Teratogenic Effects of 5-chlorodeoxyuridine on the Rat Fetus; Protection by Physiological Pyrimidines.

systems indicate that the type of halogen, fluorine, bro mine, iodine, or chlorine, substituted at the 5-position of uracil deoxyriboside greatly influences the activity and the type of inhibition of cell proliferation. 5-Fluorodeoxy uridine (FUdR) inhibits deoxyribonucleic acid (DNA) synthesis by blocking the methylation of deoxyuridylic acid to form thymidylic acid (10, 19, 20), whereas bromo deoxyuridine (BUdR), iododeoxyuridine (IUdR), and chlorodeoxyuridine (C1UdR) inhibit the incorporation of thymidine (TdR) into DNA (12, 15, 16, 20, 38, 39, 42). In addition, 5-bromouracil (BU), -iodouracil (113), and chlorouracil (CR3) substitute for the thymine of DNA (14, 19, 35, 43, 44), whereas 5-fluorouracil (FU) and chlorouracil (C1U) are incorporated in place of uradil into bacterial ribonucleic acid (RNA) (9, 13, 19, 21, 44). The use of radioactive forms of the abnormal pyrimidines on