Intestinal metabolism and absorption of cholecystokinin analogs in rats

Identification of cholecystokinin tetrapeptide amide metabolites in liver microsomes of human, Rhesus Monkey, Sprague-Dawley rat and CD1 mouse using ultra-high performance liquid chromatography coupled to high resolution mass spectrometer

Endogenous cholecystokinin tetrapeptide (CCK-4, Trp-Met-Asp-Phe-NH₂) is a fragment derived from a larger peptide hormone, cholecystokinin (or gastrin). As a panicogenic agent, CCK-4 is commonly used in clinic settings to induce panic attacks for the study of new anxiolytic drugs. However, few studies on CCK-4 metabolism have been published to date. In the present study, we investigate the metabolism of CCK-4 in liver microsomes of human (HLM), Rhesus Monkey (RMLM), Sprague-Dawley rat (RLM) and CD1 mouse (MLM) using ultra-high performance liquid chromatography coupled to a high resolution mass spetrometer. Ten metabolites, inlcuding tryptophan (M1), tryptophan amide (M2), hydroxy metabolites (M3-M5), truncated peptides (M6-M9), and CCK-4 acid (M10), were identified and 8 of them were reported for the first time. The metabolic pattern of CCK-4 in HLM was distinctly different from these in RMLM, RLM, and MLM. M2 and M9 were the major metabolites in HLM and accounted for 19.8% and 13.4% of initial CCK-4, respectively. In contrast, M2 was the major metabolite in RMLM and accounted for 41.4%, whereas M6 was the major metabolite in RLM and account for 39.1%. Three major metabolites M2, M7 and M8 in MLM accounted for 22.6%, 17.9% and 17.8% of initial CCK-4, respectively. Chemical inhibition experiment showed that aminopeptidase and/or endopeptidase hydrolysis were the major metabolic pathways in human to generate these metabolites. We further showed that cytochrome P450 were also involved in the metabolism of CCK-4 via hydroxylation, but to a less extend. These findings provide valuable information for the metabolic processes of CCK-4 among various species and an important reference basis for its safety evaluation and rational clinical application.

Cholecystokinin facilitates glutamate release by increasing the number of readily releasable vesicles and releasing probability

Cholecystokinin (CCK), a neuropeptide originally discovered in the gastrointestinal tract, is abundantly distributed in the mammalian brains including the hippocampus. Whereas CCK has been shown to increase glutamate concentration in the perfusate of hippocampal slices and in purified rat hippocampal synaptosomes, the cellular and molecular mechanisms whereby CCK modulates glutamatergic function remain unexplored. Here, we examined the effects of CCK on glutamatergic transmission in the hippocampus using whole-cell recordings from hippocampal slices. Application of CCK increased AMPA receptor-mediated EPSCs at perforant path-dentate gyrus granule cell, CA3-CA3 and Schaffer collateral-CA1 synapses without effects at mossy fiber-CA3 synapses. CCK-induced increases in AMPA EPSCs were mediated by CCK-2 receptors and were not modulated developmentally and transcriptionally. CCK reduced the coefficient of variation and paired-pulse ratio of AMPA EPSCs suggesting that CCK facilitates presynaptic glutamate release. CCK increased the release probability and the number of readily releasable vesicles with no effects on the rate of recovery from vesicle depletion. CCK-mediated increases in glutamate release required the functions of phospholipase C, intracellular Ca(2+) release and protein kinase Cgamma. CCK released endogenously from hippocampal interneurons facilitated glutamatergic transmission. Our results provide a cellular and molecular mechanism to explain the roles of CCK in the brain.

Gastrin, cholecystokinin and gastrointestinal tract functions in mammals

The aim of the present review is to synthesise and summarise our recent knowledge on the involvement of cholecystokinin (CCK) and gastrin peptides and their receptors in the control of digestive functions and more generally their role in the field of nutrition in mammals. First, we examined the release of these peptides from the gut, focusing on their molecular forms, the factors regulating their release and the signalling pathways mediating their effects. Second, general physiological effects of CCK and gastrin peptides are described with regard to their specific receptors and the role of CCK on vagal mucosal afferent nerve activities. Local effects of CCK and gastrin in the gut are also reported, including gut development, gastrointestinal motility and control of pancreatic functions through vagal afferent pathways, including NO. Third, some examples of the intervention of the CCK and gastrin peptides are exposed in diseases, taking into account intervention of the classical receptor subtypes (CCK1 and CCK2 receptors) and their heterodimerisation as well as CCK-C receptor subtype. Finally, applications and future challenges are suggested in the nutritional field (performances) and in therapy with regards to the molecular forms or in relation with the type of receptor as well as new techniques to be utilised in detection or in therapy of disease. In conclusion, the present review underlines recent developments in this field: CCK and gastrin peptides and their receptors are the key factor of nutritional aspects; a better understanding of the mechanisms involved may increase the efficiency of the nutritional functions and the treatment of abnormalities under pathological conditions.

