An open-label, phase 2 study evaluating the efficacy and safety of the anti-IGF-1R antibody cixutumumab in patients with previously treated advanced or metastatic soft-tissue sarcoma or Ewing family of tumours

BACKGROUND

Cixutumumab (IMC-A12), a fully human immunoglobulin G1 (IgG1) monoclonal antibody, exerts preclinical activity in several sarcoma models and may be effective for the treatment of these tumours.

METHODS

In this open-label, multicentre, phase 2 study, patients with previously treated advanced or metastatic rhabdomyosarcoma, leiomyosarcoma, adipocytic sarcoma, synovial sarcoma or Ewing family of tumours received intravenous cixutumumab (10mg/kg) for 1h every other week until disease progression or discontinuation. The primary end-point was the progression-free survival rate (PFR), defined as stable disease or better at 12 weeks. In each tier of disease histology, Simon's optimum 2-stage design was applied (PFR at 12 weeks P0=20%, P1=40%, α=0.10, β=0.10). Stage 1 enrolled 17 patients in each disease group/tier, with at least four patients with stable disease or better required at 12 weeks to proceed to stage 2.

RESULTS

A total of 113 patients were enrolled; all tiers except adipocytic sarcoma were closed after stage 1 due to futility. The 12-week PFR was 12% for rhabdomyosarcoma (n=17), 14% for leiomyosarcoma (n=22), 32% for adipocytic sarcoma (n=37), 18% for synovial sarcoma (n=17) and 11% for Ewing family of tumours (n=18). Median progression-free survival (weeks) was 6.1 for rhabdomyosarcoma, 6.0 for leiomyosarcoma, 12.1 for adipocytic sarcoma, 6.4 for synovial sarcoma and 6.4 for Ewing family of tumours. Among all patients, the most frequent treatment-emergent adverse events (AEs) were nausea (26%), fatigue (23%), diarrhoea (23%) and hyperglycaemia (20%).

CONCLUSIONS

Patients with adipocytic sarcoma may benefit from treatment with cixutumumab. Cixutumumab treatment was well tolerated, with limited gastrointestinal AEs, fatigue and hyperglycaemia.

Abstract P6-11-10: IBL2001: Phase I/II study of a novel dose-dense schedule of oral indibulin for the treatment of metastastic breast cancer

Background: Indibulin (ZI0-301) is a novel, oral, synthetic small molecule microtubule inhibitor which binds tubulin at a different site than taxanes and vinca alkaloids. Preclinical data demonstrate indibulin does not interact with acetylated (neuronal) tubulins and in clinical studies has not exhibited the neurotoxicity associated with other tubulin binders. Indibulin has potent antitumor activity in human cancer cell lines, including multidrug-, taxane-, and vinblastine-resistant. Norton-Simon modeling based on cell line data suggested that dose dense (dd) administration could optimize efficacy while limiting toxicity.

Methods: Eligible patients (pts) have metastatic or unresectable locally advanced breast cancer, ECOG performance status ≤ 2, adequate organ function, measurable or nonmeasurable disease and any number of prior therapies. Uncontrolled gastrointestinal malabsorption syndrome and grade 2 or higher peripheral neuropathy are the principal exclusions. Adverse events (AEs) are graded by CTCAE v. 4.0. Objective disease status is evaluated according to RECIST 1.1. The primary objective of the phase (Ph) I portion of the study is to determine the maximum tolerated dose (MTD) of indibulin when given in dd fashion 5 days treatment, 9 days rest using standard 3+3 dose escalation schema.

A phase II study evaluating the efficacy and safety of single agent IMC A12, a monoclonal antibody (MAb), against the insulin-like growth factor-1 receptor (IGF-IR), as monotherapy in patients with metastastic, asymptomatic castration-resistant prostate cancer (CRPC)

