In this multivariate analysis of a cohort of HIV-infected women over time, we reveal that antiretroviral concentrations in hair are the strongest independent predictor of virologic suppression. Levels of antiretroviral drugs in hair showed a monotonic relationship to the likelihood of viral suppression (OR for success, 4.3, 12.7, 22.9, and 59.8 for each increasing quintile of hair atazanavir concentration; each P < .001) in multivariate models. Because low hair antiretroviral concentrations can predict virologic failure prior to its development, this measurement may be useful in designing interventions aimed at prolonging the durability of cART.
Concentrations of antiretroviral drugs in hair samples may provide an integrated measure of behavior and biology. Levels of medications in hair reflect drug uptake from the systemic circulation over periods of weeks to months [12
] and capture average, as well as individual, pharmacokinetic information. Single adherence measures or plasma antiretroviral concentrations provide “snapshots” of exposure, but a level measured in hair synthesize adherence and pharmacokinetic variability over time to provide a robust exposure measure in a single assay [13
]. The value of single plasma antiretroviral levels is further limited by the so-called “white coat effect,” in which adherence transiently improves prior to clinic appointments [14
], and by an inability to define meaningful therapeutic antiretroviral ranges because of substantial interindividual pharmacokinetic variability [15
]. Therefore, despite the importance of ensuring adequate exposure to the components of HIV regimens, no gold standard exists in current practice to assess antiretroviral exposure.
Our models show that antiretroviral exposure as measured in hair far surpasses commonly used covariates to predict HIV treatment outcomes [17
]. Failed antiretroviral regimens result in substantial long-term adverse effects, including increased drug and diagnostic testing costs, as well as avoidable clinical and transmission events. Because patients who experience virologic failure on a regimen demonstrate attenuated rates of immune reconstitution on future regimens [18
], substantial efforts should be made to optimize first-line cART. Preserving responses to first-line regimens are of particular import in resource-limited settings, where the average annual cost of second-line cART regimens can be up to 8 times that of first-line regimens [19
]. The risk of viremia on therapy is highest in the first year after initiating cART and can be linked to increased HIV transmission rates [20
]. Therefore, when initiating HIV treatment, the incorporation of an effective antiretroviral exposure measure, such as hair concentrations, during initial monitoring may avert early virologic failure, blunted responses to subsequent regimens, and the need for expensive or inaccessible second-line regimens.
Previous models of outcomes with atazanavir-based regimens have failed to define precise parameters of atazanavir exposure that increase the likelihood of virologic suppression [15
]. This failure could be a result of using single plasma atazanavir concentrations in these models to define exposure instead of a longer term measure. A recent report revealed that average adherence to dosage with boosted PI regimens was a better predictor of virologic suppression than was duration or frequency of missed doses [22
]. Hair antiretroviral concentrations average daily exposure variability in a manner analogous to that of glycosylated hemoglobin A1C providing information on mean daily glucose levels in diabetic patients. A previous analysis by our group demonstrated that hair levels of antiretrovirals are more closely correlated with areas under the curve from intensive pharmacokinetic studies than are single plasma levels [23
]. Therefore, it is not surprising that hair antiretroviral measurements predict treatment outcomes with greater accuracy than do single plasma levels. Another analysis by our group, directly comparing levels of antiretrovirals in hair with plasma levels to predict treatment responses, demonstrated the superiority of hair levels [23
In addition to showing that atazanavir concentrations in hair predict virologic responses more strongly than self-reported adherence or other factors, we demonstrate that lapses in adherence, antiretroviral exposure, or virologic suppression are all associated with an increased likelihood of subsequent failure. Since adherence difficulties or the presence of detectable virus during therapy are well-known contributors to virologic failure, either state is likely to trigger corrective measures in the clinical setting. However, the inaccuracy of self-reported adherence, the lack of routine virologic monitoring in many resource-limited settings, and the fact that detectable viral loads when available may already indicate mutations [24
] all increase the appeal of finding another tool to prospectively predict failure. Our models show that low antiretroviral hair levels portend a high risk of virologic failure; hair measures in the clinical setting could therefore trigger interventions to correct either adherence or low pharmacokinetic levels (eg, through assessing drug-drug interactions or diet) to extend regimen durability (). Of note, only participants with hair atazanavir measurements in the higher quintiles during WIHS visits following a failure scenario had rates of virologic suppression similar to those in clinical trials. This supports the concept that the reasons for low drug levels in hair must be investigated and addressed by adherence intervention, change in regimen, or possibly dose increase (the latter requires additional study). Our group is currently planning a clinical trial assessing adherence interventions, regimen change, or dose modification of antiretrovirals based on hair measurements of anchor regimen components in a clinical setting.
Possible algorithm for use of atazanavir hair levels in the clinical setting. ATV, atazanavir.
As we proposed previously [7
], one possible algorithm for testing would involve measuring antiretroviral levels in hair soon after starting a new antiretroviral regimen and performing HIV viral load testing only if the hair levels fall into the lower quintiles as defined above (). Data showing that the risk of viremia on therapy is highest during the first year after initiating cART [20
] supports the use of these measures soon after regimen initiation. Data from the resource-limited setting showing that routine viral load monitoring decreases rates of virologic resistance over routine monitoring alone may argue for the use of hair measures as a surrogate for the former [25
]. If the atazanavir level in hair is ≤1.78 ng/mg (first or second quintile), the rates of virologic failure approach 50% and intensive adherence interventions (vs a pharmacokinetic evaluation for low exposure if adherence is deemed adequate) should be triggered. After a patient is receiving stable HIV therapy, antiretroviral measurements using hair need not be performed routinely but only when clinical disease progression is observed (for settings where routine CD4 cell count or viral load monitoring are not available) or when an alteration in drug exposure is predicted, such as a new drug-drug interaction, pregnancy, change in dietary patterns, change in liver or renal function, and so on.
Unlike phlebotomy, hair collection is noninvasive and does not require specific skills, sterile equipment, or specialized storage conditions. The collection of hair samples for analysis merely requires a pair of scissors, and storage is at room temperature. Hair can be stored for long periods of time prior to analysis, shipped without precautions for biohazardous materials, and analyzed economically in a high-throughput hair analysis laboratory. These features may make this monitoring tool particularly advantageous in the resource-poor setting, especially when routine viral load monitoring is prohibitively expensive. We recently applied these hair measures in a nested case-control study in 2 South African public health clinics and found that low concentrations of lopinavir in hair had a high predictive value for virologic failure in that setting [25
]. We are currently working on developing a lower cost, point-of-care method of analyzing antiretroviral levels in hair for resource-constrained settings to increase the feasibility of this tool. The results of the analyses presented here argue for the possibility of hair antiretroviral concentrations serving as a method of HIV therapeutic drug monitoring that may increase the durability of current antiretroviral regimens in a variety of settings.