ABBV-319: A CD19-targeting glucocorticoid receptor modulator antibody-drug conjugate therapy for B-cell malignancies

Glucocorticoids are key components of the current standard-of-care regimens (e.g., R-CHOP, EPOCH-R, Hyper-CVAD) for treatment of B-cell malignancy. However, systemic glucocorticoid treatment is associated with several adverse events. CD19 displays restricted expression in normal B-cells and is up-regulated in B-cell malignancies. ABBV-319 is a CD19-targeting antibody-drug conjugate (ADC) engineered to reduce glucocorticoid-associated toxicities while possessing three distinct mechanisms of action (MOA) to increase therapeutic efficacy: (1) antibody-mediated delivery of glucocorticoid receptor modulator (GRM) payload to activate apoptosis, (2) inhibition of CD19 signaling, and (3) enhanced Fc-mediated effector function via afucosylation of the antibody backbone. ABBV-319 elicited potent GRM-driven anti-tumor activity against multiple malignant B-cell lines in vitro as well as in cell line-derived xenografts (CDXs) and patient-derived xenografts (PDXs) in vivo. Remarkably, a single-dose of ABBV-319 induced sustained tumor regression and enhanced anti-tumor activity compared to repeat dosing of systemic prednisolone at the maximum tolerated dose (MTD) in mice. The unconjugated CD19 monoclonal antibody (mAb) also displayed anti-proliferative activity on a subset of B-cell lymphoma cell lines through the inhibition of PI3K signaling. Moreover, afucosylation of the CD19 mAb enhanced Fc-mediated antibody-dependent cellular cytotoxicity (ADCC), and this activity was maintained after conjugation with GRM payloads. Notably, ABBV-319 displayed superior efficacy compared to afucosylated CD19 mAb in human CD34+ PBMC-engrafted NSG-tg(Hu-IL15) transgenic mice, demonstrating enhanced anti-tumor activity when multiple MOAs are enabled. ABBV-319 also showed durable anti-tumor activity across multiple B-cell lymphoma PDX models, including non-germinal center B-cell (GCB) DLBCL and relapsed lymphoma post R-CHOP treatment. Collectively, these data support the ongoing evaluation of ABBV-319 in Phase I clinical trial (NCT05512390).

Activity of eftozanermin alfa plus venetoclax in preclinical models and patients with acute myeloid leukemia

Activation of apoptosis in malignant cells is an established strategy for controlling cancer and is potentially curative. To assess the impact of concurrently inducing the extrinsic and intrinsic apoptosis-signaling pathways in acute myeloid leukemia (AML), we evaluated activity of the TRAIL receptor agonistic fusion protein eftozanermin alfa (eftoza; ABBV-621) in combination with the B-cell lymphoma protein-2 selective inhibitor venetoclax in preclinical models and human patients. Simultaneously stimulating intrinsic and extrinsic apoptosis-signaling pathways with venetoclax and eftoza, respectively, enhanced their activities in AML cell lines and patient-derived ex vivo/in vivo models. Eftoza activity alone or plus venetoclax required death receptor 4/5 (DR4/DR5) expression on the plasma membrane but was independent of TP53 or FLT3-ITD status. The safety/tolerability of eftoza as monotherapy and in combination with venetoclax was demonstrated in patients with relapsed/refractory AML in a phase 1 clinical trial. Treatment-related adverse events were reported in 2 of 4 (50%) patients treated with eftoza monotherapy and 18 of 23 (78%) treated with eftoza plus venetoclax. An overall response rate of 30% (7/23; 4 complete responses [CRs], 2 CRs with incomplete hematologic recovery, and 1 morphologic leukemia-free state) was reported in patients who received treatment with eftoza plus venetoclax and 67% (4/6) in patients with myoblasts positive for DR4/DR5 expression; no tumor responses were observed with eftoza monotherapy. These data indicate that combination therapy with eftoza plus venetoclax to simultaneously activate the extrinsic and intrinsic apoptosis-signaling pathways may improve clinical benefit compared with venetoclax monotherapy in relapsed/refractory AML with an acceptable toxicity profile. This trial was registered at www.clinicaltrials.gov as #NCT03082209.

Abstract 2530: DELE1 loss and dysfunctional integrated stress signaling in TP53 mutated AML is a novel pathway for venetoclax resistance

Background: Venetoclax (ven) in combination with hypomethylating agents or low dose cytarabine leads to rapid and durable remission in patients (pts) with acute myeloid leukemia (AML) unfit for intensive chemotherapy (IC), however, pts with TP53 mutations (TP53mut) exhibit adverse prognosis. Here, we identify dysregulation of the integrated stress response (ISR) pathway in pts with TP53mut, specifically through the actions of DAP3 binding cell death enhancer 1 (DELE1) and its activating protease OMA1.

Methods: RNA sequencing was performed on pre-treatment bone marrow-derived mononuclear cells (BMMCs) from pts with AML ineligible for IC across four clinical trials (NCT02993523, NCT02203773, NCT03069352, and NCT02287233). CRISPR-Cas9 editing of TP53-intact AML cell lines was used to generate TP53-/-, TP53mut, DELE1-/-, OMA1-/- and TP53/DELE1 double deficient cell lines. Quantification of BCL2 and Myeloid cell leukemia 1 (MCL1) complexes with BCL2 interacting mediator (BIM) was performed using chemiluminescence assays. A drug screen to assess viability and gene expression was performed in cells treated with ven in combination with 63 drugs.

Results: In total 401 pts were included in the analyses, of which 17% harbored TP53mut and 83% were TP53wt. Analysis of genes differentially expressed (DE) between pts with TP53mut and TP53wt AML revealed 19 DE genes shared across all trials. DELE1, an ISR adaptor, was associated with genes mapping to the ISR-related eukaryotic initiation factor-α (eIF2α) in TP53mut AML BMMCs. Deletion of DELE1 or OMA1 in AML cell lines blocked eIF2α activation and induction of the transcription factor ATF4, a critical ISR effector, in response to the mitochondrial stressor FCCP, ven, and azacitidine, and resulted in ven resistance similar to that of TP53 deficient cells. All modified AML cell lines exhibited a concomitant decrease in the pro-apoptotic regulator PMAIP1, encoding NOXA, and a 14-18-fold increase in MCL1-BIM complexes following ven treatment, as well as increased MCL1 expression, compared to parental lines. Further screening of these cell lines for ven sensitizing activity revealed the BH3 mimetic S63845, which targets MCL1, as the top hit, suggesting that combined BH3 mimetics may overcome ven resistance during ISR pathway defects. As DELE1 is located on chromosome 5 (CH); the frequent loss of CH 5q among TP53mut AML may contribute to low DELE1 expression.

Conclusions: These data suggest that defective ISR signaling may be a factor in TP53mut AML treatment outcome and point to DELE1 dysregulation as a driver of ISR inactivation in pts with TP53mut AML. The ISR induces the expression of NOXA, which displaces MCL1 from BIM and lowers the apoptotic threshold. Our data identify p53, DELE1, and OMA1 as regulators of NOXA expression post ven treatment and indicate that co-targeting MCL1 in pts with TP53mut AML may be beneficial to overcome ven resistance.

Citation Format: David Sharon, Paul Jung, Yan Sun, Weiguo Feng, Ziping Yang, Valerie Robinson, Diya Mitra, Wei Liu, Pingping Zheng, Tamar Uziel, Lloyd Lam, Mark D. Minden, Jeremy Ross, Wellington Mendes, Jalaja Potluri, Andrew H. Wei, Marina Konopleva, Monique Dail, Brenda Chyla, PK Epling-Burnette. DELE1 loss and dysfunctional integrated stress signaling in TP53 mutated AML is a novel pathway for venetoclax resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2530.

