Postchallenge Administration of Brincidofovir Protects Healthy and Immune-Deficient Mice Reconstituted with Limited Numbers of T Cells from Lethal Challenge with IHD-J-Luc Vaccinia Virus

Abstract

Protection from lethality by postchallenge administration of brincidofovir (BCV, CMX001) was studied in normal and immune-deficient (nude, nu/nu) BALB/c mice infected with vaccinia virus (VACV). Whole-body bioluminescence imaging was used to record total fluxes in the nasal cavity, lungs, spleen, and liver and to enumerate pox lesions on tails of mice infected via the intranasal route with 10 5 PFU of recombinant IHD-J-Luc VACV expressing luciferase. Areas under the flux curve (AUCs) were calculated for individual mice to assess viral loads. A three-dose regimen of 20 mg/kg BCV administered every 48 h starting either on day 1 or day 2 postchallenge protected 100% of mice. Initiating BCV treatment earlier was more efficient in reducing viral loads and in providing protection from pox lesion development. All BCV-treated mice that survived challenge were also protected from rechallenge with IHD-J-Luc or WRvFire VACV without additional treatment. In immune-deficient mice, BCV protected animals from lethality and reduced viral loads while animals were on the drug. Viral recrudescence occurred within 4 to 9 days, and mice succumbed ϳ10 to 20 days after treatment termination. Nude mice reconstituted with 10 5 T cells prior to challenge with 10 4 PFU of IHD-J-Luc and treated with BCV postchallenge survived the infection, cleared the virus from all organs, and survived rechallenge with 10 5 PFU of IHD-J-Luc VACV without additional BCV treatment. Together, these data suggest that BCV protects immunocompetent and partially T cell-reconstituted immune-deficient mice from lethality, reduces viral dissemination in organs, prevents pox lesion development, and permits generation of VACV-specific memory. IMPORTANCEMass vaccination is the primary element of the public health response to a smallpox outbreak. In addition to vaccination, however, antiviral drugs are required for individuals with uncertain exposure status to smallpox or for whom vaccination is contraindicated. Whole-body bioluminescence imaging was used to study the effect of brincidofovir (BCV) in normal and immunedeficient (nu/nu) mice infected with vaccinia virus, a model of smallpox. Postchallenge administration of 20 mg/kg BCV rescued normal and immune-deficient mice partially reconstituted with T cells from lethality and significantly reduced viral loads in organs. All BCV-treated mice that survived infection were protected from rechallenge without additional treatment. In immunedeficient mice, BCV extended survival. The data show that BCV controls viral replication at the site of challenge and reduces viral dissemination to internal organs, thus providing a shield for the developing adaptive immunity that clears the host of virus and builds virus-specific immunological memory. Smallpox was eradicated following a global immunization program using live vaccinia virus (VACV) vaccine implemented by the World Health Organization (WHO). Routine smallpox vaccination was subsequently discontinued due to a low but significant risk of severe adverse reactions. A...

The efficacy and pharmacokinetics of brincidofovir for the treatment of lethal rabbitpox virus infection: A model of smallpox disease

“…The development of neutralizing antibodies to RPXV was monitored throughout this study and it was found that antibody titers, not present in any animal prior to infection, were present in all rabbits who survived to the scheduled termination with no difference in titers between placebo and BCV-treated animals. This observation is consistent with reports from other investigators that BCV did not interfere with formation of protective immunity following infection or vaccination in the mouse ectromelia and vaccinia models (Parker et al, 2014;Zaitseva et al, 2015). Hence patients infected with smallpox and treated with BCV would be expected to develop full protective immunity against subsequent smallpox infection.…”

Efficacy of three key antiviral drugs used to treat orthopoxvirus infections: a systematic review

“…In immunodeficiency studies, BCV provided partial protection to mice with moderate immunodeficiency (57-100% survived). However, it could only extend time to death in mice with severe immunodeficiency (84). Tecovirimat was protective in moderately immunocompromised mice but could only delay death when mice lacked both B and T cell immunity (95,110).…”

“…To model severe immunodeficiency, BALB/c mice lacking T cells were challenged with VV-IHD. Although all mice succumbed to disease, time to death was significantly delayed (84).…”

“…Less severe immunodeficiencies were modelled by partially reconstituting mice with T cells from healthy mice 1 day prior to infection (84). BCV was significantly protective (57-100%) and facilitated the development of strong adaptive immune responses that protected mice from re-challenge without further treatment.…”

Are We Prepared in Case of a Possible Smallpox-Like Disease Emergence?

“…A multitude of compounds have been evaluated for their ability to halt Orthopoxvirus replication. These include compounds which target host tyrosine kinases [ 123 ], viral DNA replication [ 124 , 125 ], viral transcription [ 126 , 127 ], and viral morphogenesis [ 128 , 129 , 130 , 131 , 132 , 133 ]. Amongst these compounds, two are at late stages of development.…”