Knowledge of drug concentrations in the brains of behaving subjects remains constrained on a number of dimensions, including poor temporal resolution and lack of real-time data. Here, however, we demonstrate the ability of electrochemical aptamer-based sensors to support seconds-resolved, real-time measurements of drug concentrations in the brains of freely moving rats. Specifically, using such sensors, we achieve 1.5 hours. These results demonstrate the utility of such sensors in (i) the determination of the site-specific, seconds-resolved neuropharmacokinetics, (ii) enabling the study of individual subject neuropharmacokinetics and concentration-response relations, and (iii) performing high-precision control over intracranial drug levels.

Electrochemical, aptamer-based (EAB) sensors are the first molecular monitoring technology that is (1) based on receptor binding and not the reactivity of the target, rendering it fairly general, and (2) able to support high-frequency, real-time measurements in situ in the living body. To date, EAB-derived in vivo measurements have largely been performed using three electrodes (working, reference, counter) bundled together within a catheter for insertion into the rat jugular. Exploring this architecture, here we show that the placement of these electrodes inside or outside of the lumen of the catheter significantly impacts sensor performance. Specifically, we find that retaining the counter electrode within the catheter increases the resistance between it and the working electrode, increasing the capacitive background. In contrast, extending the counter electrode outside the lumen of the catheter reduces this effect, significantly enhancing the signal-to-noise of intravenous molecular measurements. Exploring counter electrode geometries further, we find that they need not be larger than the working electrode. Putting these observations together, we have developed a new intravenous EAB architecture that achieves improved performance while remaining short enough to safely emplace in the rat jugular. These findings, though explored here with EAB sensors may prove important for the design of many electrochemical biosensors.

Continuous, in vivo drug and biomarker measurements could transform healthcare, enabling both the high-precision personalization of drug dosing and the real-time monitoring of health status. A practical realization of this vision, however, requires an improved understanding of the relationship between concentrations measured in the easily accessible dermal interstitial fluid (ISF) that correlate with the plasma concentrations that guide clinical decision making. As a preliminary step towards this goal, here we have used electrochemical, aptamer-based (EAB) sensors to perform seconds-resolved vancomycin measurements in the plasma and subcutaneous ISF of live rats. Concentrations of the antibiotic in the ISF vary rather little between different subcutaneous sites and, after the very rapid initial distribution phase is complete, they are well correlated with the plasma concentrations (mean R² = 0.88). Likewise, a simple, two-compartment, two-parameter model describes our six paired plasma and ISF drug time courses quantitatively. Together, these findings provide further evidence of the viability of the drug concentration measurements performed in the subcutaneous or dermal ISF as a less invasive approach to real-time drug monitoring in individual patients.

Little is known regarding the extent to which the barriers separating brain subcompartments lead to differential drug transport into these subcompartments and whether such differences impact the behavioral effects of drugs. Against this background, here we have adopted electrochemical aptamer-based (EAB) sensors, a seconds-resolved, in vivo molecular measurement tool, to two tasks. First, we have measured the pharmacokinetics associated with the anesthetic procaine crossing the blood-brain-barrier into the solid tissue of the hippocampus and crossing the blood-cerebrospinal fluid barrier into the lateral ventricle. Second, we have determined the extent to which procaine concentrations in these two subcompartments correlate with the behavioral response the drug elicits. Performing these studies, we find that, whereas the drug’s elimination kinetics are effectively indistinguishable between the two subcompartments, penetration into the hippocampus is statistically significantly slower than into the lateral ventricle. The relationships between drug concentration and the resulting behavioral effect also differ significantly between the two subcompartments, with the correlation between concentration and drug-induced suppression of ambulatory behavior being stronger in the hippocampus than in the lateral ventricle. This difference presumably arises because hippocampal concentrations more accurately reflect procaine exposure at the drug’s site of action.