TNO gastric–small intestinal model (TIM-1): 1, gastric secretions (lipase, pepsin and hydrochloric acid); 2, small intestinal secretions (pancreatic juice, bile, and sodium bicarbonate); 3, pH elec- trodes; 4, peristaltic valves; 5, hollow fiber membranes; 6, jejunal dialysis fluid; and 7, ileal dialysis fluid. 

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... changing vari- ables during passage through the stomach and the gut, and they give no information about drug absorption. A multicompartmental, dynamic, computer-controlled system that simulates the human GI tract, TIM-1 (1,2), was developed at TNO Nutrition and Food Research (Zeist, The Netherlands) and validated in collaboration with Equipe de Recherche Technologique ‘Conception, Ingénierie et Développement de l’Aliment et du Médicament’ (ERT CIDAM). At the present time, this in vitro system allows the closest simulation of in vivo dynamic physiological processes that occur within the lumen of the stomach and small intestine of humans. In fact, most of the artificial digestive systems developed to date have been dedicated to a single application (3–5) and include a limited number of simulated parameters. None of the in vitro digestive systems published so far (3–7) meet all of the following five requirements (8): (i) sequential use of enzymes in physiological amounts, (ii) appropriate pH for the enzymes and addition of relevant cofactors such as bile salts and coenzymes, (iii) removal of the products of digestion, (iv) appropriate mixing at each stage of digestion, and (v) physiological transit times for each step of digestion. However, the TIM-1 system indeed fulfills more than these requirements. TIM-1 was designed based on parameters and data obtained from studies on human volunteers. The main parameters of digestion, such as pH, body temperature, peristaltic mixing and transit, salivary, gastric, biliary, and pancreatic secretions, absorption of small molecules (e.g., nutrients, drugs) and water, are simulated. GI passage and successive conditions are controlled to mimic parameters in humans at different life stages (infant, adult, and elderly), different food intakes and physiological or pathological conditions (such as gastric hyperacidity or pancreatic failure). This in vitro system offers advantages above in vivo studies such as accuracy, reproducibility (no biological variation), easy manipulation, and the possibility of collecting samples at any level of the GI tract at any time during digestion. In addition, it is not impeded by ethical constraints, even when toxic compounds are involved. Experiments have been performed that show that the conditions simulated in TIM-1 are reliable, reproducible, and consistent with in vivo data (1,9). Validation experiments demonstrate the predictive value of the system with regard to the availability for absorption of minerals (10), vitamins (11) and food mutagens (12), and the survival of bacteria (13) and yeasts (14,15) in humans. The TNO artificial GI system could also be a useful tool in pharma-related studies for following the intralumenal fate of a drug compound or a probiotic strain under dynamic conditions. For instance, TIM-1 could be used to determine where and when a compound is released, what might influ- ence its release, its stability or viability and its availability for absorption, and what role the presence of food, transit time, or drug delivery systems could play in these processes. The absorption phase is simulated in TIM-1 by the use of dialysis membrane. Therefore, this system is only suited for drug compounds that are absorbed by passive diffusion and not by active transport (mucosal cells are not involved in the actual configuration of the in vitro system), except when the active transport through the mucosa is not a limiting step. The aim of the current study was to show the potential of TIM-1 when applied to research on the behavior of oral drug dosage forms and the fate of the active compound. Two model compounds were chosen: a lyophilized Lactobacillus strain used in the treatment of diarrhea, and the common analgesic and antipyretic, paracetamol (acetaminophen), in two different dosage forms, namely as immediate release (IR) and sustained-release (SR) formulation. In vivo , the Lactobacillus strain is not absorbed in the GI tract and must survive to exert its health effect whereas paracetamol is absorbed by passive diffusion for a systemic analgesic effect. First, we determined Lactobacillus survival rate in TIM-1 during simulation of infant or adult GI conditions. Second, we assessed the effects of drug delivery systems, GI transit time, and food intake on the availability for absorption of paracetamol. Lactobacillus LY/SA 1 ( Lactobacillus casei spp. rhamno- sus ) is commercialized as a probiotic strain (Bacilor ௡ ) by Lyocentre (Aurillac, France) for the treatment of diarrhea. It was supplied as a lyophilized powder for oral suspension (1500 mg per sachet). Paracetamol was obtained from Gen- farma (Maarssen, The Netherlands). The SR amylodextrin matrix tablets contains 30% active substance (16). The tablets are 9 mm in diameter. The gastric – small intestinal system TIM-1 (1,2) consists of four serial compartments simulating the stomach and the three segments of the small intestine: the duodenum, jejunum, and ileum (Fig. 1). Each compartment is formed by two connected basic units consisting of a glass jacket with a flexible wall inside. Water is pumped from a water bath into the glass jackets around the flexible walls to control the temperature inside the units (37 ° C) and the pressure on the flexible walls. Changes in the water pressure enable mixing of the chyme by alternate compression and relaxation of the flexible walls. To control the transit of the chyme, a power exponen- tial formula (f ס 1 − 2 −(t/t1⁄2) ␤ , where f represents the fraction of chyme delivered, t the time of delivery, t 1/2 the half-time of delivery, and ␤ is a coefficient describing the shape of the curve) is used for gastric and ileal delivery, as described by Elashoff et al . (17). Chyme transit is then regulated by open- ing or closing the peristaltic valves that connect the compartments. The volume in each compartment is monitored by a pressure sensor connected to the computer. The pH is com- puter-monitored and continuously controlled by secreting ei- ther water or 1 M HCl (0.25 ml/min in total) into the stomach and either electrolytes or 1 M NaHCO 3 (0.25 ml/min in total) into the small intestine. Simulated gastric (0.5 ml/min), biliary (0.5 ml/min), and pancreatic (0.25 ml/min) secretions, that is, pepsin, lipase, pancreatin, and bile salts (1), are introduced into the corresponding compartments by computer-controlled pumps. The model is equipped with hollow fiber membranes (HG 400, Hospal Cobe, France, cutoff ס 5800 Da) connected to the jejunal and ileal compartments. Water and small molecules (e.g., products of digestion, dissolved drugs) are re- moved from the lumen of the compartments by pumping dialysis fluid (10 ml/min) through the hollow fibers. This pre- vents product inhibition caused by the build-up of metabolites. Before each experiment, the system is washed with detergent, rinsed with water, and decontaminated by steaming at 100°C for 45 min. Survival Rate of Lactobacillus LY/SA 1 Two sachets of Bacilor ௡ (2.8 × 10 9 bacterial cells) were resuspended into 300 ml of infant milk (first age, Bledina ௡ ) or sterile water for in vitro experiments simulating the infant or the adult conditions, respectively. The gastric – small intestinal system was programmed with in vivo data to reproduce either average GI conditions of the infant after intake of formula milk or of the adult after intake of a glass of water (fast GI passage). The main parameters of in vitro GI conditions in the infant and in the adult are compared in Table I. In both cases, the experiments were performed either in the stomach alone (gastric digestion) or in the whole TIM-1 (gastric and small intestinal digestion). One sample was taken before introduction of the bacterial suspension into the artificial stomach. The gastric and ileal effluents were collected on ice to inhibit the activity of digestive enzymes. For gastric digestion, the collection vessels were replaced at 15, 30, 60, 90, 120, and 180 min. In the case of the gastric and small intestinal digestion, collection vessels were changed at 60, 120, 180, and 240 min and at 60, 120, 180, 240, 300, and 360 min for the infant and the adult conditions, respectively. The volumes were measured, and samples were taken for each period. Samples were diluted in physiological water and plated onto a selective medium for Lactobacillus strains (Agar MRS, pH 5). Petri dishes were incubated in anaerobic conditions at 37 ° C for 72 h, and the number of colony-forming units (CFU) was counted. The results were expressed as a percentage of the initial intake. The availability for absorption of paracetamol was estimated in TIM-1 by measuring the drug concentration in jejunal dialysis fluid, following its passive diffusion through the hollow fiber membranes connected to jejunum. To assess the effect of oral drug delivery systems on drug release, the availability for absorption of paracetamol was studied in TIM-1 after administration of the drug in powder form (IR) or as SR amylodextrin matrix tablets. The dose administered was either 500 mg of free powder or 5 tablets each containing 100 mg of paracetamol, with 200 g of water and 100 g of artificial saliva. TIM-1 was programmed to simulate the adult GI conditions after the intake of water (fast GI passage, Table I). In order to determine the drug-nutrient interaction, the availability for absorption of paracetamol was studied in TIM-1 when it was administered with either water or a standard breakfast. To constitute the two different “ meals, ” 500 mg of paracetamol powder was added to 100 g of artificial saliva and either 200 g of water or 200 g of a standard breakfast. The standard breakfast was a European continental breakfast for adults (18) and comprised the following: 10 slices of white bread and 10 slices of brown bread, 100 g of margarine (with 63 to 65% linoleic acid), 200 g of cheese with ∼ 48% fat (Gouda cheese), 150 g of strawberry jam containing 45% fruit, and 1 ...