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 restoration of the physical and biochemical properties of the IMM through its reversible association with cardiolipin (CL), a phospholipid that is unique to mitochondrial membranes (Szeto, 2008; Birk et al., 2013; Grazioli & Pugin, 2018; Mitchell et al., 2020). Specifically, this association improves membrane stability, enhances ATP synthesis in several organs including the heart, kidney, neurons, and skeletal muscle, and reduces reactive oxygen species production (Zhao et al., 2005; Manczak et al., 2010; Szeto & Schiller, 2011; Dai et al., 2013; Siegel et al., 2013; Birk et al., 2014; Brown et al., 2014; Eirin et al., 2014; Nickel et al., 2014; Szeto & Birk, 2014; Alam et al., 2015; Roshanravan et al., 2021). Elamipretide has been extensively examined in multiple preclinical studies and clinical trials for diseases involving mitochondrial dysfunction, consistently demonstrating amelioration of pathologic symptoms, including improvement in skeletal muscle strength, cardiac stroke volume, and kidney function (Manczak et al., 2010; Birk et al., 2013; Dai et al., 2013; Siegel et al., 2013; Eirin et al., 2014; Stauffer et al., 2016; Daubert et al., 2017; Saad et al., 2017; Karaa et al., 2018; Sabbah et al., 2019; Allen et al., 2020; Reid Thompson et al., 2021).
The investigational agent, 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 that require long-term administration of elamipretide (Karaa et al., 2018; Sabbah et al., 2019; Karaa et al., 2020; Reid Thompson et al., 2021). Clinical development of elamipretide has focused on subcutaneous (SC) administration. Prior studies have shown that SC elamipretide up to 80 mg once daily is generally safe and well tolerated, although most longer-term studies have used SC elamipretide 40 or 60 mg once daily [data on file, 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 these ISRs are typically mild in nature and resolved within 4 hours of elamipretide administration, they can lead to subject withdrawal or discontinuation during chronic daily administration [data on file, Stealth BioTherapeutics Inc.].
The Mas-related G-protein coupled receptor member X2 (MRGPRX2), which is expressed almost exclusively by mast cells that populate connective tissues, including the skin (Tatemoto et al., 2006; Motakis et al., 2014), has received increasing attention in recent years for its link to ISRs (McNeil 2021a). Mast cell activation in the skin causes immediate inflammation, and many therapeutic drugs associated with high frequencies of ISRs have been shown to activate MRGPRX2 directly, and thus trigger inflammation via mast cells. Notably, MRGPRX2 is preferentially activated by drugs with cationic and aromatic properties (Grimes et al., 2019), and since elamipretide has these properties, it is highly plausible that elamipretide is an MRGPRX2 agonist. While the current management of ISRs consists of alternation of daily injection sites around abdominal quadrants, interventions that target mast cell activation or the effects of mast-cell-derived mediators warrant investigation. To improve patient comfort, 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 60 mg SC elamipretide administration. The agents chosen for study (mometasone, tacrolimus, doxepin, diphenhydramine) have dosing regimens supported by the label for the product and are supported by their ability to reduce inflammatory responses by targeting mast cell activation.