The antivirals which are usually used to target herpesviruses in ME/CFS, Valtrex, Famvir and Valcyte all cross the BBB reasonably easily. Cidofovir crosses it to a degree.
Valacyclovir for Herpes Simplex Encephalitis (2011) [
10.1128/AAC.01023-10]
Pouplin et al. said:
These results suggest that acyclovir slowly crosses the blood-brain barrier and is then actively transported out of the CSF compartment and supports the idea that the plasma profile (IV versus oral) is unlikely to significantly affect the CSF/plasma ratio.
This is consistent with my own pharmacokinetic simulation. For 500mg Valaciclovir twice daily (12h-12h) over two weeks, I predicted a Cmax in the brain of ca. 6.6 μM (of the active drug). The brain eliminates the drug very quickly but accumulates it slowly over a long period. This means a tight schedule is necessary due to the half-life of Valaciclovir. 500mg is only sufficient for HSV. Depending on the targeted virus, higher concentrations will be necessary. Within the approved dosage limit of 2g, it would only be able to inhibit HSV reliably, EBV and VZV only at its peak concentrations, but not CMV or HHV (
different assay IC50s).
This is the only antiviral drug I've personally analyzed so far because it's the drug with the most data available and the drug with the smallest mutagenicity potential. It can still damage tubular cells though.
The following study is the only one on Valcyte (valganciclovir) CSF concentrations I know.
Human pharmacokinetics of the antiviral drug DHPG (1986) [
10.1038/clpt.1986.177]
Fletcher et al. said:
The CSF concentrations of DHPG compared with the corresponding model-predicted plasma concentrations indicate CSF penetration of 24% to 67%.
The 67% seems to be derived from a model-predicated plasma concentration which is off when compared to the other three samples they collected and predicted. The models in 1986 were not very sophisticated yet. I think a reasonable penetration rate is 24% to 31%. It's in the same relative range as Valacyclovir but reaches these concentrations far sooner.
In another study, the median trough ganciclovir plasma concentration for 900mg valganciclovir (24h) daily was 6.7 µM [
10.3390/antibiotics10010077]. The median peak concentration was 31 µM. The median trough could be increased by taking 900mg in a 12h-12h interval, to about 9.4 µM, which doesn't seem to be worth it, but, similarly to aciclovir, the difference in brain tissue might be greater.
But what is the targeted threshold? The FDA published the drug application
here.
Roche Laboratories Inc. said:
Ganciclovir inhibits mammalian cell proliferation (IC50) in vitro at higher concentrations ranging from 10.21 to >250 µg/mL (40 to >1000 µM). Bone marrow-derived colony-forming cells are more sensitive (IC50 = 0.69 to 3.06 µg/mL: 2.7 to 12 µM).
This explains why anemia and thrombocytopenia are associated with taking valganciclovir.
The required virustatic concentrations vary widely depending on the cell type, metabolic signaling, the type of virus variant, and how resistant the respective variant is to the drug.
In vitro comparison of currently available and investigational antiviral agents against pathogenic human double-stranded DNA viruses: A systematic literature review, Chemaly et al., licensed under
CC BY-NC-ND 4.0, [
10.1016/j.antiviral.2019.01.008]
At least, ganciclovir seems to be more effective against HHVs, even though the toxicity margin is narrow for bone marrow cells. The peak plasma concentrations from the same oral dose may also vary widely, but the trough concentration is almost always below the inhibitory concentration in bone marrow cells. At least temporarily, the bone marrow should be able to produce new progenitors and blood cells. I doubt, however, that long-term treatment with 900mg is the best option. 450mg in a 12h-12h schedule might increase the safety while still providing sufficient efficacy.
For sufficient brain concentrations, it also depends on the targeted virus. For HSV, 450mg might be sufficient if the variant isn't resistant yet. For EBV, CMV, and HHV, 450mg would most likely not help and even 900mg might still be a lottery.