Antiviral Chemistry & Chemotherapy 16:283–294 Review A survey of antiviral drugs for bioweapons Arthur J. Goff and Jason Paragas* Virology Division, United States Army Medical Research Institute of Infectious Diseases, Fort Detrick, MD, USA *Corresponding author: Tel: +1 301 619 4835; Fax: +1 301 619 2290; E-mail: [email protected] Smallpox (variola major), and the haemorrhagic mortality and cause widespread panic. With the fever viruses (filoviruses and arenaviruses) are exception of smallpox and Argentine haemor- classified as Category A biowarfare agents by the rhagic fever virus, there are no vaccines or Centers for Disease Control. Category A agents approved treatments to protect against these pose a significant risk to public health and diseases. In this review we focus on promising national security because they can be easily prophylactic, therapeutic and disease modulating disseminated by aerosol, although with the excep- drugs (see Figure 1 for select chemical structures). tion of variola, they are not easily transmitted from person to person. An attack with these Keywords: orthopoxvirus, filovirus, arenavirus, viruses would result in high morbidity and biodefence, bioterrorism, category A Introduction Bioweapons are a threat to global health. Unlike nuclear routes may produce diseases that are distinct from natural and chemical weapons, virus-based biological weapons forms of the disease. To further complicate the problem, (BW) are part of the natural global flora and fauna, with these viruses could be genetically engineered to overcome one notable exception, smallpox. In this review we will vaccines, therapeutics and detection. The combination of focus on inhibitors of arenaviruses, poxviruses, and these alternatives with a few viruses significantly amplifies filoviruses. In general there are three types of inhibitors the issues surrounding drug discovery for BW agents. needed for effective biodefence: i) Prophylactic treatments Understanding what types of compounds are effective at which enable the individual to resist infection and would inhibiting viral replication will aid in developing drugs that help limit the spread of a BW agent. Drugs of this type are address possible genetic modifications. An effective scarce and of the class A viruses, only members of the antiviral serves another important role aside from limiting Orthopoxvirus genus have promising prophylactic drug replication during an infection. In its stockpiled form, it candidates. ii) Therapeutic treatments that inhibit viral may discourage use of a particular BW agent. Furthermore, replication, reduce viral load and consequently limit the because some of these viruses are endemic in different degree of mortality and morbidity. iii) Disease modifiers parts of the world, the ability to effectively treat the infec- that do not act directly against the virus but rather target tions they cause will be a significant boon to global public aspects of the viral pathogenesis and act to transform the health. host into an inhospitable environment for viral replication. These different classes of compounds can either be specific Orthopoxviruses to the virus or broad-acting. Broad-acting compounds are attractive because there would be fewer compounds to A wealth of work has been done in a variety of systems to stockpile as contingencies. In all cases, compounds need to find drugs with antiviral activity against members of the be orally available to facilitate the rapid distribution and orthopoxvirus genus, and many drugs have been identified dosing of huge numbers of people in case of a large-scale that target different parts of the viral life cycle. attack. Orthopoxviruses are the largest known animal viruses and Infection with BW agents can be significantly different encode some 200 different gene products (Moss, 2001). than an infection with their counterparts in nature. These They replicate in the cytoplasm of infected cells and are not agents may be delivered in ways that the natural virus is not dependent upon the host cell nucleus. Possible targets for usually transmitted and possibly at infectious doses that are these viruses are the DNA polymerase, virus-encoded many times what occurs in nature. These nonstandard immune modulators, structural genes and host factors ©2005 International Medical Press 0956-3202 283 A Goff & J Paragas Figure 1. Chemical structures of selected antiviral drugs CH 3 CH3 HO O NH CH3 2 O OH O OH OH CH3 H3C N CH3 H3CO NH O N O CH 3 N O P O CH N OH OH O OH N CH OH O 3 CH3 Rifampicin1 Cidofovir1 NH2 O N O N I HO NNH 2 N N N ON HO N HO O O O OH OH OH HO OH Ribavirin1,3 5-lodo-dUrd1 6‘-lodo acetylenic Ado1,2 NH2 O N I N NH CH3 N N N HO N O HO O O S O HO N N NH2 OH H OH Adenine Arabinoside1 Methisazone1,3 Idoxuridine1 O CF 3 H C HN 3 N N N HO O O N CF3 S OH Trifluridine1 Trifluoperazine3 Structures of small molecules that exhibit activity against orthopox viruses1, filoviruses2 or arenaviruses3. Some of these compounds have broad antiviral activity as noted by the superscripts following the name of the compound. 