1.10.1 General considerations
There are currently two main antiviral drugs administered for HCM V infection and resulting disease namely aciclovir and the more potent compound ganciclovir. Antiviral therapy can be administered in three ways; either prophylactically prior to onset or detection of systemic infection, pre-emptively prior to any onset of symptoms but following detection of systemic infection, or finally as treatment of established HCMV disease. The most cost effective and ideal administration of antiviral therapy is to treat patients pre-emptively avoiding unnecessary exposure to the toxic side effects of the drugs. However, in the past this was not always possible due to unreliability of the assays for identification of systemic HCMV infection, which were primarily based on cell culture techniques which were also slow to yield results. Thus, patients with systemic infection identified by this method had in many cases already established HCM V disease. Earlier studies examining antiviral efficacy were based on treatment of HCM V disease or prophylaxis. With the advent of more rapid and reliable PCR or antigenaemia based techniques for the detection of systemic infection together with the identification of risk factors for HCMV disease, treatment of HCM V infection is progressing to pre-emptive therapy. In addition to aciclovir and the nucleoside analogue ganciclovir, there are also a number of other antiviral drugs for the treatment of HCM V such as foscamet which is used as an altemative treatment in the advent of drug resistance to ganciclovir. There are also a variety of new compounds which are at various stages of development. For the purpose of this thesis a brief description of the antivirals used only in the transplant setting follows as relevant to this thesis.
1.10.2 Aciclovir
Aciclovir (known generically as 9-[2-hydroxyethoxymethyl]guanine, with the product name Zovirax) is an acyclic analogue of deoxyguanosine. The structure of the compound is based on guanosine with the ribose sugar moiety replaced by an acyclic side chain and is shown in figure 1.9. Activity of the compound was first discovered against HSV and
VZV. In contrast, activity against HCMV is somewhat variable with many clinical isolates requiring high doses in vitro (Biron et a l, 1980, Datta at a l, 1980, Shaeffer et al 1996) T h e mechanism of action for HSV and V ZV is well defined and is mediated by the thymidine kinase (TK) gene present in both viruses as shown in figure 1.10. The compound is taken up by cells and phoshporylated to the monophosphate form by the virus encoded TK, cellular kinases subsequently phosphorylate the compound to the di and triphosphate form. The active aciclovir triphosphate acts as a competitive inhibitor for the viral DNA polymerase. Aciclovir has a single hydroxyl group on the acyclic sugar moiety corresponding to the 5'-hydroxyl group on the normal sugar of the deoxygaunosne. This feature is important as it is this hydroxyl group that allows linkage of the next incoming nucleoside in the elongating DNA chain by formation of a second phosphate ester bond. Since aciclovir lacks the 3'hydroxyl group necessary to form this 3 -5 ' phosphodiester bond aciclovir incorporation into the DNA chain leads to chain termination (Schaeffer et a l, 1996, Furman et a l, 1996). Aciclovir can be administered either topically orally or intravenously and is well tolerated in vivo following administration with few adverse effects reported. Uninfected cells only display very low levels of aciclovir which may also be a contributing safety feature of the compound. In addition, aciclovir is neither bone marrow toxic nor immunosuppressive (Steele et a l, 1980, McGuffin et al
, 1980). Plasma levels are greatest, reaching approximately 10/^g/ml following a 5mg/kg iv dose compared to only 0 .6/.^g/ml after an oral dose of 2 0 0mg, thus oral bioavailability
is approximately 15-30%. Valaciclovir the valine ester of aciclovir, which is cleaved into aciclovir and valine during a single step catalysed by valine esterase shows increased bioavailability of up to 54% . In addition, famciclovir, the prodrug of penciclovir another nucleoside analogue based on the structure of aciclovir, shows increased biovavailability up to 77% upon cleavage and oxidation to the parent compound. The safety and efficacy of famciclovir against a variety of herpes virus infections is currently under way. Aciclovir is often used for primary prophylaxis and treatment of herpes virus infections particularly H S V in seronegative transplant patients. The effects at reducing HCM V infection are somewhat limited due to the lack of efficacy of this drug against HCMV, which is presumably related to the lack of a virally encoded TK gene. However, prophylaxis in renal transplant recipients has shown to reduce the incidence of HCM V disease (Meyers
e t a l, 1988, Balfour et a l, 1989, Stratta et a l, 1992).
1.10.3 Ganciclovir
Ganciclovir (known generically as 9-[1,3-dihydroxy-propoxymethyl] guanine with the product name Cytovene) like aciclovir, is an acyclic analogue of deoxyguanosine, the
major difference is that ganciclovir has a hydroxyl methyl group (shown in figure 1.9). This apparently minor difference confers a marked improved antiviral activity against HCM V but also an increase in cellular toxicity. Ganciclovir is effective against a broad range of herpes viruses including HSV 1 and 2, VZV, EBV and HHV-6 . The mechanism of action
of the compound is the same as for aciclovir and relies on monophosphorylation by the TK gene of HSV and VZV followed by additional phosphorylation to the di and triphosphate forms by cellular enzymes as shown in figure 1.10. HCM V as mentioned previously does not possess a virally encoded TK gene. However, ganciclovir is phosphorylated to the monophosphate form by a virally encoded protein kinase the UL97 gene product (Littler et a / , 1992, Sullivan et a l, 1992). Further phosphorylation to the di and triphosphate forms is carried out by cellular kinases, and as with aciclovir, the triphosphate form is a competitive inhibitor of the viral DNA polymerase which when incorporated into the DNA chain slows down elongation. However, unlike aciclovir, ganciclovir is not an obligate chain terminator due to the presence of the 3'hydroxyl group. The lack of this safety feature possessed by aciclovir may explain the increased toxicity of the ganciclovir especially to uninfected cells where the concentration of ganciclovir is considerably higher than that of aciclovir.
Ganciclovir can be administered either orally or intravenously and recently intraocular implants have been developed for the treatment of HCM V retinitis. Following an iv injection of 5mg/kg dose of ganciclovir, the peak level in plasma reaches approximately
8//g/m l. In contrast, as with aciclovir, the oral bioavailabiliy of ganciclovir is poor at
between 5-9%.The improved activity against HCM V is partly attributable to the increased