Targeted inhibitors to oncogenic kinases demonstrate encouraging clinical responses early in the treatment course; however, most patients will relapse because of target-dependent mechanisms that mitigate enzyme-inhibitor binding or through target-independent mechanisms, such as alternate activation of survival and proliferation pathways, known as adaptive resistance. Here, we describe mechanisms of adaptive resistance in FMS-like receptor tyrosine kinase (FLT3)–mutant acute myeloid leukemia (AML) by examining integrative in-cell kinase and gene regulatory network responses after oncogenic signaling blockade by FLT3 inhibitors (FLT3i). We identified activation of innate immune stress response pathways after treatment of FLT3-mutant AML cells with FLT3i and showed that innate immune pathway activation via the interleukin-1 receptor–associated kinase 1 and 4 (IRAK1/4) complex contributes to adaptive resistance in FLT3-mutant AML cells. To overcome this adaptive resistance mechanism, we developed a small molecule that simultaneously inhibits FLT3 and IRAK1/4 kinases. The multikinase FLT3-IRAK1/4 inhibitor eliminated adaptively resistant FLT3-mutant AML cells in vitro and in vivo and displayed superior efficacy as compared to current targeted FLT3 therapies. These findings uncover a polypharmacologic strategy for overcoming adaptive resistance to therapy in AML by targeting immune stress response pathways.

Conflict of interest: MMW, JKJ, KM, DTS, and CJT are inventors of intellectual property involving NCGC1481 (WO 2018/038988 A2 [Compounds, Compositions, Methods for Treating Diseases, and Methods for Preparing Compounds]). MMW, JKJ, and CJT have assigned their rights to the NIH. DTS and KM have assigned their rights to the Cincinnati Children's Hospital Medical Center. DTS serves on the scientific advisory board of Kurome Therapeutics.

Targeted inhibitors to oncogenic kinases demonstrate encouraging clinical responses early in the treatment course, however, most patients will relapse due to target-dependent mechanisms that mitigate enzyme-inhibitor binding or through target-independent mechanisms, such as alternate activation of survival and proliferation pathways, known as adaptive resistance. One example involves the FMS-like receptor tyrosine kinase (FLT3). Activating mutations of FLT3 result in its autophosphorylation and initiation of intracellular signaling pathways, which induce abnormal survival and proliferation of leukemic cells.One of the most common mutations in acute myeloid leukemia (AML) involves the internal tandem duplication (ITD) of FLT3, which occurs in ~25% of all cases of newly diagnosed AML and confers a particularly poor prognosis. FLT3 inhibitors (FLT3i) evaluated in clinical studies as monotherapy and combination therapies have shown good initial response rates; however, patients eventually relapse with FLT3i-resistant disease. The absence of durable remission in patients treated with potent and selective FLT3i highlights the need to identify resistance mechanisms and develop additional treatment strategies. Several mechanisms contribute to resistance to selective FLT3i, including mutations in the tyrosine kinase domain of FLT3 (20-50%) or activation of parallel signaling mechanisms that bypass FLT3 signaling, referred to as adaptive resistance (30-50%). Here we describe mechanisms of adaptive resistance in FLT3-mutant AML by examining in-cell kinase and gene regulatory network responses after oncogenic signaling blockade by FLT3 inhibitors (FLT3i). Through this integrative approach, we identified activation of innate immune stress response pathways after treatment of FLT3-mutant AML cells with FLT3i. Utilizing genetic approaches, we demonstrated that innate immune pathway activation via IRAK1 and IRAK4 contributes to adaptive resistance in FLT3-mutant AML cells. The immediate nature of IRAK1/4 activation in adaptively resistant FLT3-ITD AML cells requires concomitant inhibition of these targets to avoid compensatory signaling and cell survival. Achieving optimal multi-drug combination regimens that yield extended overlapping exposure while avoiding unwanted toxicities is challenging. Therefore, we desired a small molecule inhibitor that simultaneously targeted the FLT3 and IRAK1/4 kinases to eradicate adaptively resistant FLT3-ITD AML. To overcome this adaptive resistance mechanism, we developed and optimized a novel small molecule that simultaneously inhibits FLT3 and IRAK1/4 kinases. The FLT3-IRAK1/4 inhibitor exhibited potent binding affinity for IRAK1 (KD= 2.9 nM), IRAK4 (KD= 0.3 nM), and FLT3 (KD= 0.3 nM), as well as acceptable pharmacokinetic properties in mice. Moreover, a high-resolution crystal structure demonstrates that the FLT3-IRAK1/4 inhibitor binds as a type I inhibitor (ATP-competitive binding to the active state). The FLT3-IRAK1/4 inhibitor eliminated adaptively resistant FLT3-mutant AML cell lines and patient-derived samples in vitro and in vivo, and displayed superior efficacy as compared to current targeted FLT3 therapies. Our study demonstrates that therapies that simultaneously inhibit FLT3 signaling and compensatory IRAK1/4 activation have the potential to improve the therapeutic efficacy in patients with FLT3-mutant AML. In conclusion, these findings reveal that inflammatory stress response pathways contribute to adaptive resistance in FLT3-mutant AML and suggests that this mechanism may extend to other malignant cells undergoing a stress-induced response to therapy. Disclosures Hoyt: Kurome Therapeutics: Consultancy. Berman:Astellas: Membership on an entity's Board of Directors or advisory committees, Research Funding. Levine:Qiagen: Membership on an entity's Board of Directors or advisory committees; Prelude Therapeutics: Research Funding; Amgen: Honoraria; Lilly: Honoraria; Gilead: Consultancy; C4 Therapeutics: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy; Roche: Consultancy, Research Funding; Imago Biosciences: Membership on an entity's Board of Directors or advisory committees; Isoplexis: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Research Funding; Loxo: Membership on an entity's Board of Directors or advisory committees. Rosenbaum:Kurome Therapeutics: Consultancy, Employment. Perentesis:Kurome Therapeutics: Consultancy. Starczynowski:Kurome Therapeutics: Consultancy.

Overexpression of immune-related genes is widely reported in Myelodysplastic syndromes (MDS), and chronic immune stimulation increases the risk for developing MDS. Aberrant innate immune activation, such as due to increased Toll-like receptor (TLR) signaling, in MDS can contribute to systemic effects on hematopoiesis in addition to cell-intrinsic defects on hematopoietic stem/progenitor cell (HSPC) function. This review will deconstruct aberrant function of TLR signaling mediators within MDS HSPC that may contribute to cell intrinsic consequences on hematopoiesis and disease pathogenesis. We will discuss the contribution of chronic TLR signaling to the pathogenesis of MDS based on evidence from patients and mouse genetic models.