Disposition of [14C]γ-Cyclodextrin in Germ-Free and Conventional Rats

Abstract

The absorption, disposition, metabolism, and excretion of14C-labeled γ-cyclodextrin ([14C]γ-CD) was examined in four separate experiments with Wistar rats. In experiment 1, [14C]γ-CD (25 μCi, 600 mg/kg body wt) was administered intravenously to four male and four female conventional rats. In experiment 2, [14C]γ-CD (25 μCi, 1000 mg/kg body wt) was given by gavage to four male and four female germ-free rats. In experiments 3 and 4, [14C]γ-CD (25 and 100 μCi, respectively, 1000 mg/kg body wt) was given to four male and four female conventional rats by gavage. In all experiments,14C was measured in respiratory CO2, urine, feces, contents of the gastrointestinal tract, blood, main organs, and residual carcass. The chemical identity of the14C-labeled compounds was examined by HPLC in urine (experiments 1–4), blood (experiments 1 and 4), and samples of intestinal contents (experiments 3 and 4). Recovered14C was expressed as a percentage of the administered dose. Total recovery of14C varied between about 90% (experiments 3 and 4) and 100% (experiments 1 and 2). Experiment 1 showed that about 90% of intravenously administered γ-CD is excreted in urine within 24 h. During the first 2 h after dosing, plasma14C levels decreased rapidly (t1/2, 15–20 min). The remaining 10% of the dose was probably excreted into the gastrointestinal tract with bile and saliva. In addition, some [14C]γ-CD may have been degraded by plasma and tissue amylases. Since glucose is the common product of systemic hydrolysis and intestinal digestion, it is not possible to quantitate the relative flux of [14C]γ-CD through these two pathways. Upon oral administration of [14C]γ-CD to germ-free rats (experiment 2), about 66% of the label was expired as CO2within 24 h. The rate of14C exhalation reached a maximum at 90 min after dosing and then declined steadily. In the urine, and in the contents of the cecum and colon collected at 24 h, [14C]γ-CD was not found (except for a trace in the cecum of females). In conventional rats (experiments 3 and 4), a similar, fast appearance of respiratory14CO2was observed. There was no delayed formation of14CO2due to incomplete digestion and subsequent microbial fermentation in the cecum and colon. In pooled urine collected at 4 h after dosing, a small amount of unchanged [14C]γ-CD was detected (experiment 4). From this result, it was estimated that less than 0.02% of ingested intact γ-CD was absorbed and excreted with the urine. It is concluded from the data that ingested [14C]γ-CD is rapidly and essentially completely digested in the small intestine to absorbable [14C]glucose. The absorption of intact [14C]γ-CD by passive diffusion is very low (<0.02%). Therefore, the metabolism of γ-CD resembles closely that of starch or linear dextrins.