Effectorless Fc-fusion improves FLT3L drug-like properties for cancer immunotherapy combinations.

Background: Conventional dendritic cells (cDCs), are central to antitumour immunity, but their low prevalence in tumours limits the efficacy of immunotherapies. FLT3L is a key growth factor regulating cDCs development in the bone marrow. It expands cDCs when administered exogenously, favouring antitumour T cell priming and tumour control. Currently, FLT3L pharmacokinetic (PK) and pharmacodynamic (PD) properties require daily dosing for up to 14 days, which may limit its clinical use. In the present study, we developed and characterised a therapeutic modality named FLT3L-Fc NG2LH.

Methods: We improved human FLT3L PK properties by fusing it with a modified fragment crystallisable (Fc) domain of IgG1. To prevent Fc gamma receptor (FcγR) mediated effector function, we engineered an effectorless Fc format called NG2LH, consisting of the aglycosylation substitution N297G, combined with a graft of the lower hinge region of IgG2 onto an otherwise IgG1 Fc.

Findings: FLT3L-Fc NG2LH had limited binding to FcγRs and failed to elicit antibody dependent cellular cytotoxicity (ADCC) and cellular phagocytosis (ADCP). PK/PD studies using a mouse effectorless equivalent, mFLT3L-Fc, showed that a single injection of mFLT3L-Fc leads to sustained expansion of cDCs in blood, spleen, and B16F10 tumours. When combined with polyI:C and anti-PD-L1, a single mFLT3L-Fc injection delays the growth of B16F10 tumours and reinvigorates CD8+ T cell immunity.

Interpretation: The improved properties of FLT3L-Fc NG2LH are expected to mitigate the practical limitations of FLT3L usage in the clinic, and constitute an asset for future cancer immunotherapy combination regimens leveraging cDC biology in situ.

Funding: This work was performed at, and funded by Genentech Inc. South San Francisco, CA 94080, USA.