Targeting C5
The terminal complement complex (C5b-9, MAC) generation starts with the
interaction of C5b, released from C5 convertase cleavage of C5, leading
though several steps, to the generation C5b-9 and insertion of it into
the cell plasma membrane as the membrane attack complex (MAC) that can
cause endothelial cell damage. The complex promotes inflammation by
inducing the expression of adhesion molecules and the release of
chemokines and platelet activating factor, which can ultimately lead to
dysregulation in the coagulation resulting in microvascular thrombosis.
The ability of C5b-9 to upregulate expression of leucocyte adhesion
molecules on the endothelial cell might also contribute to platelet
localization and adhesion as well as increased leucocyte adhesion and
subsequent cytokine and growth factor production (Dobrina et al., 2002).
Patients with TMA have increased production of the terminal complement
components C5a, C5b and subsequently C5b-9, due to enhanced LP, CP or AP
activation caused by defective regulation and/or excess activation
(Figure 3). Elevated levels of circulating soluble C5b-9 (sC5b-9) can be
measured in the blood of patients presenting with complement-mediated
microangiopathies. High concentration of sub-lytic MAC in target cells
may have a detrimental effect in variety of tissues like kidney, lung
and the central nervous system. Selective inhibition of C5 convertases
is one of the options to inhibit formation of C5b-9 (MAC) that has been
successfully applied to clinical practice (Figure 1). Another method to
inhibit C5 is to bind the ligand itself to prevent C5 convertases
enzymatically cleaving C5 into C5a and C5b, the method that has been
used in generating anti-C5 antibodies.
Eculizumab (Soliris, Alexion Pharmaceuticals) is a humanized murine
monoclonal antibody against C5, which prevents C5 cleavage and limits
both C5a and C5b production and prevents the generation of the terminal
complement complex C5b-9 by any of the three complement pathways.
Eculizumab was first approved for the treatment of paroxysmal nocturnal
hemoglobinuria (PNH), and the efficacy and safety of eculizumab for
treating atypical hemolytic uremic syndrome (aHUS) were demonstrated in
prospective clinical trials and was adopted for therapy in high risk
TA-TMA patients (Legendre et al., 2013). Eculizumab (off-label) has been
successfully used in HSCT recipients with severe TA-TMA and is one of
the first complement blocking agent attempted in COVID-19 patients.
Complement mediated TA-TMA occurring in HSCT recipients very closely
resembles histologic and clinical TMA presentation in subjects with
COVID-19 suffering from a hyperinflammatory syndrome. Hyperinflammatory
response in immunocompromised individuals with TA-TMA is often triggered
by viral pathogens like BK virus(Laskin et al., 2019)
influenza/parainfluenza (Bitzan & Zieg, 2018), adenovirus(Yabe et al.,
2005), HHV-6 (Belford, Myles, Magill, Wang, Myhand & Waselenko, 2004)
that is associated with very high systemic complement activation as
measured by elevated blood sC5b-9 and leading to multi-organ injury
resembling clinical and autopsy reports in SARS-CoV-2. Untreated
patients with complement mediated TA-TMA have >80%
mortality due to multi-organ failure. Eculizumab treatment significantly
improved survival as compared with untreated cohorts (66% vs 17% 1 y
post-transplant survival) (Jodele et al., 2020a; Jodele et al., 2014).
In HSCT recipients with TA-TMA pre-therapy, plasma sC5b-9 was associated
with risk of dying from TMA. Plasma sC5b-9 also correlated with
increased eculizumab drug clearance and was incorporated as one of the
variables for use in pharmacokinetic/pharmacodynamic (PK/PD) eculizumab
dosing algorithm for severely ill patients in order to achieve and to
maintain therapeutic eculizumab levels (>100 µg/ml) for
prompt control of TA-TMA (Jodele et al., 2016b).
