REVIEW
`published: 17 May 2019
`doi: 10.3389/fonc.2019.00417
`
`Mutant Isocitrate Dehydrogenase
`Inhibitors as Targeted Cancer
`Therapeutics
`
`Danielle Golub 1,2*, Nishanth Iyengar 3, Siddhant Dogra 3, Taylor Wong 1, Devin Bready 1,
`Karen Tang 2,4, Aram S. Modrek 5 and Dimitris G. Placantonakis 1,6,7,8,9*
`
`1 Department of Neurosurgery, New York University School of Medicine, NYU Langone Health, New York, NY, United States,
`2 Clinical and Translational Science Institute, New York University School of Medicine, NYU Langone Health, New York, NY,
`United States, 3 New York University School of Medicine, NYU Langone Health, New York, NY, United States, 4 Division of
`Hematology/Oncology, Department of Pediatrics, New York University School of Medicine, NYU Langone Health, New York,
`NY, United States, 5 Department of Radiation Oncology, New York University School of Medicine, NYU Langone Health, New
`York, NY, United States, 6 Kimmel Center for Stem Cell Biology, New York University School of Medicine, NYU Langone
`Health, New York, NY, United States, 7 Laura and Isaac Perlmutter Cancer Center, New York University School of Medicine,
`NYU Langone Health, New York, NY, United States, 8 Brain Tumor Center, New York University School of Medicine, NYU
`Langone Health, New York, NY, United States, 9 Neuroscience Institute, New York University School of Medicine, NYU
`Langone Health, New York, NY, United States
`
`The identification of heterozygous neomorphic isocitrate dehydrogenase (IDH) mutations
`across multiple cancer types including both solid and hematologic malignancies has
`revolutionized our understanding of oncogenesis in these malignancies and the potential
`for targeted therapeutics using small molecule inhibitors. The neomorphic mutation
`in IDH generates an oncometabolite product, 2-hydroxyglutarate (2HG), which has
`been linked to the disruption of metabolic and epigenetic mechanisms responsible
`for cellular differentiation and is likely an early and critical contributor to oncogenesis.
`In the past 2 years, two mutant IDH (mutIDH) inhibitors, Enasidenib (AG-221), and
`Ivosidenib (AG-120), have been FDA-approved for IDH-mutant relapsed or refractory
`acute myeloid leukemia (AML) based on phase 1 safety and efficacy data and continue to
`be studied in trials in hematologic malignancies, as well as in glioma, cholangiocarcinoma,
`and chondrosarcoma. In this review, we will summarize the molecular pathways and
`oncogenic consequences associated with mutIDH with a particular emphasis on glioma
`and AML, and systematically review the development and preclinical testing of mutIDH
`inhibitors. Existing clinical data in both hematologic and solid tumors will
`likewise
`be reviewed followed by a discussion on the potential
`limitations of mutIDH inhibitor
`monotherapy and potential routes for treatment optimization using combination therapy.
`
`Keywords: acute myeloid leukemia, enasidenib, glioma, IDH, isocitrate dehydrogenase, ivosidenib
`
`INTRODUCTION
`
`in
`in isocitrate dehydrogenase 1 (IDH1) and 2 (IDH2)
`The discovery of mutations
`over 80% of
`low-grade gliomas (LGGs) and secondary glioblastomas has revolutionized
`pharmaceutical approaches
`to targeted therapies and the overall glioma classification
`schema (1, 2). Driver mutations
`in IDH1 and IDH2 have been likewise identified
`acute myeloid
`leukemia
`(AML),
`chondrosarcoma, myelodysplastic
`syndromes,
`in
`and cholangiocarcinoma (3–6). Limitations
`in current
`treatment options, particularly
`to both inefficacy
`in LGG and AML, due
`and systemic
`toxicity, make mutant
`
`Edited by:
`
`Tomofusa Fukuyama,
`
`The University of Tokyo, Japan
`
`Reviewed by:
`
`Wu Xu,
`
`University of Louisiana at Lafayette,
`
`United States
`
`Pavithra Viswanath,
`
`University of California, San Francisco,
`
`United States
`
`*Correspondence:
`
`Danielle Golub
`
`danielle.golub@nyulangone.org
`
`Dimitris G. Placantonakis
`
`dimitris.placantonakis@nyulangone.org
`
`Specialty section:
`
`This article was submitted to
`
`Cancer Molecular Targets and
`
`Therapeutics,
`
`a section of the journal
`
`Frontiers in Oncology
`
`Received: 24 March 2019
`
`Accepted: 02 May 2019
`
`Published: 17 May 2019
`
`Citation:
`
`Golub D, Iyengar N, Dogra S, Wong T,
`
`Bready D, Tang K, Modrek AS and
`
`Placantonakis DG (2019) Mutant
`
`Isocitrate Dehydrogenase Inhibitors as
`
`Targeted Cancer Therapeutics.
