Pembrolizumab monotherapy for previously treated metastatic triple-negative breast cancer: cohort A of the phase II KEYNOTE-086 study

BACKGROUND

Treatment options for previously treated metastatic triple-negative breast cancer (mTNBC) are limited. In cohort A of the phase II KEYNOTE-086 study, we evaluated pembrolizumab as second or later line of treatment for patients with mTNBC.

PATIENTS AND METHODS

Eligible patients had centrally confirmed mTNBC, ≥1 systemic therapy for metastatic disease, prior treatment with anthracycline and taxane in any disease setting, and progression on or after the most recent therapy. Patients received pembrolizumab 200 mg intravenously every 3 weeks for up to 2 years. Primary end points were objective response rate in the total and PD-L1-positive populations, and safety. Secondary end points included duration of response, disease control rate (percentage of patients with complete or partial response or stable disease for ≥24 weeks), progression-free survival, and overall survival.

RESULTS

All enrolled patients (N = 170) were women, 61.8% had PD-L1-positive tumors, and 43.5% had received ≥3 previous lines of therapy for metastatic disease. ORR (95% CI) was 5.3% (2.7-9.9) in the total and 5.7% (2.4-12.2) in the PD-L1-positive populations. Disease control rate (95% CI) was 7.6% (4.4-12.7) and 9.5% (5.1-16.8), respectively. Median duration of response was not reached in the total (range, 1.2+-21.5+) and in the PD-L1-positive (range, 6.3-21.5+) populations. Median PFS was 2.0 months (95% CI, 1.9-2.0), and the 6-month rate was 14.9%. Median OS was 9.0 months (95% CI, 7.6-11.2), and the 6-month rate was 69.1%. Treatment-related adverse events occurred in 103 (60.6%) patients, including 22 (12.9%) with grade 3 or 4 AEs. There were no deaths due to AEs.

CONCLUSIONS

Pembrolizumab monotherapy demonstrated durable antitumor activity in a subset of patients with previously treated mTNBC and had a manageable safety profile.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, NCT02447003.

Abstract P3-10-09: Relationship between tumor infiltrating lymphocytes (TILs) and response to pembrolizumab (Pembro)+chemotherapy (Chemo) as neoadjuvant treatment (NAT) for triple-negative breast cancer (TNBC): phase Ib KEYNOTE-173 trial

Background:Increasing quantities of stromal TILs (sTILs) are associated with higher pathologic complete response (pCR) rates with conventional chemo in early-stage TNBC. We evaluated the association between sTILs and PD-L1 expression with response to pembro+chemo as NAT for TNBC in the KEYNOTE-173 trial (NCT02622074).

Methods: sTILs were quantified using light microscopy of H&E-stained slides from pretreatment and on-treatment (during first 3 weeks of pembro monotherapy) tumor biopsies by a pathologist blind to response data. Pretreatment PD-L1 expression was assessed using the PD-L1 IHC 22C3 pharmDx assay and reported as combined positive score (CPS). Endpoints were pCR rate by ypT0 ypN0 and ypT0/Tis ypN0 and objective response rate (ORR; RECIST v1.1) after the first 4 cycles of NAT (taxane±carboplatin+pembro) by MRI. sTILs and PD-L1 CPS were evaluated as continuous variables. Association between sTILs and PD-L1 CPS with response was assessed using logistic regression and area under the reciever operating curve (AUROC) analyses, with a 1-sided alpha level of 0.10. Correlation between PD-L1 and sTILs was assessed by Spearman's rank correlation coefficient. Multivariate analysis included sTILs (pretreatment and on-treatment) and PD-L1 CPS. Likelihood ratio tests were used to evaluate the added value of factors in predicting pCR rate.

