A Signal-seeking Phase Iia Trial of Palbociclib in Advanced Cancers With Cell Cycle Pathway Alterations – A Substudy of the Molecular Screening and Therapeutics (Most) Program

2021 ◽  
Author(s):  
Subotheni Thavaneswaran ◽  
Lucille Sebastian ◽  
Maya Kansara ◽  
Mandy Ballinger ◽  
David Espinoza ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Stable Disease ◽  
Cell Cycle Progression ◽  
Complete Response ◽  
Preceding Trial ◽  
Primary Objective ◽  
Clinical Activity ◽  
Loss Of Function ◽  
Molecular Screening ◽  
Cell Cycle Pathway

Abstract BACKGROUND: The D-type cyclin and cyclin dependent kinase 4/6 (CDK) complex phosphorylates retinoblastoma protein, thereby driving cell cycle progression. This process is blocked by inhibitors of CDK4/6. As part of the Molecular Screening and Therapeutics program, this phase IIa trial tested the clinical activity of CDK4/6 inhibitor monotherapy in tumors with cell cycle pathway alterations.PATIENTS AND METHODS: Eligible patients ≥ 18 years old, with advanced or metastatic solid cancers, along with amplification of CDK4/6, CCND1/2/3, or loss of function alterations in CDKN2A were recruited. The primary objective of this signal-seeking trial was to evaluate the clinical activity of palbociclib – a composite of objective responses and the ratio of time to progression (TTP) on palbociclib, to TTP on treatment preceding trial entry.RESULTS: Ten patients had CDK4/CDK6 or Cyclin D1 amplifications, six patients had CDKN2A deletions. After a median follow-up of 35 months, there were no objective responses. Seven patients had stable disease and one had non-complete response/non-progressive disease (non-CR/non-PD) based on evaluation of a non-target lesion. Two of these seven patients maintained stable disease for at least 6 months, as did the patient with non-CR/non-PD. Median PFS and OS were 3.5 and 11.0 months respectively. No unexpected toxicities were observed. Translational correlates yielded strategies for targeting cell cycle interactions with other molecular pathways and the immune system.CONCLUSION: Palbociclib monotherapy was not associated with any objective responses, but stable disease lasting at least 6 months was observed in 19% of patients. There was no clear relationship with alteration type or histotype. The signals of immune activation provide insights into the design of future trials, with a combination approach adding checkpoint blockade to CDK4/6 inhibition.

Phase II trial of pembrolizumab (MK-3475) in metastatic cutaneous squamous cell carcinoma: An updated analysis.

2019 ◽  
Vol 37 (15_suppl) ◽  
pp. e21015-e21015 ◽  
Author(s):  
Melinda Lynne Yushak ◽  
David H. Lawson ◽  
Monica Goings ◽  
Marjorie Mckellar ◽  
Necia Maynard ◽  
...  
Keyword(s):  
Squamous Cell ◽  
Stable Disease ◽  
Locally Advanced ◽  
Progression Free Survival ◽  
Complete Response ◽  
Primary Objective ◽  
Clinical Activity ◽  
Durable Response ◽  
Grade 3

e21015 Background: Cutaneous squamous cell carcinomas (cSCC) are generally curable with surgery and/or radiation. Unfortunately, some patients develop locally advanced or metastatic disease, with an estimated 3900 to 8700 patients dying of cSCC in 2012. Aggressive disease is often associated with immunosuppression, and no treatment has shown an improvement in overall survival (OS). Recently the FDA has approved cemiplimab-rwlc which targets PD-1. This abstract provides an updated analysis and additional followup for patients treated with pembrolizumab which also targets PD-1. Methods: Clinical activity of pembrolizumab in cSCC patients was evaluated in patients who were not curable by surgery and/or radiation. Patients were treated with pembrolizumab 200mg IV every 3 weeks for up to 2 years. Tumor response was measured every 12 weeks using RECIST version 1.1. Response rate (RR) of pembrolizumab in metastatic cSCC was the primary objective. 6-month progression-free survival (PFS) and 1 year OS were secondary objectives. Results: 11 subjects have been enrolled thus far with a median age of 70.3 years. Two were female and all were Caucasian. ORR per RECIST criteria was 64% with 18% (2) having a complete response, 36% (4) a partial response, 36% (4) progressive disease (PD), and 9% (1) stable disease. The patient with stable disease had a clinical response and was eventually able to undergo surgery that rendered him without any evidence of disease. 6 month PFS for evaluable patients was 72%. Of those patients with a response, 50% had a durable response of 18 months or greater at the time of data cut-off. No subject deaths have occurred on study. Three grade 3 related-adverse events were noted (hepatitis and pneumonitis). Conclusions: Pembrolizumab is active in advanced cSCC. Responses appear durable and comparable to what has been seen in previous studies with the FDA-approved agent, cemiplimab. No new safety concerns were noted. Additional follow up is needed in order to establish an OS benefit. Clinical trial information: NCT02964559.