`
`Front. Oncol. 9:417.
`
`doi: 10.3389/fonc.2019.00417
`
`Frontiers in Oncology | www.frontiersin.org
`
`1
`
`May 2019 | Volume 9 | Article 417
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`
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`

`Golub et al.
`
`MutIDH Inhibitors in Cancer
`
`IDH (mutIDH), and its associated molecular pathways attractive
`therapeutic targets (7–9). Major strides in developing and testing
`candidates for mutIDH inhibition have been made in the past
`few years with the FDA approvals of Ivosidenib (Tibsovo R(cid:13))
`and Enasidenib (Idhifa R(cid:13)), selective mutIDH1 and mutIDH2
`inhibitors, respectively (10, 11). While these agents have had
`some preliminary success in AML, utility in the treatment of
`IDH-mutant glioma or other IDH-mutated cancers has not been
`established (12, 13).
`IDH1 and IDH2 are homodimeric isoenzymes involved in
`a major pathway for cellular NADPH generation through the
`oxidative decarboxylation of isocitrate to α-ketoglutarate. IDH1
`is found in the cytosol and in peroxisomes, while IDH2 is a
`mitochondrial enzyme. Mutations in IDH3 isoforms, which form
`heterotetrameric complexes in mitochondria, are rarely seen in
`cancer, but there is some evidence that upregulation of wild-
`type IDH3 may contribute to various tumorigenic metabolic
`pathways (14, 15). The IDH1/2 mutations are heterozygous
`they establish a pathway for the
`and neomorphic in that
`NADPH-dependent conversion of the wild-type IDH product, α-
`ketoglutarate, to 2-hydroxyglutarate (2HG) (16). Simultaneously,
`significant decreases in NADPH production are also seen (17).
`Early structural and pharmacokinetic studies show that mutant
`IDH develops an increased affinity for both the cofactor NADPH
`and substrate α-ketoglutarate (16, 18). In the most common
`the wild-type IDH function of oxidative
`IDH1/2 mutants,
`decarboxylation of isocitrate to α-ketoglutarate is lost due to
`mutation of critical amino acid residues in the catalytic domain,
`IDH1 R132 and IDH2 R172, which are normally responsible
`for binding the β-carboxyl group of isocitrate and initiating
`catalysis (1, 16, 18). Interestingly, there is some evidence that,
`unlike the IDH1 mutant, the IDH2 mutant may not depend
`on heterodimerization with an IDH wild-type partner for 2HG
`production (19). Nevertheless, while the mutant IDH enzyme
`can exist either as a homodimer or as a heterodimer with the
`wild-type IDH within cancer cells, all reported oncogenic IDH
`mutations to date are genetically heterozygous, suggesting that
`the critical role of mutant IDH is related to its gain-of-function
`for conversion of the wild-type IDH product, α-ketoglutarate, to
`2HG (20).
`increasingly well-characterized
`Accumulation of 2HG,
`as an oncometabolite, disrupts multiple regulatory cellular
`pathways
`involving α-ketoglutarate-dependent dioxygenases
`including those involved in epigenetic remodeling and DNA
`repair
`(Figure 1)
`(21–23). Structural
`similarities between
`α-ketoglutarate and 2HG allow the latter to competitively
`occupy the same pockets as α-ketoglutarate in α-ketoglutarate-
`dependent dioxygenases (of which over 60 have been described in
`humans), without promoting enzymatic activation (22, 24–26).
`Changes in the epigenetic landscape brought on by 2HG-
`mediated disruption of the ten-eleven translocation (TET) family
`of 5-methylcytosine (5 mC) hydroxylases (DNA demethylases)
`and the JmJC domain-containing histone lysine demethylases
`(KDMs) are hypothesized to promote oncogenesis through DNA
`and histone hypermethylation and resultant
`transcriptional
`dysregulation (22, 27). The resulting global increase in DNA
`methylation in the mutIDH context is aptly named the CpG
`
`Island Methylator Phenotype (CIMP) (28, 29). Manipulating
`and reversing the oncogenic IDH-mutant methylome is the
`primary molecular endpoint for therapeutic IDH inhibition and
`2HG reduction in both glioma and AML. It remains to be seen,
`however, if 2HG reduction alone will be sufficient to reverse
`oncogenic changes to the methylome, as epigenetic memory
`persists through daughter cells via methyltransferases, a topic we
`explore further in our discussion (30, 31).
`Here, we provide an overview of the current literature on IDH
`mutations in cancer with a particular emphasis on glioma and
`AML and the potential for mutIDH as a therapeutic target in
`these contexts. We describe the current evidence for the various
`generations of mutIDH inhibitors through the drug-discovery,
`preclinical, and clinical stages and systematically review related
`past and ongoing clinical trials. We furthermore describe the
`possible adverse effects of IDH inhibitors, such as “differentiation
`syndrome,” and conclude with a discussion on the potential for
`enhancing the efficacy of IDH inhibitors in combination with
`epigenetic modification-based therapies.
