Sodium trans-[tetrachloridobis(1H-
indazole)ruthenate(III)] (NKP-1339) is a clinically investigated
ruthenium-based
metal complex, which shows promising results in solid
tumors, such as
non-small cell lung cancer,
colorectal carcinoma, and most distinctively in gastrointestinal
neuroendocrine tumors. In previous studies, fast binding to
albumin as well as
transferrin could be shown. The enhanced permeability and retention (EPR) effect, which is diversely being exploited for
tumor targeting, could therefore be applicable for
NKP-1339. Here we studied the serum dependence of its
biological activity in various methods, influencing its cellular accumulation, cytotoxicity as well as the generation of
reactive oxygen species (ROS). ROS lead to Nrf2 activation, which is known to activate
antioxidant response gene transcription.
GRP78 down-regulation on the
protein level suggests ER associated protein degradation (ERAD) as a mode of action, as
RNA levels are only mildly affected. Another important part for the mode of action is endoplasmic reticulum (ER) stress, as different factors are highly upregulated on the
protein level. For example PERK, a transmembrane receptor which is released by
GRP78 when the ER is disturbed, is upregulated and phosphorylated. EIF2α is phosphorylated, which leads to an inhibition of CAP-dependent translation and other stress responses. The
transcription factor CHOP (DDIT3), which promotes ER stress dependent apoptosis, is time and concentration dependently upregulated. Finally cytotoxicity tests could prove that inhibition of ER stress and ER stress-mediated apoptosis leads to decreased cytotoxic effects of
NKP-1339, which highlights the involvement of this mechanism in the mode of action.