Preclinical assessment of therapeutics beyond the blood-brain barrier: towards more predictive preclinical models

The blood-brain barrier (BBB) is a highly specialized neurovascular interface that preserves brain homeostasis by tightly regulating molecular exchange between blood and parenchyma. While essential for neural protection, this barrier excludes most biologics and many small molecules, making central nervous system (CNS) drug delivery a main bottleneck in neurotherapeutics (Sweeney et al., 2019; Jones and Shusta, 2007). This is why endogenous receptor-mediated transport systems such as TFRC (CD71/TfR1), CD98 (Wu et al., 2026) and pIgR (Song et al., 1994) have attracted sustained interest. Among them TFRC stands out because it is physiologically expressed on brain endothelial cells and naturally supports iron trafficking through transferrin-dependent vesicular transport (Dufès et al., 2013; Hersom et al., 2016).

TFRC as a gateway across the blood-brain barrier

Building on its biology, TFRC is now widely exploited as a molecular shuttle to transport therapeutics across the BBB. Anti-TFRC antibodies aim to bind luminal TFRC, undergo endocytosis, and then be sorted toward productive abluminal release rather than endothelial retention or lysosomal degradation (Pardridge and Boado, 2012; Bien-Ly et al., 2014). This TFRC-shuttle concept has already shaped therapeutic development for CNS disease. However, this strategy also encompasses major liabilities: endogenous transferrin competition, finite receptor capacity, peripheral TFRC expression on reticulocytes and proliferating cells, and the persistent difficulty of distinguishing true parenchymal delivery from endothelial trapping (Moos and Morgan, 2001; Hanafy et al., 2021). These issues are compounded by species-specific TFRC biology, because antibody epitopes, cross-reactivity and trafficking behavior may differ substantially between mouse and human receptors, undermining translational confidence when human-specific shuttles are tested only in wild-type rodents (Pardridge and Boado, 2012; Stocki et al., 2023).

TFRC humanized mouse model as a translational need

For this reason, humanized mouse models are no longer a convenience but a translational necessity. They allow investigators to evaluate whether a human-specific anti-TFRC therapeutic engages the intended receptor in vivo under physiological expression, reproduces the expected PK/PD profile, and achieves measurable brain exposure without misleading artefacts from murine-only binding patterns (Georgieva et al., 2020; Hanafy et al., 2021). As a result, genOway developed a humanized TFRC/CD71 knock-in mouse (genO-hTFRC), which enables the in vivo assessment of TFRC-targeted therapies for oncology and for BBB delivery, with utility for brain uptake and PK/PD evaluation of human-specific therapeutics. This model addresses the central gap created by species-specific TFRC differences: by expressing hTFRC at physiological levels it provides a receptor context that is more relevant for candidate selection, affinity tuning, safety de-risking and go/no-go decisions before progression into higher-species studies or the clinic.

Towards a predictive assessment of the function of therapeutics across the BBB

To further increase the relevance of therapeutic assessment across the BBB, a humanized TFRC model was developed on a humanized FcγR background (genO-hFcγR) or on a humanized FcRn background also expressing human serum albumin (genO-hSA/hFcRn). The resulting genO‑hTFRC/hFcγR model enables robust evaluation of Fc effector function of antibodies alongside human TFRC‑mediated BBB transport, while the genO-hTFRC/hSA/hFcRn mouse supports accurate half‑life and recycling assessment in the brain.

Taken together, TFRC has emerged as one of the most validated entry points for BBB drug delivery, but success depends less on the existence of the receptor than on precise engineering around its biology. In that context, humanized TFRC mice become strategic enablers rather than simple confirmatory tools: they help align BBB transport, systemic pharmacology and safety with the human therapeutic hypothesis.

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