`
`IDH Mutations in Glioma
`Ten years ago, our understanding of the molecular landscape
`in glioma was transformed by the first genome-wide analysis of
`somatic mutations in glioblastoma (GBM) and the identification
`of recurrent mutations in IDH1 nearly exclusively in secondary
`GBM (2). Mutations in IDH1 and IDH2 are seen in over 80%
`of lower-grade gliomas (WHO grades II and III) and secondary
`GBMs that are thought to later develop from lower-grade lesions
`(2, 32, 33). The vast majority of somatic IDH mutations (>95%)
`are seen in IDH1, and the most commonly observed IDH1
`mutation occurs at the R132 residue (1, 34). IDH2 mutations,
`which are mutually exclusive with those in IDH1 and found at
`a functionally analogous R172 residue, only represent a minority
`of somatic IDH mutations in glioma (35, 36).
`IDH-mutant gliomas are generally further categorized into
`two major subtypes: those with chromosome 1p/19q co-deletion,
`historically termed oligodendrogliomas; and those without
`1p/19q co-deletion, also known as astrocytomas (37). These two
`groups are biologically and clinically distinct. Up to 94% of IDH-
`mutant non-1p/19q co-deleted gliomas harbor loss-of-function
`TP53 mutations and 86% have inactivating ATRX mutations
`(37). Only few IDH mutant astrocytomas carry IDH wild-type
`driver mutations or copy number alterations, and those who do
`(for example CDKN2A or CDKN2B loss) are usually classified
`as IDH mutant GBM (1). These robust genomic differences
`are highly suggestive of a unique mechanism of oncogenesis
`in the IDH-mutant subgroup and furthermore imply that the
`IDH mutation is likely an early player in a cell-of-origin, which
`in its native state is capable of giving rise to both astrocyte
`and oligodendrocyte lineages. Clinically, IDH-mutant lesions
`present in a younger age group (median age in the fourth vs. the
`sixth decade of life), when compared to IDH-wild type gliomas
`(33). Furthermore, IDH mutations are well-known to be an
`independent favorable prognostic factor at all stages of glioma
`progression; for example, the median survival in IDH-mutant
`GBM is 31 months, over twice as long as the median 15-months
`survival in the wild-type counterpart (1).
`
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`
`MutIDH Inhibitors in Cancer
`
`FIGURE 1 | Schematic representation of the mutIDH1 and mutIDH2 pathways and molecular mechanisms related to oncogenesis. The neomorphic enzyme,
`mutIDH1/2, converts the wild-type IDH product, α-ketoglutarate, to the oncometabolite, 2-hydroxyglutarate (2HG) both in the cytosol and in the mitochondria. 2HG
`competitively inhibits α-ketoglutarate-dependent dioxygenases both in the cytosol and in the nucleus. 2HG-mediated inhibition of the activity of Ten-Eleven
`Translocation (TET) enzymes and histone lysine demethylases (KDM) result in global epigenetic modifications on DNA and histones, respectively, resulting in a
`hypermethylator phenotype. Inhibition of prolyl hydroxylases and lysyl hydroxylases (such as PLOD1-3) interferes with both collagen maturation and with the
`degradation pathway of Hypoxia-Inducible Factor 1α (HIF-1α), thereby post-translationally stabilizing HIF-1α. Additionally, ALKBH, responsible for repair of oxidative
`DNA damage, is also inhibited by 2HG, an effect which potentially introduces risk for increased mutational burden.
`
`Consistent with other IDH-mutant cancers, IDH-mutant
`glioma is characterized by high levels of 2HG and the resulting
`“CIMP” hypermethylator phenotype described previously. In
`glioma specifically,
`these genome-wide DNA methylation
`changes have been shown to establish “insulator dysfunction”
`or disruption of
`topologically-associated domains
`(TADs)
`and thereby directly influence key transcriptional regulatory
`pathways related to gliomagenesis (38, 39). As previously
`mentioned, analyses of clonality among glioma tumor samples
`suggests that the IDH mutation is a tumor-initiating event in
`a common progenitor cell, hypothesized by many to be derived
`from the subventricular zone stem cell niche (7, 40–42). Despite
`our enhanced understanding of the molecular pathogenesis of
`IDH-mutant glioma, however, effective treatments have yet to
`be developed and clinicians remain reliant on maximal safe
`surgical resection and various chemotherapeutic agents and
`radiation treatments to prolong survival (7). Furthermore, a
`
`unique characteristic of LGG is its diffuse and highly infiltrative
`phenotype, making surgical resection rarely curative in the
`long term. To compound the complexity of these tumors, and
`historically popular chemotherapeutic agents have been shown
`to induce hypermutant recurrent tumors (7). Recent efforts in
`developing small molecule inhibitors that target IDH mutation
`provide a new opportunity for progress in glioma treatment.