Genomic analysis and prediction of therapeutic vulnerabilities of small cell lung cancer from rovalpituzumab tesirine phase III trial (MERU)

8577

Background: Predicting variable therapeutic responses that are driven by the genetic and transcriptomic heterogeneity of Small Cell Lung Cancer (SCLC) offers an opportunity for implementing precision therapies within the cancer cells and tumor microenvironment (TME). MERU was a Phase III study of Rovalpituzumab Tesirine (Rova-T) as maintenance therapy following first-line platinum-based chemotherapy in participants with extensive stage SCLC. In this study, we comprehensively analyzed the baseline genomic data in the MERU cohort to interrogate SCLC’s heterogeneous TME and tumor-intrinsic molecular and genetic drivers for therapeutic vulnerabilities. Methods: RNA-seq and Whole-Exome-Sequencing (WES) data were collected from archival tumor samples of 306 of 740 subjects enrolled in MERU. RNA-seq reads were aligned with STAR and quantified for gene expression by HTSeq. WES reads were analyzed for somatic mutation and copy number variation using AbbVie’s tumor-only WES pipeline. TME heterogeneity was evaluated using gene-set variation analysis of pan-cancer TME gene signatures. SCLC subtyping was performed using expression of 4 transcriptional factors (TF): ASCL1, NEUROD1, POU2F3, and YAP1. A computational framework of mapping MERU transcriptome expression profile to Cancer Dependency Map (DepMap) dataset was developed to synthetically screen MERU samples’ drug sensitivity. Results: TF subtyping reveals high prevalence of SCLC-A and -N subtypes in the MERU cohort, consistent with high expression of DLL3 in these two neuroendocrine subtypes. Correlation of TME gene signature scores identified two distinct clusters in the MERU cohort with correspondingly polarized immune-suppressive and -inflamed phenotypes. The pro-inflammatory score combining IFN-gamma and TGF-beta signatures, predicted prognosis in the MERU cohort better than the previously reported SCLC-I signature (Hazard Ratio: 0.71 [95% CI 0.38-1.3] vs. 1.29 [95% CI 0.65-2.6]). WES analysis identified high prevalence of TP53 and RB1 mutations, in line with the reported prevalence of these genetic drivers in SCLC. The clinical characteristics including gender, smoking status, and prior treatment, are not significantly associated with either TF or TME subgroup. The computational drug screen framework maps 83% of MERU samples to DepMap SCLC cell lines. The correlation of subtype TF expression with drug sensitivity was highly concordant between MERU and DepMap. Collectively, this approach demonstrated that molecular subtyping can be leveraged to broadly predict drug response in SCLC patients. Conclusions: Our comprehensive genomic analysis of the MERU cohort provides new insights into SCLC heterogeneity from both tumor-intrinsic and tumor-immune interaction perspectives and shall contribute to the development of predictive biomarkers and therapeutic opportunities for SCLC.

Selective Inhibition of the Second Bromodomain of BET Family Proteins Results in Robust Antitumor Activity in Preclinical Models of Acute Myeloid Leukemia

Dual bromodomain BET inhibitors that bind with similar affinities to the first and second bromodomains across BRD2, BRD3, BRD4, and BRDT have displayed modest activity as monotherapy in clinical trials. Thrombocytopenia, closely followed by symptoms characteristic of gastrointestinal toxicity, have presented as dose-limiting adverse events that may have prevented escalation to higher dose levels required for more robust efficacy. ABBV-744 is a highly selective inhibitor for the second bromodomain of the four BET family proteins. In contrast to the broad antiproliferative activities observed with dual bromodomain BET inhibitors, ABBV-744 displayed significant antiproliferative activities largely although not exclusively in cancer cell lines derived from acute myeloid leukemia and androgen receptor positive prostate cancer. Studies in acute myeloid leukemia xenograft models demonstrated antitumor efficacy for ABBV-744 that was comparable with the pan-BET inhibitor ABBV-075 but with an improved therapeutic index. Enhanced antitumor efficacy was also observed with the combination of ABBV-744 and the BCL-2 inhibitor, venetoclax compared with monotherapies of either agent alone. These results collectively support the clinical evaluation of ABBV-744 in AML (Clinical Trials.gov identifier: NCT03360006).

Venetoclax Increases Intratumoral Effector T Cells and Antitumor Efficacy in Combination with Immune Checkpoint Blockade

The antiapoptotic protein BCL2 plays critical roles in regulating lymphocyte development and immune responses, and has also been implicated in tumorigenesis and tumor survival. However, it is unknown whether BCL2 is critical for antitumor immune responses. We evaluated whether venetoclax, a selective small-molecule inhibitor of BCL2, would influence the antitumor activity of immune checkpoint inhibitors (ICI). We demonstrate in mouse syngeneic tumor models that venetoclax can augment the antitumor efficacy of ICIs accompanied by the increase of PD-1+ T effector memory cells. Venetoclax did not impair human T-cell function in response to antigen stimuli in vitro and did not antagonize T-cell activation induced by anti-PD-1. Furthermore, we demonstrate that the antiapoptotic family member BCL-X_L provides a survival advantage in effector T cells following inhibition of BCL2. Taken together, these data provide evidence that venetoclax should be further explored in combination with ICIs for cancer therapy. SIGNIFICANCE: The antiapoptotic oncoprotein BCL2 plays critical roles in tumorigenesis, tumor survival, lymphocyte development, and immune system regulation. Here we demonstrate that venetoclax, the first FDA/European Medicines Agency-approved BCL2 inhibitor, unexpectedly can be combined preclinically with immune checkpoint inhibitors to enhance anticancer immunotherapy, warranting clinical evaluation of these combinations.This article is highlighted in the In This Issue feature, p. 1.

Abstract 3831: Selective targeting BET family BDII bromodomain with ABBV-744 and BCL-2 with venetoclax (ABT-199) is synergistic in primary acute myeloid leukemia models

Despite advances in understanding of the biology of acute myeloid leukemia (AML), cure remains elusive for the majority of patients. ABT-199 (Venetoclax) is a small-molecule BH3 mimetic that selectively inhibits BCL-2 causing cell death. ABBV-744 is a highly selective inhibitor for the BDII of BET family proteins, exhibiting greater than 300-fold more potent binding affinity to the BDII bromodomain of BRD4 relative to BDI (Warren Kati AACR 2018; Xiaoyu Lin AACR 2018). In this study, we evaluated the anti-leukemia efficacy of the concomitant BCL-2 blockade by venetoclax and of BDII inhibition with ABBV-744 in primary AML samples.

First, anti-leukemia activity of venetoclax and ABBV-744 was examined in 12 primary AML samples with diverse genomic alterations. The combination significantly enhanced cell death (61.4% ± 8.7%), as compared to the single agent treatment (51.4% ± 9.3% in venetoclax 10 nM group and 22.2% ± 3.4% in ABBV-744 50 nM group, p<0.001). ABBV-744 inhibited cell proliferation in the majority of AML cases (31.7% ± 8.2 %), and the cell growth suppression was more profound in the combination group (82.9% ± 6.9%, p<0.001). Most importantly, three of 12 patients were resistant to venetoclax, but two of these were sensitive to ABBV-744 or ABBV-744/venetoclax combination. We next performed the whole genome transcriptome analysis of pre-treatment AML cells by RNA-sequencing (RNA-seq). The samples which were sensitive to venetoclax and to the combination with ABBV-744 were characterized by high levels of BCL2 and mid-low level of MCL1 expression. In addition, low mRNA expression of AR, IL1R1 expression and high CCND1 expression correlated with response of primary AML cells to the combination of venetoclax and ABBV-744.

To test the efficacy of this regimen in vivo, we established a patient-derived xenograft (PDX) from an AML patient in NSG mice. After 21 days of therapy, flow cytometry data demonstrated significantly reduced leukemia burden in venetoclax treated group (9.5% ± 1.7%) but not in ABBV-744 group (22.3% ± 5.8%) compared to controls (30.8% ± 3.9%), with lowest tumor burden in the combination group (5.0% ± 0.8%, p<0.01). No significant impact on mice’ weight was noted, and no clinical signs of toxicity recorded over the course of therapy. The experiment is ongoing, and the survival analysis will be reported.

Next, the anti-leukemia efficacy of ABBV-744 was tested in 7 additional AML PDX models. In all of the 7 models, Combination of ABBV-744 and venetoclax treatment delayed AML progression compared to untreated mice (survival days: 141 vs 105, 275 vs 153, 62 vs 46, 136 vs 119, 138 vs 77, 129 vs 116, 94 vs 86).

In summary, combinatorial blockade of BDII bromodomain and of BCL-2 anti-apoptotic pathway facilitates apoptotic cell death and suppresses proliferation in the majority of primary AML cells and produces anti-AML activity in AML PDX models in vivo.

Citation Format: Tianyu Cai, Vinitha Kuruvilla, Xiaoyu Lin, Tamar Uziel, Xin Lu, Lu Zhang, Qi Zhang, Lina Han, Antonio Cavazos, Yu Shen, Marina Konopleva. Selective targeting BET family BDII bromodomain with ABBV-744 and BCL-2 with venetoclax (ABT-199) is synergistic in primary acute myeloid leukemia models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3831.

Targeting Lineage-specific MITF Pathway in Human Melanoma Cell Lines by A-485, the Selective Small-molecule Inhibitor of p300/CBP

Metastatic melanoma is responsible for approximately 80% of deaths from skin cancer. Microphthalmia-associated transcription factor (MITF) is a melanocyte-specific transcription factor that plays an important role in the differentiation, proliferation, and survival of melanocytes as well as in melanoma oncogenesis. MITF is amplified in approximately 15% of patients with metastatic melanoma. However, no small-molecule inhibitors of MITF currently exist. MITF was shown to associate with p300/CBP, members of the KAT3 family of histone acetyltransferase. p300 and CREB-binding protein (p300/CBP) regulate a wide range of cellular events such as senescence, apoptosis, cell cycle, DNA damage response, and cellular differentiation. p300/CBP act as transcriptional coactivators for multiple proteins in cancers, including oncogenic transcription factors such as MITF. In this study, we showed that our novel p300/CBP catalytic inhibitor, A-485, induces senescence in multiple melanoma cell lines, similar to silencing expression of EP300 (encodes p300) or MITF We did not observe apoptosis and increase invasiveness upon A-485 treatment. A-485 regulates the expression of MITF and its downstream signature genes in melanoma cell lines undergoing senescence. In addition, expression and copy number of MITF is significantly higher in melanoma cell lines that undergo A-485-induced senescence than resistant cell lines. Finally, we showed that A-485 inhibits histone-H3 acetylation but did not displace p300 at promoters of MITF and its putative downstream genes. Taken together, we provide evidence that p300/CBP inhibition suppressed the melanoma-driven transcription factor, MITF, and could be further exploited as a potential therapy for treating melanoma.

