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C J Porter

Publications and source records attributed to C J Porter.

At least 19 recordsLinked to original sources
PubMed2001

In vitro assessment of oral lipid based formulations.

In recent years there has been an increase in interest in the utility of lipid based delivery systems, at least in part as a result of the effective development of lipid based products such as Sandimmun Neoral (cyclosporin), Norvir (ritonavir) and Fortovase (saquinavir). The development pathway for lipid based formulations, however, is still largely empirical, and in vitro models that are predictive of oral bioavailability enhancement are lacking. The use of modified dissolution media, reflecting the bile salt and phospholipid levels in the intestine, has met with some success in terms of the ability to predict the bioavailability of poorly water soluble drugs and the potential bioavailability enhancing effects of food. These approaches, however, do not have the flexibility or complexity to deal with the interactions inherent in the digestion, dispersion and solubilisation of a lipid based formulation and the coincident dissolution profile of a co-administered drug. In this review, the utility of modified dissolution media to predict the impact of food on the absorption of poorly water soluble, lipophilic drugs, is explored. These dissolution based systems are subsequently contrasted with the use of lipid digestion models which have found increasing application in assessment of the interaction of digestible dose forms with the gastrointestinal milieu.

Administration, Oral↗
PubMed2001

Animal models for the study of intestinal lymphatic drug transport.

Drug transport via the intestinal lymphatic system has been shown to contribute to the absorption of a number of orally administered highly lipophilic drugs. In order to investigate this phenomenon and assist in the development of improved oral formulations, the use of appropriate animal models is required. This paper reviews the use of various animal models for this purpose, and describes in detail the conscious rat and dog models used in our laboratory. The advantages and disadvantages of both small and large animal models are explored, as well as the factors which have been found to influence the outcome of intestinal lymphatic drug transport studies with these models.

Animals↗
PubMed2001

Intestinal lymphatic drug transport: an update.

The trend towards identification of poorly water-soluble and highly lipophilic candidate drug molecules has led to an increase in interest in intestinal lymphatic drug transport. In this article we provide a brief background to the mechanism of access of drugs to the intestinal lymph and the role of lipid digestion and absorption in the stimulation of lymphatic transport. The ability of different lipid types to stimulate lymphatic drug transport, is addressed, concentrating specifically on the impact of the class, chain length and degree of unsaturation of co-administered lipids. Comment is also made as to the relevance of dosing different lipid volumes to the rat and the possible complications this may provide when trying to assess the likely extent of intestinal lymphatic transport. Recent studies are described in which the extent of lymphatic transport of a highly lipophilic antimalarial, halofantrine, was investigated after post-prandial administration to greyhound dogs. Finally the possible future directions for studies of intestinal lymphatic transport are discussed, including the use of cell culture models and genetically modified animals.

Animals↗
PubMed2001

Lymphatic transport of proteins after s.c. injection: implications of animal model selection.

Subcutaneous (s.c.) administration continues to be the main route for the delivery of protein drugs due to their poor bioavailability by most non-parenteral routes. While small drug molecules are rapidly and extensively absorbed after s.c. injection, the systemic bioavailability of protein drugs is often incomplete and variable. Given the widespread use of the s.c. route for protein drugs, surprisingly little is known about the factors that govern the rate and extent of protein absorption from the interstitial space and the role of the lymphatic system in the transport of these molecules to the systemic circulation. The few studies that have directly addressed the role of lymphatic transport in protein bioavailability are complicated by the use of methods and models that vary widely. In this review we will evaluate the available literature describing the lymphatic transport of proteins after s.c. injection and more specifically, address the impact of experimental variation (e.g. site of cannulation, animal model, anesthesia) on the interpretation of the data obtained. We will also describe in some detail the sheep model currently in use in our laboratory, which allows both estimation of the extent of uptake of protein drugs into the lymphatics draining the injection site, and quantification of the contribution of lymphatic transport to the absolute bioavailability.

