ENZYME THERAPY FOR CKD-MBD: BREAKING THE BARRIER OF VASCULAR CALCIFICATION

摘要：慢性肾脏疾病（CKD）患者的内侧动脉钙化（MAC）进展不可逆，尽管目前治疗，并导致这些患者心血管预后不良。此外，尽管其他矿化疾病的骨折风险有所改善，但CKD患者骨折的风险在过去20年中保持不变。缺乏进展表明，尽管有重大的医疗需求，目前治疗CKD-MBD中存在的矿化障碍的治疗方法是无效的。焦磷酸盐（PPi）是羟基磷灰石形成的直接抑制剂，血液透析患者血浆PPi水平降低。MAC的出现和进展与血浆焦磷酸（PPi）浓度呈负相关。此外，CKD患者还会发生骨骼代谢的改变，即慢性肾脏疾病-骨和矿物质紊乱（CKD- mbd）。在缺乏ENPP1和低血浆PPi的啮齿类动物模型中也存在骨矿物质密度降低，这支持了ENPP1/ PPi可能在CKD-MBD中存在的骨矿化障碍中发挥作用的观点。研究ENPP1/PPi在CKD中的作用的一个主要实验障碍是缺乏使体内血浆[PPi]正常化的ENPP1酶替代疗法（ERT）。我们开发了一种可溶的ENPP1生物制剂，克服了这些障碍，并在一种称为婴儿期广泛性动脉钙化（GACI）的血管钙化、低血浆PPi、FGF23升高和进行性尿毒症的致死性遗传模型中确立了其疗效。该提案的目的是确定ENPP1/PPi在CKD-MBD中存在的矿化失衡中的作用。我们的中心假设是，用ENPP1 ERT纠正低血浆[PPi]和ENPP1缺乏症将抑制内壁血管钙化，改善动脉血流动力学特性，并改善CKD-MBD中存在的骨矿化缺陷。我们的假设是基于我们的初步数据，这些数据显示CKD-MBD小鼠模型的血浆[PPi]下降了50%，并且用ENPP1 ERT治疗这些小鼠可以显著减少这些模型中的异位钙化并改善骨生物力学。我们的假设也得到了我们自己的初步数据的支持，即缺乏Enpp1会增加皮质骨矿化，从而破坏皮质骨骨细胞微环境，显著降低骨形成率，导致明显的骨质减少。我们将通过证明ENPP1 ERT在CKD-MBD小鼠模型中的有效性来验证我们的假设，通过记录治疗对这些模型中存在的异位软组织钙化增加和骨矿化减少的影响。

Abstract: The medial arterial calcifications (MAC) observed in chronic kidney disease (CKD) progress irreversibly despite current treatment, and contributes to the poor cardiovascular outcomes in these patients. In addition, the risk of bone fracture in CKD patients has remained unchanged for the last 20 years despite improvement in the fracture risk in other mineralization disorders. The lack of progress suggests that despite the significant medical need, current therapeutics treating the mineralization disorder present in CKD-MBD are ineffective. Pyrophosphate (PPi) is a direct inhibitor of hydroxyapatite formation, and patients on hemodialysis have reduced plasma PPi levels. The appearance and progression of MAC correlates inversely with plasma pyrophosphate (PPi) concentrations. In addition, patients with CKD also develop alterations in skeletal metabolism referred to as chronic kidney disease – bone and mineral disorder (CKD-MBD). Reduced bone mineral density is also present in rodent models of deficient ENPP1 and low plasma PPi, supporting the notion ENPP1/ PPi may play a role in the bone mineralization disorder present in CKD-MBD. A major experimental barrier to investigating the role of ENPP1/PPi in CKD is the lack of an ENPP1 enzyme replacement therapy (ERT) which normalizes plasma [PPi] in vivo. We have developed a soluble ENPP1 biologic agent overcoming these barriers and have established its efficacy in a lethal genetic model of vascular calcification, low plasma PPi, elevated FGF23, and progressive uremia called Generalized Arterial Calcification of Infancy (GACI). The objective of this proposal is to determine the role of ENPP1/PPi in the mineralization imbalances present in CKD-MBD. Our central hypothesis is that correcting low plasma [PPi] and ENPP1 deficiency with ENPP1 ERT will inhibit medial wall vascular calcifications, improve arterial hemodynamic properties, and improve the bone mineralization deficits present in CKD-MBD. Our hypothesis is based on our preliminary data showing that murine models of CKD-MBD drop plasma [PPi] by 50%, and that treating these mice with ENPP1 ERT significantly reduces ectopic calcifications and improves bone biomechanics in these models. Our hypothesis is also supported by our own preliminary data that Enpp1 deficiency increases cortical bone mineralization which damages the cortical bone osteocyte microenvironment and significantly decreases bone formation rates, resulting in marked osteopenia. We will validate our hypothesis by demonstrating the efficacy of ENPP1 ERT in murine models of CKD-MBD by documenting the effects of the therapeutic on the increased ectopic soft tissue calcifications and decreased bone mineralization present in these models.