Molecules and Mood Disorders: Drug Discovery and the Treatment of Depression

Antidepressant Drug Discovery: The Ways, Means, and Needs

Michael J. Owens, PhD, Associate Professor, Department of Psychiatry and Behavioral Sciences, Laboratory of Neuropsychopharmacology, Emory University School of Medicine, Atlanta, discussed the science behind antidepressant drug discovery. “In the last 15 years, since the advent of molecular neurobiology, we’ve learned a lot about the brain,” he said.

“Behavior is kind of a chemical symphony and everything happens at the synapse,” Dr. Owens said. Electrical stimuli induce the release of neurotransmitters from the presynaptic neuron. Within the synaptic cleft, these neurotransmitters can either interact with receptors on the presynaptic neuron or the postsynaptic neuron. Most antidepressants block the neurotransmitter reuptake pumps, or transporters, that are located on the outside of the cell.

Dr. Owens outlined the drug discovery process. “One of the easiest things is to make structural modifications to a drug that we know has some functional activity,” he explained. Another modification to an existing drug involves isolating the active isomer of a racemic mixture. Racemic drugs occur when the molecule has a chiral carbon. During the synthetic process, a 50:50 ratio of two mirror-image compounds is typically created, but usually only one isomer will bind well to the target. The other isomer may have no effect, cause side effects, or reduce the efficacy of the active isomer.

Another method utilizes three-dimensional computer-aided modeling. X-ray crystallography data is used to model the target structure, and drugs are designed specifically to fit targeted binding pockets. “This is a way to develop really selective and potent drugs,” said Dr. Owens. In contrast, combinatorial chemistry involves the synthesis and testing of thousands of different compounds. These random mixtures are tested in batches for the desired pharmacologic activity.

Once potential drugs are available, they are tested by in vitro pharmacology assays. In general, drugs with KD or KI values less than 10 nM are highly potent, between 10 nM and 100 nM are moderately potent, and those over 100 nM probably lack any significant physiological effects in vivo. Selectivity can be determined by conducting binding studies with a variety of different targets.

Testing then proceeds to in vivo assays which include neurobiological effects, such as electrophysiological measurement of serotonin transporter inhibition and microdialysis to measure extracellular concentrations of serotonin.

Dr. Owens used corticotropin releasing factor (CRF) as an example of how neuroscience has identified potential drug targets. “CRF integrates and orchestrates the endocrine, autonomic, and behavioral responses to stress in animals and humans,” said Dr. Owens. Preclinical studies identified a number of CRF alterations associated with depression, including an increased quantity of CRF mRNA and more CRF-positive cells in the hypothalamus. There is some clinical data to support this. CRF antagonists that have been developed are potentially novel antidepressants.

Molecular research is underway to identify novel targets. “In a normal, healthy neuron there are huge plastic changes in the connectivity of neurons each day,” explained Dr. Owens. There must be proteins involved in these cytoarchitectural changes and a resultant change in the number of connections and information that a particular neuron can receive and transmit. A potential novel target is brain-derived neurotropic factor (BDNF), which increases in the presence of all classes of antidepressants. Gene chip technology allows for the identification of changes in gene expression. As an example, Dr. Owens noted that “after haloperidol treatment, there are some genes that get really decreased and some that get really increased.” Another method is proteomics, in which proteins isolated from brain samples of individuals with different disorders are compared on two-dimensional gels. A protein can be excised from a gel and be identified by mass spectrometry. “This is a very powerful technique for looking at changes as a result of drug treatment or changes that actually represent the pathophysiology of disease,” said Dr. Owens.

The preclinical pharmacology stage lasts about three years, and of approximately 5,000 compounds developed, usually only five will advance to further trials. Clinical trials last at least 6 years, after which the new drug application can be submitted to the FDA. “In general, you get about one drug approved after about 12 years at a cost of approximately $800 million,” he said. “Efficacy and tolerability in human clinical trials will ultimately decide which of the new molecules will be used in clinical practice,” concluded Dr. Owens.

The Phenomenon of Single-Isomer Drugs

“There is a basic, important consideration in the chemistry and pharmacology of drugs, the phenomenon of stereoisomerism,” began Joseph Gal, PhD, Professor of Medicine, Pharmacology, and Pathology, Division of Clinical Pharmacology, University of Colorado School of Medicine, Denver. Any object whose mirror image is not superposable on the original is called “chiral.” Many molecules are chiral, most often because of carbon atoms that are bound to four different groups or atoms. The two mirror-image forms of a chiral molecule are called “enantiomers” or, somewhat less precisely, “isomers.” “Racemate” or “racemic mixture” refers to a substance that is a 1:1 mixture of the two enantiomers.

There are two systems to identify enantiomers: optical rotation and configuration. Optical rotation, a physical property of single enantiomers, is measured using plane-polarized light, in which the electric and magnetic components of light are in a single plane. The single enantiomer of a chiral molecule that rotates this light clockwise is called dextrorotatory, and a (+)- or the “dextro-” prefix is added to the compound’s name. The other enantiomer will rotate light counterclockwise and is called levorotatory; a (-)- or “levo-” prefix is added to its name.

Configuration refers to the actual spatial arrangement of the atoms. Amino acids and sugars are named according to the D/L system, which has nothing to do with optical rotation. Other chiral compounds are named following the R/S system. “There is no correlation between optical rotation and the configuration descriptors R/S and D/L,” explained Dr. Gal. Also, there is no correlation between the R/S and D/L descriptors. “This makes it very confusing,” he added. Single isomer drug names can be based on optical rotation (e.g., dextromethorphan), configuration (e.g., (R)-fluoxetine), or both (e.g., (S)-(+)-amphetamine). Such single-enantiomer designation may be an integral part of the generic name of the drug or added as a prefix.

