Induction of Cytotoxicity through Photorelease of Aminoferrocene

Reactive
oxygen species (ROS)-activated aminoferrocene-based anticancer prodrug
candidates successfully take advantage of intrinsically high amounts
of ROS in tumor tissues. Interestingly, the ROS-initiated activation
of these prodrug candidates leads to formation of unstable aminoferrocene
(Fc-NH2) derivatives, which decay
to iron ions. The latter catalytically increases ROS concentration
to a lethal level. In this work, we prepared light-controlled aminoferrocene
prodrug candidates by derivatizing Fc-NH2 with an o-nitrophenyl and an o-nitrobiphenyl photolabile protecting group (PLPG), respectively,
and by further conjugation to a mitochondria localization signal (MLS) peptide (Cys-d-Arg-Phe-Lys-NH2).
The resulting bioconjugates were found to be more stable and less
cytotoxic, in the dark, toward human promyelocytic leukemia cells
(HL-60) compared to Fc-NH2. Upon
light irradiation at 355 nm, both conjugates released Fc-NH2, albeit with very different photolysis
quantum yields. The o-nitrobiphenyl photocage was
in fact several orders of magnitude more efficient than the o-nitrophenyl photocage in releasing Fc-NH2. This difference was reflected by the light
irradiation experiments on the HL-60 cell line, in which aminoferrocene
conjugated with the o-nitrobiphenyl cage and the MLS displayed the highest phototoxicity index (2.5 ±
0.4) of all the compounds tested. The iron release assays confirmed
the rise in iron ion concentrations upon light irradiation of both
caged aminoferrocene derivatives. Together with the absence of phototoxicity
on the nonmalignant hTERT-immortalized retinal pigment epithelial
(hTERT RPE-1) cell line, these results indicate catalytic generation
of ROS as possible mode of action.