A new class of allosteric dopamine transporter ligands suggests therapeutic promise for chronic pain

Janowsky labFeb. 23, 2016

February's featured paper is "Rigid adenine nucleoside derivatives as novel modulators of the human sodium symporters for dopamine and norepinephrine," published in the Journal of Pharmacology and Experimental Therapeutics. The paper is published by a team led by Dr. Aaron Janowsky.

Millions of people dealing with chronic pain experience pain signals that fire in the nervous system for weeks, months or even years. 

"To date, efficacious treatment of chronic pain has not been achieved," said Amy Eshleman, Ph.D., research pharmacologist in the VA Portland Health Care System. 

"A class of chemical compounds called adenosine analogues is under development as potential therapeutic agents for treating chronic neuropathic pain and other diseases," said Aaron Janowsky, Ph.D., director of the Methamphetamine Abuse Research Center and professor of psychiatry and behavioral neuroscience in the OHSU School of Medicine. 

"I found this month's highlighted paper to be intriguing not only due to the strong science, but also due to the strong element of serendipity," said Mary Heinricher, Ph.D., associate dean for basic science in the OHSU School of Medicine. "Dopamine is implicated in a wide range of psychiatric disorders, and many drugs of abuse alter the amount of dopamine acting on neurons."

"A completely novel way"

Dr. Janowsky and colleagues are reporting on a set of adenosine analogues that were intended as ligands for adenosine receptors, but were found, entirely incidentally, to act on the dopamine transporter. Adenosine analogs may provide a completely novel way to manipulate the transporter.

"Some of these adenosine analogs bind to other targets, including the transporters for dopamine," said Dr. Janowsky. "There are many questions about how these compounds work. How does this interaction affect the transporter? Is interaction with the transporter a cause for concern when treating chronic pain with these compounds?"

"Can modifications be made to the drug structures to decrease or eliminate these interactions?" added Dr. Eshleman.

Transporters for dopamine, norepinephrine and serotonin take up their respective neurotransmitters in the extrasynaptic space and are critical for controlling the intensity and duration of signaling by the neurotransmitters. Many therapeutic compounds, including treatments for depression and ADHD, as well as several abused compounds, including cocaine and methamphetamine, elicit their effects by interacting with the transporters. 

While preclinical and clinical studies revealed that chemicals that bind to and activate the adenosine A1 and A2A receptors are effective in treating chronic pain, they also cause adverse cardiovascular side effects. 

"Work with a third adenosine receptor, A3, has not been developed, but preclinical evidence indicates that chemical activators at this adenosine receptor are also effective in treating pain, but do not cause cardiotoxicity," said Dr. Janowsky. "Development of compounds for this receptor may yield therapeutics that provide pain relief in patients suffering with chronic pain."

New class

Dr. Janowsky and colleagues' research was recently published in the Journal of Pharmacological and Experimental Therapeutics. "Our work has identified a new class of allosteric dopamine transporter ligands, rigidified adenosine derivatives. They display a very atypical pharmacological profile. They can enhance radioligand binding by increasing affinity –a property not previously encountered, or inhibit radioligand binding at the dopamine transporter and inhibit neurotransmitter uptake," he said.

Kenneth Jacobson, senior author, and a medicinal chemist at NIH, synthesized a set of sterically rigidified (N)-methanocarba adenosine derivatives and determined their affinities for the A3 adenosine receptor. 

"We found that some of these compounds actually enhanced binding of a radioligand with structural similarity to cocaine, as well as a structurally dissimilar ligand, to the dopamine and norepinephrine transporters by as much as 760 percent of control binding," said Dr. Janowsky. 

"The phenomenon of enhanced binding to the transporters is unprecedented," added Dr. Eshleman. "Contrary to effects on radioligand binding, the drugs were potent at inhibition of neurotransmitter uptake by the dopamine transporter." 

Saturation binding analysis revealed that the enhancement of binding by these compounds was due to an increase in affinity of the radioligand for the dopamine transporter, rather than an increase in the number of binding sites. 

"This result suggests an allosteric interaction with the transporter, perhaps altering the shape of the protein to allow a tighter interaction with the radioligand," said Dr. Eshleman. 

"The adenosine derivatives did not induce release of preloaded neurotransmitter, which suggests that they are not transporter substrates," said Dr. Janowsky. "This effect is transporter-selective, since there was no enhancement of binding to the serotonin transporter."

"These compounds may be useful for studying drug-transporter interactions and the mechanisms of transporter function," said Dr. Eshleman.

Next steps

Future research for Drs. Janowsky, Eshleman and Jacobson involves a two-pronged approach. "First, from the perspective of potential pain management therapies, we identified modifications that reduced dopamine transporter inhibition as an off-target effect of potent adenosine receptor agonists," said Dr. Eshleman.  "These modifications will be pursued in further drug development. Second, from the perspective of a new class of dopamine transporter modulators, these compounds will be useful tools for characterizing drug interactions and the structure and function of the transporters."

"It will be interesting to see how the nucleoside ligands fit into the transporter to modulate binding of other drugs (methamphetamine, cocaine, antidepressants) in a complex manner," concluded Dr. Janowsky. "We have explored the initial structural requirements, but have not yet eliminated adenosine receptor affinity, a goal of the project."



Rigid adenine nucleoside derivatives as novel modulators of the human sodium symporters for dopamine and norepinephrine
J Pharmacol Exp Ther. 2016 Jan 26. pii: jpet.115.229666.
Aaron Janowsky, Dilip K. Tosh, Amy J. Eshleman, Kenneth A. Jacobson

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Pictured above: front row from left: Aaron Janowsky, Ph.D., Xiao Shi, Ph.D.; second row from left: Tracy Swanson, Amy Eshleman Ph.D.; third row from left: Katherine Wolfrum, John Reed; back row from left: Nicholas Miner, Bill Schutzer and Robert Johnson.

About the OHSU School of Medicine Paper of the Month

The OHSU School of Medicine spotlights a recently published faculty research paper each month. The goals are to describe to the public the exceptional research happening at OHSU as well as inform our faculty of the innovative work underway across the school’s departments, institutes and disciplines. The monthly paper is selected by Associate Dean for Basic Research Mary Heinricher, Ph.D.