Results: Of 60 total pts, 34 had tumors evaluated for pretreatment sTILs, 52 for PD-L1 CPS, and 33 for both sTILs and CPS. On-treatment sTILs were evaluated in 31 pts. Overall pCR rates were 56.7% and 60% by ypT0 ypN0 and ypT0/Tis ypN0, respectively; ORR was 78.3%. In pts evaluated for sTILs and CPS (individually), pCR rates and ORR were comparable with overall pCR rates and ORR. There was a significant correlation between pretreatment sTILs and PD-L1 CPS (ρ=0.65, P<0.001).Higher pretreatment sTILs were significantly associated with response: ypT0 ypN0 P= 0.011; ypT0/Tis ypN0 P=0.006; ORR P=0.061. On-treatment sTILs were also significantly associated with response: ypT0 ypN0 P=0.061; ypT0/Tis ypN0 P=0.041; ORR P=0.031. Pretreatment PD-L1 CPS was significantly associated with response: ypT0 ypN0 P=0.073; ypT0/is ypN0 P=0.030; and ORR P=0.021. AUROC of pretreatment sTIL association with pCR was numerically higher than with on-treatment sTILs and PD-L1 CPS (0.69 vs 0.61 vs 0.56 for ypT0ypN0 and 0.72 vs 0.67 vs 0.62 for ypT0/Tis ypN0). Responders had higher median pretreatment sTIL levels vs nonresponders: 45% [10, 75] vs 10% [5, 20] for pCR rate by ypT0 ypN0 and 52.5% [10, 73.8] vs 10% [5, 20] for pCR rate by ypT0/Tis ypN0; 25% [5, 70] vs 10% [6.3, 27.5] for ORR. In multivariate analysis, only pretreatment sTILs were significant for both pCR endpoints (ypT0 ypN0 P=0.031; ypT0/Tis ypN0 P=0.034). Likelihood ratio tests demonstrated that for both pCR endpoints, PD-L1 CPS (P=0.683/P=0.422) and on-treatment sTILs (P=0.984/P=0.568) did not add significantly more value to pretreatment sTILs when predicting pCR.

Conclusions:Higher quantities of pretreatment sTILs and PD-L1 CPS and on-treatment sTILs were significantly associated with higher pCR rates and ORR in primary TNBC treated with pembro and NAT.

Citation Format: Loi S, Schmid P, Cortés J, Park YH, Muñoz-Couselo E, Kim S-B, Sohn J, Im S-A, Holgado E, Foukakis T, Kuemmel S, Dent R, Wang A, Aktan G, Karantza V, Salgado R. Relationship between tumor infiltrating lymphocytes (TILs) and response to pembrolizumab (Pembro)+chemotherapy (Chemo) as neoadjuvant treatment (NAT) for triple-negative breast cancer (TNBC): phase Ib KEYNOTE-173 trial [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-10-09.

Abstract OT3-04-03: KEYNOTE-756: A randomized, double-blind, phase III study of pembrolizumab versus placebo in combination with neoadjuvant chemotherapy and adjuvant endocrine therapy for high-risk early-stage ER+/HER2– breast cancer

Background:Although ER+/HER2– breast cancer has a better overall prognosis than other breast cancer subtypes, there is a high-risk subpopulation characterized by high-grade tumors and decreased sensitivity to endocrine therapy, higher responsiveness to chemotherapy and worse prognosis. A large meta-analysis of prospective studies focusing on neoadjuvant chemotherapy (NAC) for treatment of stage I-III breast cancer demonstrated that increased pathologic complete response (pCR) rates at surgery were associated with improved survival. This correlation was observed across triple-negative breast cancer (TNBC), HER2+ breast cancer, and high-grade HR+/HER2- breast cancer. Specifically, patients with a pCR after NAC had a 5-year event-free survival (EFS) rate of 90%, whereas patients who did not achieve a pCR had a 5-year EFS rate of 60%.Therefore, increasing pCR rates after NAC may have a substantial impact for patients with high-risk early-stage HR+/HER2– breast cancer. KEYNOTE-756 is a global, randomized, double-blind, phase III study of pembrolizumab (vs placebo) + chemotherapy as neoadjuvant treatment, followed by pembrolizumab (vs placebo) plus endocrine therapy as adjuvant treatment for patients with high-risk, early-stage ER+/HER2– breast cancer.

