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Page 1 of 2 The problem and the paradox Research in neuroscience in the last 20 years has allowed substantial progress in the discovery of new and potent molecules for the CNS. However, the CNS has very specific features: the vascular systems of the CNS which separate blood from the nervous tissue constitute highly impermeable physiological barriers known as the blood brain barrier (BBB) and blood-cerebrospinal fluid barrier (B-CSFB). These barriers protect efficiently the CNS from toxic molecules, viral and bacterial infections, but also restrict very efficiently the passage from blood to the nervous tissue of drugs or molecules with proven therapeutic potential. The BBB is thus impermeable to most drugs.  The BBB
To date, most of the drugs developed by the Pharmaceutical Industry to treat some CNS pathologies (schizophrenia, epilepsies, chronic pain, depression), which cross the physiological barriers are small lipid-soluble molecules (<500 Daltons). However these lipid-soluble molecules account for only 2% of the small molecules with therapeutic potential, and consequently 98% of the small molecules of pharmacological interest for treating CNS pathologies, and the vast majority of the larger molecules are difficult or impossible to exploit because of the impermeability of the barriers.
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