We are committed to the development of vehicles to safely and effectively administer therapeutic agents and to pursue new and improved disease detection technologies. To this end, we are currently pursuing research into ROMP-based polymer nanoparticles, polymer-caged liposomes, ROMP-based DNA detection, and fundamental DNA interactions.
ROMP-based Polymer Nanoparticles
There has been increased interest in replacing small-molecule drugs with nanoscale drug delivery vehicles due to the vehicles’ inherent advantages such as increased circulation time and bioavailability as well as higher specificity, resulting in less frequent dosing and the lowering of toxic side effects. Arguably, one of the most promising nanoscale drug delivery vehicles are polymer nanoparticles (PNPs). Effort in this area currently revolves around moving ROMP-based polymer nanoparticles (PNPs) towards a more applicable system for clinical research. We are aiming to create smart PNPs that can carry a high payload of multiple drugs in the core which are protected by an innocuous outer polymer shell that is functionalized with target-specific moieties and contrast agents- a “magic bullet” (Figure 1).
Figure 1: The Magic Bullet
In order to access PNPs with a high degree of drug loading, a versatile living polymerization technique is required. Ring-opening metathesis polymerization (ROMP) has been utilized to synthesize amphiphilic block copolymers composed of carbamate-linked doxorubicin and hexa(ethylene oxide)-substituted norbornene monomers. We have shown that these polymers will self-assemble into well-defined monodisperse PNPs in aqueous media and have shown efficacy in vitro (Figure 2). Additionally we have been able to incorporated imaging moieties and targeting groups on the particle surface. We are currently investigating the efficacy of our particles in vivo.
Figure 2: Doxorubicin-conjugated polymer nanoparticles (Dox-PNPs) incubated in human neuroblastoma cell lines. Doxorubicin fluorescence can be seen inside cells and after 24 h, inset shows inhibition of cellular growth by Dox-PNPs.
Biodegradable Polymer Nanoparticles
A promising improvement of the current PNP delivery system involves replacing the hydrocarbon backbone with a biodegradable polyester or polyamide. Because ROMP is used to synthesize the desired amphiphilic polymers (vide supra), ROMP active monomers, such as unsaturated lactones and lactams, are needed. There are three main characteristics of the desired monomers: (1) They must contain a strained olefin, such as a medium sized ring, (2) They must contain an acid or enzyme labile functionality such as an ester or amide group and (3) They must be modifiable at a position other than the ester or olefin such as alpha to a carbonyl. Very few example of such monomers exist in the literature today; however our group has recently developed a methodology for the synthesis of 7-membered unsaturated lactones. These relatively highly strained compounds (9-15 kcal/mol ring strain) undergo ROMP in highly concentrated conditions to yield the desired polyesters. Work is currently underway to optimize conditions for polymerization and apply the polyesters to the drug delivery system. Interestingly, because of the acidic byproducts of the polymer degradation, new and beneficial release profiles may be obtained as the small molecule therapeutic agent is released through acid catalyzed degradation of the linkage (vide supra).
Scheme 1: Retrosynthetic analysis to biodegradable ROMP monomers.
Polymer-Caged Liposomes
Another effort, in collaboration with the O’Halloran lab, is to enhance currently available technologies, such as liposomes, for drug delivery. We have developed surface-functionalizable environmental-responsive polymer-caged liposomes as stable liposome vehicles. These vehicles can be equipped with targeting groups such as folate for selective delivery of therapeutic agents to cancer cells (Scheme 2). These PCL-liposomes are currently being explored for their use as vehicles for the delivery of small-molecule chemotherapeutic agents such as doxorubicin and arsenic oxide.
Scheme 2: Synthesis and delivery of folate-conjugated PCL.
DNA Detection
As detailed above, the Nguyen group recently demonstrated that norbornene-based monomers with hydrophobic and hydrophilic functionalities can be copolymerized into amphiphilic block copolymers by ring-opening metathesis polymerization (ROMP). In aqueous media, these copolymers organize into core-shell polymer nanoparticles (PNP’s) with tunable diameters (90-300 nm). The focus of this project is on the development of a PNP-based probe such that signal amplification can be engineered into the probe to allow for readouts that are several orders of magnitude greater than those of the molecular probes commonly employed in DNA detection assays. Compared to current molecular probes, a PNP-based probe has the advantage of enfolding numerous polymer strands with each polymer strand containing a large block of reporter molecules. This significant increase of reporter molecules should give rise to an amplified signal from a single probe-binding event and remove the need for a physical amplification step. Towards this end, terthiophene (TTT) and tosylated-poly(ethylene glycol) (TsPEG) functionalized norbornene monomers have been synthesized. Respectively, the TTT and TsPEG modified monomers serve as the hydrophobic and hydrophilic blocks in the subsequently synthesized amphiphilic block copolymers. TTT is both fluorescent and redox active, thereby making it an ideal reporter moiety, while the reactivity of TsPEG facilitates DNA modification of the PNP’s periphery by amine terminated DNA. Following attachment of DNA to the PNP, the probes are used in a sandwich assay (Scheme 3).
Scheme 3: Sandwich assay using PNP probes
Small-Molecule DNA Hybrids (SMDH’s)
This project is focused on elucidating factors affecting the melting transitions of DNA aggregates. Aggregates of gold nanoparticle- and polymer-DNA hybrids show very interesting melting properties: increased melting temperature with a very sharp profile; however, the causes are not fully understood. We are currently using small molecule DNA hybrids (SMDHs) with designed structures to discern how the number of DNA strands and their collective geometries affect melting properties. With this in mind, a series of rigid small molecules based on a phenylacetylene motif have been synthesized and subsequently functionalized with oligonucleotide strands to form SMDH’s (Scheme 4). Aggregates are formed when an SMDH is duplexed with its complementary functionalized SMDH or polymer-DNA hybrid. The effect of the number of strands and their orientation on the melting properties of these aggregates has been compared to that of free-DNA duplexes.
Scheme 4: Small-Molecule DNA Hybrids (SMDH’s)