positive kb tumors compared with the control ht1080 tumors at 24 hour images in this set of experiments tumoral enhancement was also compared with the free nir2 dye at the 24 hour time point nir 2 fluorochrome did not result in appreciably higher signal than background similarly in competition studies fluorescence signal of fr positive kb tumor was reduced to that of fr negative ht1080 tumors the competition studies indicated that the availability of free folate was able to compete off the receptor binding to nir2 folate probe and that fluorochrome labelled ffolic acid can still be recognized by its receptor tumor to background contrast was measured and these ratios were plotted as a function of time after injection for the three experimental groups the resulting graph is shown in fig1 at the one hour time point all agents had similar tumor background ratios y axis and these ratios were only moderately elevated in kb tumors at 4 hours after injection a significantly higher tumor background ratio for the nir2 folate was observed when compared to the nir2 compound importantly for clinical applications tumor background ratios remained elevated with this probe for at least 24 48 hours indicating its potential utility for endoscopic and intraoperative use see fig1 the tumoral fluorescence signal was reduced rapidly after 72 hours 255 au u00b1 39 and returned to the baseline 115 au u00b1 17 in 5 days organ distribution of the probe was also examined after dissection highest fluorescence signal was observed.