50 years ago in cell biology
A virologist recalls his work on cell growth inhibition
Editor's note: Citation Classics Commentaries were written by the first authors of some of the most highly cited studies published between 1961 and 1975. The essays were originally published between 1977 and 1993 in Current Contents, a publication of the Institute for Scientific Information (ISI), now Thomson Scientific. (ISI was founded by Eugene Garfield, also the founder of The Scientist.) In this essay, published in 1980, Seymour Cohen describes the experiments that led to the development of tumor-fighting agents.
Cohen's study reported that 5-fluorouracil, a previously-discovered anti-tumor agent, blocked cell cycle replication by inhibiting thymine production in bacteria and phage, identifying a crucial target in preventing tumor growth. The paper has been cited more than 775 times, according to the ISI database, and laid the groundwork for research on potential chemotherapeutic drugs. The bulk of Cohen's research at the University of Pennsylvania, and later, at the State University of New York at Stony Brook, focused on virus multiplication and harnessing aspartyl proteases for HIV treatment. Cohen retired from SUNY in 1985. He currently lives in Woods Hole, MA., where he is working on an historical text of early American chemistry.
When G.R. Wyatt and I discovered 5-hydroxymethylcytosine (HMC) in 1952,(Biochemical J. 55:774-82, 1953) my laboratory began to study its biosynthesis. In 1953, Hazel Barner and I found that a thymine deficiency led to the death of growing bacteria, (J. Bacteriology 68:80-8, 1954) and we suggested that this might explain the antitumor effects of some antifolates. We had also observed a phage-induced synthesis of thymine and HMC in bacteria auxotrophic for thymine. By 1957, Joel Flaks and I had found that extracts of phage-infected bacteria contained large amounts of two virus-induced enzymes which made the viral pyrimidines as deoxyribonucleotides (Biochim. Biophys. Acta 25:667-8, 1957). Such extracts provided the most active sources of thymidylate synthetase, permitting a study of inhibitors of this enzyme apparently crucial to DNA synthesis and cell survival. In that year, Charles Heidelberger and Robert Duschinsky had discovered that 5-fluorouracil markedly inhibited tumors in mice, (Nature 179:663-6, 1957) and they asked me to apply our bacterial and phage systems to clarifying the mode of action of the analog on thymine synthesis. I readily accepted their invitation, with the results presented in the abstract. I also suggested to Duschinsky that fluorocytosine might be a selective antifungal agent and was pleased to learn in 1959 that fluorocytosine was specifically inhibitory to these organisms (R.Duschinsky, personal communication).
Heidelberger has extended many detailed studies with fluorouracil in human cancer and helps to lead a cancer center at USC. Duschinsky has retired to almost full-time skiing in Switzerland. As a result of experiments in 1956 with spongothymidine (J. Bacteriology 71:588-97, 1956) and our first experiences in cancer chemotherapy, I became interested in the potentialities of the D-arabinosyl nucleosides and problems of chemotherapy in general. Flaks is currently a professor of biochemistry at the University of Pennsylvania. Our other coworkers have raised families and have then returned to the laboratory.
Although it is now some 20 years since the discovery that fluorouracil provokes "thymineless death," the nature of these events is less than crystal clear. Most workers today do believe that antitumor therapy with fluorouracil, as well as agents such as inhibitors of dihydrofolate reductase, e.g., amethopterin, does produce a thymine deficiency. The importance of this effect in chemotherapy has led to increasingly detailed studies of the pure synthetase and reductase and their inhibition. The primary sequences of these key enzymes determined by an infecting parasite and host should be quite different. A thorough comparison of the parasite-and host-determined enzymes may then provide a rational approach to the development of a chemotherapy necessary to selectively inhibit or kill an infecting organism.
-Seymour S. Cohen, Department of Pharmacological Sciences, State University of New York, Stony Brook, NY, September 2, 1980
Reference: S.S. Cohen et al., "The mode of action of 5-fluorouracil and its derivatives," PNAS, 44:1004-12, 1958.