COVER STORY
SUGAR MEDICINE Researchers arefindingDRUG LEADS as they tease out pathways of cellular glycosylation STU BORMAN, C&EN WASHINGTON
THE ATTACHMENT of sugars to proteins specialized in boosting the potency of or lipids in cells is one of the most difficult therapeutic antibodies by glycosylatmolecular modifications to study because ing them in specific ways. And last year, sugar structures are so complex and variable. Merck paid $400 million for GlycoFi, a But researchers are steadily overcoming the Lebanon, N.H.-basedfirmthat developed obstacles involved, and their efforts have a yeast-based approach for optimizmade it increasingly clear that glycosylation, ing sugar structures on therapeutic as this class of reactions is known, plays key glycoproteins to improve the glycoroles in human health and disease. proteins' efficacy, duration of action, "Carbohydrate structures are absolutely and specificity. critical for biological activity," says profesThe latestfindingsand progress on sor of biochemistry and molecular biology glycan-related drug discovery was the Michael Pierce of the University of Georgia, focus of the Benzon Symposium on Athens. "So we can't ignore glycosylation." Glycosylation: Opportunities in Drug DeResearchers working at the interface velopment, in Copenhagen last month. The between glycosylation and drug discovery meeting was organized by carbohydrate indeed have been working hard to better unchemist Ole Hindsgaul and enzymologist derstand the physiological effects of changes Monica M. Palcic of Carlsberg Laboratory, in biomolecular glycosylation and to devise in Copenhagen, and glycobiologist Henbetter ways to diagnose and treat conditions rik Clausen and medicinal chemist Povl that arise when glycosylation goes awry. Krogsgaard-Larsen of Copenhagen University. The symposium was funded by Glycosylation goes wrong, for instance, the Alfred Benzon Foundation, a Copenin a number of human diseases caused by hagen-based organization that supports malfunctions in the synthesis or breakdown the medical and pharmaceutical sciences of complex sugar structures, or glycans. and related areas by sponsoring fellowScientists are trying tofindout more about ships and symposia. the mechanisms that underlie these conditions so drugs can be designed to treat patients that have them. Researchers have also been developingfinessein glycosylation chemistry to create libraries of drug candidates, to improve drug properties, and even to modify the behavior ofwhole cells. The first use of glycoproteins as drugs goes back to the 1980s, and recently major drug companies have been buying small biotech companies that specialize in creating glycoproteins with glycans tailored for specific biomedical jobs. LONGER ACTION Thefinancialscale Organon researchers of some of these acquiused a linker sitions attests to the (jagged chain) to growing importance conjugate insulin (top) of glycans in drug diswith a pentasaccharide (center) that binds the covery. In 2005, for explasma carrier protein ample, Roche paid about antithrombin III (ribbon $180 million to acquire structure). The conjugate Zurich-based GlycArt remains active longer in the bloodstream than insulin. Biotechnology, which WWW.CEN-0NLINE.ORG
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JULY 30. 2007
COVER STORY
that regulates the neuromuscular Prominent on the symposium's junction," Hart explained. "When agenda was a set of glycan-related that happens, your nerves don't • ·• conditions—congenital disorders communicate with your muscles. of glycosylation (CDGs)—that It's one of the best examples in cause human disease and death thefieldof glycobiology of the but are hardly recognized by the • ·• way sugars can regulate the func mainstream biomedical commu tion of a protein and cause human nity. CDGs are caused by genetic disease." defects that interfere with the Geneticist Robert J. Desnick of glycosylation of proteins in the FLAWED SUGARS CDGs are often-fatal glycosylation Mount Sinai Medical Center, New body. The first patients with this pathway diseases that cause a variety of errors in the York City, told Benzon attendees clinical syndrome were identified normal structures (left) of glycans on proteins (black about another important class of in 1980. Now, more than 35 CDGs curves), such as unoccupied glycosylation sites (center) glycan-related disorders, the ly are known. and missing sugars on some glycan branches (right). sosomal storage diseases (LSDs). "CDGs are underdiagnosed Gaucher disease, Fabry disease, because most physicians do not Pompe disease, and other LSDs are condi CDGs. Oral supplements of mannose are know about them," said Hudson Freeze, tions in which enzymes that break down director of glycobiology at the Burnham In effective for patients deficient in phosphoglycans for disposal are mutated or absent. stitute for Medical Research, La Jolla, Calif. mannose isomerase, and some patients The undigested biomolecules accumulate with defects in intracellular transport of "CDGs are incredibly rare diseases," in cell lysosomes, causing neurodegeneraactivated fucose respond to dietary admin commented Gerald Hart, director of bio tion and other clinical problems. istration of this sugar. logical chemistry at Johns Hopkins Uni In some LSDs, one or two amino acid Some cases of muscular dystrophy are versity School of Medicine, who chaired a also CDG-related, said Tamao Endo, leader substitutions cause a carbohydrate-de Benzon session. "The reason they're rare grading enzyme to misfold. Targeting such of the glycobiology research group at is that most people who have a genetic an enzyme with small-molecule drugs Tokyo Metropolitan Institute of Gerontol defect in any of the glycosylation pathways ogy. Researchers discovered in 1987 that called "chaperones" can sometimes tweak die while they're