Methods: Patients with T1c-2 cN1-2 or T3-4 cN0-2 grade 3 or grade 2 with Ki-67 ≥30%, invasive, ductal ER+/HER2– breast cancerwill be stratified by lymph node involvement (positive vs negative), tumor PD-L1 status (positive vs negative), ER positivity (≥10% vs <10%), and anthracycline dosing schedule (Q3W vs Q2W), and then randomized 1:1 to receive neoadjuvant treatment with pembrolizumab 200 mg Q3W or placebo in combination with paclitaxel (80 mg/m2 QW) for 4 cycles followed by (doxorubicin [60 mg/m2] or epirubicin [100 mg/m2]) plus cyclophosphamide (600 mg/m2) Q2/3W for another 4 cycles. After definitive surgery (± radiation therapy, as indicated), patients will receive adjuvant treatment with pembrolizumab (200 mg Q3W) or placebo for 9 additional administrations, in combination with endocrine therapy, which can be given for up to 10 years. Co-primary end points are pCR rate and EFS. Secondary end points are safety and overall survival. The global study will open in North America and Latin America, Europe, and Asia Pacific in the second half of 2018.

Citation Format: Cardoso F, Bardia A, Andre F, Cescon DW, McArthur H, Telli M, Loi S, Cortés J, Schmid P, Harbeck N, Denkert C, Jackisch C, Jia L, Tryfonidis K, Karantza V. KEYNOTE-756: A randomized, double-blind, phase III study of pembrolizumab versus placebo in combination with neoadjuvant chemotherapy and adjuvant endocrine therapy for high-risk early-stage ER+/HER2– breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr OT3-04-03.

Abstract OT1-02-04: SWOG S1418/NRG -BR006: A randomized, phase III trial to evaluate the efficacy and safety of MK-3475 as adjuvant therapy for triple receptor-negative breast cancer with > 1 cm residual invasive cancer or positive lymph nodes (>pN1mic) after neoadjuvant chemotherapy

Background: Patients with residual cancer after neoadjuvant chemotherapy, particularly triple negative cancers (TNBC), have poor prognosis.The SWOG S1418 / NRG BR-006 (NCT02954874) randomized, phase III trial tests the hypothesis that administration of pembrolizumab after surgery for 12 months will reduce invasive disease-free survival (IDFS) by 33% compared to observation in patients with TNBC and > 1 cm residual invasive cancer or positive lymph nodes (>pN1mic) after neoadjuvant chemotherapy.

Methods: Eligible patients ≥18 years old with triple negative breast cancer defined by ASCO/CAP guidelines and >1 cm residual invasive cancer in the breast, or any macrometastases in the lymph nodes after completion of 16-24 weeks of neoadjuvant chemotherapy. Patients may receive post-operative chemotherapy for up to 24 weeks but must be registered for screening within 35 days of completion of adjuvant chemo. Completion of radiation therapy prior to registration is allowed, but it is preferred that patients receive radiation after randomization; patients randomized to pembrolizumab will receive their XRT concomitant with pembrolizumab. Adequate organ functions: ANC > 1.5, PLT > 100, Hgb > 9, normal creatinine, Tbili < 1.5 IUNL, AST/ALT/AlkPhos < 2.5 IULN. HIV with good CD4 count is allowed. Active autoimmune disease, Hep B,C, prior immunotherapy, active immunosuppressive therapy, or live vaccines within 30 days of registration are not allowed. Five unstained slides for PDL1 staining are required for stratification. The study has a dual primary endpoint; comparison of IDFS between arms in (i) all randomized patients (1-sided a=0.01) and in PDL-1 positive patients (1-sided a=0.015). Secondary endpoints include toxicity, overall survival, distant recurrence free survival (DRFS) and quality of life measures. Patients will be randomized 1:1 with stratification for PDL1 status, T size, nodal status and adjuvant chemo (yes or no) to observation or 1 year of pembrolizumab 200mg IV q 3 weeks. The accrual goal is N=1000 patients with estimated trial duration of 8 years. Two interim analyses are planned for all randomized patients when 50% and 75% of IDFS events have occurred for early stopping for either futility or efficacy. The study was activated on 11/15/16 and 34 patients were registered as of June 9, 2017. Cancer Trials Support Unit (CTSU) sites can use “OPEN” (https://open.ctsu.org) to enroll patients to this trial.

