Glenn A. Edwards

Formulation design and bioavailability assessment of lipidic self-emulsifying formulations of halofantrine

Abstract The potential for lipidic self-emulsifying drug delivery systems (SEDDS) and self-microemulsifying drug delivery systems (SMEDDS) to improve the oral bioavailability of a poorly absorbed, antimalarial drug (Halofantrine, Hf) was investigated in fasted beagles. Hf free base, rather than the commercially available hydrochloride salt (Hf.HCl), was studied due to its much higher solubility in lipidic triglyceride solvents. The multi-component delivery systems were optimised by evaluating their ability to self-emulsify when introduced to an aqueous medium under gentle agitation, and by determination of particle size of the resulting emulsion. Optimised formulations selected for bioavailability assessment were medium-chain triglyceride SEDDS and SMEDDS, and a long-chain triglyceride SMEDDS. The relevant pharmacokinetic parameters of Hf, and its desbutyl metabolite, were determined relative to an intravenous formulation. The lipid-based formulations of Hf base afforded a six- to eight-fold improvement in absolute oral bioavailability relative to previous data of the solid Hf.HCl tablet formulation. These data indicate the utility of dispersed lipid-based formulations for the oral delivery of Hf free base, and potentially other lipophilic drugs.

Susceptibility to lipase-mediated digestion reduces the oral bioavailability of danazol after administration as a medium-chain lipid-based microemulsion formulation.

AbstractPurpose. To investigate the impact of lipidic formulation type on in vitro dispersion and digestion properties and the relationship to oral bioavailability, using danazol as a model lipophilic poorly water-soluble drug. Methods. Three lipid-based danazol formulations [a long-chain triglyceride solution (LCT-solution) and self-microemulsifying drug delivery systems (SMEDDS) based on long-chain (C18) lipids (LC-SMEDDS) and medium-chain (C8-C10) lipids (MC-SMEDDS)] were adminis- tered to fasted beagle dogs and compared with a micronized danazol formulation administered postprandially and in the fasted state. In vitro dispersion and particle size data for the two SMEDDS were compared, and the distribution/solubilization patterns of danazol across the various phases produced during in vitro digestion quantified. Results. The LCT-solution and LC-SMEDDS formulations significantly enhanced the oral bioavailability of danazol when compared to fasted administration of the powder formulation. In contrast, and despite displaying excellent dispersion properties, the MC-SMEDDS resulted in little enhancement in danazol bioavailability. In support of the in vivo findings, in vitro digestion of the medium-chain formulation resulted in significant drug precipitation when compared with the long-chain lipid formulations. Conclusions. Digestion of microemulsion preconcentrate formulations based on medium-chain lipids may limit in vivo utility when compared with similar formulations based on long chain lipids.

Increasing the proportional content of surfactant (Cremophor EL) relative to lipid in self-emulsifying lipid-based formulations of danazol reduces oral bioavailability in beagle dogs.

PurposeTo investigate the impact of a change in the proportions of lipid, surfactant and co-solvent on the solubilisation capacity of self-emulsifying formulations of danazol during in vitro dispersion and digestion studies and correlation with in vivo bioavailability in beagle dogs.MethodsFormulations from within the phase diagram of the pseudo-ternary system composed of soybean oil:maisine 35-1 (1:1 w/w), Cremophor EL and ethanol were assessed in vitro on dispersion and digestion. The relative bioavailability of danazol after administration of a series of these formulations was also determined.ResultsAll formulations formed microemulsions in the presence of water and no drug precipitation was observed on dispersion. In contrast, drug solubilisation was markedly affected by lipase-mediated digestion and a reduction in lipid (and increase in surfactant) content resulted in increased drug precipitation. Consistent with these data, the bioavailability of danazol decreased significantly when the lipid content in the formulations was reduced.ConclusionA rank-order correlation was observed between the patterns of solubilisation obtained during in vitro digestion and the in vivo performance of self-emulsifying formulations of danazol. In general a decrease in the lipid content and an increase in the proportions of surfactant and co-solvent resulted in reduced danazol bioavailability.

Examination of oral absorption and lymphatic transport of halofantrine in a triple-cannulated canine model after administration in self-microemulsifying drug delivery systems (SMEDDS) containing structured triglycerides

The potential for lipidic self-microemulsifying drug delivery systems (SMEDDS) containing triglycerides with a defined structure, where the different fatty acids on the glycerol backbone exhibit different metabolic fate, to improve the lymphatic transport and the portal absorption of a poorly water-soluble drug, halofantrine, were investigated in fasted lymph cannulated canines. Two different structured triglycerides were incorporated into the SMEDDS; 1,3-dioctanoyl-2-linoleyl-sn-glycerol (C8:0-C18:2-C8:0) (MLM) and 1,3-dilinoyl-2-octanoyl-sn-glycerol (C18:2-C8:0-C18:2) (LML). A previously optimised SMEDDS formulation for halofantrine, comprising of triglyceride, Cremophor EL, Maisine 35-1 and ethanol was selected for bioavailability assessment. The extent of lymphatic transport via the thoracic duct was 17.9% of the dose for the animals dosed with the MLM SMEDDS and 27.4% for LML. Also the plasma availability was affected by the triglyceride incorporated into the multi-component delivery system and availabilities of 56.9% (MLM) and 37.2% (LML) were found. These data indicate that the pharmaceutical scientist can use the structure of the lipid to affect the relative contribution of the two absorption pathways. The MLM formulation produced a total bioavailability of 74.9%, which is higher than the total absorption previously observed after post-prandial administration. This could indicate the utility of disperse lipid-base formulations based on structured triglycerides for the oral delivery of halofantrine, and potentially other lipophilic drugs.

