Background/Purpose: New precision medicine guided therapies targeting alternate disease driving processes are required to improve clinical outcomes in RA. The complement pathway has long been implicated in RA pathogenesis. In particular, increased levels of the Complement Factor B (CFB) breakdown product Ba are found in synovial tissue showing alternative pathway (AP) activation in a subset of RA patients. Targeting the alternative pathway by suppressing CFB would inhibit the amplification of the complement cascade while preserving capacity to remove pathogenic auto-immune complexes and opsonize pathogens. Herein, the objective was To develop a novel therapy targeting the alternative pathway of complement activation for the treatment of RA.

Methods: Complete, CFB-depleted and heat inactivated serum were treated with Zymosan for 60 min and C3a, C5a and MAC were detected by ELISA. GalNAC-conjugated siRNA were designed to suppress hepatic CFB expression and screened for CFB knockdown. AZD6912 targets human CFB, and a surrogate siRNA GalXC-CFB targets murine CFB. The CAIA model of arthritis in Balb/c mice using ArthritoMab antibody cocktail (Day 0) and LPS (Day3) was used to assess the impact of GalXC-CFB with prophylactic and therapeutic dosing regimens. A 4-point scale arthritis scoring system based on erythema and swelling of hind paws was used. At termination (Day 13) hind paws were harvested, fixed, decalcified and processed for paraffin embedding, sectioned for H&E staining, Safranin-O staining, RNAscope for C3 and CFB mRNA expression, and CD11b IHC. The pharmacology and safety profile of AZD6912, a siRNA targeting human CFB, was evaluated in non-human primates. PK/PD readouts included hepatic CFB mRNA expression measured by rtPCR, serum CFB protein detected by Western Blot and ELISA, and CAP activity using the Wieslab assay.

Results: CFB is necessary for zymosan mediated activation of the alternative complement pathway and the generation of C3a, C5a and terminal complement components in vitro. Surrogate CFB siRNA dose-dependently reduced murine hepatic mRNA expression and circulating levels of CFB in vivo. Prophylactic dosing of surrogate CFB siRNA prevented the development of CAIA. Moreover, following the emergence of inflammatory arthritis (Day 5), a single 3 mg/kg therapeutic dose resolved existent CAIA in the hind paws. Improvement in clinical scores were associated with reduced immune cell infiltrates and reduced C3 and CFB expression and cartilage erosion in the ankle joints. In non-human primate studies, monthly 2 mg/kg AZD6912 sc dosing inhibited hepatic mRNA (~80%) and serum CFB levels (~85%) resulting in sustained suppression of AP activity ( >95%) over the 16-week study period. There was no evidence of off-target mRNA knockdown in the liver, and the preliminary safety assessment was unremarkable.

Conclusion: Suppression of CFB using a GalNAC targeting siRNA demonstrated that inhibition of hepatic CFB was sufficient to reduce articular inflammation and damage in the murine CAIA model. In non-human primates AZD6912 suppressed CFB levels and complement alternative pathway activity. Dose-to-man modelling predictions and toxicology studies in cynomolgus monkeys support progression into the clinic.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/targeting-complement-factor-b-cfb-via-a-novel-sirna-therapy-azd6912-to-treat-inflammatory-arthritis/