Spiegelmers are ribonucleic acid (RNA)-like molecules built from the unnatural L- ribonucleotides. They are artificial oligonucleotides and their name is derived from the fact that they are stereochemically mirror images of natural oligonucleotides. Spiegelmers are a form of aptamers. Due to the use of L-nucleotides, they are characterized by high enzymatic stability. Spiegelmers are considered potential drugs and are currently being tested in clinical trials.

Chemical Properties

Spiegelmers are the stereochemical mirror images ( enantiomers ) of natural oligonucleotides made using L-nucleotides. Nucliec acid aptamers, including Spiegelmers, contain adenosine monophosphate (AMP), guanosine monophosphate (GMP), cytidine monophosphate (CMT) and uridine monophosphate (UMP), a phosphate group , a nucleobase and a Ribose sugar. By replacing the natural D-ribose with its enantiomer, L-ribose, artificial L-nucleotides which are the building blocks of Spiegelmers, can be made.

Biological characteristics

Like other aptamers, Spiegelmers are able to bind molecules such as peptides , proteins and low molecular weight substances. The affinity of Spiegelmers to their target molecules often lies in the pico-to nanomolar range and is thus comparable to antibodies.

Spiegelmers themselves have low antigenicity. In contrast to other aptamers,Spiegelmers have high stability in blood serum since they are less susceptible to be cleaved hydrolytically by enzymes. Nonetheless, they are excreted by the kidneys in a short time due to their low molar mass ( which is below the renal threshold).

Spiegelmers modified with a higher molar mass, such as pegylated Spiegelmers show a prolonged plasma half-life.

Production

Unlike other aptamers Spiegelmers are not directly made using the conventional systematic evolution of ligands by exponential enrichment (SELEX) method. This is because L-nucleic acids are not amenable to enzymatic methods, such as polymerase chain reaction ,used in SELEX.Therefore, the selection is done with mirrored target molecules. The process is briefly outlined below.

Reflection of the target molecule

The first step is the production of a mirror image of the target.In the case of peptides and small proteins that are produced synthetically an enantiomer is made using synthetic D-amino acids.If the target is a larger protein molecule, beyond synthetic abilities, the mirror image of an epitope is produced.

SELEX

Conventional (up to 10¹⁶different oligonucleotides) existing molecule library serves as a starting point for the subsequent SELEX process.Selection,separation and amplification using the mirror image of the target molecule is performed.

Sequencing and Synthesis

The sequence of the oligonucleotide selected using SELEX is determined with the help of DNA sequencing. This information is used for the artificial synthesis of the mirror image of the oligonucleotide, the Spiegelmer, using L-nucleotides.

Usage

Spiegelmers have been obtained for the chemokines CCL2 and CXCL12 , the complement component C5a and ghrelin.They are currently in preclinical or clinical development. They can also be used as diagnostic agents .