Abstract 4467: Synergistic antileukemic activity of JAK inhibition combined with cytarabine in acute megakaryoblastic leukemia.

Abstract Background: Acute megakaryoblastic leukemia (AMKL) remains a therapeutic challenge, with limited responsiveness to standard cytarabine-based regimens and frequent microenvironment-mediated resistance. Aberrant activation of the JAK-STAT signaling pathway has been implicated in AMKL pathogenesis but has not been fully exploited therapeutically in combination with cytarabine. Methods: We evaluated the antileukemic efficacy of combining a JAK inhibitor with cytarabine across multiple preclinical models: (i) AMKL cell lines (including CMK, CMY, MKPL1, UT7, MOLM16, Mo7e) assessing viability, apoptosis, and combination index; (ii) three-dimensional (3D) co-culture with mesenchymal cells (MSCs) modelling bone-marrow niche-mediated protection; and (iii) murine xenograft models of AMKL assessing leukemic burden and survival. Results: In cell-line experiments, the JAK inhibitor plus cytarabine demonstrated marked synergy (combination index < 0.7) with significantly increased apoptosis (Annexin V+) and reduced proliferation compared to either agent alone. In the 3D MSC co-culture model, the microenvironment protected AMKL cells from cytarabine-induced death, but this protective effect was abrogated by JAK inhibition, restoring cytarabine sensitivity especially in MOLM16 and MKPL1 cells. In vivo, combination therapy significantly reduced leukemic burden (p < 0.05) and significantly prolonged median survival compared to cytarabine alone. Mechanistically, ruxolitinib suppressed phosphorylation of STAT3 and STAT5 and exerted a synergistic effect in combination with cytarabine; however, this inhibitory effect on phosphorylation was attenuated under the 3D co-culture condition. Conclusions: Our results provide robust preclinical evidence that combining JAK inhibition with cytarabine yields synergistic antileukemic effects in AMKL, including in a niche-protected context, and significantly improves in vivo outcomes. This data strongly supports the rationale for clinical evaluation of this combination in AMKL patients, especially those with JAK-STAT pathway activation. Citation Format: Akira Shimada, Shiho Aoki, Hiroko Hayakawa, Erico Jimbo, Narumi Omika, Kaito Furuya, Hideya Asai, Hiroki Yoshinari, Hitomi Niijima, Yuta Kawahara, Kentaro Ushijima, Masakazu Mimaki, Mitsuteru Hiwatari. Synergistic antileukemic activity of JAK inhibition combined with cytarabine in acute megakaryoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4467.