DNA Linking—Novel Chemistry

The use of DNA to link together dendrimers signifies the potential for developing highly specialized drug delivery systems. We continue to make significant progress in the development of a polyamidoamine (PAMAM) dendrimer-based drug delivery system to target cancer cells. A team of multidisciplinary scientists in M- has proved that the nanometer-scale dendrimers have been engineered to be biologically functional, such as in targeting with cancer drugs (Methotrexate) cancer cells overexpressing receptors (folate or Her-2). But making these types of multifunctional nanoparticles is notoriously difficult and time-consuming, requiring months for each new molecule added to the dendrimer in sequential steps. Young Seon Choi, a Biomedical Engineering graduate student in collaboration with Dr. Thommey Thomas and Alina Kotlyar in the M-, addressed this issue by designing nanoparticle clusters comprised of two different functional dendrimers, one for imaging and the other for targeting cancer cells. Young Seon Choi earned his Ph.D. degree in August 2005 and is now a Research Fellow at the University of Florida, Gainesville.

Each of the dendrimers also carried a single-stranded, non-coding DNA. These dangling lengths of DNA, typically 34-66 bases long, found complementary sequences on other dendrimers and knitted together, forming barbell-shaped, two-dendrimer complexes with folate on one end and fluorescence on the other end. With this approach, a library of single-functional dendrimers such as targets, drugs, and contrast agents can be synthesized in parallel, rather than sequentially, and then linked together in many different combinations with the DNA strands. In the future, this bi-functional dendrimer cluster system can be readily expanded or adapted to exploit various cancer-specific biomarkers. Having an array of single-functional dendrimers, such as targets, drugs, and contrast agents, and the ability to link them together quickly and easily in many different ways would enable a clinic to offer customized multifunctional nanomedicine based on this dendrimer cluster system.

binding of DNA-linked functional dendrimers to KB cells after 1 hour incubation
Binding of DNA-linked functional dendrimers to KB cells after 1 hour incubation. The concentration-dependent saturation curve indicates specific binding of the DNA-linked cluster to KB cells over-expressing folate receptor.
binding of DNA-linked functional dendrimers to KB cells after 1 hour incubation
Competitive inhibition of the cluster agent (40 nM) by free FA (5 M).
confocal microscopic images of K B cells treated with D N A linked cluster of G 5 f i t c and G 5 f a dendrimers
Confocal microscopic images of KB cells treated with DNA-linked cluster of G5-FITC and G5-FA dendrimers (40 nM). Left panels show differential interference contrast (DIC) images which correspond to fluorescent images (right panels) of the same slide with KB cells. Compared to the control KB cells in PBS (A, B), the KB cells treated with the DNA-linked functional dendrimer cluster show the presence of the FITC signal inside the cells, indicating the binding and internalization of the cluster agent over 1 hour of incubation.