A Recombinant Antibody to EphA3 for the Treatment of Hematologic Malignancies: Research Update and Interim Phase 1 Study Results

Abstract 4893

EphA3 is a receptor tyrosine kinase important in fetal development but apparently not in healthy adults. It is expressed in hematologic malignancies including AML, CML, MDS, MPN and Multiple Myeloma and in a range of solid tumors in particular in the stromal and vascular tissue. KB004 is a high-affinity non-fucosylated, recombinant antibody to EphA3 that triggers apoptosis and has potent antibody-dependent cellular cytotoxicity (ADCC) activity against EphA3+ cells. Direct apoptosis of EphA3+ leukemic cells ex vivo was demonstrated at antibody concentrations of 10μg/mL or greater. Expression on a CD34⁺ CD38⁻ CD123⁺ cell population in primary AML patient bone marrow, and inhibition of long-term culture initiating cells by KB004, indicates that EphA3 may be present on leukemic stem cells (Palath et al 2010).

EphA3 expression in bone marrow biopsies from AML patients was characterized by immunohistochemistry (IHC). EphA3 was detected on tumor cells by IHC in 4/10 samples analyzed. EphA3 was also present on the vasculature in AML bone marrow (6/10 samples) but not in similar tissues in bone marrow biopsies (n=2) from non-leukemic individuals. This suggests tumor vasculature as a potential further therapeutic target for KB004 in AML in addition to targeting leukemia cells directly. We are further characterizing the expression level of EphA3 in different hematologic malignancies, tumor microenvironment, and at various stages of disease progression from primary human samples.

The Cynomolgus macaque was selected as a relevant species for toxicity testing since KB004 binds with equivalent affinity to human and Cynomolgus EphA3 and CD16. IHC studies demonstrated the absence of cellular expression of EphA3 in normal human or Cynomolgus monkey tissues. KB004 administered to Cynomolgus monkeys (n=44) twice weekly for 13 weeks at doses up to 100 mg/kg was well tolerated and there were no clinical or pathology adverse findings. Cardiovascular, respiratory and central nervous system functions were measured in primates after KB004 dosing 5 times over 15 days to reach steady-state exposure at doses of 10mg/kg (n=4) or 100 mg/kg (n=4). There were no KB004-related effects on body weight, food consumption, mean arterial pressure, heart rate, body temperature, neurological parameters, respiration rate, oxygen saturation, or blood gas parameters, or changes in serum concentrations of Troponin I. Qualitative evaluation of the electrocardiogram (ECG) did not reveal any electrocardiographic abnormalities and there were no effects on measured ECG intervals (i.e., duration of the QT and RR intervals, derived QTc values). The effect of KB004 on wound healing was tested in a primate incisional wound healing model in the Cynomolgus monkey. No statistically significant differences in healing were detected between control and KB004 (2×10 or 2×100mg/kg) treated animals (n=24).

A Phase I clinical study has been initiated in subjects with hematologic malignancies including AML, CML, ALL, MDS and MPN. Subjects are being assessed for EphA3 protein expression at study entry and at various time points throughout the treatment period. Study objectives are to determine a maximum tolerated dose, examine the safety and tolerability profile of KB004, obtain pharmacokinetic data, describe the immunogenicity profile, and explore cell subpopulations and pharmacodynamic effects of treatments with KB004. This is an open-label, repeat administration study of weekly IV dosing of up to 17 cycles (3 doses per 21-day cycle). Dosing levels are scheduled for 20 mg (∼0.3 mg/kg), 70 mg (∼1 mg/kg), 200 mg (∼3 mg/kg) and 700 mg (∼10 mg/kg). Therapeutic antibody levels (target level 10μg/mL) are predicted to be achieved from the first dose cohort for a portion of the dosing interval. The higher doses should provide blood levels in excess of 10 μg/mL for the entire one-week dosing interval. The first cohort (n=3) has been successfully completed, and a subject with AML remains on study having received 8 doses to date. Recruitment of the next dose cohort is ongoing. Pharmacokinetic data and tumor expression profiles will be presented.