`
`IDH Mutations in AML
`Around the same time as the identification of recurrent IDH
`mutations in glioma, Mardis et al. published the results of a
`landmark study in which they sought to pinpoint recurrent
`mutations in AML that may be associated with the pathogenesis
`of the malignancy (43). In this study, the investigators identified
`for the first time the presence of IDH1 mutations in AML
`(43). 8.5% of analyzed samples had an IDH1 mutation at the
`R132 residue (mutated to either cysteine, histidine, or serine),
`
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`
`MutIDH Inhibitors in Cancer
`
`which is also the site of the overwhelming majority of somatic
`IDH mutations in glioma (1, 34). Shortly after the discovery of
`IDH1 mutations in AML, another landmark study reported the
`first case of IDH2-mutated AML, in which the R172 residue
`was mutated to lysine (18). Further investigation of AML DNA
`samples revealed the existence of several additional cases of AML
`where the IDH2R172 residue was mutated (18). Interestingly, this
`study also found that a majority of the analyzed samples had
`IDH2 mutations (compared to IDH1 mutations) (18). This is in
`stark contrast to glioma, where the majority of IDH mutations
`are in IDH1.
`Nearly one in five cases of AML is IDH-mutant, with IDH2-
`mutant AML being more prevalent than IDH1-mutant AML
`(11–13, 44–50). The IDH2R140 mutation (in particular, the
`R140Q variant) is the most common, with the IDH1R132 and
`IDH2R172 mutations also appearing frequently in the literature
`(3, 12, 45, 46, 50–52). Other mutations include, but are not
`limited to, IDH1V71 and IDH1 SNP rs11554137, a GGC to
`GGT transversion at the glycine residue at codon position 105
`with unknown significance (47, 48, 53). Clinical and pathologic
`characteristics associated with IDH-mutant AML include normal
`karyotype (intermediate-risk cytogenetics), increased patient age,
`elevated platelet count, increased bone marrow blast percentage
`at initial presentation, increased peripheral blast percentage,
`decreased absolute neutrophil count (especially in IDH1-mutant
`AML), and concurrent mutations such as NPM1 and FLT3-
`ITD (44–47, 54). IDH1 and IDH2 mutations in AML are
`in AML, IDH
`mutually exclusive, as in glioma. Likewise,
`mutations are almost entirely mutually exclusive with TET2
`mutations, suggesting that, mechanistically, these genes aref both
`involved in DNA hypermethylation as a driver of leukemogenesis
`(3, 45–47, 54).
`It has been suggested that testing AML patients for IDH
`mutation status is simple and should be performed universally;
`however, the relationship between IDH mutation status and
`prognosis is considerably less clear and more controversial in
`AML than it is in other cancers such as glioma (46, 55). Most
`studies of IDH-mutant AML have suggested that mutIDH either
`foreshadows an adverse prognosis (given an association with
`increased blast percentage and older age at diagnosis) or is of
`little prognostic value (45, 48, 52, 55). Reported 2–3-years overall
`survival in IDH-mutant AML ranges between 51 and 89% in the
`literature; discrepancies are thought to be related to differences
`in cohort age, but some authors also argue that different specific
`IDH mutations may carry varied prognostic implications (3, 44,
`45, 47, 54, 56, 57). Interestingly, IDH mutation status may also be
`useful for the detection of residual disease and prognostication
`following treatment; several studies investigating the value of
`serum 2HG during remission in AML have found that elevated
`serum 2HG levels actually predict shortened overall survival
`(55, 58, 59).
`has
`chemotherapy
`induction/consolidation
`While
`revolutionized AML treatment strategy in the past 20 years,
`this standard-of-care universal treatment has evolved minimally
`since its introduction and is often contraindicated in elderly or
`otherwise frail patients (44). Given our enhanced understanding
`of the molecular and genetic subtypes of AML and the potential
`
`for targeted treatment, manipulation of these markers with small
`molecules may provide significant benefit. Drugs targeted to the
`mutIDH isotypes are one such example; for almost a decade,
`mutIDH inhibitors have been a focus of laboratory and clinical
`research in AML with great recent success leading to two FDA
`approvals specifically for AML indications.
`
`Drug Development and Preclinical Studies
`Multiple mutIDH inhibitors, including one pan-inhibitor and
`several specific to one mutIDH isoform, have been developed
`over the last several years. A handful of these are in use in clinical
`trials, but only two have been approved by the FDA; Enasidenib
`and Ivosidenib (10, 11). A detailed review of the structural and
`pharmacokinetic properties and relevant preclinical data for both
`FDA approved inhibitors will follow a brief discussion of other
`mutIDH inhibitors with demonstrated and repeated preclinical
`efficacy (Table 1).
