The effect of capsule size and density on transit through the proximal colon

The transit of dosage forms through the colon

Colonic transit is a subject of great relevance when considering in vivo/in vitro relationships for oral controlled release dosage forms. Our knowledge of colonic motility has first come from the clinic, where measurement of the whole gut transit of different excreted markers was used as a method of discriminating pathologies. X-ray contrast, although widely available, was used sparing due to the accumulating dosimetry associated with each exposure. Although such methods were used for swallowing studies, gamma scintigraphy allowed physicians to measure colon function with a more moderate radiation burden. The ability to label meal and dosage form separately and to measure dispersion with more certainty, prompted the use in pharmaceutical sciences; finally, the relationship between blood concentrations and transit of different sized dosage began to be understood. This mini-review considers the development of colon transit measurements and how different designs of clinical assessment assist in elucidating size and shape influence on colon transit in man.

Folate is absorbed across the colon of adults: evidence from cecal infusion of (13)C-labeled [6S]-5-formyltetrahydrofolic acid

BACKGROUND

Folate deficiency increases the risk of several human diseases. Likewise, high intakes of folate, particularly synthetic folic acid intake, may be associated with adverse health outcomes in humans. A more comprehensive understanding of the "input side" of folate nutrition may help to set dietary recommendations that strike the right balance between health benefits and risks. It is well known that the microflora in the colon produce large quantities of folate that approach or exceed recommended dietary intakes; however, there is no direct evidence of the bioavailability of this pool in humans.

OBJECTIVE

The objective was to determine whether, and to what extent, the natural folate vitamer 5-formyltetrahydrofolic acid is absorbed across the intact colon of humans.

DESIGN

During screening colonoscopy, 684 nmol (320 microg) [(13)C]glutamyl-5-formyltetrahydrofolic acid was infused directly into the cecum of 6 healthy adults. Three or more weeks later, each subject received an intravenous injection of the same compound (172 nmol). Blood samples were collected before and after each treatment. The ratio of labeled to unlabeled folates was determined in plasma by tandem mass spectrometry.

RESULTS

The apparent rate of folate absorption across the colon of a bolus dose of [(13)C]5-formyltetrahydrofolic acid infused into the cecum was 0.6 +/- 0.2 nmol/h, as determined by the appearance of [(13)C(5)]5-methyltetrahydrofolic acid in plasma. In comparison, the rate of appearance of [(13)C(5)]5-methyltetrahydrofolic acid after an intravenous injection of [(13)C(5)]5-formyltetrahydrofolate was 7 +/- 1.2 nmol/h.

CONCLUSION

Physiologic doses of natural folate are absorbed across the intact colon in humans.

Gamma scintigraphic evaluation of the fate of hydroxypropyl methylcellulose capsules in the human gastrointestinal tract

The fate (movement and disintegration) of hard novel hydroxypropyl methylcellulose (HPMC) two-piece capsules in the human gastrointestinal tract was investigated using a gamma scintigraphic imaging method. Two different prolonged-release formulations without an active ingredient were used. The capsules contained different viscosity grades of HPMC powder (HPMC K100 and HPMC K4M). The aim was to determine the main reason why the pharmacokinetic profiles of model drugs change when the diluent was changed to a higher viscosity grade. The results were compared with our previous pharmacokinetic studies with corresponding capsules containing metoclopramide hydrochloride or ibuprofen as a model drug. The first observation was that the HPMC capsules had a tendency to attach to the oesophagus. Therefore, it is recommended that the HPMC capsules as well as gelatine capsules be taken with a sufficient amount of water (150-200 ml) in an upright position and maintaining the upright position for several minutes. The viscosity grade of the HPMC did not affect the transit times of the capsules in the GI tract. The major differences between the two formulations were the complete disintegration times of the capsules and the spreading of the capsules to the large intestine. Most of the HPMC K100-based capsules were completely disintegrated during the 8h study, whereas the HPMC K4M-based capsules still exhibited plug formations in the large intestine. Also the HPMC K100-based capsules spread better to the ascending colon than the HPMC K4M-based capsules. The faster disintegration of the HPMC K100-based capsules explains the differences in the pharmacokinetic profiles of the model drugs between the HPMC K100- and K4M-based capsules in our previous studies. The main absorption site of the drugs from the capsules studied here is probably the large intestine when taken in a fasting state.

Pharmacokinetics of verapamil and norverapamil from controlled release floating pellets in humans

Pharmacokinetics of verapamil (V) in a dose of 40 mg and its metabolite norverapamil (N) from the new oral drug formulation in a form of capsule filled with floating pellets was determined. Conventional 40-mg tablets used in a medical practice served as a reference. Bioavailability studies were carried out in 12 healthy volunteers including six men and six women. In an in vitro test the pellets floated on the surface of the extraction fluid for 6 h. Mean value of maximum plasma concentration (C(max)) of V for floating pellets was 28.27 ng ml(-1) and t(max) 3.75 h. The value of the area under the concentrations versus time, AUC(0-infinity) was calculated as 364.65 ng ml(-1) h, biological half-lives of the absorption and elimination (t(0.5el)) phase were 0.5 h and 10.68 h, respectively. For the reference conventional tablets those values were 33.07 ng ml(-1), 1.21 h, 224.22 ng ml(-1) h, 0.36 h and 6.17 h, respectively. The average concentration of N in plasma was similar to that of V.

