Abstract
Modulating the complement system is a promising strategy in drug discovery for disorders with uncontrolled complement activation. Although some of these disorders can be effectively treated with an antibody that inhibits complement C5, the high plasma concentration of C5 requires a huge dosage and frequent intravenous administration. Moreover, a conventional anti-C5 antibody can cause C5 to accumulate in plasma by reducing C5 clearance when C5 forms an immune complex (IC) with the antibody, which can be salvaged from endosomal vesicles by neonatal Fc receptor (FcRn)-mediated recycling. In order to neutralize the increased C5, an even higher dosage of the antibody would be required. This antigen accumulation can be suppressed by giving the antibody a pH-dependent C5-binding property so that C5 is released from the antibody in the acidic endosome and then trafficked to the lysosome for degradation, while the C5-free antibody returns back to plasma. We recently demonstrated that a pH-dependent C5-binding antibody, SKY59, exhibited long-lasting neutralization of C5 in cynomolgus monkeys, showing potential for subcutaneous delivery or less frequent administration. Here we report the details of the antibody engineering involved in generating SKY59, from humanizing a rabbit antibody to improving the C5-binding property. Moreover, because the pH-dependent C5-binding antibodies that we first generated still accumulated C5, we hypothesized that the surface charges of the ICs partially contributed to a slow uptake rate of the C5–antibody ICs. This idea motivated us to engineer the surface charges of the antibody. Our surface-charge engineered antibody consequently exhibited a high capacity to sweep C5 and suppressed the C5 accumulation in vivo by accelerating the cycle of sweeping: uptake of ICs into cells, release of C5 from the antibody in endosomes, and salvage of the antigen-free antibody. Thus, our engineered anti-C5 antibody, SKY59, is expected to provide significant benefits for patients with complement-mediated disorders.
Highlights
Antibody engineering is in high demand for generating therapeutic antibodies with higher efficacy, safety, and convenience for patients than can be achieved by conventional antibodies
As 305LO1, with very weak affinity to C5 at acidic pH (Table 2), showed higher C5 clearance in mice compared with the other antibodies (CFA0322 and eculizumab), we evaluated the effect of pH dependency on endogenous C5 accumulation in cynomolgus monkeys using antibodies with various properties in their variable and constant regions
CFA0322 was used as the non–pH-dependent C5-binding antibody because eculizumab has minimal activity against any other primate or mammalian C5 [27], and we found that the non–pH-dependent antibody increased the concentration of endogenous C5 in plasma to 286% compared with the values at pre-administration (Fig 4A)
Summary
Antibody engineering is in high demand for generating therapeutic antibodies with higher efficacy, safety, and convenience for patients than can be achieved by conventional antibodies. Antigen accumulation in plasma is a phenomenon observed when an antibody that targets a soluble antigen in circulation is administered, and it requires a higher dose of the antibody to be administered to achieve sufficient efficacy [7]. This accumulation occurs because the antigen is synthesized continuously in vivo but the clearance of the antigen is reduced by the administered antibody [8,9], which generally has a long plasma half-life because of neonatal Fc receptor (FcRn)-mediated recycling [10]. Engineering that can further suppress the antigen accumulation without compromising the long plasma half-life of the antibody is still desired
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