The blood–brain barrier (BBB) prevents the brain uptake of most pharmaceuticals. This property arises from the epithelial-like tight junctions within the brain capillary endothelium. The BBB is anatomically and functionally distinct from the blood–cerebrospinal fluid barrier at the choroid plexus. Certain small molecule drugs may cross the BBB via lipid-mediated free diffusion, providing the drug has a molecular weight <400 Da and forms <8 hydrogen bonds. These chemical properties are lacking in the majority of small molecule drugs, and all large molecule drugs. Nevertheless, drugs can be reengineered for BBB transport, based on the knowledge of the endogenous transport systems within the BBB. Small molecule drugs can be synthesized that access carrier-mediated transport (CMT) systems within the BBB. Large molecule drugs can be reengineered with molecular Trojan horse delivery systems to access receptor-mediated transport (RMT) systems within the BBB. Peptide and antisense radiopharmaceuticals are made brain-penetrating with the combined use of RMT-based delivery systems and avidin–biotin technology. Knowledge on the endogenous CMT and RMT systems expressed at the BBB enable new solutions to the problem of BBB drug transport.

https://journals.sagepub.com/doi/pdf/10.1038/jcbfm.2012.126

Drug transport across the blood-brain barrier.

The kinetics and distribution of methotrexate in intraventricular and intrathecal cerebrospinal-fluid spaces were studied in patients with meningeal leukemia and meningeal carcinomatosis after drug administration by intravenous infusion, indwelling intraventricular subcutaneous reservoir (Ommaya), or standard lumbar puncture. Negligible ventricular concentrations followed a single intravenous dose. During an intravenous infusion (500 mg per square meter for 24 hours) the ventricular cerebrospinal-fluid concentration rose to 6 X 10–7 M. Methotrexate administered by Ommaya reservoir, at a dose of 6.25 mg per square meter, rapidly distributed in the subarachnoid space; the peak ventricular concentration of 2 X 10–4 M declined exponentially over 48 hours. Lumbar cerebrospinal-fluid concentration reached a maximum of 5 X 10–5 M four hours after injection and then fell exponentially. Administration by lumbar puncture occasionally produced epidural and subdural leakage; even with successful lumbar punct...

Methotrexate: Distribution in Cerebrospinal Fluid after Intravenous, Ventricular and Lumbar Injections

According to the traditional understanding of cerebrospinal fluid (CSF) physiology, the majority of CSF is produced by the choroid plexus, circulates through the ventricles, the cisterns, and the subarachnoid space to be absorbed into the blood by the arachnoid villi. This review surveys key developments leading to the traditional concept. Challenging this concept are novel insights utilizing molecular and cellular biology as well as neuroimaging, which indicate that CSF physiology may be much more complex than previously believed. The CSF circulation comprises not only a directed flow of CSF, but in addition a pulsatile to and fro movement throughout the entire brain with local fluid exchange between blood, interstitial fluid, and CSF. Astrocytes, aquaporins, and other membrane transporters are key elements in brain water and CSF homeostasis. A continuous bidirectional fluid exchange at the blood brain barrier produces flow rates, which exceed the choroidal CSF production rate by far. The CSF circulation around blood vessels penetrating from the subarachnoid space into the Virchow Robin spaces provides both a drainage pathway for the clearance of waste molecules from the brain and a site for the interaction of the systemic immune system with that of the brain. Important physiological functions, for example the regeneration of the brain during sleep, may depend on CSF circulation.

L. Page

Papers

Formation and absorption of cerebrospinal fluid in man.