284 ©2005 International Medical Press Antiviral drugs for bioweapons necessary for replication. Most of the drugs that display al., 2000). Cidofovir was effective in monkeys exposed by anti-orthopox activity are therapeutic agents. There are aerosol to monkeypox with a single treatment of 5 mg/kg currently no known orthopox-disease modifiers. on the day of infection (Huggins et al., 1998). Molluscum contagiosum, a skin disease caused by a Prophylactic treatments poxvirus that can be severe in HIV-infected patients, Rifampicin blocks viral assembly by interacting with the resolved completely after intravenous or topical treatment product of the D13 gene of poxviruses (Sodeik et al., 1994). with cidofovir (Meadows et al., 1997; Davies et al., 1999). Rifampicin inhibited tail lesions in mice with 1 dose of Although cidofovir is a promising anti-pox drug, a signifi- 250 mg/kg 6–24 h before infection, or repeated doses for 5 cant limitation is that it is poorly absorbed when adminis- days starting 5 days before infection. Interferon and poly- tered orally (Wachsman et al., 1996), which would limit its acrylic (interferon inducer) acid were shown to be effective utility in a medical emergency. Studies by Hostetler et al. in the tail lesion model if given 24 h before vaccinia virus (1997) showed that derivatization of acyclovir with 1-O- (VV) infection (De Clercq & De Somer, 1968). Interferon hexadecyl-glycero-3-phosphate provided a compound that was also effective in keratitis in monkeys if treatment was was 100% orally bioavailable in mice. For this reason, Kern started 15 h before viral challenge (Neumann-Haefelin et al. (2002) developed a series of ether lipid analogues of et al., 1975). cidofovir and tested their efficacy and oral bioavailability in a lethal aerosol ectromelia (mouse pox) model in mice Cidofovir (Buller et al., 2004). In addition to having in vitro IC50s that The group of drugs known as acyclic nucleoside were between 11- and 60-fold better than cidofovir, the phosphonates contains the most promising anti-pox drugs ether lipids significantly reduced mortality rates in mice. investigated to date. (S)-9-(3-hydroxy-2-phosphonyl- Treatment with a 3 mg/kg oral dose of the most potent methoxypropyl)adenine [(S)-HPMPA] is phosphorylated analogue, octadecyloxyethyl-CDV (ODE-CDV) immedi- by cellular enzymes to an active di-phosphate form which ately before viral exposure resulted in a 10% mortality rate then acts as an inhibitor of viral DNA synthesis (Votruba et as compared to 100% in control mice and mice treated with al., 1987). It has a broad range of activity against DNA unmodified cidofovir. Treatment with 5 mg/kg on days 0–4 viruses including adeno-, herpes-, hepadna-, irido-, and protected 100% (16 of 16) of animals from death. poxviruses (De Clercq et al., 1986). (S)-HPMPA is effec- tive against VV replication at an inhibitory concentration Therapeutic treatments 50% (IC50) of 0.3 µg/ml (De Clercq et al., 1986) and it Ribavirin. Ribavirin (RBV) is in a class of drugs known suppressed vaccinia tail lesion formation in mice at a dose as inosine monophosphate (IMP) dehydrogenase inhibitors. of 5–100 mg/kg/day. (S)-HPMPC (cidofovir) has an It has a relatively broad spectrum of antiviral activity, activity spectrum similar to (S)-HPMPA, however it is less inhibiting both DNA and RNA viruses (Sidwell et al., toxic, so it has surpassed its adenine counterpart in devel- 1972). IMP dehydrogenase converts IMP to XMP. This opment as an antiviral drug (De Clercq, 1987). Using a step is crucial in the synthesis of the purine mononu- neutral red uptake assay, Baker et al. (2003) found that cleotides. RBV inhibits IMP dehydrogenase by RBV 5′- cidofovir is effective against over 30 different strains of monophosphate (Streeter et al., 1973) and depletes GTP variola virus with IC50s below 30 µg/ml. and dGTP. Baker et al. (2003) found RBV to inhibit three Cidofovir has been licensed (as Vistide) for treating strains of variola in Vero cells in a neutral red uptake assay cytomegalovirus (CMV) retinitis in human immunodefi- at an IC50 of 18.4 µg/ml. ciency virus (HIV)-infected patients but is being investi- RBV has already been shown to be effective in several gated for treating other DNA virus infections, namely animal models such as topical treatment of VV keratitis in infections of poxviruses (De Clercq, 1996; De Clercq, rabbits (Sidwell et al., 1973). It is also effective against 2003). (S)-HPMPC also suppressed tail lesion formation tail lesion formation in intravenous infections with VV at in normal and SCID VV-infected mice. It suppressed viral 4, 20 or 100 mg/kg/day for 4 days immediately after infec- replication in the liver, lungs, kidneys and brain while tion (De Clercq et al., 1976).