A lipid-based liquid crystalline matrix that provides sustained release and enhanced oral bioavailability for a model poorly water soluble drug in rats

Liquid crystalline phases that are stable in excess water, formed using lipids such as glyceryl monooleate (GMO) and oleyl glycerate (OG), are known to provide a sustained release matrix for poorly water soluble drugs in vitro, yet there has been no report of the use of these materials to impart oral sustained release behaviour in vivo. In the first part of this study, in vitro lipolysis experiments were used to compare the digestibility of GMO with a second structurally related lipid, oleyl glycerate, which was found to be less susceptible to hydrolysis by pancreatic lipase than GMO. Subsequent oral bioavailability studies were conducted in rats, in which a model poorly water soluble drug, cinnarizine (CIN), was administered orally as an aqueous suspension, or as a solution in GMO or OG. In the first bioavailability study, plasma samples were taken over a 30 h period and CIN concentrations determined by HPLC. Plasma CIN concentrations after administration in the GMO formulation were only sustained for a few hours after administration while for the OG formulation, the plasma concentration of cinnarizine was at its highest level 30 h after dosing, and appeared to be increasing. A second study in which CIN was again administered in OG, and plasma samples taken for 120 h, revealed a Tmax for CIN in rats of 36 h and a relative oral bioavailability of 344% when compared to the GMO formulation (117%) and the aqueous suspension formulation (assigned a nominal bioavailability of 100%). The results indicate that lipids that form liquid crystalline structures in excess water, may have application as an oral sustained release delivery system, providing they are not digested rapidly on administration.

Hexosomes formed from glycerate surfactants—Formulation as a colloidal carrier for irinotecan

A new class of amphiphiles with a glycerate headgroup, recently shown to form reverse hexagonal phase in excess water, have been dispersed to form Hexosome dispersions comprising sub-200 nm particles retaining the internal nanostructure of the parent H(II) phase. The application of these novel materials to the development of a new injectable formulation of irinotecan was investigated. The formulation of irinotecan with a small percentage of oleic acid in oleyl glycerate permitted a clinically relevant dose of irinotecan to be dissolved in the glycerate surfactant and dispersed in aqueous medium to form an injectable particle-based dose form of irinotecan. Importantly, incorporation of irinotecan into Hexosomes at neutral pH did not result in conversion from the active lactone to the inactive carboxylate form on storage, and is hence a promising alternative to the current low pH formulation of irinotecan required to inhibit this conversion. Although release of irinotecan from the Hexosomes was shown to be virtually instantaneous from the Hexosomes on substantial dilution, the retention of the drug in lactone form at neutral pH demonstrates a potential application of these novel nanostructured particles in injectable drug delivery.

Lyotropic liquid crystalline phases formed from glycerate surfactants as sustained release drug delivery systems

A new class of surfactants with glycerate headgroups, that form viscous lyotropic liquid crystalline phases in excess water, have been investigated for their potential to provide sustained release matrices for depot drug delivery. Oleyl glycerate and phytanyl glycerate were used as representative surfactants of this new class, and their behaviour compared with that of glyceryl monooleate (GMO). The surfactants were found to form reverse hexagonal phase (H(II)) in excess water, and the matrices were loaded with a series of model hydrophobic and hydrophilic drugs, (paclitaxel, irinotecan, glucose, histidine and octreotide), and the release kinetics determined. In all cases, the release behaviour obeyed Higuchi kinetics, with linear drug release versus square root of time. The H(II) phases released model drugs slower than the GMO cubic phase matrix. The oleyl glycerate matrix was found to consistently release drug faster than the phytanyl glycerate matrix, despite both matrices being based on H(II) phase. To further demonstrate the potential utility of these materials as drug depot delivery systems, an injectable precursor formulation for octreotide was also prepared and demonstrated to provide controlled release for the peptide. The stability of the H(II) phase to likely in vivo breakdown products was also assessed.

Intestinal lymphatic transport of halofantrine occurs after oral administration of a unit-dose lipid-based formulation to fasted dogs

PURPOSE

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.

