Topical CTC-96 accelerates wart growth in rabbits infected with cottontail rabbit papillomavirus

In vitro and Animal Models for Antiviral Therapy in Papillomavirus Infections

The need for antiviral therapies for papillomavirus infections is well recognized but the difficulties of reproducing the infectious cycle of papillomaviruses in vitro has hindered our understanding of virus-cell interactions and the regulation of viral gene expression during permissive growth. Recent advances in understanding the temporal expression and function of papillomavirus proteins has enabled consideration of a targeted approach to papillomavirus chemotherapy and in particular the inhibition of viral replication by targeting the E1 and E2 proteins. There are in vitro culture systems available for the screening of new chemotherapeutic agents, since significant advances have been made with culture systems which promote epithelial differentiation in vitro. However, to date, there are no published data which show that virions generated in vitro can infect keratinocytes and initiate another round of replication in vitro. In vivo animal models are therefore necessary to assess the efficacy of antivirals in preventing and treating viral infection, particularly for the low-risk genital viruses which are on the whole refractory to culture in vitro. Although papillomaviruses affect a wide variety of hosts in a species-specific manner, the animals most useful for modelling papillomavirus infections include the rabbit, ox, mouse, dog, horse, primate and sheep. The ideal animal model should be widely available, easy to house and handle, be large enough to allow for adequate tissue sampling, develop lesions on anatomical sites comparable with those in human diseases and these lesions should be readily accessible for monitoring and ideally should yield large amounts of infectious virus particles for use in both in vivo and in vitro studies. The relative merits of the various papillomavirus animal models available in relation to these criteria are discussed.

Reversible magnetogenic cobalt complexes

A cobalt tris(2-pyridylmethyl)amine complex cycles between stable paramagnetic Co(ii) and diamagnetic Co(iii) forms with corresponding changes in the MRI contrast.

Bioreductive activation and drug chaperoning in cobalt pharmaceuticals

The potential for cobalt(III) complexes in medicine, as chaperones of bioactive ligands, and to target tumours through bioreductive activation, has been examined over the past 20 years. Despite this, chemical properties such as reduction potential and carrier ligands required for optimal tumour targeting and drug delivery have not been optimised. Here we review the chemistry of cobalt(III) drug design, and recent developments in the understanding of the cellular fate of these drugs.

Cottontail rabbit papillomavirus (CRPV) model system to test antiviral and immunotherapeutic strategies

The cottontail rabbit papillomavirus (CRPV)/rabbit model has proven to be the most versatile preclinical model to test antiviral, immunopotentiating and immunotherapeutic strategies for papilloma-virus infections. We have utilized this model for many years and have recently observed significant improvements in the utility of the model. Improvements have included various techniques to infect rabbit skin sites with strains of wild-type and mutant CRPV DNA prepared using standard molecular biological procedures. A better understanding of the virus life cycle in vivo has been gained also from these studies such that we now have several defined strains of CRPV including i) antigenically diverse strains of CRPV, ii) mutant strains of CRPV with reduced growth rates, and iii) mutant strains of CRPV that demonstrate accelerated malignant progression rates. Collectively, these mutant genomes provide laboratories with a powerful pre-clinical model to assess a variety of antiviral therapies. Many of the treatments already tested in the CRPV/rabbit model have shown parallel efficacy against HPV infections in a clinical setting. Some of our recent experiences with the CRPV/rabbit model are outlined in this brief overview.

Herpes simplex virus type 1 entry is inhibited by the cobalt chelate complex CTC-96

The CTC series of cobalt chelates display in vitro and in vivo activity against herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). The experiments described here identify the stage in the virus life cycle where CTC-96 acts and demonstrate that the drug inhibits infection of susceptible cells. CTC-96 at 50 microg/ml has no effect on adsorption of virions to Vero cell monolayers. Penetration assays reveal that CTC-96 inhibits entry of the virus independent of gC and cellular entry receptors. This observation was supported by the failure to detect the accumulation of virus-specified proteins and alpha mRNA transcripts when CTC-96 is present at the onset of infection. Moreover, virion-associated alphaTIF does not accumulate in the nucleus of cells infected in the presence of CTC-96. CTC-96 targets the initial fusion event between the virus and the cell and also inhibits cell-to-cell spread and syncytium formation. Furthermore, CTC-96 inhibits plaque formation by varicella-zoster virus and vesicular stomatitis virus as efficiently as by HSV-1. Collectively, these experiments suggest that CTC-96 is a broad-spectrum inhibitor of infection by enveloped viruses and that it inhibits HSV-1 infection at the point of membrane fusion independent of the type of virus and cellular receptors present.

