Tmprss6-ASO as a tool for the treatment of Polycythemia Vera mice

Rational In Silico Design of Selective TMPRSS6 Peptidomimetic Inhibitors via Exploitation of the S2 Subpocket

TMPRSS6 is a potential therapeutic target for the treatment of iron overload due to its role in regulating levels of hepcidin. Although potent TMPRSS6 inhibitors have been previously developed, their lack of specificity requires optimization to avoid potential side effects before pursuing preclinical development with in vivo models. Here, using computer-aided drug design based on a TMPRSS6 homology model, we reveal that the S2 position of TMPRSS6 offers a potential avenue to achieve selectivity against other members of the TTSP family. Accordingly, we synthesized novel peptidomimetic molecules containing lipophilic amino acids at the P2 position to exploit this unexplored pocket. This enabled us to identify TMPRSS6-selective small molecules with low nanomolar affinity. Finally, pharmacokinetic parameters were determined, and a compound was found to be potent in cellulo toward its primary target while retaining TTSP-subtype selectivity and showing no signs of alteration in in vitro TEER experiments.

The evolving landscape of polycythemia vera therapies

INTRODUCTION

The treatment landscape of polycythemia vera (PV) has seen major advancements within the last decade including approval of ruxolitinib in the second line setting after hydroxyurea, ropegylated interferon-α2b, and advanced clinical development of a novel class of agents called hepcidin mimetics.

AREAS COVERED

We provide a comprehensive review of the evidence discussing the risk stratification, treatment indications, role and limitations of phlebotomy only approach and pivotal trials covering nuances related to the use of interferon-α (IFN-α), ruxolitinib, hepcidin mimetics, and upcoming investigational agents including HDAC and LSD1 inhibitors.

EXPERT OPINION

The research paradigm in PV is slowly shifting from the sole focus on hematocrit control and moving toward disease modification. The discovery of hepcidin mimetics has come as a breakthrough in restoring iron homeostasis, achieving phlebotomy-independence and may lead to improved thrombosis-free survival with stricter hematocrit control. On the other hand, emerging data with IFN- α and ruxolitinib as well as combination of the two agents suggests the potential for achieving molecular remission in a subset of PV patients and long-term follow-up is awaited to validate the correlation of molecular responses with clinically relevant outcomes of progression-free and thrombosis-free survival.

Iron restriction in sickle cell disease: When less is more

Primum non nocere! Can iron deficiency, an abnormality that causes anemia, benefit people with sickle cell disease (SCD) who already have an anemia? The published literature we review appears to answer this question in the affirmative: basic science considerations, animal model experiments, and noncontrolled clinical observations all suggest a therapeutic potential of iron restriction in SCD. This is because SCD's clinical manifestations are ultimately attributable to the polymerization of hemoglobin S (HbS), a process strongly influenced by intracellular HbS concentration. Even small decrements in HbS concentration greatly reduce polymerization, and iron deficiency lowers erythrocyte hemoglobin concentration. Thus, iron deficiency could improve SCD by changing its clinical features to those of a more benign anemia (i.e., a condition with fewer or no vaso-occlusive events). We propose that well-designed clinical studies be implemented to definitively determine whether iron restriction is a safe and effective option in SCD. These investigations are particularly timely now that pharmacologic agents are being developed, which may directly reduce red cell hemoglobin concentrations without the need for phlebotomies to deplete total body iron.

Rusfertide, a Hepcidin Mimetic, for Control of Erythrocytosis in Polycythemia Vera

BACKGROUND

Polycythemia vera is a chronic myeloproliferative neoplasm characterized by erythrocytosis. Rusfertide, an injectable peptide mimetic of the master iron regulatory hormone hepcidin, restricts the availability of iron for erythropoiesis. The safety and efficacy of rusfertide in patients with phlebotomy-dependent polycythemia vera are unknown.

