I cannot find the IC50 value of 5-FU against HT-144 cancer cells, as determined via cytotoxicity analysis. I shall be thankful if someone help me with relevant literature.
Be sure to check out GDSC database. To my knowledge, it is one of the biggest (if not the biggest) database of cancer cell lines and anticancer drugs available.
There is a IC50 value reported for 5-F in HT-144. However you will have to dive into the references to have a good idea of how the value was estimated and if it fits your conditions. As a reference compared to other cell lines or treatments it is still quite useful.
This link should take you directly to where you could find the IC50 for 5-f in that cell line.