Drug Targeting Organ-Specific Strategies

2.3 BBB Biology and Pharmacology 33
of redistribution into the postvascular supernatant during processing and centrifugation
[45].
The most sensitive technique for measuring brain uptake is the intravenous bolus administration
or infusion and subsequent measurement of brain concentrations (Figure 2.4). Depending
on the pharmacokinetics of the test compound in plasma, brain sampling may be
performed after suitable circulation times ranging from minutes to hours or days.
The pharmacokinetic calculation of the unidirectional brain uptake rate, K in, after intravenous
injection, uses the relation of the brain concentration and the area under the curve of
plasma concentration, AUC:
K in = C brain/AUC (2.3)
Here C brain is the brain concentration after correction for intravascular content, and AUC
is determined between time 0 and the final sampling time. Two assumptions must hold when
interpreting the evaluation in the simple form described above: (1) the brain uptake of the
compound is linear, meaning K in is dose independent, and (2) the analysis is performed within
a time-frame where the efflux from tissue is negligible (tissue concentrations are sufficiently
low compared to plasma concentrations).Violation of these assumptions requires adjustments
in experimental design and evaluation. For example, nonlinear kinetics may be accounted
for by incorporation of a Michaelis–Menten term, while efflux can be treated by
compartmental analysis [46].
The i.v. approach has the distinct advantage of measurements being carried out under the
most physiological conditions. On the other hand, the caveats include confounding effects of
peripheral metabolism, which may give rise to artifactual brain uptake of degradation products.
To exclude such a possibility the application of suitable analytical techniques to tissue
and plasma samples is required.
Unless a test compound is available in tracer form suitable for noninvasive quantification
such as positron emission tomography (PET) or single photon emission computed tomography
(SPECT), cerebrospinal fluid sampling is often used in human studies. It needs to be remembered
that such measurements pertain to permeability at the B-CSF-B, not at the BBB,
otherwise erroneous conclusions may be derived, in particular when specific transport
processes are involved. For example the rapid penetration of the anti-HIV drug azidothymidine
into CSF [47] is due to a carrier for pyrimidine nucleotides in the choroid plexus. In contrast,
azidothymidine is subject to efflux at the BBB, which prevents it from reaching significant
concentrations in the brain [48].
A caveat should be mentioned here concerning data relating to drug transport which has
been obtained using intracerebral microdialysis. While the method is appealing for reasons
such as its ability to investigate extracellular fluid concentration–time courses and concentration-dependent
uptake rates in each single animal [49], it has limitations, in particular in
the measurement of substances with low permeability. This is a consequence of the invasive
nature of the placement of the microdialysis probe, which inevitably causes tissue damage, an
inflammatory response and gliosis surrounding the dialysis probe. Therefore, the BBB may
be locally compromised, obviously jeopardizing the valid interpretation of experimental data
[50].