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RBS2418 (Uzaribat)
ENPP1 inhibitor
Evidence Score
26
RBS2418 (uzaribat; Riboscience) is an oral, selective small-molecule inhibitor of ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), the primary extracellular enzyme that degrades 2'3'-cyclic GMP-AMP (cGAMP) — the immunotransmitter produced by cGAS upon sensing cytosolic DNA. The mechanistic rationale in SDH-deficient tumors flows from the established BRCAness phenotype. Sulkowski et al. (Nat Genet 2018, PMID 30013182; Nature 2020, PMID 32494005) demonstrated that SDH loss → succinate accumulation → inhibition of α-KG-dependent KDM4A and KDM4B histone demethylases → H3K9me3 persistence at double-strand break (DSB) sites → impaired TIP60/ATM activation → HR deficiency (BRCAness). BRCAness-positive cells accumulate unresolved DSBs → chromatin fragments and micronuclei form → cytosolic DNA engages cGAS. Mackenzie et al. (Nature 2017, PMID 28953876) established that micronuclei arising from genomic instability activate cGAS-STING, producing IFN-β. Pantelidou et al. (Immunity 2019, PMID 31076331) demonstrated in BRCA1-deficient breast tumors that BRCAness generates STING-dependent type I IFN signaling that drives cytotoxic T-cell infiltration, and that PARP inhibitor treatment amplifies this immune activation — establishing a general principle that BRCAness → cGAS-STING innate immune activation that applies to SDH-deficient BRCAness by extension. ENPP1 degrades extracellular cGAMP before it can engage STING on infiltrating dendritic cells and NK cells. Carozza et al. (Cell 2022, PMID 36265508) demonstrated that ENPP1 is the dominant cGAMP phosphodiesterase in the tumor extracellular matrix and that ENPP1 inhibition preserves tumor-derived cGAMP, enhances STING signaling in immune cells, promotes CD8+ T-cell infiltration, and produces anti-tumor efficacy in syngeneic mouse models. ENPP1 inhibition converts the BRCAness-driven cytosolic DNA signal — constitutively present in SDH-deficient tumors — into a sustained cGAMP signal that drives innate-to-adaptive immune priming. Clinical development: NCT04727138 (Phase 1/2; RBS2418 monotherapy and in combination with pembrolizumab in locally advanced or metastatic solid tumors; Riboscience; open). RBS2418 is a rational combinatorial partner with pembrolizumab (already in this engine): STING-driven IFN-β upregulates CD274 (PD-L1), creating an opportunity for PD-1 blockade to convert innate immune activation into durable adaptive anti-tumor immunity — an ENPP1i + anti-PD-1 combination strategy directly reflected in the NCT04727138 design. Key limitations: (1) No published data test RBS2418 or any ENPP1 inhibitor in SDH-deficient cell lines or animal models. (2) cGAS-STING activation from BRCAness in SDH-deficient tumors specifically (as opposed to BRCA1/2-mutant tumors) has not been directly demonstrated; the rationale is a mechanistically sound extension of established BRCAness biology (Sulkowski 2018/2020) and cGAS-STING biology in HR-deficient cells (Mackenzie 2017, Pantelidou 2019). (3) Paradoxical STING-driven PD-L1 upregulation could blunt the immune response if not paired with checkpoint blockade. (4) SDH-deficient cells engage multiple overlapping immune evasion arms (succinate-MCT1, HIF-IDO1, HIF-PD-L1); ENPP1 inhibition addresses innate cGAMP sensing but cannot alone reverse all suppressive mechanisms.
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:
ENPP1
Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1/NPC-PDE1α)
Type II transmembrane ectonucleotidase and the dominant extracellular phosphodiesterase responsible for degrading 2'3'-cyclic GMP-AMP (cGAMP) — the immunotransmitter produced by cGAS upon sensing cytosolic double-stranded DNA, including chromatin fragments and micronuclei generated from BRCAness-driven unresolved DSBs in SDH-deficient tumor cells. ENPP1 hydrolyzes cGAMP to 5'-AMP + GMP, terminating its STING-agonist activity before cGAMP can engage STING on surrounding dendritic cells, NK cells, and T cells. Carozza et al. (Cell 2022, PMID 36265508) identified ENPP1 as the primary cGAMP-degrading enzyme in the tumor extracellular matrix and demonstrated that ENPP1 inhibition increases extracellular cGAMP levels, enhances STING signaling in immune cells, increases intratumoral CD8+ T-cell infiltration, and suppresses tumor growth in syngeneic models. ENPP1 inhibition therefore converts the BRCAness-driven cytosolic DNA burden constitutively present in SDH-deficient tumors into a sustained, immune-activating cGAMP signal — complementary to the PD-1/PD-L1 checkpoint arm (pembrolizumab, Mechanism 22) and distinct from the metabolic immune suppression axes (AZD3965/MCT1, epacadostat/IDO1, Mechanism 11). RBS2418 (uzaribat, Riboscience) is the clinical-stage ENPP1 inhibitor in Phase 1/2 evaluation.
UniProt: P22413
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.