Efforts toward the continuous monitoring of molecular markers of performance

Abstract

Objectives: Technologies supporting the continuous, real-time measurement of blood oxygen saturation and plasma glucose levels have improved our ability to monitor performance status. Our ability to monitor other molecular markers of performance, however, including the hormones known to indicate overtraining and general health, has lagged. That is, although a number of other molecular markers of performance status have been identified, we have struggled to develop viable technologies supporting their real-time monitoring in the body. Here we review biosensor approaches that may support such measurements, as well as the molecules potentially of greatest interest to monitor.

Design: Narrative literature review.

Method: Literature review.

Results: Significant effort has been made to harness the specificity, affinity, and generalizability of biomolecular recognition in a platform technology supporting continuous in vivo molecular measurements. Most biosensor approaches, however, are either not generalizable to most targets, or fail when challenged in the complex environments found in vivo. Electrochemical aptamer-based sensors, in contrast, are the first technology to simultaneously achieve both of these critical attributes. In an effort to illustrate the potential of this platform technology, we both critically review the literature describing it and briefly survey some of the molecular performance markers we believe will prove advantageous to monitor using it.

Conclusions: Electrochemical aptamer-based sensors may be the first truly generalizable technology for monitoring specific molecules in situ in the body and how adaptation of the platform to subcutaneous microneedles will enable the real-time monitoring of performance markers via a wearable, minimally invasive device.

Keywords: Aptamers; Electrochemistry; Health monitoring; Pharmacokinetics; Technology; Wearable electronic devices.

Fig. 1.

Electrochemical aptamer-based (EAB) sensors are the first real-time molecular monitoring technology that is both independent of the reactivity of its targets and able to work in the complex, time-varying environments found in the body. (A) EAB sensors consist of an aptamer re-engineered to undergo reversible binding-induced folding. This is modified with a redox reporter and attached (via a self-assembled monolayer) to a gold electrode. The binding-induced conformational change causes a corresponding change in electron transfer rate that is easily monitored using electrochemistry. (B) Bundled with its counter and reference electrodes, current intravenous EAB sensors are small enough and flexible enough to be emplaced, for example, in a rat jugular vein. Using such in vivo EAB sensors, we and others have (C) collected drug pharmacokinetic timecourses and (D) dose response curves, (E) monitored the levels of endogenous metabolites, and (F) spatially resolved drug levels across a tumor. Panels C-F are adapted with permission from their respective references.

Fig. 2.

EAB sensors employ aptamers, synthetic oligonucleotides generated via an in vitro evolutionary scheme, as their recognition elements. (A) In this process, a library of random sequence, single stranded DNAs is introduced to targets of similar structure to the target of interest and any bound sequences are removed to select against sequences of poor specificity. The remaining sequences are then introduced to the target and unbound sequences are removed. The remaining sequences are then amplified and the cycle repeated until it converges on a pool of high affinity, high-specificity receptors. (B) Aptamers can be of exceptional affinity and specificity. Shown, for example, is the cross-reactivity of a mephedrone-binding aptamer with both synthetic and natural cathinone analogs. Adapted with permission from ref .

Fig. 3.

The strong square-wave frequency dependence of EAB signaling provides routes to improve signal gain and correct the drift seen in vivo. (A) Signal generation in EAB sensors is driven by binding-induced changes in the rate with which an attached redox reporter transfers electrons. (B, C) The resulting change in electron transfer rate is easily monitored using square wave voltammetry, the frequency of which can be tuned such that the sensor's signal increases (“signal on” behavior) or decreases (“signal off”) upon target binding. (D) To correct for the drift seen in vivo, a “signal on” and “signal off” frequency are selected that drift in concert. Taking the difference between these increases the observed signal and removes the drift. Adapted with permission from ref .

Fig. 4.

Overview of the feedback mechanisms involved in the hypothalamic–pituitary–adrenal (HPA) and hypothalamic–pituitary–gonadal (HPG) axes. Solid lines represent stimulation whereas dotted lines represent inhibition. Figure created with BioRender.com.