•LDCs-SLNs improve the oral bioavailability of silybin by 5–7 times in comparison with a fast-release formulation.•LDCs might be broken down in the GI tract and within enterocytes to release silybin, which is then transported to the circulation.•The contribution of absorption of integral SLNs via the M cell pathway is negligible to the bulk bioavailability•Longer lipid chains create more steric hindrance and slow degradation and biodistribution.Lipid-drug conjugates (LDCs) of a poorly soluble and poorly permeable drug silybin (SB) and lipids with different chain lengths (6C, 12C, 18C) are synthesized and formulated into solid lipid nanoparticles (SLNs). The in vivo fate of LDCs as well as SLNs is investigated by tracking either SB or LDCs or SLNs. LDCs are prone to be hydrolyzed by lipases either in simulated gastrointestinal media or in Caco-2 cell lines in a lipid chain length-dependent mode. The oral bioavailability of SB is enhanced by 5–7-fold in comparison with a fast-release formulation. No integral LDCs are detected in plasma confirms the readily degradable nature of LDCs. The absorption of LDCs by enteric epithelia and subsequent transportation into circulation might play a leading role in absorption enhancement, whereas the contribution of then M-cell pathway is not as remarkable. A shorter lipid chain favors earlier lipolysis and faster absorption along the intestine-to-circulation path.LDCs-containing SLNs can be degraded to release LDCs, which are then taken up by enterocytes, further broken down to SB, and finally transported to systemic circulation. This enterocyte-to-portal vein route accounts for a major part of enhanced oral bioavailability, whereas the contribution of the M cell/lymphatics route is not as remarkable.Download high-res image (206KB)Download full-size image

The nose-to-brain pathway has been proven to be a shortcut for direct drug delivery to the brain. However, whether and to what extent nanoparticles can be delivered through this passage is still awaiting validation with evidence. In this study, nose-to-brain transportation of nanoparticles is tracked via fluorescence bioimaging strategies using nanoemulsions (NEs) as model carriers. Identification of NEs in biological tissues is based on the on → off signal switching of a new type of environment-responsive embedded dyes, P2 and P4, and two conventional probes, DiR and coumarin-6 (C6), are embedded to represent the cargoes. Evidence for the translocation of NEs was collected either via live imaging or ex vivo histological examination in rats after nasal administration. Results suggest that NEs with a particle size of about 100 nm, either naked or coated with chitosan, have longer retention duration in nostrils and slower mucociliary clearance than larger ones. P2 signals, representing integral NEs, can be found in mucosa and trigeminal nerves for all size groups, whereas only weak P2 signals are detected in the olfactory bulb for chitosan-coated NEs of 100 nm. Confocal microscopy further confirms the translocation of integral 100 nm NEs in nasal mucosa and along the trigeminal nerve in decremental intensity. Weak signals of the P4 probe, also representing integral NEs, can be detected in the olfactory bulb but few in the brain. NEs as large as 900 nm cannot be transported to the olfactory bulb. However, the DiR or C6 signals that represent the cargoes can be found in significant amounts along the nose-to-brain pathway and finally reach the brain. Evidence shows that integral NEs can be delivered to the olfactory bulb, but few to the brain, whereas the cargoes can be released and permeated into the brain in greater amounts.

Taking advantage of its ability to deal with exogenous pathogens, the M cell passage has proven to be the most reliable pathway for entry of particulates, thus creating opportunities for oral immunization and delivery of biomacromolecules. Albeit a well-known story, the underlying mechanisms of this pathway are not yet well understood, especially concerning direct evidence of translocation of particulates. Herein, model glucan microparticles (GMs) targeting M cells are employed to track translocation through M cell pathways as well as to various organs via the systemic circulation. GMs were first labeled with a novel kind of near-infrared fluorescent water-quenching probe through encapsulation and locking by stearin. In vivo live imaging indicates prolonged residence of GMs in the gastrointestinal tract for as long as 12 h. GMs are found to be gradually absorbed from the ligated ileum segment but little from the jejunum. Histological examination using confocal laser scanning microscopy (CLSM) confirms distribution of GMs to the basolateral side of the ileum through Peyer's patches. However, no detectable fluorescence can be observed in any other organs or tissues until 12 h after administration. After 12 h, GMs can be found in the liver, spleen and lung. At 24 h, GMs accumulate in these organs with approximately 2.3% of the total amount. Repeated administration for three consecutive days augments total accumulation to as high as 4.5%. By tracking GM-bound fluorescence, the particles can be accurately located in these organs. GMs can be transported across Caco-2/Raji and Caco-2/Raji/J774A.1 co-culture monolayers, but not Caco-2 monolayers, in a time-dependent manner. As observed by CLSM, GMs can be voraciously engulfed with as many as 10–15 particles per cell. Evidence of translocation of GMs indicates that GMs can be absorbed through the M cell pathway located at Peyer's patches, especially in the ileum, and translocated to reticulo-endothelial organs.

Taking advantage of its ability to deal with exogenous pathogens, the M cell passage has proven to be the most reliable pathway for entry of particulates, thus creating opportunities for oral immunization and delivery of biomacromolecules. Albeit a well-known story, the underlying mechanisms of this pathway are not yet well understood, especially concerning direct evidence of translocation of particulates. Herein, model glucan microparticles (GMs) targeting M cells are employed to track translocation through M cell pathways as well as to various organs via the systemic circulation. GMs were first labeled with a novel kind of near-infrared fluorescent water-quenching probe through encapsulation and locking by stearin. In vivo live imaging indicates prolonged residence of GMs in the gastrointestinal tract for as long as 12 h. GMs are found to be gradually absorbed from the ligated ileum segment but little from the jejunum. Histological examination using confocal laser scanning microscopy (CLSM) confirms distribution of GMs to the basolateral side of the ileum through Peyer's patches. However, no detectable fluorescence can be observed in any other organs or tissues until 12 h after administration. After 12 h, GMs can be found in the liver, spleen and lung. At 24 h, GMs accumulate in these organs with approximately 2.3% of the total amount. Repeated administration for three consecutive days augments total accumulation to as high as 4.5%. By tracking GM-bound fluorescence, the particles can be accurately located in these organs. GMs can be transported across Caco-2/Raji and Caco-2/Raji/J774A.1 co-culture monolayers, but not Caco-2 monolayers, in a time-dependent manner. As observed by CLSM, GMs can be voraciously engulfed with as many as 10–15 particles per cell. Evidence of translocation of GMs indicates that GMs can be absorbed through the M cell pathway located at Peyer's patches, especially in the ileum, and translocated to reticulo-endothelial organs.