© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.4.3.3.2 Clearance of Ingested Particles

The possible mechanical toxicity of ingested diamond particles and related objects has already been examined in Section 15.1.1. The discussion here concerns the likely fate of immobile nanorobots or other inert particles that have been ingested into the alimentary canal (Section 8.2.3). Because of the tremendous commercial interest in creating orally-administered microparticle-based and microencapsulated pharmaceutical agents, there is a vast literature on this subject, the comprehensive review of which lies well beyond the scope of this book. There is space here to discuss only a few of the many relevant experiments and results. Most of these experiments examined the types and sizes of particles that can traverse the gut wall and the subsequent fate of the migrating particles. Our focus here is on possible particulate analogs to medical nanorobots.

In 1980, LeFevre et al [3047] administered 5.7-micron and 15.8-micron polystyrene microspheres orally to mice. They found that the larger particles did not accumulate in intestinal Peyer’s patches, mesenteric lymph nodes, or other organs of the reticuloendothelial system or in the blood, even after the maximum dosage of 8 x 10⁶ particles/day for 60 days. However, the smaller particles were found in Peyer’s patches (Section 8.2.3), mesenteric lymph nodes, and lungs after the maximum dosage of 4.5 x 10⁸ particles/day for 60 days. 77 days after terminating ingestion, the 5.7-micron particles were still present in these tissues but were not found in spleen or liver [3047]. The site of uptake for the smaller particles, which were capable of penetrating the intestinal mucosa, was the Peyer’s patches. Absorbed particles were sequestered in Peyer’s patch macrophages. Particles that escaped this sequestration were transported by lymph rather than by portal blood [3047]. Related experiments with carbon and iron oxide particles suggested that surface properties (Section 15.2.2) as well as particle size govern accumulation in Peyer’s patches [780].

Subsequent work has largely confirmed and extended this picture. Particulate matter passing through the gut lumen is continuously sampled in the gut-associated lymphoid tissues (e.g., Peyer’s patches [3036-3038]) to immunologically survey the gut content and to elicit appropriate immune reactions [3039, 3040]. In 1989, Eldridge et al [3041] found that orally administered biodegradable poly(DL-lactide-co-glycolide) microspheres of diameter 10 microns or larger were unabsorbed by the gut walls whereas microspheres smaller than 10 microns were specifically taken up into the Peyer’s patches of the gut-associated lymphoid tissue. Microspheres between 5-10 microns in diameter remained fixed in the patches for an extended period (up to 35 days) while microspheres smaller than 5 microns were disseminated within macrophages to the mesenteric lymph nodes, the blood circulation, and the spleen [3041]. Kofler et al [3042] later found that translocation of orally-administered PLG microspheres into murine Peyer’s patches was much more efficient for 0.8-micron microspheres than for 2-micron microspheres. Carr et al [3048] found that while more 2-micron particles are taken up (particularly by epithelial tissues), a greater total particle volume is translocated to lymph nodes via 6-micron particles.

In 1989-90, Jani et al [3043] fed by gavage 0.05- to 3-micron polystyrene microspheres to rats for 10 days (1.25 mg/kg dose). They confirmed the uptake of particles across the gastrointestinal tract at the Peyer’s patches and their subsequent passage via the mesentery lymph supply and lymph nodes to liver and spleen. Heart, kidney and lung showed no uptake. The smallest 0.05-micron particles were 34% absorbed; 0.1-micron particles were 26% absorbed, of which 7% (at 0.05 micron) or 4% (at 0.1 micron) was in the liver, spleen, blood and bone marrow; particles larger than 0.1 micron did not reach the bone marrow; and microspheres larger than 0.3 micron were absent from blood [3043].