5142

Background: IGF-IR-mediated signaling contributes to prostate cancer carcinogenesis and pathogenesis and may be associated with hormone resistance. IMC-A12 is a fully human IgG1 MAb that specifically targets the IGF-IR and inhibits ligand binding and signaling. In xenografts, IMC-A12 inhibits growth of both androgen-dependent and -independent prostate cancer. This phase 2, multicenter study was designed to assess the safety and antitumor activity of IMC-A12 in asymptomatic pts with metastatic CRPC who were chemotherapy-naive. Methods: Pts received IMC-A12 10 mg/kg IV every 2 wks; until evidence of progressive disease (PD), intolerable toxicity, or other withdrawal criteria were met. Radiologic evaluation was performed every 8 wks. PD by bone scan required at least 2 new lesions with confirmation at subsequent imaging per PCWG2. Results: 19 of 31 pts treated have discontinued IMC-A12, 12 due to PD. 9 of 31 pts experienced disease stabilization for ≥6 mos (range: 7.4–12.5 mos), 5 pts (3 with PSA reduction) continue on IMC-A12. The most common AEs possibly or probably related to IMC-A12, were fatigue (25.8%) and hyperglycemia (19.4%). In 4 pts cases of Grade 3 hyperglycemia was treatment related none requiring discontinuation of IMC-A12; 1 pt required insulin but continued on study. Approximately 70% of pts experienced at least transient elevation of nonfasting glucose to above normal range. Other AEs grade ≥3 at least possibly related to IMC-A12 were one case each of thrombocytopenia (requiring IMC-A12 discontinuation), hyperkalemia, fatigue, pneumonia (resulting in death), and Grade 4 pharmacokinetic and pharmacodynamic analayses are pending leukoencephalopathy (probably related; requiring IMC-A12 discontinuation). Pharmakinetic and pharmacodynamic analyses are pending. Conclusions: IMC-A12 monotherapy appears well tolerated in pts with metastatic asymptomatic CRPC. As in phase I, hyperglycemia was largely asymptomatic and manageable. Disease stabilization for > 6 months in 9 of 31 pts suggests that IMC- A12 may have modest antitumor activity. Additional studies of IMC-A12 in CRPC are planned.

[Table: see text]

Phase II randomized, open-label study of cetuximab (Cet) and bevacizumab (Bev) in combination with paclitaxel (P) and carboplatin (C) in patients with stageIII/IV non-small cell lung cancer (NSCLC)

8046^

Background: Cet, a monoclonal antibody (mAb) that specifically targets epidermal growth factor receptor (EGFR) and Bev (anti-VEGF mAb) combined with chemotherapy has resulted in superior survival compared to chemotherapy alone in some stage IV NSCLC trials. Combining both mAb's with chemotherapy might increase survival in stage IV NSCLC pts. Methods: Chemotherapy naïve advanced NSCLC patients with ECOG status <1, adequate hematologic, hepatic, and renal function received Cet (400 mg/m2 on Day 1 as initial dose and weekly thereafter at 250 mg/m2) plus Bev (15 mg/kg on Day 8 of each 3-week cycle) for 6 cycles in combination with either 6 cycles (Arm A) or 3 cycles (Arm B) of P (200 m/kg) and C (AUC=6) on Day 1 of each 3-week cycle. Patients without progressive disease (PD) after 6 cycles continued Cet until PD or other withdrawal criteria were met. Comparison of progression-free survival (PFS) for arm A vs B is the primary objective. Results: Accrual is completed. Data are available for 85 pts: 47% women, median age 65 yrs, and 88% Caucasian. Median PFS for arm A was 6.0 vs 4.2 months for arm B, hazard ratio of 0.57 for arm A relative to arm B. Objective response was Arm A=31% and B=30%. Most frequent AEs ≥Grade 3 in pts were neutropenia [9 (10.6%), A=8, B=1], fatigue [8 (9.4%), A=4, B=4], dermatitis acneiform [6 (7.1%), A=3, B=3].13 pts withdrew due to AEs (A=4, B=9), 4 were related to Cet (2 per arm). Two pts discontinued due to death (A=2, B=0) unrelated to Cet. Conclusions: Adding both mAb's to PC resulted in acceptable toxicity. The trend for superior PFS with 6 courses of PC suggests that 3 courses of PC are not optimal.

[Table: see text]

ASCO Conflict of Interest Policy and Exceptions

In compliance with the guidelines established by the ASCO Conflict of Interest Policy (J Clin Oncol. 2006 Jan 20;24[3]:519–521) and the Accreditation Council for Continuing Medical Education (ACCME), ASCO strives to promote balance, independence, objectivity, and scientific rigor through disclosure of financial and other interests, and identification and management of potential conflicts. According to the ASCO Conflict of Interest Policy, the following financial and other relationships must be disclosed: employment or leadership position, consultant or advisory role, stock ownership, honoraria, research funding, expert testimony, and other remuneration (J Clin Oncol. 2006 Jan 20;24[3]:520). The ASCO Conflict of Interest Policy disclosure requirements apply to all authors who submit abstracts to the Annual Meeting.