`
`Pan-Inhibitors
`AG-881
`AG-881 (Vorasidenib) is an orally available pan-inhibitor of
`both mutIDH1 and mutIDH2 and was the first pan-inhibitor
`developed under
`the Celgene and Agios Pharmaceuticals
`collaboration (Figure 2) (60–62). AG-881 contains a triazine
`moiety responsible for its allosteric inhibitory activity, and
`crystallography studies show that AG-881 binds mutIDH1 and
`mutIDH2 using the same allosteric pocket at the dimer interface,
`causing steric hindrance that locks the enzymes in an open,
`inactive conformation (61). Notably, the association of AG-881
`with mutIDH1, in particular with IDH1R132H, is more efficient
`than its interaction with mutIDH2 as it achieves maximal
`potency in vitro after significantly shorter incubation periods
`(61). IC50 for inhibition of 2-HG formation following 1 h of
`preincubation ranged from 6 to 34 nM in both patient-derived
`lines expressing IDH1R132C,
`and genetically- engineered cell
`IDH1R132G, IDH1R132H, IDH1R132L, or IDH1R132S. For U87 and
`TF-1 cells transfected with IDH2R140Q or IDH2R172K by lentiviral
`vector, the IC50 values following 1 h of preincubation were
`118 nM and 32 nM, respectively (62). In the same study, it was
`demonstrated that ex vivo treatment of primary human AML
`blasts with AG-881 induced myeloid differentiation (62). AG-
`881 has also been shown to effectively penetrate the blood-brain
`barrier in rodents, implicating its potential to treat both IDH-
`mutant AML and glioma patients (62). Based on this preclinical
`evidence, two multicenter clinical trials investigating the safety
`and efficacy of AG-881, one in solid tumors and the other in
`hematologic malignancies, are currently ongoing (60, 61).
`
`Specific Inhibitors
`BAY-1436032
`One of the first mutIDH1-specific inhibitors to show preclinical
`efficacy in both AML and glioma models is BAY-1436032,
`developed by Bayer. An initial screen of over 3 million
`compounds based on mutIDH enzymatic activity generated
`a small group of compounds—with IC50 ranging from 0.6
`to 17.1 µM—for further evaluation. Optimization of a lead
`compound based on differential inhibition of mutIDH1 and
`
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`Golub et al.
`
`MutIDH Inhibitors in Cancer
`
`(Continued)
`
`Celgene
`Agios,
`
`Celgene
`Agios,
`
`ratmodel)
`
`–
`
`1.11-1.48in
`model,
`mouse
`0.62-1.96in
`plasmaratio
`(brain-to-
`Yes
`
`–
`
`dailydose
`200mg
`ng•h/mLat
`7,020
`dailydose
`100mg
`ng•h/mLat
`2,746
`
`–
`
`–
`
`–
`
`67.2h
`
`UGT2B15
`UGT2B7,
`UGT1A1,
`CYP3A4,
`CYP2D6,
`CYP2C19,
`CYP2C9
`CYP1A2,
`
`-
`
`11%renal
`89%fecal
`
`–
`
`L/h
`0.74
`
`1,300ng/mL∼4h
`
`137h
`
`Celgene
`Agios,
`
`pathways
`hydrolytic
`dealkylation,
`CYP3A4,N-
`
`ratmodel)
`in
`penetrance
`(4.1%
`Yes
`
`17%renal
`77%fecal
`
`ng•h/mL
`4.3L/h117,348
`
`6,551ng/mL∼3h
`
`93h
`
`U87(IDH2R140Q):7.1nM
`TF-1(IDH2R140Q):14nM
`TF-1(IDH1R132C):3.2nM
`JJ012(IDH1R132G):6.6nM
`HT1080(IDH1R132C):4nM
`HCC-9810(IDH1R132S)0.85nM
`COR-L105(IDH1R132C):3.8nM
`HCT-116(IDH2R172K):130nM
`HCT-116(IDH1R132H):3nM
`HCT-116(IDH1R132C):22nM
`Cell-Based:
`WT/IDH2R172K:8nM(16h)
`WT/IDH2R140Q:32nM(16h)
`IDH2R172K:94nM(16h)
`IDH2R140Q:12nM(16h)
`WT/IDH1R132H:4nM
`IDH1R132L:34nM
`IDH1R132S:6nM
`IDH1R132G:17nM
`IDH1R132C:19nM
`IDH1R132H:6nM
`Biochemical:
`
`U87(IDH2R172K):1,590nM
`U87(IDH2R140Q):10nM
`(IDH2R172K):980nM
`TF-1(IDH2R140Q):20nMTF-1
`HCT-116(IDH2R172K):530nM
`Cell-Based:
`WT/IDH2R172K:10nM
`WT/IDH2R140Q:30nM
`IDH2R172K:400nM
`IDH2R140Q:100nM
`Biochemical(at16h):
`
`HCCC-9810(IDH1R132S):12nM
`COR-L105(IDH1R132C):15nM
`HT1080(IDH1R132C):8nM
