Efficacy of an Anti-CD22 Antibody-Monomethyl Auristatin E Conjugate in a Preclinical Xenograft Model of Precursor B Cell Acute Lymphoblastic Leukemia

Precursor B cell acute lymphoblastic leukemia (preB ALL) is the most common childhood cancer, as well as the leading cause of childhood cancer-related mortality. Despite overall progress in treatment, certain types of patients with preB ALL have a dismal prognosis with an overall survival of 30%. In addition, current approaches predispose these young patients to late effects, including secondary malignancies. Therefore, more efficacious and less toxic approaches are needed. Targeted therapy for leukemia has the potential to reduce off-target effects, thus minimizing toxicity and late effects and improving efficacy.

Monoclonal antibodies (mAbs) have proven utility in leukemia therapy based on their ability to specifically target the leukemic clone and minimize off target effects. However, mAbs are generally not adequate as single agents because of limited efficacy. Antibody drug conjugates (ADCs) provide a method to deliver a potent toxin to the interior of antigen-positive tumor cells. CD22 is an ideal target for ADC-mediated therapeutics for B-cell malignancies because 1) there is high CD22 expression (more than 90%) in B-cell type ALL and 2) CD22 undergoes rapid internalization upon mAb binding. In this study, we evaluated the anti-CD22 (aCD22) mAb as a vehicle for the targeted delivery of Monomethyl Auristatin E (mMAE), a derivative of the cytotoxic tubulin modifier auristatin E.

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Treatments and outcome of the animals

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Treatments and outcome of the animals

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First, we assessed the in vitro cytotoxicities of the aCD22 mAb-mMAE in preB ALL cell lines Reh and JM1. MTS assay showed that IC50 doses of aCD22 mAb-mMAE were 0.7nM and 1nM in Reh and JM1, respectively. Next, we assessed in vivo therapeutic efficacy of the aCD22 mAb-mMAE in a pre-clinical xenograft animal model of preB ALL, using a primary leukemia sample which was confirmed to be CD22 positive. Age matched female NOD/SCID/IL2Rg-/- (NSG) mice were randomly assigned to 4 treatment groups (n=8 per group): 1) PBS, 2) free mMAE (0.165mg/kg), 3) free aCD22 mAb (7.335mg/kg), and 4) aCD22 mAb-mMAE conjugate (7.5mg/kg). The dose of free aCD22 mAb and free mMAE was equivalent to those of each component in the aCD22 mAb-mMAE conjugate. Five million leukemia cells were inoculated per mouse via intra-bone marrow injection. Twenty four hours after leukemia inoculation, animals started receiving weekly iv treatments for 3 weeks. When compared to controls (PBS, free aCD22 mAb or mMAE treatments), the treatment with the aCD22 mAb-mMAE conjugate increased the median survival time of the mice by two fold (Figure. PBS vs. aCD22 mAb-mMAE p<0.005, free mMAE vs. aCD22 mAb-mMAE p<0.05, free aCD22 mAb vs. aCD22 mAb-mMAE p<0.05, by Gehan-Breslow-Wilcoxon test). Leukemia-related death was confirmed by necropsy. Harvested leukemia cells were assessed by flow cytometry and found to be HLA and CD22 positive. During treatment, the mice in all the treatment groups remained healthy and active, and did not lose weight. Toxicity was assessed with weekly CBC and chemistry panels which revealed no significant toxicity.

In conclusion, we demonstrated that aCD22 mAb-mMAE is efficacious in a preclinical preB ALL xenograft mouse model.