Enabling In Vivo Optical Imaging of an Osmium Photosensitizer by Micellar Formulation by Drashti Shah, Menitte Eroy, John Fakhry, Azophi Moffat, Kevin Fritz, Houston D. Cole, Colin G. Cameron, Sherri A. McFarland, and Girgis Obaid
Received: 14 October 2022 / Revised: 3 November 2022 / Accepted: 7 November 2022 / Published: 10 November 2022
1. Introduction
Photodynamic therapy (PDT) is a minimally invasive modality used in a variety of cancer and non-cancer indications. Its therapeutic action relies on the selective accumulation of a photosensitizer (PS) molecule in diseased tissue followed by the light-activated production of cytotoxic and biomodulatory reactive molecular species (RMS). Such RMS molecules include, but are not limited to, singlet oxygen, hydroxyl radicals, hydrogen peroxide, superoxide anions and peroxynitrite anions [1]. A distinguishing feature of PDT is the ability to simultaneously perform multiparametric optical imaging to personalize and guide treatment [2]. This is made possible by the intrinsic fluorescence or phosphorescence properties of many PSs in pre-clinical and clinical development. In vivo luminescence imaging is often used to estimate PS accumulation in tissue to inform the optimal injected dose and ideal timing for photoactivation to maximize tissue selectivity. In addition, photobleaching of the luminescence signal has been used pre-clinically and clinically to guide PDT dosimetry implicitly [2,3,4,5]. Here, implicit dosimetry refers to the measurement of the indirect consequences of PDT that imply that an effective PDT dose has been administered, whereas explicit dosimetry refers to more direct measurements of the applied PDT dose [5]. Explicit PDT dose metrics include the light fluence and fluence rate, PS tissue concentration, oxygen concentration, and singlet oxygen concentration to model the applied PDT dose [6]. Regardless, luminescence imaging of the PS itself plays a central role in PDT dosimetry and personalization.
Metal-based PSs are emerging as highly versatile, potent, and promising PDT agents [7]. Of note, our ruthenium-based photosensitizer (TLD1433) has successfully completed phase I in clinical trials for non-muscle invasive bladder cancer (NMIBC) [7] and is currently in a Phase 2 study. We also recently reported a hypoxia-active Os(II)-based PS, (rac-[Os(phen)2(IP-4T)](Cl)2, referred to as ML18J03) with potent submicromolar phototherapeutic efficacy under hypoxic conditions [8]. Certain Os(II) complexes such as ML18J03 and its close relatives have the potential for eliciting tumor tissue phototoxicity at greater depths owing to their longer wavelength absorption windows. In addition, these Os(II) PSs are designed specifically to exhibit prolonged excited state lifetimes for increased RMS generation, with both visible and NIR wavelengths of light [8,9]. As a result, ML18J03 has a very high singlet oxygen quantum yield (95% from the lowest lying 3ILCT state). We have shown that ML18J03 exhibits excellent in vivo tolerability with maximum tolerated doses (MTD) exceeding 200 mg/kg [9]. Despite these attractive features, ML18J03 is not suitable for in vivo optical imaging because its luminescence is almost completely suppressed in an aqueous solution due to aggregation-induced quenching [10]. Novel approaches are therefore required to harness the luminescence imaging capabilities of ML18J03 as well as other promising Os(II) complexes.
A variety of delivery systems exist that have been demonstrated to effectively carry hydrophobic, aggregation-prone PS molecules [1,11]. Among these, micelles show promise due to their relatively low toxicity, high biocompatibility, small diameters that favor tumor penetration, and prolonged circulation times [12,13,14]. Tseng et al. reported fluorinated Ce6-loaded PFFA polymeric micelles that show promising therapeutic results in vitro [15]. P3H2 polymeric micelles, with a peptide targeting HER-2 receptor in breast cancer cells, were also found to be a promising drug delivery vehicle in 3D cell models [16]. Along with this, pH/glutathione (GSH) responsive nano-prodrug micelles demonstrate in vivo efficacy for MRI-guided tumor PDT [17]. We previously showed that DSPE-mPEG2000 micelles are an efficient and robust platform that stably encapsulate a hydrophobic conjugate of the PS benzoporphyrin derivative and cholesterol, even when liposomes fail to do so [18]. Our recent study has shown that ML18J03, when formulated in 10.2 nm DSPE-mPEG2000 micelles, exhibits low dark toxicity, submicromolar EC50 values in hypoxia, retains its photoactivity in normoxia, and provides a significant reduction in inter-assay variability of ML18J03 [10]. Of particular significance to this study, the formulation of ML18J03 into DSPE-mPEG2000 micelles increases its luminescence quantum yield in PBS from 9.8 × 10−5 to 1.3 × 10−3, Figure 1 [10]. As such, in this study, we explore the capability of micellar formulation to enable luminescence imaging of ML18J03 in vivo in orthotopic AT-84 head and neck tumors and assess their tumor selectivity following intravenous administration.