Effects of storage conditions on the performance of an electrochemical aptamer-based sensor.

The electrochemical aptamer-based (EAB) sensor platform is the only molecular monitoring approach yet reported that is (1) real time and effectively continuous, (2) selective enough to deploy in situ in the living body, and (3) independent of the chemical or enzymatic reactivity of its target, rendering it adaptable to a wide range of analytes. These attributes suggest the EAB platform will prove to be an important tool in both biomedical research and clinical practice. To advance this possibility, here we have explored the stability of EAB sensors upon storage, using retention of the target recognizing aptamer, the sensor's signal gain, and the affinity of the aptamer as our performance metrics. Doing so we find that low-temperature (-20 °C) storage is sufficient to preserve sensor functionality for at least six months without the need for exogenous preservatives.

The Use of Xenonucleic Acids Significantly Reduces the In Vivo Drift of Electrochemical Aptamer-Based Sensors.

Electrochemical aptamer-based sensors support the high-frequency, real-time monitoring of molecules-of-interest in vivo. Achieving this requires methods for correcting the sensor drift seen during in vivo placements. While this correction ensures EAB sensor measurements remain accurate, as drift progresses it reduces the signal-to-noise ratio and precision. Here, we show that enzymatic cleavage of the sensor's target-recognizing DNA aptamer is a major source of this signal loss. To demonstrate this, we deployed a tobramycin-detecting EAB sensor analog fabricated with the DNase-resistant "xenonucleic acid" 2'O-methyl-RNA in a live rat. In contrast to the sensor employing the equivalent DNA aptamer, the 2'O-methyl-RNA aptamer sensor lost very little signal and had improved signal-to-noise. We further characterized the EAB sensor drift using unstructured DNA or 2'O-methyl-RNA oligonucleotides. While the two devices drift similarly in vitro in whole blood, the in vivo drift of the 2'O-methyl-RNA-employing device is less compared to the DNA-employing device. Studies of the electron transfer kinetics suggested that the greater drift of the latter sensor arises due to enzymatic DNA degradation. These findings, coupled with advances in the selection of aptamers employing XNA, suggest a means of improving EAB sensor stability when they are used to perform molecular monitoring in the living body.

High-Affinity Aptamers for In Vitro and In Vivo Cocaine Sensing.

The ability to quantify cocaine in biological fluids is crucial for both the diagnosis of intoxication and overdose in the clinic as well as investigation of the drug's pharmacological and toxicological effects in the laboratory. To this end, we have performed high-stringency in vitro selection to generate DNA aptamers that bind cocaine with nanomolar affinity and clinically relevant specificity, thus representing a dramatic improvement over the current-generation, micromolar-affinity, low-specificity cocaine aptamers. Using these novel aptamers, we then developed two sensors for cocaine detection. The first, an in vitro fluorescent sensor, successfully detects cocaine at clinically relevant levels in 50% human serum without responding significantly to other drugs of abuse, endogenous substances, or a diverse range of therapeutic agents. The second, an electrochemical aptamer-based sensor, supports the real-time, seconds-resolved measurement of cocaine concentrations in vivo in the circulation of live animals. We believe the aptamers and sensors developed here could prove valuable for both point-of-care and on-site clinical cocaine detection as well as fundamental studies of cocaine neuropharmacology.

High-precision monitoring of and feedback control over drug concentrations in the brains of freely moving rats.

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 <4 µM limits of detection and 10-s resolution in the measurement of procaine in the brains of freely moving rats, permitting the determination of the pharmacokinetics and concentration-behavior relations of the drug with high precision for individual subjects. In parallel, we have used closed-loop feedback-controlled drug delivery to hold intracranial procaine levels constant (±10%) for >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.

A tight squeeze: geometric effects on the performance of three-electrode electrochemical-aptamer based sensors in constrained, in vivo placements.

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.

Real-Time, Seconds-Resolved Measurements of Plasma Methotrexate In Situ in the Living Body.

Dose-limiting toxicity and significant patient-to-patient pharmacokinetic variability often render it difficult to achieve the safe and effective dosing of drugs. This is further compounded by the slow, cumbersome nature of the analytical methods used to monitor patient-specific pharmacokinetics, which inevitably rely on blood draws followed by post-facto laboratory analysis. Motivated by the pressing need for improved "therapeutic drug monitoring", we are developing electrochemical aptamer-based (EAB) sensors, a minimally invasive biosensor architecture that can provide real-time, seconds-resolved measurements of drug levels in situ in the living body. A key advantage of EAB sensors is that they are generalizable to the detection of a wide range of therapeutic agents because they are independent of the chemical or enzymatic reactivity of their targets. Three of the four therapeutic drug classes that have, to date, been shown measurable using in vivo EAB sensors, however, bind to nucleic acids as part of their mode of action, leaving open questions regarding the extent to which the approach can be generalized to therapeutics that do not. Here, we demonstrate real-time, in vivo measurements of plasma methotrexate, an antimetabolite (a mode of action not reliant on DNA binding) chemotherapeutic, following human-relevant dosing in a live rat animal model. By providing hundreds of drug concentration values, the resulting seconds-resolved measurements succeed in defining key pharmacokinetic parameters, including the drug's elimination rate, peak plasma concentration, and exposure (area under the curve), with unprecedented 5 to 10% precision. With this level of precision, we easily identify significant (>2-fold) differences in drug exposure occurring between even healthy rats given the same mass-adjusted methotrexate dose. By providing a real-time, seconds-resolved window into methotrexate pharmacokinetics, such measurements can be used to precisely "individualize" the dosing of this significantly toxic yet vitally important chemotherapeutic.