Aptamers
What are aptamers?
Aptamers are short single-stranded DNA molecules that bind to desirable targets with high affinity and high specificity by folding into tertiary structures. Aptamers are obtained via a standard method called Systematic Evolution of Ligands by EXponential enrichment (SELEX). Almost all the aptamers that are selected through SELEX exhibit high affinity to targets with a dissociation constant (Kd) in the micromolar to nanomolar range. Therefore, aptamers are also denoted as “chemical antibodies”, which show excellent potential for drug delivery, molecular probes development, biosensor and tumor biomarker discovery.
SELEX process
SELEX starts by binding a pool of aptamers to the target cells. The aptamer pool consists of 10^(13) random synthetic 40-nucleotide long single-stranded DNA sequences. Aptamers bound to target cells can be enriched after 10 – 15 rounds of PCR amplification. Our company has used a modified SELEX process called Cell-SELEX for selecting our aptamers that are targeting EBV-infected cells. In combination with in vivo-SELEX, the identified aptamers have been further selected based on their affinity and their accumulative potential to EBV-positive tumors in mice models.
Chemodrug
conjugation
Chemotherapy drugs, such as gemcitabine (Gem) and fluorouracil (5-FU), have similar structures to natural nucleotide and can be enzymatically incorporated into the aptamers. These therapeutic drugs can be released in situ after hydrolysis of the aptamers and escape from the lysosomes by active nucleotide transporters, such as equilibrative nucleoside transporter 3 (ENT3). The conjugation of Gem and 5-FU into our EBV-targeting aptamers creates bifunctional aptamers with both enhanced binding affinity and toxicity.