Funding: NIH/NCI U10CA180888, U10CA180819, CA180868; and in part by Merck, Sharpe & Dohme, Corporation.

Citation Format: Pusztai L, Barlow WE, Ganz PA, Henry NL, White J, Jagsi R, Mammen JMV, Lew D, Mejia J, Karantza V, Aktan G, Sharon E, Korde L, Hortobagyi GN, Mamounas E. SWOG S1418/NRG -BR006: A randomized, phase III trial to evaluate the efficacy and safety of MK-3475 as adjuvant therapy for triple receptor-negative breast cancer with > 1 cm residual invasive cancer or positive lymph nodes (>pN1mic) after neoadjuvant chemotherapy [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr OT1-02-04.

The Pak4 protein kinase is required for oncogenic transformation of MDA-MB-231 breast cancer cells

The Pak4 protein kinase, normally expressed at low level in the mammary gland, is commonly overexpressed in breast cancer. Overexpression of Pak4 transforms mouse mammary epithelial cells in vitro and renders these cells tumorigenic in athymic mice in vivo. Here we show that Pak4 is also required for oncogenic transformation of the human breast cancer cell line MDA-MB-231. These high Pak4-expressing human breast cancer cells form highly disorganized three-dimensional (3D) structures in vitro and readily give rise to orthotopic xenograft tumors in nude mice. We have found that when Pak4 levels are reduced, MDA-MB-231 cells exhibit decreased proliferation and migration in vitro, as well as gross restoration of normal 3D mammary acinar organization, the latter in association with a strong induction of apoptosis. Similarly, Pak4 knockdown suppresses MDA-MB-231 breast xenograft tumor formation in nude mice in vivo. These results indicate that Pak4 has a key role in the oncogenic transformation of breast cells.

Keratins in health and cancer: more than mere epithelial cell markers

Keratins are the intermediate filament (IF)-forming proteins of epithelial cells. Since their initial characterization almost 30 years ago, the total number of mammalian keratins has increased to 54, including 28 type I and 26 type II keratins. Keratins are obligate heteropolymers and, similarly to other IFs, they contain a dimeric central α-helical rod domain that is flanked by non-helical head and tail domains. The 10-nm keratin filaments participate in the formation of a proteinaceous structural framework within the cellular cytoplasm and, as such, serve an important role in epithelial cell protection from mechanical and non-mechanical stressors, a property extensively substantiated by the discovery of human keratin mutations predisposing to tissue-specific injury and by studies in keratin knockout and transgenic mice. More recently, keratins have also been recognized as regulators of other cellular properties and functions, including apico-basal polarization, motility, cell size, protein synthesis and membrane traffic and signaling. In cancer, keratins are extensively used as diagnostic tumor markers, as epithelial malignancies largely maintain the specific keratin patterns associated with their respective cells of origin, and, in many occasions, full-length or cleaved keratin expression (or lack there of) in tumors and/or peripheral blood carries prognostic significance for cancer patients. Quite intriguingly, several studies have provided evidence for active keratin involvement in cancer cell invasion and metastasis, as well as in treatment responsiveness, and have set the foundation for further exploration of the role of keratins as multifunctional regulators of epithelial tumorigenesis.

