Comparisons between Two Monoclonal Antibodies That Bind to the Same Antigen but Have Differing Affinities: Uptake Kinetics and 125I-Antibody Therapy Efficacy in Multicell Spheroids

Abstract

It has been predicted that low affinity antibodies (Abs) should pen-etrate into tumors more readily than high affinity Abs. However, the absolute uptake and residence time of a high affinity Ah may be better. It is, therefore, not clear whether a high affinity Ab would have a therapeutic advantage. This is particularly relevant with 125I radioImmunotherapy, where targeting of every cell is important This study compared the uptake kinetics and toxicity in multicell spheroids of two marine monoclonal Abs labeled with 1251.17-1A was produced by immunization with a human colon cancer cell line and has an affinity of 5.15 X 107 M-1. 323/A3 was produced by immunization with a human breast cancer cell line and has an affinity of 1.87 X 109 M-”. Binding of both Abs to LS174T spheroids was similar at 4°C, but binding of 17-1A was 8-10-fold less than that of 323/A3 at 37°C. Despite this difference, the toxicity of 12SI-17-1A in spheroids after 7 days of incubation was similar to that of 125I-323/A3. Autoradiography showed that 17-1A penetrated the spheroids much more deeply and evenly than did 323/A3. It appears that much of the radiation dose to spheroids treated with, 25I-323/A3 was wasted because of the uneven Ab distribution. This study demonstrates the potential advantage of using Abs of lower affinity for 125I radioimmunotherapy, because of their more even distribution. It also suggests that a large number of binding sites. © 1992, American Association for Cancer Research. All rights reserved.