Nucleic acids are emerging as important drug targets and versatile therapeutic agents because they fold into complex three-dimensional structures capable of expressing many enzymatic activities and because they “digitally” interfere with the flow of genetic information from DNA to proteins. A number of studies demonstrated that nucleic acids also constitute attractive materials for nanotechnology because these macromolecules can be easily programmed to carry out specific functions through the incorporation of aptamers. These novel “smart” molecules can be selected from random pools of both RNA and DNA molecules based on their ability to bind metals, small organic compounds and even entire cells. Our strategy is to exploit the aptamer's response to different physical and chemical triggers such as temperature, affinity, pH, enzymes, to produce controlled and extended drug release profiles.
We investigated effects of derivatization of 15-nm gold nanoparticles (AuNPs) with single-stranded DNA. These molecules were covalently attached to the surface of AuNPs via thiol-gold bonds. The number of DNA molecules attached to AuNP (number density) is affected by both the salt and DNA concentration. We determined the optimal salt and DNA concentration for thiol-gold bonding as 0.5 M and 4 µM, respectively, by quantifying the number density as a function of salt and DNA concentration using 32P-labeled DNA derivatives. At optimal conditions, we were able to immobilize 101 +/- 11 DNA strands per 15-nm AuNPs. Changes in number density influenced the mean diameter of DNA derivatized AuNps which was determined using dynamic light scattering. The mean diameter of DNA-derivatized AuNPs was also affected by conformational changes of DNA strands. Our studies revealed that short DNA strands exist in a stretched conformation while longer DNAs adopt coiled conformation in AuNPs functionalized with single-stranded DNA of different lengths.
Single-stranded DNA was used as an anchor for attaching aptamers to AuNPs. In these experiments, we used a novel DNA aptamer that binds to the antibiotic neomycin. We tested two neomycin aptamer derivatives and their drug binding affinity constants were determined using Surface Plasmon Resonance (SPR). Upon washing the drug bound aptamer on the surface of SPR chip with water, we observed an increase in drug release with an increase in temperature. Complete release of neomycin from its DNA aptamer was observed at 41oC. This drug release can be attributed to a yet uncharacterized conformational change in the DNA aptamer. Temperatures higher than 60oC were needed to break bonds between the neomycin aptamer and its anchor. We also demonstrated that approximately 35 neomycin molecules can be carried by the 15-nm derivatized AuNP. Two DNA aptamer derivatives attached to gold nanoparticles via a DNA anchor displayed different temperature release profiles. On derivatizing the AuNPs with a combination of drug bound aptamer and derivatives, we obtained tailored affinity based drug release profiles.
These results suggest that a relatively wide-range of release profiles can be achieved from particle carriers. Also, multiple therapeutic release at different rates can be achieved from the surface functionalized nanoparticle carriers, which would be a significant advancement in the field.