The need
There are 14.7 million people in the UK affected by neurological conditions including Parkinson’s disease, epilepsy, dementia, brain cancer and many others, and approximately 600,000 are newly diagnosed each year. The cost to the NHS is estimated at £4.4 billion, with an occupation of 12.7 million hospital bed days per year.
Intensive effort is being put into developing new treatments for these conditions, and those based on new drug formulations face a common problem – the so-called blood-brain barrier. This is the barrier between the blood vessels or capillaries in the brain and the cells (eg. neurons) and other organelles that make up the brain tissue. Under normal circumstances it is very important as it prevents toxins or other pathogens that may be circulating in our blood from infecting cells in the brain tissue. The downside of this is that the barrier will also prevent the vast majority of drug compounds from reaching targeted cells when treating mental or neurological disorders.
Drug development efforts would benefit from a means by which compounds could be screened early in their development pipeline on their ability to permeate through the blood-brain barrier and reach the targeted cells.
The solution
NPL’s NanoSIMS imaging facility is a unique mass spectrometry imaging method capable of pinpointing the location of drug compounds at subcellular levels. When atom(s) in potential drug compounds are replaced with an isotope of the same element but with lower natural abundance (known as stable isotope labeling), the biological action of the drug will not be affected since the chemistry has not changed. By generating high resolution isotope ratio images using the NanoSIMS, we can pinpoint those regions with isotopic ratios in excess of the expected values from natural abundances and are signatures of the locations of the stable isotope labeled drug compound.
Compounds can be additionally or alternatively tagged by conjugation of metal nanoparticles less than a few nm in diameter, though this in principle could alter the compound’s ability to permeate the barrier.
In this case study, NPL labelled the test compound with both a stable isotope and metal nanoparticles (< 5 nm in diameter). The sample was an ultramicrotomed section of mouse brain.
As the NanoSIMS gradually removes material from the sample during analysis, through depth measurements can be achieved. In the video below, the Carbon/Nitrogen image (left) shows a capillary (dark area) and membrane at its periphery (bright area). The isotope ratio image (middle) shows the location of the compound by the appearance (and disappearance) of the green/yellow/red colors as the video runs. Blue is the natural ratio. The signal on the metal image (right) indicates the distribution of the metal nanoparticles tagged to the compound (which should match the regions showing elevated isotope ratios). As the section through the capillary is obliquely cut, we see the position of the membrane move with change in depth as the video runs. In this particular case, the compound seems to be caught up in the barrier and does not penetrate the brain tissue.
Figure 1: From left to right CN image, isotope image, metal image
The impact
This type of analysis gives researchers working in the field of drug development for treatment of neurological disorders valuable information early in the R&D pipeline. The video indicates that the compound is not likely a viable candidate worth pursuing. This knowledge enables valuable resources to be directed to other candidates, which will help to reduce costs and speed development of more promising compounds with the ultimate goals of reducing the debilitating effects of neurological disorders and the burden they place on NHS resources. The NanoSIMS capability is now being used by a number of leading pharmaceutical companies to support their decision-making around drug attrition.
This new capability was developed under the NPL MedAccel programme.