ALBERT

Description: CD123 (IL-3Rα) is a phenotypic marker of putative leukemic stem cells (LSC) in AML (Jordan, Leukemia2000;14:1777). We and others have found that CD34+38− cells from AML patients (pts) express high levels of CD123, in contrast to absence of expression on CD34+38− cells in normal individuals. Binding of CD123 by monoclonal antibody (mAb) 7G3 inhibits IL-3-dependent signalling and proliferation in vitro. In a NOD-SCID xenograft model 7G3 inhibits human AML engraftment, but not normal human hematopoiesis (Lock ASH2007; Abs161). CSL360, a recombinant chimeric IgG1 mAb derived from 7G3, binds the same epitope. CSL360 concentrations ≥ 0.1μg/mL in vitro inhibited 90% AML cell growth in the presence of supraphysiological IL-3 levels. Preclinical toxicology studies with doses up to 100 mg/kg weekly × 4 in cynomolgus monkeys showed no CSL360-related effects in clinical signs, hematology, chemistry, urinalysis, gross pathology or histopathology. A Phase 1 study of safety, pharmacokinetics (PK) and bioactivity of CSL360 in relapsed, refractory or high risk AML began in March 2007. Pts receive 12 weekly iv infusions if not withdrawn early due to treatment-related toxicity or disease progression. Additional treatments may be given to pts who achieve a response. Bone marrow aspirates/trephine samples are obtained at screening, after dose 3 and before doses 5 and 11. More than 180 infusions have been administered to 26 AML pts (21 M, 5 F; 17 de novo, 8 MDS-related, 1 treatment-related AML) in 5 dose level cohorts: 0.1, 0.3, 1.0, 3.0 and 10 mg/kg. There is no intra-patient dose escalation. PK parameters over the dose range, estimated in 19 pts over 7 days after doses 1 and 4, were linear with dose-proportional increases in the AUC and Cmax; dose 1 Cmax ranged from 0.62 – 287.33 μg/mL and dose 4 Cmax from 1.02 – 178.22 μg/mL. CSL360 mean plasma half-life (dose 1, 83 ± 33 h; dose 4, 117 ± 59 h) appears to be independent of dose and treatment number. Dose 1 systemic clearance (0.21 ± 0.16 L/h) and volume of distribution (0.39 ± 0.22 L/kg) were relatively low, consistent with this size IgG. In all pre-treatment samples anti-CSL360 antibody titers were negative, determined by enzyme immunoassay. Anti-CSL360 antibodies were detected post-treatment in 8/12 pts; these antibodies have not been fully quantified or characterised. CSL360 has been well tolerated; a MTD has not been defined. Seven pts received all 12 doses, 13 pts were withdrawn due to progressive disease or investigator’s decision, 3 pts were withdrawn in association with infections, 2 pts withdrew consent, and 1 pt is ongoing. Three serious adverse events have been considered possibly related or related to CSL360: 1 invasive fungal infection (Gr 5), and 2 infusion reactions (Gr 2; hospitalised). Other adverse events are consistent with expectations for the disease population. Of 8 pts in the 3 mg/kg and 10 mg/kg cohorts who are evaluable for response after ≥ 4 doses, 1 complete response (CR) has been observed. A 22 yr old male, de novo FAB M1 cytogenetically normal AML, who had relapsed post-2 allogeneic SCT, achieved a morphologic leukemia free state after 3 doses (3.0 mg/kg) and CR after 12 doses, sustained for 〉 9 weeks. The pt received 17 doses before withdrawal to treat co-morbidities. Flow cytometry studies with anti-CD123 antibodies demonstrated dose-dependent CSL360 coating of both AML blasts and LSC. Saturation of target antigen on marrow and blood cells was observed 1 day after dosing at 0.3mg/kg, associated with decreased expression of CD123 detected by an antibody to a different epitope. At higher dose levels saturation of CD123 was maintained 7 days post dosing, associated with ongoing reduction in surface CD123 expression. In a representative sample, plasma from a pt treated at 10 mg/kg specifically inhibited IL-3-induced proliferation of AML blasts ex vivo, indicating sufficient circulating concentration of CSL360 to inhibit IL-3 mediated effects in vivo. Effects of CSL360 on proliferation and apoptosis of AML cells in treated patients are being investigated. These preliminary results show anti-CD123 mAb therapy with CSL360 is safe and tolerable; biological effects have been demonstrated; a sustained CR was achieved in 1 advanced, refractory AML pt. The study is continuing, with 20 evaluable patients to be accrued and treated at 10 mg/kg weekly; at this dose level the PK and correlative assays predict that complete blockade of IL-3 signalling through CD123 can be achieved in vivo.

