Improvements to target-guided synthesis herald novel enzyme inhibitors
The hunt for small-molecule inhibitors of enzyme function remains an immensely wasteful process. Now, US researchers are using the molecular structure of target enzymes as a scaffold to assemble inhibitors. This works for several enzymes, they report.
Unwanted side-reactions have hindered the use of target-guided synthesis, but Barry Sharpless' team at the Scripps Research Institute, La Jolla, US, has introduced a non-biological mechanism designed not to interfere with enzyme function.
The method, termed in situ click chemistry, uses the cycloaddition of azides with alkynes as the 'click' mechanism that locks two halves of the desired structure together. Thus, two molecules with weak affinity to different structural features of the target enzyme can be linked to produce a high-affinity bivalent inhibitor. A combinatorial approach including different lengths of hydrocarbon linker chains between the functional sites and the clicking parts allows chemists to screen several combinations for the optimal geometric arrangement.Sharpless' team used the method to find a new inhibitor of the enzyme acetylcholinesterase (AChE), which plays a key role in the transmission of nerve signals. The inhibitor, syn-TZ2PA6, originated from a tacrine azide moiety including a 2-carbon linker, combined with a phenylphenanthridinium alkyne building block including a chain of six carbons. A crystal structure of the inhibitor reveals a multivalent binding mechanism, with the tacrine moiety binding inside the active site cleft, the phenanthridinium recognising a peripheral site, and the triazole function resulting from the click reaction engaging in specific hydrogen bonding with amino acid residues lining the active site cleft.1
Hartmuth Kolb's group, also at the Scripps Research Institute, has collaborated to extend the range of the method and identified three additional AChE inhibitors.2 The group has expanded this work to the metabolic enzyme carbonic anhydrase (CA), a drug target in the treatment of glaucoma. Of 24 theoretical combinations, 11 compounds were identified as hits as they formed in the presence but not the absence of CA. All 11 were found to bind the enzyme more tightly than the original reactants. They also compared favourably with the CA-inhibitors currently used as drugs.3
1 Y Bourne et al, Proc. Natl. Acad. Sci. USA, 2004, 101, 1449
2 R Manetsch et al, J. Am. Chem. Soc., 2004, 126, 12809
3 V P Mocharla et al, Angew. Chem. Int. Ed., 2005, 44, 116