Abstract 800: ABBV-744, a first-in-class and highly selective inhibitor of the second bromodomain of BET family proteins, displays robust activities in preclinical models of acute myelogenous leukemia

Many small-molecule inhibitors that target both bromodomains of the BET family proteins (pan BET inhibitors) are undergoing studies in clinical trials. Emerging data are beginning to suggest that clinical responses to these pan BET inhibitors in subsets of hematologic malignancies may be modest and short lived, perhaps due, at least in part, to tolerability issues that limit dosing levels. We hypothesized that selective inhibition of four of the eight bromodomains in BET family proteins might retain the anticancer activities in certain tumor subsets while alleviating some of the tolerability liabilities of pan BET inhibitors, thus possibly providing better therapeutic benefits. ABBV-744 is a highly selective inhibitor for the second bromodomain (BDII) of the four BET family proteins, exhibiting greater than 300-fold more potent binding affinity to the BDII bromodomain of BRD4 relative to the first bromodomain (BDI) of BRD4. In contrast to the broad antiproliferative activities observed with pan BET inhibitors, ABBV-744 only displayed significant antiproliferative activities in a limited number of cancer cell lines, including AML and androgen receptor (AR)-positive prostate cancer. Studies in AML xenograft models demonstrated antitumor efficacy for ABBV-744 that was comparable to the pan-BET inhibitor ABBV-075 but with improved tolerability. Taken together, these results suggest that ABBV-744 could be a promising second-generation BET inhibitor for AML therapy.

Affiliation: Oncology Discovery, AbbVie Inc., 1 North Waukegan Rd., North Chicago, IL 60064. Disclosures: All authors are employees of AbbVie. The design, study conduct, and financial support for this research were provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the publication.

Citation Format: Xiaoyu Lin, Xiaoli Huang, Richard Bellin, Emily Faivre, Paul Hessler, Lloyd Lam, Mai Ha Bui, Denise Wilcox, Tamar Uziel, Debra C. Ferguson, Terrance J. Magoc, Daniel H. Albert, Keith F. McDaniel, Warren Kati, Yu Shen. ABBV-744, a first-in-class and highly selective inhibitor of the second bromodomain of BET family proteins, displays robust activities in preclinical models of acute myelogenous leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 800.

Abstract 4960: First-in-class, highly BDII-selective BET family inhibitor ABBV-744 displays potent anti-tumor activity in androgen receptor positive prostate cancer models and an improved tolerability profile

First generation BET inhibitors, including ABBV-075, bind with nearly equimolar affinity to two highly conserved bromodomains (BDI and BDII) in the N-terminus of BRD2, BRD3, BRD4, and BRDt. These “pan” inhibitors compete with the natural acetylated lysine substrates of BDI and BDII to displace BET family proteins from chromatin. In so doing, pan-BET inhibitors generally block transcription and induce potent anti-tumor activity across a wide range of pre-clinical models. Consequently, numerous phase I clinical trials have been initiated with pan-BET inhibitors. The initial reports of responses in the leading indication of AML have been encouraging; however, thrombocytopenia and other dose limiting toxicities may prevent realization of a therapeutic dose in most solid tumor indications. We hypothesized that selective inhibition of BDII but not BDI of BET family proteins would improve tolerability while retaining efficacy. Medicinal chemistry efforts produced ABBV-744, a potent inhibitor specific for BDII of BRD2/3/4, with >250x differential binding preference for BDII over BDI and excellent drug-like properties. In striking contrast to the broad range of cell growth inhibition effected by pan BET inhibitor ABBV-075, BDII-selective ABBV-744 anti-proliferative activity was largely but not exclusively restricted to AML and androgen receptor (AR) positive prostate cancer cell line models, including models of Enzalutamide resistance. RNA-Seq and qPCR revealed that ABBV-744 was a potent and selective inhibitor of the AR transcription pathway. ChIP-Seq and ChIP-qPCR revealed a BDII-inhibitor displacement of BRD4 from AR-occupied enhancers and promoters, consistent with a BRD4 BDII-specific dependency of AR to sustain an oncogenic gene expression program. In vivo, doses of ABBV-744 at fractions of its MTD in LNCaP and MDA-PCa-2b xenograft models induced tumor growth inhibition that was comparable to that observed with ABBV-075 when dosed at its MTD. Together, these in vitro and in vivo results provide proof of concept that selective BDII BET family inhibitors may have improved tolerability relative to pan-BET inhibitors while maintaining tumor growth inhibition in AR positive prostate cancer. The design, study conduct, and financial support for this research were provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the publication.

Citation Format: Emily J. Faivre, Denise Wilcox, Mai Ha-Bui, Paul Hessler, Vasudha Sehgal, Xin Lu, Tamar Uziel, Gaurav Mehta, Daniel H. Albert, Keith McDaniel, Warren Kati, Yu Shen. First-in-class, highly BDII-selective BET family inhibitor ABBV-744 displays potent anti-tumor activity in androgen receptor positive prostate cancer models and an improved tolerability profile [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4960.

Author Correction: Discovery of a selective catalytic p300/CBP inhibitor that targets lineage-specific tumours

In the originally published version of this Letter, the authors Arthur F. Kluge, Michael A. Patane and Ce Wang were inadvertently omitted from the author list. Their affiliations are: I-to-D, Inc., PO Box 6177, Lincoln, Massachusetts 01773, USA (A.F.K.); Mitobridge, Inc. 1030 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (M.A.P.); and China Novartis Institutes for BioMedical Research, No. 4218 Jinke Road, Zhangjiang Hi-Tech Park, Pudong District, Shanghai 201203, China (C.W.). These authors contributed to the interpretation of results and design of compounds. In addition, author 'Edward A. Kesicki' was misspelled as 'Ed Kesicki'. These errors have been corrected online.

Abstract LB-A23: Discovery of a potent catalytic p300/CBP inhibitor that targets lineage-specific tumors

The dynamic and reversible acetylation of proteins catalyzed by histone acetyltransferases (HATs) and histone deacetylases (HDACs) is a major epigenetic regulatory mechanism of gene transcription associated with multiple diseases. While HDAC inhibitors are approved to treat certain cancers, progress on the development of drug-like HAT inhibitors has lagged. The HAT paralogs p300 and CBP (p300/CBP) are key transcriptional co-activators essential for a multitude of cellular processes and also implicated in human pathological conditions, including cancer. Current p300/CBP HAT domain inhibitors including natural products, bi-substrate analogs (Lys-CoA) and the widely utilized C646 lack potency or selectivity. Here, we describe A-485, a potent, selective and drug-like p300/CBP catalytic inhibitor. We show the first high resolution (1.95Å) co-crystal structure of a small molecule bound to the catalytic active site of p300 and demonstrate that A-485 is acetyl-CoA competitive. A-485 selectively inhibited proliferation across lineage-specific tumor types, including several hematological malignancies and androgen receptor-positive prostate cancer. A-485 inhibited the androgen receptor transcriptional program in both androgen sensitive and castrate resistant prostate cancer and inhibited tumor growth in a castration resistant xenograft model. These results demonstrate the feasibility of selectively targeting the catalytic activity of histone acetyltransferases.

Citation Format: Kenneth D. Bromberg, Loren M. Lasko, Clarissa G. Jakob, Wei Qiu, Debra Montgomery, Enrico L. Digiammarino, Todd M. Hansen, Roberto M. Risi, Robin R. Frey, Vlasios Manaves, Bailin Shaw, Mikkel Algire, Paul Hessler, Lloyd T. Lam, Tamar Uziel, Emily Favire, Debra Ferguson, Fritz G. Buchanan, Ruth L. Martin, Maricel Torrent, Saul H. Rosenberg, Michael R. Michaelides, Albert Lai. Discovery of a potent catalytic p300/CBP inhibitor that targets lineage-specific tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-A23.