`U87(IDH1R132H):19nM
`Cell-Based:
`IDH1R132L:13nM
`IDH1R132S:12nM
`IDH1R132G:8nM
`IDH1R132C:13nM
`IDH1R132H:12nM
`Biochemical:
`
`IDH2R172K
`IDH2R140Q
`IDH1R132L
`IDH1R132S
`IDH1R132G
`IDH1R132C
`IDH1R132H
`
`hindrance)
`conformation(steric
`inactiveenzymedimer
`stabilizationofopen,
`inhibitionvia
`non-competitive
`Allosteric,
`
`inhibitor
`Pan-
`
`(61,62,69)
`AG-881
`
`IDH2R172K
`IDH2R140Q
`
`hindrance)
`conformation(steric
`inactiveenzymedimer
`stabilizationofopen,
`inhibitionvia
`non-competitive
`Allosteric,
`
`mutIDH2
`
`(1,12,51)
`(AG-221)1
`ENASIDENIB
`
`IDH1R132L
`IDH1R132S
`IDH1R132G
`IDH1R132C
`IDH1R132H
`
`site
`cofactor(Mg)binding
`competitiveinhibitorvia
`Reversible,allosteric,
`
`mutIDH1
`
`(10,13,68)
`(AG-120)
`Ivosidenib
`
`SPONSOR(S)
`
`Metabolism
`
`Penetrates
`
`Modeof
`
`AUC
`
`Cl
`
`Timeto
`
`Cmax
`
`T1/2
`
`BBB
`
`elimination
`
`Cmax
`
`IC50
`
`mutations
`
`Susceptible
`
`Mechanism
`
`Target
`
`Compound
`
`TABLE1|MutIDHinhibitors.
`
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`

`Golub et al.
`
`MutIDH Inhibitors in Cancer
`
`(Continued)
`
`(Germany)
`School
`Medical
`Hannover
`
`Forma
`
`Celgene
`Agios,
`
`Novartis
`
`Celgene
`Agios,
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`murinemodels)
`in
`0.29–0.61
`plasmaratio
`(brain-to-
`Yes
`
`–
`
`gliomaxenografts)
`inmouse
`accumulate
`(shownto
`Yes
`
`–
`
`-
`
`–
`
`-
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`i.p.dosing)
`modelvia
`(inmouse
`ng•h/mL
`208,000
`
`–
`
`–
`
`Bayer
`
`–
`
`mousemodel)
`in
`0.08–0.38
`plasmaratio
`(brain-to-
`Yes
`
`–
`
`–
`
`model)
`(inrat
`L/h/kg
`0.15
`
`BBB
`
`elimination
`
`–
`
`Cmax
`
`SPONSOR(S)
`
`Metabolism
`
`Penetrates
`
`Modeof
`
`AUC
`
`Cl
`
`Timeto
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`model)
`rat
`3.1h(in
`
`Cmax
`
`T1/2
`
`U87(IDH2R140Q):11nM
`TF-1(IDH2R140Q):18nM
`Cell-Based:
`IDH2R140Q/WT:4nM
`IDH2R140Q:23nM
`Biochemical(at16h):
`
`MCF10A(IDH1R132H):20nM
`HCT-116(IDH1R132H):24nM
`Cell-Based:
`IDH1R132C:28nM
`IDH1R132H:27nM
`Biochemical:
`
`HT1080(IDH1R132C):480nM
`U87(IDH1R132H):70nM
`Cell-Based:
`IDH1R132C:160nM
`IDH1R132H:70nM
`Biochemical:
`
`IDHR123L:3nM
`IDHR123S:16nM
`IDHR123G:4nM
`IDHR123C:5nM
`IDHR123H:5nM
`AML(patient-derived)
`(IDH1R132H):13nM
`NCH551b(GBM)
`HT1080(IDH1R132C):135nM
`HCT-116(IDH1R132H):47nM
`LN299(IDH1R123H):73nM
`Cell-Based:
`IDH1R132C:15nM
`IDH1R132H:15nM
`Biochemical:
`
`IDH1R132L
`IDH1R132S
`IDH1R132G
`IDH1R132C
`IDH1R132H
`
`hindrance)
`conformation(steric
`inactiveenzymedimer
`stabilizationofopen,
`inhibitionvia
`non-competitive
`Allosteric,
`
`mutIDH1
`
`(52,63)
`1436032
`BAY-
`
`IC50
`
`mutations
`
`Susceptible
`
`Mechanism
`
`Target
`
`Compound
`
`TABLE1|Continued
`
`Frontiers in Oncology | www.frontiersin.org
`
`–
`
`–
`
`–
`
`60h
`
`(IDH1R132C):1000nM
`HOXA9(mousebonemarrow)
`Cell-Based:
`
`IDH1R132C
`
`pocket
`bindsisocitrate-binding
`Competitiveinhibition,
`
`mutIDH1
`
`(76,77)
`HMS-101
`
`–
`
`–
`
`–
`
`FT-2102(75)mutIDH1
`
`May 2019 | Volume 9 | Article 417
`
`IDH2R140Q
`
`IDH1R132C
`IDH1R132H
`
`hindrance)
`conformation(steric
`inactiveenzymedimer
`stabilizationofopen,
`inhibitionvia
`non-competitive
`allosteric,
`Slow-tightbinder,
`
`hindrance)
`conformation(steric
`inactiveenzymedimer
`stabilizationofopen,
`inhibitionvia
`non-competitive
`Allosteric,
`
`mutiDH2
`
`(72–74)
`AGI-6780
`
`IDH305(71)mutIDH1
`
`6
`
`IDH1R132C
`IDH1R132H
`
`substratebindingsite
`alpha-KGatthe
`competitiveinhibitorto
`Reversible,allosteric,
`
`mutIDH1
`
`(66,67,70)
`AGI-5198
`
`Rigel Exhibit 1014
`Page 6 of 25
`
`

`

`Golub et al.
