Verapamil Increases Rhodamine 123 Laser Phototherapy of Drug-Resistant Human Sarcoma Cells

Abstract

It has been reported previously that rhodamine 123 (Rh123) can sensitize carcinomas and several other types of human tumor cells for inactivation by laser illumination. However, sarcoma cells often were found to be resistant to Rh123. In the present study, we observed that verapamil (Vp) reverses this drug resistance in human HT1080 fibrosarcoma cells and allows Rh123 laser phototherapy. Intracellular Rh123 levels were measured by spectrophotometric absorbance at 400–600 nm after extraction of dye from cells with butanol. HT1080 cells incubated with 10 μg/ml Rh123 and 0–1 μM Vp had less than 50 ng Rh123/ml uptake with no increase over time up to 8 hr. However at 5, 10, and 100 μM Vp retention of Rh123 increased by 8 hr to peak levels of 200, 290, and 480 ng, respectively. Drug-sensitive TE671 fibrosarcoma cells treated in the same manner had linear Rh123 uptake in the absence of Vp. Argon laser and Rh123 treatment of Vp-sensitized HT1080 fibrosarcoma cells reduced tumor cell proliferation as measured by [³H]thymidine uptake over 80% compared to no effect on these cells in the absence of Vp. Western blotting with antibody to the multiple drug-resistance P-glycoprotein confirmed strong over expression by HT1080 compared to TE671 fibrosarcoma cells. These results show Vp increases Rh123 and laser sensitivity by blocking the P-glycoprotein efflux pump in the human sarcoma cells and suggests Rh123 laser phototherapy may be possible in patients with multiple drug-resistant tumors.