Abstract 3812: JAK2 and PTPN11 mutations as potential biomarkers for BCL-xL inhibition as monotherapy and in combination therapy for acute myeloid leukemia

Acute Myeloid Leukemia (AML) is a heterogeneous disease where many distinct functional mutations have been identified. While mutations in FLT3, NRAS, NPM1, and IDH genes have been well characterized in de novo AML, a subset of AML associated with leukemic transformation of chronic myeloproliferative neoplasms (MPNs) are enriched in mutated JAK2 and PTPN11. MPNs are a set of disorders characterized by the chronic and abnormal overproduction of blood cells which can ultimately progress to AML. Targeted therapy against mutated JAK2 and PTPN11 with small molecule inhibitors against the JAK-STAT and RAS-ERK pathways, respectively, are being actively investigated in AML. However, acquired resistance and non-durable responses are already being demonstrated in monotherapy treatment. Thus, AML that has arisen from leukemic transformation of MPNs remains an area of unmet medical need. This need is underscored by the fact that these patients suffer from poor outcomes and low response rates to standard chemotherapy. Together, these results indicate the need for rational combinations in this population.

In this study, we sought to better define the dependencies of AML cells on BCL-2 family members by screening AML cell lines with selective small molecule inhibitors to the BCL-2 family members. The screen revealed that the majority of AML cell lines depend on BCL-2, or BCL-2 together with MCL1 for survival. Intriguingly, a subset of AML cell lines is exquisitely sensitive to BCL-xL inhibition. Treating these cell lines with BCL-xL-selective inhibitor or with navitoclax, a dual BCL2/BCL-xL inhibitor induces apoptosis. These BCL-xL inhibitor-sensitive cell lines express high levels of BCL-xL, but not BCL-2 and MCL1. Genomic analysis of BCL-xL inhibitor-sensitive AML cell lines revealed that this subset is also enriched in JAK2V16F and PTPN11 mutations. We further demonstrate positive combination effect between BCL-xL inhibitor or navitoclax with JAK inhibitor (ruxolitinb) and two MEK inhibitors (trametinib and AZD6244) in cell lines with JAK2V16F and PTPN11 mutations, respectively. Together, these results demonstrate the potential utility of a BCL-xL inhibition as a combination partner in JAK2 and PTPN11 mutated cancers. Importantly, JAK2 and PTPN11 mutations in addition to expression of BCL-xL, BCL-2, and MCL1 could also be further tested as potential biomarkers for the utility of BCL-xL inhibition based treatments for patients with these mutations.

Citation Format: Richard J. Bellin, Valerie Hilgenberg, Relja Popovic, Tamar Uziel, Lloyd T. Lam. JAK2 and PTPN11 mutations as potential biomarkers for BCL-xL inhibition as monotherapy and in combination therapy for acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3812. doi:10.1158/1538-7445.AM2017-3812

Vulnerability of Small-Cell Lung Cancer to Apoptosis Induced by the Combination of BET Bromodomain Proteins and BCL2 Inhibitors

Ten percent to 15% of all lung cancers are small-cell lung cancer (SCLC). SCLC usually grows and metastasizes before it is diagnosed and relapses rapidly upon treatment. Unfortunately, no new targeted agent has been approved in the past 30 years for patients with SCLC. The BET (bromodomain and extraterminal) proteins bind acetylated histones and recruit protein complexes to promote transcription initiation and elongation. BET proteins have been shown to regulate expression of key genes in oncogenesis, such as MYC, CCND2, and BCL2L1 Here, we demonstrate that approximately 50% of SCLC cell lines are exquisitely sensitive to growth inhibition by the BET inhibitor, ABBV-075. The majority of these SCLC cell lines underwent apoptosis in response to ABBV-075 treatment via induction of caspase-3/7 activity. ABBV-075 enhanced the expression of proapoptotic protein BIM and downregulated antiapoptotic proteins BCL2 and BCLxl to a lesser extent. Furthermore, BET inhibition increased BCL2-BIM complex, thus priming the cells for apoptosis. Indeed, strong synergy was observed both in vitro and in vivo when cotreating the cells with BET inhibitor and the BH3-mimetic, BCL2 inhibitor venetoclax (ABT-199). ABBV-075 interaction with venetoclax positively correlated with BCL2 expression. Taken together, our studies provide a rationale for treating SCLC with BET and BCL2 inhibitors in tumors with high BCL2 protein expression. Mol Cancer Ther; 16(8); 1511-20. ©2017 AACR.

Preclinical Characterization of BET Family Bromodomain Inhibitor ABBV-075 Suggests Combination Therapeutic Strategies

ABBV-075 is a potent and selective BET family bromodomain inhibitor that recently entered phase I clinical trials. Comprehensive preclinical characterization of ABBV-075 demonstrated broad activity across cell lines and tumor models, representing a variety of hematologic malignancies and solid tumor indications. In most cancer cell lines derived from solid tumors, ABBV-075 triggers prominent G₁ cell-cycle arrest without extensive apoptosis. In this study, we show that ABBV-075 efficiently triggers apoptosis in acute myeloid leukemia (AML), non-Hodgkin lymphoma, and multiple myeloma cells. Apoptosis induced by ABBV-075 was mediated in part by modulation of the intrinsic apoptotic pathway, exhibiting synergy with the BCL-2 inhibitor venetoclax in preclinical models of AML. In germinal center diffuse large B-cell lymphoma, BCL-2 levels or venetoclax sensitivity predicted the apoptotic response to ABBV-075 treatment. In vivo combination studies uncovered surprising benefits of low doses of ABBV-075 coupled with bortezomib and azacitidine treatment, despite the lack of in vitro synergy between ABBV-075 and these agents. The in vitro/in vivo activities of ABBV-075 described here may serve as a useful reference to guide the development of ABBV-075 and other BET family inhibitors for cancer therapy. Cancer Res; 77(11); 2976-89. ©2017 AACR.

HEXIM1 as a Robust Pharmacodynamic Marker for Monitoring Target Engagement of BET Family Bromodomain Inhibitors in Tumors and Surrogate Tissues

An increasing number of BET family protein inhibitors have recently entered clinical trials. It has been reported that attempts of monitoring target engagement of the BET bromodomain inhibitor OTX015 using literature-described putative pharmacodynamic markers, such as c-Myc, BRD2, etc., failed to detect pharmacodynamic marker responses in AML patients treated at active dose and those with clinical responses. Here, we report the identification and characterization of HEXIM1 and other genes as robust pharmacodynamic markers for BET inhibitors. Global gene expression profiling studies were carried out using cancer cells and surrogate tissues, such as whole blood and skin, to identify genes that are modulated by BET family proteins. Candidate markers were further characterized for concentration- and time-dependent responses to the BET inhibitor ABBV-075 in vitro and in vivo HEXIM1 was found to be the only gene that exhibited robust and consistent modulation by BET inhibitors across multiple cancer indications and surrogate tissues. Markers such as SERPINI1, ZCCHC24, and ZMYND8 were modulated by ABBV-075 and other BET inhibitors across cancer cell lines and xenograft tumors but not in blood and skin. Significant downregulation of c-Myc, a well-publicized target of BET inhibitors, was largely restricted to hematologic cancer cell lines. Incorporating well-characterized pharmacodynamic markers, such as HEXIM1 and other genes described here, can provide a better understanding of potential efficacy and toxicity associated with inhibiting BET family proteins and informs early clinical decisions on BET inhibitor development programs. Mol Cancer Ther; 16(2); 388-96. ©2016 AACR.

Exploitation of Castration-Resistant Prostate Cancer Transcription Factor Dependencies by the Novel BET Inhibitor ABBV-075

Competitive inhibitors of acetyl-lysine binding to the bromodomains of the BET (bromodomain and extra terminal) family are being developed for the treatment of solid and hematologic malignancies. The function of BET family member BRD4 at enhancers/superenhancers has been shown to sustain signal-dependent or pathogenic gene expression programs. Here, the hypothesis was tested that the transcription factor drivers of castration-resistant prostate cancer (CRPC) clinical progression, including the androgen receptor (AR), are critically dependent on BRD4 and thus represent a sensitive solid tumor indication for the BET inhibitor ABBV-075. DHT-stimulated transcription of AR target genes was inhibited by ABBV-075 without significant effect on AR protein expression. Furthermore, ABBV-075 disrupted DHT-stimulated recruitment of BET family member BRD4 to gene-regulatory regions cooccupied by AR, including the well-established PSA and TMPRSS2 enhancers. Persistent BET inhibition disrupted the composition and function of AR-occupied enhancers as measured by a reduction in AR and H3K27Ac ChIP signal and inhibition of enhancer RNA transcription. ABBV-075 displayed potent antiproliferative activity in multiple models of resistance to second-generation antiandrogens and inhibited the activity of the AR splice variant AR-V7 and ligand-binding domain gain-of-function mutations, F877L and L702H. ABBV-075 was also a potent inhibitor of MYC and the TMPRSS2-ETS fusion protein, important parallel transcription factor drivers of CRPC.

IMPLICATIONS

The ability of BET family inhibitor ABBV-075 to inhibit transcription activation downstream of the initiating events of transcription factors like AR and TMPRSS2:ETS fusion proteins provides a promising therapeutic option for CRPC patients who have developed resistance to second-generation antiandrogens. Mol Cancer Res; 15(1); 35-44. ©2016 AACR.

