Winston Churchill The various procedures in which radioactive isotopes play a major role have been applied to many studies and investigations in the fields of biology and medicine. In fact, most of the concepts of modern biology that we have been discussing in this booklet owe their discovery to the judicious use of radioisotopes. To illustrate how radioisotopes can be used to solve a practical problem, we have chosen a typical example, the investigation, at a molecular level, of the effectiveness of an anti-cancer drug. Several drugs that exert a beneficial effect, at least temporarily, on the course of certain cancers have been used by doctors for several years. Most of them were discovered empirically, that is, by accident, during routine trials against cancers. Doctors know they work but do not always know how. They would also like to know the mechanism of the drugs’ action at the molecular level so that the knowledge might open the way to the discovery of other drugs more effective against cancer and less toxic against normal cells. The following experiment shows how the molecular effect of an anti-cancer drug is studied. [Illustration: Figure 28 _Technician preparing tissues for comparative studies._] Cells growing in tissue cultures are often used to test anti-cancer drugs (see Figure 28). These cells, derived from human cell lines, are grown in glass or plastic bottles as a suspension in a nutrient medium. To begin, a culture is divided into halves. To one half is added the anti-cancer drug Actinomycin D. The other half will continue to grow without addition of other substances and will serve as a control, or comparison. After a suitable time has elapsed for the drug to act on the cultured cells, similar portions of the drug-treated cells and the control cells will be tested in several ways. One portion of each kind of cells is incubated with ³H-thymidine to determine the effect of the drug on DNA synthesis. Two other portions are incubated with ³H-cytidine to study the effect on RNA synthesis. Another pair will be tested with ¹⁴C-leucine to investigate protein synthesis. The effect of the drug, of course, is determined by comparing the untreated control with the drug-treated culture. The biochemical, autoradiographic, and counting techniques that we described previously are all used to determine the uptake of the radioisotopes into the cell’s components. Chromatography is used to ascertain if the drug has changed the concentration of precursors (thymidine, cytidine, or leucine) in the nutrient medium, since a change in these could produce misleading results. Finally, if the drug is found to have an effect on RNA, we can investigate the type of RNA that is affected by centrifuging phenol-purified RNA.