embryos." Patients who errant forms of the protein ct-dystroglycan it into a better fold and thereby enhance its survive are those who retain sufficient or a complete absence of the protein was activity considerably, Desnick said. residual glycosylation activity in the defec associated with muscular dystrophy. But Amicus Therapeutics, in Cranbury, N.J., tive pathway. is trying to develop chaperones as LSD Only about 600 patients worldwide have recently acquired evidence indicates that some forms of muscular dystrophy are treatments. The company currently has ever been diagnosed with CDGs. Symp caused not by problems with the protein candidate compounds in clinical trials for toms are mild to severe and range from Gaucher disease, Fabry disease, and Pompe neurological disorders and changes in body per se but instead with the sugars associ ated with it. disease. shape to blood-clotting problems and liver Many glycans express their functional "If α-dystroglycan is not properly glyco and kidney disease. roles byway of binding and molecularsylated, then it doesn't form the complex Therapies are available for only two
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JULY 30, 2007
COVER STORY
recent human clinical trial, galactosylation proved ineffective at preventing clearance of platelets that had been refrigerated for 48 hours. "Adding galactose promotes another platelet clearance mechanism," Hoffmeister concluded. "Sialylation is the next logical step," a clinical trial of which "will be performed in the near future," she said. Also, ZymeQuest, of Beverly, Mass., has licensed the O-GlcNAcshielding approach and is trying to develop it. The work shows that carbohydrates "are not just decorations but have a real impact on health care," Ernst said. Masking O-GlcNAcs on platelets "could be instantly important in solv ing a serious medical problem." Hart noted that increased GlcNAcylation—glycosylation with O-GlcNAc, a process his group discovered in 1984—promotes insulin resistance, a hallmark of diabetes, and that it helps protect cells from stress, inhibits protein degradation, and can either boost or curb gene transcription. GlcNAcylation thus
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diabetic glucose toxicity, cancer, and other conditions. "The ubiquitous roles of GlcNAc ylation in signaling, transcription, and cytoskeletal functions are an un explored avenue for drug discovery," Hart said. Monsanto and other com panies once had programs to develop inhibitors of O-GlcNAc transferase, the enzyme that catalyzes GlcNAcyla tion, he said, "but many of these were dropped." In part, this is because O-GlcNAc transferase is so essential, raising fears that inhibiting it could cause serious side effects, he said. But targeting drugs to specific OGlcNAc transferase interactions in a highly selective manner might reduce the potential for side effects, Hart said. For example, he noted, inhibiting an O-GlcNAc transferase interaction with a specific nuclear receptor activator might lower inappropriately high glu cose levels in diabetics without affect ing normal GlcNAcylation. Changes in the branching structure of cell-surface oligosaccharides can also
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COVER STORY
"Carbohydrate structures are absolutely critical for biological activity. So we can't ignore glycosylation." porter is reduced, its branching becomes less complex, and the glycoprotein is not retained on the cell surface. This causes a glucose-transport deficiency that makes
the cells unresponsive to glucose and un able to secrete insulin when they should, thereby causing a diabetes-like condition. Marth and coworkers found that high-fat
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diets also knock down the enzyme's ex pression, showing how such diets might lead to diabetes. Marth's group also recently demonstrat ed a way in which cell-surface glycans can act as immune indicators of self or nonself. The researchers found that reduced amannosidase-II activity changes the com position of vertebrate cell-surface glycans, favoring the incorporation of mannose at the expense of other sugars. The resulting high-mannose glycans resemble foreign (microbial or invertebrate) cell-surface glycans and can therefore elicit an immune response or cause an autoimmune disease like lupus. The mammalian forms of cell-surface glycans "keep us from looking like mi crobes and other organisms," Marth said. "Should we don the wrong glycan cloak, one that mimics these organisms, autoim mune disease can develop as our body tries tofightitself. It is a novel mechanism of autoimmune disease." Research on rheumatoid arthritis, an other autoimmune disease, reveals that carbohydrate recognition may play an important role there as well. Inflamma tion specialist Rikard Holmdahl of Lund University and coworkers found that rheu matoid arthritis can be induced in mice by injecting them with collagen with modified glycosylation patterns. Immune system Τ cells in the mice recognize the glycosyl ation variations, causing the autoimmune condition. Human rheumatoid arthritis patients were sensitive to collagen with the same types of glycosylation changes, suggesting that such structures could be starting points for the development of rheumatoid arthritis vaccines. In the future, the number of such glycan-focused drug targets is likely to grow considerably. "Probably when we look at carbohydrate-based drug discovery 10 years from now, we will smile about our efforts, just as we smile today about those from 10 years ago," Ernst said. "A number of carbohydrate-based drugs are now on the market, and thefieldis just explod ing in all directions. It's more and more fascinating. When you list all the thera peutic possibilities you can think of with carbohydrate-based drugs, it's absolutely endless." •
