The Impact of the Blood-Brain Barrier on Stroke Drug Development

The central nervous system (CNS) maintains a tight in vivo balance through the blood-brain barrier (BBB), which regulates the movement of ions, nutrients, and waste products between the blood and brain tissue. While the BBB plays a critical protective role in maintaining a stable environment in the brain, it is also a formidable barrier to drug delivery. This unique challenge is particularly important in stroke research, as many promising drug candidates fail to reach the brain due to poor blood-brain barrier permeability. In fact, drugs that act on the central nervous system have the highest attrition rate in drug development, with 98% of drug candidates being discontinued due to their inability to effectively cross the blood-brain barrier. Difficulty in brain penetration is a serious impediment to the success of stroke therapy. There is therefore an urgent need to understand how drugs enter the brain, to detect the therapeutic concentrations at which drugs reach their intended site of action, and to develop innovative approaches to overcoming the blood-brain barrier in stroke therapy.

Fig. 1 Example of ischaemic damage followed by BBB breakdown and brain penetration of a neuroprotective drug. (Greenhalgh, et al., 2011)

Related Services

• Trans-BBB PK Analysis in Stroke Drug Discovery and Development

Ace Therapeutics has a team of experienced scientists who specialize in BBB research in stroke. Ace Therapeutics offers a variety of methods to analyze the ability of stroke drugs to cross the BBB from the blood stream, measure their concentration in the central nervous system, and predict their functional capacity. Our skilled research directors can help design optimal strategies and customize protocols to advance a client's stroke drug discovery program.

Methods for Measuring Pharmacokinetics Across the BBB in Stroke Drug Discovery

Sampling and Detection in Pharmacokinetics

Sampling CSF

Sampling ventricular CSF provides an effective measure of free drug concentrations in the brain. This can be done post-mortem or through repeated in vivo sampling from the cisterna magna. CSF concentrations generally follow the same profile as brain interstitial concentrations, although this is not always the case. Additionally, CSF concentrations have been shown to correlate with both brain interstitial drug levels and behavioral changes, making it a useful tool for assessing drug efficacy and distribution in the brain.

Sampling Brain Interstitial Fluid

Tissue microdialysis is a powerful technique for assessing brain interstitial fluid, crucial for understanding stroke drug pharmacokinetics (PK). Tissue microdialysis permits measurement of the concentration of a compound in brain interstitial fluid over time, allowing Cmax, half-life (t1/2), and area under the curve (AUC) to be calculated. It offers exceptional anatomical precision, enabling the placement of microdialysis probes into specific brain regions, which is important because drug distribution varies across different brain areas. Microdialysis also helps in optimizing dosing regimens by directly assessing drug concentrations at the site of action, which improves drug efficacy, reduces side effects, and enhances the therapeutic ratio. Additionally, it can be used to correlate pharmacokinetic profiles with pharmacodynamic effects, linking changes in drug concentration to physiological responses, such as neurotransmitter fluctuations.

Fig. 2 The principles of microdialysis: perfusate is pumped through a semi-permeable membrane. (Greenhalgh, et al., 2011)

Pharmacokinetics Imaging

Direct sampling from the brain or CSF presents significant challenges, but recent advances in imaging techniques have introduced new tools to assess drug penetration in the brain, particularly in the context of stroke. These innovative imaging methods offer non-invasive alternatives, enabling researchers to track drug distribution and concentration in real-time, providing valuable insights into the pharmacokinetics of potential therapeutic agents.

Positron Emission Tomography (PET) PK

PET is a powerful imaging technique used to measure drug concentration in tissues following intravenous injection of a radiolabeled drug. The method involves dynamic imaging of the target area and other regions, alongside plasma sampling for PK analysis. The data obtained are fitted to a compartment model using differential equations, which enables the simulation of drug kinetics for different dosing regimens, formulations, or routes of administration. PET has proven particularly useful for assessing the brain penetration of drugs. PET has also been used to measure CNS receptor occupancy following peripheral administration.

While PET offers high sensitivity and specificity, there are challenges, such as the resolution of preclinical PET scanners (typically 1-1.5 mm), which may limit its use in small animal studies. Co-registration with other imaging modalities like CT or MRI can improve precision. Additionally, the partial volume effect can lead to inaccurate measurements of regions with low or high uptake. The short half-lives of common isotopes like [¹¹C] and [¹⁸F] (20 and 110 minutes, respectively) can also limit the duration of PK studies, although longer-lived isotopes like [⁶⁴Cu] or [⁸⁶Zr] can be used in preclinical studies.

Fig. 3 PET imaging confirming biodistribution, pharmacokinetics, and metabolism of [¹⁸F]IL-1Ra in the rat. (Greenhalgh, et al., 2011)

Magnetic Resonance Spectroscopy (MRS) PK

MRS is a technique that measures the distribution of a drug and its metabolites by recording the unique resonance frequencies of individual atoms within the drug. While MRS offers several advantages over PET, including the ability to differentiate between intracellular and extracellular drug concentrations, it has some limitations. MRS has lower sensitivity (10^-4 mol/L) and poor spatial resolution, which can make it challenging for small animal studies (e.g., rodents). However, it is particularly useful for studying stable drugs with longer half-lives, enabling a more extended PK profile characterization.

1. Greenhalgh, A. D., et al. (2011). Translational pharmacokinetics: challenges of an emerging approach to drug development in stroke. Expert Opinion on Drug Metabolism & Toxicology, 7(6), 681-695.