CytoLight Labeling as a Practical Extracellular Vesicle Analysis Platform for Quantitation, Phenotyping and Tracking Across In-Vitro and In-Vivo Models

Reliable fluorescent labeling of extracellular vesicles (EVs) requires efficient discrimination of bona fide EVs from non-vesicular noise while avoiding dye-associated artifacts. Conventional dyes commonly introduce limitations including aggregation, membrane intercalation, fluorescence transfer and variable performance across EV preparations, distorting EV quantification, phenotyping, uptake analysis and biodistribution tracking. We evaluated CytoLight Rapid Red, a commercially available luminal dye developed for live-cell imaging, as an artifact-minimizing strategy for EV labeling, surface-marker phenotyping, uptake analysis and in vivo tracking, benchmarking its performance against PKH26, carboxyfluorescein succinimidyl ester (CFSE) and ExoBrite across multiple analytical platforms. CytoLight generated stable, EV-associated fluorescence with no detectable aggregation or measurable change in EV size or scatter characteristics under the conditions tested. By fluorescence-mode nanoparticle tracking analysis (NTA) and nano-flow cytometry, CytoLight yielded robust and reproducible staining across EV preparations, while CFSE showed a bimodal distribution consistent with esterase-dependent activation, and PKH26 generated particles of unidentified origin. Gating on CytoLight-positive events increased tetraspanin-positive frequencies 1.97–2.42-fold in HEK293FT-derived EVs and 1.25–1.43-fold in natural killer cell-derived EVs. In uptake assays, CytoLight yielded EV-associated signals exceeding labeled phosphate-buffered saline and protein-matched EV-depleted serum controls. In vivo, CytoLight-labeled EVs produced significantly higher organ-level fluorescence accumulation than controls, consistent with commonly reported EV-biodistribution patterns. Our findings support the feasibility of CytoLight as a practical, aggregation-resistant strategy for EV-labeling and as a positive-selection gate for fluorescence-based EV analysis. CytoLight enabled reliable particle quantification, surface-marker phenotyping, EV-associated uptake analysis and in vivo fluorescence biodistribution studies across multiple analytical platforms, offering a practical, scalable alternative to established EV dyes for standardized and reproducible EV research.