Short Communication AN EXAMINATION OF THE INTERPLAY BETWEEN ENTEROCYTE-BASED METABOLISM AND LYMPHATIC DRUG TRANSPORT IN THE RAT

The current study has examined whether drugs that are transported to the systemic circulation via the intestinal lymph (and therefore associate with lipoproteins within the enterocyte) are accessible to enterocyte-based metabolic processes. The impact of changes to the mass of lipid present within the enterocytebased lymph lipid precursor pool (LLPP) on the extent of enterocyte-based drug metabolism has also been addressed. Low (5 mg oleic acid/h) or high [20 mg oleic acid/5.2 mg lyso-phosphatidylcholine/h] lipid dose formulations containing halofantrine (which is lymphatically transported and metabolized) or dichlorodiphenyltrichloroethane (DDT) (which is lymphatically transported and relatively metabolically inert) and radiolabeled oleic acid were infused into the duodenum of lymph duct-cannulated rats. After 5 h, drug and radiolabeled oleic acid were removed from the infusions, allowing calculation of the first order turnover rate constants describing drug and oleic acid transport from the LLPP into lymph from the washout profiles. In one group of animals, bolus doses of ketoconazole were also administered to inhibit cytochrome P450based metabolism. The rate constant describing halofantrine transport from the LLPP into the lymph was lower than that of oleic acid, whereas these differences were abolished in the presence of ketoconazole. DDT and oleic acid exhibited similar turnover rate constants. The data therefore suggest that enterocyte-based metabolism removes halofantrine from the LLPP before transport into the lymph. Furthermore, enhancing the lymphatic transport of halofantrine by coadministration of larger quantities of lipid reduced the difference between the turnover rate constant for halofantrine and oleic acid and seemed to reduce the extent of enterocyte-based metabolism. After oral delivery, drugs may be transported from the intestine to the systemic circulation via the intestinal lymph or the portal blood (Porter and Charman, 2001a). Drugs that are significantly transported via the intestinal lymphatic system usually do so in association with the lipid-rich lipoprotein fraction of lymph (Sieber et al., 1974; Porter and Charman, 2001a). For this reason, only highly lipophilic drugs (typically log P 5, triglyceride solubility 50 mg/g) (Charman and Stella, 1986a), which can associate with developing lipoproteins during transport through the enterocyte, are appreciably transported via the intestinal lymphatic system. Relatively little has been published detailing the processes that dictate the association of lipophilic drugs with enterocyte-based lipoproteins; however, a recent study from our laboratory (Trevaskis et al., 2006) has shown that the size and dynamics of the LLPP seems to dictate the rate and extent of lymphatic transport of a highly lipophilic drug (halofantrine). The LLPP consists of multiple droplets of lipid situated in the smooth endoplasmic reticulum (SER) and Golgi of the enterocyte. The lipids in the LLPP are destined for assembly into lipoproteins, which are subsequently transported from the enterocyte to the systemic circulation via the intestinal lymphatic system. The LLPP in the SER and Golgi is distinct from the lipid droplets that are located in the enterocyte cytoplasm. These cytoplasmic lipids are primarily transported to the systemic circulation via the portal vein (Tipton et al., 1989; Nevin et al., 1995). Interestingly, the previous study from our laboratory showed that, whereas drug transport into the lymph was, in large part, a function of the mass of lipid in the LLPP available to solubilize drug (where fatty acid was used as a marker for lipid), the first order rate constant describing halofantrine transport from the LLPP into the lymph was significantly lower than the equivalent rate constant describing fatty acid transport (Trevaskis et al., 2006). Because the fractional turnover of fatty acid from the LLPP was more rapid than that of drug, the data suggested that halofantrine was removed from the LLPP, not only by transfer into the lymph, but also by an additional mechanism. The hypothesis underlying the current study was that the differences in halofantrine and lipid turnover may be accounted for by loss of halofantrine from the LLPP as a result of enterocyte-based metabo-