Delivering therapeutic drugs to the brain has long been a major challenge in medicine, largely because of the blood–brain barrier, a dense layer of cells that lines blood vessels in the brain and blocks most substances from entering. Focused ultrasound (FUS), combined with gas-filled microbubbles, has emerged as a promising method to temporarily open this barrier and allow drugs to reach their targets. However, until now, it was unclear how the size of a drug affects how much of it actually gets through.
Researchers at the University of Virginia have now addressed this question using a novel MRI technique called quantitative susceptibility mapping (QSM). Their findings, published in the journal Radiology, challenge the long-held assumption that smaller drug molecules always cross the barrier more efficiently. Instead, they found that delivery follows a bell-shaped curve, with an optimal particle size range of 15 to 23 nanometers.
The blood–brain barrier is a protective mechanism that only allows essential nutrients and oxygen to pass, blocking pathogens and toxins. But this same defense also prevents many life-saving therapies from reaching the brain. Brain tumors add another layer of difficulty with their own blood–tumor barrier. Focused ultrasound works by injecting microbubbles into the bloodstream; when exposed to ultrasound waves, these bubbles expand and contract, creating temporary gaps in the barrier that allow drugs to slip through.
Despite the technique's promise, measuring exactly how much drug crosses the barrier has been difficult. Traditional imaging methods like fluorescence and PET scans have limitations in sensitivity or resolution. Standard MRI techniques are time-consuming and often not suited for larger therapeutic agents used in immunotherapy or gene therapy. The University of Virginia team, led by researcher Matthew Hoch, developed a method using QSM, which exploits the distortion of local magnetic fields by different materials. By injecting iron-based nanoparticles that produce measurable shifts in the MRI signal's phase, the researchers could generate a 3D map of magnetic susceptibility and calculate the concentration of nanoparticles in brain tissue with high precision.
In their experiments, the team tested four different particles ranging from 2.3 nanometers to 45 nanometers in size. The smallest was a gadolinium-based contrast agent, while the others were iron oxide nanoparticles of 15, 23, and 45 nanometers, sizes that mirror those of potential therapeutic candidates for immunotherapy and gene therapy. Using healthy mice and mice with glioma brain tumors, they applied focused ultrasound to open the blood–brain barrier and then measured how much of each particle reached the brain tissue.
The results were surprising. In healthy mice, delivery increased 2.6-fold from the smallest particle to the 15-nanometer nanoparticle, remained steady at 23 nanometers, and then dropped 2.5-fold at 45 nanometers, returning to levels comparable to the smallest agent. This indicates that the optimal size range is between 15 and 23 nanometers, contradicting the idea that smaller is always better.
The researchers suggest that this pattern reflects a balance between two factors: smaller particles can more easily squeeze through the barrier, but larger particles stay in circulation longer, giving them more time to cross. In mice with brain tumors, the presence of the blood–tumor barrier added another variable, but the overall trend was similar.
These findings have significant implications for the design of neurotherapeutics and the clinical application of focused ultrasound. By identifying the optimal particle size for delivery, researchers can better tailor drug carriers, such as nanoparticles or liposomes, to maximize treatment efficacy for brain disorders, including tumors, Alzheimer's disease, and other neurological conditions. The novel QSM-based measurement technique also provides a powerful tool for future research, allowing precise quantification of drug delivery in preclinical models.
