biostudies-literatureUnknownLiu YNCI NIH HHSEngineering red blood cells (RBCs) has been widely explored for drug delivery, imaging, vaccination, and other applications. However, effective strategies to directly engineer RBCs <i>in vivo</i> still do not exist. Here, for the first time, we report successful metabolic glycan labeling of RBCs <i>in vivo</i>. We demonstrate that systemically administered azido-sugars can metabolically label circulating RBCs with azido groups in the form of glycoproteins and glycolipids, with contributions from the labeling of both mature RBCs in the bloodstream and RBC precursor cells in the bone marrow. The RBC labeling efficiency can be improved by optimizing the choice and dose and dosing frequency of azido-sugars. The surface azido tags on RBCs can persist for >42 days in mice (nearly the lifespan ofbioRxiv : the preprint server for biology2025.10.05.676041https://www.ebi.ac.uk/biostudies/studies/S-EPMC12632469biostudies-literature<i>In Vivo</i> Metabolic Tagging and Targeting of Circulating Red Blood Cells.PMC12632469R01 CA274738R21 CA270872Baskaran DLiu YLam FNguyen DHan JWang HWang YBhatta RZhou Jfalse<i>In Vivo</i> Metabolic Tagging and Targeting of Circulating Red Blood Cells.Engineering red blood cells (RBCs) has been widely explored for drug delivery, imaging, vaccination, and other applications. However, effective strategies to directly engineer RBCs <i>in vivo</i> still do not exist. Here, for the first time, we report successful metabolic glycan labeling of RBCs <i>in vivo</i>. We demonstrate that systemically administered azido-sugars can metabolically label circulating RBCs with azido groups in the form of glycoproteins and glycolipids, with contributions from the labeling of both mature RBCs in the bloodstream and RBC precursor cells in the bone marrow. The RBC labeling efficiency can be improved by optimizing the choice and dose and dosing frequency of azido-sugars. The surface azido tags on RBCs can persist for >42 days in mice (nearly the lifespan of2025-01-01T00:00:00Z2025 Oct2026-06-11T03:12:09.014Z2026-06-11T03:08:08.272ZS-EPMC126324694127887110.1101/2025.10.05.676041