Initial testing (stage 1) of tazemetostat (EPZ-6438), a novel EZH2 inhibitor, by the Pediatric Preclinical Testing Program

BACKGROUND

Tazemetostat (EPZ-6438) is a selective inhibitor of the histone methyltransferase EZH2 and currently in clinical development for non-Hodgkin lymphoma and genetically defined tumors.

PROCEDURES

Tazemetostat was tested against the Pediatric Preclinical Testing Program (PPTP) solid tumor xenografts using a dose of 400 mg/kg administered twice daily by oral gavage for 28 days. H3K27me3:H3 ratios were determined in control and treated tumors.

RESULTS

Tazemetostat induced significant differences in event-free survival (EFS) distribution compared with control in nine of 30 (30%) of the xenografts studied. Significant differences in EFS distribution were observed in five of seven (71%) rhabdoid tumor xenograft lines compared with four of 23 (17%) nonrhabdoid xenograft lines (chi-square [χ² ] test P = 0.006). Tazemetostat induced tumor growth inhibition meeting criteria for intermediate and high EFS treated-to-control (T/C) activity in two of 25 (8%) and one of 25 (4%) xenografts, respectively. Intermediate and high activity for the EFS T/C metric was observed exclusively among rhabdoid tumor xenografts (three of five rhabdoid tumor vs 0 of 22 nonrhabdoid tumors (χ² test P < 0.001). One rhabdoid tumor xenograft (G401) showed stable disease. For one rhabdoid tumor (G401), delayed tumor regression to tazemetostat was noted following 1 week of tumor growth. Tazemetostat induced significant reduction of H3K27me3 levels in the majority of tumors compared with controls.

CONCLUSIONS

Tazemetostat demonstrated significant antitumor activity in rhabdoid tumor models but showed no consistent activity against any other histology. Tazemetostat reduced H3K27me3 levels irrespective of tumor response. Further preclinical testing to evaluate tazemetostat in combination with other anticancer agents is warranted.

Nedd8-activating enzyme inhibitor MLN4924 provides synergy with mitomycin C through interactions with ATR, BRCA1/BRCA2, and chromatin dynamics pathways

MLN4924 is an investigational small-molecule inhibitor of the Nedd8-activating enzyme currently in phase I clinical trials. MLN4924 induces DNA damage via rereplication in most cell lines. This distinct mechanism of DNA damage may affect its ability to combine with standard-of-care agents and may affect the clinical development of MLN4924. As such, we studied its interaction with other DNA-damaging agents. Mitomycin C, cisplatin, cytarabine, UV radiation, SN-38, and gemcitabine demonstrated synergy in combination with MLN4924 in vitro. The combination of mitomycin C and MLN4924 was shown to be synergistic in a mouse xenograft model. Importantly, depletion of genes within the ataxia telangiectasia and Rad3 related (ATR) and BRCA1/BRCA2 pathways, chromatin modification, and transcription-coupled repair reduced the synergy between mitomycin C and MLN4924. In addition, comet assay demonstrated increased DNA strand breaks with the combination of MLN4924 and mitomycin C. Our data suggest that mitomycin C causes stalled replication forks, which when combined with rereplication induced by MLN4924 results in frequent replication fork collisions, leading to cell death. This study provides a straightforward approach to understand the mechanism of synergy, which may provide useful information for the clinical development of these combinations.

Abstract C233: Activation of p53 by the NEDD8 activating enzyme inhibitor MLN4924 is independent of Cdt1 and involves the cullin-RING ligase CRL4-DTL

NEDD8-activating enzyme (NAE) is an E1 involved in the activation of a large family of E3 ubiquitin ligases termed the cullin-RING ligases (CRLs) through conjugation of cullin proteins with the ubiquitin-like modifier NEDD8. Polyubiquitination of CRL substrates targets them for degradation by the 26S proteasome. NAE thereby regulates the stability of many proteins including those required for cancer cell growth and survival. MLN4924 is an investigational small molecule inhibitor of NAE with 2-3 orders of magnitude selectivity over the related E1’s ubiquitin-activating enzyme (UAE) and sumo-activating enzymes (SAE; Ref. 1). Recently, a genome-wide siRNA screen in the A375 melanoma cell line identified key DNA damage response genes affecting sensitivity of cells to MLN4924, many of which were associated with the p53 pathway which activates a p21-dependent intra-S-phase checkpoint (2). A comparison of the effects of TP53 RNAi in A375 cells to genetic deletion of TP53 in HCT116 cells (HD 104-001 cells, Horizon Discovery Ltd) suggests that p21 induces a cell cycle arrest within 24 h, whereas analysis of the p53-regulated transcriptional signature indicates that proapoptotic proteins are up-regulated at later times. Surprisingly, stabilization of p53 by MLN4924 appears independent of the replication licensing factor Cdt1, ATR activation, or p53 neddylation, but rather involves the E3 ligase complex Cul4A/B-Ddb1-DTL. Furthermore, Nutlin-3induced ubiquitination of p53 could be blocked by MLN4924, suggesting that Mdm2 and DTL may cooperate in the ubiquitination of p53.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C233.