Spurious hypoxaemia in a patient with leukaemia and extreme leucocytosis

Rapid consumption of oxygen by leucocytes can result in erroneous diagnosis of severe hypoxaemia in patients with extreme leucocytosis. We report a case of chronic myeloid leukaemia, extreme leucocytosis and arterial hypoxaemia which was out of proportion to the clinical and radiological evidence of lung disease. The pseudohypoxaemia was confirmed by pulse oximeter and became less significant after successful reduction of leucocyte counts following leucophoresis and chemotherapy. Serial arterial blood gas analysis also demonstrated a slower initial rate of decay of PaO2 as the leucocyte count decreased with treatment.

A physicochemical basis for the extensive intestinal lymphatic transport of a poorly lipid soluble antimalarial, halofantrine hydrochloride, after postprandial administration to dogs

The highly lipid soluble, free-base form of halofantrine (Hf base; approximately 50 mg/mL in triglyceride lipids), a highly lipophilic (calculated log P approximately 8.5) antimalarial, has recently been shown to undergo significant intestinal lymphatic transport (54% of administered dose) after postprandial administration to dogs. In contrast, the clinically available hydrochloride salt of Hf (Hf small middle dot HCl), was not considered to be a likely substrate for lymphatic transport because its solubility in long-chain triglyceride lipids is low (< 1 mg/mL). This paper reports the lymphatic transport of Hf after postprandial administration of Hf.HCl, which was surprisingly high at 47% of the administered dose, and not significantly different from that of Hf base. It was postulated that partial conversion of solubilized Hf.HCl to the highly lipid soluble Hf base within the intestinal lumen might account for the extensive lymphatic transport. However, as Hf is a tertiary amine with an expected pK(a) of > 10, at gastrointestinal pH, the fraction of Hf present as the free base form is likely to be extremely low. Physicochemical studies exploring the solubility and pK(a) of Hf suggest that Hf.HCl was extensively solubilized following fed administration. When solubilized in representative fed state mixed micellar solutions, its apparent pK(a) was 6.92 and considerably lower than anticipated for a tertiary amine. It appears that the extensive lymphatic transport of Hf observed after postprandial administration of Hf.HCl was likely to be due to the conversion of solubilized Hf.HCl to the free base. Therefore, in addition to indicators such as log P and triglyceride solubility, factors such as drug solubilization in representative fed state intestinal conditions and the possible conversion to the un-ionized form should be considered when predicting the potential lymphatic transport of salts of poorly water soluble acids and bases.

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.

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.

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.

Metabolism of halofantrine to its equipotent metabolite, desbutylhalofantrine, is decreased when orally administered with ketoconazole

Halofantrine (Hf) is a highly lipophilic antimalarial with poor and erratic absorption. Published data indicates that the oral bioavailability of Hf was increased 3-fold in humans and 12-fold in dogs when administered postprandially; however, the proportional formation of the active desbutyl metabolite (desbutylhalofantrine, Hfm) decreased 2.4-fold in humans and 6.8-fold in dogs (Milton et al., Br. J. Clin. Pharmacol. 1989, 28, 71-77; Humberstone et al., J. Pharm. Sci. 1996, 85, 525-529). The current study was undertaken to confirm the putative involvement of CYP3A4 in the N-dealkylation of Hf to Hfm by administering Hf with and without ketoconazole (KC), a specific CYP3A4 inhibitor, and measuring the resulting plasma concentration profiles of Hf and Hfm. The plasma Hfm/Hf AUC(0-72 h) ratio after fasted oral administration of Hf without KC was 0.56, whereas the ratio after fasted oral administration with KC was less than 0.05. It is likely that both hepatic and prehepatic (enterocyte-based) CYP3A4 contributed to metabolism of Hf to Hfm after oral administration. Interestingly, the low plasma Hfm/Hf AUC ratios observed after fasted administration of Hf with KC were similar to the low values previously observed when Hf was administered postprandially (despite increased Hf absorption). The mechanism(s) by which postprandial administration of Hf led to a decrease in its metabolism are unknown, but based on the current data, could include inhibition of CYP3A4-mediated metabolism by components of the ingested meal. Other possibilities include a lipid-induced postprandial recruitment of intestinal lymphatic transport or avoidance of metabolism during transport through the enterocyte into the portal blood. Further studies are required to determine the relative contributions by which these different processes may decrease the presystemic metabolism of Hf.