Transport Across the BBB

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

In silico modeling of BBB permeability started with the assumption that the majority of drugs are transported across the BBB by passive diffusion. Absence of the data regarding P-glycoprotein (P-gp)-mediated active transport as well as the limited understanding of the behavior of drugs as substrates or inhibitors of these P-gp transporters has restricted the people from generating BBB models based on active transport [42],... 

Smaller peptides may be transported across the BBB either by nonspecific fluid-phase endocytosis or RMT mechanisms. It may also be possible for them to use a peptide-specific transporter protein, directly inserted into the cell membrane in a similar manner to the solute transporters, which flips them across the membrane [43,56]. In some cases the receptor that transduces the signal at the cell membrane may also act as the transporter for the peptide and is co-opted to initiate RMT, or another transport system in other cases the membrane transporter for a peptide may be quite distinct in structure from the receptor that transduces signals at the cell membrane from a signaling peptide [45]. 

An attractive strategy to improve CNS drug delivery is to link a nontransportable drug with a vector to the BBB. These moieties can work as molecular Trojan horses to transport across the BBB attached proteins, DNA molecules, and drug micro- and nanocarriers facilitating their penetration through the BBB. The choice of a vector moiety and a type of a linker is crucial for the success of this method of drug delivery. 

The objective of pharmacologically based strategies is to turn water-soluble (and thus poorly penetrable) compounds into lipid-soluble ones, thereby increasing their passive transport across the BBB. A number of approaches have been investigated ... 

One of the simplest methods of improving the uptake of a dmg to the brain involves the conversion of the drag to a more lipophilic prodrug (see Section 1.3.4.1). Using this approach, moderate increases in transport across the BBB have been achieved for enkephalin analogs and TRH. 

As discussed above, certain nutrients are taken up into the brain by carrier-mediated systems. If a dmg possesses a molecular structure similar to that of a nutrient which is a substrate for carrier-mediated transport (Table 13.1), the pseudo-nutrient dmg may be transported across the BBB by the appropriate carrier-mediated system. For example, the dmg L-dopa crosses the BBB via the neutral amino acid carrier system. Other neutral amino acid dmgs that are transported through the BBB on this transport system are a-methyldopa, a-methylparatyrosine, and phenylalanine mustard. 

Alternatively, monoclonal antibodies (Mabs) to the relevant receptors can be used as transport vectors. Anti-insulin (Mab83-7 and Mab83-14) and anti-transferrin (0X26) receptor antibodies have been proposed as efficient and selective BBB transport vectors. The antitransferrin receptor antibody binds to a site removed from the transferrin binding site and therefore does not compete with endogenous transferrin for transport across the BBB. Studies using radiolabeled peptides have shown that significant uptake of a... 

The BBB is known to transport several cytokines in the blood-to-brain direction. For example, the BBB transports the IL-l s, IL-6, and TNF by three separate transport systems. Additionally, nerve growth factor, brain derived neurotrophic factor, interferons, neurotrophins, and leukemia inhibitory factor (Poduslo and Curran, 1996 Pan et al., 1997b Pan et al., 1998b Pan et al., 1998a) are also transported across the BBB. In some cases, the same gene which gives rise to a cytokine s receptor also produces the cytokine s transporter, whereas in other cases the receptor and transporter are immunologic ally distinct proteins (Banks and Kastin, 1992 Pan and Kastin,... 

Figure 47.5. Effect of HRP modification with Pluronic block copolymer on transport across the BBB in an in vitro and in vivo models. A HRP conjugated with Pluronic P85 via the biodegradable bond B confocal microphotograph of BBMEC monolayers treated with rhodamine-labeled HRP and Pluronic-HRP for 2h C blood-to-brain transport of HRP and Pluronic-HRP in mice.

Drug transport to the brain depends on various parameters. The amount of drag available for transport across the BBB depends upon its systemic pharmacokinetics [represented by absorption, distribution, metabolism, and elimination (ADME) see also Figure 1]. For drags that can easily pass the BBB, blood flow is a limiting factor, whereas for other drags, BBB-permeability is restrictive. In addition, the cardiac output to the brain seems not to be the main determinant for blood flow, but rather the local blood flow and the capillary flow area. In vivo capillary flow was... 

Research on dmg transport across the BBB and its functionality has been very much enhanced by the availability of in vitro BBB endothelial/astrocyte (co)-culture systems. The use of such systems allows a detailed investigation of BBB-related phenomena at the (sub)-cellular level and in the absence of feedback systems from the rest of the body. This makes it much easier to study in vitro BBB transport and BBB functionality by (pharmacological) intervention techniques, such as the application of receptor agonists and antagonists, blockers of transporters and enzymes, antisense and (anti)gene approaches, and the influence of disease. Recently, BBB (co)-culture systems have been reviewed and discussed with respect to their use in BBB-related research (30,31). 

The ABC transporter P-gp protects the brain from accumulation of lipophilic compounds by active efflux transport across the BBB. Miillauer et al. [715] investigated the suitability of the radio-labeled Pgp inhibitors [nC]elacridar and [nC]tariquidar to visualize P-gp density in rat brain with PET. The small P-gp binding signals obtained with [nC]elacridar and [nC]tariquidar limit the applicability of these tracers to measure cerebral P-gp density. 

TNF-a is released from virtually all brain parenchymal cells after trauma, hypoxia, epilepsy, neuro-AIDS, and inflammation [73]. Interestingly, TNF-a is not only specifically transported across the BBB but also modulates the functions of the specialized endothelial cells lining the BBB [74—76]. TNF-a transport across the BBB follows a circadian rhythm, and strikingly ABCBl expression at the BBB also displays a circadian rhythm [71, 77]. Upon receptor binding, TNF-a probably affects ABCBl expression by activating NFkB, which binds to the proximal promoter of ABCBl and activates its transcription [78]. 

To further evaluate the potential utility of the Ga-complex as an in vivo marker of Pgp-mediated transport activity, the complexes were injected in wild-type and mdrla/lb (-/-) knoekout mice to study the drug transport across the BBB. As previously mentioned, Pgp is expressed in the luminal surfaces of brain endothelial cells preventing the entry of amphipathic compounds into the central nervous system. Therefore, the mdrla/lb (-/-) knockout mice offer an interesting model to evaluate the applicability of radiotracers for in vivo transport activity of Pgp. At 5 min after injection, the brain uptake of Ga-complex in Pgp-knockout miee was 10-fold higher compared with that in the wild-type mice. As the cerebral blood flow did not differ significantly between the wild-type and the knockout mice, differences on initial brain uptake and retention of Ga-complex were not attributed to changes in cerebral perfusion. 

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