Optimising CDC
CDC has been recognised as an important mechanism of action for some therapeutic mAbs such as anti-CD2024–26. Thus, strategies to optimise Fc-mediated complement activation are currently being developed.
Intrinsically, due to its naturally occurring pentameric and hexameric forms, IgM demonstrates the highest capacity for complement activation. However, IgM has not received much attention in therapeutic mAbs development and only a few tumour-targeting IgM mAbs have been evaluated in clinical trials27; most prominently with PAT-SM6 receiving orphan drug designation by EMA and FDA for multiple myeloma28,29.
Among IgG subclasses, IgG1 and IgG3 are good complement activators. Although IgG3 seems to be more potent, manufacturing issues, instability and shorter in vivo half-life make it a less attractive candidate for drug development. A way to combine the advantages of both IgG1 (favourable manufacturing characteristics) and IgG3 (enhanced CDC) was achieved through the construction of IgG1/IgG3 chimeric antibodies30. The optimal construct, called 113F, combined the CH1 and the hinge of IgG1 with the CH2 of IgG3 and a CH3 which was partly of IgG3 and partly of IgG1 origin. The nonfucosylated version of this chimeric antibody showed enhanced CDC and ADCC comparable to nonfucosylated IgG1, in addition to preserved protein A binding, important for the purification process. The improved efficacy of this chimeric construct was confirmed in cynomolgus monkeys where an anti-CD20 113F antibody construct showed greater B-cell depletion if compared to IgG1 (both antibodies were nonfucosylated for improved ADCC). This study indicates that the combination of optimised complement activation and A/I ratio represents a promising strategy for the improvement of tumour-depleting antibodies.
Other strategies for enhanced complement activation include the introduction of point mutations to improve IgG1 binding to C1q16, a key component for the initiation of the complement cascade. Importantly, mutations that potentiate CDC can be combined with ADCP and ADCC-enhancing mutations in a single IgG131, thus broadening the effector function of these antibodies. Finally, the mutations that favour IgG hexamer formation also significantly enhance C1q fixation and thus CDC32,33. However, currently it remains to be seen whether these Fc mutations that enhance CDC in in vitro and inex vivo studies translate into improved clinical efficacy.