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.

The kinetics with which chemotherapeutics distribute into solid tissues, including their sites of both action and toxicity, remains poorly characterized. This is due to the limited temporal resolution of traditional methods of measuring drug concentrations in the body, all of which employ sample collection (e.g., via a blood draw or microdialysis) followed by benchtop analysis. Here, we have used electrochemical aptamer-based (EAB) sensors to perform simultaneous, 12 s resolution, nanomolar-precision measurements of the chemotherapeutic doxorubicin in the jugular vein (plasma) and subcutaneous space (interstitial fluid) of live rats. The resulting data sets identify predictively strong correlations between its plasma and solid-tissue pharmacokinetics in terms of both cumulative (area under the curve) and maximum exposure. In contrast, the correlations between delivered body-mass-adjusted and body-surface-area-adjusted doses and drug exposure in both the plasma and solid tissue are relatively poor. The latter observation highlights the need for therapeutic drug monitoring, and the former observation shows the potential value of employing subcutaneous EAB sensors as a convenient, minimally invasive, high-precision means of performing such monitoring. The high time density of our two-compartment data sets also provides unprecedented opportunities to model the distribution of a drug from the central compartment to a distal physiological compartment. We find that the preferred description of doxorubicin transport into the solid tissues for five of our six data sets is a three-compartment model composed of the vein (plasma), the interstitial fluid, and an unobserved third compartment distal to the interstitial fluid, with this additional compartment presumably representing intracellular fluid.

To overcome early cancer detection challenges, diagnostic tools enabling more sensitive, rapid, and noninvasive detection are necessary. An attractive cancer target for diagnostic blood tests is human Ecto-NOX disulfide–thiol exchanger 2 (ENOX2), expressed in most human cancer types and regularly shed into blood sera. Here, we developed an electrochemical DNA-based (E-DNA) biosensor that rapidly detects physiologically relevant levels of ENOX2. To identify ENOX2-binding aptamers that could potentially be used in a biosensor, recombinantly expressed ENOX2 was used as a binding target in an oligonucleotide library pull-down that generated a highly enriched ENOX2-binding aptamer. This candidate aptamer sensitively bound ENOX2 via gel mobility shift assays. To enable this aptamer to function in an ENOX2 E-DNA biosensor, the aptamer sequence was modified to adopt two conformations, one capable of ENOX2 binding, and one with disrupted ENOX2 binding. Upon ENOX2 introduction, a conformational shift to the ENOX2 binding state resulted in changed dynamics of a redox reporter molecule, which generated a rapid, significant, and target-specific electrical current readout change. ENOX2 biosensor sensitivity was at or below the diagnostic range. The ENOX2 E-DNA biosensor design presented here may enable the development of more sensitive, rapid, diagnostic tools for early cancer detection.

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.