). To date, biomarkers of response or resistance to abemaciclib have not been identified, although reports of induced histologic changes that are suggestive of tumor differentiation and increased lymphocytic infiltration were observed in some cases. 35 e incomplete availability of biopsy samples could potentially bias the biologic findings for Ki-67 and c-PARP. When EoT biopsies were not taken (n = 38), this often occurred with incomplete treatment (n = 29; 76%). Excluding these cases could overstate the proportion who responded; however, there were an approximately equal number of cases in which Ki-67 was unevaluable as a result of scant tumor in the biopsy. A similar level of Ki-67 suppression would be expected in these cases compared with the evaluable population and so would not be expected to bias our findings.\ \ Other trials that featured Ki-67 as an end point have observed similar evaluable proportions. In the NeoMONARCH study, 138 (61.9%) of 223 patients were evaluable for Ki-67 compared with 190 (61.9%) of 307 in our trial. Analyses of Ki-67 and c-PARP levels between baseline and week 2 and from week 2 to EoT in PALLET were conducted post hoc and did not adjust for multiple testing and so should be cautiously interpreted. Nonetheless, such findings match our expectations that the addition of palbociclib to letrozole would increase the suppression of cell proliferation.\ \ In conclusion, the PALLET trial demonstrated that the addition of palbociclib to letrozole markedly enhanced the suppression of malignant cell proliferation as measured by Ki-67 expression, yet did not increase tumor shrinkage as determined by clinical ultrasound. Correlating biomarkers of antiproliferative response in the context of a randomized neoadjuvant study will be important in determining which patients may derive the most benefit from CDK4/6 inhibitors in ongoing adjuvant studies in early BC.\ \ Funding\ \ SUPPORT Funded by Pfizer with additional core support to Institute of Cancer Research Clinical Trials and Statistics Unit in the United Kingdom from Cancer Research UK (C1491/A15955) and from the National Surgical Adjuvant Breast and Bowel Project Foundation in North America.Support also provided by National Institute for Health Research funding to the Royal Marsden and Institute of Cancer Research Biomedical Research Centre. Technologies, Eli Lilly Speakers' Bureau: Roche, Novartis, Genomic Heath, Pfizer Research Funding: Genentech (Inst), Novartis (Inst), Pfizer (Inst), AbbVie (Inst), Merck (Inst), Eli Lilly (Inst), RNA Diagnostics (Inst) Travel, Accommodations, Expenses: Roche, GlaxoSmithKline, Novartis, Eli Lilly Louise Provencher Consulting or Advisory Role: Eli Lilly, Pfizer, Roche, Novartis Research Funding: Pfizer, Roche, Novartis, Merck, GlaxoSmithKline, Odonate Therapeutics (Inst) Andr é Robidoux Consulting or Advisory Role: AstraZeneca, Roche, Eisai, Novartis, Genomic Health, Pfizer Speakers' Bureau: Pfizer, Genomic Health Research Funding: Novartis (Inst), Roche (Inst), Amgen (Inst), AstraZeneca (Inst) Travel, Accommodations, Expenses: Novartis, Genomic Health, Pfizer Mothaffar Rimawi Consulting or Advisory Role: Genentech, Novartis, Macrogenics, Daiichi Sankyo Research Funding: Pfizer (Inst) Robert C. Stein Stock and Other Ownership Interests: GlaxoSmithKline Honoraria: Novartis Consulting or Advisory Role: Teva Pharmaceuticals Speakers' Bureau: Novartis, Roche Michael Thirlwell Consulting or Advisory Role: Taiho Pharmaceuticals Research Funding: Puma Pharmaceuticals, Synthon, Novartis Travel, Accommodations, Expenses: Novartis David Dolling Research Funding: Bayer (Inst), Astellas Pharma (Inst), AstraZeneca (Inst), Aventis Pharma (Inst), Janssen Diagnostics (Inst) Travel, Accommodations, Expenses: Pfizer James Morden Research Funding: Pfizer (Inst) Travel, Accommodations, Expenses: Pfizer Claire Snowdon Travel, Accommodations, Expenses: Pfizer Sophie Perry Employment: Astellas Pharma (I), GlaxoSmithKline (I) Research Funding: Pfizer (Inst) Travel, Accommodations, Expenses: Pfizer Leona M. Batten Research Funding: Pfizer (Inst) Travel, Accommodations, Expenses: Pfizer Lisa K. Jeffs Research Funding: Institute of Cancer Research Travel, Accommodations, Expenses: Institute of Cancer Research Andrew Dodson Consulting or Advisory Role: MSD Oncology (I) Research Funding: Pfizer Travel, Accommodations, Expenses: Pfizer Vera Martins Research Funding: Pfizer (Inst) Arjun Modi Research Funding: Pfizer (Inst) C. Kent Osborne Stock and Other Ownership Interests: Genetex Consulting or Advisory Role: AstraZeneca, Genentech, Ventana Medical Systems, Eli Lilly, Tolmar Pharmaceuticals Patents, Royalties, Other Intellectual Property: Royalties for coeditor of\ \ Failed to load PDF:\ \ StripeM-Inner\ \ Paper sources\ \ Abstract screening pilot\ \ Abstract screening results\ \ Extraction pilot\ \ Extraction results\ \ Research report\ \ * * *\ \ Modify setup