Receptor-Mediated Transcytosis at the BBB
The blood-brain barrier is not an impenetrable wall — it is a selectively permeable interface with defined transcytosis pathways. Biopathio engineers LNP surface chemistry to exploit transferrin receptor (TfR) and LRP1-mediated entry into the CNS.
The BBB Challenge: Tight Junctions, Efflux Pumps, Immune Surveillance
The blood-brain barrier is formed by cerebrovascular endothelial cells connected by tight junction complexes (claudin-5, occludin, ZO-1) that restrict paracellular transport. Three independent barriers limit nanoparticle CNS access:
- Tight junctions: paracellular permeability is restricted to molecules < 0.5 nm
- Efflux pumps (P-gp, BCRP, MRPs): actively export small molecules and some nanoparticles back into blood
- Immune surveillance: pericytes and microglia monitor the neurovascular unit and can trigger nanoparticle clearance
Standard passive diffusion strategies fail at all three barriers. Transcytosis bypasses all of them by exploiting the endogenous vesicular transport system the BBB uses for nutrient import.
Transferrin Receptor and LRP1 as Transcytosis Entry Points
Transferrin receptor 1 (TfR1, CD71) is expressed at 5–10× higher levels on brain endothelial cells than on peripheral endothelium. It mediates iron transport into the CNS via clathrin-mediated endocytosis and subsequent transcytosis. Biopathio's surface-modified LNPs carry a targeting ligand that engages TfR1 without saturating the receptor or competing with endogenous transferrin.
LRP1 (low-density lipoprotein receptor-related protein 1) represents a complementary transcytosis pathway. LRP1 is activated by ApoE-mimetic peptides and is expressed constitutively on BBB endothelium. Biopathio is developing a dual-targeting approach (TfR1 + LRP1) to improve CNS delivery efficiency across varied animal model backgrounds where TfR1 expression may vary.
In Vitro BBB Model Results
Primary evaluation uses the hCMEC/D3 immortalized human cerebrovascular endothelial cell line in transwell culture. This model reproduces tight junction formation (TEER > 50 Ω·cm²) and P-gp efflux activity. Data presented here are representative synthetic values from Biopathio's internal preclinical work.
| LNP Formulation | Surface Modification | Transcytosis Efficiency (4h) | Cytotoxicity (EC₅₀) |
|---|---|---|---|
| CNS-LNP-TfR | TfR1 targeting peptide | 38.2 ± 4.8 % | > 200 nM |
| CNS-LNP-LRP1 | ApoE-mimetic peptide | 31.5 ± 5.1 % | > 200 nM |
| CNS-LNP-Dual | TfR1 + LRP1 dual targeting | 62.3 ± 7.2 % | > 180 nM |
| Standard LNP (control) | No targeting modification | 8.1 ± 2.3 % | > 200 nM |
Evaluating BBB delivery options for your program?
Discuss your CNS target and our transcytosis approach with our formulation team.