“Although their chemical formulas look identical in two dimensions, drug enantiomers are, from a biological standpoint, different substances,” said Dr. Gal. A simple example of this is limonene: (R)-limonene smells like oranges and (S)-limonene smells like lemons. The reason for such differences, explained Dr. Gal, is that the biological macromolecular targets of drugs —proteins, nucleic acids, carbohydrates, lipids—are created by nature in single-isomer form, and the two enantiomers of a chiral drug may interact differently with a target. The different interactions in turn could elicit different effects.

In practice, drug enantiomers often differ significantly in activity and/or disposition. For example, the beta blocker propranolol is a racemic mixture but only the (S) enantiomer has beta-blocking activity. Another example is propoxyphene: the (+) enantiomer is an analgesic and the (-) enantiomer is an antitussive. In other cases, such as DOPA, one enantiomer is too toxic to be included in the drug product. Metabolism is another consideration with racemic drugs because enzymes are often selective for one of the two enantiomers.

“Based on scientific and clinical evidence, in the majority of cases the single-isomer form is preferred,” said Dr. Gal. The many likely advantages of single isomers over racemates include: reduced dosage, improved efficacy, reduced side effects, reduced metabolic burden, simpler pharmacology, clearer pharmacokinetics, and better-defined relationship between drug concentration and effect.

In most cases, these advantages are realized. In 1992 the FDA issued guidelines that strongly encourage the development of single enantiomers over racemates. “Racemic drugs may still appear, but the overwhelming result of the new regulations is that the majority of new chiral drugs will appear in the
single isomer form,” Dr. Gal said. As evidence of this, in 1999 the sale of single-isomer drugs topped $100 billion and accounted for approximately one-third of the total worldwide drug sales.

Dr. Gal summarized the science and technology specifically required for the development of new single-isomer drugs. The three-dimensional aspects of drug-receptor interactions need to be understood. Efficient methods for the chemical synthesis of single-isomer drugs is another prerequisite, and techniques to measure the individual enantiomers in the presence of each other, especially in biological fluids, is also of fundamental importance. Dr. Gal indicated that great progress has been made in the last two decades in all of these areas.

There are two main approaches for the discovery of new single-isomer drugs. The first is called “chiral switch,” in which a single isomer is developed from an existing racemate. Successful chiral switch drugs include dexmethylphenidate for ADHD, escitalopram for depression, and levobupivacaine for local anesthesia. There have been some failed chiral switches. For instance, the single-isomer form of the antihypertensive drug labetalol, called dilevalol, is severely hepatotoxic and was withdrawn from the market.

Alternatively, single-isomer drugs may be new chemical entities. These may be a new chemical structure or a modification of an existing drug. Strategies for designing new chemical entities include classical structure-activity relationship studies, combinatorial chemistry, and computer-aided drug design. “Not every single-isomer candidate will reach the clinic, but there is no doubt that the move to single-isomer agents is an important step forward in the search for better and safer drugs,” concluded Dr. Gal.

The State of the Art in Treating Depression: 2002

“Antidepressants are still a little magical,” said Andrew A. Nierenberg, MD, Associate Professor of Psychiatry, Harvard Medical School. “Even though we talk about what's going on with the neurons, we still don't quite have the full picture.”

To try to achieve remission, many providers go to the maximally effective dose as quickly as the patient can tolerate it. Currently, an adequate trial is anywhere from 8 to 12 weeks, although many abandon a trial after 4 weeks. “Databases suggest that, out of all the responders who will respond at 12 weeks, 20% of them will respond between the eighth and twelfth weeks,” Dr. Nierenberg stated. Of those who respond to treatment, most respond by the second or fourth week. Often, patients who respond still have symptoms. Data strongly suggest that responders should continue treatment for 4 to 9 months after an acute response.

Dr. Nierenberg displayed a long list of available antidepressants and asked: “why do we need new stuff?” The limitations of current antidepressants include: side effects, incremental dosing, delayed onset of action, infrequent remission, frequent residual symptoms, and low adherence.
Classes of antidepressants that are on the horizon are uptake inhibitors, receptor modulators, hormone regulators, peptides, combinations, and alternative treatments. Dr. Nierenberg provided examples of each class. The single isomer uptake inhibitor escitalopram is more effective than its racemic parent citalopram. Atamoxatine is a norepinephrine uptake inhibitor for ADHD which will probably be released. Dual neurotransmitter uptake inhibitors include duloxetine, which outperforms paroxetine, and milnacipran.

Receptor modulators include the 5HT-1A agonists gepirone and flesinoxan and 5HT-1D blockers. CRF antagonists and estradiol are in the hormone regulator class as are the antiglucocorticoids mifepristone (RU-486), ketoconazole, and dexamethasone. Netamiftide is a novel peptide that is given subcutaneously in two 5-day dosing cycles. The effects of netamifitide last for months.

It is common to combine SSRI’s and mirtzapine, venlafaxine and mirtzapine, SSRI’s and bupropion, triple treatments, and antidepressants plus atypical antipsychotics. “There really are no data for a lot of these things that we do,” he stated. One small study found that 64% of patients who were nonrespon-sive to SSRI’s responded to a SSRI plus mirtazapine.

Dr. Nierenberg concluded by reviewing some potential alternative treatments for depression. St. John’s Wort remains a controversial antidepressant. S-adneosyl-L-methionine is touted as an antidepressant, but there are no data to support this. There is conflicting data that inositol helps unipolar or bipolar depression. Eicosapentaenoic acid, an omega-3 fatty acid, was found to effectively treat breakthrough depression in 60% of the patients who were on antidepressants.

All contents Copyright © 1999 - 2002 Medical Association Communications