Chiral Chemistry Enables Firms To Try New Twists On Old Drugs

Sidebar: New Ways To Separate Enantiomers Photo: Malboro Photo SUCCESS IN SEPARATION: Sepracor researchers found that the S-enantiomer of Prozac worked well in preventing migraine, says Dean Handley. A trend in the pharmaceutical industry toward increasing use of chiral chemistry techniques is leading to safer new drugs, while coaxing novel uses from existing blockbuster compounds. These techniques separate drugs that occur in two mirror-image molecular forms, called enantiomers. Louis Pasteur discovered such dual compounds that consist of two enantiomers, called racemates, in 1848. INTRICATE ROLES: Louisiana State's Isiah Warner notes that determining the side effects of enantiomers is not always a straightforward task. Unraveling the roles of enantiomers is not always straightforward. "In many cases, the mirror-image enantiomer of an active compound is innocuous. In other cases, it may be extremely toxic and cause side effects. And together, the enantiomers might produce effects that are not common to either alone," says Isiah Warner, chairman of the chemistry department at Louisiana State University in Baton Rouge. Developing racemic (mixed-enantiomer) drugs is on the way out, industry analysts say, because pharmaceutical companies must evaluate the individual enantiomers and the combination. "Nearly every pharmaceutical company we deal with doesn't even talk about using a racemate, because it costs more to develop. The company has to put three products through clinical trials, not one," explains Roger Pettman, vice president for sales and marketing at ChiRex Inc. of Wellesley, Mass. ChiRex supplies pure intermediate compounds to pharmaceutical companies, which use these products for their chiral pharmaceuticals. Improved synthetic chemical methods developed over the past decade have made large-scale production of single-enantiomer drugs commercially viable. And sales of these drugs are predicted to soar. "Single-enantiomer drugs sold for $61 billion worldwide in '95. That number will be $90 billion in the year 2000," predicts Michael Cannarsa, director of chiral chemistry consulting at Technology Catalysts International Corp. in Falls Church, Va. In 1995, 70 percent of drugs were single-enantiomer. By the year 2000, that figure will rise 8 percent, according to the seventh annual edition of Optically Active Chemical Compounds: Worldwide Technology and Market Assessment, a report published in 1996 by Technology Catalysts. The growing emphasis on single-enantiomer drugs means jobs in pharmaceutical firms, the 50 or so fine chemical companies that manufacture intermediates, and the few companies that revamp existing drugs. The last category includes ChiRex; Sepracor Inc. of Marlborough, Mass.; Celgene Corp. in Warren, N.J.; and Chiroscience Group PLC of Cambridge, U.K. Chiral drug development has traditionally been a source of employment for chemists, but opportunities for life scientists will increase, predicts Cannarsa. ChiRex is typical in the types of expertise currently sought. "We hire organic and analytical chemists for our technology development who have expertise in synthetic chemistry, asymmetric synthesis, and biotransformation," says Pettman. Biotransformation is the study of changes to a drug in the body, which involves a working knowledge of pharmacology, toxicology, and biochemistry, particularly enzymology. New life science positions will likely require expertise in microbial and molecular genetics, to improve upon enzymes used to separate enantiomers. For example, Celgene as well as Genencor Corp. of Rochester, N.Y., currently have projects that use genetic alteration to boost activities of certain enzymes in bacteria. INCREASED OUTPUT: Celgene's Sol Barer points out that agrichemical manufacturing plants could double production capacity by developing chirally pure compounds. Life science opportunities may also widen as chiral chemistry applications diversify. Celgene, for example, has recently spun off a separate division, called Celgro Corp., which will tackle the potential $5 billion market in racemic agrichemicals, according to Sol Barer, president of Celgene. "If we can develop chirally pure versions of current mixed agrichemicals, we can cut by 50 percent the amount needed in the field. For most manufacturing plants, this could double capacity," Barer adds. And Cannarsa sees future jobs in developing more carbohydrate-based single-enantiomer drugs. "Discoveries in the interactions of carbohydrates with cell membranes will lead to more carbohydrate-based products in chiral drug development," he forecasts. In addition, applications of chiral chemistry in the flavor and fragrance industries could require expertise of botanists to identify interesting natural products that are candidates for synthetic versions. Racemates result when the atoms composing a molecule can assume more than one arrangement in three-dimensional space. "Enantiomers occur when one carbon has four different substituents, and the molecule can exist in two forms in space," explains Dean Handley, director of scientific