Multispectral singlet oxygen and photosensitizer luminescence dosimeter for continuous photodynamic therapy dose assessment during treatment

High Sensitivity Singlet Oxygen Luminescence Sensor Using Computational Spectroscopy and Solid-State Detector

This paper presents a technique for high sensitivity measurement of singlet oxygen luminescence generated during photodynamic therapy (PDT) and ultraviolet (UV) irradiation on skin. The high measurement sensitivity is achieved by using a computational spectroscopy (CS) approach that provides improved photon detection efficiency compared to spectral filtering methodology. A solid-state InGaAs photodiode is used as the CS detector, which significantly reduces system cost and improves robustness compared to photomultiplier tubes. The spectral resolution enables high-accuracy determination and subtraction of photosensitizer fluorescence baseline without the need for time-gating. This allows for high sensitivity detection of singlet oxygen luminescence emission generated by continuous wave light sources, such as solar simulator sources and those commonly used in PDT clinics. The value of the technology is demonstrated during in vivo and ex vivo experiments that show the correlation of measured singlet oxygen with PDT treatment efficacy and the illumination intensity on the skin. These results demonstrate the potential use of the technology as a dosimeter to guide PDT treatment and as an analytical tool supporting the development of improved sunscreen products for skin cancer prevention.

Singlet Molecular Oxygen: from COIL Lasers to Photodynamic Cancer Therapy

Translation of experimental techniques from one scientific discipline to another is often difficult but rewarding. Knowledge gained from the new area can lead to long lasting and fruitful collaborations with concomitant development of new ideas and studies. In this Review Article, we describe how early work on the chemically pumped atomic iodine laser (COIL) led to the development of a key diagnostic for a promising cancer treatment known as photodynamic therapy (PDT). The highly metastable excited state of molecular oxygen, a¹Δ_g, also known as singlet oxygen, is the link between these disparate fields. It powers the COIL laser and is the active species that kills cancer cells during PDT. We describe the fundamentals of both COIL and PDT and trace the development path of an ultrasensitive dosimeter for singlet oxygen. The path from COIL lasers to cancer research was relatively long and required medical and engineering expertise from numerous collaborations. As we show below, the knowledge gained in the COIL research, combined with these extensive collaborations, has resulted in our being able to show a strong correlation between cancer cell death and the singlet oxygen measured during PDT treatments of mice. This progress is a key step in the eventual development of a singlet oxygen dosimeter that could be used to guide PDT treatments and improve outcomes.

Multispectral singlet oxygen luminescent dosimetry (MSOLD) for Photofrin-mediated Photodynamic Therapy

Direct detection of singlet-state oxygen ([1O2]) constitutes the holy grail dosimetric method for type II PDT, a goal that can be quantified using multispectral singlet oxygen dosimetry (MSOLD). However, the short lifetime and extremely weak nature of the singlet oxygen signal produced has given rise to a need to improve MSOLD signal-to-noise ratio. This study examines methods for optimizing MSOLD signal acquisition, specifically employing an orthogonal arrangement between detection and PDT treatment light, consisting of two fiber optics - connected to a 632-nm laser and an InGaAs detector respectively. Light collected by the InGaAs detector is then passed through a filter wheel, where spectral emission measurements are taken at 1200 nm, 1240 nm, 1250 nm, 1270 nm, and 1300 nm. The data, after fitting to the fluorescence background and a gaussian-fit for the singlet oxygen peak, is established for the background-subtracted singlet oxygen emission signal. The MSOLD signal is then compared with the singlet oxygen explicit dosimetry (SOED) results, based on direct measurements of in-vivo light fluence (rate), in-vivo Photofrin concentration, and tissue oxygenation concentration. This study focuses on validating the sensitivity and minimum detectability of MSOLD signal in various in-vitro conditions. Finally, the MSOLD device will be tested in Photofrin-mediated PDT for mice bearing Radiation-Induced Fibrosarcoma (RIF) tumors.

Effect of intermittency factor on singlet oxygen and PGE2 formation in azulene-mediated photodynamic therapy: A preliminary study

In photodynamic therapy, intermittent irradiation modes that incorporate an interval between pulses are believed to decrease the effect of hypoxia by permitting an interval of re-oxygenation. The effect of the irradiation intermittency factor (the ratio of the irradiation pulse time to the total irradiation time) on singlet oxygen formation and inflammatory cytokine production was examined using azulene as a photosensitizer. Effects of difference intermittency factor on singlet oxygen formation and inflammatory cytokine were examined. Azulene solutions (1/10 μM) were irradiated with a 638-nm 500 mW diode laser in fractionation (intermittency factor of 5 or 9) or continuous mode using 50 mW/cm² at 4 or 8 J/cm². Singlet oxygen measurement was performed using a dimethyl anthracene probe. Peripheral blood mononuclear cells (PBMC) were stimulated by 10 ng/ml rhTNF-α for 6 h, before addition of 1 and 10 μM azulene solutions and irradiation. PGE₂ measurement was undertaken using a human PGE₂ ELISA kit. Kruskal-Wallis with Dunn Bonferroni test was used for statistical analyses at p < 0.05.Irradiation of 1 μM azulene+4 J/cm²+intermittency factor of 9 increased singlet oxygen 3-fold (p < 0.0001). Irradiation of 10 μM azulene at either 4 J/cm²+intermittency of 9 or 8 J/cm²+intermittency factor of 5 reduced PGE₂ expression in PBMCs to non-inflamed levels. Thus, at 50 mW/cm², 10 μM azulene-mediated photodynamic therapy with a high intermittency factor and a low energy density generated sufficient singlet oxygen to suppress PGE₂ in Inflamed PBMCs.