Due to immediate need for clinical strategies how to manage vigorous
complement activation an in SARS-CoV-2 infected patients, we could adopt
some of currently available knowledge in complement mediated TA-TMA in
HSCT recipients. Eculizumab can be considered in COVID-19 population due
to significant amount of knowledge using this drug in critically ill
patients like HSCT recipients with TA-TMA, acceptable toxicity profile,
and lack of interference with T-cell mediated anti-viral responses
(Jodele et al., 2020a). Importantly, a first case study which applied
eculizumab therapy COVID-19 patients suffering from ARDS or severe
pneumonia resulted in successful recovery of all patients with reduction
in inflammation (Diurno et al., 2020). Four subjects with confirmed
severe, COVID-19-associated pneumonia with oxygen requirement and
radiologic evidence of bilateral pneumonia were offered eculizumab.
Despite patients presenting with a rapidly worsening respiratory
function, all of them showed a marked clinical improvement within the
first 48 hours after the first eculizumab dose with the median therapy
time of 12.8 days to disease resolution. This first encouraging data
warrant further evaluation in a larger cohort of COVID-19 patients.
Monitoring blood sC5b-9 can potentially serve as a helpful surrogate
marker for enhanced C5 production in COVID-19 patients as the turnover
of C5 will determine the clearance of eculizumab. In a high inflammatory
state as can be seen in severely ill HSCT recipients with TA-TMA or
COVID-19 patients, there is an acute phase response of the liver with
massive C5 production and additional C5 production by activated
circulating inflammatory cells and injured endothelial cells. Under such
conditions, there are more target C5 molecules generated and more
eculizumab is required as it forms immune complexes with the increased
number of C5 molecules. Eculizumab serum concentration, sC5b-9 and CH50
monitoring tests are clinically available that can be adopted for PK/PD
guided eculizumab dosing in COVID-19 patients as described in HSCT
population (Jodele et al., 2016b). All patients receiving complement
blockers should additionally receive antimicrobial prophylaxis
appropriate for the prevention of meningococcal infection, as the
available meningococcal vaccine does not provide adequate protection
(Bouts, Monnens, Davin, Struijk & Spanjaard, 2011; Struijk, Bouts,
Rijkers, Kuin, ten Berge & Bemelman, 2013). Complement blockade using
eculizumab with appropriate antimicrobial prophylaxis was shown to be
safe in immunocompromised HSCT recipients(Jodele et al., 2016a). Of
note, last year Alexion launched a variant of eculizumab, i.e.
ravulizumab, with longer plasma residence than eculizumab, allowing a
less frequent dosing interval of 8 weeks(Kulasekararaj et al., 2019; Lee
et al., 2019).
In addition to Eculizumab, several other C5-targeting antibodies have
been developed, as genetic C5 variants have been resulting in poor
binding and response to eculizumab inhibition (Nishimura et al., 2014).
For example, Roche and Chugai developed SKY/RO7112689, which works in
patients with the C5 variant p.Arg885His and long-lasting C5 inhibition
properties (Fukuzawa et al., 2017). Also, Novartis (LFG316) and
Regeneron (puzelimab/REGN3918) generated anti-C5 antibodies that are
currently in clinical development.
A tick-derived C5 inhibitor nomacopan (Coversin, Akari therapeutics) is
another complement inhibitory molecule that targets C5 and prevents the
release of C5a and formation of C5b–9, although in a different way than
eculizumab (Jore et al., 2016). In contrast to eculizumab, it is a
recombinant small protein from the Ornithodros moubata tick. In addition
to C5, it targets leukotriene/LTB4 that disrupts cell trafficking (in
particular neutrophils) (Figure 2), cytokine release and provides a link
between complement activation and coagulopathy with demonstrated
effectiveness in a range of ARDS and sepsis pre-clinical models
including those induced by viral infection like influenza H1N1(Garcia et
al., 2013). It is likely that C5a and LTB4 together account for many of
the pro-inflammatory effects associated with pulmonary inflammation and
TMA (Figure 2) as observed COVID-19 patients with severe courses.
Nomacopan is a small protein that is administered as continues
subcutaneous infusion, which will potentially provide continuous
complement blockade in the circulation. It demonstrated promising
complement modulating response in TA-TMA and is now being examined in
phase III clinical trials (Goodship et al., 2017).
Although terminal complement blockade provided significant improvement
in TA-TMA, a more tailored approach targeting C5a or C5aR1 might be
sufficient to cope with the deleterious, pro-inflammatory effects of
overactivated complement.