In vivo monitoring of singlet oxygen using delayed chemiluminescence during photodynamic therapy

It is known that singlet oxygen ((1)O(2)) is the main factor mediating cytotoxicity in photodynamic therapy (PDT). The effectiveness of a PDT treatment is directly linked to the (1)O(2) produced in the target. Although the luminescence from (1)O(2) is suggested as an indicator for evaluating photodynamic therapy, the inherent disadvantages limit its potential for in vivo applications. We have previously reported that chemiluminescence can be used to detect (1)O(2) production in PDT and have linked the signal to the cytotoxicity. We further our investigation for monitoring (1)O(2) production during PDT. The lifetime of 3,7-dihydro-6-{4-[2-(N(')-(5-fluoresceinyl)thioureido)ethoxy]phenyl}-2-methylimidazo {1,2-a} pyrazin-3-one-chemiluminescence (FCLA-CL) is evaluated, and the results show that it is much longer than that of direct luminescence of (1)O(2). A gated measurement algorithm is developed to fully utilize the longer lifetime for a clean measurement of the CL without the interference from the irradiation light. The results show that it is practically feasible to use the technique to monitor the (1)O(2). Compared to the direct (1)O(2) luminescence measurement, our new technique is sensitive and can be realized with a conventional optical detector with excellent signal-to-noise ratio. It thus provides a means for real-time in vivo monitoring of (1)O(2) production during PDT.