A trial with 3'-azido-2',3'-dideoxythymidine and human interferon-α in cats naturally infected with feline leukaemia virus

Antiviral therapy in cats progressively infected with feline leukaemia virus: lessons from a series of 18 consecutive cases from Australia

BACKGROUND

It is doubtful that any of the treatments proposed for feline leukaemia virus (FeLV) infection are effective, despite the entity being described 60 years ago.

METHODS

Eighteen pet cats with progressive FeLV infections were recruited in Australia. One or more antiviral drugs were trialled in 16 cats, while two FeLV-infected cats were not handleable and served as untreated controls. Six cats were administered RetroMAD1™ only (0.5 mg/kg orally twice daily), a commercially available recombinant chimeric protein with proposed antiretroviral activity. Three cats were administered the integrase inhibitor raltegravir only (10-15 mg/kg orally twice daily), a drug used as a component of highly effective antiretroviral therapy for human immunodeficiency virus (HIV-1) infection. Three cats were administered RetroMAD1™ and raltegravir concurrently, and four cats were administered raltegravir and the reverse transcriptase inhibitor zidovudine (AZT, 5 mg/kg orally twice daily) concurrently. FeLV RNA and p27 antigen loads were measured at two timepoints (T1-2 months and T3-5 months) during therapy and compared to baseline (pretreatment) levels, to assess the response to therapy using linear modelling. The median survival time (MST) of the cats from commencement of FeLV treatment to death was also determined and compared between treatments.

RESULTS

The MST for the 16 FeLV-positive cats which received antiviral therapy was 634 days, while the MST from FeLV diagnosis to death for the two untreated control cats was 780 days. In cats treated with RetroMAD1™, FeLV viral load decreased from T0 to T1-2 months (median viral load reduced from 1339 × 106 to 705 × 106 copies/mL plasma; P = 0.012), but MST was reduced compared to cats not given RetroMAD1™ (426 days vs 1006 days; P = 0.049). Cats treated with raltegravir and AZT had no significant changes in FeLV viral load over time, but p27 antigen load was decreased from T0 to T3-5 months in cats treated with raltegravir (median p27 antigen level reduced from 50.2% to 42.7%; P = 0.005). All other results were not significantly affected by the treatment provided. Importantly, statistically significant and substantial associations were found between age at FeLV diagnosis and survival time (P = 0.046, R2 = 18.6) and between FeLV viral load at T0 and survival time (P = 0.004, R2 = 44.4). Younger cats, and cats with higher levels of pretreatment FeLV RNA, had reduced survival times. Cats treated with RetroMAD1™ were typically younger (median age 2.0 vs 8.0 years), likely explaining the observed reduction in MST. A significant association was found between FeLV viral load and p27 antigen load at T0 (P = 0.015, R2 = 32.9).

CONCLUSIONS

Results from this small case series do not provide convincing support for the use of RetroMAD1™, raltegravir or AZT, alone or in combination, for the treatment of cats progressively infected with FeLV. The changes observed were biologically insignificant. Age and FeLV viral load at diagnosis are useful prognostic markers, and p27 antigen concentration can be used to predict viral load. Larger field trials should be performed examining antiretroviral therapy in FeLV-positive cats with progressive infections, preferably using three or more drugs from at least two classes, as is standard with human antiretroviral therapy. Future studies would be easier in countries with a higher prevalence of FeLV infections than Australia.

Undetectable proviral DNA and viral RNA levels after raltegravir administration in two cats with natural feline leukemia virus infection

Feline leukemia virus (FeLV) infection was discovered over 50 years ago; however, the serious clinical changes associated with FeLV infection still have great importance in the diagnosis, prevention, and clinical management of symptomatic patients. Progressive infection with FeLV leads to a reduction in the patient's life expectancy and quality of life. This report describes the use of an antiretroviral integrase inhibitor, raltegravir, in two cats with natural FeLV infection. Raltegravir was administered orally at a dose of 40 mg/cat every 12 h in both cases. In case one, 13 weeks after starting raltegravir, RNA loads were undetectable, while proviral DNA loads were still detectable. In case two, proviral DNA loads were undetectable after 32 weeks of medication, while RNA loads were undetectable throughout the treatment. No adverse effects or laboratory test abnormalities were detected with the use of raltegravir in either patient. The patients are currently clinically healthy, still receiving the drug, and are under close observation. To our knowledge, this is the first report describing the use of raltegravir in naturally infected FeLV-positive cats and its effects on circulating viral load. Moreover, the patients described here were followed-up for a longer period than those in previously reported cases.

Interferon therapies in small animals

Interferons (IFNs) are cytokines that play an important role in the immune response of animals and humans. A number of studies reviewed here have evaluated the use of human, canine and feline IFNs as treatments for infectious, inflammatory and neoplastic disease in dogs and cats. Recombinant canine IFN-γ is deemed an efficacious therapy for canine atopic dermatitis. Recombinant feline IFN-ω is effective against canine parvoviral enteritis and has also been recommended for canine atopic dermatitis. Based on limited evidence, recombinant canine IFN-α could be a topical treatment option for dogs with gingivitis and keratoconjunctivitis sicca. Conclusive evidence is lacking for other diseases and large randomised controlled trials are needed before IFNs can be recommended for other indications.

