Photodynamic therapy (
PDT) is a promising
tumor therapy which utilizes
reactive oxygen species (ROSs) to cause
tumor cells death. 5-aminolevulinic
acid (ALA) and two of its
esters are FDA-approved
photosensitizers. However, their clinical application suffers from their instability and lack of
tumor selectivity. In addition, the overexpression of
glutathione (GSH) in some
tumor cells reduces the
PDT efficiency due to the ROS-scavenging ability of GSH. In this work, we present three multifunctional ALA derivates with the characteristics of dual-targeting and GSH depletion to improve the
therapeutic effect of ALA-based
PDT. The general structure of these compounds consists of an ALA methyl
ester (ALA-OMe) moiety that can metabolize to photosensitive protoporphyin IX (
PpIX) inside the cells, a
biotin group for targeting
biotin receptor-positive
tumor cells and a
disulfide bond-based self-immolative linker which can be activated by GSH to liberate ALA-OMe. Simultaneously, the reaction between the
disulfide bond and GSH also depletes intracellular GSH, causing
tumor cells more vulnerable to ROSs. All three compounds exhibited high stability under physiological conditions. In vitro experiments demonstrated that the more lipophilic compounds 1 and 2 were much more efficient in inducing
PpIX production in
biotin receptor-overexpressed HeLa cells as compared with their parent compound (ALA-OMe). And the
PpIX generation induced by compounds 1 and 2 was positively correlated with the overexpression of
biotin receptor and GSH level in
tumor cells. More importantly, the GSH depletion ability of them significantly increased their
phototoxicity. Furthermore, in comparison with ALA-OMe, compound 2 showed much higher in vivo efficiency in
PpIX production. All the results demonstrate that the combination strategy of dual-targeting and GSH depletion can be used to concurrently enhance the
tumor-specificity and anti-
tumor efficiency of ALA-based
PDT. And this strategy may be used for designing other ALA-based
photosensitizers with higher
tumor-specificity and better
therapeutic effects.