Diverse Drug Classes Partition into Human Sweat: Implications for Both Sweat Fundamentals and for Therapeutic Drug Monitoring

Therapeutic drug monitoring to optimize drug therapy typically relies on the inconvenience of repeated plasma sampling. Sweat is a potential alternative biofluid convenient for sampling. However, limited information exists regarding the range of drugs excreted in sweat and their correlation with plasma concentrations. This study evaluated drugs in sweat and plasma of an ambulatory clinical cohort. Pilocarpine-induced sweat was collected from ambulatory participants at a single instance using an absorbent nylon mesh, followed by concurrent blood sampling for ratio and correlation analyses. In a model drug study, the pharmacokinetics of acetaminophen in sweat and plasma were compared. Of the 14 drugs and 2 metabolites monitored in the clinical study, all compounds were present in sweat and plasma; however, the sweat-to-plasma ratio varied substantially across the drugs. Opioids and methocarbamol demonstrated the highest concentrations in sweat, sometimes exceeding plasma concentrations. Selected antidepressants and muscle relaxants were also detected in sweat at a 2-10-fold dilution to the plasma. Others, such as gabapentin and pregabalin, were highly diluted (>30-fold) in sweat compared with plasma. Together, these data suggest that molecular attributes, specifically hydrophobicity (logP) and charge state at physiologic pH (7.4), enable reasonable prediction of sweat-to-plasma drug correlation. These findings demonstrated that sweat could be used as an alternative biofluid for therapeutic drug monitoring. The findings also suggest that although it has been broadly accepted that small hydrophobic molecules most likely have a strong plasma correlation, there is a small window of hydrophobicity and charge state that permits sweat partitioning.