Drug delivery using poly(ethylene glycol) derivatives


Drug delivery

(1) Tumor-Targeted Byfunctional Nanoparticles for Cancer Therapy and Tumor Imaging

Cancer ranks as one of the leading causes of death in industrialized countries. Cancer is defined by three characteristics: (1) A group of cells displaying uncontrolled growth; (2) These cells then intrude and destruct adjacent tissues; and (3) In some cases, spreading to other locations in the body via lymph and/or blood. Despite significant efforts in the field of oncology, the cancer therapy remains a challenge. Chemotherapy is primarily used in treating hematological malignancies and metastasized tumors. A severe problem with cancer chemotherapy is that chemotherapeutics cause terrible damage to normal tissues when used at doses required to eradicate cancer cells. Thus, strategies to selectively attack cancer cells could significantly enhance the therapeutic efficacy and diminish their toxicity to the body.

Nanoparticle carries have been widely studied for selective delivery of drugs to cancer tumors through two mechanisms: (1) passive targeting; and (2) active targeting. Passive targeting refers to the accumulation of drug or drug-carrier system at a particular site due to physicochemical/pharmacological properties of cells/tissues. Nanoparticles were found to have enhanced permeability and retention in solid tumors, referred to as EPR effect. Unlike the tight blood vessels in most normal tissues, adjacent endothelial cells lining the interior surface in blood vessels to tumors have gaps as large as 600-800 nm. Drug carriers in the nanometer size range can extravasate through these gaps into the tumor interstitial space. Tumors have impaired lymphatic drainage, thus the carriers accumulate in tumors. Active targeting to tumor can be achieved by ligand-receptor or antibody-antigen interactions. This may also lead to receptor-mediated cell internalization of drug carrier system. Nanoparticles offer versatility for targeting tumors through surface engineering for specific interactions of ligand-proteins/receptors expressed on cancer cells.

A number of polymers have been tried in formulating biodegradable nanoparticles. Among them, polylactide (PLA) and poly (D, L-lactide-co-glycolide) (PLGA) have been most extensively studied for drug delivery applications. PLGA-PLA-based polymers have a number of desirable properties for drug delivery including their biodegradability, biocompatibility, and approval of use by FDA for human body. To have the nanoparticles remain in the blood circulation for long is needed for their extravasations in the tumor vasculature. Thus, it is essential to avoid their engulfment by the reticuloendothelial system (RES). It was found that particles smaller than 100 nm and coated with hydrophilic polymers such as polyethylene glycol (PEG) are effective in avoiding their uptake by the RES.

Imaging of cancer tumor is the key for guiding clinical treatment and monitoring the efficacy of therapeutics. The use of tumor-targeted nanoparticles for image contrast and enhancer can identify specific biomarkers, in addition to their passive accumulation in cancer tissue. This has the potential to improve detection, classification, and treatment of cancer. Cancer detection through procedures such as Magnetic Resonance Imaging (MRI) and optical-based imaging have been improved by the use of these nanoparticles.

In searching for bifunctionalized anticancer drugs, we found that anticancer alkylating agent, such as chlorambucil, after conjugated with tempol has synergized potency to cause cell death compared with the original anticancer agent. Besides their role in increasing the drug's potency, the nitroxide radical in tempol also provides a magnetic probe for EPR (electron paramagnetic resonance), NMR (nuclear magnetic resonance) and MRI (magnetic resonance imaging) characterizations. Molecular interaction of the bifunctionalized anticancer drug with DNA studied using NMR techniques and delivery of the drugs with tumor-targeted nanoparticles and cancer tumor MRI imaging are the current focus of this research. Long term goal of this research is to invent innovative drug delivery systems for cancer chemotherapy.

Synthesis of chlorambucil-tempol adduct.

Overlay of NOESY (blue) and TOCSY (red) 2D NMR spectra to assign the chemical shifts of self-complementary 5'-GAAGTACTTC-3' dsDNA in a 50% D2O, 50% d3-CAN solvent.

Model of chlorambucil-Tempol adduct interacts with 5'-GAAGTACTTC-3' dsDNA derived from NEOSY and TOCSY NMR. The nitrogen mustard alkylating group interacts with G(N7) and tempol moiety interacts with sugars near the DNA backbone.

(2) Hydrogel materials as drug delivery depot

Self-assemblies of fluoroalkyl-ended poly(ethylene glycol) (Rf-PEG) and its variation are studied as drug-delivery system. Rf-PEG hydrogel was designed with properties of biocompatibility and biodegradability. The hydrophobic cores of the micelles formed by the fluoroalkyl groups ended on both sides of the PEG chains hold together the physical network in an aqueous environment. Rf-PEG can dissociate in biocompatible organic solvents to produce a low viscosity, injectable formulation. The coexistence of the gel phase and the sol phase predicts the potential for the Rf-PEG system to be developed as drug delivery depot to the body with controlled and sustained release pattern. The purpose of our research is to elucidate the fundamental mechanisms of drug delivery by means of NMR techniques of relaxation, self-diffusion and microimaging for designing advanced Rf-PEG hydrogel drug-delivery system.

Mechanism of electron spin induced 19F relaxation NMR.

Result of molecular dynamics modeling using proximity restrains from electron spin induced 19F relaxation NMR. The space-filled yellow-colored molecule shows the location of chlorambucil-tempol anticancer drug in the IPDU shell in a ball and stick notation for the Rf-IPDU-PEG units.

Proliferation assay of human KB cancer cell showing the controlled release
of chlorambucil-tempol adduct (CT) from the Rf-PEG hydrogel.

Pulse sequence of Pulsed Field Gradient Spin Echo (PFGSE) diffusion NMR (up)
and the schematic presentation of diffusion reduced NMR signal.

Chemical structures of anticancer drug 5-Fluorouracil (FU) and its
hydrophobic analog 1,3-dimethyl-5- fluorouracil (DMFU).

Echo attenuations for FU and NMFU in water, PEG and 6KC6 Rf-PEG showing the
location confined diffusion of MDFU in the Rf-PEG hydrogel.

NMR Microscopy of water diffusion to 6KmC6 Rf-PEG compressed tablet (both images above).