Synthesis, Characterization, and Tumor Targeting of Novel pH Sensitive Nanoparticles

Abstract

When particles approach a nanoscale size, they exhibit novel properties not seen in their bulk counterparts. Therefore, nanoparticles are widely used in the diagnosis and treatment of cancer as imaging, delivery, and therapeutic agents. However, the main challenge is to obtain stable, biocompatible, and tumor-targeting nanomaterials so that they preferentially accumulate in cancerous tissue. Many contemporary therapeutic and imaging approaches to cancer treatment are based on specific tumor-targeting biomarkers such as antibody or vitamin binding. Although there are significant successes, the inter- and intra-heterogeneity of tumors, their complexity, and their ability to adapt to new conditions often create parts of a tumor that do not express enough of the biomarker to be targeted. On the other hand, extracellular tumor acidity, resulting from the metabolic alterations of cancer cells, attracts a lot of attention as a novel general tumor biomarker. pH (Low) Insertion Peptides (pHLIP® peptides), discovered in our laboratory, represent a novel class of pH-responsive tumor-targeting delivery agents. The molecular mechanism of a pHLIP® peptide’s action is based on the protonation of Asp/Glu residues, which enhances the peptide’s hydrophobicity and promotes the membrane-associated folding and insertion of a transmembrane helix. It was shown that pHLIP® peptides have excellent targeting of acidic tumors and specific delivery of imaging and therapeutic agents to cancer cells within tumors. Therefore, it is expected that coating nanoparticles with pHLIP® peptides would enhance the stability of nanomaterials, promote uptake of nanomaterials by cancer cells, and/or facilitate in the release of therapeutic and diagnostic molecules to the cytoplasm of cancer cells. The main goal of this work was the development of novel pHLIP® peptide coated nanomaterials for cancer therapy. More specifically, we introduced and investigated: a family of pHLIP® peptides (along with K-pHLIP, the pH-insensitive counterpart to pHLIP®); fusogenic, PEGylated liposomes coated with pHLIP® peptide for the delivery of therapeutic molecules; specific delivery to tumors of nanogold® clusters conjugated to the pHLIP® peptide; novel pHLIP® and PEG coated spherical and spiked gold nanoparticles A variety of rationally designed pHLIP® variants were characterized. It was shown how tuning the biophysical properties of the peptide–lipid bilayer interactions alter: i) tumor-targeting, ii) distribution in organs, and iii) blood clearance. Also presented, is the current understanding of the main principles of pHLIP® design. All pHLIP® coated nanomaterials in this work that have been administered to cancerous cells have shown a pH-dependent interaction in vitro and/or in vivo. A novel type of