Antibody Conjugation Strategies for Ultrasound Microbubbles: A Review of Bioconjugation Chemistry and Clinical Translation.

Antibody-conjugated microbubbles have gained significant attention in biomedical imaging and therapy due to their unique acoustic properties and molecular targeting capabilities. These gas-filled microbubbles, already adopted in modern clinical workflows as ultrasound contrast agents, can be functionalized with specific antibodies to target disease biomarkers. Such targeted microbubbles enable ultrasound-based molecular imaging of tumors, cardiovascular lesions, and inflammation by binding to sites of interest. They can also serve as platforms for targeted drug and gene delivery. This review summarizes the core principles of contrast-enhanced ultrasound (CEUS); details current microbubble composition, acoustic response (harmonic generation, cavitation thresholds), and in vivo kinetics (stability, circulation time); and provides an overview of antibody-based targeting strategies. We compare noncovalent methods such as avidin-streptavidin to covalent approaches including NHS ester, maleimide-thiol, and click chemistry, evaluating their relative stability, conjugating efficiency, and translation potential. Practical considerations such as immunogenicity, orientation, and retention of antibodies are discussed in the context of conjugation chemistry. This review offers a mechanistic and translational perspective on antibody-conjugated microbubbles and underscores the current limitations of conjugation chemistries, while outlining the hurdles that must be addressed to enable future clinical translation.