1 Biodefense Clinical Research Branch, Office of Clinical Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892; 2 Department of Aerobiology and Product Evaluation, U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID), 1425 Porter St., Fort Detrick, Maryland 21702, USA
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Abstract |
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Keywords: monkeypox , airborne infections , antiviral therapy , aerosol therapy , cidofovir , vaccination , biodefence
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Introduction |
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Several considerations indicate that antiviral prophylaxis would be a valuable addition to our defences against smallpox. Because the success of global eradication led to the end of universal vaccination, more than half the world's population is now unprotected against the disease. Large amounts of vaccine are being stockpiled, but current plans do not call for initiating mass vaccination until after an outbreak has been detectedmeaning that some persons may already have been infected by the time they receive the vaccine, when immunization may be only partially protective.2,5 In addition, currently available vaccines pose a risk of serious complications in persons with atopic dermatitis or cell-mediated immunodeficiency, and may not induce protective responses in the latter.6 Concern that the virulence of variola virus, the agent of smallpox, could be increased by genetic manipulation, enabling it to overcome vaccine-induced immunity, is also a driving force for developing effective drug prophylaxis.7
A two-pronged approach of vaccination and antiviral prophylaxis would resemble measures already in place for influenza. Vaccines are the basic means of limiting a flu epidemic, but protection is also available in the form of antiviral drugs: an aerosolized compound (zanamivir) delivered from a portable inhaler, or oral medications (oseltamivir and amantadine). Such treatment, begun either before or after exposure to an influenza patient, can be highly effective in preventing or mitigating illness and blocking disease transmission.8,9 The stockpiling of these drugs has been urged in preparation for a possible flu pandemic.9
As presently employed, cidofovir is an intravenous medication that requires increased hydration and other measures to prevent nephrotoxicity, so it would be difficult to administer the drug on a large scale during a smallpox epidemic. In this article, we describe recent proof-of-concept studies indicating that cidofovir could also be administered by aerosol or in an orally available form, that treatment would be effective in preventing disease if given before or after exposure to smallpox, and that these approaches would be safer than intravenous therapy. Our own studies have evaluated the protective efficacy of aerosolized cidofovir in mice,10,11 whereas others have synthesized orally available forms of the compound and given them to mice by gavage.1216 After reviewing basic information on cidofovir and the pathogenesis of smallpox, we present the results of these proof-of-concept studies, then discuss whether cidofovir prophylaxis would interfere with simultaneous vaccination.
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Cidofovir and poxviral infections |
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Cidofovir is not toxic to cells at the low levels needed to block viral replication, but intravenous treatment can cause the rapid accumulation of damaging concentrations in the kidneys unless special precautions are taken.24 Efforts to adapt cidofovir for smallpox prophylaxis aim to avoid this problem, either by delivering a low dose of aerosolized drug to the respiratory tract, or by taking advantage of the gradual absorption of a larger orally administered dose to provide sufficient time for renal excretion.
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Smallpox pathogenesis: implications for antiviral prophylaxis |
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The 1012-day incubation period ends when the release of cytokines and virus from infected cells into the bloodstream causes fever, malaise and development of a vesiculopustular rash in the skin and oropharynx (lesion formation at these sites is favoured by a virus-encoded epidermal growth factor).2,5 In nearly half of cases, the circulating virus titre is low enough to produce discrete pocks separated by areas of normal skin, but in a small percentage of patients, much higher levels of viral replication result in a shortened incubation period and rapidly lethal haemorrhagic disease.
The ability of innate and adaptive immune responses to restrict viral replication during the incubation period appears to play the major role in determining the outcome of infection.5 If given early enough after exposure to smallpox, vaccination may stimulate the development of cross-protective immunity and reduce the severity of subsequent illness. Aerosolized or oral cidofovir may provide an additional means of blocking the development of clinical smallpox, either by creating a barrier to infection in the respiratory tract, or by preventing the internal spread of virus.