Description: Background: Most patients with acute myeloid leukemia (AML) die from relapsed disease, including those who attain complete remission (CR). Post-remission treatment designed to kill chemoresistant leukemia stem cells (LSC) and eradicate minimal residual disease (MRD) could delay or prevent relapse. The Interleukin-3 Receptor alpha chain (IL3Rα/CD123) is overexpressed on LSC and AML blasts compared to normal hematopoietic cells. CSL362 is a fully humanized anti-CD123 monoclonal antibody, engineered for greater antibody-dependent cell-mediated cytotoxicity (ADCC) by high affinity for NK cell CD16. CSL362 has potent activity against AML cells in vitro and in mouse xenograft models. In this Phase 1 study we evaluated the safety, PK and immunogenicity of repeat doses of CSL362 as post-remission treatment for AML patients. Exploratory endpoints include effects on CD123+ normal blood cells (basophils and pDC), NK cells, levels of serum cytokines, CSL362 binding to CD123 on monocytes, and MRD. Methods: Eligibility criteria include CD123+ AML in 1st or 2nd CR /CRp, adverse risk factors (disease history, cytogenetics, molecular mutations or MRD), and no plan for allogeneic stem cell transplantation. Intravenous CSL362 is given over 3 hours every 14 days x 6 doses. Final evaluation for safety is at week 16 and for remission status is at week 24. Sequential cohorts of 3 - 6 pts have been treated at 5 dose levels, from 0.3 to 9.0 mg/kg. Serum is assayed centrally for cytokines, PK and immunogenicity. Standardized whole blood biomarker assays are performed at each site by flow cytometry, and analyzed centrally. Bone marrow is sampled at baseline, weeks 5 and 12. Results: To date, 25 pts (16M, 9F) median age 66 yr (range 32-83 yr) have received 118 infusions of CSL362; 15 pts received all 6 doses, 6 pts relapsed on study, 1 pt was withdrawn for an SAE (infusion reaction); 3 pts are ongoing. Adverse events related to CSL362 in ≥ 10% of pts (CTCAE V4 grades) were infusion reactions (12 pts; 15 grade 1-2, 2 grade 3 events), increased C-reactive protein (3 pts; 4 grade 2 events), hypertension (3 pts; 6 grade 1-2, 2 grade 3 events), and hypotension (3 pts; 2 grade 2, 1 grade 3 events). There have been 3 dose limiting toxicities (1 hypertension, 2 infusion reactions), and 3 related SAEs (2 infusion reactions, 1 transient delirium). Infusion reactions have been manageable with supportive care. A protocol amendment to allow hydrocortisone premedication has been effective to prevent or reduce the severity of reactions. At follow-up ≥ 6 months from first dose, 10 of 20 evaluable pts maintained CR. Median duration of CR from start of CR and ongoing at last follow-up was 34+ weeks [range 26 – 52+ weeks]. Of 6 evaluable pts who were MRD positive at baseline, 3 converted to negative on study. Biomarker assays show rapid, complete in vivo depletion of basophils and pDC (cells that have high expression of CD123) at all dose levels, ≤ 6 hours post-dose. Depletion is sustained for ≥ 15 days at doses ≥ 3.0 mg/kg. NK cells (which do not express CD123) in blood fall transiently then recover from day 3 post-dose to ≥ baseline by day 8. CSL362 saturation of CD123 on monocytes (cells that have low CD123 expression) in vivo occurs rapidly post-dose at all dose levels and is sustained ≥ 15 days at dose levels ≥ 3.0 mg/kg. Serum CSL362 concentration demonstrated a nonlinear PK profile with slower clearance and longer half-lives at higher doses. PK profiles at doses above 3 mg/kg seem to be dose proportional. The half-life for the two higher dose levels is 4-5 days. Cytokine assays showed increased levels of GM-CSF, IFN-γ, IL-1β, IL-6, IL-8, TNF-α, IL-15 and IL-1RA after the 1st dose, returning to baseline by day 8. Levels after dose 6 were lower than after the dose 1. Low level anti-CSL362 antibody titers were detected coincident with trough concentrations of CSL362 in 6 of 18 pts tested thus far, with no effects on PK or association with adverse events. Conclusion: CSL362 is safe and well tolerated in AML pts with CR/CRp and high risk of relapse. Potent, targeted ADCC of normal cells (basophils and pDC) that highly express CD123 is evident at all dose levels tested, with durable depletion at ≥ 3.0 mg/kg. At dose levels ≥ 3.0 mg/kg, saturation of CD123 on marker monocytes is durable for the inter-dose interval. These data indicate it is likely that dose levels ≥ 3.0 mg/kg are required to maximize ADCC against residual AML. A phase 2 study of CSL362 is planned. Figure 1 Figure 1. Disclosures Roboz: Glaxo SmithKline: Consultancy; Celgene: Consultancy; Agios: Consultancy; Novartis: Consultancy; Astra Zeneca: Consultancy; Sunesis: Consultancy; Teva Oncology: Consultancy; Astex: Consultancy. Busfield:CSL Limited: Employment. Barnden:CSL Limited: Employment. Sedgmen:CSL Limited: Employment. Ghosh:CSL Limited: Employment. Hosback:CSL Limited: Employment. Davis:CSL Limited: Employment. Dyson:CSL Limited: Employment. Dasen:CSL Behring: Employment. DeWitte:CSL Behring: Employment, Equity Ownership. Bensen-Kennedy:CSL Behring: Employment.