Experimental methods for studying drug uptake in the head and brain

The promise and pitfalls of intranasally administering psychopharmacological agents for the treatment of psychiatric disorders

Accumulating research demonstrates the potential of intranasal delivery of psychopharmacological agents to treat a range of psychiatric disorders and symptoms. It is believed that intranasal administration offers both direct and indirect pathways to deliver psychopharmacological agents to the central nervous system. This administration route provides a unique opportunity to repurpose both old drugs for new uses and improve currently approved drugs that are indicated for other administration routes. Despite this promise, however, the physiology of intranasal delivery and related assumptions behind the bypassing of the blood brain barrier is seldom considered in detail in clinical trials and translational research. In this review, we describe the current state of the art in intranasal psychopharmacological agent delivery research and current challenges using this administration route, and discuss important aspects of nose-to-brain delivery that may improve the efficacy of these new therapies in future research. We also highlight current gaps in the literature and suggest how research can directly examine the assumptions of nose-to-brain delivery of psychopharmacological agents in humans.

CEREBRAL UPTAKE OF DRUGS IN HUMANS

1. Drugs that target the central nervous system (CNS) are under-represented in the pharmacopoeia because of the difficulties of overcoming passive and active defences of the blood-brain barrier (BBB). Methods have been developed in drug discovery to make decisions about whether a compound crosses the BBB. Less is known about how the rate and extent of CNS penetration of a drug affects its clinical behaviour, largely because of past difficulties in measuring the cerebral uptake of drugs in humans. Three methods for doing so are reviewed. 2. Microdialysis is sometimes used as a clinical tool for monitoring the brain in neurointensive care and opportunistic pharmacological studies are possible. The method is relatively cheap and simple, measures free drug concentrations and is better suited to characterizing slowly changing brain drug concentrations. 3. Measuring cerebral drug uptake using positron emission tomography imaging requires the use of short-lived isotopes (labelled drug or labelled receptor ligand). Data with high spatial and temporal resolution can be collected, but the method requires expensive infrastructure. 4. Jugular bulb catheters collect pure brain venous blood and are sometimes placed for neuromonitoring. Cerebral drug uptake is inferred from the arterial to cerebral venous concentration difference. The method is relatively cheap and simple and allows global brain concentrations to be estimated. It is better suited to characterizing rapidly changing brain concentrations and effects. 5. The cerebral kinetics of a cerebro-active drug can make substantial contributions to its clinical behaviour. For example, loperamide is a peripherally acting opioid that has little CNS effect due to the active efflux transport of loperamide from the brain back into the blood by transporters including P-glycoprotein. The opioids alfentanil and fentanyl differ in their duration of action largely because of differences in their cerebral distribution volume rather than differences in systemic kinetics. The onset of anaesthesia of the intravenous anaesthetic propofol is governed almost completely by the kinetics of the first-pass passage of the drug through the brain and is more affected by changes in cerebral blood flow than hepatic clearance. 6. Continuing to exploit and develop these methods may provide new avenues to enhance the safety and efficacy of cerebro-active drugs in clinical practice.

Barriers, nephrotoxicology and chronic testing in vitro

In many organs of the human body, there are effective physiological barriers which contribute to regulation of the uptake, transport and secretion of endogenous and exogenous materials. ECVAM is involved in the development of several in vitro models for detecting damage to various barriers, for example, the renal epithelium, the intestinal barrier, and the blood-brain barrier, after acute and chronic exposure to chemicals and products of various kinds. Long-term toxicity testing is an important issue in toxicology. At present, there are no generally accepted in vitro models available for replacing chronic testing in animals. Under chronic exposure conditions, the cellular response is larger than that which can be predicted in the standard cytotoxicity models. Therefore, the approach to predicting chronic toxicity will need to involve more-complex in vitro models. Several in vitro long-term toxicity systems currently available are under evaluation.

High-intensity focused ultrasound selectively disrupts the blood-brain barrier in vivo

High-intensity focused ultrasound (HIFU) has been shown to generate lesions that destroy brain tissue while disrupting the blood-brain barrier (BBB) in the periphery of the lesion. BBB opening, however, has not been shown without damage, and the mechanisms by which HIFU induces BBB disruption remain unknown. We show that HIFU is capable of reversible, nondestructive, BBB disruption in a targeted region-of-interest (ROI) (29 of 55 applications; 26 of 55 applications showed no effect); this opening reverses after 72 h. Light microscopy demonstrates that HIFU either entirely preserves brain architecture while opening the BBB (18 of 29 applications), or generates tissue damage in a small volume within the region of BBB opening (11 of 29 applications). Electron microscopy supports these observations and suggests that HIFU disrupts the BBB by opening capillary endothelial cell tight junctions, an isolated ultrastructural effect that is different from the mechanisms through which other (untargeted) modalities, such as hyperosmotic solutions, hyperthermia and percussive injury disrupt the BBB.