Animals↗
PubMed2001

A conscious dog model for assessing the absorption, enterocyte-based metabolism, and intestinal lymphatic transport of halofantrine.

Postprandial administration of halofantrine (Hf), an important antimalarial, leads to 3- and 12-fold increases in oral bioavailability in humans and beagles, respectively, and corresponding 2.4-fold and 6.8-fold decreases in metabolic conversion to desbutylhalofantrine (Hfm). Factors contributing to the decreased postprandial metabolism of Hf could include inhibition of presystemic CYP3A metabolism by food components and/or recruitment of the intestinal lymphatics as an absorption pathway. Although previous rat studies confirmed Hf base is a substrate for lymphatic transport, it is difficult to extrapolate such data to higher species, as the largely constant bile flow in a rat precludes attainment of representative pre- and postprandial states, and formulations administered to rats are often not relevant to higher species. These limitations have now been addressed by development of a conscious dog model that allows simultaneous study of intestinal lymphatic and nonlymphatic drug absorption and aspects of enterocyte-based drug metabolism. After oral administration of 100 mg Hf base, the mean fasted and postprandial lymphatic transport was 1.3% and 54% of the administered dose, respectively. Comparison of portal and systemic plasma Hfm concentration profiles suggested enterocyte-based conversion of Hf to Hfm; however, the proportion of Hf metabolized to Hfm was similar after fasted or postprandial administration. Hence, it appears that the previously observed decrease in the postprandial metabolism of Hf is largely a consequence of significant postprandial intestinal lymphatic transport (which bypasses first pass hepatic metabolism). This new dog model will facilitate identification of the key factors that impact bioavailability, lymphatic transport, and metabolic profiles of highly lipophilic drugs.

Administration, Oral↗
PubMed2001

Characterisation and quantification of medium chain and long chain triglycerides and their in vitro digestion products, by HPTLC coupled with in situ densitometric analysis.

The development of new and simple high performance thin layer chromatography (HPTLC) assays for the quantification of medium chain triglycerides (MCT, tricaprylin) and long chain triglycerides (LCT, triolein) and their lipolytic products, bile salts (BS) and phospholipids (PL) are described. Different classes of lipids (PL, BS, fatty acids, monoglycerides, diglycerides, and triglycerides) were separated on a single silica gel 60 HPTLC plate by Automated Multiple Development (AMD) methods using a Camag AMD 2. Post-chromatographic staining of long chain lipids (triolein, diolein, monoolein, and oleic acid), PL and BS with a solution of copper sulphate-phosphoric acid and medium chain lipids (tricaprylin, dicaprylin, monocaprylin, and caprylic acid) with a solution of ammonium molybdate-perchloric acid allowed visualisation of the lipids. Lipids were quantified by in situ spectrodensitometric measurements using a Camag TLC scanner 3. The intra- and inter-assay accuracy was between 83 and 115% and the assay was precise to within a CV of less than 20% over a range of 0.1-1 and 5-50 microg for long chain lipids and medium chain lipids, respectively. The methods have been employed to study the kinetics of triolein and tricaprylin lipolysis in an in vitro lipid digestion model commonly used to assess the digestibility of novel oral lipid-based formulations.

Chromatography, Thin Layer↗
PubMed2001

Lymphatic absorption is a significant contributor to the subcutaneous bioavailability of insulin in a sheep model.

PURPOSE: This study was conducted to explore the role of the peripheral lymphatics in insulin absorption following subcutaneous (SC) administration using a sheep model that allows continuous collection of peripheral lymph and simultaneous assessment of systemic bioavailability. METHODS: In a parallel group design, soluble human insulin (0.5 IU/kg) was administered by bolus SC injection into the interdigital space of the hind leg of non-cannulated control sheep, and sheep in which the efferent popliteal lymph duct was cannulated. A separate group received a bolus IV injection (0.15 IU/kg). Blood was sampled from all animals, and lymph was collected continuously over 12 h postdosing. Samples were assayed for insulin by ELISA. RESULTS: The SC bioavailability of insulin in control sheep was 31.5+/-3.2%, which was significantly higher than when the peripheral lymph was continuously collected (18.4+/-1.7%). In the lymph-cannulated animals, 17.3+/-1.0% of the dose was collected in peripheral lymph. CONCLUSIONS: Based on the direct measurement of insulin in regional lymph and on the decrease in the systemic bioavailability when regional lymph was continuously collected, the results demonstrate that lymphatic absorption contributed significantly to the overall insulin bioavailability following SC administration to sheep.