The role of gamma-scintigraphy in oral drug delivery

The gastrointestinal tract is usually the preferred site of absorption for most therapeutic agents, as seen from the standpoints of convenience of administration, patient compliance and cost. In recent years there has been a tendency to employ sophisticated systems that enable controlled or timed release of a drug, thereby providing a better dosing pattern and greater convenience to the patient. Although much about the performance of a system can be learned from in vitro release studies using conventional and modified dissolution methods, evaluation in vivo is essential in product development. The non-invasive technique of gamma-scintigraphy has been used to follow the gastrointestinal transit and release characteristics of a variety of pharmaceutical dosage forms. Such studies provide an insight into the fate of the delivery system and its integrity and enable the relationship between in vivo performance and resultant pharmacokinetics to be examined (pharmacoscintigraphy).

Soft drugs. 18. Oral and rectal delivery of loteprednol etabonate, a novel soft corticosteroid, in rats--for safer treatment of gastrointestinal inflammation

PURPOSE

As a safe anti-inflammatory corticosteroid, the utility of loteprednol etabonate (LE) for the treatment of gastrointestinal inflammation, via oral and rectal administration, was investigated in rats.

METHODS

In vivo, LE solution and suspension were orally administered (20 mg/kg), and various LE preparations (solution, suspension & suppository) were applied in rectal loops (0.2 mg per loop). In vitro, various GI tissues were used to study the stability and partition of LE.

RESULTS

After oral administration of LE solution, LE reached the upper GI tract effectively, but not the colon, due to absorption and/or decomposition. In suspension, LE reached most of the GI tract (except rectum) in 8 hr and showed little absorption. After rectal applications, LE remained intact in the rectal loop for more than five hours with a slow rate of disappearance, however, LE distributed in the rectal membrane to some extent. The concentrations of LE and its inactive metabolites in plasma after both oral and rectal administrations were lower than the detection limit (0.1 microgram/ml) at anytime during the experiments. In vitro, LE in solution was stable in stomach, but not in cecum, due to the hydrolysis by the cecal resident micro flora. In solution, LE distributed into the mucosal membranes efficiently (about 2.5-4.0 micrograms/g tissue).

CONCLUSIONS

The results suggest that LE can be orally or rectally delivered in the GI tract for the topical treatment of the inflammatory bowel disease.

Targeting drugs to the colon: delivery systems for oral administration

The oral delivery of drugs to the colon has applications in a variety of therapeutic areas. This review is concerned with the approaches taken to achieve a universal system for delivery. The design of such a system requires the identification and exploitation of a unique feature of the colonic environment. The use of transit times, pH and bacterial enzymes are critically assessed. In addition, the system must provide protection for the drug during transit to the colon. Upper gastro-intestinal physiology and the transit of pharmaceuticals through these regions are reviewed with reference to their relevance in achieving site specificity.

Evaluation of an enteric-coated naproxen pellet formulation

An enteric-coated, pellet formulation of naproxen has been evaluated in eight healthy subjects. Each volunteer was dosed with 153Sm-labelled, enteric-coated pellets on two occasions, once whilst fasted and once after breakfast. Gastrointestinal transit was followed using gamma scintigraphy and drug absorption compared with that from uncoated naproxen pellets dosed on a separate occasion. The pH in the stomach and intestines was monitored using radiotelemetry capsules. Gastric emptying was delayed by dosing after breakfast, but small intestinal transit of the enteric-coated formulation was the same on both occasions. The highest pH recorded from the stomach was 4.0 and in all subjects the pH rose to at least 7.3 in the small intestine. The onset of drug absorption was fastest from the uncoated formulation and slowest from the coated pellets taken after breakfast. The total amount of drug absorbed was the same on all three occasions.

Microbially controlled drug delivery to the colon

The human gastrointestinal tract consists of a highly complex ecosystem of aerobic and anaerobic microorganisms that plays a significant role in the metabolism of nutrients as well as drugs. In the colon, bacteria ferment various types of substrates that are not susceptible to digestion in the small intestine. This arouses interest in specific drugs, drug delivery systems, and prodrugs that escape small bowel digestion, arrive intact, and are absorbed or degraded in the large bowel. For the past forty years, experience has been gained with the azo prodrug of 5-amino salicylic acid, salazopyrine, which is cleaved by colonic bacteria to its parent drug. Some laxative drugs were also reported to degrade into active metabolites in the colon. Lately equally interesting and more sophisticated microbial controlled delivery systems, have been developed based on similar principles.