hLL2-Cys-PBD, a New Site-Specifically Conjugated, Pyrrolobenzodiazepine (PBD) Dimer-Based Antibody Drug Conjugate (ADC) Targeting CD22-Expressing B-Cell Malignancies.

We previously described the development of hLL2-PBD, an ADC composed of the humanized monoclonal antibody epratuzumab (hLL2) against human CD22, stochastically conjugated to SG3249, a cathepsin B-cleavable valine-alanine PBD payload. SG3249, also known as tesirine, is a potent and clinically validated PBD payload currently being employed in the clinic in three ADCs: the CD25-targeted ADCT-301, the CD19-targeted ADCT-402, both for the treatment of lymphoma and leukemia, and the DDL3-targeted rovalpituzumab tesirine for the treatment of small cell lung cancer. hLL2-PBD demonstrated potent and specific in vitro and in vivo activity in the CD22-positive human Burkitt's lymphoma-derived cell lines Ramos, Daudi and WSU-DLCL2. Here, we describe the development and pre-clinical characterization of hLL2-cys-PBD, an ADC composed of an engineered version of epratuzumab (hLL2-cys), site-specifically conjugated to the PBD payload tesirine. The average drug to antibody ratio of hLL2-cys-PBD is 1.74.

In vitro, hLL2-cys-PBD showed potent and specific cytoxtoxic activity against a panel of CD22-expressing human lymphoma cell lines, including Ramos, Daudi, WSU-DLCL2 and SU-DHL-4. Moreover, hLL2-cys-PBD was efficiently internalized by CD22-expressing cells and, consistent with the proposed mode of action of PBD dimer warheads, it induced DNA inter-strand cross-linking after a two hour exposure period, as measured by the modified single cell gel electrophoresis (comet) assay. In contrast, a non-CD22-targeted, site-specific, PBD-containing control ADC did not yield appreciable DNA cross-links.

In vivo, hLL2-PBD showed potent, dose-dependent, and durable anti-tumor activity in subcutaneously implanted Ramos and WSU-DLCL2 xenograft models. In the Ramos model, a single dose of hLL2-cys-PBD at 1 mg/kg induced complete regression (CR) in all ten treated mice, with 9/10 tumor free survivors (TFS) at the end of the study on day 60. In the WSU-DLCL2 model, a single dose of hLL2-cys-PBD at 3 mg/kg resulted in 3/10 partial regression, 7/10 CR and 2/10 TFS at the end of the study on day 60. The toxicity of hLL2-cys-PBD was evaluated in rat (single dose study) and in cynomolgus monkey (repeat dose study). In rat, hLL2-cys-PBD was well tolerated at 2 mg/kg. In cynomolgus monkey, hLL2-cys-PBD elicited the expected rapid B-cell depletion and it was well tolerated at 0.6 mg/kg.

Together, these data demonstrate that hLL2-cys-PBD has potent anti-tumor activity against CD22-expressing hematological tumors and warrants further development of this ADC into the clinic.