Dendritic Block Copolymer Micelles as New Targeted Drug Delivery Systems

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A barrier to the effective delivery of cancer chemotherapy drugs is the transport of the toxic drug to specific tumor cells, ensuring high uptake of drug by cancer cells while avoiding noncancerous ones. One means of accomplishing this method is the use of ligand groups added to the exterior of a nano-scale drug carrier; however, much is not understood about maximizing these ligand interactions to achieve orders of magnitude improvements in efficacy and reduced side effects. The primary objective of this work is the synthesis of new amphiphilic linear-dendritic block copolymers that self-assemble in the solution state to generate stable micelles with highly branched, dense dendritic groups in the exterior shell. Due to the unique nature of the dendritic outer block, these micelles will act as vessels with a highly tunable 3D presentation of ligand, enabling the creation of delivery nanoparticles with homo- or heterogeneous surfaces that enable cluster presentation of ligand. Biological studies of cellular interactions with ligands indicate that not only the valency but spatial factors such as branching mode and the localized clustering of groups are important in influencing binding and downstream signaling processes. This important aspect of targeting may have potentially significant impact on all kinds of nanoparticle delivery systems, from liposomes to inorganic nanoparticles. If this capability can be transferred to drug delivery vehicles, it may greatly impact efficacy and specificity of targeted delivery. Preliminary findings with the proposed systems suggest that ligand clustering can lead to significantly higher micelle cellular uptake compared to homogeneous distribution of ligand. The first specific aim is the synthetic optimization of a linear-dendritic diblock copolymer series generated in the Hammond group, consisting of a biocompatible linear polypeptide hydrophobic block and a hydrophilic polyester dendron to which targeting ligands will be conjugated via PEG linkers. Linear backbone modification, direct drug conjugation and photocrosslinkable groups within the hydrophobic core will be investigated to enhance drug loading and sustained release of drug. Micelle stability and drug release studies will be performed to determine release characteristics of each system under physiological and endosomal conditions. Results of these studies will be used to guide further synthesis of the copolymer and to ultimately select the most promising copolymer systems for further study. The second specific aim of this work is to examine the role of valency and ligand clustering through the use of mixed micelle clusters to significantly enhance efficacy in the delivery of cytotoxic drugs in vitro, with specific emphasis on peptide sequence, LyP-1, discovered using phage display methods, which exhibits a highly specific recognition to p32 surface receptors on a subset of tumor cells and tumor lymphatic cells including breast carcinomas. This system will be of particular therapeutic and clinical relevance, and will be used to address the universality of impact of valency and clustering in ligand presentation by extending the concept to different receptor types. Results of this Aim will be compared with recently obtained results from folate targeted micelles to determine differences in the significance of clustering. In the third specific aim, the best micellar-drug nanoparticle formulations from this analysis will be evaluated using an in vivo animal model. These studies will focus on both folate and LyP-1 as specific tumor ligands with potential clinical interest, with a focus on pharmacokinetics, biodistribution as a function of time, organ specificity, accumulation of drug carriers, intratumor uptake, and tumor inhibition as well as cytotoxicity.