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.