Table 1.

Antiorthopoxvirus Activities^a

compd	efficacy (EC50,c μM)					toxicity (CC50,e μM) Neutral Red uptake
	VVb CPE	VVb PR	CPVd TK+ lacZ	CPVb PR	CPVd TK− lacZ
cidofovir	3.2	24 ± 12	3.3 ± 1.1	40 ± 6.1	5.2 ± 3.9	>317 ± 0
2	0.6	4.6 ± 2.0	0.8 ± 0.1	2.0 ± 0.3	28 ± 2.7	>300 ± 0
5-iodo-2′-deoxyuridine		6.0 ± 0.2	0.4 ± 0.1	2.0 ± 0.2	27 ± 3	>260a

^aProcedures adapted from Kern et al.³⁴ Assays were performed according to the procedures described previously^35–37 for activity against VV and CV and for cytotoxicity (Neutral Red uptake assay) in human foreskin fibroblast (HFF) cells. Briefly, to determine efficacy, initial cytopathogenic effect (CPE) assays were performed in 96-well plates seeded with HFF cells. Varying concentrations of drug were added to monolayers of HFF cells and challenged with VV or CV at 1000 PFU per well (incubation at 37 °C for 7 days). Confirmatory assays involving plaque reduction (PR) assays were performed using HFF cells seeded in six-well plates 2 days prior to use and infected with VV or CV by the addition of 20–30 PFU per well. Plates were incubated for 1 h. Various concentrations of drug were then added to triplicate wells, and plates were incubated at 37 °C for 3 days. Toxicity was evaluated using uninfected HFF cells seeded in 96-well plates incubated with various concentrations of drug for 7 days at 37 °C.

^bVirus used for challenge: VV (Copenhagen) or CV (Brighton).

^cValues are the mean (standard deviation of two or more assays.

^dCV strains δ crmA (TK⁺) and TK:GFP lacZ (TK⁻) were obtained from Pete Turner (University of Florida, Gainesville, FL) and were described previously.³⁸ Values were obtained using a β-galactosidase assay to determine antiviral activity.

^eCC₅₀: concentration that causes a cytotoxic effect (as ascertained by Neutral Red uptake) on 50% of uninfected cells.