For clinical trials that began accrual on or after April 29, 2004, ASCO's Policy places some restrictions on the financial relationships of principal investigators (J Clin Oncol. 2006 Jan 20;24[3]:521). If a principal investigator holds any restricted relationships, his or her abstract will be ineligible for placement in the 2009 Annual Meeting unless the ASCO Ethics Committee grants an exception. Among the circumstances that might justify an exception are that the principal investigator (1) is a widely acknowledged expert in a particular therapeutic area; (2) is the inventor of a unique technology or treatment being evaluated in the clinical trial; or (3) is involved in international clinical oncology research and has acted consistently with recognized international standards of ethics in the conduct of clinical research. NIH-sponsored trials are exempt from the Policy restrictions. Abstracts for which authors requested and have been granted an exception in accordance with ASCO's Policy are designated with a caret symbol (^) in the Annual Meeting Proceedings.

For more information about the ASCO Conflict of Interest Policy and the exceptions process, please visit www.asco.org/conflictofinterest .

EGFR expression by immunohistochemistry (IHC) and response to chemotherapy and cetuximab in squamous cell carcinoma of the head and neck (SCCHN)

6024

Background: There is modest activity for cetuximab in recurrent SCCHN. Molecular markers for patient selection are desirable. Burtness et al (JCO 23:8646, 2005) reported that high EGFR expression predicted for tumor resistance to chemotherapy (CT) with or without cetuximab. We have studied an independent cohort of patients from the IMC-9816 study (Herbst et al, JCO 23:5578, 2005) using the same EGFR immunoreactivity staining technique. Methods: Slides were prepared from formalin fixed paraffin embedded tissue with the DAKO kit, with staining intensity graded on an ordinal scale 0–3 and staining density assessed according to the percentage of cells stained. High expression was defined as staining intensity 3 + on = 80% of cells. Tumor samples were available in 77 pts from the initial cohort of 132 patients with recurrent SCCHN who received cisplatin-based CT. Of the latter, 76 patients were assessed to have stable (SD 51) or progressive disease (PD 25) and then treated with cetuximab and cisplatin. An amendment permitted further enrollment of patients with PD after off study cisplatin therapy (N 54). Of a total of 130 pts who received cetuximab and cisplatin, tumor samples were available from 86. Results: There was no association of response with EGFR expression. Responses to CT are tabulated. Of the 86 SD and PD pts receiving CT and cetuximab, responses were observed in 3 of 44 and 7 of 42 pts with low and high EGFR expression, respectively (p=0.191). In a survival analysis of patients treated with cetuximab, the hazard ratio for subjects with high EGFR was 0.835, p=0.40. Conclusions: Conventional IHC, even when supplemented by staining intensity and staining density, does not predict for responsiveness of SCCHN to CT, with or without cetuximab. Future studies may require more quantitative methods, possibly subcellular localization, and measurement of other EGFR pathway targets, ligand content and probably “down-stream” effectors.

No significant financial relationships to disclose.

[Table: see text]

Heterogeneity of epidermal growth factor receptor (EGFR) expression and variation in immunohistochemistry (IHC) testing may affect access to EGFR-targeted therapy in patients with advanced colorectal cancer (CRC)

10104

Introduction: Cetuximab, an IgG1 MAb directed at the EGFR, has demonstrated benefit in pts with CRC and is currently indicated in EGFR expressing CRC. However, previous reports show heterogeneous EGFR expression between anatomic sites and suggest that results from IHC testing vary between labs. To assess this variability and the resultant impact on pt access to EGFR-targeted therapy, we compared EGFR results reported to clinicians by hospital and reference labs with those obtained at a single central lab. Methods: Formalin-fixed, paraffin-embeded CRC specimens submitted through the Targeted Diagnostics Advocacy Program (tdap) were assayed using the Dako EGFR pharmDx kit and scored using the FDA-approved package insert. If the initial specimen tested EGFR negative (−), additional specimens were requested for assay. Prior EGFR results, lab, and specimen ID were documented through review of anatomic pathology reports. Results: Of 332 evaluable CRC pts tested, 245 pts (74%) were previously reported as EGFR(−). After retesting, 167 (68%) pts were found to be EGFR The originally tested tumor block was retested in 82 pts and 41 pts (50%) were found to be EGFR (+). Retest results were similar for both hospital and reference labs. Of 66 pts with >1 specimen from the same anatomic site tested, 21 (32%) were discordant (at least 1 (+) and one (−) result). Results from different anatomic sites were compared for 65 pts, 34 (52%) of which were discordant (+). Overall, 231 pts (70%) tested EGFR(+). Conclusion: This study shows marked heterogeneity in EGFR expression by IHC both within and across anatomic sites from the same pt as well as variability in both hospital and reference pathology labs. This variability undoubtedly has an adverse impact on pt access to EGFR-targeted therapy. The impact of EGFR heterogeneity may be reduced through the assay of multiple specimens.

