Drug Targeting Organ-Specific Strategies

dergo transcytosis [39] underlines the difference between vesicular transport and carrier-mediated
uptake.
Absorptive-mediated endocytosis and transcytosis of macromolecules through the BBB is
related to receptor-mediated uptake, although not as specific. The process applies to certain
lectins, for example wheat germ agglutinin [40], and to cationic proteins. Transport is triggered
by glycoprotein binding (lectins) or by ionic interactions between negative charges on
the endothelial plasma membrane and positive surface charges on the proteins. Both native
proteins (histones) and chemically modified proteins (cationized albumin and IgG) can undergo
absorptive-mediated transcytosis. While the physiological function of that process at
the BBB remains to be identified, it offers a potential strategy for drug targeting/delivery
[41].
2.3.2 Techniques for Measurement of Brain Uptake
2.3.2.1 In vivo Methods
For the correct interpretation of brain uptake studies in general and for the pharmacokinetic
validation of a given delivery strategy, it is necessary to be familiar with characteristics and
limitations of the applied technique [41]. In vivo methods remain the gold standard, as there
are still no cell culture models available that fully represent the barrier characteristics.
Quantitative measurement of diffusional uptake and carrier-mediated transport of nutrients
and drugs in experimental animals was greatly facilitated with the introduction of Oldendorf’s
brain uptake index (BUI) [42]. Test and reference tracers are injected as an intraarterial
bolus into the carotid artery of the anaesthetized animal. After 5 s the animal is killed
and the brain is removed for radioactivity counting. This method measures the ratio of the
unidirectional brain extraction, E, of the test substance and of the reference ([ 3 H]-water,
[ 14 C]-butanol), which are labelled with different isotopes, during a single passage through the
brain capillary bed:
BUI = E test/E reference
2.3 BBB Biology and Pharmacology 31
(2.1)
Advantages of the method include technical simplicity, and control over the composition
of the injection fluid, making the technique suitable for competition and saturation experiments.
Provided that the absolute value of E reference is known, the absolute E test may be calculated
and, with the independently determined value of cerebral blood flow, F, E test may be
converted into a permeability surface area (PS) product. The latter conversion follows from
the application of the Kety–Renkin–Crone equation of capillary physiology:
E =1–e PS/F (2.2)
The major drawback of the BUI is its limited sensitivity for measurement of compounds
with low extraction.
Sensitivity was improved by at least two orders of magnitude with the internal carotid
artery perfusion technique [43].An outline of the method is given in Figure 2.4. Here, the extraction
can be measured over a time frame of 15 s to 10 min or more, while maintaining the