In vivo anti-papillomavirus activity of nucleoside analogues including cidofovir on CRPV-induced rabbit papillomas

A series of nucleoside analogues were tested for in vivo anti-papillomavirus activity using the cottontail rabbit papillomavirus (CRPV) domestic rabbit model. Compounds were delivered either topically, injected into growing papillomas, or delivered subcutaneously at a site remote from the papillomas. Compounds tested included cidofovir [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine] (HPMPC); cyclic HPMPC (cHPMPC); cyclopentenylcytosine (CPE-C); lobucavir [1R(1alpha,2beta,3alpha)]-9-[2, 3-bis(hydroxymethyl)cyclobutyl]guanine; 9-((2-phosphonylmethoxy)propyl)adenine (PMPA); adefovir 9-((2-phosphonylmethoxy)ethyl)adenine(PMEA) and cyclopropyl 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (cyclopropylPMEDAP). Dose response curves and time-course treatments were included for most compounds tested. Strong anti-viral activity was detected using cidofovir and cHPMPC when delivered either topically or by the intralesional route. Complete cures were obtained using 1% (w/v) topical cidofovir at dosing schedules of twice daily for 8 weeks beginning at 4 weeks after CRPV infection, which represents a time when papillomas were clearly visible. Complete cures of large established papillomas were obtained by intralesional injection of 1% cidofovir three times per week for 8 weeks. Topical treatments with adefovir had strong anti-viral activity, cyclopropyl PMEDAP had moderate anti-viral activity, and CPE-C, PMPA and lobucavir showed no effects. These data indicate that certain nucleoside analogues have strong in vivo anti-papillomavirus activity and that the CRPV/rabbit model is a good model for assessing clinical responses of anti-viral treatments for patients with HPV disease.

Topical effects of cidofovir on cutaneous rabbit warts: treatment regimen and inoculum dependence

The present study examined topical effects of cidofovir on cutaneous rabbit warts. Based on an inoculum-dependency study, each New Zealand White rabbit was inoculated with a high and low titer of cottontail rabbit papillomavirus (CRPV) at four sites on each dorsolateral area. Inoculation with 50 ID(50) induced papillomas at 100% of the inoculation sites within 16+/-1 days, and the wart growth curve plateaued within approximately 7 weeks. With an inoculum of 5 ID(50), 80% of the inoculated sites developed papillomas within 21+/-1 days and their size plateaued at a later time. Cidofovir was applied topically twice daily on the inoculated sites at a concentration of 1% for 18 days, starting at three different time points. In the first experiment, treatment was initiated 7 days post-inoculation. One of the inoculated sides received cidofovir or the vehicle, PBS, while the other side was left untreated. With this treatment regimen, cidofovir significantly delayed the time of onset and the growth rate of papillomas induced with the high titer of inoculum. It completely prevented papilloma-induction on the sites inoculated with the low titer of CRPV. Reversible side-effects of cidofovir were observed on the directly treated area including erythema, necrosis, and flaking. Both therapeutic and side-effects were limited to the sites of direct exposure. In the second experiment, one of the two sides in each group of rabbits received cidofovir or vehicle starting on day 29 post-inoculation. With this treatment regimen, cidofovir significantly reduced wart growth against the low titer only. Topical treatment initiated on day 49 post-inoculation was not effective on warts initiated with either viral titer. These results demonstrated that topical cidofovir could be very effective against papillomavirus-induced wart growth if it is initiated early during the infection, especially against low titers of inoculum.