Cats presenting with clinical signs of ocular herpesvirus infection often appear uncomfortable, demonstrating blepharospasm, excessive lacrimation, and sometimes conjunctivitis, chemosis, acute corneal ulceration, and chronic stromal keratitis. Clinicians and owners alike experience frustration in witnessing this significant discomfort given that there are no clinically proven, easily used treatments available. Although a number of topical antiviral ophthalmic medications have been used empirically to treat feline herpesvirus type 1 (FHV-1), all require frequent application, and no antiviral drug has ever been developed specifically for FHV-1 or for use in cats.
Herpesviruses are large DNA viruses with a short replication cycle that infect through mucosal and epithelial surfaces. Once a host is infected, there is lifelong latency, principally in neurons. In cats, FHV-1 is the most common cause of ocular surface disease. Despite the widespread use of FHV-1 vaccines, which reduce the frequency and severity of FHV-1 associated disease, many cats still develop herpetic ocular disease, and this can be recurrent throughout life.
In this study, a feline whole corneal explant model was used to evaluate three topical antiviral drugs for their efficacy in inhibiting FHV-1 replication. The corneas used in the study were harvested from recently deceased cats unrelated to this study; these cats had been vaccinated against FHV-1 but never exposed to an active infection. Two of the drugs, cidofovir and acyclovir, are anti-herpes nucleoside analogues, while raltegravir is a retroviral integrase inhibitor.
Cidofovir and acyclovir are used clinically to treat FHV-1 ocular infection in cats, and have been studied in vitro in two-dimensional feline cell cultures. In addition, cidofovir was shown in one study to cause a 33% reduction in FHV-1 viral titers in FHV-1-infected cats, compared to FHV-1-infected control cats not treated with this drug. The oral prodrug of acyclovir, valacyclovir, is toxic to cats, but topical acyclovir has been used to treat drug-resistant herpetic corneal ulcers. In one study, acyclovir was used successfully to treat cats with ocular FHV-1 when used five times daily for 21 days. Much higher concentrations of acyclovir compared to cidofovir are required to efficiently inhibit FHV-1 replication in the feline corneal explant model; it is unknown if long-term topical administration of acyclovir might be toxic to cats.
Raltegravir, a retroviral integrase inhibitor used in the treatment of human immunodeficiency virus, is also capable of blocking replication of all three families of herpesviruses. It appears to be safe to administer systemically to cats.
The corneal explants were infected with FHV-1 and cultured at either 34⁰C or 37⁰C; ultimately all experiments were done with corneas cultured at 34⁰C, as viral growth was found to be fourfold higher at this temperature compared to 37⁰C. Feline corneas in the living animal have a temperature of approximately 33⁰C, so ocular herpesviruses are probably adapted to grow at temperatures lower than normal core body temperatures. Those corneas infected with FHV-1 demonstrated epithelial thinning and detachment, compared to mock-infected corneas, whch displayed normal, multi-layer stratified squamous epithelium. Viral antigen staining was demonstrated in the FHV-1 infected corneas but not in the mock-infected corneas.
Both cidofovir and acyclovir inhibited FHV-1 replication in the whole corneal explant model. Treatment of the FHV-1 infected corneas with these drugs resulted in a reduction in viral load of 98±3% with cidofovir and 73±20% with acyclovir. Both of these drugs caused a significant reduction in virus production by 36 hours post-treatment. By 48 hours post-treatment, cidofovir resulted in the complete absence of extracellular infectious virus. Treatment with these drugs limited the reduction in corneal epithelial thickness observed after FHV-1 infection.
Initially, the investigators determined that raltegravir was not toxic in vitro to Crandell-Reese feline kidney cells or to feline corneal epithelium in explants. Then, the antiviral properties of raltegravir were investigated in the corneal explant model system; FHV-1 infected corneas were treated with 500 µM raltegravir or left untreated every 24 hours for 2 days. The raltegravir was found to significantly inhibit FHV-1 replication and prevent corneal epithelial thinning caused by FHV-1 in the whole corneal explant model system. In contrast to the nucleoside analogues tested, which needed to be added at 12-hour intervals, raltegravir inhibited FHV-1 replication in the feline corneal explant system when added to the system every 24 hours. As client-patient compliance can be a significant problem in the clinical treatment of ocular FHV-1 infections, raltegravir may be a promising new treatment for this frustrating and painful condition, because it was shown to be effective when given once daily. [PJS]