Preliminary Screening and In Vitro Confirmation of Orthopoxvirus Antivirals

The lack of antiviral drugs for the treatment of orthopoxvirus disease represents an unmet medical need, particularly due to the threat of variola virus (the causative agent of smallpox) as an agent of biowarfare or bioterrorism (Henderson, 283:1279-1282, 1999). In addition to variola, monkeypox, cowpox, and vaccinia viruses are orthopoxviruses of concern to human health (Lewis-Jones, 17:81-89, 2004). Smallpox vaccination, using the closely related vaccinia virus, is no longer provided to the general public leading to a worldwide population increasingly susceptible not only to variola but to monkeypox, cowpox, and vaccinia viruses as well. Orthopoxviruses share similar life cycles (Fenner et al., WHO, Geneva, 1988), and significant nucleotide and protein homology, and are immunologically cross-protective against other species within the genus, which was the basis of the highly successful vaccinia virus vaccine. These similarities also serve as the basis for screening for antivirals for dangerous pathogens such as variola and monkeypox virus using generally safer viruses such as cowpox and vaccinia. Methods for preliminary screening and initial characterization of potential orthopoxvirus antivirals in vitro, using vaccinia virus as a relatively safe surrogate for more pathogenic orthopoxviruses, are described herein. They include candidate identification in a viral cytopathic effect (CPE) assay as well as evaluation of the antiviral activity in inhibition assays to determine mean effective (or inhibitory) concentrations (EC₅₀ or IC₅₀). These assays were utilized in the identification and early characterization of tecovirimat (ST-246) (Yang et al., 79:13,139-13,149, 2005). These initial steps in identifying and characterizing the antiviral activity should be followed up with additional in vitro studies including specificity testing (for other orthopoxviruses and against other viruses), single-cycle growth curves, time of addition assays, cytotoxicity testing, and identification of the drug target.