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Cidofovir Treatment of Variola (Smallpox) in the Hemorrhagic Smallpox Primate Model and the IV Monkeypox Primate Model

JW Huggins1, SH Zwiers1 , RO Baker1, LE Hensley1 , T Larsen2, MJ Martinez2 , PB Jahrling3

1Viral Therapeutics Branch, 2Veterinary Pathology Division, and 3Headquarters, US Army Medical Research Institute of Infectious Diseases, Fort Detrick, USA

Please direct all queries to the authors at the addresses given.

Cidofovir is capable of inhibiting smallpox and other orthopoxviruses in vitro and can successfully treat mice that are lethally infected with cowpox by either the intranasal or true small particle aerosol routes. Aerosol infection of cynomolgus monkeys with high doses (107 pfu) of the Zaire 76 strain of monkeypox produced a lethal fibrinonecrotic bronchopneumonia, which could be successfully treated with cidofovir.

To access the efficacy of cidofovir against smallpox, two models of intravenous infection of cynomolgus monkeys with variola (smallpox) were developed based on challenge dose. The extent of the characteristic pox rash of smallpox and monkeypox correlates with disease severity in man. Intravenous infection with 107 pfu of monkeypox produces a lethal model that faithfully reproduces the rash lesional disease characteristic of smallpox and monkeypox, while similar studies with 108 pfu the Harper strain of variola produced a similar lesional disease with > 250 lesions (WHO category “grave”) and 33% mortality (day 11), but increasing the virus challenge dose 10-fold (2 X 1010 genomes) resulted in a 100% acutely lethal disease (MTD 4 days) that more closely mimics hemorrhagic smallpox, with virus levels in organs 1,000 to 10,000 fold greater.

This model of hemorrhagic smallpox was used to demonstrate successful prophylaxis with cidofovir, but the overwhelming nature of the infection makes the hemorrhagic smallpox model inappropriate to determine the treatment window for classical smallpox. To determine if a lesional model could be successfully treated, we use the uniformly lethal IV monkeypox model that corresponds more closely to the 108pfu of variola model to show that cidofovir prophylaxis provided complete protection, showed no signs of illness and controlled virus replication in blood, while the placebo-treated animal had >850 lesions and levels of virus in blood > 107 genomes/ ml and died on day 12. Based on these studies we believe the lesional model of smallpox produced by 108 pfu of Harper strain is the most appropriate primate model for drug evaluation.

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