Abstract B189: Impact of site of conjugation on efficacy and pharmacokinetics of anti-NaPi3b antibody drug conjugate variants.
Authors: Zhang Crystal , Roth Leslie , Cook Ryan , Tsai Siao Ping , Tibbitts Jay , Venook Rayna , Miao Li , Rubinfeld Bonnee , Lin Kedan 
Published: June 2012

Introduction: Optimization of antibody-drug conjugates (ADCs) with therapeutic potential involves several critical parameters, including selection of the antibody, linker, cytotoxic drug, and drug-to-antibody ratio (DAR). To minimize the heterogeneity from conjugation, the antibody can be engineered at different conjugation sites to achieve a defined DAR at specific sites. Anti-NaPi3b-vc-MMAE (anti-NaPi3b-vc-E) is an ADC targeting sodium dependent phosphate transporter expressed in lung and ovarian cancers. Construction and production of the THIOMAB variant of antibody was reported previously. Briefly, a cysteine residue was engineered at Ala118 position of heavy chain (HC) or Val205 position of light chain (LC) to produce its THIOMAB HC and LC variants, respectively. The MMAE conjugates were produced with a drug to antibody ratio (DAR) of two. The purpose of this study was to evaluate the site of conjugation on ADC efficacy and antibody pharmacokinetics.

Methods: The anti-tumor activity of anti-Napi3b ADC variants was evaluated in a mouse xenograft model of human ovarian cancer OVCAR3-X2.1 after single dose administration in a range of doses. The unconjugated antibodies including anti-NaPi3b (control) and anti-NaPi3b (HC and LC) and their respective ADC variants were administered intravenously to mice and rats; and the pharmacokinetic (PK) parameters measured by a total antibody assay were estimated by a two-compartmental model.

Results: Based on comparisons of tumor growth inhibition, both LC and HC anti-Napi3b-vc-E showed clear dose-dependent inhibitory activity when given at 3 to 12 mg/kg versus the vehicle group. Non-NaPi3b-binding ADCs did not significantly affect tumor growth, indicating targeted anti-tumor activity. The activities of LC and HC variants were similar. Dose proportional pharmacokinetics of total antibody following administration of anti-NaPi3b-vc-E (HC and LC) were observed in mice and rats over the dose range tested. Total antibody clearance of the three unconjugated antibodies (control, thio HC, and thio LC) was comparable. Conjugation with MMAE only modestly increased the total antibody clearance in both LC and HC conjugates when compared to unconjugated antibodies. Overall, PK behavior of the HC and LC variants were similar.

Conclusion: The results demonstrated that ADC variants with different sites of conjugation exhibited similar anti-tumor activities, which provided additional choice of conjugation sites for individual antibody. Cysteine mutation in either HC or LC of the unconjugated antibodies did not affect the PK when compared to the control. In addition, conjugation did not significantly change the overall clearance of the antibody, and the site of MMAE conjugations did not have a significant impact on the PK.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B189.