HSV infections of the CNS remain a significant cause of morbidity and mortality in humans, in spite of therapy with ACV. For example, with neonatal HSV-2 infections of the CNS the mortality is low at only 5% but well over 50% of survivors suffer from significant neurologic sequelea [12
]. Similarly, with HSV-1 encephalitis of older children and adults the mortality increases to 25% and the morbidity to greater than 50%[5
]. Thus, new therapeutic approaches are necessary to improve the outcome of these devastating infections. The current studies contained in this communication indicate that CMX001 may provide an alternative to ACV either by itself or in combination with ACV.
Historically, intranasal infections in mice utilizing HSV-1 have been considered to be an excellent model for human herpes encephalitis as it uses a natural route of infection and has been predictive for the efficacy of both vidarabine and ACV in treatment of human HSV infections [20
]. Viral replication first occurs in the nasopharynx, then spreads to the CNS by olfactory and trigeminal nerves and has served to model both adult encephalitis and neonatal CNS disease. Viral replication first is detected in the olfactory bulbs and spreads throughout the CNS by neural routes from the day after inoculation until death in untreated mice by day 7. Intranasal inoculation of BALB/c mice with HSV-2 has been used as a model of disseminated neonatal herpes with CNS involvement occurring later after infection and an extended mean day to death of approximately 9 days. These studies illustrate the importance of utilizing both HSV-1 and -2 in efficacy evaluations since HSV encephalitis in adults is predominantly due to HSV-1 and in neonates about 70% of cases are due to HSV-2. These models have been predictive for efficacy of vidarabine and ACV in treatment of both HSV type-1 and -2 infections in humans.
CDV despite being a potent inhibitor of HSV replication in vitro and in experimental animal infections has not been utilized for the treatment of HSV infections in humans due to its lack of oral activity [39
] and its nephrotoxicity [40
]. However, the drug has been used for the treatment of ACV-resistant HSV infections [41
]. Several lipid conjugates of CDV have been synthesized that are active orally in animals, have reduced toxicity and have multi-fold enhanced activity against a variety of viruses including a number of the herpesviruses [24
]. The enhanced activity of the lipid conjugates appear to be due to their greater uptake into cells, possibly including the CNS, resulting in a significant increase in the amount of intracellular CDV [42
]. The conjugate that has received the most interest is the hexadecyloxypropyl analog of CDV, originally termed HDP-CDV, which is currently under development as CMX001. The interest in this molecule has been in part due to its being highly active against orthopoxviruses, which are suspected agents of bioterror.
In previous studies using cell cultures, CMX001 was about 100-fold more potent as an inhibitor of HSV replication than ACV[30
]. In the experimental infections presented in these studies, CMX001 was highly effective in preventing mortality in mice infected with HSV-1 or HSV-2 when treatments were delayed until 48 to 72 h post viral inoculation, respectively by reducing viral replication in the CNS as well as in lung, liver, spleen and kidney. Importantly, CMX001 was more efficacious than ACV in reducing viral replication in CNS in either HSV-1 or HSV-2 infected mice. Additionally, no virus replication was detected in tissues after cessation of therapy with CMX001, whereas, a rebound in virus replication was observed after cessation of therapy with ACV. In these studies, CMX001 given at 5 mg/kg once daily was more efficacious than ACV at 100 mg/kg given twice daily and suggests that CMX001 may have potential for use in the treatment of herpes encephalitis, neonatal herpes or other severe HSV infections in humans.
When CMX001 is delivered orally, it has a very unique tissue distribution profile compared to parenterally administered CDV, which does not cross the blood brain barrier. High levels of drug were found very early in lung, liver and spleen but not kidney. CMX001 had little effect on the microtubules of the kidney and no apparent toxicity [43
]. When mice were dosed with 5 mg of CMX001/kg in a quantitative drug distribution study the levels in the CNS were below the level of quantitation, however, detectable levels were observed in mice given 10 mg/kg of drug. In the antiviral studies reported in this paper the highest dose administered was 5 mg/kg once daily for 5 days. Given that the half-life of the activated antiviral form of CMX001, cidofovir-diphosphate, is 6.5 days (unpublished results), drug may accumulate in the CNS after seven daily doses, and reach levels in excess of those seen in the distribution study after a single 10 mg/kg dose. In these studies CMX001 had a significant effect on HSV replication in the CNS when therapy was delayed until 24 h post viral inoculation. This would suggest that only a small of amount of drug is needed in the CNS given its in vitro potency of about 0.03 μg/ml and/or that viral replication is also being inhibited in peripheral tissues and preventing spread to the CNS. The results obtained in these studies suggest that there is, in fact, virus already replicating in parts of the CNS when therapy was initiated and that deliverable drug can, in fact, inhibit replication in the CNS resulting in total clearance of virus from the CNS. The lack of viral persistence after treatment with CMX001 would be in contrast to previous studies showing that mice treated with valacyclovir had persistant virus detected in trigeminal ganglia and brain stem tissues [44
]. It should also be pointed out that the drug distribution studies were carried out in uninfected mice and it is certainly feasible that in infected mice additional amounts of drug may have crossed compromised blood/brain or blood/cerebral spinal fluid barriers compared to those observed in uninfected mice. In previous human studies, ACV provided as oral valacyclovir using 1 gram three times daily provided 20% of the serum concentration entering the CSF with active transport out of the CSF compartment[45
Considering the continuing mortality and long term sequelae of adults and infants infected with HSV and treated aggressively with ACV, CMX001 holds promise for improved outcomes. Its unique cellular uptake and biodistribution may prove more beneficial in particular with clinical cases of encephalitis. In addition, combination studies with CMX001 and ACV both in vitro and in vivo have been carried out and a synergistic interaction without additive toxicity has been demonstrated [46
]. This combination may improve considerably the outcome and long term well being of patients infected with these and other herpesviruses and should be considered for use in these infections.