MRI-guided ultrasound delivers drug to brain
The blood-brain barrier prevents harmful substances from entering the brain, but it
also obstructs the passage of beneficial drugs, necessitating invasive procedures
to deliver them. The breast cancer drug Herceptin — and other antibody-based
drugs — cannot cross the barrier, but now researchers have attempted to deliver
it across the blood-brain barrier of mice using a focused ultrasound transducer,
while guiding the transducer and monitoring the results with MRI.
Kullervo Hynynen’s group at Brigham and
Women’s Hospital and at Harvard Medical School, both in Boston, were the authors
of the study. They chose Herceptin because of its effectiveness in treating metastatic
breast cancer. However, it cannot reach tumors that metastasize to the brain unless
doctors use invasive methods to deliver it. As they demonstrated in a previous paper,
ultrasonic waves create minute, temporary openings in the blood-brain barrier when
used at an appropriately low pressure.
The scientists built their own ultrasound
transducer, which, unlike diagnostic ultrasound, was focused and used a longer 10-ms
burst of ultrasonic waves. They operated the transducer at 0.69 MHz because low
frequencies are ideal for focusing through the skull and disrupting the barrier.
They used a GE Healthcare MRI scanner
to focus the ultrasound and to measure the amount of drug delivered. They chose
the imaging method because it has good soft tissue contrast and spatial resolution.
They aimed the ultrasound at the brain while using a gradient-echo MRI sequence,
and they observed the brain using the T1-weighted fast-spin echo method. The concentration
of the MRI contrast agent is proportional to the signal intensity of the T1-weighted
sequences, whereas gradient echo is fast and can detect temperature elevations.
They assessed tissue damage resulting from the ultrasound by histological examination
of the mouse brains.
The researchers opened the barrier
at a minimum pressure of 0.6 MPa and did not detect tissue hemorrhage at this pressure.
At 0.8 MPa, blood cells leaked in small, scattered areas.
In eight of nine of the control mice,
the amount of Herceptin that entered the brain was below the limit of detection.
However, 1032 ng/g of tissue entered the brain in one of the control mice because
of experimental uncertainty and biological variation. Using ultrasound at 0.6 MPa
and 0.8 MPa, 1504 and 3257 ng of the drug per gram of tissue entered the brain,
The researchers believe that they can
translate this method to human treatment. They previously evinced that a focused
array can safely disrupt the barrier in humans, and they believe that they can use
this technique to deliver other antibodies to the brain, such as those against b-amyloid,
which some evidence suggests may cure Alzheimer’s disease.
PNAS, Aug. 1, 2006, pp. 11719-11723.
MORE FROM PHOTONICS MEDIA