[Table: see text]

[Table: see text]

EGFR expression using immunohistochemistry (IHC) testing as a tool for selecting patients (pts) for treatment with cetuximab

13000

Background: EGFR expression, as determined by IHC, is currently used to select patients for cetuximab therapy. Based on prior studies in colorectal cancer patients, approximately 60 to 75% of patients express EGFR. There is increasing evidence that EGFR expression is not predictive of response to cetuximab therapy, and does not properly select patients who might benefit from such therapy (Chung et al 2005, Saltz et al 2005, Scartozi et al 2004, NCCN 2005). Such selection limits access to a considerable number of patients who might otherwise benefit. Methods: A clinical trial of cetuximab (Erbitux®) monotherapy is being conducted in 60 EGFR-undetectable patients with metastatic colorectal cancer at 14 sites in the US and Canada to explore the relationship between EGFR expression and cetuximab activity. Results: As of January 5, 2006, 112 patients have been screened. Of these patients, 33 (29%) were EGFR-undetectable and continued screening for study enrollment; 52 (46%) tested positive for EGFR expression; and 2 (2%) did not have enough tissue to evaluate EGFR status and were not enrolled onto the trial. The remaining 25 pts (22%), were initially found to be EGFR-undetectable by IHC testing at local labs, but were subsequently identified as EGFR-positive after reevaluation at a highly experienced, centralized laboratory. Conclusion: The majority of patients tested for EGFR expression are tested using the EGFR pharmDx™ IHC assay. Results of the IHC-based assay for EGFR expression are highly dependent upon sample preparation and the methods used in conducting the assay. Variability in methods among labs may result in poor identification of pts expressing EGFR. This finding, together with the growing evidence that EGFR expression is not predictive of response to cetuximab therapy, indicate that the current routine practice of tumor IHC EGFR testing for the purpose of selecting cetuximab therapy may be inappropriate and pts who could potentially benefit from cetuximab therapy are being excluded from a treatment option.

[Table: see text]

Correction of cross-linker sensitivity of Fanconi anemia group F cells by CD33-mediated protein transfer

Studies have previously described the feasibility of receptor-mediated protein transfer in a cell culture model of Fanconi anemia (FA) group C. This study explores the versatility of this approach by using an antibody single-chain fusion protein to correct the phenotypic defect in FA group F cells. A 68.5-kd chimeric protein (His-M195FANCF) was expressed, consisting of a His tag, a single-chain antibody to the myeloid antigen CD33, and the FANCF protein, as well as a 43-kd His-FANCF fusion protein lacking the antibody motif, in Escherichia coli. The nickel-agarose-purified His-M195FANCF protein bound specifically to the surface of HeLa cells transfected with CD33 and internalized through vesicular structures. The fusion protein, but not CD33, sorted to the nucleus, consistent with the known nuclear localization of FANCF. No similar binding or internalization was observed with His-FANCF. Pretreatment of the transfected cells with chloroquine abolished nuclear accumulation, but there was little change with brefeldin A, indicating a minimal if any role for the Golgi apparatus in mediating transport from endosomes to the cytosol and the nucleus. The intracellular half-life of His-M195FANCF was approximately 160 minutes. Treatment of CD33-transfected FA group F lymphoblastoid cells with 0.1 mg/mL His-M195FANCF conferred resistance to mitomycin C. No similar protection was noted in CD33(-) parental cells or CD33(+) FA cells belonging to groups A and C. These results demonstrate that antibody-directed, receptor-mediated protein transfer is a versatile method for the delivery of biologically active proteins into hematopoietic cells.