Citation Format: Jonathan L. Blank, Xiaozhen J. Liu, Hugues Bernard, Hua Liao, Katherine Cosmopoulos, Eric S. Lightcap. Activation of p53 by the NEDD8 activating enzyme inhibitor MLN4924 is independent of Cdt1 and involves the cullin-RING ligase CRL4-DTL. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C233.

Abstract B92: Nedd8-activating enzyme inhibitor MLN4924 provides synergy in nonclinical models with mitomycin C through interactions with ATR, BRCA1/BRCA2 and chromatin dynamics pathways

MLN4924 is an investigational small molecule inhibitor of the Nedd8-activating enzyme (NAE) currently in Phase 1 clinical trials. MLN4924 induces DNA damage via rereplication in most cell lines. This distinct mechanism of DNA damage may affect its ability to combine with standards of care, including other DNA damaging agents. We studied the interaction of MLN4924 with other DNA damaging agents in a panel of 4 cell lines and found that mitomycin C, cisplatin, carboplatin, cytarabine, ultraviolet radiation, SN-38, and gemcitabine demonstrated synergy in combination with MLN4924 in at least 1 cell line. Further testing in xenograft-bearing mice demonstrated synergy of MLN4924 with mitomycin C and with carboplatin, and additivity with gemcitabine. Based in part on this data, MLN4924 is currently being evaluated in a Phase 1b trial (NCT01862328) with 3 combination arms: MLN4924 + carboplatin and paclitaxel, MLN4924 + gemcitabine, and MLN4924 + docetaxel. To evaluate the mechanism of synergy between MLN4924 and mitomycin C, in vitro experiments with RNAi were performed. Depletion of genes within the ATR and BRCA1/BRCA2 pathways, chromatin modification, and transcription-coupled repair reduced the synergy between mitomycin C and MLN4924. Our data suggest that mitomycin C causes stalled replication forks, which when combined with rereplication induced by MLN4924, results in frequent replication fork collisions, leading to cell death. This study provides a straightforward approach to understand the mechanism of synergy, which may be applied to additional combinations currently under clinical evaluation.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B92.

Citation Format: Eric S. Lightcap, Khristofer Garcia, Jonathan L. Blank, David C. Bouck, Xiaozhen J. Liu, Greg Hather, Allison Berger, Katherine Cosmopoulos, Michael P. Thomas, Mike Kuranda, Michael D. Pickard, Ray Liu, Syamala Bandi, Peter G. Smith. Nedd8-activating enzyme inhibitor MLN4924 provides synergy in nonclinical models with mitomycin C through interactions with ATR, BRCA1/BRCA2 and chromatin dynamics pathways. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B92.

Novel DNA damage checkpoints mediating cell death induced by the NEDD8-activating enzyme inhibitor MLN4924

MLN4924 is an investigational small-molecule inhibitor of the NEDD8-activating enzyme (NAE) in phase I clinical trials. NAE inhibition prevents the ubiquitination and proteasomal degradation of substrates for cullin-RING ubiquitin E3 ligases that support cancer pathophysiology, but the genetic determinants conferring sensitivity to NAE inhibition are unknown. To address this gap in knowledge, we conducted a genome-wide siRNA screen to identify genes and pathways that affect the lethality of MLN4924 in melanoma cells. Of the 154 genes identified, approximately one-half interfered with components of the cell cycle, apoptotic machinery, ubiquitin system, and DNA damage response pathways. In particular, genes involved in DNA replication, p53, BRCA1/BRCA2, transcription-coupled repair, and base excision repair seemed to be important for MLN4924 lethality. In contrast, genes within the G(2)-M checkpoint affected sensitivity to MLN4924 in colon cancer cells. Cell-cycle analysis in melanoma cells by flow cytometry following RNAi-mediated silencing showed that MLN4924 prevented the transition of cells from S-G(2) phase after induction of rereplication stress. Our analysis suggested an important role for the p21-dependent intra-S-phase checkpoint and extensive rereplication, whereas the ATR-dependent intra-S-phase checkpoint seemed to play a less dominant role. Unexpectedly, induction of the p21-dependent intra-S-phase checkpoint seemed to be independent of both Cdt1 stabilization and ATR signaling. Collectively, these data enhance our understanding of the mechanisms by which inhibition of NEDD8-dependent ubiquitination causes cell death, informing clinical development of MLN4924.