ANCHOR (OP-104): Melflufen plus dexamethasone (dex) and bortezomib (BTZ) in relapsed/refractory multiple myeloma (RRMM)—Optimal dose, updated efficacy and safety results.

2021 ◽  
Vol 39 (15_suppl) ◽  
pp. 8037-8037
Author(s):  
Roman Hajek ◽  
Luděk Pour ◽  
Miquel Granell ◽  
Vladimir Maisnar ◽  
Paul G. Richardson ◽  
...  
Keyword(s):  
Partial Response ◽  
Cardiac Failure ◽  
Alkylating Agents ◽  
Phase 1 ◽  
Optimal Dose ◽  
Complete Response ◽  
Primary Objective ◽  
Clinical Activity ◽  
Development Of Resistance ◽  
Grade 3

8037 Background: Development of resistance to standard treatments for RRMM highlights the need for novel therapies. Melphalan flufenamide (melflufen) is a first-in-class peptide-drug conjugate (PDC) that leverages aminopeptidases and rapidly releases alkylating agents inside tumor cells. Melflufen + dex showed clinical activity and an acceptable safety profile in HORIZON (Richardson et al. J Clin Oncol. 2020 Dec 9 [Epub]). This is an update of the BTZ arm of the phase 1/2a ANCHOR study (NCT03481556). Methods: Patients (pts) with RRMM were intolerant or refractory to a prior IMiD, with 1-4 prior lines of therapy (LoTs). Prior treatment with a proteasome inhibitor (PI) was allowed, but pts could not be refractory to PIs in the last LoT. Melflufen (30, 40, or 20 mg intravenously; d 1 of each 28-d cycle) was administered with BTZ (1.3 mg/m2 subcutaneous) + oral dex (20 mg on d 1, 4, 8, and 11 and 40 mg on d 15 and 22; dex dose reduced if aged ≥ 75 y). The primary objective in phase 1 was to determine the optimal phase 2 dose of melflufen for this combination. Results: As of the data cutoff date (October 19, 2020), 13 pts received melflufen (30 mg, n = 6; 40 mg, n = 7) + dex and BTZ. In the 30 mg and 40 mg cohorts, respectively, median age was 78.5 y (range, 70-82) and 70.0 y (range, 61-76); median prior LoTs was 3.5 (range, 2-4) and 2.0 (range, 1-4); 33% and 50% of evaluable pts had high-risk cytogenetics; 83% and 71% were refractory to last LoT; 100% and 86% received a prior PI; 33% and 14% were refractory to PIs. In the 30 mg and 40 mg cohorts, respectively, median treatment duration was 6.5 mo (range, 1.4-29.0) and 8.7 mo (range, 2.1-19.6); 4 (67%) and 4 pts (57%) were still on treatment; 2 and 3 pts discontinued (30 mg: progressive disease [PD] and other [1 pt each]; 40 mg: adverse event [AE], lack of efficacy, and PD [1 pt each]). Confirmed overall response rate in the 30 mg and 40 mg cohorts, respectively, was 50% (1 very good partial response [VGPR] and 2 partial response [PR]) and 71% (1 complete response, 3 VGPR, and 1 PR). Most common grade 3/4 treatment-related AEs (TRAEs) were thrombocytopenia (30 mg: 50%; 40 mg: 100%) and neutropenia (30 mg: 33%; 40 mg: 71%); grade 3/4 nonhematologic TRAEs were infrequent; 3 pts discontinued study treatment due to treatment-emergent AEs (30 mg: cardiac failure chronic and osteolysis [1 pt each]; 40 mg: thrombocytopenia [1 pt]). Serious TRAEs occurred in 2 pts (33%) in the 30 mg cohort (neutropenia and pneumonia [1 pt], syncope [1 pt]) and 1 pt (14%) in the 40 mg cohort (thrombocytopenia and neutropenia). No dose-limiting toxicities occurred at either dose level. Fatal AEs occurred in 1 pt in the 30 mg cohort (cardiac failure chronic; unrelated to study treatment). Conclusions: ANCHOR determined that the optimal dose of melflufen is 30 mg + dex and BTZ; results showed clinical activity in heavily pretreated pts. Recruitment is ongoing; updated data will be presented. Clinical trial information: NCT03481556.