The protein kinase Pak4 disrupts mammary acinar architecture and promotes mammary tumorigenesis

The Pak4 serine/threonine kinase is highly expressed in many cancer cell lines and human tumors. While several studies have addressed the role for Pak4 in transformation of fibroblasts, most human cancers are epithelial in origin. Epithelial cancers are associated not only with changes in cell growth, but also with changes in the cellular organization within the three dimensional (3D) architecture of the affected tissues. Here we used immortalized mouse mammary epithelial cells (iMMECs) as a model system to study the role for Pak4 in mammary tumorigenesis. iMMECs are an excellent model system for studying breast cancer they can grow in 3D-epithelial cell culture, where they form acinar structures that recapitulate in vivo mammary morphogenesis. While Pak4 is expressed at low levels in wild type iMMECs, it is overexpressed in response to oncogenes, such as oncogenic Ras and Her2/neu. Here we found that overexpression of Pak4 in iMMECs leads to changes in 3D acinar architecture that are consistent with oncogenic transformation. These include decreased central acinar cell death, abrogation of lumen formation, cell polarity alterations, and deregulation of acinar size and cell number. Furthermore, iMMECs overexpressing Pak4 form tumors when implanted into the fat pads of athymic mice. Our results suggest that overexpression of Pak4 triggers events that are important for the transformation of mammary epithelial cells. This is likely to be due to the ability of Pak4 to inhibit apoptosis and promote cell survival, and thus subsequent uncontrolled proliferation, and to its ability to deregulate cell shape and polarity.

Thermodynamic studies of the core histones: stability of the octamer subunits is not altered by removal of their terminal domains

We have investigated the role of the labile terminal domains of the core histones on the stability of the subunits of the protein core of the nucleosome by studying the thermodynamic behavior of the products of limited trypsin digestion of these subunits. The thermal stabilities of the truncated H2A-H2B dimer and the truncated (H3-H4)/(H3-H4)(2) system were studied by high-sensitivity differential scanning calorimetry and circular dichroism spectroscopy. The thermal denaturation of the truncated H2A-H2B dimer at pH 6.0 and low ionic strength is centered at 47.3 degrees C. The corresponding enthalpy change is 35 kcal/mol of 11.5 kDa monomer unit, and the heat capacity change upon unfolding is 1.2 kcal/(K mol of 11.5 kDa monomer unit). At pH 4.5 and low ionic strength, the truncated (H3-H4)/(H3-H4)(2) system, like its full-length counterpart, is quantitatively dissociated into two truncated H3-H4 dimers. The thermal denaturation of the truncated H3-H4 dimer is characterized by the presence of a single calorimetric peak centered at 60 degrees C. The enthalpy change is 25 kcal/mol of 10 kDa monomer unit, and the change in heat capacity upon unfolding is 0.5 kcal/(K mol of 10 kDa monomer unit). The thermal stabilities of both types of truncated dimers exhibit salt and pH dependencies similar to those of the full-length proteins. Finally, like their full-length counterparts, both truncated core histone dimers undergo thermal denaturation as highly cooperative units, without the involvement of any significant population of melting intermediates. Therefore, removal of the histone "tails" does not generally affect the thermodynamic behavior of the subunits of the core histone complex, indicating that the more centrally located regions of the histone fold and the extra-fold structured elements are primarily responsible for their stability and responses to parameters of their chemical microenvironment.

Interaction of Nickel(II) with histones: in vitro binding of nickel(II) to the core histone tetramer

The absorption spectra of Ni(II) bound to the core histone tetramer, (H3-H4)2, of chicken erythrocytes in 500 mM NaCl + 100 mM phosphate (pH 7.4) were recorded. A charge transfer band was seen at 317 nm, characteristic of a bond between Ni(II) and the sulfur atom of Cys-110 of histone H3. The conditional affinity constants for Ni(II) binding at pH 7.4 for low and high Ni(II) saturation (log Kc = 4.26 +/- 0.02 and 5.26 +/- 0.11 M-1, respectively) were calculated from spectrophotometric titrations with the use of this band. The binding of Ni(II) to (H3-H4)2 is proposed to involve the Cys-110 and His-113 of different H3 molecules within the tetramer. The competition between histones and low-molecular-weight chelators for Ni(II) in the cell nucleus, histidine and glutathione, is discussed on the basis of the above results, indicating that histone H3 is very likely to bind Ni(II) dissolved intracellularly from phagocytosed particulate nickel compounds.

