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The Skaggs Institute
for Chemical Biology


Scientific Report 2008




Click Chemistry and Biological Activity

K.B. Sharpless, V.V. Fokin, J. Culhane, J. Fotsing, S. Grecian, N. Grimster, J. Hein, T. Horneff, J. Kalisiak, K. Korthals, S.-W. Kwok, S. Pitram, J. Raushel, B. Stump, J. Tripp, C. Valdez, T. Weide

We aim to discover new chemical processes that allow rapid and efficient synthesis of molecules with a desired function from diverse building blocks. Since 1996, the support of the Skaggs Institute for Chemical Biology has been instrumental to the development of the concept and the applications of click chemistry. Click chemistry embodies an attitude that function matters most and that tools that enable researchers to achieve function are to be prized. Thus, click chemistry relies on the use of a few near-perfect chemical reactions for the synthesis and assembly of specially designed building blocks. These building blocks have a high built-in energy content that drives a spontaneous, selective, and irreversible linking reaction with complementary sites in the reactive partners.

The power of click chemistry lies in the ability to rapidly generate novel structures that may not necessarily resemble known biologically active compounds. For discovering compounds that may be useful as drugs, this strategy provides a means for the rapid exploration of the chemical space. For optimization of compounds, the method enables rapid structure-activity profiling through generation of analog libraries. Click chemistry does not replace existing methods for drug discovery; rather it complements and extends them. It works well in conjunction with structure-based design and combinatorial chemistry techniques.

Click chemistry is both enabled and constrained by its reliance on a few nearly perfect reactions, and this characteristic raises concerns about limitations on the access of click chemistry to chemical diversity. However, the pool of druglike compounds may be as large as 1063. Currently, only a few million compounds that fulfill these criteria are known, implying that only an infinitesimal part of the potential medicinal chemistry universe has been explored so far.

These facts have staggering implications for drug discovery. First and foremost, most molecules with useful properties remain to be discovered. Second, the majority of useful new compounds likely will be found in unconventional structure space. Thus, with click chemistry, we have the interesting proposition that greater diversity can be achieved with fewer reactions, because it is not the number of reactions that is important, but the reach of the reactions, which is determined by the tolerance to variations in the nature of their components. Click chemistry approaches have already proved themselves in biomedical research, ranging from synthetic chemistry to bioconjugation strategies, polymer chemistry, and materials science.

Rapid Modification of Antibiotics to Overcome Resistance

Although macrolides, including erythromycin A have been widely prescribed for more than 50 years, the emergence of widespread bacterial resistance to these molecules is a serious and expanding problem. Third-generation macrolides, such as telithromycin, have been developed in recent years as effective means to overcome resistant bacterial strains. However, despite these efforts, only a few macrolide candidates with activity against methicillin-resistant Staphylococcus aureus (MRSA) have been identified to date. Clearly, the medical need for new antibiotics to combat strains of MRSA is urgent.

In our collaboration with scientists at the Kitasato Institute, Tokyo, Japan, we have reexamined the activity of various derivatives of erythromycin A against 12 types of gram-positive bacteria, including macrolide-resistant strains, and 1 gram-negative organism. We found that 11,12-di-O-iso-butyryl-8,9-anhydroerythromycin A 6,9-hemiketal has moderate activity against 4 strains of MRSA and 2 strains of vancomycin-resistant enterococci (VRE). Further modification of an alkynylated derivative of this lead compound by using the copper-catalyzed azide-alkyne cycloaddition, the flagship click reaction, quickly led to identification of several triazole-containing erythromycin A analogs with improved activity against MRSA and VRE strains (Fig. 1). These promising antibacterials are currently undergoing further evaluation.
Fig. 1. Novel erythromycin A derivatives with activity against strains of MRSA and VRE. MIC = minimum inhibitory concentration.

