(49d) Using Nucleic Acid Aptamers to Modulate Specificity, Binding Affinity, and Drug Quantity to Create an Individualized Cancer Treatment Approach

Our work uses a gold nanoparticle core coated with DNA strands and aptamers in order to bind cancer cells via an avidity driven process. This results in the creation of a flexible and versatile nanocarrier with detection, drug carrying, and specific recognition capabilities. There are many advantageous properties of gold nanoparticles (AuNps) that can be applied to our DNA based targeting and drug delivery approach. Most notably their detection by x-rays and well studied binding of DNA with a thiol-bond modification. In addition, they are easy to synthesize, modify, characterize, and known to not elicit an immune response. We combine the properties of AuNps with recently developed DNA aptamers in order to target and treat cancer cells. This allows us to construct a nanocarrier with the ability to modulate quantity of drug and specificity towards targeting of distinct cancer cells.

The first step is to functionalize 15-nm gold nanoparticles with single stranded DNA strands (anchor DNAs) modified with a thiol group on the 5â?? end. Successful attachment of anchor DNAs is validated by surface plasmon resonance and gel electrophoresis. A 15-nm Np is chosen because with the addition of DNA strand attachment to the surface, the particle size is capable of both passive and active targeting. Previous work in our lab has shown that we can bind up to 101 +/- 8 of our anchor DNAs to a 15-nm AuNp using optimized experimental procedures. Since the theoretical maximum is 107 DNA strands, this means we are capable of attaching the entire range of DNA possible. By increasing the size of the AuNps, we can produce nanocarriers that carry more anchor DNAs, which will result in an increased number of aptamers and higher payloads of drug to the surface of the cancer cells. Alternatively, we can produce smaller nanocarriers that can be more readily taken up by cancer cells.

Cancer cell specific aptamers can be equipped with a single-stranded segment that is complementary to the single-stranded DNA anchor. Such design produces a general drug nanocarrier that can be readily programmed with either one or several different cancer-specific aptamers. Additionally, aptamer strands can be engineered to include drug-binding regions in desired locations and quantities. This is done by creating a double-stranded DNA region that will allow for intercalation of chemotherapeutic drugs, such as daunomycin. The length of this region determines how many drugs can bind. Furthermore, the release rate of daunomycin can be regulated using different triplet base pair sequences for varying binding affinity of daunomycin to the intercalating region. This property allows us to control the location, quantity, and release rate of the drug on our nanoparticle platform.

Presently, we have been able to attach a maximum of ~1,100 molecules of drug to a 15-nm gold nanoparticle with our current design, which is much greater than the average drug loading of any 15-nm nanoparticle found in the literature. This was accomplished due to our ability to control the specific amount of anchor DNAs bound to the Np surface, as well as the length of the drug-binding region. Current release studies have indicated that we are able to control the release rate of daunomycin through temperature and by manipulating the triplet base pair binding sequences. In vitro viability studies have indicated that our nanocarrier is more efficient at killing cancer cells than free drug at the same concentration. Additionally, our studies show that daunomycin is releasing from the nanocarrier.

Currently, we are investigating regulation of the specificity and binding constant of our nanocarrier by modulating use of the aptamers. By attaching multiple DNA aptamers, the resulting nanoparticle is expected to act as a â??molecular octopusâ? increasing the binding constant of our platform by utilizing avidity, and is able to target different markers on the surface of a heterogeneous cancer tumor. Our preliminary results indicate that by manipulating the size of the nanocarrier, the structure of the DNA aptamers, and its drug-binding segment, we have created an effective yet flexible, versatile, highly tunable platform that is capable of recognizing a specific cancer tumor, and is able to deliver controlled amounts of chemotherapeutics dictated by health status to individual patients.