METHODS

In part 1 of the international, phase 2 REVIVE trial, we enrolled patients in a 28-week dose-finding assessment of rusfertide. Part 2 was a double-blind, randomized withdrawal period in which we assigned patients, in a 1:1 ratio, to receive rusfertide or placebo for 12 weeks. The primary efficacy end point was a response, defined by hematocrit control, absence of phlebotomy, and completion of the trial regimen during part 2. Patient-reported outcomes were assessed by means of the modified Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) patient diary (scores range from 0 to 10, with higher scores indicating greater severity of symptoms).

RESULTS

Seventy patients were enrolled in part 1 of the trial, and 59 were assigned to receive rusfertide (30 patients) or placebo (29 patients) in part 2. The estimated mean (±SD) number of phlebotomies per year was 8.7±2.9 during the 28 weeks before the first dose of rusfertide and 0.6±1.0 during part 1 (estimated difference, 8.1 phlebotomies per year). The mean maximum hematocrit was 44.5±2.2% during part 1 as compared with 50.0±5.8% during the 28 weeks before the first dose of rusfertide. During part 2, a response was observed in 60% of the patients who received rusfertide as compared with 17% of those who received placebo (P = 0.002). Between baseline and the end of part 1, rusfertide treatment was associated with a decrease in individual symptom scores on the MPN-SAF in patients with moderate or severe symptoms at baseline. During parts 1 and 2, grade 3 adverse events occurred in 13% of the patients, and none of the patients had a grade 4 or 5 event. Injection-site reactions of grade 1 or 2 in severity were common.

CONCLUSIONS

In patients with polycythemia vera, rusfertide treatment was associated with a mean hematocrit of less than 45% during the 28-week dose-finding period, and the percentage of patients with a response during the 12-week randomized withdrawal period was greater with rusfertide than with placebo. (Funded by Protagonist Therapeutics; REVIVE ClinicalTrials.gov number, NCT04057040.).

Novel potential therapeutics to modify iron metabolism and red cell synthesis in diseases associated with defective erythropoiesis

Under normal conditions, iron metabolism is carefully regulated to sustain normal cellular functions and the production of hemoglobin in erythroid cells. Perturbation to the erythropoiesis-iron metabolism axis can result in iron imbalances and cause anemia or organ toxicity. Various congenital and acquired diseases associated with abnormal red cell production are characterized by aberrant iron absorption. Several recent studies have shown that improvements in red blood cell production also ameliorate iron metabolism and vice versa. Many therapeutics are now under development with the potential to improve a variety of hematologic diseases, from β-thalassemia and iron-refractory iron deficiency anemia to anemia of inflammation and polycythemia vera. This review summarizes selected mechanisms related to red cell production and iron metabolism and describes potential therapeutics and their current uses. We also consider the potential application of the discussed therapeutics on various diseases, alone or in combination. The vast repertoire of drugs under development offers new opportunities to improve the clinical care of patients suffering from congenital or acquired red blood cell disorders with limited or no treatment options.

Normal and dysregulated crosstalk between iron metabolism and erythropoiesis

Erythroblasts possess unique characteristics as they undergo differentiation from hematopoietic stem cells. During terminal erythropoiesis, these cells incorporate large amounts of iron in order to generate hemoglobin and ultimately undergo enucleation to become mature red blood cells, ultimately delivering oxygen in the circulation. Thus, erythropoiesis is a finely tuned, multifaceted process requiring numerous properly timed physiological events to maintain efficient production of 2 million red blood cells per second in steady state. Iron is required for normal functioning in all human cells, the erythropoietic compartment consuming the majority in light of the high iron requirements for hemoglobin synthesis. Recent evidence regarding the crosstalk between erythropoiesis and iron metabolism sheds light on the regulation of iron availability by erythroblasts and the consequences of insufficient as well as excess iron on erythroid lineage proliferation and differentiation. In addition, significant progress has been made in our understanding of dysregulated iron metabolism in various congenital and acquired malignant and non-malignant diseases. Finally, we report several actual as well as theoretical opportunities for translating the recently acquired robust mechanistic understanding of iron metabolism regulation to improve management of patients with disordered erythropoiesis, such as anemia of chronic inflammation, β-thalassemia, polycythemia vera, and myelodysplastic syndromes.

