Diphtheria toxin receptor-targeted brain drug delivery

Brain drug delivery is limited by the blood-brain barrier (BBB). We have newly identified the membrane-bound precursor of heparin-binding epidermal growth factor (HB-EGF), which is also known as the diphtheria toxin receptor (DTR), as a well characterized internalizing transport receptor on the BBB for the targeting of drugs to the brain. This transport receptor has several unique advantages. It makes use of a non-toxic endogenous transport mechanism called receptor-mediated endocytosis, with proven cargo-carrying properties across the BBB (e.g., brain delivery (transcytosis) of large proteins and liposomes containing drugs); the receptor has no endogenous ligands and thus neither competition from endogenous ligands, nor blockade of transport to the brain of essential nutrients is to be expected; the membrane bound receptor is constitutively expressed on the BBB, neurons, and glial cells; receptor expression is highly amplified in disease conditions and thus allows for site-specific disease targeting; and the biological activity of the receptor can be modulated by a variety of pharmacologically active compounds (like heparin and proteinase inhibitors). Furthermore, the targeting technology makes use of a non-toxic mutant of diphtheria toxin (known as CRM197) as the receptor-specific carrier protein. This carrier protein has several unique advantages as well. It is a well characterized protein (i.e., known receptor binding domain, conjugation sites, manufacturing process), and it has already been successfully marketed for human use to millions of people in vaccination programs, and recently also in anti-cancer trials, with a proven carrier efficacy and excellent safety profile. We have been able to demonstrate proof-of-principle data in our cell culture model of the BBB, as well as in guinea pigs with this novel brain drug targeting technology, including: functional expression of DTR; safety of CRM197 carrier protein; transport efficacy of CRM197 carrier protein conjugated directly to horseradish peroxidase (HRP, serving as a 40 kDa 'model' protein drug); and specific in vivo brain uptake of DTR-targeted HRP. In conclusion, the DTR seems to be a human applicable, safe, and effective uptake receptor for the targeting of drugs to the brain.