In a cohort of HIV-infected women initiating new PI-based combination regimens, the strongest independent predictor for virologic response in adjusted analyses was hair concentrations of the anchor drug. Monitoring drug exposure may be important for chronically-administered medications when drug exposure is variable and the consequences of treatment failure are high. Interest in therapeutic drug monitoring using hair levels has recently extended to anticonvulsant[15
] and psychotropic[38
] medications. Similar to ARVs, the therapeutic index of these medications is narrow, intra-individual variation in drug levels is significant[10
], patient adherence greatly influences efficacy, drug-drug interactions alter pharmacokinetics[43
], and administration of these medications is often lifelong, making dose optimization important.
Despite its potential importance, there is no gold standard or even optimal method for the assessment of drug exposure in the field of HIV therapeutics. Self-reported adherence can be a poor surrogate for assessing ARV exposure in situations where inaccuracy of reporting is likely, drug formulations are unpredictable and parameters of bioavailability or clearance vary. Single or infrequent plasma levels of ARVs represent only a brief snapshot of drug exposure and have not consistently contributed to improving treatment outcomes. Hair levels of drug may be superior to plasma measurements in providing a prolonged assessment of drug exposure, analogous to the advantage of hemoglobin A1C (HbA1C) monitoring over single glucose levels in predicting long term outcomes of diabetes mellitus[20
Another research group has examined levels of a indinavir in hair samples and correlated these levels with virologic responses[16
]; concentrations of indinavir (IDV) in hair showed a stronger correlation with virologic suppression than plasma IDV levels in 43 HIV-infected patients [19
]. This study had several limitations: relatively large thatches of hair were required to measure IDV concentrations, which may limit the acceptability of repeated collection; lengthy sample preparation procedures were required prior to hair analysis; the relative value of hair concentrations versus self-reported adherence to HAART was not assessed; and PIs in more prevalent use in the current era were not studied.
Our study is the first to demonstrate quantification of the most commonly prescribed PIs in small hair samples (approximately 1-5mg or between 10-20 strands) using similar methods to measuring plasma levels. LC-tandem MS is a well-described method for drug level determination[27
], available in commercial laboratories, and can be performed in laboratories distant from sites of hair collection. The collection of small amounts of hair may be more acceptable to patients and our sample preparation and analysis methods are simple and inexpensive.
Unlike phlebotomy, hair collection is noninvasive and does not require specific skills, sterile or designated equipment, or storage materials. The collection of hair samples for analysis of ARV levels merely requires a pair of scissors and aluminum foil for storage. The drug-protein complex in hair is highly stable, so that hair does not require immediate processing after collection. Hair can be stored for indefinite periods of time at room temperature and shipped without precautions for biohazardous materials, offering additional feasibility advantages over blood levels for TDM. These features of this monitoring tool may make hair measurement of ARVs a useful method for assessing exposure in the developing world, where hair levels can be collected on-site and sent to outside laboratories for analysis. This approach may also be helpful when specimen collection is difficult, such as in pediatrics or when drug exposure is unpredictable, such as during pregnancy[44
] or with multiple drug-drug interactions.
Assessing hair exposure of ARVs in resource-constrained settings may be cost-effective when HIV RNA quantification is too expensive for routine monitoring. In these settings, treatment failure may be detected late, after the accumulation of multiple viral resistance mutations[45
]. The cost of hair collection is nominal; non-biohazardous shipping costs are inexpensive; and a high through-put hair analysis laboratory can perform the test economically. One possible algorithm for testing would involve measuring hair ARV levels a few months after starting a new ARV regimen and only performing HIV viral load testing if the hair levels fall below the range observed to predict virologic success in treated populations. After a patient is on stable HIV therapy, hair ARV measurements need not be performed routinely, but only when clinical disease progression is observed 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, etc.
The association of both adherence and hair levels of protease inhibitors with virologic control in our models indicates that exposure is a function of both behavioral and biologic factors. Drug levels measured in hair therefore add unique information not provided by measures of adherence. Careful assessment of adherence is indicated, and consideration of biologic factors that impede bioavailability or increase clearance should be considered, such as concomitant use of an interacting drug. Low levels of drug in hair in patients who profess optimal adherence may also prompt more comprehensive pharmacokinetic evaluation.
Prolonging the success of current and novel HIV medications is important in treating the burgeoning HIV epidemic worldwide. We have developed methods to monitor HIV drug exposure in hair and shown these levels to be the strongest predictor of treatment response in multivariate modeling. The field of HIV diagnostics lacks an optimal method for assessing exposure to medications, and hair levels have the potential of addressing that gap. Further study of this tool for therapeutic drug monitoring is indicated to demonstrate its utility in enhancing treatment responses in the global HIV setting.