A Sense of Balance

By Eric Sauter

The blood-brain barrier was first noticed by Paul Ehrlich, a 19th century German scientist, who, after injecting dyes into the bloodstream, discovered that they leaked out everywhere except into the brain. He pushed the experiment a bit further, injecting the dye directly into the brain itself. This time, while the brain picked up the dye, nothing else did. Still, it would be well into the 1960s for the existence of the blood-brain barrier to be confirmed.

The blood-brain barrier consists of what has been estimated as hundreds of miles of capillaries crammed thick with cells that restrict what gets through. In this case, only size and solubility matters. Molecules like oxygen—fat-soluble molecules—move easily through the capillary wall lipids. The blood-brain barrier lets through oxygen but not bacteria. Nicotine, alcohol, and caffeine, no problem. But the blood-brain barrier will not allow large therapeutic molecules into the brain, a spectacular problem when it comes to administering drugs to treat neurodegenerative diseases such as Parkinson and Alzheimer's diseases.

The blood-brain barrier will, however, let through small soluble molecules, like the kind just produced by Phil LoGrasso and his colleagues in the Translational Research Institute at Scripps Florida, a division of The Scripps Research Institute.

In a new study appearing in the January 14, 2010 print edition of Journal of Medicinal Chemistry, LoGrasso, an associate professor and senior director for drug discovery at Scripps Florida, and his team have developed a number of potent and highly selective small molecules that not only pass the blood-brain barrier but are very good at inhibiting a key kinase that plays an essential role in neurodegenerative disorders.

There has been a lot of evidence over the decade that points to c-jun-N-terminal kinase (JNK) as a good therapeutic target for the treatment of neurodegenerative disease. In fact, several studies with animal models have shown that loss of JNK protects against neurodegeneration. Inhibitors of JNK have been one of LoGrasso's main targets since he joined Scripps Florida in 2005.

"These are the first JNK selective inhibitors that have demonstrated an ability to penetrate the blood-brain barrier," LoGrasso said. "JNK is a good target to inhibit for diseases like Parkinson's, Alzheimer's, and stroke. The compounds we developed are highly potent, plus they inhibit reactive oxygen species—or ROS production. This is a free radical known to be one of the potential causes for neurodegenerative disease."

One particular class of compounds, the aminopyrimidines, however, has been shown to be effective kinase inhibitors in other studies, and LoGrasso and his colleagues developed a compound similar in structure.

"It has taken some time," LoGrasso said. "Our progress has been good but the blood-brain element adds to the difficulty. In addition, many of these kinase inhibitors don't have the greatest solubility, so you have some significant constraints right from the start."

The Balancing Act

The key is balance. As in any medicinal chemistry program, multiple parameters need to be balanced to produce a compound with the desired properties. Drugs that target the brain are especially challenging because brain penetration must be incorporated while still maintaining a wide range of other, often incompatible (but not insurmountable) functions—potency, selectivity, stability, and good pharmacokinetic properties, in other words how the drug is absorbed and distributed throughout the body.

"You always have to maintain potency and selectivity," LoGrasso said, "but some of the chemical modifications needed to maintain those often prevent the compound from crossing the blood-brain barrier."

The other key point is that any successful compound could not inhibit a class of enzymes known as P450, a widespread superfamily of proteins involved in hormone synthesis and metabolism, among others. The P450 proteins are also involved in drug metabolism.

"When developing these compounds, it has to be an iterative process," LoGrasso said, referring to the slow and repetitive cycles that eventually close in on the desired result. "You do this, while not inhibiting the P450 enzymes, which is crucial, and looking for compounds that inhibit reactive oxygen species generation. We had to try to balance what we had in all those aspects. Some of them had great brain penetration but they inhibit P450 enzymes, while others had terrible brain penetration but did not inhibit P450 enzymes."

Of the many compounds reported in that study, the primary compound, known as 9L, was shown to be 12 times more potent in cells than the most potent compound highlighted by a 2006 study reporting a series of aminopyridine-based JNK inhibitors, 32 times more potent than the least potent compound highlighted.

Compound 9L also struck a good balance between being able to cross the blood-brain barrier, as well as having potent cell inhibition of JNK and a high level of selectivity, which would minimize any potential side effects.

"The compound was also very good in reducing reactive oxygen species production in cells," he said. "We feel that this class of compounds represents good candidates for testing in vivo models of neurodegeneration."

This study is part of a $7.6-million multi-year grant that was award to LoGrasso in 2008 by the National Institutes of Neurological Disorders and Stroke (NINDS) to develop the next generation of kinase inhibitors to treat Parkinson's Disease. The grant will enable Scripps Research and potential partners to file for an investigational new drug (IND)—the first step in the lengthy clinical trials process required by the Food and Drug Administration (FDA)—for a compound to treat neurodegeneration in Parkinson's disease.

The first author of the study, "Synthesis, Biological Evaluation, X-ray Structure, and Pharmacokinetics of Aminopyrimidine c-jun-N-terminal Kinase (JNK) Inhibitors," is Ted Kamenecka of The Scripps Research Institute. Other authors include Rong Jiang, Xinyi Song, Derek Duckett, Weimin Chen, Yuan Yuan Ling, Jeff Habel, John D. Laughlin, Jeremy Chambers, Mariana Figuera-Losada, Michael D. Cameron, Li Lin and Claudia H. Ruiz of The Scripps Research Institute, Scripps Florida. For more information, see http://pubs.acs.org/doi/abs/10.1021/jm901351f

The study was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health.

Send comments to: mikaono[at]scripps.edu

A new class of compounds developed by the LoGrasso lab passes the blood-brain barrier to target neurodegenerative disease.