Table 3.

A summary of different techniques for measuring brain uptake of compounds along with their advantages and disadvantages.

Technique	Advantages	Disadvantage
Single Time Point Analysis	• Physiological approach. • Low technical difficulty since the process only involves blood and brain collection after IV injection. • Widely used for brain permeability measurements using a passive permeability marker.	• Only suitable for studying the unidirectional transfer of a solute from blood to the brain. • Transporter-mediated influx/efflux cannot be understood. • Bulk flow-mediated efflux may be an additional confounding factor for test compounds having low permeability.
Multiple time point analysis (Patlak Plot)	• Physiological approach • Widely used clinically for measuring gadolinium uptake in patients.	• Requires more ‘n’ compared to the single time point analysis. • Only suitable for studying the unidirectional transfer of a solute from blood to the brain. • Transporter-mediated influx/efflux cannot be understood. • Bulk flow-mediated efflux may be an additional confounding factor for test compounds having low permeability.
Brain Uptake Index	• The procedure can be performed very quickly. • The technical difficulty is moderate. • Infusate compositions can be changed to meet requirements.	• Insensitive for molecules having low permeability • Transporter-mediated influx/efflux cannot be understood.
In-situ brain perfusion technique	• Very widely used procedure for understanding brain uptake of compounds. • The procedure can be performed very quickly. • Better sensitivity compared to the Brain Uptake Index. • Infusate composition as well as the flow rate can be changed to meet study requirements. • The impact of transporters on drug uptake can be understood by using inhibitors along with compounds being studied.	• Complex surgical procedures necessary.
Brain microdialysis	• The best technique for accurate measurement of unbound drug concentrations in the CNS. • The technique allows sampling from multiple brain regions. • Measurements at steady-state drug concentrations allow an understanding of interactions of the test compounds with influx/efflux transporters in the CNS. • The procedure has been used clinically in patients with traumatic brain injury and glioblastoma. • In combination with pharmacokinetic modeling and simulation, measurements using this technique were clinically translatable from monkeys to humans.	• Highly time-consuming and labor-intensive technique. • Data may not be accurate unless the probes recoveries are appropriately studied. • Not suitable for highly lipophilic compounds
Brain Slice technique	• An ex-vivo technique allowing rapid measurement of volume distribution of compounds in the brain. • More accurate compared to using brain homogenates. • Results can be utilized to complement other in vivo studies when a microdialysis study is not feasible.	• Accuracy is within 3 folds of microdialysis results.
Quantitative Autoradiography	• Suitable for studying the distribution of radiolabeled compounds in the brain. • A good spatial resolution allowing the understanding of how drug uptake varies in different regions of the brain	• Evaluation is time-consuming and labor-intensive. • Radiolabeled compounds may behave differently compared to unlabeled compounds