In vitro studies show that ganciclovir, cidofovir and foscarnet should be effective against HHV-6. Tokimasa et al. (2002) reported a lower rate of HHV-6 reactivations in patients receiving ganciclovir as CMV prophylaxis. Wang et al. (1996) showed in an epidemiological study that patients who received high-dose acyclovir had lower HHV-6 DNA levels and were less likely to suffer from a delayed marrow engraftment. However, in other studies, no effect on the viral load by acyclovir has been seen (Ljungman et al., 2000; Zerr et al., 2005).
Antiviral therapy with ganciclovir or foscarnet has been shown to lead to reduction in HHV-6 viral load in CSF (Zerr et al., 2002) and blood (Mendez et al., 2001; Zerr et al., 2002). Ganciclovir is also able to reduce HHV-6 viral load in saliva (Ljungman et al., 2001).
Both ganciclovir and foscarnet have been reported being effective against HHV-6 meningo-encephalitis after transplantation and the superiority of either ganciclovir or foscarnet over the other has not been established. In a review of HHV-6 encephalitis in transplant recipients, cure was documented in 7/8 patients who received ganciclovir or foscarnet for at least 7 days as compared to 0/4 in those who did not receive these drugs or received them for less than 7 days (p — 0.01) (Singh and Paterson, 2000). Table 2 shows outcome with different antiviral therapy in published and unpublished cases of HHV-6 encephalitis in SCT patients (Drobyski et al., 1994; Mookerjee and Vogelsang, 1997; Bosi et al., 1998; Cole et al., 1998; Rieux et al., 1998; Tsujimura et al., 1998; Bethge et al., 1999; De Almeida Rodrigues et al., 1999; Wang et al., 1999; Ljungman et al., 2000; Tiacci et al., 2000; Zerr et al., 2001; MacLean and Douen, 2002; Yoshida et al., 2002; Hentrich et al., 2005). Co-morbid clinical conditions such as renal failure or marrow suppression may also dictate whether ganciclovir or foscarnet is employed as therapy for HHV-6. The choice of the antiviral drug might be made based on the side-effect profile. Foscarnet may be preferable in patients with marrow suppression because it does not possess the myelosuppressive effect of ganciclovir. All 3 available agents are nephrotoxic so the choice in patients with renal dysfunction is not obvious.
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