CX-5461 (Senhwa Biosciences) is a small-molecule G-quadruplex (G4) DNA stabilizer. G4 structures are four-stranded helices formed at guanine-rich sequences throughout the genome — at telomeres, oncogene promoters (c-MYC, c-KIT, VEGF), replication origins, and non-B-DNA sites. CX-5461 binds and stabilizes these structures with nanomolar affinity, physically blocking replication fork progression through G4 sites. Stalled forks collapse into single-stranded DNA gaps and double-strand breaks (DSBs) that require BRCA1/2-mediated homologous recombination (HR) for repair. Xu et al. (Nat Commun 2017, PMID 28211448) established the foundational mechanistic and preclinical evidence: CX-5461 is selectively lethal in BRCA1/2-deficient breast and ovarian cancer cell lines and in polyclonal patient-derived xenograft (PDX) models, including tumours resistant to PARP inhibition. The BRCA and NHEJ pathways are required for repair of CX-5461-induced DNA damage — failure to repair leads to selective lethality. This directly identifies BRCAness as the biomarker of sensitivity. Mechanistic rationale in SDH-deficient tumors rests on the established BRCAness phenotype. Sulkowski et al. (Nat Genet 2018, PMID 30013182; Nature 2020, PMID 32494005) demonstrated that SDH loss causes succinate accumulation → competitive inhibition of α-KG-dependent KDM4A and KDM4B histone demethylases → H3K9me3 persistence at DSB sites → impaired TIP60 acetyltransferase and ATM kinase recruitment → HR repair deficiency (BRCAness) in all SDH-deficient cells regardless of BRCA1/2 germline status. This succinate-driven constitutive BRCAness renders SDH-deficient cells functionally equivalent to BRCA1/2-mutant cells with respect to CX-5461 sensitivity. In ATRX-null/ALT-positive SDHB-metastatic PPGL (~30–40% of metastatic cases, PMID 42230482), ATRX loss further elevates the G4 DNA burden: ATRX normally resolves G4 structures and R-loops at replication forks; ATRX deficiency leads to G4 accumulation at telomeric and non-telomeric sites. This amplifies CX-5461 sensitivity beyond the BRCAness baseline — a compounded vulnerability from SDH-driven BRCAness PLUS ATRX-driven G4 overload. This mechanism is non-redundant with ceralasertib (Mechanism 13), which inhibits ATR kinase to exploit ATRX-null dependence on ATR-mediated fork stabilization; CX-5461 stabilizes the upstream G4 structures themselves, creating the initial DNA damage rather than preventing the cellular response to it. Mechanistic distinctions from other DDR drugs in the engine: - PARP inhibitors (olaparib, niraparib): trap PARP at SSBs → SSBs collapse into DSBs at forks → BRCAness-selective lethality via DSB generation at forks. CX-5461 generates DSBs directly via G4 stabilization — a distinct upstream source of HR-requiring damage. Xu et al. 2017 specifically demonstrated CX-5461 activity in PARP inhibitor-resistant BRCA-deficient tumors. - ART558 (POLQ inhibitor): blocks alt-EJ/TMEJ DSB repair backup — acts after DSB generation, not at DSB generation. - Elimusertib (DNA-PKcs inhibitor): blocks c-NHEJ backup repair — also acts after DSB generation. - Prexasertib (CHK1 inhibitor): prevents replication fork stabilization checkpoint — acts at checkpoint, not DSB generation. - Ceralasertib (ATR inhibitor): inhibits ATR kinase to exploit ATRX-null/ALT fork stabilization dependency — acts at ATR signaling response, not G4 stabilization. CX-5461 occupies the upstream position in the synthetic lethality chain — it generates the G4-induced DSB load that BRCAness-positive SDH-deficient cells cannot resolve. Clinical development: NCT02719977 (Phase 1; Senhwa Biosciences/BC Cancer; BRCA1/2-deficient hematologic malignancies; opened May 2016) established first-in-human pharmacology specifically selecting for the BRCAness patient population. NCT03914288 (Phase 1b; BRCA1/2-mutated solid tumours) extended evaluation to solid tumor histologies including gynecological, breast, and other BRCA-deficient cancers. Key limitations: (1) No published data directly test CX-5461 in SDH-deficient GIST, PPGL, or RCC cell lines or xenograft models; the rationale extrapolates from the Sulkowski BRCAness mechanism (PMID 30013182, 32494005) and the Xu et al. G4/BRCA-selective lethality data (PMID 28211448). (2) Clinical trial data from NCT02719977 and NCT03914288 are in BRCA-germline-mutant cancers, not succinate-driven BRCAness — direct SDH-deficient cohort data are absent. (3) CX-5461 has been reported to inhibit RNA polymerase I (rDNA transcription); the relative contribution of G4 stabilization versus Pol I inhibition to cytotoxicity remains incompletely resolved, though Xu et al. 2017 provided direct evidence for the G4 mechanism. Evidence_score 30 (preclinical) reflects a strong mechanistic chain anchored by the Xu et al. G4/BRCA-selective lethality data and the Sulkowski SDH-BRCAness mechanism, combined with Phase 1 clinical pharmacology in BRCA-deficient patients, in the absence of SDH-specific experimental validation.