In 1996, Tabata et al [3050] administered biodegradable poly(D,L-lactic acid) microspheres from 0.6-26 microns orally to mice and found that the amount of microspheres taken up into Peyer’s patches increased with size up to 11 microns, then decreased at larger sizes, falling to zero at 21 microns or larger (the next lowest size tested being 15 microns). After being taken up into the Peyer’s patches, particles larger than 5 microns remained trapped there whereas particles 5 microns or smaller were transported to the spleen. Also in 1996, Damge et al [3044] injected microspheres of size 1-5 microns (1.44 x 10⁹ particle dose) and 5-10 microns (0.183 x 10⁹ particle dose) into the ileal lumen of adult rats. The number of microspheres found in the mesenteric vein increased rapidly, reaching a maximum after 4 hours for both sizes, then decreasing more rapidly for larger particles. A total of 12.7% of small particles and 0.11% of large particles were ultimately absorbed, mainly after crossing the intestinal mucosa at the site of the Peyer’s patches [3044]. A few small microspheres were occasionally found in the epithelial cells, and only the smallest particles were recovered in the liver, lymph nodes, spleen, and basement membranes [3044]. In another 1996 study [3045], 6-micron microcrystalline cellulose particles exhibited no translocation through the intestinal wall at doses up to 5 gm/kg-day (~10¹⁰ particles/kg-day) for 90 days.

In 1997, Porter et al [3046] injected 0.2- to 20-micron microspheres into chicken intestinal lumens. No uptake of 6-, 10- or 20-micron microspheres was observed in any intestinal segment, into epithelium and lamina propria, after 1 hour. Microspheres 2 microns or smaller were taken up equally by most intestinal segments. After 1 hour, 0.2-, 0.5- and 2-micron microspheres were oriented along the brush border of epithelial cells and microsphere uptake into the epithelium and lamina propria was observed in the duodenum, ileum, cecum, cecal tonsil, and colon [3046]. Peyer’s patch tissue had 2-200 times higher microparticle uptake than in adjacent non-patch tissue, and the uptake efficiency for 0.1-micron particles was 15-250 times higher than for 1- to 10-micron particles [3049]. Mathiowitz et al [2592] also found that 0.3- to 2-micron copolymer-coated microspheres could slip between mucosal epithelial cells, entering the lymphoid tissue of Peyer’s patches, the bloodstream, and eventually both spleen and liver.

In 1998, Beier and Gebert [3039] injected 3.4-micron yeast particles into pig gut lumen at Peyer’s patches. They found that the particles were transcytosed out of the lumen through the gut epithelium via membranous (M) cells in a few hours, without significant phagocytosis by intraepithelial macrophages. The particles then migrated down to and across the basal lamina in 2.5-4 hours, whereupon they were quickly phagocytosed and transported out of the Peyer’s patch domes [3039].

Aside from particle size [3047-3050] and surface modifications [3043, 3051], other important factors in the absorption of microspheres in the gut include particle dosage [3052, 3057], dosage duration [3053, 3054], the age of the animal [3055-3057], and diet* [3058-3060]. LeFevre et al [3055] administered 1.8-micron latex microspheres orally to young and old mice for 25 days and found that old mice accumulated more particles in Peyer’s patches, and fewer in lungs, than young mice, though all mice contained measurable particles in mesenteric lymph nodes and Peyer’s patch-free intestinal segments (cf. Simon et al [3056]). A similar study by Seifert et al [3057] using 1-micron polystyrene microspheres counted the number of particles present in thoracic duct lymph (since particles are preferentially transported in the lymph), and found a larger particle uptake by older than younger animals. Uptake was also dose-dependent in Seifert’s study: the thoracic lymph contained 5 particles/cm³ of lymph after an intraduodenal administration of 3.7 x 10⁵ particles, rising to 221 particles/cm³ for a total duodenal dose of 3.7 x 10⁹ particles [3057]. Diet also matters. Simon et al [3060] found that the number of 2-micron polystyrene microspheres retained in the gut lumen of rats fed a liquid diet was greater than the number of particles retained when rats were fed a solid diet. Larger volumes of water given with 0.87-micron particles increased the rapidity of appearance and number of particles in the bloodstream [3058]. A quantitative study of the translocation of latex microparticles across the epithelium of the rat small intestine and the microsphere uptake rate to internal organs, also by Simon’s group [3053], found that particle number increased with time in spleen, kidney, lung, liver, and brain, but decreased with time in mesenteric lymph node and heart tissues. Uptake and translocation of 1.82-micron latex particles may begin as early as 5-10 minutes after administration in the gut [3054, 3061]. Micron-sized intestinal bacteria are also readily translocated from the gut to mesenteric lymph nodes by macrophages [3062].

* Interestingly, food-ingested foreign DNA is not completely degraded in mouse gut and segments up to 976 bp can reach peripheral blood leukocytes, liver cells, and cells from spleen including B cells, T cells, and macrophages [3063].

Last updated on 30 April 2004