Investigation of underlying mechanisms and novel therapies for human acquired and congenital disorders of hematopoiesis

National Heart, Lung, And Blood Institute

Investigators

Linked publications & trials

Abstract

We developed a translational and clinical research program investigating the efficacy and safety of in vivo stimulation of hematopoiesis via the drug eltrombopag (EPAG), a small molecular that binds to the c-mpl receptor on hematopoetic stem and progenitor cells, demonstrating efficacy in refractory severe aplastic anemia, de novo severe aplastic anemia, moderate aplastic anemia, and myelodysplasia. During the current reporting period, we focused on predictors for response and molecular/cytogenetic progression and outcomes, participating in collaborative research with the group of Dr Neal Young on these topics. A major focus that resulted from our initial program studying the role of EPAG in marrow failure was the striking response to the drug in a patient with Diamond-Blackfan anemia, an inherited severe hypoproliferative anemia shown to result from mutations resulting in haploinsufficiency of one of a a group of ribosomal protein genes. This response was surprising, and led us to investigate the mechanism and pursue EPAG as a possible new therapy for DBA. Based on data generated in all of our EPAG bone marrow failure trials, we observed that EPAG is a potent iron chelator. In our large cohort study, we found that EPAG treatment results in rapid and clinically-significant iron unloading. Response rates and relapse rates were not impacted by initial iron status, suggesting that in aplastic anemia the activity of EPAG is linked to HSC stimulation not iron unloading. However, several patients have required oral iron supplementation while on long-term EPAG to avoid clinically-relevant iron deficiency (Young et al, 2022) The observation that EPAG reversed anemia in the inherited ribosomopathy DBA in the single DBA patient enrolled in the moderate AA/cytopenia trial along with new knowledge regarding possible DBA pathophysiology led us to hypothesize that the possible effectiveness of EPAG in DBA may be due to the potent intracellular chelating activity of EPAG. Recent laboratory studies suggest that erythroid development is inhibited in DBA due to slowed protein synthesis in erythroid progenitors, with a resulting imbalance in global chain production versus heme biosynthesis, leading to free heme/increased intracellular iron and toxic accumulation of reactive oxygen species. We have designed and now completed accrual and follow-up for a clinical trial to investigate the safety and activity of EPAG in DBA (20-H-0021). Only 1/15 patients responded, however the majority of patients required dose reductions or drug discontinuation due to thrombocytosis. The now two total patient responses is encouraging regarding the underlying hypothesis regarding how heme depletion might improve erythroid output in DBA. This work has now been published (Duncan et al, BJH, 2024). We are now focusing on a new drug that can slow heme synthesis without inducing thrombocytosis. Bitopterin in an oral Gly1T glycine transport inhibitor, It slows heme synthesis in erythroid precursors that are dependent on glycine for the first step in producing heme. We have shown that this drug is active in improving DBA erythroid progenitor maturation in vitro and in vivo in murine models. This trial has completed enrollment of the first cohort of 15 DBA patients in a mini/max trial design. None have reached response criteria, although we saw evidence for stimulation of erythropoiesis without any unexpected toxicities, however the dosing schedule we believe was suboptimal, reaching likely erythroid-inhibitory doses and thus reversing any chance of response, based on additional information from our patients and from murine models. We have thus designed a new trial of bitopertin in patients with earlier stage/steroid-responsive but dependent disease, utilizing a slower dose escalation plateauing at lower doses. We are carrying out correlative laboratory and imaging studies to assess iron status and mechanism of EPAG and bitopertin action on samples from patients enrolled in the initial DBA trial. We have begun to investigate the mechanisms of increased risk of hematologic malignancies and clonal progression, as well as somatic rescue, in banks of samples from patients with DBA and with telomeropathies. Large scale screening for acquired somatic mutations that could contribute to malignant progression and/or somatic rescue is being performed using a sensitive error-corrected sequencing panel targeting all clonal hematopoiesis, myeloid malignancy, iron metabolism and telomere biology genes. We have accumulated over 130 samples, the largest DBA cohort ever studied and sequencing for germ line mutations, CNV and somatic mutations in both clonal hematopoiesis as well as a rationally-designed panel of genes linked to erythropoiesis and iron metabolism is ongoing. We have discovered a number of new causative mutations and CNVs and thus far find a low risk of clonal hematopoiesis. We are also creating in vitro and murine models to investigate POT1 as a common somatic mutation in patients with telomeropathies.