A single-valent long-acting human CD47 antagonist enhances antibody directed phagocytic activities

Many cancer cells express CD47 as a 'don't eat me' signal to mask their presences from immune recognition and destruction. Such a signal is transmitted when CD47 binds to the signal regulatory protein-α (SIRPα) on macrophages to cut the phagocytic reaction. Most recent studies have focused on developing CD47 blocking agents with different affinities and avidities in order to optimize the therapeutic window between efficacy and toxicities involving normal cells expressing CD47. We described in this study a new design to fuse one CD47 binding domain of SIRPα with a pharmacokinetics modifying domain F8. The resulted single valent long-acting CD47 antagonist SIRPα-F8 was able to bind to CD47 and disrupt CD47-SIRPα axis. However, by itself it cannot trigger endocytosis and has no effect on tumor growth. Only when used in combination with the anti-CD20 mAbs, there were greatly improved phagocytic activities towards CD20 positive cancer cells. In vivo the combination also resulted in better tumor growth inhibition comparing to the vehicle control group. In addition, we showed that the F8 fusion bound to hFcRn only inside endosomes at pH 6.0, enabled hFcRn mediated recycling and thus greatly extended the circulation half-life in hFcRn knock-in mice. Taken together, the SIRPα-F8 design may suggest a new option to improve the therapeutic index of antibody treatment in clinical use towards tumors.