Abstract 4694: ABBV-075, a novel BET family inhibitor, disrupts critical transcription programs that drive prostate cancer growth to induce potent anti-tumor activity in vitro and in vivo

The novel bromodomain inhibitor, ABBV-075, is being tested in a Phase I study for the treatment of solid tumors. Here, we show that potent inhibition of the BET (bromodomain and extra-terminal) family with ABBV-075 is a highly efficacious therapy in pre-clinical models of prostate cancer. The single-digit to low nanomolar anti-proliferative IC50s and potent in vivo tumor growth inhibition of ABBV-075 is mediated in part via inhibition of androgen receptor (AR)-dependent transcription. Prostate tumor incidence and CRPC clinical progression are driven by aberrant activation of the AR transcription program. Gene expression profiling and qPCR results indicate that ABBV-075 inhibited DHT-stimulated transcription of AR target genes without significant effect on AR protein expression. Further, ABBV-075 disrupted DHT-stimulated recruitment of the BET family member BRD4 to gene regulatory regions co-occupied by AR, including the well-established PSA and TMPRSS2 enhancers. Persistent BET inhibition led to the disassembly of AR occupied enhancers as measured by a reduction in AR and H3K27Ac ChIP signal and additionally downregulated enhancer RNA (eRNA) transcription. ABBV-075 displayed potent anti-proliferative activity in multiple models of resistance to the second generation anti-androgen Enzalutamide, including the F876L, L702H AR ligand binding domain mutations and the AR-V7 splicing variant. In addition to blocking the transcription activation downstream of AR, ABBV-075 is also a potent inhibitor of MYC and the TMPRSS2-ETS fusion proteins. Thus, we propose that ABBV-075 may provide a promising therapeutic option for CRPC patients who have developed resistance to second-generation anti-androgens.

Citation Format: Emily J. Faivre, Denise M. Wilcox, Paul Hessler, Tamar Uziel, Paul Tapang, Terry Magoc, Daniel H. Albert, Guowei Fang, Saul Rosenberg, Keith McDaniel, Keith McDaniel, Warren Kati, Yu Shen. ABBV-075, a novel BET family inhibitor, disrupts critical transcription programs that drive prostate cancer growth to induce potent anti-tumor activity in vitro and in vivo. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4694.

Abstract 4718: ABBV-075, a novel BET family bromodomain inhibitor, represents a promising therapeutic agent for a broad spectrum of cancer indications

Small molecule inhibitors of the bromodomain and extraterminal domain (BET) proteins have emerged as a promising option for cancer therapy. ABBV-075 is a potent and selective BET family bromodomain inhibitor that recently entered Phase 1 clinical trials. It binds bromodomains of BRD2/4/T with similar affinities (Ki of 1-2.2 nM), but exhibits roughly 10-fold weaker potency towards BRD3 (Ki of 12.2 nM). ABBV-075 is highly selective for 18 bromodomain proteins tested (Kd > 1 μM; more than 600-fold selectivity vs. BRD4) and has moderate activity towards CREBBP (Kd = 87 μM; 54-fold selectivity vs. BRD4). ABBV-075 exhibited robust single agent activity in cell viability assays across cancer cell lines derived from solid tumors, leukemia and lymphomas. Further characterization of cancer cell responses to ABBV-075 indicated that ABBV-075 manifested diverse mechanisms of action in different cancer settings. These include 1): disruption of cell cycle control leading to G1 arrest followed by senescence, 2) inhibition of oncogenesis drivers leading to apoptosis and 3) potentially targeting tumor microenvironment to provide additional therapeutic benefit. Consistent with its broad spectrum of activities in vitro, ABBV-075 has comparable or superior efficacies to standard of care agents in flank xenograft mouse models of non-small-cell and small cell lung cancers, pancreatic, breast, prostate, head & neck cancers, multiple myeloma, diffuse large B cell lymphoma and leukemia. These results support the development of ABBV-075 in diverse hematological malignancies and solid tumor indications.

Citation Format: Aparna Sarthy, Leiming Li, Daniel H. Albert, Xiaoyu Lin, Warder Scott, Emily Faivre, Mai H. Bui, Xiaoli Huang, Denise M. Wilcox, Terry Magoc, Fritz G. Buchanan, Paul Tapang, George S. Sheppard, Le Wang, Steve D. Fidanze, John Pratt, Dachun Liu, Lisa Hasvold, Paul Hessler, Tamar Uziel, Lloyd Lam, Ganesh Rajaraman, Guowei Fang, Steven W. Elmore, Saul H. Rosenberg, Keith McDaniel, Warren Kati, Yu Shen. ABBV-075, a novel BET family bromodomain inhibitor, represents a promising therapeutic agent for a broad spectrum of cancer indications. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4718.

Abstract 3770: HEXIM1 as a pharmacodynamic marker for monitoring target engagement of ABBV-075

Bromodomain and extra-terminal (BET) family are dual bromodomain-containing proteins that play an important role in transcription regulation. ABBV-075 is a small molecule BET bromodomain inhibitor currently in phase 1 clinical trials. Here we report the identification of Hexim1 and other BET-responsive genes as robust pharmacodynamic markers for monitoring ABBV-075 target engagement in tumors and in surrogate tissues such as whole blood and skin. Transcription profiling of cancer cell lines for their responses to BET inhibitors identified a set of 9 BET-regulated genes, including Hexim1, that responded to BET inhibition across cancer cell lines and in xenograft tumors. Further characterization of BET-responsive genes in surrogate tissues such as mouse skin, mouse/human PBMCs and whole blood revealed that the mRNA level of Hexim1 exhibited the best response to BETi treatment across different settings. A dose-dependent increase of Hexim1 expression was detected in the whole blood of ABBV-075 treated tumor bearing mice, and the Hexim1 response was closely correlated with the plasma drug concentration and largely reflected the anti-tumor efficacy at various dose levels of ABBV-075. Hexim1 is part of a protein/RNA complex that sequesters pTEFb and prevents its recruitment by BRD4. It also reportedly mediates the anti-proliferative activity of BRD4 in AML. Taken together, Hexim1 could serve as a functional relevant pharmacodynamic marker for monitoring ABBV-075 target engagement in animal models and in the clinical setting.

Citation Format: Xiaoli Huang, Xiaoyu Lin, Leiming Li, Rongqi Wang, Lisa Roberts, Paul Hessler, Tamar Uziel, Lloyd Lam, Terry Magoc, Daniel H. Albert, Steven W. Elmore, Guowei Fang, Saul H. Rosenberg, Keith McDaniel, Warren Kati, Yu Shen. HEXIM1 as a pharmacodynamic marker for monitoring target engagement of ABBV-075. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3770.

Abstract 3085: The BET family bromodomain inhibitor ABBV-075 targets multiple pathogenesis factors in multiple myeloma and exhibits robust in vivo efficacies as a single agent and in combination with bortezomib

Small molecule inhibitors of the bromodomain and extraterminal domain (BET) proteins represent a potential new class of cancer therapeutic agents. ABBV-075 is a potent and selective BET family bromodomain inhibitor that was recently advanced to Phase 1 studies. Here we report the in vitro and in vivo characterization of ABBV-075 in preclinical models of multiple myeloma. ABBV-075 exhibited significant anti-proliferative activities and triggered robust apoptosis across many multiple myeloma cell lines, including cell lines that were engineered to become resistant to bortezomib. As reported for other BET inhibitors, ABBV-075 potently inhibited super-enhancer regulated potential cancer drivers such as Myc, IRF8 etc. In addition to directly impacting multiple myeloma cell growth and survival, ABBV-075 also diminished stroma cell derived IL-6 secretion and prevented HUVEC cell proliferation in vitro. Given the importance of angiogenesis and the survival factor IL-6 for multiple myeloma pathogenesis, ABBV-075 could potentially produce significant therapeutic benefits in myeloma by simultaneously targeting multiple critical pathogenesis mechanisms for this disease. ABBV-075 caused significant tumor growth delay in the OPM2 flank tumor model as a monotherapy. Combining ABBV-075 with Bortezomib provided further therapeutic benefit compared with the treatment of either of the two agents individually. Taken together, our results suggest that ABBV-075 could be a promising option to explore for the treatment of multiple myeloma.

Citation Format: Tamar Uziel, Xiaoyu Lin, Emily Faivre, Aparna Sarthy, Daniel H. Albert, Leiming Li, Denise Wilcox, Xiaoli Huang, Terry Magoc, Lloyd Lam, Steven W. Elmore, Keith McDaniel, Warren Kati, Yu Shen. The BET family bromodomain inhibitor ABBV-075 targets multiple pathogenesis factors in multiple myeloma and exhibits robust in vivo efficacies as a single agent and in combination with bortezomib. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3085.

