Transport pathways across the blood-brain barrier for waste clearance and drug delivery

The blood-brain barrier (BBB) displays a highly organized and complex structure, which is important for maintaining brain homeostasis and protecting the brain from foreign molecules or pathogens. Receptor-mediated transcytosis (RMT) is one of the main delivery pathways across the BBB for molecules that cannot pass the barrier via, e.g. paracellular diffusion. For understanding the treatment options in neurodegenerative diseases such as Alzheimer´s disease (AD), it is important to investigate transport pathways and mechanisms at the BBB for a potential delivery of drugs, antibodies or other compounds across the BBB. This review provides an overview of the different transport variants across the BBB and how they can be targeted in order to promote internalization or secretion into or out of the brain. Therefore, we want to focus on two characterized proteins: the low-density lipoprotein receptor-related protein 1 (LRP1), which is a key mediator of amyloid β (Aβ) clearance from the brain during AD, and transferrin receptor 1 (TfR1), which is already used as a target for antibody-delivery into the brain. Additionally, this review discusses two other important proteins, which have been less frequently addressed in research regarding transport mechanisms: P-glycoprotein (P-gp) as another transporter at the BBB and proprotein convertase subtilisin/kexin type 9 (PCSK9), a well-known regulator of cholesterol homeostasis which promotes the degradation of the low-density lipoprotein receptor (LDLR) and LRP1. For these four main proteins, we aim to highlight existing approaches for targeting or inhibiting the aforementioned receptors or proteins. The approaches enable a higher penetration of the BBB, a better distribution in the brain, and ultimately fewer side effects of antibodies or nanoparticles. Here, we include lecanemab, trontinemab, dual TfR/CD98hc shuttles, evolocumab and alirocumab, immunoliposomes and other nanoparticles targeting TfR1 or LRP1. We will further highlight approaches which differ from these common ideas and demonstrate the current state of the art regarding drug delivery and waste clearance across the BBB.