Iron homeostasis governs erythroid phenotype in polycythemia vera

Polycythemia vera (PV) is a myeloproliferative neoplasm driven by activating mutations in JAK2 that result in unrestrained erythrocyte production, increasing patients' hematocrit and hemoglobin concentrations, placing them at risk of life-threatening thrombotic events. Our genome-wide association study of 440 PV cases and 403 351 controls using UK Biobank data showed that single nucleotide polymorphisms in HFE known to cause hemochromatosis are highly associated with PV diagnosis, linking iron regulation to PV. Analysis of the FinnGen dataset independently confirmed overrepresentation of homozygous HFE variants in patients with PV. HFE influences the expression of hepcidin, the master regulator of systemic iron homeostasis. Through genetic dissection of mouse models of PV, we show that the PV erythroid phenotype is directly linked to hepcidin expression: endogenous hepcidin upregulation alleviates erythroid disease whereas hepcidin ablation worsens it. Furthermore, we demonstrate that in PV, hepcidin is not regulated by expanded erythropoiesis but is likely governed by inflammatory cytokines signaling via GP130-coupled receptors. These findings have important implications for understanding the pathophysiology of PV and offer new therapeutic strategies for this disease.

JAK2 V617F allele burden in polycythemia vera: burden of proof

Polycythemia vera (PV) is a hematopoietic stem cell neoplasm defined by activating somatic mutations in the JAK2 gene and characterized clinically by overproduction of red blood cells, platelets, and neutrophils; a significant burden of disease-specific symptoms; high rates of vascular events; and evolution to a myelofibrosis phase or acute leukemia. The JAK2V617F variant allele frequency (VAF) is a key determinant of outcomes in PV, including thrombosis and myelofibrotic progression. Here, we critically review the dynamic role of JAK2V617F mutation burden in the pathogenesis and natural history of PV, the suitability of JAK2V617F VAF as a diagnostic and prognostic biomarker, and the utility of JAK2V617F VAF reduction in PV treatment.

TMPRSS6 as a Therapeutic Target for Disorders of Erythropoiesis and Iron Homeostasis

TMPRSS6 is a serine protease highly expressed in the liver. Its role in iron regulation was first reported in 2008 when mutations in TMPRSS6 were shown to be the cause of iron-refractory iron deficiency anemia (IRIDA) in humans and in mouse models. TMPRSS6 functions as a negative regulator of the expression of the systemic iron-regulatory hormone hepcidin. Over the last decade and a half, growing understanding of TMPRSS6 biology and mechanism of action has enabled development of new therapeutic approaches for patients with diseases of erythropoiesis and iron homeostasis.ClinicalTrials.gov identifier NCT03165864.

Hepcidin and its multiple partners: Complex regulation of iron metabolism in health and disease

The peptide hormone hepcidin is central to the regulation of iron metabolism, influencing the movement of iron into the circulation and determining total body iron stores. Its effect on a cellular level involves binding ferroportin, the main iron export protein, preventing iron egress and leading to iron sequestration within ferroportin-expressing cells. Hepcidin expression is enhanced by iron loading and inflammation and suppressed by erythropoietic stimulation. Aberrantly increased hepcidin leads to systemic iron deficiency and/or iron restricted erythropoiesis as occurs in anemia of chronic inflammation. Furthermore, insufficiently elevated hepcidin occurs in multiple diseases associated with iron overload such as hereditary hemochromatosis and iron loading anemias. Abnormal iron metabolism as a consequence of hepcidin dysregulation is an underlying factor resulting in pathophysiology of multiple diseases and several agents aimed at manipulating this pathway have been designed, with some already in clinical trials. In this chapter, we assess the complex regulation of hepcidin, delineate the many binding partners involved in its regulation, and present an update on the development of hepcidin agonists and antagonists in various clinical scenarios.

