Targeting complement amplification at the level of C3
As aptly put by the Lambris lab, C3 serves as the ”swiss army-knife” of the complement proteins. C3b generated from C3 in response to CP, LP or alternative pathway (AP) activation can amplify the initial complement activation by either pathway, when it forms the C3bB complex that can be cleaved by the serine protease factor D (FD) resulting in the self-amplifying C3bBb convertase (Ricklin, Reis, Mastellos, Gros & Lambris, 2016). This amplification loop will feed the cleavage of many molecules of C3 when not appropriately controlled by complement regulator proteins (Figures 1 and 3). The available data suggest that such control by complement inhibitors of the regulator of complement activation (RCA) family is disturbed in patients developing ARDS and TMA following infection with highly pathogenic coronaviruses, in particular in African Americans (Figure 3). All of these RCA proteins harbor complement control protein domains. Some of them are membrane-bound such as complement receptor 1 (CR1/CD35) membrane cofactor protein (MCP/CD46), decay-accelerating factor (DAF/CD55), whereas other are found in the circulation (FH, C4 binding protein (C4BP)). Mechanistically, the RCA proteins either destabilize the C3 convertases or serve as co-factors for Factor I (FI)-mediated degradation of C3b to iC3b and C3dg, which no longer contribute to the formation of the amplification loop. As implied by the name, CD55 accelerates the decay of the convertase, whereas CD46 mediates degradation of C3b. CR1, FH and C4BP exert both functions.
Several compounds have been developed that target the C3 convertase either by targeting molecules that are critical for assembly (FB, FD, MASP-3) or the destabilization of the convertase complex and degradation of C3b (CR1, FH). These compound have recently been discussed in detail in two excellent reviews (Mastellos, Ricklin & Lambris, 2019; Ricklin, Mastellos, Reis & Lambris, 2018). Prima vista, FD is an attractive target in COVID-19 infection, given that the proteolytic cleavage of FB by this serine protease is a crucial step to ignite the amplification loop. Also, plasma levels are relatively low, although high plasma turnover might pose a challenge. Thus, it is not surprising that small molecule inhibitors (Novartis, Achillion) and FD-antibodies (Genentech) have been generated and tested in several clinical trials. Also, a MASP-3-specific antibody (OMS906) has been developed by Omeros to block the conversion of por-FD to FD (Dobo et al., 2016; Hayashi et al., 2019). However, several other serine proteases can cleave C3 including elastase from neutrophils or proteases of the coagulation, the kinin and the fibrinolysis system. As the substrate specificity of these proteases for C3 is much lower than for their cognate substrates, the impact of such proteases under homeostatic conditions is probably minor. However, under pro-thrombotic conditions such as TMA, when control systems are exhausted and intravascular protease inhibitor concentration is low, such proteases are likely to cleave C3 and drive AP activation (Ekdahl et al., 2019). In light of these consideration, blocking of systemic AP activation by specific targeting of FD or MASP-3 seems difficult in patients suffering from ALI/ARDS with TMA or multiorgan failure.
As an alternative to FD, FB-targeting intervention has been developed by Ionis with Roche as a partner. They use a ligand-conjugated antisense drug to reduce the production of FB, which is now in phase II trials for IgA nephropathy and age-related macular degeneration (AMD). At this point it is unclear, how efficient this drug would attenuate FB production in a severe systemic inflammatory disease state. We will only briefly touch molecules that destabilize the C3 convertase as clinical development of some of these molecules has either been discontinued (TP10 or TT30, extracellular variants of CR1)(Lazar et al., 2007; Risitano et al., 2012) or are still in pre-clinical development (mini-FH, Amyndas)(Schmidt et al., 2013).
As an alternative approach to target C3, conversion to C3 convertase has been selected by the Lambris lab. They selected a peptide from a phage library, compstatin, that prevents the binding of C3 to the assembled convertase independent of its origin (Sahu, Kay & Lambris, 1996). Through several round of iteration, the affinity of this compound for C3 has been increased by more than 3 orders of magnitude as compared with the original compound resulting in CP40, which served as drug candidate for AMY-101 (Amyndas), which is now in phase II trials for periodontal disease and C3G (Mastellos, Ricklin & Lambris, 2019). This approach of direct C3 targeting is attractive, as it is supposed to block virus-induced LP and CP activation as well as LP/CP-driven activation of the amplification loop at the bottleneck of all pathways (Figure 1). However, C3 is one of the most abundant plasms proteins with a concentration in the range of 1.5 mg/ml. Thus, high amounts of inhibitor would be required to efficiently reduce circulating C3. The high turnover of C3 under strong inflammatory conditions would add to this problem. Finally, it remains to be determined whether compstatin derivatives would also prevent the cleavage of C3 by all circulating or cell derived serine proteases as outlined above. Despite these challenges, direct C3 targeting appears an attractive target in severe infection with highly pathogenic coronaviruses.
The list of compounds that specifically target the interaction of the small cleavage product of C3, the C3a anaphylatoxin with its C3aR is short. The selective nonpeptide C3aR antagonist SB 290157 has been generated almost 20 years ago(Ames et al., 2001). It has been used in with different success in several preclinical models to target C3aR. However, no clinical development has been pursued. During the past 10 years, the Fairlie lab has developed sophisticated approaches to design small molecule agonist and antagonists from proteins including C3a (Reid et al., 2013). In this context, they have recently reported on the new compound JR14a, a very potent C3aR antagonist that is 100-fold more potent than SB290157 (Rowley et al., 2020). This molecule awaits broad preclinical testing in animal models of inflammation.