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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. Mol Cancer Ther; 13(6); 1625–35. ©2014 AACR.

The ubiquitin-proteasome system (UPS) comprises a network of enzymes that is responsible for maintaining cellular protein homeostasis. The therapeutic potential of this pathway has been validated by the clinical successes of a number of UPS modulators, including proteasome inhibitors and immunomodulatory imide drugs (IMiDs). Here we identified TAK-243 (formerly known as MLN7243) as a potent, mechanism-based small-molecule inhibitor of the ubiquitin activating enzyme (UAE), the primary mammalian E1 enzyme that regulates the ubiquitin conjugation cascade. TAK-243 treatment caused depletion of cellular ubiquitin conjugates, resulting in disruption of signaling events, induction of proteotoxic stress, and impairment of cell cycle progression and DNA damage repair pathways. TAK-243 treatment caused death of cancer cells and, in primary human xenograft studies, demonstrated antitumor activity at tolerated doses. Due to its specificity and potency, TAK-243 allows for interrogation of ubiquitin biology and for assessment of UAE inhibition as a new approach for cancer treatment.

Multiple pathways have been proposed to explain how proteasome inhibition induces cell death, but mechanisms remain unclear. To approach this issue, we performed a genome-wide siRNA screen to evaluate the genetic determinants that confer sensitivity to bortezomib (Velcade (R); PS-341). This screen identified 100 genes whose knockdown affected lethality to bortezomib and to a structurally diverse set of other proteasome inhibitors. A comparison of three cell lines revealed that 39 of 100 genes were commonly linked to cell death. We causally linked bortezomib-induced cell death to the accumulation of ASF1B, Myc, ODC1, Noxa, BNIP3, Gadd45α, p-SMC1A, SREBF1, and p53. Our results suggest that proteasome inhibition promotes cell death primarily by dysregulating Myc and polyamines, interfering with protein translation, and disrupting essential DNA damage repair pathways, leading to programmed cell death. Cancer Res; 70(11); 4318–26. ©2010 AACR.

Radiation therapy, as a primary therapy or as a combination partner, is used in half of all worldwide cancer treatments. Research is ongoing to identify agents which potentiate the effects of ionizing radiation (IR) in tumor cells. Because IR causes DNA double strand breaks (DSBs), inhibition of DNA damage repair mechanisms could enhance the effects of radiation. DNA repair at DSBs is mediated by the non-homologous end-joining (NHEJ) and homologous recombination (HR) pathways, both of which rely on the post-translational modification of proteins by ubiquitin (Ub). A phosphorylation and ubiqutination cascasde at DSBs results in Ub-dependent recruitment of 53BP1 and BRCA1 complexes. We have identified a first in class investigational drug, TAK-243 (MLN7243), which targets the ubiquitin activating enzyme, UAE (UBA1), the enzyme responsible for activating > 99% of all cellular Ub. Previously, TAK-243 was shown to inhibit mono-Ub of PCNA and FANCD2, key proteins within the translesion synthesis (TLS) and Fanconi Anemia (FA) DNA repair pathways, and also to inhibit Ub transfer to UBC13, an E2 ubiquitin-conjugating enzyme utilized in DSB repair. We hypothesized that TAK-243 would prevent repair of DSBs and thereby potentiate IR-induced cell death. Here we show that TAK-243 pre-treatment potentiates the effect of IR on HCT-116 cells in a colony formation assay in vitro. To link this combination benefit to the disruption of DNA damage repair, we demonstrate that TAK-243 pre-treatment blocks the IR-induced recruitment of 53BP1 to sites of DNA damage both in vitro and in vivo. In a patient-derived xenograft (PDX) model of non-small cell lung cancer, formation of IR-induced 53BP1 foci is inhibited when TAK-243 is dosed 1 hour before beam-focused radiation exposure. In contrast, levels of IR-induced pH2Ax are not significantly changed by TAK-243 treatment, suggesting that TAK-243 does not prevent formation or detection of DSBs, but rather acts downstream to prevent DNA damage repair. Additive-to-synergistic effects on tumor growth inhibition were observed in several xenograft models treated with the combination of TAK-243 and beam-focused IR, with persistent tumor regressions noted in some NSCLC and breast cancer models. The results of our experiments provide a mechanistic rationale for combining radiation with TAK-243 in the clinical setting. Currently, TAK-243 is being evaluated in a solid tumor phase I clinical trial evaluating safety, tolerability, pharmacokinetics, pharmacodynamics and anti-tumor activity (NCT02045095). Citation Format: Michael A. Milhollen, Judy Qiuju Shi, Tary Traore, Jessica Huck, Darshan Sappal, Kenichi Iwai, Akihiro Ohashi, Claudia Rabino, Jennifer A. Duffy, Eric Lightcap, Yuko Ishii, Jeffrey Ciavarri, Neil Bence, Allison J. Berger, Marc L. Hyer. TAK-243, a small molecule inhibitor of the ubiquitin activating enzyme (UAE), disrupts DNA damage repair and sensitizes tumor cells and xenografts to ionizing radiation. [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 3719.

