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The in vivo fate and absorption mechanisms of lipid-based drug delivery systems have not been well deciphered due to complexity of the gastrointestinal physiology and lack of functional tools.For this end, environment-responsive near-infrared (NIR) aza-BODIPY dyes capable of fluorescence quenching in water are explored to visualize the in vivo fate of a model lipid-based nanocarriers, solid lipid nanoparticles (SLNs).The water-quenching effect of the dyes is confirmed to be sensitive and persisted for at least 24 h.In vitro lipolysis indicates quick degradation of SLNs within 20 min, which is in correlation with alkaline compensation results.The fluorescence quenching is confirmed to be synchronous to SLNs degradation and be able to mirror the actual lipolysis.The in vivo fate of SLNs was monitored using water-quenching NIR fluorescent probe P2 after gastric gavage administration in a series of animal models.In vivo live imaging in nude mice indicates predominant digestion of SLNs within 2 h and complete digestion within 4 h, which correlates well to in vitro data.Rekindling of quenched dyes by mixed micelles is observed in vitro, but not in vivo.In contrast, SLNs encapsulating another NIR dye DiR show persistent fluorescence both in vitro and in vivo despite of significant lipolysis.It is envisaged that water-quenching fluorescence dyes can be used as probes to monitor the in vivo fate of lipid-based delivery systems.Furthermore, in vivo live imaging in mice indicated very fast degradation of SLNs under both fed and fasted conditions, but significantly delayed degradation under high-fat feeding and lipase-inhibiting conditions.Imaging of dissected GI tract segments gave unexpected findings that the majority of SLNs reside in the stomach, whereas in small intestine SLNs could be degraded very quickly.Both high-fat feeding and lipase inhibition lead to significantly slowed gastric emptying rate and prolonged retention of SLNs in the GI tract.No distribution of SLNs in organs or tissues other than the GI tract was observed.In situ perfusion results confirmed strong adhesion of SLNs as well as SMMs that simulates the digestive products of SLNs to the intestinal wall.However, cross-section observation using fluorescent microscope eliminate the possibility of permeation of intact vehicles across the epithelia.Cellular uptake of SLNs and SMMs by Caco-2 and Caeo-2/HT29-MTX cell lines further confirmed bioadhesion of the vehicles to cell membranes.Trans-monolayer permeation study indicated that both vehicles cannot be transported across the monolayers.The presence of a mucus layer due to co-culturing with HT29-MTX cells works as a barrier to the interaction of the nanocarriers with cell membranes.SMMs and BS-decoration both enhanced bioadhesion and cellular uptake, indication enhanced bioadhesion due to the presence of physiological components.Taking all evidence together, the chance of absorption of intact SLNs via oral delivery seems to be little.