Evaluation of the safety, tolerability, pharmacokinetics, and pharmacodynamics of long-term mitapivat dosing in subjects with stable sickle cell disease

镰状细胞病(SCD)是一种多系统疾病，与急性临床事件和进行性器官损害有关。由镰状红细胞引起的微血管闭塞引起的阵发性疼痛是最常见的急性并发症，也是住院的主要原因。SCD的管理策略发展非常缓慢，急性疼痛的治疗仍然局限于阿片类药物的支持性治疗。在2017年FDA批准L谷氨酰胺(Endari)以及2019年批准Crizanlizumab(Adakveo)和voxelotor(Oxbryta)之前，唯一批准的SCD治疗方法是羟基尿素(HU)，表明可以减少急性疼痛危象的频率，但并不是普遍有效。 由于SCD的根本原因是脱氧血红蛋白S(HBS)的聚合，因此有充分的理由探索能够抑制或减少其自身聚合过程的药物。2 HBS只有在脱氧时才会聚合；其氧合受几个因素的影响，其中一个关键因素是红细胞中2，3-二磷酸甘油酸酯(2，3-DPG)的浓度。细胞内2，3-DPG的增加减少了氧结合，稳定了脱氧型(T型)的血红蛋白。此外，2，3-DPG浓度的增加降低了红细胞内pH，进一步促进了HBS的聚合。2，3-DPG是糖酵解途径的中间底物，是红细胞产生三磷酸腺苷(ATP)的唯一来源。丙酮酸激酶(PK)是糖酵解最后一步的关键酶，它将磷酸烯醇式丙酮酸转化为丙酮酸，生成红细胞膜上50%的三磷酸腺苷，维持红细胞膜的完整性。PK活性降低会导致上游酶底物的积累，包括2，3-DPG，它有利于聚合，因为它稳定了脱氧型(T型)的血红蛋白。在患有SCD的人类中，甚至在通常没有症状的镰刀携带者中，氧亲和力降低将有利于HBs及其聚合的脱氧，从而导致镰刀状。事实上，PK缺乏和镰刀状细胞特征的结合导致了急性镰状综合征，此前曾有两例报道。 目前减少HBS聚合的方法包括通过多种策略和药物诱导胎儿血红蛋白(HbF)，通过增加血红蛋白对氧气的亲和力(例如体素运动)来靶向HBS聚合。增加红细胞PK(PKR)活性，导致细胞内2，3-DPG浓度下降，为抑制HBS聚合和急性镰刀痛提供了一个新的和潜在的有吸引力的治疗靶点。Mitapivat(AG-348)是一种口服生物利用的PKR小分子变构激活剂，目前正在进行针对PK缺乏患者的II/III期临床试验(NCT02476916、NCT03548220/AG348-C-006；NCT03559699/AG348-C-007)以及正在进行的针对非输血依赖型地中海贫血患者的II期临床试验(NCT03692052)。最近发表的在PK缺乏的受试者中的结果看起来很有希望，该药物的安全性可以接受。临床前MITAPIVAT数据的概述和其他数据支持在所有测试的多个递增剂量下，与糖酵解途径激活一致的血液糖酵解中间产物的剂量依赖变化，支持MITAPIVAT在SCD治疗中的潜在作用。 我们最近完成了我们的第一阶段研究(NCT04000165，研究19-H-0097)，以确定多次递增剂量的米他匹特在SCD患者中的临床安全性和耐受性。我们观察到，对于所有受试者和10名受试者来说，每日两次(BID)的米达帕特剂量高达50毫克(BID)，以及10名受试者100毫克BID，其安全性可接受，其不良反应范围与在PKD患者中观察到的不良反应相当。此外，我们的第一阶段研究已经提供了米达匹特治疗SCD疗效的初步证据，在大多数SCD受试者中观察到血红蛋白水平的上升和溶血标志物的下降，一些受试者报告的症状有主观改善。本研究的目的是评估长期服用稳定剂量的米塔帕特治疗SCD的安全性和耐受性。

Sickle cell disease (SCD) is a multisystem disorder associated with episodes of acute clinical events and progressive organ damage. Episodic pain, triggered by microvascular vaso-occlusion induced by sickled red blood cells, is the most common acute complication and the leading cause of hospitalization. Management strategies for SCD have evolved very slowly, and treatment of acute pain is still limited to supportive therapy with opioid medication. Until the FDA approval of Lglutamine (Endari) in 2017 and the more recent approvals of crizanlizumab (Adakveo) and voxelotor (Oxbryta) in 2019, the only approved therapy for SCD was hydroxyurea (HU), indicated to reduce frequency of acute painful crises but which is not universally effective. As the root cause of SCD is polymerization of deoxy-hemoglobin S (HbS), there is a strong rationale for exploring agents that could inhibit or reduce the polymerization process itself.2 HbS polymerizes only when deoxygenated; its oxygenation is influenced by a few factors, one key factor being the 2,3- diphosphoglycerate (2,3-DPG) concentration in the RBC. Increased intracellular 2,3-DPG decreases oxygen binding and stabilizes the deoxygenated form (T form) of hemoglobin. In addition, increased 2,3-DPG concentration decreases intraerythrocyte pH, further promoting HbS polymerization. 