Dynamic Light Scattering for Assessing Aggregation in [ ¹⁷⁷ Lu]Lu-DOTA-Trastuzumab: A Quality Control Approach for Radioimmunotherapy

Radioimmunotherapy (RIT) delivers radionuclides to tumors through antibody-based targeting. However, radiolabeling with high-linear energy transfer (LET) isotopes like Lu-177 can induce aggregation, reducing antibody stability and bioactivity. In this study, we evaluated [¹⁷⁷Lu]Lu-DOTA-Trastuzumab, a HER2-targeting radioimmunoconjugate, to assess how conjugation chemistry, radiolabeling conditions, and purification methods affect aggregation and therapeutic performance. Dynamic light scattering (DLS) was used to monitor aggregation throughout formulation. Higher chelator-to-antibody ratios and increased radioactivity caused significant aggregation, which was not detected by conventional SEC-HPLC or radio-TLC quality control methods. Compared to the diagnostic isotope Cu-64, the therapeutic isotope Lu-177 exhibited markedly greater aggregation, reflecting the stronger radiolytic stress from high LET isotopes. Aggregation inversely correlated with HER2-specific uptake and tumor accumulation in vivo. Centrifugal membrane filtration removed aggregates more effectively than PD-10 columns. In HER2-positive mouse models, aggregate-free [¹⁷⁷Lu]Lu-DOTA-Trastuzumab achieved higher tumor accumulation and resulted in ∼65% tumor growth inhibition by day 21. These findings demonstrate that DLS is a sensitive and practical quality control tool. Given the greater aggregation risk in therapeutic radiolabeling, aggregate monitoring is especially critical for ensuring the safety, consistency, and effectiveness of radioimmunotherapy agents.