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Ulevostinag
MK-1454
Cyclic dinucleotide STING agonist (intratumoral)
Evidence Score
25
Ulevostinag (MK-1454; Merck) is a synthetic cyclic dinucleotide analogue of cGAMP that directly binds and activates STING (STING1/TMEM173) in tumor-infiltrating immune cells and tumor cells. It bypasses the upstream cGAS sensing step entirely, inducing STING conformational changes that recruit TBK1 → IRF3 phosphorylation → IFN-β and ISG transcription → innate immune priming of the tumor microenvironment. In SDH-deficient tumors, the mechanistic rationale flows from the established BRCAness phenotype. Sulkowski et al. (Nat Genet 2018, PMID 30013182; Nature 2020, PMID 32494005) demonstrated that SDH loss causes succinate accumulation → inhibition of α-KG-dependent KDM4A/KDM4B histone demethylases → H3K9me3 persistence at double-strand break (DSB) sites → impaired TIP60/ATM recruitment → HR deficiency (BRCAness). BRCAness-positive cells accumulate chromosomal instability from unrepaired DSBs, leading to mis-segregation during mitosis and formation of micronuclei — chromosomal fragments enclosed in abnormal nuclear membranes. Mackenzie et al. (Nature 2017, PMID 28738408) demonstrated that rupture of the micronuclear envelope exposes chromatin to the cytoplasm, where cGAS rapidly accumulates and is activated, producing 2′3′-cGAMP that activates STING → TBK1 → IRF3 → IFN-β. Ulevostinag directly amplifies the terminal effector (STING activation) of this BRCAness-driven innate immune chain, independently of upstream cGAS sensing efficiency. Mechanistic caveat: Liu et al. (Nature 2018, PMID 30356214) identified a distinct nuclear pool of cGAS that, following DNA damage, translocates to the nucleus and suppresses HR by interacting with PARP1 and impairing the PARP1-Timeless complex — a pro-tumorigenic function. This nuclear cGAS effect is mechanistically separate from cytoplasmic/micronuclear cGAS (which activates STING); ulevostinag acts directly at STING and is entirely independent of nuclear cGAS biology. Clinical anchor: Harrington et al. (Clin Cancer Res 2025, PMID 40499147) reported Phase I (NCT03010176) results of intratumoral ulevostinag ± pembrolizumab in 156 patients with advanced solid tumors or lymphomas. Dose-dependent pharmacokinetics were established; circulating IP-10, IFNγ, and IL-6 showed transient elevation at 2–8 h post-injection, directly confirming STING activation and type I IFN response in vivo. The recommended Phase 2 dose was 540 µg intratumoral. In a Phase II expansion in HNSCC (NCT04220866), combination with pembrolizumab yielded 4/8 CR/PR vs 1/10 with pembrolizumab monotherapy, suggesting STING activation can potentiate checkpoint immunotherapy responses. Key limitations: (1) Intratumoral administration — ulevostinag is delivered by direct tumor injection; diffusely metastatic SDH-deficient tumors may not be accessible. (2) No SDH-specific data exist — all published data are in unselected solid tumor populations; BRCAness-enriched patient selection is theoretically motivated but untested. (3) The BRCAness → chromosomal instability → micronuclei → cGAS-STING chain is plausible from established BRCAness biology and the Mackenzie 2017 paper, but has not been directly quantified in SDH-deficient models. (4) Low ORR in the ulevostinag Phase I monotherapy arm (no confirmed PRs in 22 patients) suggests that biomarker-selected populations — potentially including BRCAness-positive tumors — may be needed for clinical efficacy. (5) Evidence_score 25 (theoretical) reflects a well-anchored multi-step mechanistic chain (PMID 30013182, 32494005, 28738408, 40499147) with no direct SDH-specific experimental validation.
Succinate accumulation competitively inhibits the α-KG-dependent histone demethylases KDM4A and KDM4B (JMJD2A/B), which normally erase repressive H3K9me3 marks at sites of DNA double-strand breaks. When KDM4B is inhibited, H3K9me3 hypermethylation persists at break sites, blocking recruitment of TIP60 acetyltransferase and ATM kinase — both required for DNA end-resection and initiation of homology-directed repair (HDR/HR). The result is a 'BRCAness' phenotype: SDH-deficient tumor cells have impaired HR capacity despite wild-type BRCA1/2. Sulkowski et al. (Nat Genet 2018, PMID: 30013182) directly demonstrated HR deficiency and olaparib hypersensitivity in cells and tumors from SDH-deficient hereditary paraganglioma/PPGL patients; Sulkowski et al. (Nature 2020, PMID: 32494005) dissected the KDM4B/H3K9me3 chromatin mechanism.
