Unique insights into the intestinal absorption, transit, and subsequent biodistribution of polymer-derived microspheres

Abstract

Polymeric microspheres (MSs) have received attention for their potential to improve the delivery of drugs with poor oral bioavailability. Although MSs can be absorbed into the absorptive epithelium of the small intestine, little is known about the physiologic mechanisms that are responsible for their cellular trafficking. In these experiments, nonbiodegradable polystyrene MSs (diameter range: 500 nm to 5 µm) were delivered locally to the jejunum or ileum or by oral administration to young male rats. Following administration, MSs were taken up rapidly (≤ 5 min) by the small intestine and were detected by transmission electron microscopy and confocal laser scanning microscopy. Gel permeation chromatography confirmed that polymer was present in all tissue samples, including the brain. These results confirm that MSs (diameter range: 500 nm to 5 µm) were absorbed by the small intestine and distributed throughout the rat. After delivering MSs to the jejunum or ileum, high concentrations of polystyrene were detected in the liver, kidneys, and lungs. The pharmacologic inhibitors chlorpromazine, phorbol 12-myristate 13-acetate, and cytochalasin D caused a reduction in the total number of MSs absorbed in the jejunum and ileum, demonstrating that nonphagocytic processes (including endocytosis) direct the uptake of MSs in the small intestine. These results challenge the convention that phagocytic cells such as the microfold cells solely facilitate MS absorption in the small intestine.

Keywords: oral delivery; uptake mechanism.

Fig. 1.

TEM images of 200- (A–C) and 500-nm (D and E) polystyrene MSs 5 min after local delivery to the rat jejunum. Arrows indicate the location of MSs. (Scale bar, 500 nm.)

Fig. 2.

CLSM images of a single villus 1 h after delivering fluorescent PC-red 500-nm polystyrene MSs to the jejunum is shown with DAPI stain (A), FM 1-43 stain (B), and with illumination of fluorescent PC-red MSs (C). (D) Overlay of all three channels (A–C) is shown. In D, red arrows show where MSs are located within goblet cells; white arrows highlight where MS have penetrated beyond the epithelial cell layer.

Fig. 3.

Effect of polystyrene MS diameter on total uptake (expressed as a percentage of total administered dose) 5 h after local delivery to the jejunum or ileum or 5 h after oral administration. n = 4 animals for each cohort.

Fig. 4.

(A) Effect of time on the uptake of polystyrene microspheres in the jejunum and ileum is shown. The biodistribution of 500-nm MSs 1, 3, and 5 h after local delivery to the jejunum (B) and 1-μm MSs 1, 3, and 5 h after local delivery to the ileum (C) are also shown. D shows quantitative resident time analysis of 500-nm and 1-μm MSs 5 h after oral administration.

Fig. 5.

(A) Total MS uptake is expressed as a percentage of the total administered dose following local delivery of 500-nm polystyrene MSs to the jejunum in the absence of drug (control) and with CPZ, PMA, and CytD. (B) Total MS uptake is expressed as a percentage of the total administered dose 5 h after local delivery of 1-µm polystyrene MSs to the ileum in the presence of CPZ, colchicine, and CytB and in the absence of drug.