Past, present, and future technologies for oral delivery of therapeutic proteins

Biological drugs are usually complex proteins and cannot be orally delivered due to problems related to degradation in the acidic and protease-rich environment of the gastrointestinal (GI) tract. The high molecular weight of these drugs often results in poor absorption into the periphery when administered orally. The most common route of administration for these therapeutic proteins is injection. Most of these proteins have short serum half-lives and need to be administered frequently or in high doses to be effective. So, difficulties in the administration of protein-based drugs provides the motivation for developing drug delivery systems (DDSs) capable of maintaining therapeutic drug levels without side effects as well as traversing the deleterious mucosal environment. Employing a polymer as an entrapment matrix is a common feature among the different types of systems currently being pursued for protein delivery. Protein release from these matrices can occur through various mechanisms, such as diffusion through or erosion of the polymer matrix, and sometimes a combination of both. Encapsulation of proteins in liposomes has also been a widely investigated technology for protein delivery. All of these systems have merit and our worthy of pursuit.

Possible degradative process of cholecystokinin analogs in rabbit jejunum brush-border membrane vesicles

Our recent work on the intestinal metabolism and absorption of cholecystokinin analogs, sulfated C-terminal octapeptide (CCK8; Asp-Tyr(SO(3)H)-Met-Gly-Trp-Met-Asp-Phe(NH(2)) = DY(SO(3)H)MGWMDF(NH(2))) and tetrapeptide (CCK4; Trp-Met-Asp-Phe(NH(2)) = WMDF(NH(2))), was extended to investigate the degradative process of these analogs using rabbit jejunum brush-border membrane vesicles and to find a better enzyme-inhibitor system for intestinal absorption of peptide drugs. Various enzyme inhibitors and a lower pH buffer were applied to discover the major enzyme(s) involved in each process. Metabolic pathways showing degradative processes were proposed for both analogs. The major cleavage site occurs at the W(1)-M(2) for CCK4. At least three metabolic pathways occur independently for CCK8 and appear at peptides bonds between G(4)-W(5), M(6)-D(7), and D(7)-F(NH(2))(8). Many different enzymes of aminopeptidase, endopeptidase, angiotensin-converting enzyme, metalloenzyme, and others were involved in each process. Identification of more specific yet safe enzyme inhibitors and co-administration of various these inhibitors may lead to further enhancement in intestinal peptide absorption when administered orally.

Protein drug oral delivery: the recent progress

Rapid development in molecular biology and recent advancement in recombinant technology increase identification and commercialization of potential protein drugs. Traditional forms of administrations for the peptide and protein drugs often rely on their parenteral injection, since the bioavailability of these therapeutic agents is poor when administered nonparenterally. Tremendous efforts by numerous investigators in the world have been put to improve protein formulations and as a result, a few successful formulations have been developed including sustained-release human growth hormone. For a promising protein delivery technology, efficacy and safety are the first requirement to meet. However, these systems still require periodic injection and increase the incidence of patient compliance. The development of an oral dosage form that improves the absorption of peptide and especially protein drugs is the most desirable formulation but one of the greatest challenges in the pharmaceutical field. The major barriers to developing oral formulations for peptides and proteins are metabolic enzymes and impermeable mucosal tissues in the intestine. Furthermore, chemical and conformational instability of protein drugs is not a small issue in protein pharmaceuticals. Conventional pharmaceutical approaches to address these barriers, which have been successful with traditional organic drug molecules, have not been effective for peptide and protein formulations. It is likely that effective oral formulations for peptides and proteins will remain highly compound specific. A number of innovative oral drug delivery approaches have been recently developed, including the drug entrapment within small vesicles or their passage through the intestinal paracellular pathway. This review provides a summary of the novel approaches currently in progress in the protein oral delivery followed by factors affecting protein oral absorption.