Discussion
The aim of this preliminary phase 1 study was to evaluate the potential
efficacy of interventions that might be used to mitigate ISRs following
the SC administration of 60mg elamipretide in healthy subjects, and any
potential impact on PK and safety. We focused on treatments that target
mast cells or factors upstream or downstream to their activation, due to
our preclinical data that strongly suggest that elamipretide-associated
ISRs are caused by direct activation of mast cells through MRGPRX2.
Overall, mometasone appeared to mitigate pseudo-allergic reactions
observed following SC elamipretide, reducing pruritus and
induration/swelling with no effect on PK. Ice application ameliorated
early injection site pain and itching, but reduced the absorption of
elamipretide. Diphenhydramine demonstrated potential in reducing
induration/swelling but caused sedation in some cases. Tacrolimus and
doxepin demonstrated little impact on reported ISRs compared to
elamipretide alone; therefore, they are not recommended for ISR
mitigation. There were no significant changes in elamipretide or M1/M2
metabolite plasma exposures with any of the treatments except for ice
application, which reduced exposure to elamipretide and its metabolites.
Although ice application and diphenhydramine showed some reduction in
ISR signs and symptoms, mometasone revealed the most promise. Reductions
in pruritus and induration/swelling demonstrated by mometasone enabled
patients to be more comfortable with elamipretide treatment. In
addition, mometasone may further improve elamipretide tolerability by
reducing scratching and subsequent scratching-related skin damage. In
some subjects, the hydrocolloid occlusive dressing applied over the
mometasone ointment resulted in redness of the covered area (injection
site area), confounding the ability to identify erythema due to ISRs.
ISR photographs proved useful in deciphering erythema grading in
subjects where the pattern of redness appeared to be related to the use
of the occlusive dressing, although the ISR erythema may have been
overreported in the mometasone treatment arm.
Despite ice application and diphenhydramine reducing some ISR
signs/symptoms, both presented undesirable effects. Ice application
reduced early injection site pain and itching, but reduced the
Cmax and AUC0-6h of elamipretide and its
metabolites (M1 and M2), possibly due to vasoconstriction. Neither M1
nor M2 are biologically active and the potential reduction in plasma
concentrations of these metabolites is not therefore anticipated to have
a clinical impact [DOF, Stealth BioTherapeutics Inc.]. Although the
potential ramification on efficacy of the disruption in absorption of
the active parent drug is unclear, lowering plasma exposure of
elamipretide is not desirable, making ice application less appropriate.
Similarly, diphenhydramine showed some potential in reducing
induration/swelling, but a significant incidence (50%) of mild
somnolence was reported in this treatment arm. While second generation
antihistamines were not included in this study, the observations with
diphenhydramine suggest that other antihistamines that are less sedating
could provide utility in mitigating ISRs.
Based on mometasone and antihistamine outcomes, further investigation is
warranted to include separate/combined interventions with oral
second-generation antihistamines (such as fexofenadine) and topical
mometasone without an occlusive dressing. Fexofenadine, a selective
peripheral H1 receptor antagonist, does not readily cross the
blood–brain barrier, thereby causing less drowsiness in comparison to
first-generation antihistamines, such as diphenhydramine. Dermatology
and allergy organizations issued a common guideline on chronic urticaria
management, recommending the regular use of second-generation
antihistamines as first-line treatment. Fexofenadine appears safe and
well tolerated; daily doses can be titrated upwards in case of no
improvement (Zuberbier 2018). Chronic urticaria could be considered the
closest model to chronic ISRs in terms of available data on therapeutic
management. Because elamipretide is injected daily at alternating sites,
applications with mometasone 2x/day (one prior and one post-injection)
appears to be a more practical alternative to the use of mometasone with
occlusive dressing.
Our focus on mast cells arose from preclinical experiments which
established that elamipretide, in addition to its intended function,
also acts as an agonist of the human G protein-coupled receptor MRGPRX2.
MRGPRX2 is primarily expressed by mast cells, which are constitutive
residents of the skin and other tissues, trigger rapid tissue
inflammation, and mediate many of the symptoms of allergic diseases. We
reasoned that MRGPRX2 might be involved because it can be activated by
many cationic peptides and small molecules with properties similar to
elamipretide and which also cause ISRs and other pseudo-allergic
reactions (McNeil 2021a). We have several pieces of evidence to support
the hypothesis that elamipretide-associated ISRs are due to MRGPRX2
activation. First, elamipretide triggers intracellular signaling
pathways, as measured by calcium mobilization, in cell lines forced to
express MRGPRX2 or its mouse ortholog Mrgprb2, but not in unmodified
cell lines that do not natively express the receptors (Figures 2 A, B).
Notably, this could be blocked by a molecule reported to inhibit MRGPRX2
signaling (Figure 2C). MRGRPX2 activation by elamipretide is
physiologically relevant, as the calculated EC50 is over
1,000-fold lower than the injection concentration in our clinical trial.
Second, elamipretide causes intracellular signaling, again as measured
by calcium flux, in primary mouse mast cells from wild type but not
Mrgprb2 knockout mice (Figure 3). Third, SC elamipretide injection into
mouse hind paws triggered rapid swelling via fluid extravasation from
the bloodstream, similar to human ISRs, in wild type mice, while
extravasation was nearly absent in mice lacking Mrgprb2 (Figure 4).
These effects almost certainly are due to the parent drug, as
elamipretide’s metabolites have no effect on MRGPRX2 signaling (Figures
2 D, E).
While itch produced in humans by injection of MRGPRX2 agonists can be
blocked by antihistamines (Hasbak 2006), development of inhibitors is
still in its infancy with no candidates in clinical trial yet (McNeil
2021a). Indeed, a combination of H1 and H2 histamine receptor
antagonists have demonstrated efficacy in blocking MRGPRX2-driven
systemic and local reactions (McNeil 2021b) and could be considered in
future elamipretide-driven ISR studies. Although the topical steroid
mometasone reduced pruritus and induration/swelling following SC
elamipretide in this study, its anti-inflammatory mechanism is unclear
but is thought to act by inhibition of the arachidonic acid pathway
(Spada 2018; ELOCON® Prescribing Information, 2018).
In addition to anti-inflammatory effects, topical steroids, such as
mometasone, possess anti-mitotic, immunosuppressive, and
vasoconstrictive effects (Gabros 2021), which may have played a role in
mitigating ISR signs in this study.
Overall, the data collected in this study support prior findings that SC
elamipretide is generally safe and well-tolerated (Karaa 2020; Thompson
2021; Mettu 2022). With the exception of data relating to ISRs, very few
TEAEs were identified in this study. The only TEAE seen in more than one
subject was that of somnolence in the diphenhydramine treatment arm.
Given that somnolence is a well-known side effect of diphenhydramine
(Sicari/Zabbo 2021), this adverse event was considered likely related to
diphenhydramine and not to elamipretide treatment.
A limitation of the study was the relatively low number of subjects
evaluated (N=10), which impacted the potential to demonstrate
statistically significant results with respect to efficacy of
mitigation. This study was not powered to show a statistically
significant difference in the ISR profile between treatment arms but was
meant to identify signals that could warrant further investigation in
other clinical settings.