Pharmacodynamic Activity and Preclinical Safety of CSL362, a Novel Humanised, Affinity Matured Monoclonal Antibody Against Human Interleukin 3 Receptor

Abstract 1524

CSL362, a novel humanized, affinity matured monoclonal antibody (mAB), is currently under development at CSL Limited. CSL362 neutralizes Interleukin-3 (IL-3) by binding to the human IL-3 receptor alpha chain (IL-3Ra/CD123). Furthermore, CSL362 has been engineered to bind to FcγRIIIa receptor (CD16) with high affinity and engages with effector cells of the innate immune system to elicit antibody-dependent cellular cytotoxicity (ADCC) against target cells expressing IL-3Ra such as AML blasts and leukemic stem cells.

A non-clinical program was conducted to investigate the general safety and toxicity profile as well as the pharmacokinetic and pharmacodynamic properties of CSL362 in non-human primates. Cynomolgus monkeys (n=6 per dose cohort) were administered weekly intravenous infusions of CSL362 at doses of either 0, 3, 10, 30 or 100 mg/kg for 5 weeks with a 6 week recovery period. The potential of CSL362 to induce hypersensitivity or allergic reactions, its impact on safety pharmacology variables as well as local tolerance were assessed. Furthermore, the possible adverse effects of CSL362 on the proliferation, differentiation and maturation of bone marrow derived hematopoetic progenitors were explored. The potential for immunotoxicity of CSL362 was assessed by means of peripheral blood immunophenotyping, T cell-dependent antibody response measurements after keyhole limpet hemocyanin-immunization, measurements of natural killer cell (NK), macrophage and granulocyte function, and quantification of serum cytokine release. The ability to ablate cells expressing IL-3Ra was also determined by evaluating the numbers of basophils, plasmacytoid dendritic cells (pDCs) and monocytes in the peripheral blood. Plasma concentrations of CSL362 were determined to evaluate the pharmacokinetic characteristics. In parallel, the specific immune response against CSL362 was monitored throughout the 11 week study duration.

Plasma level determination of CSL362 revealed adequate exposure and a PK profile similar to other humanized mABs. Specific anti-CSL362 antibody formation was observed in 18 % of the study animals. Treatment with CSL362 did not result in any anaphylactic or allergic reaction and had no adverse effect on systemic toxicity parameters or safety pharmacology variables measured. As predicted from in vitro and ex vivo studies, CSL362 produced a marked, dose-dependent and sustained depletion of peripheral blood basophils and pDCs which was associated with a transient decrease in NK cell numbers. No adverse effects of CSL362 on peripheral blood monocytes or pluripotent progenitor cells in the bone marrow was observed, furthermore the function of normal peripheral blood cells expressing IL-3Ra remained unaffected by CSL362. Intriguingly, a clear dose-dependent trend to NK cell recovery was observed following the final dose at week 5 during the subsequent recovery period which preceded the reappearance of basophils and pDC's. Together with the outcome of a tissue-cross reactivity study this proof of specific pharmacodynamic activity confirmed the Cynomolgus monkey as a relevant species. Overall, treatment with CSL362 was very well tolerated and 100 mg/kg, the highest tested dose, was designated the no-observed-adverse-effect-level.

In conclusion, the results from this non-clinical safety program demonstrate an excellent tolerance and favorable safety profile of CSL362. This study supports the further clinical development of this mAB in the remission maintenance of patients with CD123-positive acute myeloid leukemia.