Absorption↗
PubMed2001

Pulmonary atresia with ventricular septal defect and persistent airway hyperresponsiveness.

OBJECTIVE: We and others have observed significant hyperinflation and airflow obstruction after the surgical repair of pulmonary atresia and ventricular septal defect. This study sought to objectively characterize this problem and determine the prevalence of airway hyperresponsiveness in these patients. METHODS: We performed a prospective study of children and young adults with pulmonary atresia and ventricular septal defect between June 1996 and December 1998. The participants were stratified into 2 distinct molecular genotypes on the basis of chromosome 22q11.2 microdeletion. A clinical diagnosis of asthma and an objective assessment of airway hyperresponsiveness were determined by means of an asthma inventory scale and methacholine challenge testing, respectively. Thirty-three patients were enrolled. Thirteen had velocardiofacial syndrome, each with chromosome 22q11.2 microdeletion. RESULTS: None of the nonsyndromic patients had evidence for haploinsufficiency. Overall, 66.7% (22/33) met criteria for a clinical diagnosis of airway hyperresponsiveness: 62% (8/13) from the microdeletion genotype and 70% (14/20) from the nonsyndromic group. CONCLUSIONS: We have identified an extremely strong association between pulmonary atresia and ventricular septal defect and persistent airway hyperresponsiveness. Haploinsufficiency at chromosome 22q11.2 did not contribute to this predilection for airway hyperresponsiveness. These results provide a basis to anticipate persistent respiratory difficulties after operations in patients with pulmonary atresia and ventricular septal defect. Moreover, this at-risk patient population may yield unique insights into fundamental mechanisms involved in the pathogenesis of airway hyperresponsiveness.

Adolescent↗
PubMed2001

The impact of P-glycoprotein efflux on enterocyte residence time and enterocyte-based metabolism of verapamil.

P-glycoprotein (P-gp) can limit the intestinal permeability of a number of compounds and may therefore influence their exposure to metabolizing enzymes within the enterocyte (e.g. cytochrome P450 3A, CYP 3A). In this study, the intestinal metabolic profile of verapamil, the influence of P-gp anti-transport on the cellular residence time of verapamil, and the impact of this change in residence time on the extent of enterocyte-based metabolism have been investigated in-vitro, utilizing segments of rat jejunum and side-by-side diffusion chambers. Verapamil exhibited concentration-dependent P-gp efflux and CYP 3A metabolism. The P-gp efflux of verapamil (1 microM) increased the cellular residence time across the intestinal membrane (approximately 3-fold) in the mucosal to serosal (m to s) direction relative to serosal to mucosal (s to m), yielding significantly greater metabolism (approximately 2-fold), presumably as a result of the prolonged exposure to CYP 3A. Intestinal metabolism of verapamil generated not only norverapamil, but resulted also in the formation of an N-dealkylated product (D-617). Norverapamil and D-617 accumulated significantly in mucosal chambers, relative to serosal chambers, over the time course of the experiment. Based on these in-vitro data, it was apparent that P-gp efflux prolonged the cellular residence time of verapamil (m to s) and therefore increased the extent of intestinal metabolism, and also played a role in metabolite secretion from within the enterocyte.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2001

Breath-holding spells associated with significant bradycardia: successful treatment with permanent pacemaker implantation.