Hepcidin mimetics in polycythemia vera: resolving the irony of iron deficiency and erythrocytosis

PURPOSE OF REVIEW

Development of hepcidin therapeutics has been a ground-breaking discovery in restoring iron homeostasis in several haematological disorders. The hepcidin mimetic, rusfertide, is in late-stage clinical development for treating polycythemia vera patients with a global phase 3 trial [NCT05210790] currently underway. Rusfertide serves as the first possible noncytoreductive therapeutic option to maintain haematocrit control and avoid phlebotomy in polycythemia vera patients. In this comprehensive review, we discuss the pathobiology of dysregulated iron metabolism in polycythemia vera, provide the rationale for targeting the hepcidin-ferroportin axis and elaborate on the preclinical and clinical trial evidence supporting the role of hepcidin mimetics in polycythemia vera.

RECENT FINDINGS

Recently, updated results from two phase 2 clinical trials [NCT04057040 & NCT04767802] of rusfertide (PTG300) demonstrate that the drug is highly effective in eliminating the need for therapeutic phlebotomies, normalizing haematological parameters, repleting iron stores and relieving constitutional symptoms in patients with polycythemia vera. In light of these findings, additional hepcidin mimetic agents are also being evaluated in polycythemia vera patients.

SUMMARY

Hepcidin agonists essentially serve as a 'chemical phlebotomy' and are poised to vastly improve the quality of life for phlebotomy requiring polycythemia vera patients.

Functionally impaired isoforms regulate TMPRSS6 proteolytic activity

TMPRSS6 is a type II transmembrane serine protease involved in iron homeostasis expressed as 4 isoforms in humans. TMPRSS6 isoform 2 downregulates hepcidin production by cleaving hemojuvelin and other surface proteins of hepatocytes. The functions of catalytically impaired isoforms 3 and 4 are still unknown. Here we demonstrate that TMPRSS6 isoforms 3 and 4 reduce the proteolytic activity of isoform 2 and uncover the ability of isoforms to interact. Moreover, we identified 49 potential protein partners common to TMPRSS6 isoforms, including TfR1, known to be involved in iron regulation. By co-expressing TMPRSS6 and TfR1, we show that TfR1 is cleaved and shed from the cell surface. Further, we demonstrate that TMPRSS6 isoforms 3 and 4 behave as dominant negative.

Treatment and Clinical Endpoints in Polycythemia Vera: Seeking the Best Obtainable Version of the Truth

Polycythemia vera (PV) is a Philadelphia chromosome-negative myeloproliferative neoplasm driven by the JAK2 V617F (or rarely exon 12) mutation. Its natural history can extend over a few decades, and therefore treatment planning is predicated on continual re-assessment of traditional risk features (age, prior thrombosis) to evaluate the need for cytoreduction besides foundational therapy with low-dose aspirin and stringent phlebotomy. Shorter- and longer-term patient goals should be considered in light of several variables such as co-morbid conditions (especially cardiovascular risk factors), disease symptoms, and the risk-benefit profile of available drugs. While hydroxyurea has been the pro forma choice of cytoreduction for many practitioners over the last half-century, the more recent regulatory approvals of ruxolitinib and ropeginterferon-alfa-2b, based on phase 3 randomized trials, highlight an expanding portfolio of active drugs. Obtaining high-level evidence for short-term clinical trial endpoints such as hematocrit control, symptom burden/quality of life, splenomegaly, and JAK2 V617F allele burden lies within the timeline of most studies. However, in many cases, it may not be possible to adequately power trials to capture significant differences in the typically low event rates of thrombosis, as well as longer-horizon endpoints such as evolution to myelofibrosis and acute myeloid leukemia, and survival. This Perspective highlights the challenges of addressing these data gaps and outstanding questions in the emerging treatment landscape of PV.