Antiviral activity of 2-chloro-2'-deoxyadenosine (2-CdA).

The effect of 2-chloro-2'-deoxyadenosine (cladribine, 2-CdA) on replication of herpes simplex type 1 (HSV) virus in Vero cells (green african monkey kidney line) was evaluated and compared with that of acyclovir. 2-CdA up to 600 micrograms/ml was not cytotoxic. ED50 for 2-CdA was approximately 5 micrograms/ml and ED100 was approximately 70 micrograms/ml, compared with 0.16 microgram/ml and 1.6 micrograms/ml for acyclovir, respectively. DNA cell cytometry suggested the progressive S-phase block at concentrations of 2-CdA 0.14 microgram/ml and higher. The results suggest that 2-CdA is not activated by HSV-encoded kinase, and a weak antiviral effect of the drug results from its interference with DNA replication following activation by kinases of host cells.