N-ethoxybenzylimidazoles: a novel acid-sensitive linker for controlled release of therapeutics from drug delivery systems.

Abstract

Abstract Abstract #2156 Background:
 Drug delivery systems (DDS) are useful for cancer therapies because they can target tumors and cancer cells more specifically than cancer therapeutics alone. An important component of a DDS is the triggering mechanism for drug release. This presentation will highlight a new class of N-linked imidazoles as potential acid-sensitive cleavable linkers for use in cancer DDSs. These acid-sensitive linkers are designed to exploit the lower extracellular pH of some tumors and the endosomes and lysosomes within cells to trigger the controlled release of therapeutic agents from drug delivery vessels. Cleavage of N-ethoxybenzylimidazoles (NEBIs) exhibit a 10-fold increase in the rate of hydrolysis in mild aqueous acidic solutions (at pH = 5.5) compared to solutions at normal, physiological pH. The rate of hydrolysis can be tuned to range from minutes to months through the addition of electron donating or withdrawing groups.
 The NEBI linker can be used to conjugate a cytotoxic agent to a carrier. Carriers for this DDS include molecules such as tumor specific antibodies, polymers/nanoparticles, peptides, and ligands, that can target cancer cells or accumulate around tumors. Cytotoxic agents include FDA approved small molecules and chemotherapeutics.
 Methods and Materials:
 The NEBI was developed into a bifunctional crosslinker containing a carboxylic acid and an azide for conjugation of cancer therapeutics to carriers. For simplicity, we used Human Serum Albumin (HSA) as our model carrier and doxorubicin as our model cytotoxic agent. The NEBI was conjugated to doxorubicin through a simple amide coupling. The NEBI was then conjugated to an alkyne containing HSA via “Click Chemistry.”
 Results:
 We were able to load approximately 1-2 doxorubicin molecules per HSA. Imaging studies have shown that our HSA conjugates to doxorubicin via a NEBI linker (HSA-NEBI-dox) localize in lysosomes. Cytotoxicity studies show HSA-NEBI-dox is cytotoxic while HSA conjugates to doxorubicin via a stable linkage is not.
 Discussion:
 We were able to develop methods for conjugating doxorubicin to HSA. The techniques used here can be applied to conjugate other carriers and other cancer therapeutics. We will continue to develop the NEBI linker and demonstrate how the linker can be used to conjugate trastuzumab to doxorubicin. We chose use trastuzumab, a monoclonal antibody, as a carrier because it is known to bind to the HER2 receptor. HER2 is over expressed in 25%-30% of breast cancers. Trastuzumab not only targets cancer cells that are over expressing the HER2 receptor, but binding of trastuzumab to the HER2 receptor can trigger receptor mediated endocytosis, which can lead to controlled release of doxorubicin within the cell.
 
 Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 2156.