A Phase 1 Dose Escalation Study of the Pyruvate Kinase Activator Mitapivat (AG-348) in Sickle Cell Disease.

Painful vaso-occlusive crisis (VOC) remains the most common reason for presenting to the Emergency Department and hospitalization in patients with sickle cell disease (SCD). Although two new agents have been approved by the Food and Drug Administration for treating SCD, they both target to reduce the frequency of VOC. Results from studies investigating various approaches to treat and shorten VOC have so far been generally disappointing. In this paper, we will summarize the complex pathophysiology and downstream events of VOC and discuss the likely reasons for the disappointing results using monotherapy. We will put forward the rationale for exploring some of the currently available agents to either protect erythrocytes un-involved in the hemoglobin polymerization process from sickling induced by the secondary events, or a multipronged combination approach that targets the complex downstream pathways of VOC.

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Vaso-occlusive crisis in sickle cell disease: a vicious cycle of secondary events.

To the Editor: Sickle cell disease (SCD) is one of the most common and debilitating inherited blood disorders affecting millions of people worldwide. SCD is characterized by a single nucleotide mutation in the β-globin gene, HBB:c.20A>T (p.Glu6Val), leading to the production of sickle hemoglobin (HbS). HbS polymerization upon deoxygenation results in poorly deformable sickled red blood cells (RBCs) with a shortened lifespan. These RBCs cause extremely painful vaso-occlusive crises (VOCs), chronic hemolytic anemia, and progressive end-organ damage. New therapeutic options have evolved very slowly despite the medical need for safe and effective therapies that both reduce VOCs and improve anemia. None of the US Food and Drug Administration (FDA) approved therapies for SCD are universally effective nor globally available, and they are limited by their drug costs and safety profiles. Mitapivat (AG-348), an investigational, first-in-class, oral, small-molecule allosteric activator of pyruvate kinase (PK), holds promise as an antisickling agent in addition to its use in PK deficiency (US FDA approved Pyrukynd USPI, 2022; NCT02476916, NCT03548220, NCT03559699, NCT03853798), and thalassemia (NCT03692052, NCT04770779, NCT04770753). PK is a key enzyme in RBC metabolism, generating adenosine triphosphate (ATP) to maintain energy homeostasis, thereby membrane integrity and deformability, and modulating 2,3-DPG levels (Figure S1). In SCD, an important metabolic feature directly associated with HbS polymerization is an increased intracellular 2,3-DPG level which promotes deoxygenation of hemoglobin (Hb) by lowering its oxygen affinity. Ex vivo treatment of RBCs of SCD patients with mitapivat restored PK activity and thermostability, increased ATP/2,3-DPG ratio, and markers of RBC sickling. Based on this and phase 1 study results (NCT04000165), we hypothesized that increased glycolytic flux through treatment with mitapivat decreases 2,3-DPG levels and increases ATP levels, thereby inhibiting HbS polymerization, RBC sickling, and ameliorating hemolytic anemia in patients with SCD. The aim of this ongoing phase 2, investigator-initiated, open-label, monocenter study (the ESTIMATE study) was to evaluate the safety and efficacy and provide proof of concept for mitapivat treatment in patients with SCD. Patients were enrolled at the Center for Benign Hematology, Thrombosis and Hemostasis Van Creveldkliniek, University Medical Center Utrecht, Utrecht, The Netherlands. Key inclusion criteria included patients ≥16 years with a diagnosis of SCD (homozygous HbS [HbSS], HbS/β or HbS/β thalassemia), prior VOCs (1–10 in the 12 months prior to study treatment) or SCD related complications (e.g., osteonecrosis, nephropathy, retinopathy, leg ulcer, acute chest syndrome, acute hemolytic crisis), and a Hb level >6.1 and ≤11.1 g/dL during screening. VOCs were defined as acute pain episodes that resulted in a healthcare provider contact and treatment with oral or parenteral narcotics, or parenteral non-steroidal anti-inflammatory drugs, acute chest syndrome, priapism, hepatic or splenic sequestration. Patients receiving hydroxyurea had to be on a stable dose for at least 3 months prior to study treatment. Major exclusion criteria were regular RBC transfusions and significant comorbidities conferring an unacceptable risk for participating. The study comprised different phases: a screening phase, an 8-week dosefinding period, a 52-week fixed-dose extension period, a 1-to-2-week taper phase to prevent hemolysis after drug interruption or withdrawal, and a safety follow-up visit 4 weeks after the last dose (Figure S2). In the here reported 8-week dose-finding period, the initial dosing of mitapivat was 20 mg twice daily. Patients received a maximum of two sequential dose escalations (i.e., from 20 to 50 mg and 100 mg twice daily) after study visit days 14 and 28 unless dosage limiting side effects or an excessive Hb response exceeding 12.9 g/dL occurred. This was based on phase 1 studies and (ongoing) phase 2–3 studies in patients with PK deficiency and thalassemia. In total nine patients were enrolled at the final analysis of the dose-finding period in December 2021 (Figure S3). Baseline characteristics were: median age of 22 years (range 16–59 years), six patients (6/9, 67%) were female and six (6/9, 67%) were on stable-dose hydroxyurea (Table S1). Seven patients (7/9, 78%) had homozygous HbSS, one patient (1/9, 11%) had HbS/β thalassemia and one patient (1/9, 11%) had HbS/β thalassemia. The median VOC episodes experienced was three (range 0–7) with a median of ten days (range 0–42 days) of hospitalization for SCD-related complications in the 24 months prior to screening. Eight patients (8/9, 89%) completed the dose-finding period, with a median compliance rate of 98% (range 94%–100%) based on diaries and pill counts. The safety analysis, evaluated by frequency and severity of adverse events according to Common Terminology Criteria for Adverse Events version 5.0 (National Cancer Institute, Bethesda, MD), included all enrolled patients. The majority of treatment-emergent adverse events (TEAEs) in the dose-finding period were grade 1 and transient (Table S2). The most common reported TEAEs included headache (n = 4, 44%), transaminase increase (alanine aminotransferase increased in n = 4 [44%]; aspartate aminotransferase increased in n = 2 [22%]) and dyspepsia (n = 2, 22%). No treatment-related TEAEs grade 3 or higher were reported. The non-treatment-related TEAE grade 4 was a serious adverse event of a urinary tract infection with Received: 10 March 2022 Revised: 25 March 2022 Accepted: 28 March 2022

