In vitro tests of the validity of singlet oxygen luminescence measurements as a dose metric in photodynamic therapy.

Singlet oxygen ((1)O(2)) is widely believed to be the major cytotoxic agent involved in photodynamic therapy (PDT). We showed recently that measurement of the weak near infrared luminescence of (1)O(2) is possible in cells in vitro and tissues in vivo. Here, we investigated the relationship between the integrated luminescence signal and the in vitro PDT response of AML5 leukemia cells sensitized with aminolevulinic acid-induced protoporphyrin IX (PpIX). Sensitized cell suspensions were irradiated with pulsed 523 nm laser light at average fluence rates of 10, 25, or 50 mWcm(-2) and, (1)O(2) luminescence measurements were made throughout the treatment. Cell survival was measured with either propidium iodide-labeled flow cytometry or colony-forming assay. The PpIX concentration in the cells, the photobleaching, and the pO(2) in the cell suspensions were also monitored. There were large variations in cell survival and (1)O(2) generation in different experiments due to different controlled treatment parameters (fluence and fluence rate) and other uncontrolled factors (PpIX synthesis and oxygenation). However, in all of the cases, cell kill correlated strongly with the cumulative (1)O(2) luminescence and allowed direct estimation of the (1)O(2) per cell required to achieve a specific level of cell kill. This study supports the validity and potential utility of (1)O(2) luminescence measurement as a dosimetric tool for PDT, as well as confirming the likely role of (1)O(2) in porphyrin-based PDT.