Follow-Up of Viral Parameters in FeLV- or FIV-Naturally Infected Cats Treated Orally with Low Doses of Human Interferon Alpha

Specific treatments for the long-life infections by feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are either toxic, expensive or not too effective. Interferon α (IFN-α) is an immunomodulatory molecule which has been shown in vitro to decrease the release of infective particles. The aim of this study was to follow the progress of the clinical score and viral parameters of FeLV- and FIV-naturally infected privately owned cats treated with recombinant human IFN-α (rHuIFN-α, Roferon-A). Twenty-seven FeLV-infected cats (FeLV+) and 31 FIV-infected cats (FIV+) were enrolled in the study. Owners were instructed to orally administer 1 mL/day of 60 IU rHuIFN-α/mL in alternating weeks for four months. Blood samples were taken at the beginning of the study (M0), mid-treatment (M2), end of treatment (M4), and 6-10 months later (M10). Clinical status at these time points improved notably with rHuIFN-α treatment, regardless of the initial severity of the disease, an effect which lasted throughout the study in most animals (15 of the 16 FeLV+ symptomatic cats; 20 of the 22 FIV+ symptomatic cats) improved markedly their clinical situation. In FeLV+ cats plasma antigenemia (p27CA), reverse transcriptase (RT) activity, and proviral load decreased at M2 and M4 but increased again at M10 ("rebound effect"). The level of antigenemia or RT activity was below the detection limits in FIV+ cats, and the effect on proviral load was less marked than in FeLV+ cats. Taken together, these results indicate that rHuIFN-α is a good candidate for treating FeLV+ cats, but the "rebound effect" seen when treatment was discontinued suggests that additional studies should be conducted to clarify its effect on progression of the infection in cats.

Efficacy of antiviral chemotherapy for retrovirus-infected cats: What does the current literature tell us?

GLOBAL IMPORTANCE

The two feline retroviruses, feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV), are global and widespread, but differ in their potential to cause disease.

VIRAL INFECTION - FIV

FIV, a lentivirus that shares many properties with human immunodeficiency virus (HIV), can cause an acquired immune deficiency syndrome, which predisposes cats to other infections, stomatitis, neurological disorders and tumours. Although secondary infections are common, specific opportunistic infections or acquired immunodeficiency virus-defining infections, such as those that occur with HIV, are not commonly reported in FIV-infected cats. In most naturally infected cats, FIV does not cause a severe clinical syndrome; with appropriate care, FIV-infected cats can live many years before succumbing to conditions unrelated to their FIV infection. Thus, overall survival time is not necessarily shorter than in uninfected cats, and quality of life is usually high over many years or lifelong.

VIRAL INFECTION - FELV

FeLV, an oncornavirus, is more pathogenic than FIV. Historically, it was considered to account for more disease-related deaths and clinical syndromes in cats than any other infectious agent. Recently, the prevalence and importance of FeLV have been decreasing, mainly because of testing and eradication programmes and the use of FeLV vaccines. Progressive FeLV infection can cause tumours, bone marrow suppression and immunosuppression, as well as neurological and other disorders, and leads to a decrease in life expectancy. However, with appropriate care, many FeLV-infected cats can also live several years with a good quality of life.

PRACTICAL RELEVANCE

A decision regarding treatment or euthanasia should never be based solely on the presence or absence of a retrovirus infection. Antiviral chemotherapy is of increasing interest in veterinary medicine, but is still not used commonly.

EVIDENCE BASE

This article reviews the current literature on antiviral chemotherapy in retrovirus-infected cats, focusing on drugs that are currently available on the market and, thus, could potentially be used in cats.

Evaluation of the effect of short-term treatment with the integrase inhibitor raltegravir (Isentress) on the course of progressive feline leukemia virus infection

Cats persistently infected with the gammaretrovirus feline leukemia virus (FeLV) are at risk to die within months to years from FeLV-associated disease, such as immunosuppression, anemia or lymphoma/leukemia. The integrase inhibitor raltegravir has been demonstrated to reduce FeLV replication in vitro. The aim of the present study was to investigate raltegravir in vivo for its safety and efficacy to suppress FeLV replication. The safety was tested in three naïve specified pathogen-free (SPF) cats during a 15 weeks treatment period (initially 20mg then 40mg orally b.i.d.). No adverse effects were noted. The efficacy was tested in seven persistently FeLV-infected SPF cats attained from 18 cats experimentally exposed to FeLV-A/Glasgow-1. The seven cats were treated during nine weeks (40mg then 80mg b.i.d.). Raltegravir was well tolerated even at the higher dose. A significant decrease in plasma viral RNA loads (∼5×) was found; however, after treatment termination a rebound effect was observed. Only one cat developed anti-FeLV antibodies and viral RNA loads remained decreased after treatment termination. Of note, one of the untreated FeLV-A infected cats developed fatal FeLV-C associated anemia within 5 weeks of FeLV-A infection. Moreover, progressive FeLV infection was associated with significantly lower enFeLV loads prior to infection supporting that FeLV susceptibility may be related to the genetic background of the cat. Overall, our data demonstrate the ability of raltegravir to reduce viral replication also in vivo. However, no complete control of viremia was achieved. Further investigations are needed to find an optimized treatment against FeLV. (250 words).