Upstream event:
SDH loss → succinate accumulation → competitive inhibition of KDM4A/KDM4B (α-KG-dependent H3K9me3 demethylases) → H3K9me3 persistence at DNA double-strand break sites → impaired TIP60/ATM recruitment → defective DNA end-resection → HR deficiency
Downstream effects:
The BRCAness phenotype established in all SDH-deficient tumors (Sulkowski et al. Nat Genet 2018, PMID 30013182; Nature 2020, PMID 32494005) generates constitutive replication stress and chromosomal instability. Stalled, unrepaired replication forks lead to chromosomal mis-segregation during mitosis and formation of micronuclei — fragments of chromatin enclosed in abnormal nuclear membranes. Mackenzie et al. (Nature 2017, PMID 28738408) demonstrated that cGAS (cyclic GMP-AMP synthase; CGAS/MB21D1) localises to ruptured micronuclei and is activated by the exposed chromatin, producing 2′3′-cGAMP. This second messenger binds and activates STING (stimulator of interferon genes; STING1/TMEM173), which recruits TBK1 and activates IRF3 and NF-κB, driving IFN-β and type I interferon-stimulated gene (ISG) expression — an innate immune programme that can prime antitumor adaptive immunity. STING agonists bypass the cGAS sensing step entirely by directly binding and activating STING, amplifying this innate immune response in the SDH-deficient tumour microenvironment. Note: Liu et al. (Nature 2018, PMID 30356214) showed that a distinct nuclear pool of cGAS suppresses homologous recombination via PARP1 interaction; STING agonists act downstream of and independently from this nuclear cGAS pool. A second, complementary druggable node sits on the same axis: ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) is the dominant extracellular hydrolase for 2′3′-cGAMP. By degrading secreted cGAMP before it reaches STING on neighbouring dendritic and stromal cells, ENPP1 blunts paracrine STING signalling; ENPP1 inhibition preserves the cGAMP pool, sustains STING activation, and promotes dendritic-cell maturation and cytotoxic T-cell cross-priming.
Upstream event:
SDH loss → BRCAness (succinate → KDM4A/KDM4B inhibition → H3K9me3 at DSBs → HR deficiency) → chromosomal mis-segregation → micronuclei formation → micronuclear envelope rupture → cytoplasmic chromatin exposure → cGAS activation → cGAMP → STING → TBK1 → IRF3/NF-κB → IFN-β / ISG expression
Downstream effects:
STING1
Stimulator of interferon genes protein (STING; TMEM173)
ER-resident transmembrane adaptor protein (also known as TMEM173, MPYS, MITA, ERIS). STING binds cyclic dinucleotides including cGAMP (produced by cGAS upon cytoplasmic dsDNA sensing) and bacterial CDNs, then translocates to the Golgi where it recruits and activates TBK1. TBK1 phosphorylates IRF3, which dimerises and translocates to the nucleus to drive IFN-β and type I interferon-stimulated gene (ISG) transcription. STING also activates NF-κB for pro-inflammatory cytokine expression. In SDH-deficient tumors, the BRCAness phenotype generates chromosomal instability and micronuclei that activate cGAS → STING (Mackenzie et al. Nature 2017, PMID 28738408). Pharmacological STING agonists (ulevostinag/MK-1454; Merck) directly bind and activate STING, bypassing upstream cGAS and amplifying innate immune signaling in the tumour microenvironment. Clinical Phase I/II (NCT03010176) demonstrated dose-dependent STING target engagement (IP-10, IFNγ, IL-6 transient elevation) and manageable tolerability in advanced solid tumors (Harrington et al. Clin Cancer Res 2025, PMID 40499147).
UniProt: Q86WV6
Tumor Type Applicability
Evidence from PubMed, OpenTargets, and ChEMBL will appear here once external data integration is enabled.
Coming in Phase 3
For research exploration only — not medical advice. Consult your doctor before acting on any information.
Have Claude analyze this drug's repurposing potential for SDH-deficient diseases.