A Minimally Invasive, Subcutaneous Biosensor Reliably Estimates Plasma Methotrexate Exposure and Quantifies Plasma-to-Tissue Transport Kinetics

Electrochemical aptamer-based (EAB) sensors enable high-frequency, real-time measurements of drug concentrations in situ in the body, offering a potential means of rendering therapeutic drug monitoring (TDM) as convenient, timely, and precise as the continuous glucose monitor has rendered the measurement of blood sugar. Fully realizing this vision, however, will require improved understanding of the relationship between drug concentrations in the interstitial fluid (ISF) and in the plasma, as the latter are the basis of nearly all clinical decision-making. Toward this end, here we have used an EAB sensor against the chemotherapeutic methotrexate, a drug that requires monitoring to optimize efficacy and minimize toxicity, to perform simultaneous, seconds-resolved concentration measurements in the plasma and subcutaneous ISF of live rats. Noncompartmental analysis reveals that, while body-mass-adjusted dose is a poor predictor of methotrexate exposure (as area under the curve) in either bodily compartment (R2 ≤ 0.35), exposure in the ISF strongly predicts plasma exposure (R2 = 0.85, p = 0.00002). The latter, predictive relationship is sufficiently strong that dosing guided by minimally invasive ISF measurements is twice as accurate as body-mass adjusted dosing at achieving a desired systemic methotrexate exposure. Leveraging the high temporal density of these paired data sets, we also applied compartmental modeling to characterize methotrexate transport between plasma and tissues. Across all 12 paired data sets, the preferred model is a two-compartment differential transport model that allows for asymmetric exchange between plasma and the ISF, providing a significantly more accurate description of the observed kinetics than simple passive diffusion.