`
`MutIDH Inhibitors in Cancer
`
`wild-type IDH1 enzymes resulted in BAY-1436032, an allosteric
`inhibitor that binds at the IDH dimer interface (Figure 2)
`(63). Interestingly, BAY-1436032 demonstrates potent inhibition
`of all known IDH1R132 mutants with nearly equal efficacy in
`both human-derived AML cells (IC50 3–16 nM) and genetically
`engineered cell
`lines representative of solid tumors (IC50
`13–135 nM) (52, 63). Additionally, reduced proliferation and
`induction of differentiation was seen in vitro in both IDH-mutant
`AML and glioma cell lines. In AML cell lines, BAY-1436032
`demonstrated some efficacy in reducing histone methylation
`as well, but multiple studies have failed to show changes in
`histone or DNA methylation status in glioma models (52,
`63). In vivo, however, BAY-1436032 effectively penetrates the
`blood-brain barrier and has shown prolonged survival in mice
`with IDHR132H astrocytoma xenographs (52, 64). Two dose
`escalation and expansion phase I trials, for AML and solid
`tumors (including glioma) respectively, are currently ongoing
`but initial results have yet to be reported (ClinicalTrials.gov
`NCT03127735, NCT02746081).
`
`AGI-5198
`A collaboration between Celgene Corporation and Agios
`Pharmaceuticals
`for
`research in cancer metabolism-based
`therapeutics starting in early 2010 has generated a host
`of mutIDH inhibitors through a high-throughput screening
`campaign (65). One of the earliest, and most well-studied
`mutIDH1-specific inhibitors is AG-5198, a phenyl-glycine-based
`compound (Figure 2) (66). In early characterization studies,
`AGI-5198 showed up to 90% 2HG reduction in IDH1R132H
`U87 xenografts (IC50 0.07 µM in vitro) (66, 67). Rohle et al.
`re-demonstrated the efficacy of AG-5198 in inhibiting 2HG
`production in patient-derived glioma xenografts and additionally
`showed that AGI-5198 promotes expression of markers for
`differentiation, decreases cellular proliferation and decreases
`histone methylation in the same cell line. However, based on
`methylation array data, global DNA methylation contributing to
`the glioma CIMP phenotype was notably unchanged after AGI-
`5198 treatment (67). A key study by Johannessen et al. using
`AGI-5198 in an inducible mutIDH1 knock-in human astrocyte
`model by Johannessen et al. puts these mixed findings into
`larger context: mutIDH1 inhibition appears to have a small
`effective time frame, since the role in of mutIDH in gliomagenesis
`likely changes from “driver” to “passenger.” (80). Early and
`persistent exposure to AGI-5198 prior to inducing mutIDH1
`resulted in reduced 2HG, blocked histone modifications such as
`methylation, and decreased cellular proliferation; however, just 4
`days after the oncogenic insult, the drug was rendered incapable
`of reversing or blocking the genetic and phenotypic changes
`rendered by mutIDH (80). Work by Tateishi et al. complements
`these findings: In multiple patient-derived IDH1R132H glioma
`tumorsphere lines, treatment with the S-enantiomer of AGI-5198
`counterintuitively resulted in modest but consistent increases
`in cellular proliferation, despite successful depletion of 2HG.
