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

Abstract

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.