A novel persistence associated EBV miRNA expression profile is disrupted in neoplasia

We have performed the first extensive profiling of Epstein-Barr virus (EBV) miRNAs on in vivo derived normal and neoplastic infected tissues. We describe a unique pattern of viral miRNA expression by normal infected cells in vivo expressing restricted viral latency programs (germinal center: Latency II and memory B: Latency I/0). This includes the complete absence of 15 of the 34 miRNAs profiled. These consist of 12 BART miRNAs (including approximately half of Cluster 2) and 3 of the 4 BHRF1 miRNAs. All but 2 of these absent miRNAs become expressed during EBV driven growth (Latency III). Furthermore, EBV driven growth is accompanied by a 5–10 fold down regulation in the level of the BART miRNAs expressed in germinal center and memory B cells. Therefore, Latency III also expresses a unique pattern of viral miRNAs. We refer to the miRNAs that are specifically expressed in EBV driven growth as the Latency III associated miRNAs. In EBV associated tumors that employ Latency I or II (Burkitt's lymphoma, Hodgkin's disease, nasopharyngeal carcinoma and gastric carcinoma), the Latency III associated BART but not BHRF1 miRNAs are up regulated. Thus BART miRNA expression is deregulated in the EBV associated tumors. This is the first demonstration that Latency III specific genes (the Latency III associated BARTs) can be expressed in these tumors. The EBV associated tumors demonstrate very similar patterns of miRNA expression yet were readily distinguished when the expression data were analyzed either by heat-map/clustering or principal component analysis. Systematic analysis revealed that the information distinguishing the tumor types was redundant and distributed across all the miRNAs. This resembles “secret sharing” algorithms where information can be distributed among a large number of recipients in such a way that any combination of a small number of recipients is able to understand the message. Biologically, this may be a consequence of functional redundancy between the miRNAs.

miRNAs are small (∼22 bp) RNAs. They play central roles in many cellular processes. Epstein-Barr virus (EBV) is an important human pathogen that establishes persistent infection in nearly all humans and is associated with several common forms of cancer. To achieve persistent infection, the virus infects B cells and uses a series of discrete transcription programs to drive these B cells to become memory B cells – the site of long term persistent infection. It was the first human virus found to express miRNAs of which there are at least 40. The functions of a few of these miRNAs are known but their expression in latently infected normal and neoplastic tissues in vivo have not been described. Here we have profiled EBV miRNAs in a wide range of infected normal and neoplastic tissue. We demonstrate that there are indeed latency program specific patterns of viral miRNA expression and that these patterns are disrupted in EBV associated tumors implicating EBV miRNAs both in long term persistence and in oncogenesis.

Immunoglobulin deficiencies and susceptibility to infection among homozygotes and heterozygotes for C2 deficiency

About 25% of C2-deficient homozygotes have increased susceptibility to severe bacterial infections. C2-deficient homozygotes had significantly lower serum levels of IgG2, IgG4, IgD, and Factor B, significantly higher levels of IgA and IgG3 and levels of IgG1 and IgM similar to controls. Type 1 (28 bp deletion in C2 exon 6 on the [HLA-B18, S042, DR2] haplotype or its fragments) and type II (non-type I) C2-deficient patients with increased susceptibility to bacterial infection had significantly lower mean levels of IgG4 (p < 0.04) and IgA (p < 0.01) than those without infections (who had a higher than normal mean IgA level) but similar mean levels of other immunoglobulins and Factor B. Of 13 C2-deficient homozygotes with infections, 85% had IgG4 deficiency, compared with 64% of 25 without infections. IgD deficiency was equally extraordinarily common among infection-prone (50%) and noninfection-prone (70%) homozygous type I C2-deficient patients. IgD deficiency was also common (35%) among 31 type I C2-deficient heterozygotes (with normal or type II haplotypes), but was not found in 5 type II C2-deficient heterozygotes or 1 homozygote. Thus, C2 deficiency itself is associated with many abnormalities in serum immunoglobulin levels, some of which, such as in IgG4 and IgA, may contribute to increased susceptibility to infection. In contrast, IgD deficiency appears not to contribute to increased infections and appears to be a dominant trait determined by a gene or genes on the extended major histocompatibility complex (MHC) haplotype [HLA-B 18, S042, DR2] (but probably not on type II C2-deficient haplotypes) similar to those previously identified on [HLA-B8, SC01, DR3] and [HLA-B18, F1C30, DR3].