Extreme dark cytotoxicity of Nile Blue A in normal human fibroblasts

Early reports using mouse models indicated that Nile Blue A (NBA) is taken up more efficiently by tumor cells than normal tissue and retards tumor gro wth. NBA also shows both dark toxicity and phototoxicity of human tumor cel ls in vitro. However, studies on the dark toxicity of NBA and the effects o f NBA-mediated photodynamic treatment in normal human cells are lacking. In the current study we have examined the cytotoxicity of NBA in normal human fibroblasts, spontaneously immortalized Li-Fraumeni Syndrome (LFS) cells a nd three different human tumor cell lines. The normal human fibroblasts sho wed extreme sensitivity to NBA compared with LFS cells and the human tumor cell lines. Treatment with 0.1 mug/mL of NBA for 1 h reduced the colony for mation of normal human fibroblasts by greater than 95%, but had no signific ant effect on the colony formation of LFS cells. No significant numbers of apoptotic cells were detected in either normal human fibroblasts or LFS cel ls following this drug concentration. Thus, unlike photodynamic therapy wit h some other photosensitizers, the dark toxicity of NBA was not caused by a poptosis. Although the drug uptake was higher in normal human fibroblasts c ompared with LFS cells, the difference in sensitivity between normal human fibroblasts and LFS cells could not be accounted for by the difference in d rug uptake alone. In addition, we could not detect any significant photocyt otoxic effect of NBA in either normal human fibroblasts or LFS cells for a drug concentration of 0.05 mug/mL at light exposures of up to 6.7 J/cm(2). These data indicate an extreme sensitivity of normal human fibroblasts to N BA and an inability to produce a significant photocytotoxic effect on human cells using NBA concentrations that have relatively low toxicity for norma l human fibroblasts.