Aptamer

Aptamers (from Latin aptus, fit and gr meros, region) are short single-stranded DNA or RNA oligonucleotides ( 25-70 bases) or peptides that can bind to a specific molecule about its 3D structure. Is that a peptide aptamer, one speaks of a peptide aptamer.

Description

Aptamers bind to proteins, such as growth factors and bacterial toxins, low-molecular substances, such as amino acids and antibiotics and also virus particles. Aptamers have dissociation constants in the pico - to nanomolar range. Accordingly, they bind with similar strength such as antibodies to their target molecules. This high specificity is achieved by the 3D structure of the oligonucleotide exactly to the binding partner around folds ( "adaptive binding "). The most important interactions in addition to the fit are electrostatic interactions, hydrogen bonding and base stacking ( engl. base stacking ).

Aptamers are artificially made ​​( in vitro), according to the criterion of the highest possible specific binding affinity. To do this, create large random libraries of oligonucleotides of different base sequence, in the order of up to several sequences per micromol. (In an oligonucleotide length of 40 to 70 bases significantly more variations would be possible to, but such amounts can not be produced ). From these sequences are the " systematic evolution of ligands by exponential enrichment " ( selection of ligands by exponential enrichment, SELEX ®) filtered out those that bind most strongly the desired molecule. The aptamer candidates are mixed with immobilized ligand and the unbound washed away. What remains are candidates who have a high affinity for the target molecule. This is propagated by PCR and begins a new cycle of binding and washing away the weaker bound candidates. After several passages to obtain a two- or oligonucleotides, which are referred to as aptamers.

Aptamers combine the favorable properties of small molecules and antibodies. These properties include inter alia

• High specificity and affinity
• Chemical stability
• Low immunogenicity
• The ability of the target protein -protein interactions

In contrast to monoclonal antibodies, aptamers are chemically synthesized and not biologically expressed, which represents a significant cost advantage in their synthesis. In the synthesis can be various modifications such as incorporation of fluorescent reporter molecules or affinity tags, are made. Is an aptamer conjugated to polyethylene glycol (PEG), such as may to prevent originally very small molecules are filtered too fast or excreted by the kidney. This allows the elimination half-life from the minute range significantly increase hours. The enzymatic stability of aptamers can be also by using chemically modified or stereochemically mirror-image nucleotides ( spiegelmers ) improve.

Applications

Through their ability to function of individual proteins in the cell off target, aptamers are regarded as promising molecular tools. Aptamers are used as therapeutics in medical diagnostics and environmental analysis.

Particularly in oncology are possible with aptamers promising therapeutic approaches. At present there is no drug -based aptamers for the treatment of cancer. Several clinical studies have however begun. End of 2006, based on the aptamer technology cocaine rapid test was presented.

Age-related macular degeneration ( AMD)

In December 2004, was approved in the U.S. pegaptanib as a drug, which since May 2006 after the European approval in February 2006 in Germany in the trade. It is approved for the treatment for wet age-related macular degeneration ( AMD). Pegaptanib is a 27-mer RNA aptamer, which was developed with the aim of highly specifically and with high affinity to the growth factor VEGF (Vascular Endothelial Growth Factor) to bind.