Abstract 2577: Analyzing specific and nonspecific tissue uptake of antibodies by SPECT imaging and non-linear compartmental modeling
Authors: Cho Nicholas , Spinosa Phillip , Mandikian Danielle , Rosario Geoffrey Del , Yu Shang-Fan , Ho Jason , Ferl Greg , Boswell Charles A. 
Published: March 2024

Abstract
Understanding the mechanisms driving specific target engagement versus off-target nonspecific cellular uptake of antibodies can help inform design and/or delivery strategies that maximize therapeutic indices. Receptor binding kinetics, rates of receptor-mediated internalization and nonspecific pinocytosis are often measured through in vitro cell culture models, but there have been fewer efforts to collectively measure these cellular parameters in vivo. Single photon emission computed tomography (SPECT) imaging is a powerful method of estimating these parameters by quantifying both free/bound (intact) and internalized/catabolized antibody using non-residualizing halogen labels and residualizing radiometal-chelates, respectively. Modeling longitudinal in vivo imaging data of non-residualizing antibody can inform receptor occupancy (RO), while residualizing tracer kinetics can provide additional insights into catabolism due to pinocytosis and receptor-mediated internalization in the intracellular spaces. Here, we describe SPECT imaging of tumor-targeting trastuzumab (anti-HER2) and non-targeting anti-glycoprotein D (anti-gD) antibody, each labeled with non-residualizing ([125I]SIB) and residualizing radiometal-chelate (111In-DOTA) radionuclides in HER2+ xenograft-bearing mice. A non-linear compartmental model was fitted to SPECT imaging data to characterize RO of 10 mg/kg of anti-HER2 out to 7 days post dosing as well as to estimate rate constants of pinocytosis and receptor-mediated internalization of anti-HER2 in tumor. Longitudinal SPECT imaging demonstrated statistically significant differences in tumor drug exposure between 125I- and 111In-labeled anti-HER2 relative to anti-gD (p &lt; 0.01). Tracer kinetics of anti-HER2 and associated RO simulations in tumor showed rapid extravasation from blood to tumor with peak RO of 25% at 13 hours post administration followed by a reduction in RO, reaching ~10% by day 7. Simulations for free, bound and catabolized tracer kinetics from our fitted model showed that anti-HER2 in the tumor interstitium was primarily receptor-bound with minimal pinocytosis of unbound anti-HER2 as well as gradual receptor-mediated catabolism, reaching a peak concentration of 113 µg/mL 7 days post dosing. Rate constants of nonspecific pinocytosis and HER2-mediated receptor internalization were estimated to be 0.0145 and 0.036 hours−1, respectively. Our model allowed us to successfully capture anti-HER2 antibody receptor binding and specific and nonspecific internalization over time in vivo. These data and analyses demonstrate the power of SPECT imaging using both non-residualizing and residualizing radiolabels combined with compartmental modeling to better characterize the different biological states (free, bound, and catabolized) of antibody within interstitial and intracellular compartments.
Citation Format: Nicholas Cho, Phillip Spinosa, Danielle Mandikian, Geoffrey Del Rosario, Shang-Fan Yu, Jason Ho, Greg Ferl, Charles A. Boswell. Analyzing specific and nonspecific tissue uptake of antibodies by SPECT imaging and non-linear compartmental modeling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2577.