Clinical results of VELCADE® (bortezomib) For Injection have prompted evaluation of other enzymes within the ubiquitin proteasome system (UPS) as druggable targets for human cancer. We have identified a first in class investigational drug, TAK-243 (MLN7243), which targets the ubiquitin activating enzyme, UAE (UBA1), an essential cellular enzyme responsible for activating > 99% of all cellular ubiquitin. Ubiquitin is involved in multiple cellular processes including ubiquitin-dependent protein turnover, cell cycle progression, regulation of apoptosis, protein localization and response to DNA damage. Experiments combining targeted siRNA knockdown with TAK-243 identified DNA damage repair genes necessary for UAE inhibitor-induced cell death. A more focused approach revealed TAK-243 treatment blocked essential monoubiquitination events within the Translesion synthesis (TLS), Fanconi Anemia (FA) and Homologous recombination (HR) pathways. Inhibition of UAE prevented mono-ubiquitin signaling of key mediators within these pathways, including PCNA and FANCD2, by blocking formation of their specific E2-ubiquitin thioesters. In vitro cell-based assays combining TAK-243 with ultraviolet (UV) and radiation, both known to induce DNA damage, yielded inhibition of cell growth and enhanced DNA damage as observed through colony formation assays and Comet assay detection, respectively. Xenograft tumor bearing mice were treated with carboplatin or docetaxel, combined with TAK-243, to evaluate combination benefits in vivo. Synergistic and additive anti-tumor combination benefits were observed in animals treated with TAK-243 + carboplatin and TAK-243 + docetaxel. These important mechanistic in vitro and in vivo studies indicate the dependency of ubiquitination signaling in DNA damage repair and provide a mechanistic rationale for combining radiation, carboplatin or docetaxel with TAK-243 in the clinical setting. Currently, TAK-243 is being evaluated in a solid tumor phase I clinical trial evaluating safety, tolerability, pharmacokinetics, pharmacodynamics and anti-tumor activity (ClinicalTrials.gov identifier: NCT02045095). Citation Format: Michael A. Milhollen, Judi Shi, Tary Traore, Jessica Huck, Darshan Sappal, Jennifer Duffy, Eric Lightcap, Yuko Ishii, Jeff Ciavarri, Paul Fleming, Neil Bence, Marc L. Hyer. The small molecule UAE inhibitor TAK-243 (MLN7243) prevents DNA damage repair and reduces cell viability/tumor growth when combined with radiation, carboplatin and docetaxel. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A164.

Millennium Pharmaceuticals, Inc. is dedicated to the discovery and development of novel oncology therapeutics in the area of protein homeostasis. Here we report the identification and characterization of compounds that target the ubiquitin activating enzymes, UBA1 and UBA6. These compounds are mechanism based inhibitors that inactivate the ubiquitin E1 enzymes by forming a ubiquitin compound adduct that remains tightly associated with the E1 adenylate binding site. Treatment of cells with these inhibitors results in cellular effects consistent with known Uba1 biology including rapid loss of E2 ubiquitin thioesters, loss of total ubiquitin conjugates, and accumulation of many ubiquitin proteasome system substrates. Following prolonged treatment, cells primarily arrest in the G2 phase of the cell cycle and ultimately undergo apoptosis. Reflecting the extensive cellular roles of ubiquitin, the compounds also impact global protein turnover, ER stress and DNA damage repair. UBA1 inhibition impairs ubiquitination of PCNA and the Fanconia Anemia protein FANCD2 leading to defective repair of UV induced DNA damage. UBA1 inhibition impacts numerous biological pathways relevant to cancer, results in apoptosis in vitro and is capable of inhibiting tumor growth in mouse xenografts in vivo. These data implicate UBA1 as a target for the treatment of cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C82.