Molecules that are lipid soluble and have a molecular weight under a 500 Dalton threshold are able to cross the BBB in pharmacologically significant amounts (Pardridge, 2001). All present-day CNS drugs in clinical practice fulfill these dual criteria of (1) lipid solubility and (2) molecular weight under a 500 Dalton threshold. If a molecule lacks both of these criteria, it is unlikely that the molecule will cross the BBB in pharmacologically significant amounts, unless the molecule has affinity for one of the CMT or RMT systems in the BBB. In the absence of this, the molecule will need a brain drug delivery system if the drug is to be used as a neuropharmaceutical. Since more than 98% of the drugs that emanate from high throughput receptor-based CNS drug discovery programs will not cross the BBB, the presence of the BBB poses a significant problem for future CNS drug development. On this basis, it is important that there be parallel progress in both CNS drug discovery and CNS drug delivery so that these two pathways can be merged in the overall CNS drug development process. Unfortunately, less than 1% of present-day CNS drug development is devoted to CNS drug delivery, and more than 99% is applied to CNS drug discovery.
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