Cell cycle-dependent expression and nucleolar localization of hCAP-H

Condensin is a conserved 13S heteropentamer composed of two nonidentical structural maintenance of chromosome (SMC) family proteins, in Xenopus XCAP-C and XCAP-E, and three regulatory subunits, XCAP-D2, XCAP-G, and XCAP-H. Both biochemical and genetic analyses have demonstrated an essential role for the 13S condensin complex in mitotic chromosome condensation. Further, a potential requirement for condensin in completion of chromatid arm separation in early anaphase is demonstrated by the mutational phenotypes of the Drosophila homologues of XCAP-H, barren and XCAP-C, DmSMC4. In this study we have investigated the expression and subcellular distribution of hCAP-H, the human homolog of XCAP-H, in order to better understand its cellular functions. Transcription of hCAP-H was restricted to proliferating cells with highest expression during the G(2) phase of the cell cycle. In contrast, cellular hCAP-H protein levels were constant throughout the cell cycle. hCAP-H was found to be associated with mitotic chromosomes exhibiting a nonuniform but symmetric distribution along sister chromatids. The symmetry of hCAP-H association with sister chromatids suggests that there are sequence-dependent domains of condensin aggregation. During interphase hCAP-H, -C, and -E, have distinct punctate nucleolar localization, suggesting that condensin may associate with and modulate the conformation and function of rDNA. hCAP-H association with condensed chromatin was not observed in the early phase of chromosome condensation when histone H3 phosphorylation has already taken place. This finding is consistent with the hypothesis that histone H3 phosphorylation precedes condensin-mediated condensation.

Fanconi anemia group C protein prevents apoptosis in hematopoietic cells through redox regulation of GSTP1

The Fanconi anemia group C protein (FANCC) plays an important role in hematopoiesis by ensuring the survival of hematopoietic progenitor cells through an unknown mechanism. We investigated the function of FANCC by identifying FANCC-binding proteins in hematopoietic cells. Here we show that glutathione S-transferase P1-1 (GSTP1) interacts with FANCC, and that overexpression of both proteins in a myeloid progenitor cell line prevents apoptosis following factor deprivation. FANCC increases GSTP1 activity after the induction of apoptosis. GSTP1 is an enzyme that catalyzes the detoxification of xenobiotics and by-products of oxidative stress, and it is frequently upregulated in neoplastic cells. Although FANCC lacks homology with conventional disulfide reductases, it functions by preventing the formation of inactivating disulfide bonds within GSTP1 during apoptosis. The prevention of protein oxidation by FANCC reveals a novel mechanism of enzyme regulation during apoptosis and has implications for the treatment of degenerative diseases with thiol reducing agents.

Functional analysis of the putative peroxidase domain of FANCA, the Fanconi anemia complementation group A protein

Fanconi anemia (FA) is an autosomal recessive disorder manifested by chromosomal breakage, birth defects, and susceptibility to bone marrow failure and cancer. At least seven complementation groups have been identified, and the genes defective in four groups have been cloned. The most common subtype is complementation group A. Although the normal functions of the gene products defective in FA cells are not completely understood, a clue to the function of the FA group A gene product (FANCA) was provided by the detection of limited homology in the amino terminal region to a class of heme peroxidases. We evaluated this hypothesis by mutagenesis and functional complementation studies. We substituted alanine residues for the most conserved FANCA residues in the putative peroxidase domain and tested their effects on known biochemical and cellular functions of FANCA. While the substitution mutants were comparable to wild-type FANCA with regard to their stability, subcellular localization, and interaction with FANCG, only the Trp(183)-to-Ala substitution (W183A) abolished the ability of FANCA to complement the sensitivity of FA group A cells to mitomycin C. By contrast, TUNEL assays for apoptosis after exposure to H2O2 showed no differences between parental FA group A cells, cells complemented with wild-type FANCA, and cells complemented with the W183A of FANCA. Moreover, semiquantitative RT-PCR analysis for the expression of the peroxide-sensitive heme oxygenase gene showed appropriate induction after H2O2 exposure. Thus, W183A appears to be essential for the in vivo activity of FANCA in a manner independent of its interaction with FANCG. Moreover, neither wild-type FANCA nor the W183A mutation appears to alter the peroxide-induced apoptosisor peroxide-sensing ability of FA group A cells.

Cancer predisposition caused by elevated mitotic recombination in Bloom mice

Bloom syndrome is a disorder associated with genomic instability that causes affected people to be prone to cancer. Bloom cell lines show increased sister chromatid exchange, yet are proficient in the repair of various DNA lesions. The underlying cause of this disease are mutations in a gene encoding a RECQ DNA helicase. Using embryonic stem cell technology, we have generated viable Bloom mice that are prone to a wide variety of cancers. Cell lines from these mice show elevations in the rates of mitotic recombination. We demonstrate that the increased rate of loss of heterozygosity (LOH) resulting from mitotic recombination in vivo constitutes the underlying mechanism causing tumour susceptibility in these mice.