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Modelling antiviral prophylaxis in laboratory animals |
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Orally available cidofovir derivatives
The intestinal absorption of cidofovir can be greatly enhanced by attaching an ether lipid residue to the phosphonate group (Figure 1d).1214 Not only is uptake increased from <1% for cidofovir to >98% for cidofoviralkoxyalkanol esters such as octadecyloxyethyl (ODE)cidofovir, but these compounds are also taken up much more efficiently into cells. They then remain in the cytoplasm for some time before the ester bond is cleaved, increasing the drug half-life to some 810 days. As a result, alkoxyalkanol esters of cidofovir are 50100-fold more active against variola and other poxviruses than the parent compound.1214
Initial testing of orally available cidofovir derivatives in mice showed that a single dose, given as long as 5 days before infection, protected against lethal intranasal cowpox virus challenge.15 A low daily dose (5 mg/kg) of ODEcidofovir, the most active compound, also protected mice against aerosolized ectromelia virus when treatment was begun on the day of infection.16 Treatment completely blocked viral replication in the liver and spleen.
Data from mice also indicate that treatment with orally available cidofovir derivatives would pose less risk of nephrotoxicity than intravenous cidofovir therapy. In the experiment shown in Figures 2(a and b), parenteral injection of cidofovir caused rapid drug accumulation in the kidneys, whereas a similar dose of orally administered ODEcidofovir was absorbed much more gradually from the intestinal tract, resulting in a 30-fold lower peak renal concentration.13 Even though most of an oral dose is taken up by the liver, distribution of the remainder to the spleen, lungs and other organs should provide tissue levels capable of blocking variola replication. Initial studies have found no evidence of hepatotoxicity, but additional testing will be required.
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Aerosol prophylaxis aims to block the initiation of smallpox infection by delivering cidofovir to cells lining the respiratory tract and to adjacent lymph nodes. Initial studies in mice showed that a single dose of 0.55 mg/kg of aerosolized cidofovir prevented lethal intranasal or aerosol cowpox virus infection and gave equal or better protection than a subcutaneous dose of 100 mg/kg.10 Treatment was equally effective when given the day before or on the day of virus challenge. Subcutaneous injection of radiolabelled cidofovir caused rapid drug accumulation in the kidneys (Figure 2c), but aerosol delivery of a similar dose resulted in retention of a significant fraction in pulmonary tissues and a 15-fold lower peak renal concentration (Figure 2d). 11 One day after treatment, the level of cidofovir in the lungs of aerosol-treated mice was three times higher than in the kidneys, whereas in subcutaneously injected mice the kidney concentration was 75 times higher than in the lungs.
These findings suggest that a single dose of aerosolized cidofovir could protect the respiratory tract against poxviral infection for a number of days, without risk of renal injury. Aerosol delivery of a compound such as ODEcidofovir might further extend protection for as long as several weeks. Our studies revealed no evidence of pulmonary toxicity, but further testing will be required. Experience with the use of cidofovir in the human respiratory tract is so far limited to the treatment of laryngeal papillomas by injection,25 supplemented in one case by aerosol (R. Snoeck, personal communication).
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Effect of cidofovir treatment on vaccination |
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Conclusion |
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Acknowledgements |
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Footnotes |
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References |
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2 . Fenner, F., Henderson, D., Arita, I., et al. (1988). Smallpox and its Eradication. World Health Organization, Geneva, Switzerland.
3 . De Clercq, E., Neyts, J. & Clercq, E. D. (2003). Therapy and short-term prophylaxis of poxvirus infections: historical background and perspectives. Antiviral Research 57, 2533.[CrossRef][ISI][Medline]
4 . Baker, R., Bray, M. & Huggins, J. (2003). Potential antiviral therapeutics for smallpox, monkeypox and other orthopoxvirus infections. Antiviral Research 57, 1323.[CrossRef][ISI][Medline]
5 . Bray, M. & Buller, R. (2004). Looking back at smallpox. Clinical Infectious Diseases 38, 8829.[CrossRef][ISI][Medline]
6 . Bray, M. (2003). Pathogenesis and potential antiviral therapy of complications of smallpox vaccination. Antiviral Research 58, 10114.[CrossRef][ISI][Medline]
7
.
Jackson, R., Ramsay, A., Christensen, C. et al. (2001). Expression of mouse interleukin-4 by a recombinant ectromelia virus suppresses cytolytic lymphocyte responses and overcomes genetic resistance to mousepox. Journal of Virology 75, 120510.
8 . Osterhaus, A. & de Jong, J. (1999). The control of influenza: antivirals as an adjunct to vaccines. Vaccine 18, 77980.[CrossRef][ISI][Medline]
9 . Monto, A. (2003). The role of antivirals in the control of influenza. Vaccine 21, 1796800.[CrossRef][ISI][Medline]
10 . Bray, M., Martinez, M., Kefauver, D. et al. (2002). Treatment of aerosolized cowpox virus infection in mice with aerosolized cidofovir. Antiviral Research 54, 12942.[CrossRef][ISI][Medline]
11
.
