Comparison of multispectral singlet oxygen luminescence dosimetry and singlet oxygen explicit dosimetry in artificial...

Direct detection of singlet-state oxygen ( ${{}^{1}O}_2$ ) is a critical objective in Type II photodynamic therapy (PDT) due to its pivotal role in mediating therapeutic effects. Although multispectral singlet oxygen dosimetry (MSOLD) has demonstrated the capability to detect ${{}^{1}O}_2$ luminescence both in vitro and in vivo, there remains no standardized method for accurately quantifying reactive singlet oxygen, ${}^1[O_2{]}_{\mathrm{rx}}$ , based on these measured signals. By contrast, the singlet oxygen explicit dosimetry (SOED) model offers a robust framework for calculating ${}^1[O_2{]}_{\mathrm{rx}}$ . Demonstrating that ${{}^{1}O}_2$ luminescence obtained through MSOLD can reliably quantify ${}^1[O_2{]}_{\mathrm{rx}}$ , as achieved by the SOED model, is essential for advancing the accuracy and applicability of PDT dosimetry.

We aim to evaluate the accuracy and reliability of MSOLD in quantifying ${{}^{1}O}_2$ concentrations from measured ${{}^{1}O}_2$ luminescence in benzoporphyrin derivative (BPD)-mediated PDT. The performance of MSOLD is assessed by comparing its results with those derived from the SOED model.

A continuous-wave 690 nm laser was used to excite a nanoparticle formulation of BPD, tradename Visudyne^® in methanol at varying concentrations (2 to $6\mathrm{mg}/L$ ). The singlet oxygen luminescence was measured using an InGaAs spectrometer and analyzed using a singular value decomposition algorithm. Near-infrared singlet oxygen emission at $\sim 1270\mathrm{nm}$ was extracted as ${{}^{1}O}_2$ luminescence. Real-time singlet oxygen spectra were collected over 900 s using a 1.5 mm diameter fiber optic. Ground-state oxygen concentration was measured with a commercial oxygen probe, photosensitizer concentration was determined with a custom-made contact probe, and photon fluence rate was assessed with an isotropic detector. ${}^1[O_2{]}_{\mathrm{rx}}$ was then calculated based on the SOED model.

The extracted singlet oxygen ( ${{}^{1}O}_2$ ) luminescence exhibited clear concentration-dependent trends, with higher BPD concentrations producing stronger ${{}^{1}O}_2$ luminescence. Over time, the ${{}^{1}O}_2$ luminescence decayed due to photosensitizer bleaching. In addition, a strong linear correlation was observed between the ${{}^{1}O}_2$ luminescence measured via MSOLD and the reactive oxygen species (ROS) concentrations calculated using the SOED model. We also investigated the impact of tissue optical properties on singlet oxygen luminescence detection and developed correction factors to account for their variations.

We demonstrate that singlet oxygen ( ${{}^{1}O}_2$ ) detected through MSOLD can reliably quantify ROS concentrations in BPD-mediated PDT with accuracy comparable to the SOED model, which requires separate measurements of light fluence rate, photosensitizer concentration, and oxygen availability, followed by modeling to estimate the amount of reactive singlet oxygen. By contrast, MSOLD can also be a more cost-effective, simpler, and faster alternative to SOED as it directly measures the singlet oxygen luminescence to quantify reactive singlet oxygen. Under appropriate correction for tissue optical properties, MSOLD presents a promising, robust, and direct dosimetry solution for clinical PDT applications.