Abstract 4692: Targeting the BET bromodomain proteins and anti-apoptotic protein BCL2 in small cell lung cancer

10% to 15% of all lung cancers are small cell lung cancer (SCLC), named for the size of the cancer cells when seen under a microscope. SCLC often starts in the bronchi near the center of the chest. It usually grows and spreads quickly to distant parts of the body before it is diagnosed. Unfortunately, no new treatment has been identified in the past 30 years for patients with SCLC. The BET (bromodomain and extra-terminal) proteins bind acetylated histones and recruit protein complexes to promote transcription initiation and elongation. In hematologic cancers, BET proteins have been shown to regulate expression of MYC and other oncogenic transcription factors that drive disease pathology. Pharmacologic inhibition of BET binding to acetylated proteins has been shown to inhibit tumor growth in MYC-dependent cancers, such as acute myeloid leukemia, multiple myeloma and neuroblastoma. Here, we demonstrate that ∼40% of SCLC cell lines are exquisitely sensitive to growth inhibition by the BET inhibitor ABBV-075. Whereas most SCLC cell lines undergo cell cycle arrest with ABBV-075 treatment, a few cell lines undergo apoptosis. Sensitivity does not correlate with MYC status (amplification/expression). However, downregulation of anti-apoptotic protein BCLxL and BCL2 by ABBV-075 was observed in sensitive SCLC cells. Synergy was observed when co-treating BET inhibitor and BCL2 inhibitors, venetoclax (ABT-199) or navitoclax (ABT-263), and it positively correlated with BCL2 expression. Thus, high BCL2 protein expression could potentially be a biomarker predictive of these therapeutic combinations. The potential higher synergy with venetoclax may be explained by downregulation of BCLxL with BET inhibition and potent activity of venetoclax against BCL2. Our studies have provided evidence for treating SCLC by BET inhibition and/or combination with BCL2 inhibitors and a rationale for the clinical development of BET inhibitors in this disease with high unmet medical need.

Disclosures: All authors are employees of AbbVie. The design, study conduct, and financial support for this research were provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the publication.

Citation Format: Lloyd T. Lam, Xiaoyu Lin, Emily Faivre, Ziping Yang, Xiaoli Huang, Ricky Bellin, Xin Lu, Yu Shen, Tamar Uziel. Targeting the BET bromodomain proteins and anti-apoptotic protein BCL2 in small cell lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4692.

Abstract 420: Translational strategy for targeting MCL1 amplified tumors with CDK9 inhibitor

BCL-2, BCL-XL, and MCL-1 are members of the prosurvival family of proteins that regulate the mitochondrial apoptotic pathway. These proteins are often amplified or overexpressed in multiple tumor types. Previously, we showed that non-small cell lung cancer (NSCLC) cell lines with low BCL-XL expression (high MCL-1 / BCL-XL ratio) are MCL-1-dependent. TCGA data shows MCL1 amplification is one of the frequent genetic events in NSCLC adenocarcinoma (20%) and breast cancer (15%), while BCL2 is frequently amplified in activated B-cell like diffuse large B cell lymphoma (11%) and BCL2L1 in colorectal cancer (15%). We hypothesize that cancer cell lines with MCL1 amplification and/or high MCL-1 / BCL-XL ratio depend on MCL-1 for survival and are sensitive to inhibition of CDK9, a component of the transcriptional elongation complex that regulates MCL-1 expression. In this study, we demonstrate that NSCLC, triple negative breast cancer (TNBC) and ovarian cell lines with amplified MCL1 or low BCL-XL expression (high MCL-1 / BCL-XL expression ratio) are MCL-1-dependent, and are sensitive to dinaciclib, a CDK9 inhibitor. Cell lines with high BCL-XL expression could be re-sensitized to dinaciclib when co-treated with BCL2/BCL-XL inhibitor navitoclax (ABT-263) or BCL-XL-selective inhibitor A-1155463, suggesting BCL-XL as a resistance factor. Indeed, exogenous expression of BCL-XL rescues sensitive cell lines from dinaciclib. As reported in TCGA, ovarian, NSCLC and TNBC patients with MCL1 amplification also have high MCL-1 expression, and we hypothesized that these patients to be sensitive to CDK9 inhibitors. We developed a fluorescence in situ hybridization (FISH) assay to detect MCL1 amplification which can potentially select patients that may benefit from dinaciclib treatment. We show that fifteen percent of NSCLC patients have high MCL1 amplification, similar to published data. In addition, we have identified other predictive biomarkers associated with sensitivity in TNBC cell lines from gene expression analysis. In conclusion, we have developed a translational strategy for identifying MCL-1-dependent cancers that may be sensitive to CDK9 inhibitors in the clinic.

Disclosures:

All authors are employees of AbbVie. The design, study conduct, and financial support for this research were provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the publication

Citation Format: Ziping Yang, Ricky Bellin, Paul Hessler, Xin Lu, Tamar Uziel, Lloyd T. Lam. Translational strategy for targeting MCL1 amplified tumors with CDK9 inhibitor. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 420.

Abstract 4706: ABBV-075 exhibits robust in vitro and in vivo activities against the ABC and GCB subtypes of DLBCL

Diffuse Large B-Cell Lymphoma (DLBCL) consists of activated B-cell-like (ABC), germinal center B cell-like (GCB), and primary mediastinal B cell lymphoma (PMBL) subtypes. Among these, the ABC subtype of DLBCL often harbors mutations that cause abnormal NF-κB activation, which subsequently activates genes important for cell survival and disease progression in ABC DLBCL. The bromodomain and extraterminal domain (BET) protein BRD4 binds acetylated NF-κB and regulates NF-κB dependent transcription. Accordingly, targeting BET family proteins using small molecule inhibitors such as ABBV-075 might provide therapeutic benefit in ABC DLBCL by blocking the NF-κB pathway. In this study, we examined the activity of ABBV-075 in both the ABC and GCB subtypes of DLBCL. As expected, ABBV-075 diminished NF-κB reporter activity, down-regulated NF-κB target genes, inhibited proliferation and survival of ABC DLBCL cells in vitro, and delayed the growth of ABC DLBCL tumors in vivo. Interestingly, ABBV-075 also exhibited robust anti-proliferative activities in GCB DLBCL cells in vitro and inhibited the growth of GCB DLBCL tumors in vivo. Further characterization of the responses of GCB DLBCL cells to ABBV-075 indicated that ABBV-075 induced strong apoptosis in GCB DLBCL cells that are resistant to the Bcl-2 inhibitor venetoclax (ABT-199), but rarely in venetoclax-sensitive GCB DLBCL cell lines. Our results indicate that ABBV-075 could be active against both the ABC and GCB subtypes of DLBCL. The complementary activity of ABBV-075 and ABT-199 further suggests that ABBV-075 maybe an interesting option for ABT-199 resistant/refractory DLBCL patients.

Citation Format: Xiaoyu Lin, Xiaoli Huang, Aparna Sarthy, Terry Magoc, Daniel Albert, Lloyd Lam, Tamar Uziel, Xin Lu, Mai-Ha Bui, Emily Faivre, Denise Wilcox, Steven Elmore, Keith McDaniel, Warren Kati, Yu Shen. ABBV-075 exhibits robust in vitro and in vivo activities against the ABC and GCB subtypes of DLBCL. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4706.

Abstract 4738: The BET family bromodomain inhibitor ABBV-075 is a promising therapeutic agent for acute myeloid leukemia and myelodysplastic syndrome

Small molecule inhibitors of the bromodomain and extraterminal domain (BET) proteins exhibit interesting activities in preclinical models of various cancer indications, and several of these inhibitors are currently under clinical investigation. ABBV-075 is a potent and selective BET family bromodomain inhibitor that recently entered Phase 1 studies. Upon characterizing cellular responses to BET inhibitors across a large panel of cancer cell lines, we identified AML/MDS as one of the few cancer indications where BET inhibitors triggered robust apoptosis in cancer cell lines and in patient-derived cancer cells. The induction of apoptosis by BET inhibitors in AML/MDS cells may be partly attributed to their abilities to down regulate Bcl-XL and Bcl-2, and combining BET inhibitors with the Bcl-2 inhibitor venetoclax (ABT-199) resulted in robust cytotoxicity in AML/MDS cells. ABBV-075 exhibited significant antitumor efficacy as a monotherapy in flank xenograft models of AML and MDS. Furthermore, combining low doses of ABBV-075 with the standard of care agent azacitidine in the SKM1 model led to significant tumor regression, and the combination regimen was better tolerated than BETi monotherapy at doses that produced a similar degree of therapeutic benefit. Expression profiling of SKM1 tumors from mice treated with the ABBV-075/azacitidine combination or each of these agents as monotherapies revealed that ABBV-075 and azacitidine regulated a common set of biologic pathways. The combination of ABBV-075/azacitidine led to a more robust impact on these common pathways, which may partially contribute to the enhanced efficacy of the ABBV-075/azacitidine combination in the SKM-1 model.