`Furthermore, it was observed that mice with recurrent mutIDH
`glioblastoma xenografts had equitable survival and developed
`similar size tumors between those treated and not treated with
`the AGI-5198 S-enantiomer (81). The presentation of mixed
`
`GlaxoSmithKline
`
`–
`
`–
`
`Merck
`
`–
`
`mousemodel)
`>1in
`plasmaratio
`(brain-to-
`Yes
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`–
`
`(EC50):85nM
`HT1080(IDH1R132C)
`Cell-Based:
`IDH1R132G:2.9nM
`IDH1R132C:3.8nM
`IDH1R132H:4.6nM
`Biochemical:
`
`BT142(IDH1R132H):∼5nM
`HT1080(IDH1R132C):∼50nM
`90-100nM
`MOG-G-UVW(IDH1R132H):
`Cell-Based:
`IDH1R132H:5nM
`Biochemical:
`
`IDH1R132G
`IDH1R132C
`IDH1R132H
`
`IDH1R132C
`IDH1R132H
`
`SPONSOR(S)
`
`Metabolism
`
`Penetrates
`
`Modeof
`
`AUC
`
`Cl
`
`Timeto
`
`Cmax
`
`T1/2
`
`BBB
`
`elimination
`
`Cmax
`
`IC50
`
`mutations
`
`Susceptible
`
`hindrance)
`conformation(steric
`inactiveenzymedimer
`stabilizationofopen,
`inhibitionvia
`non-competitive
`Reversible,allosteric,
`
`GSK321(79)mutIDH1
`
`–
`
`MRK-A(78)mutIDH1
`
`Mechanism
`
`Target
`
`Compound
`
`TABLE1|Continued
`
`Frontiers in Oncology | www.frontiersin.org
`
`7
`
`May 2019 | Volume 9 | Article 417
`
`Rigel Exhibit 1014
`Page 7 of 25
`
`

`

`Golub et al.
`
`MutIDH Inhibitors in Cancer
`
`FIGURE 2 | Chemical structures of mutIDH inhibitor compounds reviewed. MutIDH1 inhibitors: Ivosidenib (AG-120), BAY-1436032, AGI-5198, IDH305, FT-2102,
`HMS-101, MRK-A, GSK321. MutIDH2 inhibitors: Enasidenib (AG-221), AGI-6780. Pan-inhibitors: AG-881.
`
`results in glioma models is likely due to the fact that certain
`tumorigenic processes are uncoupled from the IDH1 mutation,
`and further studies into combination treatment (discussed in a
`later section) may be warranted. Popular hypotheses to explain
`this changing role of mutIDH include the generation of an
`epigenetic memory that is indelible by its enzymatic inhibition
`alone and the accumulation of additional mutations during
`tumor evolution. The poor pharmacodynamic profile of AGI-
`5198 due to its rapid metabolism and clearance has precluded its
`use in clinical trials (68).
`
`IDH305
`demonstrated
`inhibitor with
`Another mutIDH1-specific
`preclinical efficacy is IDH305, a pyrimidin-5-yl-oxazolidine-
`2-one compound recently developed by Novartis (Figure 2)
`(71). IDH305 was developed from efforts to optimize Novartis’s
`first published mutIDH inhibitor, IDH889, as this “parent”
`
`drug’s high intrinsic clearance, high plasma protein binding,
`and poor solubility posed significant challenges to further
`clinical development (82). X-ray crystallography reveals that
`IDH305 binds to an allosteric binding pocket to stabilize the
`mutIDH1 enzyme in a catalytically inactive conformation (71).
`In preclinical characterization testing, IDH305 demonstrated
`efficacious 2HG reduction in an IDH1R132H colorectal cancer
`cell line (IC50 24 nM), low liver microsomal clearance values,
`and substantial brain penetrance in murine models (71). IDH305
`has moved into clinical testing in humans with IDH-mutant
`glioma, AML/MDS, and other solid tumors, and phase 1
`safety data in all tumor types is promising (ClinicalTrials.gov
`NCT02381886) (83).
`
`AGI-6780
`The first mutIDH2-specific inhibitor to come out of development
`was AGI-6780, developed as part of
`the Celgene/Agios
`
`Frontiers in Oncology | www.frontiersin.org
`
`8
`
`May 2019 | Volume 9 | Article 417
`
`Rigel Exhibit 1014
`Page 8 of 25
`
`

`

`Golub et al.
`
`MutIDH Inhibitors in Cancer
`
`collaboration (72). AGI-6780 is a urea sulfonamide inhibitor
`of the IDH2R140Q mutant enzyme specifically and exhibits
`non-competitive inhibition with respect
`to substrate, and
`uncompetitive inhibition with respect to the NADPH cofactor,
`operating at an allosteric site at the enzyme’s dimer interface
`(Figure 2) (72, 73). In early pharmacokinetic studies, AGI-6780
`demonstrated time-dependent potency for 2HG reduction in
`both mutant homodimer and mutant/wild-type heterodimer
`enzyme contexts as well as in more organic cellular contexts (IC50
`11nM in IDH2R140Q-overexpressing U87 cells) (72). The study of
`AGI-6780’s biological effects is limited to AML-related contexts.
`The capability for erythropoietin-induced differentiation of TF-1
`erythroleukemia cells with IDH2R140Q overexpression is restored
`after AGI-6780 treatment in vitro, and the same effect was seen
`in patient-derived primary IDH2-mutant AML blood and bon