452 RADIATION RESEARCH 160, 452–459 (2003) 0033-7587/ 03 $15.00 q 2003 by Radiation Research Society. All rights of reproduction in any form reserved. Blood Flow Dynamics after Photodynamic Therapy with Verteporfin in the RIF-1 Tumor Bin Chen, a Brian W. Pogue, a,b,1 Isak A. Goodwin, a Julia A. O’Hara, c Carmen M. Wilmot, b John E. Hutchins, d P. Jack Hoopes a,d and Tayyaba Hasan b a Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755; b Wellman Laboratories of Photomedicine, Massachusetts General Hospital, Department of Dermatology, Harvard Medical School, Boston, Massachusetts 02114; c Department of Radiology, Dartmouth Medical School, Hanover, New Hampshire 03755; and d Department of Surgery, Dartmouth Medical School, Lebanon, New Hampshire 03756 Chen, B., Pogue, B. W., Goodwin, I. A., O’Hara, J. A., Wil- mot, C. M., Hutchins, J. E., Hoopes, P. J. and Hasan, T. Blood Flow Dynamics after Photodynamic Therapy with Verteporfin in the RIF-1 Tumor. Radiat. Res. 160, 452–459 (2003). In the present study, the effects of photodynamic therapy (PDT) with verteporfin on tumor blood flow and tumor re- growth were compared as verteporfin distributed in different compartments within the RIF-1 tumor. Tissue distribution of verteporfin was examined by fluorescence microscopy, and blood flow measurements were taken with a laser Doppler system. It was found that, at 15 min after drug administration, when verteporfin was mainly confined within the vasculature, PDT induced a complete arrest of blood flow by 6 h after treatment. PDT treatment at a longer drug–light interval (3 h), which allowed the drug to diffuse to the tumor intersti- tium, caused significantly less flow decrease, only to 50% of the initial flow in 6 h. A histological study and Hoechst 33342 staining of functional tumor vasculature confirmed the pri- mary vascular damage and the decrease in tumor perfusion. The regrowth rate of tumors treated with 15-min interval PDT was 64% of that of the control group. However, when tumors were treated with 3-h interval PDT, the regrowth rate was not significantly different from that of the control, indi- cating that only the 15-min interval PDT caused serious dam- age to the tumor vascular bed. These results support the hy- pothesis that temporal pharmacokinetic changes in the distri- bution of the photosensitizer between the tumor parenchyma and blood vessels can significantly alter the tumor target of PDT. q 2003 by Radiation Research Society INTRODUCTION Photodynamic therapy (PDT) involves systemic or topi- cal application of a photosensitizing drug, followed by ir- radiation with light of a wavelength which activates the photosensitizer, resulting in destruction of targeted tissues (1, 2). Clinically, PDT is a burgeoning treatment modality for diseases including late-stage esophageal cancer (3, 4), 1 Address for correspondence: HB 800, Dartmouth College, Hanover, NH 03755; e-mail: Brian.Pogue@dartmouth.edu. age-related macular degeneration (5, 6 ), and non-hyperker- atotic actinic keratosis (7 ). The mechanisms of action for PDT are complex, depending upon the photosensitizer used, the tissue being treated, and the treatment conditions. However, it is generally accepted that tumor destruction is the result of interplay among direct cytotoxicity, vascular damage, and immune reaction (1, 2). Verteporfin (benzoporphyrin derivative monoacid ring A, BPD-MA) is a second-generation photosensitizer with fast body clearance and strong light absorption at a long wave- length, which significantly increases light penetration and reduces skin photosensitization (8). Verteporfin has been approved for the treatment of age-related macular degen- eration and is under investigation for cancer treatment. A study by Fingar et al. indicated that vascular stasis, as dem- onstrated by the reduction in red blood cell column diam- eter and fluorescein exclusion assay, induced by vertepor- fin-based PDT was dependent on the interval between in- jection of the photosensitizer and light irradiation (9). They showed that irradiation delivered 5 min after injection of the photosensitizer led to a high degree of vascular stasis and a greater tumor cure rate compared to irradiation 30 min or 3 h after injection of the photosensitizer. Similarly, Kurohane et al. demonstrated that PDT with verteporfin using a 15-min drug–light interval induced hemostasis and a decrease in blood volume. However, hemostasis was not observed with 3-h interval PDT, and a significant increase in blood volume was detected (10). In both of these studies, the changes in tumor blood flow after verteporfin-PDT were not measured. In the previous study using EPR oximetry to measure RIF-1 tumor pO 2 after verteporfin-PDT, we found a large increase in tumor oxygenation during and immediately after light treatment at 3 h after drug administration, whereas tumors treated with 15-min interval PDT showed little or no increase in tumor pO 2 (11). It is still not clear why PDT at long intervals increases tumor oxygenation. This could be due to direct cellular damage leading to arrest of cell respiration (12). Thus the local pO 2 level is elevated be- cause oxygen is no longer being consumed. Changes in