affairs at Sepracor. The pharmaceutical challenge is that in a chemistry lab, a racemic compound is a 50:50 mixture of enantiomers (or 25:25:25:25, if it has two chiral centers). In an organism, chiral molecules are single enantiomers. "Life is essentially asymmetrical. Enzymes recognize left- or right-handed molecules. Most carbohydrates are the dextro [right-handed] form, whereas amino acids are the levo [left-handed] form," Barer. TRUE IDENTITIES: "There are several drugs where one enantiomer thought to be inactive has its own activity," notes University of Illinois medical chemist John Fitzloff. If one enantiomer is therapeutic, removing its mirror image halves the dose and may eliminate side effects. "The basic assumption was that one enantiomer was active while the other was inactive, but that was shown not to be the case. There are several drugs where one enantiomer thought to be inactive has its own activity," notes John Fitzloff, an associate professor of medical chemistry at the University of Illinois at Chicago. Albuterol, a bronchodilator used to treat acute asthma, is a racemic drug that is improved without one enantiomer. The drug is sold as Ventolin from Middlesex, U.K.-based Glaxo Wellcome PLC, and Proventil, from Schering-Plough Corp. in Kenilworth, N.J., and yields $1.2 billion a year globally, according to information from Sepracor, which plans to have its own version of the drug available by 1998. In addition to producing jitteriness and irregular heartbeat, repeated use worsens the asthma (W.O. Spitzer et al., New England Journal of Medicine, 326:501-6, 1992). In 1994, Sepracor patented the active enantiomer, which works at one-eighth the dose of albuterol, halves incidence of side effects, and does not worsen asthma. The new-and-improved version is expected to hit the market in 1998. Barbara Hoffman, a biotech analyst with Vector Securities International in Denver, predicts that sales will top $500 million by the year 2000. Photo: Larry Laszlo THERAPEUTIC BOON: "Patients will be the ones to ulimately benefit from the development of single-enantiomer drugs," declares biotech analyst Barbara Hoffman. Sometimes removing a dangerous enantiomer doesn't help. This happens with thalidomide, the drug used to alleviate morning sickness in the early 1960s in Europe. Thalidomide's harmful enantiomer led to the birth of 10,000 children with severe limb defects. Even if the helpful enantiomer of thalidomide is administered alone, the body converts it into the two forms. In a best-of-both-worlds situation, separating a racemate reveals hidden talents of its components. Consider the racemic drug fluoxetine, better known as the antidepressant Prozac, from Indianapolis-based Eli Lilly and Co. "Eli Lilly and Co. funded a study that showed that racemic Prozac had no effect on migraine. Sepracor separated the enantiomers, initiated pharmacological testing, ran a pilot clinical study, and found that the S-enantiomer worked remarkably well in preventing migraine," says Handley. Food and Drug Administration (FDA) policy published in 1992 (Chirality, 4:338-340) strongly urges companies to evaluate racemates and enantiomers for new drugs. Even though in many cases a single-enantiomer drug is safer than a racemate, an effective racemate can still be marketed. "If a firm has data supporting the safety and efficacy of a racemate, we'd have to approve it," notes Thomas McGinnis, associate director of pharmaceutical affairs at FDA. COMPREHENSIVE TESTS: Most pharmaceutical companies routinely test racemates and both enantiomers, maintains Wyeth-Ayerst's Magid Abou-Gharbia. Many pharmaceutical companies, however, routinely tested the three forms of a racemic drug before the 1992 FDA urging. "We have always resolved racemates and prepared both enantiomers, then tested both and determined if there was complete justification for producing a racemic drug," maintains Magid Abou-Gharbia, vice president of chemical sciences at the Wyeth-Ayerst Research in Monmouth Junction, N.J. Despite widespread adherence to the 1992 guidelines, FDA may toughen its stance soon. "FDA has publicly stated that they believe the agency needs a stricter policy," notes Hoffman. "Guidelines should be amended to encourage and in some cases require development of single-enantiomer drugs. The policy now is really a preference." The January 1997 market removal of terfenadine, a popular nonsedating antihistamine sold as Seldane by Hoechst Marion Roussel Inc. of Kansas City, Mo., illustrates FDA's tightening policy (K.S. Brown, The Scientist, May 12, 1997, page 1). In 1992, Seldane was found to interact with the antibiotic erythromycin and the antifungal ketoconazole, producing life-threatening cardiac arrhythmia. FDA instituted a "black box" warning on packages stating that there are serious adverse side effects, according to Hoffman. Meanwhile, Sepracor researchers were at work on a variation of the single-enantiomer theme to improve Seldane. In 1993, Hoechst Marion Roussel licensed rights from Sepracor to market fexofenadine, which was launched in July 1996 as Allegra. The new drug is an active metabolite