1. Biodelivery: Polymer Nanocarriers for Targeted Drug Delivery and Gene Delivery to Cancer

Drug Delivery

Cancer has dethroned heart disease as the top killer among Americans under the age of 85. Most patients, although initially responsive, eventually develop and succumb to drug-resistant metastases. For example, the success of typical postsurgical regimens for ovarian cancer is limited by primary tumors being intrinsically or becoming refractory to treatment. First-line treatment yields about 30% complete pathologic remission and an overall response rate of 75%, but the disease usually recurs within 2 years of the initial treatment. Thus, drug resistance is a major obstacle to the successful cancer chemotherapy, particularly at advanced stages.

Cancer cells have many intrinsic and acquired drug resistance mechanisms to mitigate the cytotoxic effects of anti-cancer drugs (Figure 1). These mechanisms include the loss of surface receptors or transporters to slow drug influx, cell-membrane-associated multidrug resistance to remove drugs, specific drug metabolism or detoxification, intracellular drug sequestration, overexpression of Src tyrosine kinase and splicing factor SPF45, increased DNA-repair activity, altered expression of oncogenes and regulatory proteins and increased expression of antiapoptotic genes and mutations to resist apoptosis, and etc.

Our research in this area is focused on using active nanocarriers to deliver drugs to the specific subcellular targets to overcome cancer drug resistance for high therapeutic efficacy. Generally, we start from design and synthesis of new stimulus-responsive multifunctional polymers and fabrication of programmed or active nanocarriers. These nanocarriers are then tested in vitro and in vivo.

Figure 1. Illustration of some drug resistance mechanisms of cancer cells

The fist system is cancer-targeted lysosomal triggered fast release nanoparticles (Figure 2). In vitro and in vivo evaluation shows that drugs in these nanoparticles have higher anticancer activity than free and conventional nanoparticle-encapsulated drugs. This work is highly recognized as one of the four “most intriguing” work of 2006-2Q selected by CAS from over 200,000 documents per quarter.

Figure 2.The numbers of tumors on intestine/ mesentery (per cm2) of the nude mice. Cisplatin dose was 10 mg/kg/treatment. Mice were treated twice at fourth and the fifth weeks after inoculation of SKOV-3 cells. Data represent mean value ± S.E.

The second system is nuclear localization nanoparticles for nuclear drug delivery (Figure 3). The central hypothesis is that delivery of drugs to the immediate vicinity of the anticancer drug targets ? the nuclear DNA? can circumvent both of the cell-membrane associated multidrug resistance and the intracellular drug resistance mechanisms. The big challenge is how to activate the nuclear localization agents only inside cancer cells. We developed a charge-reversal technique and successfully solved the problem (Angewandte Chemie International Edition, 2007, 46, 4999-5002). Highlightedhttp://www.nanowerk.com/spotlight/spotid=2113.php

Figure 3. Nuclear localization of the PEI-based charge-reversal nanoparticles observed by confocal scanning laser microscopy after cultured with SKOV-3 cells for 24 h at 37 oC. The nuclei were stained with DRAQ5 (blue). The nanoparticles loaded with PKH26 were assigned to red. Pink spots were nanoparticles colocalized in the nuclei.
Gene Delivery

In polymer-mediated gene delivery, cationic polymers generally complex plasmids to compact them into nanoparticles and to shield their negative charges for effective cellular internalization. Tight packing is also needed for DNA trafficking to the nucleus and protection from degradation by enzymes. However, this tight complexation has been found as one of the major barriers to efficient DNA transcription because in the nucleus the complexed DNA is inaccessible for the transcription machine. Facilitated dissociation of the complexes using short, reversibly crosslinked, degradable, or low positively-charged cationic polymers or charge-reversible amphiphiles has been shown to significantly enhance transgenic efficiency.

Our research in this area is rational design of polymers that can deliver loosely packed or even free DNA (Scheme 1 and Figure 4) into the nucleus for high transfection efficiency. Our ultimate goal is to develop polymer gene therapy for cancer or other diseases.

Scheme 1. Virion-mimicking nanocapsule formation via a pH-controlled hierarchical self-assembly of the PCL/PDEA/PEG terpolymer brush and DNA. The PDEA chains were positively charged by protonation at pH 5 (a); They complexed with DNA and formed a hydrophilic core; the hydrophobic PCL chains collapsed on the core, forming a membrane surrounding the core; the hydrophilic PEG chains were incompatible with the hydrophobic PCL layer and thus were extended in the aqueous solution, forming the hydrophilic outer layer (b); After the solution pH was raised to pH 7.2, the PDEA chains were deprotonated, became neutral and insoluble, and thus dissociated from the DNA, leaving free DNA in the core (c) (Angewandte Chemie International Edition, 2008, 2008, 47:1260-1264).
Synthesis and applications of biodegradable dendrimers

Polyester dendrimers are attractive for in vivo delivery of bioactive molecules due to their biodegradability, but their synthesis generally requires multistep reactions with intensive purifications. A highly efficient approach to the synthesis of dendrimers by simply “sticking” generation by generation together is achieved by combining kinetic or mechanistic chemoselectivity with click reactions between the monomers. In each generation, the targeted molecules are the major reaction product as detected by MALDI-TOF MS. The only separation needed is to remove the little unreacted monomer by simple precipitation or washing. This simple click-like process without complicated purification is particularly suitable for the synthesis of custom-made polyester dendrimers. Currently, we are further improving this method for accelerated synthesis and using the dendrimers in in vivo gene and drug delivery as well as the magnetic resonance imaging (Journal of American Chemical Society 2009, 131 (41), 14795–14803).

Scheme 2. Sequential click coupling of asymmetrical monomers for facile polyester dendrimer synthesis