Citation Format: Mai H. Bui, Xiaoyu Lin, Xiaoli Huang, Leiming Li, Aparna Sarthy, Daniel Albert, Terry Magoc, Lloyd Lam, Paul Hessler, Tamar Uziel, Steven Elmore, Keith McDaniel, Warren Kati, Yu Shen. The BET family bromodomain inhibitor ABBV-075 is a promising therapeutic agent for acute myeloid leukemia and myelodysplastic syndrome. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4738.

Abstract 1434: A mouse model of the most aggressive subgroup of human medulloblastoma

Human medulloblastomas are molecularly classified into four major subgroups. Among them, the MYC-subgroup, the most aggressive and the least curable is characterized by amplification or overexpression of the c-MYC gene. While mouse models for the human SHH-subgroup have advanced the development of targeted treatments for this subgroup, the absence of a preclinical model for the MYC-subgroup has severely limited our understanding of its biology and treatment. We have now generated the first mouse model of MYC-subgroup medulloblastoma that faithfully recapitulate the transcriptome, histopathology, and clinical behavior of the human disease. This mouse model was generated by orthotopic transplantation of percoll-purified Myc-transduced cerebellar cells from postnatal day 6-7 Trp53-deficient mice into the cortices or the cerebellum of immunocompromised recipient animals. While >2 x 105 cells from the mouse SHH-tumors are required to induce a tumor, 100 MYC-tumor cells were sufficient, suggesting that most MYC-tumor cells are tumor initiating cells. MYC-tumors can be passaged indefinitely as neurospheres that express markers associated with stem cells, including Prom1, Lgr5, Oct4, Nanog, and Sox2 and induce secondary medulloblastomas with similar characteristics as the primary tumors, after orthotopic transplant into the cortices of recipient mice. This novel medulloblastoma model should significantly advance our efforts to develop new therapeutic modalities and to determine the origin of this deadly childhood cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1434. doi:1538-7445.AM2012-1434

Abstract SY10-03: Cell of origin in pediatric medulloblastomas

Medulloblastoma is the most common malignant brain tumor in children with a median age of 3 to 7 years old. Human medulloblastomas are molecularly classified into four major subgroups including two with constitutive activation of developmental signaling pathways Sonic Hedgehog/Patched defining the SHH-subgroup and Wnt for the WNT-subgroup, subgroup 3 with amplification or overexpression of c-MYC and subgroup 4 with as yet undefined genetic anomalies. To date, all medulloblastomas regardless of their subgroup are treated with the same therapeutic regimen with different outcome. If children with WNT-subgroup tumors fair well, those with subgroup 3 medulloblastoma, the most aggressive form of the disease have a dismal prognosis.

Most mouse models of medulloblastoma developed to date mimic the human SHH-subgroup. However, we recently developed two new mouse models that faithfully recapitulate the human WNT-subgroup and subgroup 3 (MYC). The SHH-subgroup and subgroup 3 medulloblastomas were derived from cerebellar granule progenitors in the external granule layer which sustain mutations in the SHH pathway associated with MYCN overexpression, or C-MYC amplification, respectively. In contrast, the WNT-subgroup medulloblastoma with mutations in β-Catenin originate from the lower rhombic lip, the germinal neuroepithelium surrounding the floor of the 4th ventricle, suggesting that each subgroup may originate from specific neuronal precursors within the developing cerebellum.

Mouse subgroup 3 medulloblastomas, but not SHH- or WNT-subgroup tumors can be passaged indefinitely as neurospheres that express markers characterizing stem cells, including Lgr5, Oct4, Nanog, and Sox2 and induce secondary medulloblastomas with similar characteristics as the primary tumors, after orthotopic transplant into the cortices of recipient mice.

These novel medulloblastoma mouse models should significantly advance our efforts to develop new therapeutic modalities for this deadly childhood cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr SY10-03. doi:1538-7445.AM2012-SY10-03

A mouse model of the most aggressive subgroup of human medulloblastoma

Medulloblastomas that display a large cell/anaplastic morphology and overexpress the cellular c-MYC gene are highly aggressive and carry a very poor prognosis. This so-called MYC-subgroup differs in its histopathology, gene expression profile, and clinical behavior from other forms of medulloblastoma. We generated a mouse model of MYC-subgroup medulloblastoma by transducing Trp53-null cerebellar progenitor cells with Myc. The cardinal features of these mouse medulloblastomas closely mimic those of human MYC-subgroup tumors and significantly differ from mouse models of the Sonic-Hedgehog- and WNT-disease subgroups. This mouse model should significantly accelerate understanding and treatment of the most aggressive form of medulloblastoma and infers distinct roles for MYC and MYCN in tumorigenesis.

Abstract 3444: Enforced expression of MycN and C-Myc induces different medulloblastoma subtypes

Medulloblastoma (MB), a tumor of the cerebellum, is the most common malignant brain tumor of childhood. MBs are classified histologically and molecularly into four distinct major subgroups, two of which correspond to the deregulation of major developmental pathways, Sonic Hedgehog (SHH) and WNT. Among the two other subgroups, one correlates with high levels of C-MYC expression. Using orthotopic transplantation in the cerebellum or cortex of naïve recipient mice, we showed that enforced expression of MycN in cerebellar granule neuron progenitors (CGNPs) isolated from the cerebellum of postnatal (P) day 6-7 Cdkn2c-/-, Trp53-/-, Atoh1-GFP mice induced MBs with a SHH “signature”. Interestingly, overexpression of C-Myc in the same CGNPs induced MBs with Large Cell Anaplastic (LCA) features and mRNA and microRNA profiles distinct from the SHH subgroup. In vitro analysis revealed that tumor cells were resistant to Shh antagonists, such as cyclopamine and BMP4. Ongoing experiments are focusing on understanding the differences between MycN and C-Myc function in CGNPs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3444. doi:10.1158/1538-7445.AM2011-3444

Genetic Alterations in Mouse Medulloblastomas and Generation of TumorsDe novofrom Primary Cerebellar Granule Neuron Precursors

Mice lacking p53 and one or two alleles of the cyclin D-dependent kinase inhibitor p18(Ink4c) are prone to medulloblastoma development. The tumor frequency is increased by exposing postnatal animals to ionizing radiation at a time when their cerebella are developing. In irradiated mice engineered to express a floxed p53 allele and a Nestin-Cre transgene, tumor development can be restricted to the brain. Analysis of these animals indicated that inactivation of one or both Ink4c alleles did not affect the time of medulloblastoma onset but increased tumor invasiveness. All such tumors exhibited complete loss of function of the Patched 1 (Ptc1) gene encoding the receptor for sonic hedgehog, and many exhibited other recurrent genetic alterations, including trisomy of chromosome 6, amplification of N-Myc, modest increases in copy number of the Ccnd1 gene encoding cyclin D1, and other complex chromosomal rearrangements. In contrast, medulloblastomas arising in Ptc1(+/-) mice lacking one or both Ink4c alleles retained p53 function and exhibited only limited genomic instability. Nonetheless, complete inactivation of the wild-type Ptc1 allele was a universal event, and trisomy of chromosome 6 was again frequent. The enforced expression of N-Myc or cyclin D1 in primary cerebellar granule neuron precursors isolated from Ink4c(-/-), p53(-/-) mice enabled the cells to initiate medulloblastomas when injected back into the brains of immunocompromised recipient animals. These "engineered" tumors exhibited gene expression profiles indistinguishable from those of medulloblastomas that arose spontaneously. These results underscore the functional interplay between a network of specific genes that recurrently contribute to medulloblastoma formation.

The CDK Inhibitor p18Ink4c is a Tumor Suppressor in Medulloblastoma

Medulloblastoma (MB) is the most common malignant pediatric brain tumor which is thought to originate from cerebellar granule cell precursors (CGNPs) that fail to properly exit the cell cycle and differentiate. Although mutations in the Sonic Hedgehog (Shh) signaling pathway occur in 30% of cases, genetic alterations that account for MB formation in most patients have not yet been identified. We recently determined that the cyclin D-dependent kinase inhibitor, p18(Ink4c), is expressed as CGNPs exit the cell cycle, suggesting that this protein might play a central role in arresting the proliferation of these cells and in timing their subsequent migration and differentiation. In mice, disruption of Ink4c collaborates independently with loss of p53 or with inactivation of the gene (Ptc1) encoding the Shh receptor, Patched, to induce MB formation. Whereas loss of both Ink4c alleles is required for MB formation in a p53-null background, Ink4c is haplo-insufficient for tumor suppression in a Ptc(1+/-) background. Moreover, MBs derived from Ptc(1+/-) mice that lack one or two Ink4c alleles retain wild-type p53. Methylation of the INK4C (CDKN2C) promoter and complete loss of p18(INK4C) protein expression were detected in a significant fraction of human MBs again pointing toward a role for INK4C in suppression of MB formation.

