Functional in vivo Blockade of the GM-CSF Receptor in Cynomolgus Monkey with CAM-3001, A Human Anti-GM-CSF Receptor Antibody in Development for the Treatment of Patients with Rheumatoid Arthritis

Suzanne Cohen1, Matthew McCourt1, Phillip Monk3, Tim Meyers1, Cathy Drinkwater2, Andrew Nash2, Ian Anderson1, Matthew Sleeman1. 1MedImmune Ltd, Cambridge, United Kingdom, 2CSL, Melbourne, Australia, 3Synairgen, Southampton, United Kingdom.

GM-CSF is a key cytokine involved in the pathogenesis of rheumatoid arthritis (RA). Recently several clinical trials have started to investigate this axis with antibody programmes either to GM-CSF or its receptor. Using a model of GM-CSF-induced hematologic responses in cynomolgus monkeys, we demonstrate the in vivo activity of CAM-3001, an antibody to the GM-CSFRα chain that is currently in clinical development for the treatment of RA.

Four treatment groups of five male cynomolgus monkeys received PBS or CAM-3001 (1, 10 or 30 mg/kg) as a 30 minute IV infusion prior to GM-CSF administration. The first dose of GM-CSF was given 30 minutes following the end of antibody dosing and animals were dosed subcutaneously twice daily (approximately 8 hours apart) for three consecutive days with 5mcg/kg recombinant human GM-CSF. Blood for haematology (complete blood count with differential) and serum for determination of antibody concentration was collected prior to each antibody infusion, at 30 minutes and 4 hours after GM-CSF dosing on day 1, four hours after GM-CSF dosing on days 2 and 3, and on Days 4, 6 and 8. Antigen capture ELISA was used to determine CAM-3001 levels in sera. Human GM-CSF receptor Fc fusion was used as the capture reagent. CAM-3001 was detected with secondary antibody (sheep anti-human IgG4 peroxidase) / TMB. Statistical analyses were performed with GraphPad Prism 5 software, using a 2 way ANOVA with repeated measures. Bonferroni post-tests were performed to compare treatments with control PBS group.

Recombinant human GM-CSF (5mcg/kg) was administered twice daily for 3 consecutive days to cynomolgus monkeys that had previously received PBS or 1, 10 or 30 mg/kg CAM-3001. The first dose of GM-CSF stimulated a rapid and transient drop (margination) in circulating leukocyte (69% decrease from baseline) and neutrophil counts (84% reduction from baseline) in the control PBS group. This margination response was significantly inhibited in the presence of CAM-3001 after dosing at 1, 10 or 30mg/kg (p<0.001). A similar trend was noted for basophils, monocytes and eosinophils (62%, 78%, and 98% reductions from baseline respectively that were reversed by CAM-3001). Continued administration of GM-CSF over 3 days stimulated a leukocytosis, with increases in neutrophils (3.6-fold), basophils (2-fold) and eosinophils (5.3-fold) peaking at days 3, 4 and 4 respectively. Increases in numbers of platelets at days 6-8 (2-fold) were also observed. These responses were completely inhibited at 10 and 30 mg/kg of CAM-3001 (p<0.001) and attenuated at 1mg/kg of CAM-3001. Serum concentrations of CAM-3001 were consistent with the animals having been dosed appropriately with 1, 10 or 30mg/kg.

At all three dose levels, CAM-3001 effectively attenuated or fully inhibited GM-CSF-mediated leukocyte margination and subsequent leukocytosis in vivo. No adverse events were observed. These results suggest that CAM-3001 has the potential to suppress GM-CSF-driven pathologies in diseases such as rheumatoid arthritis.