Photo-induced uncaging of a specific Re(i) organometallic complex in living cells

Abstract

In the last decades, a large number of organometallic complexes have shown promising anti-proliferative activity towards different cancer cell lines. However, these compounds generally had low cellular uptake and low selectivity towards cancer cells over healthy cells. The use of external triggers (e.g. light, ultrasound, temperature, etc.) to modify the cytotoxic effect of a prodrug and the coupling of a targeting vector (e.g. peptides, antibodies, etc.) to a drug were found to be very successful techniques to tackle these drawbacks. Here, we envisioned combining these two methods, namely an external trigger (i.e. light activation) and a targeting vector, in an organometallic compound. More specifically, a Re(I) tricarbonyl N,N-bis(quinolinoyl) complex (Re-NH 2) was derivatised with a photo-labile protecting group (PLPG) to cage Re-NH 2 by formation of Re-PLPG. For organelle/cellular specificity, Re-PLPG was then further coupled to a nuclear localization sequence (NLS) or a bombesin peptide derivative to give Re-PLPG-NLS or Re-PLPG-Bombesin, respectively. Photolysis experiments in PBS buffer (pH 7.4) demonstrated that Re-NH 2 was completely photo-released from Re-PLPG-NLS and Re-PLPG-Bombesin using a very low irradiation dose (1.2 J cm À2). To the best of our knowledge, these are the first two examples of the selective photo-release of an intact organometallic compound from a bioconjugate. Of high interest, both derivatives showed toxicity comparable to that of cisplatin towards cervical cancer cells (HeLa) upon light irradiation, although the phototoxic index (PTI) varied greatly with the targeting peptide. The cell death mechanism of Re-PLPG-NLS was explored using different techniques, including fluorescence microscopy, ICP-MS, gel electrophoresis, flow cytometry and transmission electron microscopy (TEM). It could be demonstrated that HeLa cells treated with Re-PLPG-NLS in the dark and upon irradiation showed severe cell stress (nucleolar segregation, pyknosis and vacuolation). The data obtained from an Annexin V/propidium iodide (PI) assay indicated that, after an early apoptotic stage, the onset induced by Re-PLPG-NLS led to cell death, with features ascribable to late apoptosis and necrosis, which were more marked for the treatment involving irradiation.

Underestimated Potential of Organometallic Rhenium Complexes as Anticancer Agents

“…The introduced ligands can be designed to incorporate other functional moieties such as targeting biomolecules. 28,29,34,35,39,41,44,46,47,50,51 However, the key property of the Re organometallic complexes is the possibility to prepare "hot" analogues that can be applied for radio-imaging as well as for therapy . Besides 186 Re and 188 Re isotopes, 99m Tc can often be used with the same set of ligands as the cold Re due to the similarity in the coordination chemistry of these group 7 congeners.…”

“…39 A non-negligible percentage of endosomal accumulation (about 20%) has also been detected for compound 7i 39 As a note of caution, although fluorescent microscopy imaging is an important tool in the investigation of the cellular behavior of Re organometallic complexes, their luminescence can sometimes be quenched in cells. 39,44,47 Due to this problem, direct measurements of Re content by inductively coupled plasma mass spectrometry (ICP-MS) or atomic absorption spectroscopy (AAS) 60 can be a more accurate method to quantify the cellular uptake and distribution of Re complexes.…”

“…43,44,46,47 Most of them act as photodynamic therapy (PDT) photosensitizers (PS) -i.e. they transfer the energy received from light to oxygen (present in tissues) to generate reactive oxygen species (ROS).…”

Lightening up Ruthenium Complexes to Fight Cancer?

“…[16][17][18][19] Over the past few years, our group, in collaboration with several colleagues, has investigated the use of metal complexes in the fields of PDT and PACT. [20][21][22][23][24][25][26][27][28] In particular, we focused our attention on Ru(ii) complexes. Ruthenium compounds are nowadays considered as the potential successors to Pt-based anticancer drugs.…”

Peptide-mediated vectorization of metal complexes: conjugation strategies and biomedical applications

“…The activity of these conjugates was improved by using a photocaged strategy. 63 An o-nitrophenyl photolabile linker (PLPG) was incorporated between the Re(I) complex and the corresponding peptide sequence (Figure 6c). Re-NH 2 was selectively photoreleased from Re-PLPG-NLS and Re-PLPG-bombesin conjugates and, upon irradiation, both metallopeptides showed IC 50 values similar to those of cisplatin against HeLa cells.…”