A Highly Sensitive Immunoassay to Enable Quantification of a Nanobody-Based Imaging Agent in Human Serum

Immuno-positron emission tomography (i-PET) is a non-invasive imaging technique that combines the specificity of monoclonal antibodies with the sensitivity of positron emission tomography to visualize and quantify the distribution of target antigens in vivo, providing detailed spatial information about the presence and localization of specific biomarkers. Due to the crucial role that cytotoxic T cells play in antitumor responses, a new tracer consisting of a humanized anti-CD8 nanobody labeled with fluorine-18 was developed to inform immuno-oncology treatments and support medical decision-making. Nanobodies are single-domain antibodies with low molecular weight and fast peripheral blood clearance, both of which are advantageous properties for same-day imaging. However, these unique characteristics pose bioanalytical challenges when developing clinical pharmacokinetic (PK) assays, including the need for high assay sensitivity. This manuscript focuses on overcoming bioanalytical challenges related to sensitivity and matrix interference during the development of a method to quantify this novel anti-CD8 nanobody tracer in human serum. Out of the three immunoassay platforms evaluated (ELISA, SMCxPRO™ and Gyrolab®), a Gyrolab method was ultimately selected due to its superior sensitivity, equal detectability of both conjugated and unconjugated forms of the nanobody and its ability to minimize matrix interference. By selecting the right assay format, along with the appropriate critical reagents for capture and detection, matrix interference was diminished. This novel PK method was successfully qualified demonstrating acceptable performance across all parameters. The acquired bioanalytical insights gained could be applied to nanobody-derived conjugates or other modalities that require high sensitivity in the clinical settings.