The pyrrolo[2,3-d]pyrimidine nucleoside antibiotics tubercidin, toyocamycin, and sangivamycin and the synthetic analogues 5-chloro-, 5,6-dichloro-, 5-bromo-, 6-bromo-, 5,6-dibromo-, 5-iodo-, 5-(1-hydroxyethyl)-, 5-(1-methoxyethyl)-, (E)-5-(2-bromoethenyl)-, (E)-5-(2-cyanoethenyl)-, 5-(2-buten-1-yl)-, 5-(3-hydroxypropyl)-, and 5-butyltubercidin were evaluated for their antiviral properties against six RNA viruses and three DNA viruses in HeLa cell, primary rabbit kidney cell, and Vero cell cultures. Most of the derivatives had substantial activity against the RNA viruses, with the least activity shown by 6-bromo-, 5,6-dichloro-, and 5,6-dibromotubercidin. The C-5 substituted derivatives were quite toxic for the host cells. 5-(1-Hydroxyethyl)-, 5-(1-methoxyethyl)-, and 5-(2-buten-1-yl)tubercidin were more selective against reovirus type 1, parainfluenza virus type 3 and Coxsackie virus B4 than tubercidin and the 5-halotubercidins. When tested for in vivo activity against Coxsackie B4 virus infection in newborn NMRI mice, 5-(1-hydroxyethyl)- and 5-(1-methoxyethyl)tubercidin caused a significant decrease in the mortality rate at a dose level of 100 micrograms per mouse. The inhibitory effects on L-1210 cell growth were also determined, and toyocamycin (ID50 = 0.006 micrograms/mL) was found to be the most active compound. This study demonstrates the significance of structural modification at C-5 and the potential of C-5 substituted analogues of tubercidin as biologically active agents.
