Targeted delivery plays an essential role in the detection, diagnosis and treatment of life-threatening diseases, including cancers1-3
. A challenging aspect facing the delivery strategy relates to the timely control of the drug release after uptake by the targeted cell1, 4-7
. Most of the release mechanisms currently being explored involve chemical and enzymatic reactions which are triggered passively under the influence of specific internal cellular factors (). We have been interested in designing an orthogonal release approach in which an external tool such as light8-13
is applied to actively trigger the drug release. The present study aims to investigate a photon-based external approach for the release of methotrexate (MTX) as the model cancer drug.
A proposed schematic illustrating the concept of cancer targeting drug delivery. The drug release can be controlled by the mechanism triggered by an endogenous factor (low pH, reduction, enzymes), or an external tool such as light.
MTX belongs to a class of antifolate molecules and constitutes one of the clinically approved anticancer drugs1, 16-17
. It is a potent inhibitor (Ki
= 0.058 nM16
) of dihydrofolate reductase (DHFR18-19
) localized in the cytoplasm. Despite its proven therapeutic value in the treatment of certain cancers20
and rheumatoid arthritis21
, MTX suffers from its non-selective cytotoxicity that contributes to lower its therapeutic index 17
. As the approach frequently applied to overcome such a therapeutic limitation, targeted delivery provides a route for facilitating the MTX uptake by a cancer cell, and as a consequence, for enhancing its therapeutic index4, 6-7, 22
. This strategy relies on a macromolecular system designed in a rational way such that the therapeutic payloads are carried by a system that is conjugated with a targeting ligand for binding to the cancer cell5-6, 23-27
. Such delivery strategy has already led to a number of successful applications for MTX5, 28
and for other anti-cancer therapeutic agents represented by cisplatin6
, and paclitaxel4, 23
. In each of these cases, the drug molecule is delivered by a nanometer-sized carrier based on dendritic macromolecules22
, and metallic nanoparticles6
. Despite the rapid progress achieved in this field, there are certain technical aspects that deserve further optimization—in particular, in the method of drug release. Most of the current release methods rely on either a chemical or enzymatic cleavage reaction of the linker that tethers the drug molecule4, 6-7
. Such methods are incorporated by an ester-based or amide-based linker which is cleaved hydrolytically at the acidic subcellular compartments, such as the endosomes and lysosomes (pH 4–5), where the drug carriers are internalized6-7
. In addition, there are other specialized linkers based on di-sulfide4
, and nitroheterocycle32
which are cleavable differently through bioreductive mechanisms. Despite the differences among such release mechanisms, it is common that the drug release occurs passively only in response to environmental and pathophysiological factors.
This study aims to investigate the orthogonal method, which allows the active release of drug molecules through application of an external trigger. We employed photochemistry as the orthogonal means to control the release of MTX. Our approach is based on the concept of photocaging33
, in which a biologically active molecule (ligand, drug) is temporarily inactivated by protecting it with a photocleavable group (“photocaged”). This caged molecule releases its parent species in an actively controlled manner only when its photosensitive protective group is cleaved by UV irradiation. The focus of this release method has mainly been on chemical and biological problems, such as the spatiotemporal control of cell signaling processes34-36
, and it was only recently applied to drug delivery8, 11-13
. The photocleavable linkers that are applicable for the current application are comprised of those based on o
-nitrobenzyl (ONB)33, 35, 37
, and benzophenone39
. illustrates a schematic for controlled drug release triggered by a photochemical mechanism where a drug molecule attached to the photocleavable linker is released upon UV irradiation8
. In the present study, we selected the ONB group as the core of the photocleavable linker and developed linker chemistry that is applicable for the drug attachment and conjugation to poly(amidoamine) (PAMAM) dendrimer as the delivery platform. Here, we report the photochemical mechanism of MTX release with the ONB-linker strategy and analyze a set of key basic parameters that determine the kinetics of the drug release.
A schematic for photon-based cleavage of an o-nitrobenzyl (ONB) linker as the mechanism that enables the controlled drug release