Abstract 51: Characterization of the mechanism of action, pharmacodynamics and preclinical safety of ADCT-402, a pyrrolobenzodiazepine (PBD) dimer-containing antibody-drug conjugate (ADC) targeting CD19-expressing hematological malignancies (2017)

Abstract ADCT-402, currently in Phase I clinical trials for B-cell hematological malignancies, is an ADC composed of a recombinant humanized IgG1 against human CD19, stochastically conjugated via a cleavable linker to a PBD dimer cytotoxin (DAR of 2.3). PBD dimers, DNA minor groove interstrand cross-linking agents, are gaining increasing attention and are currently being tested as the ADC warheads in several clinical trials. ADCT-402 has potent and targeted cytotoxicity against a panel of human lymphoma and leukemia cell lines in vitro. In vivo, ADCT-402 demonstrates dose-dependent antitumor activity against Burkitt’s lymphoma xenograft models. Moreover, ADCT-402 is markedly superior to maytansinoid- and auristatin-based CD19-targeting ADCs in the Ramos xenograft model. In a rat toxicology study, a single dose of ADCT-402 at 2 mg/kg is well tolerated with a favorable PK profile and excellent stability in vivo. The current study aimed to further define the mechanism of action (MOA) of ADCT-402 and validate its pharmacology and preclinical safety for clinical development. CD19 is a clinically validated target with restricted normal tissue expression and a widespread expression in the majority of B-cell malignancies. Importantly, we show here the consistent expression of CD19 in matched samples (initial diagnosis and relapsed/refractory) from panels of lymphoma patients, indicating that relapsed/refractory patients are appropriate for treatment with ADCT-402. ADCT-402 was shown to be efficiently internalized by CD19+ cells in vitro. Moreover, in line with the PBD dimer MOA, following a 2 hour exposure to ADCT-402, DNA interstrand cross-links reached a peak between 8 - 12 hours and persisted for up to 36 hours post-treatment. In contrast, the peak of cross-link formation for the PBD dimer warhead alone was observed immediately after 2 hour incubation, while a non-targeted PBD-ADC did not yield any appreciable DNA cross-links. In SCID mice s.c. implanted with Ramos cells, a single dose of ADCT-402 was administered at 0.33 or 1 mg/kg. Twenty-four hours after treatment, excised tumors showed a dose proportional increase in intensity of staining by an anti-PBD payload antibody, as well as in DNA cross-linking and in γ-H2AX formation. In contrast, no DNA cross-linking was observed in matched lymphocyte samples. The toxicity of ADCT-402 was further evaluated in a repeat dose cynomolgus monkey study. ADCT-402 was clinically well tolerated with an acceptable off-target safety profile. The PK of the ADC was consistent with normal antibody clearance with a half-life of about 12 to 17 days. These data confirm the MOA of ADCT-402 and provide relevant pharmacodynamic assays and preclinical safety assessment to guide the clinical development of this promising ADC in B-cell malignancies. Citation Format: Francesca Zammarchi, Simon Corbett, Karin Havenith, Narinder Janghra, Konstantinos Kiakos, Teresa Marafioti, David G. Williams, Simon Chivers, Phil W. Howard, John A. Hartley, Patrick H. van Berkel. Characterization of the mechanism of action, pharmacodynamics and preclinical safety of ADCT-402, a pyrrolobenzodiazepine (PBD) dimer-containing antibody-drug conjugate (ADC) targeting CD19-expressing hematological malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 51. doi:10.1158/1538-7445.AM2017-51