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发的发作性疼痛是最常见的急性并发症和住院的主要原因。SCD的管理策略发展非常缓慢，急性疼痛的治疗仍然限于阿片类药物的支持治疗。在FDA于2017年批准L谷氨酰胺（Endari）以及最近于2019年批准crizanlizumab（Adakveo）和voxelotor（Oxbryta）之前，唯一批准的SCD疗法是羟基脲（HU），用于降低急性疼痛危象的频率，但并非普遍有效。 由于SCD的根本原因是脱氧血红蛋白S（HbS）的聚合，因此有很强的理由探索能够抑制或减少聚合过程本身的药物。2 HbS仅在脱氧时聚合;其氧合受几个因素影响，其中一个关键因素是RBC中的2，3-二磷酸甘油酸（2，3-DPG）浓度。增加的细胞内2，3-DPG降低氧结合并稳定血红蛋白的脱氧形式（T形式）。此外，增加的2，3-DPG浓度降低红细胞内pH，进一步促进HbS聚合。2，3- DPG是糖酵解途径中的中间底物，是RBC中产生三磷酸腺苷（ATP）的唯一来源。丙酮酸激酶（PK）是糖酵解最后一步的关键酶; PK将磷酸烯醇丙酮酸转化为丙酮酸，产生50%的总红细胞ATP，这对维持RBC膜的完整性至关重要。降低的PK活性导致上游酶底物的积累，包括2，3-DPG，其有利于聚合，因为其稳定脱氧形式（T形式）的血红蛋白。在患有SCD的人中，甚至在通常无症状的镰状携带者中，降低的氧亲和力将有利于HbS的脱氧及其聚合，从而镰状化。事实上，PK缺陷和镰状细胞性状的组合导致急性镰状综合征先前已在两个病例中报道。 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（Hb F），所述多种策略和药物通过增加血红蛋白对氧的亲和力（例如voxelotor）靶向HbS聚合。增加红细胞PK（PKR）活性，导致细胞内2，3-DPG浓度降低，为阻碍HbS聚合和急性镰状疼痛提供了新的且潜在有吸引力的治疗靶点。Mitapivat（AG-348）是一种口服生物可利用的PKR小分子变构激活剂，目前正在PK缺乏症患者中进行II/III期临床试验（NCT 02476916、NCT 03548220/ AG348-C-006; NCT 03559699/AG 348- C-007），以及在患有非输血依赖性地中海贫血的人类中进行的II期临床试验（NCT 03692052）中。最近发表的PK缺陷受试者的结果似乎很有希望，药物的安全性特征是可接受的。临床前mitapivat数据和其他数据的概述支持血液糖酵解中间体的剂量依赖性变化，与在所有多次递增剂量下检测的糖酵解途径激活一致，支持mitapivat在治疗SCD中的潜在作用。 我们最近完成了I期研究（NCT 04000165，研究19-H-0097），以确定多次递增剂量的mitapivat在SCD受试者中的临床安全性和耐受性。我们观察到所有受试者的mitapivat剂量高达50 mg每日两次（BID）和10名受试者的100 mg BID的安全性特征可接受，其不良反应范围与PKD患者中观察到的不良反应相当。此外，我们的I期研究提供了mitapivat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升高和溶血标志物降低，一些受试者报告的症状主观改善。本研究的目的是评价SCD受试者接受稳定剂量的mitapivat长期治疗的安全性和耐受性。

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.