The tumor suppressors Ink4c and p53 collaborate independently with Patched to suppress medulloblastoma formation

Recurrent genetic alterations in human medulloblastoma (MB) include mutations in the sonic hedgehog (SHH) signaling pathway and TP53 inactivation (approximately 25% and 10% of cases, respectively). However, mouse models of MB, regardless of their initiating lesions, generally depend upon p53 inactivation for rapid onset and high penetrance. The gene encoding the cyclin-dependent kinase inhibitor p18(Ink4c) is transiently expressed in mouse cerebellar granule neuronal precursor cells (GNPs) as they exit the cell division cycle and differentiate. Coinactivation of Ink4c and p53 provided cultured GNPs with an additive proliferative advantage, either in the presence or absence of Shh, and induced MB with low penetrance but with greatly increased incidence following postnatal irradiation. In contrast, mice lacking one or two functional Ink4c alleles and one copy of Patched (Ptc1) encoding the Shh receptor rapidly developed MBs that retained wild-type p53. In tumor cells purified from double heterozygotes, the wild-type Ptc1 allele, but not Ink4c, was inactivated. Therefore, when combined with Ptc1 mutation, Ink4c is haploinsufficient for tumor suppression. Methylation of INK4C (CDKN2C) was observed in four of 23 human MBs, and p18(INK4C) protein expression was extinguished in 14 of 73 cases. Hence, p18(INK4C) loss may contribute to MB formation in children.

Requirement of the MRN complex for ATM activation by DNA damage

The ATM protein kinase is a primary activator of the cellular response to DNA double-strand breaks (DSBs). In response to DSBs, ATM is activated and phosphorylates key players in various branches of the DNA damage response network. ATM deficiency causes the genetic disorder ataxia-telangiectasia (A-T), characterized by cerebellar degeneration, immunodeficiency, radiation sensitivity, chromosomal instability and cancer predisposition. The MRN complex, whose core contains the Mre11, Rad50 and Nbs1 proteins, is involved in the initial processing of DSBs. Hypomorphic mutations in the NBS1 and MRE11 genes lead to two other genomic instability disorders: the Nijmegen breakage syndrome (NBS) and A-T like disease (A-TLD), respectively. The order in which ATM and MRN act in the early phase of the DSB response is unclear. Here we show that functional MRN is required for ATM activation, and consequently for timely activation of ATM-mediated pathways. Collectively, these and previous results assign to components of the MRN complex roles upstream and downstream of ATM in the DNA damage response pathway and explain the clinical resemblance between A-T and A-TLD.

Ataxia-telangiectasia: chronic activation of damage-responsive functions is reduced by alpha-lipoic acid

Cells from patients with the genetic disorder ataxia-telangiectasia (A-T) are hypersensitive to ionizing radiation and radiomimetic agents, both of which generate reactive oxygen species capable of causing oxidative damage to DNA and other macromolecules. We describe in A-T cells constitutive activation of pathways that normally respond to genotoxic stress. Basal levels of p53 and p21(WAF1/CIP1), phosphorylation on serine 15 of p53, and the Tyr15-phosphorylated form of cdc2 are chronically elevated in these cells. Treatment of A-T cells with the antioxidant alpha-lipoic acid significantly reduced the levels of these proteins, pointing to the involvement of reactive oxygen species in their chronic activation. These findings suggest that the absence of functional ATM results in a mild but continuous state of oxidative stress, which could account for several features of the pleiotropic phenotype of A-T.

Ataxia-telangiectasia locus: sequence analysis of 184 kb of human genomic DNA containing the entire ATM gene

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder involving cerebellar degeneration, immunodeficiency, chromosomal instability, radiosensitivity, and cancer predisposition. The genomic organization of the A-T gene, designated ATM, was established recently. To date, more than 100 A-T-associated mutations have been reported in the ATM gene that do not support the existence of one or several mutational hotspots. To allow genotype/phenotype correlations it will be important to find additional ATM mutations. The nature and location of the mutations will also provide insights into the molecular processes that underly the disease. To facilitate the search for ATM mutations and to establish the basis for the identification of transcriptional regulatory elements, we have sequenced and report here 184,490 bp of genomic sequence from the human 11q22-23 chromosomal region containing the entire ATM gene, spanning 146 kb, and 10 kb of the 5'-region of an adjacent gene named E14/NPAT. The latter shares a bidirectional promoter with ATM and is transcribed in the opposite direction. The entire region is transcribed to approximately 85% and translated to 5%. Genome-wide repeats were found to constitute 37.2%, with LINE (17.1%) and Alu (14.6%) being the main repetitive elements. The high representation of LINE repeats is attributable to the presence of three full-length LINE-1s, inserted in the same orientation in introns 18 and 63 as well as downstream of the ATM gene. Homology searches suggest that ATM exon 2 could have derived from a mammalian interspersed repeat (MIR). Promoter recognition algorithms identified divergent promoter elements within the CpG island, which lies between the ATM and E14/NPAT genes, and provide evidence for a putative second ATM promoter located within intron 3, immediately upstream of the first coding exon. The low G+C level (38.1%) of the ATM locus is reflected in a strongly biased codon and amino acid usage of the gene.

Ataxia-telangiectasia: structural diversity of untranslated sequences suggests complex post-transcriptional regulation of ATM gene expression

Mutations in the ATM gene are responsible for the multisystem disorder ataxia-telangiectasia, characterized by neurodegeneration, immune deficiency and cancer predisposition. While no alternative splicing was identified within the coding region, the first four exons of the ATM gene, which fall within the 5'untranslated region (UTR), undergo extensive alternative splicing. We identified 12 different 5'UTRs that show considerable diversity in length and sequence contents. These mRNA leaders, which range from 150 to 884 nucleotides (nt), are expected to form variable secondary structures and contain different numbers of AUG codons. The longest 5'UTR contains a total of 18 AUGs upstream of the translation start site. The 3'UTR of 3590 nt is contained within a single 3'exon. Alternative polyadenylation results in 3'UTRs of varying lengths. These structural features suggest that ATM expression might be subject to complex post-transcriptional regulation, enabling rapid modulation of ATM protein level in response to environmental stimuli or alterations in cellular physiological states.

A human gene (DDX10) encoding a putative DEAD-box RNA helicase at 11q22-q23

A human gene encoding a putative RNA helicase, designated DDX10, was identified 400 kb telomeric to the ataxia-telangiectasia gene at chromosome 11q22-q23. The predicted amino acid sequence shows very high similarity to a subgroup of DEAD-box RNA helicases involved in ribosome biogenesis. This novel gene encodes a 3.2-kb transcript in a variety of human tissues. A processed pseudogene of DDX10 was detected at chromosome 9q21-q22. We observed a rare trinucleotide repeat length polymorphism within the coding sequence of DDX10.

Genomic Organization of the ATM gene

The ATM gene was recently identified and found to be responsible for the genetic disorder ataxiatelgiectasia. The major ATM transcript is 13 kb. Using long-distance PCR, we determined the genomic structure of this gene and identified all of its exon-intron boundaries. The ATM gene spans approximately 150 kb of genomic DNA and consists of 66 exons. The initiation codon falls within exon 4. The last exon is 3.8 kb and contains the stop codon and a 3'-untranslated region of about 3600 nucleotides.

Predominance of null mutations in ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder involving cerebellar degeneration, immunodeficiency, chromosomal instability, radiosensitivity and cancer predisposition. The responsible gene, ATM, was recently identified by positional cloning and found to encode a putative 350 kDa protein with a Pl 3-kinase-like domain, presumably involved in mediating cell cycle arrest in response to radiation-induced DNA damage. The nature and location of A-T mutations should provide insight into the function of the ATM protein and the molecular basis of this pleiotropic disease. Of 44 A-T mutations identified by us to date, 39 (89%) are expected to inactivate the ATM protein by truncating it, by abolishing correct initiation or termination of translation, or by deleting large segments. Additional mutations are four smaller in-frame deletions and insertions, and one substitution of a highly conserved amino acid at the Pl 3-kinase domain. The emerging profile of mutations causing A-T is thus dominated by those expected to completely inactivate the ATM protein. ATM mutations with milder effects may result in phenotypes related, but not identical, to A-T.

A single ataxia telangiectasia gene with a product similar to PI-3 kinase

A gene, ATM, that is mutated in the autosomal recessive disorder ataxia telangiectasia (AT) was identified by positional cloning on chromosome 11q22-23. AT is characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, cancer predisposition, radiation sensitivity, and cell cycle abnormalities. The disease is genetically heterogeneous, with four complementation groups that have been suspected to represent different genes. ATM, which has a transcript of 12 kilobases, was found to be mutated in AT patients from all complementation groups, indicating that it is probably the sole gene responsible for this disorder. A partial ATM complementary DNA clone of 5.9 kilobases encoded a putative protein that is similar to several yeast and mammalian phosphatidylinositol-3' kinases that are involved in mitogenic signal transduction, meiotic recombination, and cell cycle control. The discovery of ATM should enhance understanding of AT and related syndromes and may allow the identification of AT heterozygotes, who are at increased risk of cancer.