Pharmacological Targeting of the Heme-Regulated Inhibitor Kinase Overcomes Ineffective Erythropoiesis in Patients with Myelodysplastic Syndromes with Ringed Sideroblasts and Sickle Cell Disease

The integrated stress response (ISR), a cytoprotective pathway activated upon phosphorylation of the eukaryotic translation initiation factor 2 (eIF2α), is critical for translational control in response to various stress conditions. The eIF2α kinases heme-regulated inhibitor (HRI), protein kinase R (PKR), PKR-like endoplasmic reticulum kinase, and general control non-depressible 2 are induced by heme depletion, viral infection, endoplasmic reticulum stress, and amino acid starvation, respectively.

We previously showed that the SF3B1 mutation–induced aberrant splicing of heme mitochondrial transporters, which trap heme in mitochondria, results in HRI pathway activation, abnormal terminal erythroid differentiation, and ringed sideroblastic formation (Adema et al., Blood Cancer Discov. 2022). The CRISPR/Cas9-mediated depletion of HRI improved the terminal erythropoiesis of erythroblasts from myelodysplastic syndromes with ringed sideroblasts (MDS-RS) by increasing the expression of mitochondrial heme transporters, which suggests that the pharmacological inhibition of HRI can rescue ineffective erythropoiesis and anemia in patients with MDS-RS. The inhibition of the HRI pathway is also one of the most promising therapeutic strategies for sickle cell disease (SCD). In sickle cells, HRI inhibition represses BCL11A and increases gamma globin (HBG1/2) expression, producing fetal hemoglobin.

We partnered with MD Anderson's Institute of Applied Cancer Science to identify potent, selective, and orally bioavailable small-molecule inhibitors of HRI. Using a virtual screening and leveraging structural insights from other ISR kinase family inhibitor programs, we designed and synthesized more than 200 diverse compounds. To develop lead HRI inhibitors and optimize their ability to inhibit HRI’s function, we established a robust screening platform that includes biochemical and cellular assays to monitor HRI’s phosphorylation of eIF2α on serine 51 and assess candidate inhibitors’ impact on differentiation and gene expression in MDS-RS primary samples.

Two compounds, IACS-18148 and IACS-77717, inhibited HRI kinase catalytic activity with half-maximal inhibitory concentration (IC₅₀) values of 7.2 and 1.3 nM and had 140- and 650-fold selectivity for HRI over the other ISR family members, respectively. In CRISPR/Cas9-edited K562 cells with the SF3B1^K700E knock-in mutation, treatment with IACS-18148 or IACS-77717 (0.01-1 µM) for 30 min potently inhibited eIF2α phosphorylation and ATF4 expression, which confirmed these drugs’ target engagement (IC₅₀= 88nM or 46nM, respectively).

Erythroblasts derived from CD34⁺ cells isolated from MDS-RS SF3B1-mutant patients (n=3) cultured in erythroid differentiation conditions for 8 days and then treated with IACS-18148 for 5 days had a higher co-expression of the CD71 and CD235a markers of terminal differentiation than those treated with vehicle (80.3% ± 18.9 vs. 50.9% ± 17.5; P=0.022, respectively). Improved erythroid differentiation was associated with decreased eIF2α phosphorylation and increased expression of genes involved in heme mitochondrial transport, metabolism, and globin synthesis. In CD34⁺ cell–derived erythroblasts from SCD patients, IACS-18148 and IACS-77717 decreased BCL11A by 1.6- and 1.2-fold and increased HBG1/2 expression by 3.2- and 4.6-fold, respectively, after 13 days of differentiation.

Together, these data demonstrate that compounds targeting HRI signaling overcome ineffective erythropoiesis in MDS-RS and rescue fetal hemoglobin production in SCD and thus may overcome anemia and transfusion needs in patients with these diseases.