Roy, C., Baker, R., Washburn, K. et al. (2003). Aerosolized cidofovir is retained in the respiratory tract and protects mice against intranasal cowpox virus challenge. Antimicrobial Agents and Chemotherapy 47, 29337.
12
.
Kern, E., Hartline, C., Harden, E. et al. (2002). Enhanced inhibition of orthopoxvirus replication in vitro by alkoxyalkyl esters of cidofovir and cyclic cidofovir. Antimicrobial Agents and Chemotherapy 46, 9915.
13 . Ciesla, S., Trahan, J., Wan, W. et al. (2003). Esterification of cidofovir with alkoxyalkanols increases oral bioavailability and diminishes drug accumulation in kidney. Antiviral Research 59, 16371.[CrossRef][ISI][Medline]
14
.
Aldern, K., Ciesla, S., Winegarden, K. et al. (2003). Increased antiviral activity of 1-O-hexadecyloxypropyl-[2-(14)C]cidofovir in MRC-5 human lung fibroblasts is explained by unique cellular uptake and metabolism. Molecular Pharmacology 63, 67881.
15
.
Quenelle, D., Collins, D., Wan, W. et al. (2004). Oral treatment of cowpox and vaccinia virus infections in mice with ether lipid esters of cidofovir. Antimicrobial Agents and Chemotherapy 48, 404412.
16 . Buller, R., Owens, G., Schriewer, J. et al. (2004). Efficacy of oral active ether lipid analogs of cidofovir in a lethal mousepox model. Virology 318, 47481.[CrossRef][ISI][Medline]
17 . Neyts, J. & De Clercq, E. (1993). Efficacy of (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl) cytosine for the treatment of lethal vaccinia virus infections in severe combined immune deficiency (SCID) mice. Journal of Medical Virology 41, 2426.[ISI][Medline]
18 . Bray, M., Martinez, M., Smee, D. et al. (2000). Cidofovir protects mice against lethal aerosol or intranasal cowpox virus challenge. Journal of Infectious Diseases 181, 109.[CrossRef][ISI][Medline]
19 . Smith, K. & Skelton, H. (2002). Molluscum contagiosum: recent advances in pathogenic mechanisms, and new therapies. American Journal of Clinical Dermatology 3, 53545.[Medline]
20 . Geerinck, K., Lukito, G., Snoeck, R. et al. (2001). A case of human orf in an immunocompromised patient treated successfully with cidofovir cream. Journal of Medical Virology 64, 5439.[CrossRef][ISI][Medline]
21 . Smee, D., Sidwell, R., Kefauver, D. et al. Characterization of wild-type and cidofovir-resistant strains of camelpox, cowpox, monkeypox, and vaccinia viruses. Antimicrobial Agents and Chemotherapy 46 1329-35.
22 . Ho, H., Woods, K., Bronson, J. et al. (1992). Intracellular metabolism of the antiherpes agent (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine. Molecular Pharmacology 41, 197202.[Abstract]
23 . Hao, Z., Stowe, E., Ahluwalia, G. et al. (1993). Characterization of 2,3-dideoxycytidine diphosphocholine and 2,3-dideoxycytidine diphosphoethanolamine. Prominent phosphodiester metabolites of the anti-HIV nucleoside 2,3-dideoxycytidine. Drug Metabolism and Disposition 21, 73844.[Abstract]
24 . Cundy, K. (1999). Clinical pharmacokinetics of the antiviral nucleotide analogues cidofovir and adefovir. Clinical Pharmacokinetics 36, 12743.[ISI][Medline]
25 . Snoeck, R., Wellens, W., Desloovere, C. et al. (1998). Treatment of severe laryngeal papillomatosis with intralesional injections of cidofovir [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine]. Journal of Medical Virology 54, 21925.[CrossRef][ISI][Medline]
26 . Smee, D., Bailey, K. & Sidwell, R. (2004). Topical cidofovir is more effective than parenteral therapy for treatment of progressive vaccinia in immunocompromised mice. Journal of Infectious Diseases 190, in press.
27 . Sutter, G. & Staib, C. (2003). Vaccinia vectors as candidate vaccines: the development of modified vaccinia virus Ankara for antigen delivery. Current Drug Targets for Infectious Disorders 3, 26371.