of Seldane. Both drugs are racemates, but while Seldane's alter-ego is cardiotoxic, this isn't the case for Allegra. "By using a metabolite strategy, we produced a safer and effective drug without a racemic switch," says Sepracor's Handley. Chemists have known about racemates since Pasteur, at 26 years of age, told the Paris Academy of Sciences how he used tweezers to separate two types of crystals of salts of tartaric acid, which rotate polarized light clockwise or counterclockwise. The technique of resolving racemates into two salts is still used. Mass spectroscopy and high-performance liquid chromatography are also used to separate enantiomers based on small differences in physical properties. In a second technique, asymmetric synthesis, a chemical reaction is used that yields one enantiomer. "You take a nonchiral starting material and go directly to a single isomer using a catalyst. This is the most direct way" to obtain a single enantiomer, according to Pettman. A third approach borrows from biology by using enzymes. Because the proteins of life are one-sided, enzymes preferentially catalyze reactions that produce one enantiomer. Compared with inorganic catalysts, enzymes are "environmentally friendly," says Barer, and the reactions they catalyze usually take fewer steps. But enzyme-producing cell cultures cannot withstand the rigors of an industrial process, and enzyme extracts are notoriously impure. Two companies have tackled the limitations of enzyme-based systems. "Celgene has developed a novel enzyme system that combines the selectivity of biology with the convenience and flexibility of chemistry," explains Barer. "We culture microorganisms, take the enzymes out and stabilize them, and use them as one would in a conventional chemistry catalytic process." And Altus Biologics Inc. of Cambridge, Mass., purifies enzyme extracts with "cross-linked enzyme crystals," or CLEC technology. "We take the enzyme and crystallize it, and chemically cross-link the crystals so that they are insoluble," explains Jim Lalonde, staff scientist at Altus. "Crude enzyme preparations are fine for cleaning clothes or making cheese, but not for pharmaceuticals," he adds. As pharmaceutical researchers continue to perfect standard ways of separating enantiomers and academic researchers discover new methods (see story below), drugs will become safer. Concludes Hoffman: "Patients will be the ones to ultimately benefit from development of single-enantiomer drugs. If an enantiomer introduces unnecessary side effects, why not take it out?" Modified Micelles: "In order to separate chiral molecules, you need another chiral species," says Isiah Warner, chairman of the chemistry department at Louisiana State University in Baton Rouge. Warner uses fatty spheres called micelles, which form as fatty acid tails face inward and heads face outward in solution. He links a single enantiomer of the amino acid valine to the fatty acids, creating micelles that bind certain other single enantiomers. It is an old idea with a twist, Warner says: He uses gamma rays to fuse the tails. "Normally, if you go below a certain concentration, micelles break up. If you fuse the tails, they can't break," he explains. This is useful because separating some enantiomers occurs only at low concentrations. Serendipitous Stirring: In 1990, Dilip Kondepudi, a professor of chemistry at Wake Forest University in Winston-Salem, N.C., was scrutinizing the effect of impurities on crystallization of sodium chlorate. A film of fine crystals would form at the surface of the solution, like the skin on chocolate pudding. To dispel the layer, Kondepudi tried constantly stirring the solution-and was flabbergasted. "The crystals all turned polarized light either clockwise or counterclockwise," he recalls. Publication of Kondepudi's finding caused quite a stir (D. Kondepudi, R. Hegstrom, Science, 250:975-6, 1990). Today Kondepudi studies the effect of stirring on mixtures of amino acids. "You can find an impurity that slows crystallization of one enantiomer and crystallizes out the other. There is a window of time when one enantiomer crystallizes out, and the other is still in solution. If you can catch that window, you can separate them," he says. Computerized Predictions: Predicting single-enantiomer activities before separating a racemate would save time in pharmaceutical research. A multinational team used computer modeling to distinguish enantiomers of bromochlorofluoromethane (CHFClBr), which bind with slightly different strengths to a molecular "cage" called cryptophane (A. McCammon, Journal of the American Chemical Society, 119:3818-23, 1997). "Our approach would help identify which [enantiomeric] version is the most important for screening new drugs," explains team member James Briggs, a professor of chemistry at the University of California, San Diego. -R.L. Ricki Lewis, a freelance science writer based in Scotia, N.Y., is the author of several biology textbooks. She can be reached online at 76715.3517@compuserve.com.

Chiral Chemistry Enables Firms To Try New Twists On Old Drugs

Interested in reading more?

Become a Member of