Introduction
Elamipretide is an aromatic-cationic tetrapeptide that readily penetrates the cell membrane and transiently localizes to the inner mitochondrial membrane (IMM) where energy (adenosine triphosphate [ATP]) production occurs, thereby improving mitochondrial function by restoring the physical and biochemical properties of the IMM by irreversibly associating with cardiolipin (CL), a mitochondrial-membrane-unique phospholipid (Szeto 2008; Birk 2013; Grazioli/Pugin 2018; Mitchell 2020). This association improves membrane stability, enhances ATP synthesis in several organs including the heart, kidney, neurons, and skeletal muscle, and reduces reactive oxygen species (ROS) production (Zhao 2005; Manczak 2010; Szeto/Schiller 2011; Dai 2013; Siegel 2013; Birk 2014; Brown 2014; Eirin 2014; Nickel 2014; Szeto/Birk 2014; Alam 2015; Roshanravan 2021). Elamipretide has been extensively examined in multiple preclinical and clinical studies for diseases involving mitochondrial dysfunction, consistently demonstrating amelioration of pathologic symptoms, including improvement in skeletal muscle strength, cardiac stroke volume, and kidney function (Manczak 2010; Birk 2013; Dai 2013; Siegel 2013; Eirin 2014; Stauffer 2016; Daubert 2017; Saad 2017; Karaa 2018; Sabbah 2019; Allen 2020; Reid Thompson 2021).
Elamipretide is being developed for the treatment of patients with a variety of diseases, such as Barth syndrome (BTHS) and Primary Mitochondrial Myopathy (PMM), among others, in which genetic abnormalities affecting the mitochondria lead to life-long symptoms requiring long-term elamipretide administration (Karaa 2018; Sabbah 2019; Karaa 2020; Reid Thompson 2021). Clinical development of elamipretide has focused on subcutaneous (SC) administration. Prior studies showed that SC elamipretide up to 80mg once daily is generally safe and well tolerated, although most longer-term studies have used SC elamipretide 40mg or 60mg once daily [DOF, Stealth BioTherapeutics Inc.]. However, injection site reactions (ISRs) were reported in the majority of subjects receiving treatment. In multiple-dose clinical trials lasting >8 days, ISRs were relatively common in subjects: injection site erythema (47%), pruritus (45%), pain (22%), induration (19%), swelling (14%), urticaria (13%), bruising (12%), hemorrhage (6%), and mass (6%). Although typically mild in nature, resolving within 4hours of elamipretide administration, they can lead to subject withdrawal or discontinuation during chronic daily administration [DOF, Stealth BioTherapeutics Inc.].
The Mas-related G-protein coupled receptor member X2 (MRGPRX2), expressed almost exclusively by mast cells that populate connective tissues, including the skin (Tatemoto 2006; Motakis 2014), has relatively recently been linked to ISRs (McNeil 2021a). Skin mast-cell-activation causes immediate inflammation, and many therapeutic drugs associated with high frequencies of ISRs have been shown to activate MRGPRX2 directly, thereby triggering inflammation via mast cells. MRGPRX2 is preferentially activated by drugs with cationic and aromatic properties (Grimes 2019), such as elamipretide, making it an MRGPRX2 agonist. While the current management of ISRs consists of alternating daily injection sites around abdominal quadrants, interventions that target mast cell activation or the effects of mast-cell-derived mediators warrant investigation. The aim of the present study was to evaluate the efficacy of potential interventions used to mitigate elamipretide-induced ISRs, identify any role of the MRGPRX2 receptor in elamipretide ISRs, and further understand the PK and safety of 60mg SC elamipretide administration. The study agents (mometasone, tacrolimus, doxepin, diphenhydramine) have dosing regimens supported by the product label and their ability to reduce inflammatory responses by targeting mast cell activation.