Cloning and analysis of the mouse Fanconi anemia group A cDNA and an overlapping penta zinc finger cDNA

Despite the cloning of four disease-associated genes for Fanconi anemia (FA), the molecular pathogenesis of FA remains largely unknown. To study FA complementation group A using the mouse as a model system, we cloned and characterized the mouse homolog of the human FANCA cDNA. The mouse cDNA (Fanca) encodes a 161-kDa protein that shares 65% amino acid sequence identity with human FANCA. Fanca is located at the distal region of mouse chromosome 8 and has a ubiquitous pattern of expression in embryonic and adult tissues. Expression of the mouse cDNA in human FA-A cells restores the cellular drug sensitivity to normal levels. Thus, the expression pattern, protein structure, chromosomal location, and function of FANCA are conserved in the mouse. We also isolated a novel zinc finger protein, Zfp276, which has five C(2)H(2) domains. Interestingly, Zfp276 is situated in the Fanca locus, and the 3'UTR of its cDNA overlaps with the last four exons of Fanca in a tail-to-tail manner. Zfp276 is expressed in the same tissues as Fanca, but does not complement the mitomycin C (MMC)-sensitive phenotype of FA-A cells. The overlapping genomic organization between Zfp276 and Fanca may have relevance to the disease phenotype of FA.

Investigation of Fanconi anemia protein interactions by yeast two-hybrid analysis

Fanconi anemia is a chromosomal breakage disorder with eight complementation groups (A-H), and three genes (FANCA, FANCC, and FANCG) have been identified. Initial investigations of the interaction between FANCA and FANCC, principally by co-immunoprecipitation, have proved controversial. We used the yeast two-hybrid assay to test for interactions of the FANCA, FANCC, and FANCG proteins. No activation of the reporter gene was observed in yeast co-expressing FANCA and FANCC as hybrid proteins, suggesting that FANCA does not directly interact with FANCC. However, a high level of activation was found when FANCA was co-expressed with FANCG, indicating strong, direct interaction between these proteins. Both FANCA and FANCG show weak but consistent interaction with themselves, suggesting that their function may involve dimerisation. The site of interaction of FANCG with FANCA was investigated by analysis of 12 mutant fragments of FANCG. Although both N- and C-terminal fragments did interact, binding to FANCA was drastically reduced, suggesting that more than one region of the FANCG protein is required for proper interaction with FANCA.

Cellular Werner phenotypes in mice expressing a putative dominant-negative human WRN gene

Mutations at the Werner helicase locus (WRN) are responsible for the Werner syndrome (WS). WS patients prematurely develop an aged appearance and various age-related disorders. We have generated transgenic mice expressing human WRN with a putative dominant-negative mutation (K577M-WRN). Primary tail fibroblast cultures from K577M-WRN mice showed three characteristics of WS cells: hypersensitivity to 4-nitroquinoline-1-oxide (4NQO), reduced replicative potential, and reduced expression of the endogenous WRN protein. These data suggest that K577M-WRN mice may provide a novel mouse model for the WS.

Localization of the Bloom syndrome helicase to punctate nuclear structures and the nuclear matrix and regulation during the cell cycle: comparison with the Werner's syndrome helicase

The Bloom (BLM) and Werner's (WRN) syndrome proteins may regulate recombination and DNA repair. Using a novel polyclonal antibody to human BLM, we detected the 170-kda BLM antigen in wild-type but not Bloom syndrome cells. BLM was localized to punctate nuclear structures. The level of BLM but not WRN was 3.6 fold-higher in G(1)/S-synchronized fibroblasts than in G(0)-synchronized fibroblasts. BLM-positive cells invariably expressed topoisomerase IIalpha, whereas topoisomerase IIbeta was expressed constitutively. Transfections of BLM deletion mutants demonstrated that the C-terminal domain of BLM mediated nuclear entry and the central helicase domain was necessary for producing the punctate pattern. By subcellular fractionation, BLM was found primarily in high-salt extracts of the nucleoplasm and the nuclear matrix and was enriched in G(1)/S-synchronized cells compared with G(0)-synchronized cells. There was no interaction between BLM and WRN or topoisomerases IIalpha and IIbeta in fibroblasts. These results demonstrate that BLM is targeted to specific nuclear structures and that its expression is enhanced during cell growth. The known nucleolar localization of WRN, its invariant expression during the cell cycle, and the lack of interaction between BLM and WRN suggest distinct roles for BLM and WRN in processes such as DNA repair and recombination.