Safety and efficacy of mitapivat, an oral pyruvate kinase activator, in sickle cell disease: A phase 2, open‐label study

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Revisiting anemia in sickle cell disease and finding the balance with therapeutic approaches.

Polymerization of deoxygenated sickle hemoglobin (HbS) leads to erythrocyte sickling. Enhancing activity of the erythrocyte glycolytic pathway has anti-sickling potential as this reduces 2,3-diphosphoglycerate (2,3-DPG) and increases ATP, factors that decrease HbS polymerization and improve erythrocyte membrane integrity. These factors can be modulated by mitapivat, which activates erythrocyte pyruvate kinase (PKR) and improves sickling kinetics in SCD patients. We investigated mechanisms by which mitapivat may impact SCD by examining its effects in the Townes SCD mouse model. Control (HbAA) and sickle (HbSS) mice were treated with mitapivat or vehicle. Surprisingly, HbSS had higher PKR protein, higher ATP, and lower 2,3-DPG levels, compared to HbAA mice, in contrast with humans with SCD, in whom 2,3-DPG is elevated compared to healthy subjects. Despite our inability to investigate 2,3-DPG-mediated sickling and hemoglobin effects, mitapivat yielded potential benefits in HbSS mice. Mitapivat further increased ATP without significantly changing 2,3-DPG or hemoglobin levels, and decreased levels of leukocytosis, erythrocyte oxidative stress, and the percentage of erythrocytes that retained mitochondria in HbSS mice. These data suggest that, even though Townes HbSS mice have increased PKR activity, further activation of PKR with mitapivat yields potentially beneficial effects that are independent of changes in sickling or hemoglobin levels. 

http://manuscript.elsevier.com/S1079979622000171/pdf/S1079979622000171.pdf

Stephanie van Straaten

Papers

Decreased activity and stability of pyruvate kinase in sickle cell disease: a novel target for mitapivat therapy