In Situ Click Chemistry

Although click chemistry allows rapid assembly of diverse collections of molecules that may serve as lead structures, further evolution of the molecules is traditionally achieved by iterative cycles of screening for biological activity and synthetic modification. Can direct involvement of the target, usually a specific receptor or enzyme, in the selection and evolution of possible drug candidates accelerate this drug discovery cycle? The aim of using in situ click chemistry is to engage an enzyme in the selection and covalent assembly of its own “best-fitting” inhibitor. Although the concept has been previously tested by several researchers, the in situ click chemistry approach is unique because it relies on the completely bioorthogonal 1,3-dipolar cycloaddition of organic azides and alkynes. This highly exergonic reaction produces 5-membered nitrogen heterocycles, 1,2,3-triazoles, which are exceedingly stable to acidic and basic hydrolysis and to severe reduction-oxidation conditions. At the same time, the triazoles produced can actively participate in hydrogen-bonding, dipole-dipole, and π-stacking interactions.

The efficacy of in situ click chemistry has been demonstrated by the discovery of novel, highly potent inhibitors of acetylcholinesterase, carbonic anhydrase, and HIV protease. During the past year, we extended the approach to more challenging targets. Among these are the enzyme β -secretase, which is involved in the progression of Alzheimer’s disease; several members of the vast kinase family; metalloproteases; and nicotinic acetylcholine receptors, the family of ligand-gated ion channels responsible for key neurotransmission events.

Publications

Finn, M.G., Kolb, H.C., Fokin, V.V., Sharpless, K.B. Concept and applications of click chemistry from the standpoint of advocates. Kagaku to Kogyo 60:976, 2007.

Hawker, C.J., Fokin, V.V., Finn, M.G., Sharpless, K.B. Bringing efficiency to materials synthesis: the philosophy of click chemistry. Aust. J. Chem. 60:381, 2007.

Kalisiak, J., Sharpless, K.B., Fokin, V.V. Efficient synthesis of 2-substituted-1,2,3-triazoles. Org. Lett. 10:3171, 2008.

Kwok, S.-W., Hein, J.E., Fokin, V.V., Sharpless, K.B. Regioselective synthesis of either 1H- or 2H-1,2,3-triazoles via Michael addition to α,β-unsaturated ketones. Heterocycles 76:1141, 2008.

Liu, Y., Dìaz, D.D., Accurso, A.A., Sharpless, K.B., Fokin, V.V., Finn, M.G. Click chemistry in materials synthesis, III: metal-adhesive polymers from Cu(I)-catalyzed azide-alkyne cycloaddition. J. Polym. Sci. A Polym. Chem. 45:5182, 2007.

Radic, Z., Manetsch, R., Fournier, D., Sharpless, K.B., Taylor, P. Probing gorge dimensions of cholinesterases by freeze-frame click chemistry. Chem. Biol. Interact. 175:161, 2008.

Sugawara, A., Sunazuka, T., Hirose, T., Nagai, K., Yamaguchi, Y., Hanaki, H., Sharpless, K.B., Omura, S. Design and synthesis via click chemistry of 8,9-anhydroerythromycin A 6,9-hemiketal analogues with anti-MRSA and -VRE activity. Bioorg. Med. Chem. Lett. 17:6340, 2007.

Van der Eycken, E., Sharpless, K.B. Click chemistry. QSAR Comb. Sci. 26:1115, 2007.

Vestberg, R., Malkoch, M., Kade, M., Wu, P., Fokin, V.V., Sharpless, K.B., Drockenmuller, E., Hawker, C.J. Role of architecture and molecular weight in the formation of tailor-made ultrathin multilayers using dendritic macromolecules and click chemistry. J. Polym. Sci. A Polym. Chem. 45:2835, 2007.

Yoo, E.J., Ahlquist, M., Bae, I., Sharpless, K.B., Fokin, V.V., Chang, S. Mechanistic studies on the Cu-catalyzed three-component reactions of sulfonyl azides, 1-alkynes and amines, alcohols, or water: dichotomy via a common pathway J. Org. Chem. 73:5520, 2008.

 

K. Barry Sharpless, Ph.D.
W.M. Keck Professor of Chemistry

Sharpless Web Site