Development of a liposomal near-infrared fluorescence lactate assay for human blood

175 Years of Bilirubin Testing: Ready for Point-of-Care?

Bilirubin was first detected in blood in 1847 and since then has become one of the most widely used biomarkers for liver disease. Clinical routine bilirubin testing is performed at the hospital laboratory, and the gold standard colorimetric test is prone to interferences. The absence of a bedside test for bilirubin delays critical clinical decisions for patients with liver disease. This clinical care gap has motivated the development of a new generation of bioengineered point-of-care bilirubin assays. In this Perspective, recently developed bilirubin assays are critically discussed, and their translational potential evaluated.

Portable Near-Infrared Fluorometer for a Liposomal Blood Lactate Assay

In sepsis, plasma lactate is a key biomarker of disease severity, prognosis, and treatment success. However, the median time to result for clinical lactate tests is 3 h. We recently reported a near-infrared fluorescent (NIRF) blood lactate assay that relies on a two-step enzymatic reaction in a liposomal reaction compartment. This assay was optimized in human blood and was capable of quantifying lactate in fresh capillary blood from human volunteers at clinically relevant concentrations in 2 min. However, these studies were performed with a tabletop fluorescence plate reader. For translation to the point of care, the liposomal lactate assay needs to be combined with a small portable NIR fluorometer. Portable NIR fluorometers were successfully used for the analysis of skin and soil samples, but reports for blood metabolite assays are scarce. We aimed at testing the performance of the liposomal lactate assay in combination with a commercial small portable NIR fluorometer. First, we tested the fluorophore of the liposomal lactate assay using the NIR dye sulfo-cyanine 7; we observed strong fluorescence signals and high linearity. Second, we performed the liposomal lactate assay in lactate-spiked human arterial blood using the portable fluorometer as the detector and observed strong and highly linear lactate sensing at clinically relevant lactate concentrations after 2 min. Finally, spiking fresh mouse blood with three clinically relevant lactate concentrations led to a significantly different response to all three concentrations after 5 min. These results highlight the usefulness of the tested portable NIR fluorometer for the liposomal lactate assay and motivate a clinical evaluation of this rapid and easy-to-use lactate assay.

Vesicular Diagnostics: A Spotlight on Lactate- and Ammonia-Sensing Systems

Liposomes are a highly successful drug delivery system with over 15 FDA-approved formulations. Beyond delivering drugs, lipid and polymer vesicles have successfully been used for diagnostic applications. These applications range from more traditional uses, such as releasing diagnostic agents in a controlled manner, to leveraging the unique membrane properties to separate analytes and provide isolated reaction compartments in complex biological matrices. In this Spotlight on Applications, I highlight the complexities in the development and translation of diagnostic vesicles with two case studies, a liposomal reaction compartment for lactate sensing and a transmembrane pH-gradient polymersome for ammonia sensing.

A Selective Fluorescent l-Lactate Biosensor Based on an l-Lactate-Specific Transcription Regulator and Förster Resonance Energy Transfer

Selective detection of l-lactate levels in foods, clinical, and bacterial fermentation samples has drawn intensive attention. Many fluorescent biosensors based on non-stereoselective recognition elements have been developed for lactate detection. Herein, the allosteric transcription factor STLldR from Salmonella enterica serovar Typhimurium LT2 was identified to be stereo-selectively respond to l-lactate. Then, STLldR was combined with Förster resonance energy transfer (FRET) to construct a fluorescent l-lactate biosensor FILLac. FILLac was further optimized by truncating the N- and C-terminal amino acids of STLldR between cyan and yellow fluorescent proteins. The optimized biosensor FILLac_10N0C exhibited a maximum emission ratio change (ΔR_max) of 33.47 ± 1.91%, an apparent dissociation constant (K_d) of 6.33 ± 0.79 μM, and a limit of detection of 0.68 μM. FILLac_10N0C was applied in 96-well microplates to detect l-lactate in bacterial fermentation samples and commercial foods such as Jiaosu and yogurt. The quantitation results of FILLac_10N0C exhibited good agreement with that of a commercial l-lactate biosensor SBA-40D bioanalyzer. Thus, the biosensor FILLac_10N0C compatible with high-throughput detection may be a potential choice for quantitation of l-lactate in different biological samples.

Next-Generation Diagnostic Wound Dressings for Diabetic Wounds

Chronic lower extremity wounds (diabetic foot ulcers) are a serious and prevalent complication of diabetes. These wounds exhibit low healing rates and present a high risk of amputation. Current diagnostic options for foot ulcers are limited to macroscopic wound analysis such as wound depth, implicated tissues, and infection. Molecular diagnostics promises to improve foot ulcer diagnosis, staging, and assessment of the treatment response. In this perspective, we report recent progress in understanding the pathophysiology of diabetic wound healing and point to recently emerged novel molecular targets for wound diagnostics. We discuss selected diagnostic wound dressings under preclinical development that detect one or several inflammatory markers, bacterial secretions, hyperglycemia, and mechanical stress. We also highlight key translational challenges of investigational diagnostic bandages for diabetic foot ulcers.