Oxygen monitoring during 5-aminolaevulinic acid induced photodynamic therapy in normal rat colon - Comparison of continuous and fractionated light regimes

Currently, the clinical use of 5-aminolaevulinic acid (ALA) induced protopo rphyrin IX (PPIX) for photodynamic therapy (PDT) is Limited by the maximum tolerated oral ALA dose (60 mg/kg). Attempts have been made to enhance this treatment modality without increasing the administered dose of ALA. One wa y to do this is through light dose fractionation, where the irradiation is interrupted at a particular point for a short period of time. This can prod uce up to three times more necrosis than with the same light dose delivered without a break. An oxygen microelectrode was employed to study the effect of continuous and fractionated light regimes on the level of oxygen in the colon of normal Wistar rats during ALA PDT. A rapid decline in p(O2) occur red close to the irradiation fibre as soon as the light dose commenced. Wit h the fractionated regime, a partial recovery in p(O2) was observed during the dark interval which was reversed soon after the second light fraction c ommenced. We have shown that the level of tissue oxygen at the treatment si te is affected differently when the Light dose is fractionated, than when c ontinuous illumination is employed. This factor may at least partially expl ain the difference in outcome of these two treatment regimes. Further, oxyg en measurements might prove to be a useful way of monitoring PDT treatments if they can predict whether tissue is likely to be viable following treatm ent. (C) 2000 Elsevier Science B.V. All rights reserved.