Thermodynamic studies of the core histones: pH and ionic strength effects on the stability of the (H3-H4)/(H3-H4)2 system

The self-associative behavior and the thermal stability of the H3/H4 histone complex was studied in low-ionic strength conditions by several physicochemical techniques, including differential scanning calorimetry and circular dichroism spectroscopy. At neutrality, the major molecular species present in solution is the (H3-H4)2 tetramer. Its thermodynamic properties cannot be studied directly though, since its thermal denaturation is completely irreversible even at the lowest salt concentrations. However, a complete thermodynamic analysis can be performed at low ionic strength and pH 4.5, where the (H3-H4)2 tetramer is quantitatively dissociated into two H3-H4 dimers and where almost complete reversibility of the thermal transitions is attained. The unfolding transition temperature of the 26.5 kDa H3-H4 dimer increases as a function of both the ionic strength of the solvent and the total protein concentration. The thermal denaturation of the H3-H4 dimer is characterized by the presence of a single calorimetric peak, centered at 58 degrees C, with a corresponding enthalpy change of 25 kcal/mol of a 13 kDa monomer unit and a change in heat capacity upon unfolding of about 0.6 kcal/(K mol of 13 kDa monomer unit). The complex between histones H3 and H4 (tetramer or dimer) is stable between pH 9.5 and 3.0. At pH 1.5, the system is almost completely unfolded at all temperatures. At low ionic strengths and pH values between 5.0 and 2.5, the H3-H4 dimer behaves as a highly cooperative system, melting as a single unit; i.e. individual H3 and H4 folded monomers are not detectable during the treatment. The two-state mechanism accounting for the unfolding of the H3-H4 dimer at pH 4.5 is the same as that described for the H2A-H2B dimer at neutrality. Just like for the H2A and H2B histones, the H3 and H4 polypeptides are properly folded only when assembled as H3-H4 dimers or in higher-order histone assemblies. Therefore, coupling along the interfaces of the two chains within the heterodimer is the major factor contributing to the stabilization of the secondary and tertiary structures of the chains as well as of the histone dimers.

Thermodynamic studies of the core histones: ionic strength and pH dependence of H2A-H2B dimer stability

The thermal stability of the core histone dimer H2A-H2B has been studied by high-sensitivity differential scanning calorimetry and circular dichroism spectroscopy. The unfolding transition temperature of the 28 kDa H2A-H2B dimer increases as a function of both the ionic strength of the solvent and the total protein concentration. At neutral pH and physiological ionic strength, the thermal denaturation is centered at about 50 degrees C with a corresponding enthalpy change of about 40 kcal/mol of 14 kDa monomer unit and an excess heat capacity of about 1.4 kcal/(K.mol) of 14 kDa monomer unit. The H2A-H2B dimer is stable mainly between pH 5.5 and 10.5. Below pH 4.0, the system is unfolded at all temperatures. The thermodynamic analysis is performed at low ionic strength where almost complete reversibility is attained, since higher salt conditions seem to promote aggregation and irreversibility of the transitions. Analysis of the data shows that at low ionic strength and pH values between 6.5 and 8.5, the H2A-H2B dimer behaves as a highly cooperative system, melting as a single unit without any detectable intermediates of dissociated, yet folded, H2A and H2B monomers. This is consistent with the observed protein concentration dependence of the midpoint of the thermal denaturation. The two-state unfolding process can be described by the general scheme AB-->2U, indicating that the individual H2A and H2B polypeptides are folded, stable entities only when complexed as the H2A-H2B dimer and that the major contribution to the stabilization of the dimer derives from the coupling between the H2A and H2B interfaces.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.