Mutant-selective JAK2V617F inhibition via JH2 pseudokinase domain

Current understanding

Mutant-selective JAK2V617F inhibitors target the JH2 pseudokinase domain deep pocket, where the V617F mutation resides, rather than the ATP-binding catalytic JH1 domain engaged by pan-JAK inhibitors (ruxolitinib, fedratinib, pacritinib, momelotinib) and Type II conformational JAK2 inhibitors (AJ1-11095). The mechanistic rationale is that JH2-directed allosteric inhibition preferentially suppresses the constitutively activated mutant kinase while sparing wild-type JAK2 signaling required for normal hematopoiesis, potentially yielding a wider therapeutic index and clonal selectivity not achievable with active-site inhibitors. PRT12396 (Prelude Therapeutics) is the first-in-class agent of this kind to receive FDA IND clearance, with Phase 1 dosing anticipated Q2 2026; clinical efficacy and on-target validation in patients remain to be established.

Evidence log

• February 3 — Prelude Therapeutics Receives FDA Clearance of Investigational New Drug Application for PRT12396: FDA IND clearance for PRT12396 (JH2-directed mutant-selective JAK2V617F inhibitor) confirms the class’s advance into clinical development, with Phase 1 dosing anticipated Q2 2026. (novelty: 3)
• 2026-09-24 — Selective dependency of CALR-mutant myeloproliferative neoplasms on TYK2 signaling.: cross-connection with tyk2-signaling-calr-mpn: Complementary mutant-selective pharmacology axes: JH2-targeted JAK2V617F inhibitors address JAK2-driven disease, while TYK2 inhibition (deucravacitinib) selectively addresses the CALR/MPL-driven disease that JAK2V617F-selective agents spare. (novelty: 5)
• 2026-08-01 — Type II JAK2 Inhibitor Gets Off to a Strong Start.: Early phase I efficacy of the type II JAK2 inhibitor AJ1-11095 in MF sharpens the contrast with the JH2-directed mutant-selective PRT12396 approach, since both bid to succeed type I catalytic inhibitors but via mechanistically distinct binding modes (conformational trap vs. pseudokinase allostery). (novelty: 4)
• 2026-06-13 — Lilly to present initial clinical data for first-in-class type II JAK2 inhibitor in patients with previously treated myelofibrosis at the 2026 EHA Annual Meeting: EHA 2026 readout of AJ1-11095 (type II JAK2 inhibitor) in type I JAKi-failed MF provides a contrasting clinical-stage benchmark to the JH2-directed mutant-selective approach (PRT12396), underscoring that type II conformational and JH2 allosteric strategies are mechanistically distinct routes to overcoming type I JAKi resistance. (novelty: 4)
• 2026-06-02 — Genomic insights transform diagnosis, prognosis, and therapy in BCR::ABL1-negative myeloproliferative neoplasms.: Review highlights mutation-selective JAK2 therapeutic approaches as a key emerging direction in genomically-guided MPN therapy, supporting the rationale for JH2-directed mutant-selective inhibitors. (novelty: 3)
• December 6 — Prelude Therapeutics Presents Data at 2025 ASH Annual Meeting from its Myeloproliferative Neoplasm Programs: ASH 2025 preclinical data on PRT12396 show anti-MPN activity superior to ruxolitinib while sparing wild-type JAK2-mediated cytokine signaling, with IND filing and Phase 1 initiation anticipated Q1 2026 — providing the first preclinical efficacy benchmark for this JH2-targeted mutant-selective approach. (novelty: 4)

Open questions

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