A comprehensive review of humanized mice applications in regulatory submissions for cell and gene therapy products

The emergence of cell and gene therapies has transformed the therapeutic landscape, offering curative potential for a range of previously intractable diseases. However, their biological complexity and patient-specific mechanisms of action present significant challenges for preclinical evaluation, particularly in modeling human responses and predicting safety outcomes. Traditional animal models often lack translational fidelity, prompting the adoption of humanized immunodeficient mice, including those engrafted with human immune cells, as more predictive in vivo platforms. These models enable the assessment of pharmacodynamics, biodistribution, and immunotoxicity in a human-relevant context. This review critically explores the integration of humanized mice into regulatory submissions for cell and gene therapy products, highlighting their utility across proof-of-concept, pharmacokinetic, toxicology, and tumorigenicity studies. We also address key limitations of the different models, including variability in engraftment efficiency, immune reconstitution, and lifespan, as well as challenges in standardization and regulatory acceptance. Future directions include refining humanized mouse models to better mimic human physiology, incorporating pathological endpoints, and aligning with 3R principles and new methodological approaches. By enhancing the translational relevance of nonclinical data, humanized mice are poised to play an increasingly strategic role in early safety assessment and successful development of advanced therapies.