Targeted and stimuli-responsive polymers as chemotherapeutic delivery systems
Author(s)Zaman, Noreen Tasneem
Massachusetts Institute of Technology. Dept. of Chemical Engineering.
Jackie Y. Ying.
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Successful administration of chemotherapeutic agents for cancer treatment requires a balance between the efficacy and the safety of the drug. This often limits physicians to a very narrow therapeutic window. To avoid the harmful side-effects, chemotherapeutic agents may be administered at a suboptimal dose. This is not only a less effective treatment, but can lead to the development of drug resistance by cancerous cells. The therapeutic window can be increased through targeted, stimuli-responsive delivery, which increases the drug concentration at the diseased site, and releases or activates the drug only when it reaches the target. Cancer is a highly variable disease occurring in many organs. There is a need for delivery systems that are easily adaptable for a number of targets in different forms of cancers, and that can accommodate various cytotoxic drugs. The motivation of this project was to develop flexible synthesis procedures for the targeted delivery of chemotherapeutic agents. In this work, we have synthesized and tested three drug delivery systems. The first system is a dextran-based polymer conjugate designed to preferentially deliver doxorubicin to hepatocytes. Doxorubicin has been conjugated to dextran of different molecular weights, with varying degrees of galactose substitution. The degree of doxorubicin substitution was maintained by performing the conjugation of doxorubicin and galactose in two sequential steps. The synthesis scheme was simple, efficient and easily adaptable to other therapeutic agents and targeting moieties with free amine groups. In cell culture studies on target hepatocytes, the dextran-doxorubicin-galactose (DDG) conjugates showed lower toxicity compared to doxorubicin, increased toxicity with higher molecular weight polymers, and greater toxicity with higher degree of galactose substitution.(cont.) Experiments in the control cell lines showed increased toxicity for higher molecular weight polymers; however, there was no effect due to the presence of galactose. At diameters of 15-40 nm, the polymer conjugates were too large to enter the cell nuclei in large quantities; however, a sufficient amount of doxorubicin entered the nuclei to cause cell death. The higher molecular weight polymers were more effective as they had a higher chain loading of doxorubicin. In spite of significant uptake of the targeted conjugates, the cytotoxicity of the first system was limited since the doxorubicin remained attached to the polymer. For the second system, pH-sensitive dextran-doxorubicin conjugates of different molecular weights were synthesized. The doxorubicin was attached to the dextran backbone through a hydrazone bond. These polymer conjugates were stable at a physiological pH of 7.4, but released over 70% of the attached doxorubicin within 24 h at a pH of 5.0. The rate of release was found to be faster for the lower molecular weight polymers. In cell culture studies, the conjugates showed significant cytotoxicity. The effect of lower chain loading of doxorubicin for the lower molecular weight polymers was offset by the rapid initial release; these polymers showed slightly greater toxicity. Live confocal microscopy indicated that the conjugates were internalized by cells within minutes after incubation. Since release of doxorubicin from the conjugates was much slower than cellular trafficking, it is possible that the conjugates went through multiple endocytosis and exocytosis cycles before the doxorubicin was released. Doxorubicin from the dextran-hydrazone-doxorubicin (DHD) conjugates was found to localize almost exclusively in the nuclei of cells.(cont.) Since doxorubicin attached to dextran with a stable bond showed limited localization in the nuclei, this indicated that doxorubicin from the acid-labile conjugates was released after internalization by cells. The cytotoxicity of the DHD conjugates was significantly greater than the stable DDG conjugates due to the release of doxorubicin inside cells. In the third system, the targeting and pH-sensitivity functionalities were combined by expressing galactose on an amphiphilic, temperature- and pH-sensitive copolymer of Nisopropylacrylamide (NIPAAm), N,N-dimethylacrylamide (DMAAm) and 10-undecenoic acid (UA). The polymer self-assembled in aqueous medium, and was used to encapsulate paclitaxel. Various synthesis parameters were adjusted to yield polymers that achieved high drug loading and rapid release in a temperature- and pH-responsive manner. The appropriate lower critical solution temperature (LCST) was obtained by adjusting the content of DMAAm and UA to change the hydrophilicity of the polymer. The hydrophilicity of UA was dependent on pH and thus, made the polymer pH-sensitive. Galactose was attached to the end-group of the copolymer to target it to hepatocytes. The drug loading, particle size and release rate were affected by the polymer molecular weight. Paclitaxel was encapsulated in particles that released nearly 100% of the drug within 24 h at a pH of 5.0 at 370C. In the target hepatocyte cell line, the stimuli-responsive, galactose-expressing particles were significantly more toxic than the non-stimuli-responsive as well as the non-targeted particles. In the control cell line, the presence of galactose did not have any effect on cytotoxicity. In summary, we have synthesized three targeted drug delivery systems. The DDG conjugates successfully targeted hepatocytes by expressing galactose. The DHD conjugates would be retained in tumors due to the enhanced permeability and retention effect, and release the drug at the target site.(cont.) The galactose-targeted paclitaxel-loaded particles synthesized from the temperature- and pH-sensitive polymer achieved a remarkable increase in toxicity in the target cell line, while maintaining base toxicity in the control cell line. Both the amount of drug delivered and the rate of release were found to be important in the efficacy of the drug delivery vehicles.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2008.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Chemical Engineering
Massachusetts Institute of Technology