Establishing a Robust and Reliable Response from a Potent Osmium Based Photosensitizer via Lipid Nanoformulation.

Osmium (Os) based photosensitizers (PSs) are a unique class of non-tetrapyrrolic metal-containing PSs with red-shifted absorption. We recently reported a highly potent Os(II) PS, rac-[Os(phen) 2 (IP-4T)](Cl) 2, referred to as ML18J03 in this study, with light EC 50 values as low as 20 pM. ML18J03 also exhibits low dark toxicity and submicromolar light EC 50 values in hypoxia. However, owing to its longer oligothiophene chain, ML18J03 is not completely water soluble and forms 1-2 μm sized aggregates in PBS containing 1% DMSO. This aggregation causes a wide interassay variability in PDT efficacy, and thus unreliable and irreproducible reports of activity. To that end, we utilized PEG-modified DPPC liposomes (138.1 nm diameter) and DSPE-mPEG 2000 micelles (10.2 nm diameter) as lipid nanoformulation vehicles to mitigate aggregation of ML18J03 and found that the spectroscopic properties important to biological activity were maintained or improved. Importantly, the lipid formulations decreased the interassay variance between the reported EC 50 values by almost twenty-fold, with respect to the unformulated ML18J03 when using broadband visible light excitation (P= 0.0276). Herein, lipid formulations are presented as reliable platforms for more accurate in vitro photocytotoxicity quantification for PSs prone to aggregation such as ML18J03 and will be useful for assessing their in vivo PDT effects.