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Different intermittency factors can stimulate ROS formation differently.

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Relative high intermittency factor with azulene induces high ROS formation.

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Relative high intermittency factor with low energy density inhibits PGE₂ production.

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Azulene-based photodynamic therapy suppresses inflammation.

Validation of multispectral singlet oxygen luminescence dosimetry (MSOLD) for photofrin-mediated photodynamic therapy

Accurate dosimetry is crucial for the ongoing development and clinical study of photodynamic therapy (PDT). Current dosimetry standards range from less accurate methods involving measurement of only light fluence and photosensitizer concentration during treatment, to significantly improved methods such as singlet oxygen explicit dosimetry (SOED), a macroscopic model that includes an additional important parameter in its dosimetric calculations: ground-state oxygen concentration ([³O₂]). However, neither of these models is a method of direct dosimetry. Multispectral singlet oxygen luminescence dosimetry (MSOLD) shows promise in this regard but requires significant improvement in signal quality and remains to be validated in a clinical setting. In this study, we validate a linearly increasing MSOLD signal with an InGaAs photodiode detector for increasing concentration (0 mg/kg to 200 mg/kg) in tissue-simulating phantoms containing photofrin, calculating a calibration curve based on 1270 nm peak-intensity signal and area under the curve for background-subtracted singlet oxygen emission. Additionally, we validate MSOLD against the current clinical dosimetry standard, SOED, through simultaneous measurement of SOED parameters and MSOLD signal for varying concentrations (50 μM - 300 μM). Finally, we investigate the effects of using very high gain amplification on InGaAs photodiode detectors to amplify the MSOLD signal for use in clinical models. We show that a calibration curve relating photosensitizer concentration (PS) and MSOLD signal can be established. Additionally, we demonstrate good correlation between MSOLD signal and SOED-calculated [¹O₂]_rx. However, we show that when using high amplification on InGaAs photodiodes for long illumination times, the inherent instability in these detectors becomes apparent.

High optical-throughput spectroscopic singlet oxygen and photosensitizer luminescence dosimeter for monitoring of photodynamic therapy

We report a high light-throughput spectroscopic dosimeter system that is able to noninvasively measure luminescence signals of singlet oxygen (¹ O₂ ) produced during photodynamic therapy (PDT) using a CW (continuous wave) light source. The system is based on a compact, fiber-coupled, high collection efficiency spectrometer (>50% transmittance) designed to maximize optical throughput but with sufficient spectral resolution (~7 nm). This is adequate to detect ¹ O₂ phosphorescence in the presence of strong luminescence background in vivo. This system provides simultaneous acquisition of multiple spectral data points, allowing for more accurate determination of luminescence baseline via spectral fitting and thus the extraction of ¹ O₂ phosphorescence signal based solely on spectroscopic decomposition, without the need for time-gating. Simultaneous collection of photons at different wavelengths improves the quantum efficiency of the system when compared to sequential spectral measurements such as filter-wheel or tunable-filter based systems. A prototype system was tested during in vivo PDT tumor regression experiments using benzoporphyrin derivative (BPD) photosensitizer. It was found that the treatment efficacy (tumor growth inhibition rate) correlated more strongly with ¹ O₂ phosphorescence than with PS fluorescence. These results indicate that this high photon-collection efficiency spectrometer instrument may offer a viable option for real-time ¹ O₂ dosimetry during PDT treatment using CW light.

Singlet oxygen luminescence detector based on low-cost InGaAs avalanche photodiode

Molecular oxygen excited to singlet state (Singlet oxygen, ¹O₂) becomes highly reactive and cytotoxic chemical. ¹O₂ is commonly generated by photoexcitation of dyes (photosensitizers), including the photodynamic therapy and diagnostics of cancer. However, the formation of singlet oxygen is often unwanted for various light-sensitive compounds, e.g. it causes the photobleaching of fluorescent probes. In either case, during a development of new photosensitive chemicals and drugs there is a need to evaluate the amount of ¹O₂ formed during photoexcitation. The direct approach in measuring the amount of singlet oxygen is based on the detection of its luminescence at 1270 nm. However, this luminescence is usually weak, which implies the use of highly sensitive single-photon detectors. Thus the existing instruments are commonly complicated and expensive. Here we suggest an approach and report a device to measure the ¹O₂ luminescence using low-cost InGaAs avalanche photodiode and simple electronics. The measurements can be performed in stationary (not time-resolved) mode in organic solvents such as tetrachloromethane (CCl₄), ethanol and DMSO. In particular, we performed spectral-resolved measurements of the singlet oxygen luminescence in CCl₄ with the device and demonstrated high complementarity to literature data. The simple setup allows to evaluate the efficiency (or speed) of singlet oxygen generation and hence facilitates the development and characterization of new photosensitizers and other photosensitive chemicals.

Detection of singlet oxygen luminescence for experimental corneal rose bengal photodynamic antimicrobial therapy

Rose bengal photodynamic antimicrobial therapy (RB-PDAT) treats corneal infection by activating rose bengal (RB) with green light to produce singlet oxygen (¹O₂). Singlet oxygen dosimetry can help optimize treatment parameters. We present a ¹O₂ dosimeter for detection of ¹O₂ generated during experimental RB-PDAT. The system uses a 520 nm laser and an InGaAs photoreceiver with bandpass filters to detect ¹O₂ luminescence during irradiation. The system was validated in RB solutions and ex vivo in human donor eyes. The results demonstrate the feasibility of ¹O₂ dosimetry in an experimental model of RB-PDAT in the cornea.