Phase I/II Study of Ipilimumab for Patients With Metastatic Melanoma

2008 ◽  
Vol 26 (36) ◽  
pp. 5950-5956 ◽  
Author(s):  
Jeffrey S. Weber ◽  
Steven O’Day ◽  
Walter Urba ◽  
John Powderly ◽  
Geoff Nichol ◽  
...  
Keyword(s):  
Phase I ◽  
Metastatic Melanoma ◽  
Complete Response ◽  
Pharmacokinetic Profile ◽  
Maximum Tolerated Dose ◽  
Primary Objective ◽  
Clinical Activity ◽  
Multiple Doses ◽  
Group A ◽  
Group B

PurposeThe primary objective of this phase I/II study was to determine the safety and pharmacokinetic profile of either transfectoma- or a hybridoma-derived ipilimumab. Secondary objectives included determination of a maximum-tolerated dose and assessment of clinical activity.Patients and MethodsEighty-eight patients with unresectable stage III or IV melanoma with at least one measurable lesion were treated. Mean age was 59 years, with 65% male and 35% female patients, and 79% of patients had received prior systemic therapy. Single doses of ipilimumab up to 20 mg/kg (group A, single dose), multiple doses up to 5 mg/kg (group A, multiple dose), and multiple doses up to 10 mg/kg (group B) were administered.ResultsSingle dosing up to 20 mg/kg of transfectoma antibody was well tolerated, as were multiple doses up to 10 mg/kg without a maximum-tolerated dose. In group B, dose-limiting toxicity was seen in six of 23 melanoma patients. Grade 3 or 4 immune-related adverse events (irAEs) were observed in 14% of patients (12 of 88 patients), and grade 1 or 2 irAEs were seen in an additional 58%. The half-life of ipilimumab was 359 hours. In group B, there was one partial response (23+ months), one complete response (21+ months), and seven patients with stable disease (SD), for a disease control rate of 39%. Two patients in group B with SD had slow, steady decline in tumor burden that was ongoing at 1 year of observation.ConclusionIpilimumab has activity in patients with metastatic melanoma. Late responses were observed in patients with prolonged SD.

SCL-mediated regulation of the cell-cycle regulator p21 is critical for murine megakaryopoiesis

Blood ◽  
2011 ◽  
Vol 118 (3) ◽  
pp. 723-735 ◽  
Author(s):  
Hedia Chagraoui ◽  
Mira Kassouf ◽  
Sreemoti Banerjee ◽  
Nicolas Goardon ◽  
Kevin Clark ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Transcriptional Control ◽  
Cell Cycle Progression ◽  
Cellular Proliferation ◽  
Loss Of Function ◽  
Cycle Progression ◽  
Cell Cycle Regulator ◽  
Nuclear Changes ◽  
Cytoplasmic Maturation

Abstract Megakaryopoiesis is a complex process that involves major cellular and nuclear changes and relies on controlled coordination of cellular proliferation and differentiation. These mechanisms are orchestrated in part by transcriptional regulators. The key hematopoietic transcription factor stem cell leukemia (SCL)/TAL1 is required in early hematopoietic progenitors for specification of the megakaryocytic lineage. These early functions have, so far, prevented full investigation of its role in megakaryocyte development in loss-of-function studies. Here, we report that SCL critically controls terminal megakaryocyte maturation. In vivo deletion of Scl specifically in the megakaryocytic lineage affects all key attributes of megakaryocyte progenitors (MkPs), namely, proliferation, ploidization, cytoplasmic maturation, and platelet release. Genome-wide expression analysis reveals increased expression of the cell-cycle regulator p21 in Scl-deleted MkPs. Importantly, p21 knockdown-mediated rescue of Scl-mutant MkPs shows full restoration of cell-cycle progression and partial rescue of the nuclear and cytoplasmic maturation defects. Therefore, SCL-mediated transcriptional control of p21 is essential for terminal maturation of MkPs. Our study provides a mechanistic link between a major hematopoietic transcriptional regulator, cell-cycle progression, and megakaryocytic differentiation.