2,3- DPG is an intermediate substrate in the glycolytic pathway, the only source of adenosine triphosphate (ATP) production in RBCs. Pyruvate kinase (PK) is a key enzyme in the final step of glycolysis; PK converts phosphoenolpyruvate to pyruvate, creating 50% of the total red cell ATP that is essential for maintaining integrity of the RBC membrane. Reduced PK activity leads to accumulation of the upstream enzyme substrates, including 2,3-DPG which favors polymerization as it stabilizes the deoxygenated form (T form) of hemoglobin. In humans with SCD, and even in sickle carriers who are generally asymptomatic, reduced oxygen affinity will favor deoxygenation of HbS and its polymerization, and thus sickling. Indeed, the combination of PK deficiency and sickle cell trait causing an acute sickling syndrome has been previously reported in two cases. Current approaches to reduce HbS polymerization include fetal hemoglobin (Hb F) induction via multiple strategies and drugs that targets HbS polymerization through increasing affinity of hemoglobin for oxygen (e.g. voxelotor). Increasing red cell PK (PKR) activity, leading to a decrease in intracellular 2,3-DPG concentration, presents a new and potentially attractive therapeutic target for thwarting HbS polymerization and acute sickle pain. Mitapivat (AG-348) is an orally bioavailable small molecule allosteric activator of PKR, currently being studied in Phase II/III clinical trials in humans with PK deficiency (NCT02476916, NCT03548220 / AG348-C-006; NCT03559699 / AG348- C-007), as well as in an ongoing Phase II clinical trial in humans with non-transfusion-dependent thalassemia (NCT03692052). The recently published results in PK deficient subjects appear promising, and the safety profile of the drug was acceptable. Overview of the preclinical mitapivat data and other data support dose-dependent changes in blood glycolytic intermediates consistent with glycolytic pathway activation at all multiple ascending doses tested, supporting the potential role of mitapivat in the treatment of SCD. We recently completed our Phase I study (NCT04000165, Study 19-H-0097) to determine the clinical safety and tolerability of multiple escalating doses of mitapivat in subjects with SCD. We have observed an acceptable safety profile for mitapivat doses up to 50 mg twice daily (BID) for all subjects and 100 mg BID for ten subjects, with a range of adverse effects comparable to those observed in PKD patients. Furthermore, our Phase I study has provided preliminary evidence of efficacy for mitapivat in SCD, with increases in hemoglobin level and decreases in hemolytic markers observed in the majority of SCD subjects and subjective improvements in symptoms reported by some subjects. The objective of the present study is to evaluate the safety and tolerability of long-term treatment with a stable dose of mitapivat in subjects with SCD.