Approximately 500,000 new cases of genital herpes are reported each year, and it is estimated that 30 million Americans are affected by this currently incurable disease. Similarly, it is estimated that there is an annual incidence of 500,000 new cases of herpes simplex gingivostomatitis and at least 100 million Americans suffer from recurrent herpes labialis. Overall the prevalence of seropositive individuals in the general population is approximately 70-80%. Although recurrent herpes simplex virus infections are the most prevalent of all herpesvirus infections, there is a need to develop more specific forms of therapy for diseases such as herpes simplex encephalitis, keratoconjunctivitis, herpetic whitlow and disseminated herpes infections of neonates and immunocompromised hosts.
The incidence of encephalitis is low (one case in 250,000 individuals per year), yet with existing therapy, the mortality rate is as high as 40% and approximately 50% of the survivors are left with severe neurological sequelae. Ocular infections are neither rare nor trivial. They are usually caused by herpes simplex virus (HSV) type 1 (HSV-1) and are a leading cause of blindness in many countries of the world. Herpetic whitlow is an occupational hazard of nurses, dentists and physicians which begins with erythema and tenderness of the distal segments of the fingers and is followed by coalescence and enlargement of the vesicles. An accompanying lymphangitis and lymphadenopathy of the draining lymphatics is a common feature. Neonatal HSV infection is usually encountered as a consequence of a child being born through an infected birth canal. The incidence of the disease is approximately 1 in 10,000 births. Mortality in babies with limited infection can be as high as 20% while mortality of neonates from disseminated infection, even with current therapy, can approach 75% and many survivors have significant neurological impairment.
Currently, nucleoside analogs are clearly the preferred therapeutic agents for HSV infections. A number of pyrimidine deoxyribonucleoside compounds have a specific affinity for the virus-encoded thymidine (dCyd) kinase enzyme. The specificity of action of these compounds confines the phosphorylation and antiviral activity of these compounds to virus-infected cells. A number of drugs from this class, e.g., 5-iodo-dUrd (IDU), 5-trifluoro-methyl-dUrd (FMAU), 5-ethyl-dUrd (EDU), (E)-5-(2-bromovinyl)-dUrd (BVDU), 5-iodo-dCyd (IDC), and 5-trifluoromethyl-dUrd (TFT), are either in clinical use or likely to become available for clinical use in the near future. IDU is a moderately effective topical antiviral agent when applied to HSV gingivostomatitis and ocular stromal keratitis, however, its use in controlled clinical studies of HSV encephalitis revealed a high toxicity associated with IDU treatment. Although the antiviral specificity of 5-arabinofuranosyl cytosine (Ara-C) was initially promising, its clinical history has paralleled that of IDU. The clinical appearance of HSV strains which are deficient in their ability to synthesize the viral thymidine kinase has generated further concern over the future efficacy of this class of compounds.
The utility of a number of viral targets has been defined for anti-HSV compound development. Studies with thiosemicarbazone compounds have demonstrated that inhibition of the viral ribonucleotide reductase enzyme is an effective means of inhibiting replication of HSV in vitro. Further, a number of purine nucleosides which interfere with viral DNA replication have been approved for treatment of human HSV infections. 9-(.beta.-D-arabinofuranosyl) adenine (Ara-A) has been used for treatment of HSV-1 keratitis, HSV-1 encephalitis and neonatal herpes infections. Reports of clinical efficacy are contradictory and a major disadvantage for practical use is the extremely poor solubility of Ara-A in water. 9-(2-hydroxyethoxymethyl) guanine (Acyclovir, ACV) is of major interest. In humans, ACV has been used successfully in the therapy of localized and disseminated HSV infections. However, there appear to be both the existence of drug-resistant viral mutants and negative results in double-blind studies of HSV-1 treatment with ACV. ACV, like Ara-A, is poorly soluble in water (0.2%) and this physical characteristic limits the application forms for ACV. The practical application of purine nucleoside analogs in an extended clinical situation suffers from their inherently efficient catabolism, which not only lowers the biological activity of the drug but also may result in the formation of toxic catabolites.
All of the effective anti-HSV compounds currently in use or clinical testing are nucleoside analogs. The efficacy of these compounds is diminished by their inherently poor solubility in aqueous solutions, rapid intracellular catabolism and high cellular toxicities. An additional caveat to the long-term use of any given nucleoside analog is the recent detection of clinical isolates of HSV which are resistant to inhibition by nucleoside compounds which were being administered in clinical trials. Antiviral oligonucleotides offer the potential of better compound solubilities, lower cellular toxicities and less sensitivity to nucleotide point mutations in the target gene than those typical of the nucleoside analogs.
It is apparent that new routes to the diagnosis and therapy of herpesvirus infections are greatly desired. It is particularly desired to provide compositions and methods for therapy which are, at once, highly effective and free of serious side effects. Thus, the provision of oligonucleotide therapies for herpesvirus infections in accordance with this invention satisfies the long-felt need for such therapies.