scholarly journals Ras controls growth, survival and differentiation in the Drosophila eye by different thresholds of MAP kinase activity

Development ◽  
2001 ◽  
Vol 128 (9) ◽  
pp. 1687-1696 ◽  
Author(s):  
K. Halfar ◽  
C. Rommel ◽  
H. Stocker ◽  
E. Hafen
Keyword(s):  
Cell Cycle ◽  
Cell Cycle Progression ◽  
Kinase Activity ◽  
Photoreceptor Cells ◽  
Loss Of Function ◽  
Cellular Functions ◽  
Partial Loss ◽  
Cycle Progression ◽  
Mapk Activity ◽  
Dividing Cells

Ras mediates a plethora of cellular functions during development. In the developing eye of Drosophila, Ras performs three temporally separate functions. In dividing cells, it is required for growth but is not essential for cell cycle progression. In postmitotic cells, it promotes survival and subsequent differentiation of ommatidial cells. In the present paper, we have analyzed the different roles of Ras during eye development by using molecularly defined complete and partial loss-of-function mutations of Ras. We show that the three different functions of Ras are mediated by distinct thresholds of MAPK activity. Low MAPK activity prolongs cell survival and permits differentiation of R8 photoreceptor cells while high or persistent MAPK activity is sufficient to precociously induce R1-R7 photoreceptor differentiation in dividing cells.

scholarly journals The RASSF6 Tumor Suppressor Protein Regulates Apoptosis and Cell Cycle Progression via Retinoblastoma Protein

2018 ◽  
Vol 38 (17) ◽  
Author(s):  
Shakhawoat Hossain ◽  
Hiroaki Iwasa ◽  
Aradhan Sarkar ◽  
Junichi Maruyama ◽  
Kyoko Arimoto-Matsuzaki ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Tumor Suppressor ◽  
Cell Cycle Arrest ◽  
Cell Cycle Progression ◽  
Target Genes ◽  
Loss Of Function ◽  
Cycle Progression ◽  
P53 Expression ◽  
Cycle Arrest ◽  
Hct116 Cells

ABSTRACT RASSF6 is a member of the tumor suppressor Ras association domain family (RASSF) proteins. RASSF6 is frequently suppressed in human cancers, and its low expression level is associated with poor prognosis. RASSF6 regulates cell cycle arrest and apoptosis and plays a tumor suppressor role. Mechanistically, RASSF6 blocks MDM2-mediated p53 degradation and enhances p53 expression. However, RASSF6 also induces cell cycle arrest and apoptosis in a p53-negative background, which implies that the tumor suppressor function of RASSF6 does not depend solely on p53. In this study, we revealed that RASSF6 mediates cell cycle arrest and apoptosis via pRb. RASSF6 enhances the interaction between pRb and protein phosphatase. RASSF6 also enhances P16INK4A and P14ARF expression by suppressing BMI1. In this way, RASSF6 increases unphosphorylated pRb and augments the interaction between pRb and E2F1. Moreover, RASSF6 induces TP73 target genes via pRb and E2F1 in a p53-negative background. Finally, we confirmed that RASSF6 depletion induces polyploid cells in p53-negative HCT116 cells. In conclusion, RASSF6 behaves as a tumor suppressor in cancers with loss of function of p53, and pRb is implicated in this function of RASSF6.

scholarly journals MicroRNAs in the miR-106b Family Regulate p21/CDKN1A and Promote Cell Cycle Progression

2008 ◽  
Vol 28 (7) ◽  
pp. 2167-2174 ◽  
Author(s):  
Irena Ivanovska ◽  
Alexey S. Ball ◽  
Robert L. Diaz ◽  
Jill F. Magnus ◽  
Miho Kibukawa ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Cell Cycle Progression ◽  
Kinase Inhibitor ◽  
Phenotype Microarray ◽  
Loss Of Function ◽  
Cycle Progression ◽  
Multiple Tumor ◽  
Cyclin Dependent Kinase Inhibitor ◽  
Expression Of Genes ◽  
Tumor Types

