Abstract 4504: Plasma pharmacokinetics and tumor accumulation in mice of IMGN779, an antibody-drug conjugate for acute myeloid leukemia
Authors: Lai Katharine C. , Shah Prerak , Sikka Surina , Sun XiuXia , LaLeau Rassol , Whiteman Kathleen R. , Johnson-Modafferi Holly , Wilhelm Alan , Audette Charlene , Wang Lintao , Bogalhas Megan E. , Keating Thomas A. , Chari Ravi 
Published: August 2015

IMGN779, an antibody-drug conjugate (ADC) consisting of the anti-CD33 antibody, Z4681A, linked to the potent DNA-alkylating agent, DGN462, via a charged disulfide linker, sulfo-SPDB, is in development for the treatment of acute myeloid leukemia (AML). IMGN779 is highly active in vitro against AML cell lines and primary patient AML cells and causes complete regression of AML xenograft tumors at non-toxic doses in vivo.

To understand the fate of IMGN779 upon uptake, we performed metabolism studies on cultures of ADC-treated AML cells in vitro, and distribution studies in vivo, following the plasma clearance and tumor accumulation of the ADC in mice. The antigen-mediated binding, uptake, and degradation of IMGN779 by AML cells in culture were measured using a radiolabeled conjugate. CD33-targeted degradation of IMGN779 was observed in treated cultures, with metabolites detected within the cells, some as DNA-adducts, and in the media following efflux from cells. Approximately 40% of initially-bound IMGN779 was measured as protein-free degraded species after 22 h incubation at 37 °C, with an additional ∼40% measured as DNA-associated species.

To evaluate plasma pharmacokinetics (PK), mice were injected with [3H]propionate radiolabeled Z4681A antibody (Ab), or with IMGN779 that was radiolabeled either on the Ab portion of the ADC ([3H]propionate) or on the DGN462 moiety. Total Ab and intact ADC concentrations were determined in plasma samples collected from 2 min to 28 days post-injection. Overlapping clearance profiles for Z4681A and Ab-labeled IMGN779 were observed with half-lives (t½) of ∼18 d, which demonstrated that conjugation with DGN462 did not alter Ab PK. A faster clearance of [3H]DGN462-labeled IMGN779 was observed, with a plasma t½ of 4.5 d, indicating that DGN462 is released from IMGN779 in circulation. Plasma samples were CD33-affinity captured and analyzed by LC/MS to determine the mass-distribution profile of IMGN779. A decrease in relative abundances of antibody species associated with a high number of DGN462 molecules from 2 min- 3 d supported the plasma clearance results.

To determine the tumor localization of IMGN779 and its metabolites, the radioactivity in excised HL60/QC xenografts dosed with 5 mg/kg [3H]IMGN779 or non-targeting [3H]ADC was measured at 6, 24 and 48 h after treatment. The total amount of ADC (intact and metabolized) present was similar, but slightly higher for IMGN779 versus non-targeting ADC samples. However, the amount of both extractable metabolites and DNA-bound species was higher in tumor samples from mice treated with IMGN779 versus non-targeting ADC, indicating CD33-targeted metabolism in the tumor.

From these studies, we conclude IMGN779 demonstrates CD33 target-mediated generation of DGN462 metabolites, and also exhibits DNA-modification consistent with the mechanism of action of its effector molecule, DGN462.

Citation Format: Katharine C. Lai, Prerak Shah, Surina Sikka, XiuXia Sun, Rassol LaLeau, Kathleen R. Whiteman, Holly Johnson-Modafferi, Alan Wilhelm, Charlene Audette, Lintao Wang, Megan E. Bogalhas, Thomas A. Keating, Ravi Chari. Plasma pharmacokinetics and tumor accumulation in mice of IMGN779, an antibody-drug conjugate for acute myeloid leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4504. doi:10.1158/1538-7445.AM2015-4504