OBJECTIVE: To determine whether children with pallid breath-holding spells associated with bradycardia can be treated safely and successfully with permanent pacemaker implantation. METHODS: The records of pediatric patients who had apparent breath-holding spells and associated bradycardia and were treated with permanent pacemaker implantation were reviewed. RESULTS: Ten pediatric patients with apparent breath-holding spells associated with bradycardia were treated with a permanent ventricular demand pacemaker at the Mayo Clinic between 1985 and 1995. Patients had onset of symptoms between ages 6 days and 12 months and presented for evaluation between ages 12 months and 5 years. Duration of spells was 15 seconds to 10 minutes. Medications to prevent spells were unsuccessful. Electrocardiograms documented asystolic pauses of 1.7 to 24 seconds (mean: 11.9 seconds). Permanent ventricular demand pacemakers were implanted at 10 months to 5 years of age (median: 14.5 months): 9 endocardial and 1 epicardial. Three patients required pacemaker revision. At follow-up of 38 to 170 months (median: 65.5), 5 patients had complete resolution of spells, 2 had only mild color change without loss of consciousness or seizure activity, and 3 continued to have minor brief spells. CONCLUSIONS: Permanent pacemaker therapy for children with pallid breath-holding spells associated with severe bradycardia is safe, efficacious, and warranted.

Apnea↗
PubMed2000

The formulation of Halofantrine as either non-solubilizing PEG 6000 or solubilizing lipid based solid dispersions: physical stability and absolute bioavailability assessment.

A non-solubilizing solid dispersion formulation (polyethylene glycol 6000) and two solubilizing solid dispersions (Vitamin E TPGS and a Gelucire 44/14/Vitamin E TPGS blend) containing the antimalarial, Halofantrine (Hf), were formulated for bioavailability assessment in fasted beagles to determine if the oral absorption of Hf can be enhanced by these delivery systems. Solid dispersions comprising varying proportions of drug to carrier were prepared by the fusion method. Whilst the non-solubilizing formulation was assessed according to its dispersion characteristics, the solubilizing solid dispersions were assessed by their ability to form microemulsions upon dispersion. Studies in fasted beagles showed that the solid dispersions afforded a five- to seven-fold improvement in absolute oral bioavailability when compared with the commercially available tablet formulation. The delivery of Hf in either a solubilizing or non-solubilizing solid dispersion did not result in significant differences in oral bioavailability. The physical stability of the solid dispersions was studied using differential scanning calorimetry and X-ray powder diffraction.

Antimalarials↗
PubMed2000

Factors limiting the oral bioavailability of N-acetylglucosaminyl-N-acetylmuramyl dipeptide (GMDP) and enhancement of absorption in rats by delivery in a water-in-oil microemulsion.

The bioavailability (BA) of radio-labelled N-acetylglucosaminyl-N-acetylmuramyl dipeptide (GMDP) was low when administered by oral gavage as an aqueous solution to conscious male Sprague-Dawley rats (8.3+/-4.4% (mean+/-S.D., n=3)). To assess the likely factors contributing to the poor BA of GMDP, the stability of GMDP in the lumen of the gastrointestinal (GI) tract was examined in vitro, using ex vivo GI contents. GMDP was degraded by the contents of the small intestine, caecum and large intestine but was more stable in stomach contents. The permeability coefficient (p(app)) of GMDP in isolated sections of rabbit ileum was 1.67x10(-6) cm/s in the mucosal to serosal direction and was not significantly different in the serosal to mucosal direction, indicating that GMDP is poorly permeable and passively transported across the intestinal wall. First pass metabolism was considered to be unlikely to be the primary limitation to the oral bioavailability of GMDP and therefore, that the oral bioavailability of GMDP was likely limited by instability in the lumen of the gastrointestinal tract and low intestinal permeability. A water-in-oil (w/o) microemulsion formulation subsequently developed to address these problems was trialed in a preliminary bioavailability study in rats and enhanced the bioavailability of GMDP ten-fold when administered intraduodenally, indicating that w/o microemulsions may represent a viable mechanism for enhancing the bioavailability of poorly GI-stable and poorly permeable peptide-based molecules.

