Pan-BET bromodomain inhibitor that competes with acetylated histones for binding to the BRD2, BRD3, and BRD4 bromodomains. The mechanistic rationale in SDH-deficient tumors builds directly on two converging lines of evidence. First, succinate-mediated inhibition of KDM6A (UTX) and KDM6B (JMJD3) causes global H3K27me3 accumulation (Mechanism 9), which compresses residual active chromatin (H3K27ac-marked enhancers) into spatially denser, more concentrated super-enhancer hubs — a structural reorganization that makes those hubs disproportionately dependent on BRD4 for transcription. Second, the 2026 Nature Medicine Phase 2 trial (Merriam et al., Nat Med 2026, PMID: 42191879) provided direct empirical evidence of pathological super-enhancer formation in SDH-deficient GIST: SDH-loss-driven DNA hypermethylation disrupts CTCF insulator elements at the FGF3/FGF4 locus, converting these silenced oncogenes into an ectopic super-enhancer that drives autocrine FGFR1 signaling. BRD4 is required for the transcriptional output of precisely this type of ectopic super-enhancer. Loven et al. (Cell 2013, PMID: 23582323) established that BET inhibitors preferentially displace BRD4 from super-enhancers over typical enhancers, a selectivity that arises because super-enhancers are densely acetylated and more sensitive to BRD4 dosage. Birabresib would therefore act 'upstream' of rogaratinib — disrupting super-enhancer maintenance itself rather than only the downstream FGFR signaling output, and potentially suppressing other ectopically activated super-enhancers beyond the FGF3/FGF4 locus that arise from the same CIMP epigenomic remodeling. Birabresib Phase 1b/2 trial in haematological malignancies and NUT carcinoma (NCT01713582) established pharmacokinetics and early efficacy signals in super-enhancer-addicted tumors. Key limitation: no published SDH-deficient-specific preclinical validation for BRD4 inhibition exists; the evidence chain is mechanistic inference from (a) established BRD4/super-enhancer biology, (b) confirmed ectopic super-enhancer pathology in SDH-GIST (PMID 42191879), and (c) the analogous BET-inhibitor sensitivity observed in other H3K27me3-overloaded cancer contexts. Dedicated SDH-deficient preclinical experiments are the critical next step.