ACE2 Fusions with Immunoglobulin Fc
Despite an ongoing clinical trial of wild type sACE2 in
Europe42 and the studies reviewed above demonstrating
the realization of picomolar affinity decoy receptors, it is likely that
low sACE2 serum stability will limit its successful application as a
therapeutic. The serum half-life of dimeric sACE2 in vivo is on the
order of hours55,56, thereby requiring frequent dosing
to maintain sACE2 in circulation. As a consequence, it is questionable
whether sACE2 can be safely administered in an outpatient setting. Serum
stability of biologic drugs is greatly increased by fusion of the
therapeutic protein with the Fc region of IgG57. Serum
proteins are turned over by constant internalization and lysosomal
degradation, especially by endothelial cells. Internalized IgG-Fc
engages the neonatal Fc receptor (FcRn) within endosomal compartments to
direct its trafficking back to the plasma membrane, thereby returning
the protein to the serum (Figure 3A). The previously highlighted works
describing the discovery of high affinity sACE2
variants43,49,50,52 have already extended their
findings to fusions with IgG1 Fc, and we believe that sACE2-Fc fusions
will be the standard going forward for the production of high-affinity
decoy receptors with excellent pharmacokinetics.
Recombinant human sACE2 has been developed over nearly two decades as a
therapeutic for lung inflammation and acute respiratory distress
syndrome (ARDS). Its administration has been shown to be protective
against cardiovascular58 and pulmonary pathologies and
to treat hypoxemia in multiple animal
models59,60,61,62,63, and it has acceptable safety
without causing episodes of hypotension in healthy and mechanically
ventilated human subjects56,64. However, while sACE2
demonstrated the expected in vivo biochemical activity in ARDS patients,
there was no statistically significant improvement in clinical outcomes
in a phase 2 trial. The clinicians commented that “…lower plasma
concentrations (of sACE2) over a longer duration may be more effective
as a result of more sustained production of” ACE2
products64. These findings motivated the first
published report of an sACE2-Fc fusion tested in vivo (in mice), in
which the serum half-life was extended to over a week with substantially
higher maximum serum activity63. Recombinant sACE2-Fc
mitigated hypertension following injection of the vasopressor (and ACE2
substrate) angiotensin II for up to 7 days, whereas unfused sACE2 lost
function in serum within hours. The Fc fusion protein reduced blood
pressure and cardiac and renal fibrosis in a transgenic mouse model for
hypertension and albuminuria in which renin is massively
overexpressed63. This work demonstrated serum
longevity of sACE2 fused to IgG1-Fc as well as the potent physiological
effects of ACE2 catalytic activity.
More recently with the emergence of SARS-CoV-2, multiple groups have
explored IgG1-Fc fusions of sACE2 and consistently find long serum
half-life without adverse toxicity in small
animals53,38. The Fc region of IgG subclasses engage
multiple Fc receptors to effect function, including not only FcRn to
mediate serum stability and uptake into the lungs, but also a suite of
inflammatory Fc γ receptors (FcγR)65. These receptors
are expressed on a variety of cell types, especially in the immune
system, and are responsible for effector functions such as
antibody-dependent cellular cytotoxicity, cytokine release, antigen
presentation and co-stimulation (Figure 3A). Until different Fc fusion
proteins are properly assessed in controlled in vivo experiments, it is
unclear whether these effector functions will aid in clearance and
recovery from SARS-CoV-2 infection. Iwanaga et al have hypothesized that
interactions of a sACE2-Fc biologic with inflammatory FcγRs may
exacerbate inflammation and cytokine storms in COVID-19
patients53. Consequently, two leucine residues in the
IgG1-Fc moiety were mutated to alanines (L234A and L235A). These
so-called LALA mutations disrupt FcγR interactions while leaving FcRn
binding intact for high serum stability. The LALA variant of sACE2-Fc
had long serum half-life, was distributed to lung tissues, and retained
virus neutralization efficacy.