The relationship between tumor oxygenation and the effectiveness of photodynamic therapy (PDT) was studied in vitro and in vivo using the RIF mouse tumor model. The oxygen dependence of photodynamic inactivation of RIF cells, which had been exposed to 25 mg/kg porphyrin (dihematoporphyrin ether) in vivo, isolated and illuminated in vitro, was determined. No cell kill was achieved under anoxic conditions, full effect was reached at 5% O2, and the half value of cell inactivation was found to be at 1% O2. Tumor hypoxia was assessed after in vivo γ-irradiation of control and PDT-treated tumors by in vitro clonogenic assay of cell radiosensitivity. In vitro control experiments established that the radio-sensitivity of PDT-surviving RIF cells was identical to that of untreated control cells. RIF tumors of treatment size (80–120 mg) contained no detectable hypoxic tumor cell fraction. PDT treatment consisting of i.p. injection of 10 mg/kg dihematoporphyrin ether 24 h prior to 45 J/cm2 of 630 nm light, rendered approximately 9% of tumor cells severely hypoxic within 10 min of treatment time. An illumination period of 30 min (135 J/cm2) induced a hypoxic tumor cell fraction of 17%, which increased to 47% within 1 h posttreatment. Despite the prompt induction of tumor hypoxia during PDT light treatment, the tumors proved highly curable (81% cures) under the present treatment conditions (depilation of tumor area, 10 mg/kg dihematoporphyrin ether i.p., 135 J/cm2). Considering the reduced effectiveness of photodynamic cell kill at low oxygen concentrations, the rapid induction of tumor hypoxia by PDT itself, and the high tumor cure rate, it has to be concluded that in the RIF tumor hypoxic tumor cells are inactivated by a mechanism other than direct photodynamic cytotoxicity, and are thus not limiting to PDT tumor response.

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This investigation was supported by USPHS Grant CA-42278, awarded by the National Cancer Institute, Department of Health and Human Services.

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