ABSTRACT microRNAs in the miR-106b family are overexpressed in multiple tumor types and are correlated with the expression of genes that regulate the cell cycle. Consistent with these observations, miR-106b family gain of function promotes cell cycle progression, whereas loss of function reverses this phenotype. Microarray profiling uncovers multiple targets of the family, including the cyclin-dependent kinase inhibitor p21/CDKN1A. We show that p21 is a direct target of miR-106b and that its silencing plays a key role in miR-106b-induced cell cycle phenotypes. We also show that miR-106b overrides a doxorubicin-induced DNA damage checkpoint. Thus, miR-106b family members contribute to tumor cell proliferation in part by regulating cell cycle progression and by modulating checkpoint functions.

A Phase 1 Dose-Escalation Study of the Novel Cell Cycle Active Agent SNS-595 in Advanced Leukemias.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 1962-1962
Author(s):  
Jeffrey E. Lancet ◽  
Judith E. Karp ◽  
Nancy Havrilla ◽  
Ute Hoch ◽  
Jeffrey Silverman ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Bone Marrow ◽  
Dose Escalation ◽  
Phase 1 ◽  
Active Agent ◽  
Complete Response ◽  
Clinical Activity ◽  
Dose Escalation Study ◽  
Acute Leukemias ◽  
Prior Therapy

Abstract SNS-595 is a novel, cell cycle active cytotoxic naphthyridine analog that induces G2 arrest in a variety of preclinical tumor models. We initiated an escalating-dose phase 1 trial of SNS-595, administered as a weekly x 3 (arm A) or twice weekly x 2 bolus (arm B), in patients with advanced or refractory acute leukemias. Objectives: The primary objectives were to:establish safety, tolerability, and MTD of SNS-595 given on each schedule,characterize pharmacokinetics (PK) of SNS-595 when given on these schedules. Secondary objectives were:assessment of clinical activity,exploration of potential biomarkers. Methods: SNS-595 was administered as a slow IV push on days 1, 8, 15 (arm A) or days 1, 4, 8, 11 (arm B). Minimum cycle length was 42 days (arm A) and 39 days (arm B). Additional cycles were permitted if patients achieved stable disease or better. The starting dose was 18 mg/m2/d on arm A, and 9 mg/m2/d on arm B and escalated by cohort using a modified Fibonacci schema. PK analyses for SNS-595 were performed on plasma samples collected during cycle 1. Pretreatment peripheral blood and bone marrow aspirate samples were collected for exploratory analyses of the level and functional activity of the DNA damage repair proteins DNA-PK and MSH2. Results: To date, 21 patients have been enrolled and are evaluable in the live database, including 13 patients assigned to arm A and 8 assigned to arm B, 12 males and 9 females with a median age of 64 years. Diagnoses included AML (19 patients) and ALL (2 patients). All patients had disease refractory to or relapsed from prior therapy (median 3 prior regimens (range 1–6)). Dose escalation has proceeded to 50 mg/m2/d (arm A) and 19 mg/m2/d (arm B). No dose-limiting toxicities have been observed to date. Non-dose limiting toxicities included nausea/vomiting, diarrhea, and mucositis . Grade 4 neutropenic fever was observed in only one patient. Plasma exposures at the first two dose levels in each arm increased linearly, resulting in AUCs of 5.5 – 17.8 ughr/mL for 9–27 mg/m2 doses. CL, Vss, and terminal half-lives were similar to those reported previously in solid tumor patients, and averaged ~2 L/hr/m2, 58 L/m2, and 23 hr, respectively. No patients have achieved complete response to date, although 6 patients (distributed across all dosing groups) experienced >50% reductions in peripheral blasts following cycle 1. Conclusion: SNS-595 appears to be well-tolerated in patients with advanced leukemias, with preliminary and promising signs of clinical activity as measured by decreases in leukemic blasts. Bone marrow ablation has not yet been achieved; patient accrual and dose-escalation are ongoing.

Clinical efficacy and toxicity data on phase I study of fosbretabulin in combination with everolimus in neuroendocrine tumors.