Acetylmuramyl-Alanyl-Isoglutamine↗
PubMed2000

fas-fas-ligand antigen expression and its relationship to increased apoptosis in acute renal transplant rejection.

BACKGROUND: To examine the role of fas-fas-ligand interaction and apoptosis in acute transplant rejection. METHODS: Pre- and posttransplant renal allograft biopsies were stained by in situ 3-end labeling of DNA for detection of apoptotic cells (TUNEL) and immunohistochemistry techniques were used for demonstration of fas and fas-ligand antigen expression. RESULTS: Posttransplantation apoptosis was significantly increased in acute rejection and acute tubular necrosis (P<0.0001) compared to preimplantation biopsies and biopsies taken from grafts showing dysfunction not attributed to rejection. Fas and fas-ligand expression was demonstrated predominantly in the tubular epithelium. In preimplant biopsies fas was expressed in 11% (4/37) of cases; posttransplantation expression increased to: 44% (8/18) acute rejection, 63% (5/8) acute tubular necrosis, and 38% (5/13) dysfunction without evidence of rejection. Fas-ligand was expressed by 30% (11/37) of preimplant biopsies, posttransplantation expression was reduced in all groups: 17% (3/18) acute rejection, 13% (1/8) acute tubular necrosis, delayed xenograft rejection and 15% (2/13) dysfunction without evidence of rejection. A correlation with fas-1 expression preimplantation and a subsequent absence of acute rejection post transplant was noted (P<0.001). CONCLUSIONS: Apoptosis is a feature of acute rejection and acute tubular necrosis. Fas expression is uncommon preimplantation and increases non-specifically post transplant. Fas-1 was expressed by a third of preimplantation biopsies and expression was lost non-specifically post transplant. The expression of fas-ligand preimplantation correlated with an absence of acute rejection episodes posttransplant, suggesting some degree of immune privilege. These data suggest that the fas-fas-1 mediated pathway does not play a specific role in apoptosis during acute rejection. We were unable to find any evidence that the fas-fas-1-mediated pathway has a role in the increased apoptosis seen during acute rejection.

Adult↗
PubMed2000

Central mutation databases--a review.

The Internet has been a key component in the coordination of the diverse group of scientists involved in the Human Genome Project. Nowhere has this contribution been more critical than in the maintenance and exchange of information about genetic variation and mutation. Whereas the majority of DNA sequence is generated and stored by a relatively few sites, a far greater number of researchers investigate the variations in that sequence from sites scattered worldwide. It falls to central databases to utilize the Internet to assemble data from these sites and make them available to the greater human genomic community.

Databases, Factual↗
PubMed2000

Systemic availability and lymphatic transport of human growth hormone administered by subcutaneous injection.

Degradation of human growth hormone (hGH) at the injection site has previously been implicated as the basis for its reduced systemic availability following subcutaneous (SC) administration. The goal of these studies was to develop an animal model which would allow mass balance calculations to (i) quantify the loss at the injection site and (ii) determine the role of the lymphatics in the transport of subcutaneously-administered hGH. The animal model utilized a sheep and enabled simultaneous sampling of blood and collection of either peripheral lymph (via the efferent duct of the popliteal lymph node draining the injection site) or central lymph (via the thoracic lymph duct). In non-lymph cannulated sheep, the systemic availability of hGH following SC dosing was 58.4 +/- 9.1% (mean +/- SEM) relative to an intravenous (IV) control. The availability of hGH decreased to approximately 30-40% when either peripheral or central lymph was collected indicating that a proportion of the dose was transported via the lymph. The fraction of the administered dose collected in peripheral lymph was 61.7 +/- 8.5% (mean +/- SEM), whereas only 8.6 +/- 1.3% was collected in central lymph. These results suggested that loss of hGH within the lymphatics contributed significantly to its reduced systemic availability following SC administration. The total recovery (sum of the systemic availability and the cumulative amount recovered in lymph) of hGH was approximately 93% of the dose in the peripherally-cannulated group indicating that loss at the injection site was minimal.

Animals↗