The failure to detect enfuvirtide in CSF in the face of therapeutic levels in blood that are more than 100-fold higher than the limit of detection indicates that this therapeutic peptide is excluded from crossing the blood-brain and blood-CSF barriers and does not appreciably enter the CSF. Our reconstruction studies with spiked samples suggest that there may be as much as a 20 percent loss of the compound in CSF with storage, freezing and thawing at low concentrations, and results may also have been influenced by differences in protein binding in blood and CSF, since enfuvirtide is about 90% bound to plasma proteins, chiefly albumin [3
], and CSF binding is therefore presumed to be much lower (not directly measured). Even correcting for these factors, the CSF levels were uniformly low and likely cell exposure was below the effective therapeutic range. Our findings are paralleled by those of an earlier report demonstrating failure of enfuvirtide to cross the blood-testis barrier and reach therapeutic concentrations in semen [12
While for some other antiretroviral drugs there may be additional factors that importantly influence in vivo
efficacy compared to findings in cell culture (for example, in the case of some nucleosides differences in intracellular metabolism and substrate competition [13
] or in cell type [14
] or activation [15
] may be important), enfuvirtide is active at the cell surface, and therefore the extracellular concentration determines its antiviral activity. Hence, to the extent that local antiviral effect is important in reducing CSF (and, more broadly, CNS) HIV-1 infection, enfuvirtide is predicted not to contribute directly to viral suppression in this compartment, though it is important to caution that neither drug access to nor infection of the CSF space and brain parenchyma are identical. Likewise, without appreciable concentrations of the drug in the CSF, this compartment is less likely to serve as a site for the selection of resistance mutations, though it is possible that low, but undetectable levels in CSF might have contributed to local selection. Also, while concern about CNS toxicity of antiretroviral drugs has increased, if brain parenchymal penetration is similar to that of the CSF, enfuvirtide likely has little potential for causing neurotoxicity.
Here, we report a case in which virus rebounded to a greater extent in CSF than in the plasma. The virus which emerged during this time exhibited genotypic evidence of resistance to enfuvirtide. Notably, the sequences that showed characteristic resistance mutations were identical in the plasma and CSF, suggesting a common source. Given that the enfuvirtide concentrations were measurable only in plasma, this ‘spike’ in CSF HIV-1 RNA in the setting of CSF pleocytosis likely resulted from temporally proximate (almost certainly some time after starting enfuvirtide, and perhaps close to the time of the spike) seeding (transitory infection) and local amplification by activated CD4 T cells among the CSF cell reaction. This would be consistent with analysis of Harrington and colleagues using the heteroduplex tracking assay that suggested the origin of CSF HIV-1 in short-lived cells [16
Another of the four subjects (7071) exhibited a similar isolated CSF spike (CSF HIV RNA 8,320 copies per mL, CSF WBC to 33 cells per µL, and plasma HIV RNA levels 26 copies per mL); genotypic resistance testing was not successful in this subject. These two cases, in which virus appeared to replicate better in CSF than plasma while on a stable regimen, were highly unusual in our experience in subjects without neurological disease. In the longitudinal study from which these samples were taken, out of 435 total evaluations (36 with subjects taking enfuvirtide) in 101 subjects on antiretroviral therapy, we found only 13 examples where CSF HIV-1 RNA levels exceeded those of plasma. Seven of these 13 were from subjects 7044 and 7071 included in this report, indicating a high association with enfuvirtide treatment (p <0.0001, χ2
= 37.1), while the others were in subjects starting or stopping therapy during longitudinal follow up, and thus on unstable regimens (unpublished). While this effect also likely relates to the high level of multidrug resistance that led to the selection of these subjects to receive enfuvirtide, the properties of enfuvirtide may also have contributed. One can speculate that enfuvirtide, even in the face of resistance mutations in the illustrated subject, still had a partial effect on systemic HIV-1 replication but none within the CSF in these cases, resulting in a disproportionate rise in this compartment. While resistance to the other drugs may also have contributed, their higher CSF penetration may have given them a lesser role in the increase in CSF HIV-1 RNA. A similar pharmacologic argument has been made by our group for the failure of enfuvirtide to prevent maternal to fetal transmission even as the mother’s plasma HIV-1 RNA levels remained undetectable [17
On the other hand, the spike of CSF HIV-1, if indeed of recent plasma origin, also may, paradoxically, support an effect of non-CSF penetrating drugs on suppressing CSF HIV-1 infection. To the extent that CSF HIV-1 involves continued seeding by systemic infection, then systemically effective drugs can reduce the viral burden despite poor penetration. Systemic therapy may also reduce immune activation or other factors that modulate infection in the CNS [18
]. Thus, the impact of enfuvirtide on CNS infection may vary with the clinical setting and dynamics of seeding and local viral replication.
Without a clear understanding of these dynamics, what can be recommended regarding the use of enfuvirtide in relation to CNS infection in the individual patient? First, enfuvirtide should continue to be used in settings where it can contribute to overall antiviral effect, basing its use on available treatment choices and its effects on systemic HIV-1 infection. Fear that it is not suppressing CNS infection should not deter its use in most settings, particularly since it is now given principally to patients with multidrug resistance as a component of combination treatment that may include more than three drugs. Even in cases where HIV-1 encephalitis is responsible for active CNS injury manifesting as the AIDS dementia complex, it may be used because of its contributions to treating the systemic infection that provides ‘renewal’ of CNS infection. Our findings in CSF may not fully translate into whether enfuvirtide reaches the brain parenchyma, and even if brain penetration is generally similarly restricted, it may be higher in patients with HIV-1 encephalitis due to their characteristic disruption of the blood-brain barrier. Where it is suspected that CNS treatment is less potent than systemic treatment, it may be worthwhile to measure CSF HIV-1 RNA levels and, if there is local escape, test its resistance profile, including resistance to enfuvirtide, and tailor modifications of treatment accordingly.