Intestinal Lymphatic Transport of Halofantrine Occurs After Oral Administration of a Unit-Dose Lipid-Based Formulation to Fasted Dogs

AbstractPurpose. To examine whether the small quantities of lipid present in unit-dose microemulsion formulations comprising medium- (C8-10) or long-chain (C18) glyceride lipids can stimulate the intestinal lymphatic transport of halofantrine (Hf), a model lymphatically transported drug. Methods. Hf (50 mg) was administered to thoracic lymph duct- and cephalic vein-cannulated fasted greyhound dogs. Drug was formulated as a single soft gelatin capsule containing approximately 1 g of a microemulsion preconcentrate based on either medium- or long-chain glycerides. Thoracic lymph was collected, and systemic plasma samples taken over 10 h postdose. Results. The extent of lymphatic transport of Hf after administration of the long-chain lipid formulation was high (28.3% of dose), and significantly higher than that seen after administration of the medium-chain formulation (5.0% of dose). Plasma levels of Hf were not significantly different across the two formulations when assessed by AUC0-10h. Conclusions. This is the first study to demonstrate that the small amounts of lipid present within a single lipid-based dose form can support substantial intestinal lymphatic transport in the fasted state. Furthermore, microemulsions based on long-chain glycerides appear to be more effective with respect to lymphatic transport than the equivalent medium-chain formulation.

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.

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.

A highly elastic tissue sealant based on photopolymerised gelatin

Gelatin is widely used as a medical biomaterial because it is readily available, cheap, biodegradable and demonstrates favourable biocompatibility. Many applications require stabilisation of the biomaterial by chemical crosslinking, and this often involves derivatisation of the protein or treatment with cytotoxic crosslinking agents. We have previously shown that a facile photochemical method, using blue light, a ruthenium catalyst and a persulphate oxidant, produces covalent di-tyrosine crosslinks in resilin and fibrinogen to form stable hydrogel biomaterials. Here we show that various gelatins can also be rapidly crosslinked to form highly elastic (extension to break >650%) and adhesive (stress at break >100 kPa) biomaterials. Although the method does not require derivatisation of the protein, we show that when the phenolic (tyrosine-like) content of gelatin is increased, the crosslinked material becomes resistant to swelling, yet retains considerable elasticity and high adhesive strength. The reagents are not cytotoxic at the concentration used in the photopolymerisation reaction. When tested in vivo in sheep lung, the photopolymerised gelatin effectively sealed a wound in lung tissue from blood and air leakage, was not cytotoxic and did not produce an inflammatory response. The elastic properties, thermal stability, speed of curing and high tissue adhesive strength of this photopolymerised gelatin, offer considerable improvement over current surgical tissue sealants.

Atrial electrophysiology is altered by acute hypercapnia but not hypoxemia: implications for promotion of atrial fibrillation in pulmonary disease and sleep apnea.

BACKGROUND Chronic pulmonary disease and sleep apnea have been associated with the development of atrial fibrillation (AF). OBJECTIVE The purpose of this study was to characterize the atrial electrical changes that occur with hypercapnia and hypoxemia and to determine their role in AF development. METHODS Seventeen sheep (6 control, 5 hypercapnia, 6 hypoxemia) underwent open chest electrophysiologic evaluation under autonomic blockade. A 64-electrode endocardial basket catheter was positioned in the right atrium, and 2 x 128 electrode epicardial plaques were sutured to the right atrial and left atrial appendages to determine atrial refractoriness (effective refractory period [ERP]) at 9 sites and 5 cycle lengths, conduction time to fixed points on each plaque, and AF vulnerability. RESULTS Hypercapnia was associated with a 152% lengthening of ERP from baseline and increased conduction time. ERPs rapidly returned to baseline, but recovery of conduction was delayed at least 117 +/- 24 minutes following resolution of hypercapnia. AF vulnerability was reduced during hypercapnia (with increased ERP) but increased significantly with subsequent return to eucapnia (when ERP normalized but conduction time remained prolonged). No significant changes in ERP, atrial conduction time, or AF vulnerability occurred in hypoxemic or control groups. CONCLUSION Differential recovery of ERP and conduction that occurs following hypercapnia might account for the increased vulnerability to AF observed in the phase after return to eucapnia. This may explain in part the increased prevalence of AF in pulmonary disease and sleep apnea.

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.