2019 ◽  
Vol 37 (15_suppl) ◽  
pp. 4114-4114 ◽  
Author(s):  
Aman Chauhan ◽  
Susanne M. Arnold ◽  
Jianrong Wu ◽  
Rashmi T Nair ◽  
Stacey A. Slone ◽  
...  
Keyword(s):  
Phase I ◽  
Clinical Efficacy ◽  
Stable Disease ◽  
Phase I Study ◽  
Safety Data ◽  
Primary Objective ◽  
Water Soluble ◽  
Clinical Activity ◽  
Open Label ◽  
Gi Symptoms

4114 Background: Fosbretabulin, a synthetic, water-soluble, phosphorylated prodrug of the natural product combretastatin A4 (CA4P), initially isolated from the bark of the South African bush willow, Combretum caffrum, is the lead compound in a class of agents termed vascular disrupting agents (VDAs). Everolimus, an mTOR inhibitor, is FDA approved for the management of well-differentiated NETs. A Phase I trial combining fosbretabulin and everolimus to determine the recommended Phase II trial dose (RP2D), safety data and early clinical efficacy in metastatic GEPNET patients was conducted. Methods: An investigator-initiated, single center, open-label, phase I study involving GEPNETs incorporated partial order continual reassessment method (PO-CRM) to define the dose escalation. The primary objective was to establish the maximum tolerated dose (MTD) of the combination of everolimus and fosbretabulin in NETs that have progressed after at least one prior regimen for metastatic disease. Secondary objective included identifying the safety profile of the combination using NCI CTCAE4 reporting criteria. Patients received daily oral everolimus (2.5 mg, 5 mg, 7.5 mg, and 10 mg). Fosbretabulin was administered IV 60 mg/m2 either q3 weekly or q weekly based on PO-CRM. Patients were treated for 12 weeks with all combinations. RECIST 1.1 was used to evaluate radiological responses at 3 month. Results: Of the 17 patients enrolled, 16 completed the 12-week trial. One patient was not evaluable due to noncompliance. No DLTs were observed at day 21. The highest dose of 10 mg daily oral everolimus in combination with weekly 60mg/m2 IV fosbretabulin is the RP2D. No grade 4 or 5 toxicities were noted. Grade 3 toxicities were seen in 5 patients; abdominal pain and hyperglycemia (not related to study drug), fatigue (possibly related), decreased lymphocyte count and anemia (related). Several patients had delay in treatment due to grade 2 AE’s (GI symptoms, rash, thrombocytopenia) and one patient was unable to complete treatment due to pneumonitis. All evaluable patients except one had stable disease at 3 months. One patient showed SD but non target lesion demonstrated PD. One patient had > 30% decrease in tumor size but overall sum of lesions showed SD. A detailed table with all grade toxicities and waterfall plot of RR will be presented at the meeting. Conclusions: Ten mg PO daily everolimus plus 60 mg/m2 fosbretabulin IV weekly is the RP2D. Early clinical data suggests clinical activity and stable disease in all but one patient at 3 months. Clinical trial information: NCT0301429.

scholarly journals Promyelocytic leukemia nuclear bodies behave as DNA damage sensors whose response to DNA double-strand breaks is regulated by NBS1 and the kinases ATM, Chk2, and ATR

2006 ◽  
Vol 175 (1) ◽  
pp. 55-66 ◽  
Author(s):  
Graham Dellaire ◽  
Reagan W. Ching ◽  
Kashif Ahmed ◽  
Farid Jalali ◽  
Kenneth C.K. Tse ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Dna Damage ◽  
Cellular Response ◽  
Cell Cycle Progression ◽  
Nuclear Body ◽  
Promyelocytic Leukemia ◽  
S Phase ◽  
Nuclear Bodies ◽  
Structural Basis ◽  
Loss Of Function

The promyelocytic leukemia (PML) nuclear body (NB) is a dynamic subnuclear compartment that is implicated in tumor suppression, as well as in the transcription, replication, and repair of DNA. PML NB number can change during the cell cycle, increasing in S phase and in response to cellular stress, including DNA damage. Although topological changes in chromatin after DNA damage may affect the integrity of PML NBs, the molecular or structural basis for an increase in PML NB number has not been elucidated. We demonstrate that after DNA double-strand break induction, the increase in PML NB number is based on a biophysical process, as well as ongoing cell cycle progression and DNA repair. PML NBs increase in number by a supramolecular fission mechanism similar to that observed in S-phase cells, and which is delayed or inhibited by the loss of function of NBS1, ATM, Chk2, and ATR kinase. Therefore, an increase in PML NB number is an intrinsic element of the cellular response to DNA damage.

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