BOSTON-For the past forty years, drug discovery efforts were guided by a basic principle: any compound considered for work in the brain needed to have a nitrogen atom built into its structure. Consequently, all drugs targeting the brain-psychoactive, anti-parkinsonian, anti-epileptic drugs and many more- contain this nitrogen atom.
By faithfully following this untested dogma, researchers in academia and the pharmaceutical industry have needlessly limited themselves in their search for new medicines, contends Bertha Madras. The associate professor of psychobiology at Harvard Medical School and her collaborators report in the December issue of the journal Synapse that the nitrogen is not indispensable after all. Indeed, the researchers found a novel group of compounds without nitrogen that recognizes the same molecular targets as do common drugs like Prozac and Ritalin. What's more, these experimental compounds appear to be as powerful as their traditional counterparts.
"This study has literally shredded the old premise, which should have been examined a long time ago. It forces a revision of current concepts of drug binding domains," says Madras. The work opens up new classes of chemical compounds to drug discovery, especially for the treatment of attention deficit hyperactivity disorder (ADHD), depression, and cocaine addiction. In addition, it means that there may be many natural compounds active in our brains that scientists never detected because they did not know how to look for them.
Madras' study focuses on brain molecules called dopamine transporters. These proteins reside in the plasma membrane of the nerve ending at the synapse, the tiny gap that neurotransmitters must cross before relaying neural impulses. After the neurotransmitter has done its job, the transporter binds and removes it from the synaptic gap to prevent overstimulation. Dopamine-one of the brain's so-called monoamine neurotransmitters-contains a nitrogen, as do the other members of its class, serotonin and norepinephrine. Drug discovery efforts were guided by the attempt to mimic the natural products, therefore the common feature of the transmitter structure-the nitrogen-was deemed the core requirement for drug candidates.
This assumption was so basic that researchers never tested it, and it was convenient since nitrogen makes compounds easy to dissolve, crystallize and handle as drugs in powder form. Madras did not intend to test the premise, either, she readily admits. As often happens in science, she was pursuing an entirely different question. She was after the holy grail in drug addiction research: to find a treatment for cocaine addiction. Since cocaine acts by blocking the dopamine transporter, stopping the transmitter's removal from the synapse and thus causing rapid surges of dopamine in the brain, such a drug would need to block the action of cocaine while leaving normal dopamine transport untouched.
To find such a compound, Madras teamed up with her long-term collaborator Peter Meltzer, a synthetic chemist and president of the Woburn-based company Organix Inc., who designed and prepared a series of novel, structurally unusual compounds that replaced the common nitrogen with an oxygen atom. When the researchers tested the compounds on monoamine transporters in monkey and human brain tissue, they found that these unorthodox compounds not only bound specifically to dopamine, serotonin, and norepinephrine transporters, but that they were about as potent as current drugs. One of the new compounds, called tropoxane, recognized the dopamine transporter with higher potency than do cocaine and Ritalin, the most commonly prescribed drug for ADHD. It also binds the serotonin transporter with potencies similar to the antidepressants Prozac and desipramine, the researchers report. "We are right in the therapeutic range," says Madras.
That tropoxane and its cousins can compare with marketed drugs is even more significant given that tropoxane represents only a lead compound, or starting material, for drug discovery. Frequently, the lengthy fine-tuning process of drug development further increases the potency and specificity of such lead compounds.
But real drugs need to do more than bind a target: They must affect its function. While Madras cautions that these are still early days, she hints that her compounds-dubbed non-amines, as opposed to the traditional monoamines- probably do so. Her team reports that non-amines block dopamine transport in a cell culture experiment, and preliminary tests with living animals indicate that the compounds produce biochemical and behavioral effects comparable to those of other dopamine transporter blockers.
While the non-amines' ability to block dopamine transport for now dashes Madras' original hope of finding a cocaine blocker that still allows dopamine to work, it has led the research team to something more interesting, she says.
The results suggest that there may be an additional, entirely different mechanism for how drugs recognize targets in the brain that was overlooked until now. The traditional premise assumed that the nitrogen in drugs forms a strong ionic bond with a counterion on the brain's target molecule, as do the neurotransmitters themselves. Madras' results indicate that instead, non-amines- and potentially many more compounds-interact with transporters by aligning the fatty parts of their structure with similar fatty components on the transporter in what is called a molecular stacking interaction. In fact, she adds, nature uses this binding mode already: some hormones and pheromones home in on their targets that way.
Madras suggests that researchers should exploit this new binding mode for drug discovery, because drugs need not necessarily imitate neurotransmitters in every way. Neurotransmitters have evolved to cross the watery synaptic gap in milliseconds, stimulate the neuron on the other side, and then become sequestered by the transporter and metabolized quickly. Nitrogen confers the biochemical features important for these steps. But drugs do not have to act and disappear that quickly, quite to the contrary: A slower mode of action might result in a long-acting drug that the patient would not need to take every few hours, she says.
But will the non-amines really make better drugs? It is too early to answer this question, cautions Madras. Much work remains to be done to study the toxicology and pharmacology of these compounds.
"I feel that we opened a small window in a dark house and see that there is a whole new vista. And now, let's walk and see how far it takes us," she says.
Additional authors of the Synapse article are Zdenek Pristupa, Hyman Niznik, both of Clarke Institute of Psychiatry in Toronto, Canada; Anna Liang, Paul Blundell, and Mario Gonzalez, at Organix, Inc.