The difficulty is selecting for the hybridomas, since the majority of cells after the fusion are myeloma cells that did not fuse. The solution to this problem was to exploit purine and pyrimidine salvage pathways in a selection scheme. However B cells grow poorly in culture. Thus, the hybridoma cells outgrow the B cells and the unfused myeloma cells die. Fluoro compounds and treatment of cancer Enzyme mechanisms often require the removal of a proton from a specific site on a substrate.
Fluorine is so electronegative that it makes a very poor leaving group.
By substituting F for H at a critical site, the enzyme can become trapped in a ternary complex that won't release the fluorine. This is illustrated in box 22-1 page 713. Thymidylate synthase is needed for DNA synthesis. 5 fluoro uracil gets converted to 5 fluoro-dUMP by the normal route, but thymidylate synthase cannot extract the F and it becomes permanently inhibited. This blocks DNA synthesis.