Os(II) Oligothienyl Complexes as a Hypoxia-Active Photosensitizer Class for Photodynamic Therapy

Abstract

Hypoxia presents a challenge to anticancer therapy, reducing the efficacy of many available treatments. Photodynamic therapy is particularly susceptible to hypoxia, given that its mechanism relies on oxygen. Herein, we introduce two new osmium-based polypyridyl photosensitizers that are active in hypoxia. The lead compounds emerged from a systematic study of two Os­(II) polypyridyl families derived from 2,2′-bipyridine (bpy) or 4,4′-dimethyl-2,2′-bipyridine (dmb) as coligands combined with imidazo­[4,5-f]­[1,10]­phenanthroline ligands tethered to n = 0–4 thiophenes (IP-nT). The compounds were characterized and investigated for their spectroscopic and (photo)­biological activities. The two hypoxia-active Os­(II) photosensitizers had n = 4 thiophenes, with the bpy analogue 1-4T being the most potent. In normoxia, 1-4T had low nanomolar activity (half-maximal effective concentration (EC50) = 1–13 nM) with phototherapeutic indices (PI) ranging from 5500 to 55 000 with red and visible light, respectively. A sub-micromolar potency was maintained even in hypoxia (1% O2), with light EC50 and PI values of 732–812 nM and 68–76, respectively currently among the largest PIs for hypoxic photoactivity. This high degree of activity coincided with a low-energy, long-lived (0.98–3.6 μs) mixed-character intraligand charge-transfer (3ILCT)/ligand-to-ligand charge-transfer (3LLCT) state only accessible in quaterthiophene complexes 1-4T and 2-4T. The coligand identity strongly influenced the photophysical and photobiological results in this study, whereby the bpy coligand led to longer lifetimes (3.6 μs) and more potent photo-cytotoxicity relative to those of dmb. The unactivated compounds were relatively nontoxic both in vitro and in vivo. The maximum tolerated dose for 1-4T and 2-4T in mice was greater than or equal to 200 mg kg–1, an excellent starting point for future in vivo validation.

Fine‐Feature Modifications to Strained Ruthenium Complexes Radically Alter Their Hypoxic Anticancer Activity†

“…This result is consistent with other recent findings by our group that demonstrate that seemingly innocuous structural changes can have a very significant influence on a compound’s photocytotoxicity in normoxia and in hypoxia (28,29). The present study not only validates the findings from our previous report regarding the activity of 1 and the inactivity of 5 , but it explores in greater depth the apparent dichotomy of these two NIP isomers.…”

“…Since they were designed as PCT agents for low‐oxygen conditions, the compounds were evaluated in hypoxia (1% O 2 ) using a protocol similar to the normoxic screen but with hypoxia maintained up to and during the light treatment. Our standard hypoxia assay protocol (28–30) involves preincubating the cells under hypoxic conditions prior to compound addition, applying a film over the well plates before removal from the hypoxia chamber for light treatment outside the hypoxic chamber, and incubating the cells in normoxia after irradiation (since the resazurin cell viability assay used for quantifying live versus dead cells is based on metabolic respiration). We have confirmed that this is a robust method for testing photocytotoxicity in hypoxia (28–30), and provides low oxygen tension at the time of illumination.…”

“…Importantly, PCT mechanisms do not necessarily rely on oxygen, a feature that could be exploited in hypoxic tumors. While few compounds to date have been shown to present good hypoxic activity (26–32), efforts are actively underway to design improved PCT agents with high photocytotoxicity in low‐oxygen conditions.…”

It Takes Three to Tango: The Length of the Oligothiophene Chain Determines the Nature of the Long‐Lived Excited State and the Resulting Photocytotoxicity of a Ruthenium(II) Photodrug

“…The TA spectra change substantially for Ru‐ip‐2T through Ru‐ip‐4T , which have very strong and much more structured excited state absorptions with maxima at 550, 630, and 680 nm, respectively. This systematic red‐shift of the excited state absorption with increasing n , by over 100 nm, is characteristic of oligothiophene‐based 3 IL states [5,17] . Thus, the ns TA spectra indicate that the character of the long‐lived state changes from 3 MLCT for Ru‐ip‐0T and Ru‐ip‐1T to predominantly 3 IL with additional thiophene rings.…”

“…This band corresponds to the ππ* 1 IL transition associated with the oligothiophene [16] . These oligothiophene‐localized transitions have been computed to have substantial charge transfer character and are best described as intraligand charge transfer (ILCT) states, but are referred to herein more generally as IL [17,18] …”