TABLE 1.
Characteristics of the Techniques Used to Estimate Brain Uptake of Drugs
| Technique | Measured/Estimated Parameter | Advantages | Disadvantages/Caveats |
|---|---|---|---|
| In vivo methods | |||
| Intravenous injection/brain sampling | Influx; Influx/efflux | Most physiological approach; highest sensitivity; low technical difficulty | May require good analytical tools to exclude metabolite uptake and careful pharmacokinetic analysis to discriminate unidirectional uptake versus bidirectional transfer |
| Brain uptake index | Influx | Fast procedure; moderate technical difficulty; permits wide range of modifications of injectate composition; artifacts by metabolism largely excluded | Relatively insensitive (compared with intravenous injection and brain perfusion) |
| Brain perfusion | Influx | Higher sensitivity compared with BUI; permits modification of both perfusate composition and flow rates; artifacts by peripheral metabolism excluded | Technically more difficult than intravenous experiments and BUI |
| Quantitative autoradiography | Influx | Excellent spatial resolution | Time-consuming evaluation; no proof of integrity of tracer |
| External registration: MRI, SPECT, PET | Influx/efflux | Noninvasive and applicable in humans; allows time course measurements in individual subjects | Expensive equipment (MRI, PET) and tracers (PET); limited sensitivity (MRI) and availability of labeled tracers (MRI, PET); poor spatial resolution for small animals (SPECT) |
| Microdialysis | Influx/efflux | Allows time course measurements in individual subjects; samples well suited for subsequent analytical procedures | Technically involved; in vivo probe calibration required for valid quantitative evaluation;local damage to BBB integrity |
| CSF sampling | Influx/efflux | Readily accessible for sampling; applicable to humans | Reflects permeability of B-CSF-B and CSF fluid dynamics rather than BBB |
| In vitro methods | |||
| Fresh isolated brain microvessels | Binding, uptake, efflux | Representing the in vivo expression of transporters and efflux systems at the BBB | Transcellular passage cannot be measured |
| EC membrane vesicles | Carrier-mediated transport | Allows distinction of luminal versus abluminal transport activity | Large amounts of source material required, laborious preparation |
| Endothelial cell culture | |||
| Primary cultures, cell lines | Receptor binding; uptake; luminal to abluminal transfer (and opposite direction) | Permeability screening experiments (feasible with primary EC from bovine/porcine sources); effect of culture conditions on BBB transport properties may be studied (e.g., astroglial factors, serum effects, inflammatory stimuli, hypoxia/aglycemia) | No system yet able to represent in vivo condition with respect to barrier tightness and BBB specific transporter expression; multitude of models makes comparison of results between studies difficult |
| In silico models | CNS active (+/−); Log BB; Log PS | Screening of large compound libraries (depending on model selection and computational resources); screening of virtual libraries | Many current models based on data, which may not represent BBB permeability as such (log BB; CNS activity); still very limited data bases for BBB transport (log PS models) |
| Rules of thumb | |||
| Classification models | |||
| QSAR |