Photoacoustic imaging of the dynamics of a dye-labeled IgG4 monoclonal antibody in subcutaneous tissue reveals a transient decrease in murine blood oxygenation under anesthesia

Significance: Over 100 monoclonal antibodies have been approved by the U.S. Food and Drug Administration (FDA) for clinical use; however, a paucity of knowledge exists regarding the injection site behavior of these formulated therapeutics, particularly the effect of antibody, formulation, and tissue at the injection site. A deeper understanding of antibody behavior at the injection site, especially on blood oxygenation through imaging, will help design improved versions of the therapeutics for a wide range of diseases.

Aim: The aim of this research is to understand the dynamics of monoclonal antibodies at the injection site as well as how the antibody itself affects the functional characteristics of the injection site [e.g., blood oxygen saturation (${\mathrm{sO}}_2$)].

Approach: We employed triple-wavelength equipped functional photoacoustic imaging to study the dynamics of dye-labeled and unlabeled monoclonal antibodies at the site of injection in a mouse ear. We injected a near-infrared dye-labeled (and unlabeled) human IgG4 isotype control antibody into the subcutaneous space in mouse ears to analyze the injection site dynamics and quantify molecular movement, as well as its effect on local hemodynamics.

Results: We performed pharmacokinetic studies of the antibody in different regions of the mouse body to show that dye labeling does not alter the pharmacokinetic characteristics of the antibody and that mouse ear is a viable model for these initial studies. We explored the movement of the antibody in the interstitial space to show that the bolus area grows by $\sim 300\%$ over 24 h. We discovered that injection of the antibody transiently reduces the local ${\mathrm{sO}}_2$ levels in mice after prolonged anesthesia without affecting the total hemoglobin content and oxygen extraction fraction.

Conclusions: This finding on local oxygen saturation opens a new avenue of study on the functional effects of monoclonal antibody injections. We also show the suitability of the mouse ear model to study antibody dynamics through high-resolution imaging techniques. We quantified the movement of antibodies at the injection site caused by the interstitial fluid, which could be helpful for designing antibodies with tailored absorption speeds in the future.