inhibition of the latter leading to less energy produced by the Krebs cycle. Due to diminished ATP production, the parasite is immobilized and eventually dies. Some parasites have evolved some resistance to albendazole by having a different set of acids constituting β-tubulin, decreasing the binding affinity of albendazole. Some parasites (especially filarial nematodes) live in symbiosis with Wolbachia, a type of intracellular parasite bacteria. In such cases the Wolbachia are necessary to the survival of the parasitic worms. Elimination of Wolbachia from these filarial nematodes generally results in either death or sterility of the host nematode. Pharmacokinetics Oral absorption of albendazole varies among species, with 1–5% of the drug being successfully absorbed in humans, 20–30% in rats, and 50% in cattle. The absorption also largely depends on gastric pH. People have varying gastric pHs on empty stomachs, and thus absorption from one person to another can vary wildly when taken without food. Generally, the absorption in the GI tract is poor due to albendazole's low solubility in water. It is, however, better absorbed than other benzimidazole carbamates. Food stimulates gastric acid secretion, lowering the pH and making albendazole more soluble and thus more easily absorbed. Oral absorption is especially increased with a fatty meal, as albendazole dissolves better in lipids, allowing it to cross the lipid barrier created by the mucus surface of the GI tract. To target intestinal parasites, albendazole is taken on an empty stomach to stay within the gut. Absorption is also affected by how much of the albendazole is degraded within the small intestine by metabolic enzymes in the villi. The pharmacokinetics of albendazole differ slightly between men and women: women have a lower oral clearance and volume of distribution, while men have a lower serum peak concentration. Albendazole undergoes very fast first-pass metabolism in all species, such that the unchanged drug is undetectable in plasma. Most of it is oxidized into albendazole sulfoxide (also known as ricobendazole and albendazole oxide) in the liver by cytochrome P450 oxidases (CYPs) and a flavin-containing monooxygenase (FMO), which was discovered later. In humans, the cytochrome P450 oxidases are thought to include CYP3A4 and CYP1A1, while those in the rats are thought to be CYP2C6 and CYP2A1. Oxidation to albendazole sulfoxide by FMO produces R(+) enantiomers, while oxidation the cytochromes and by some enzymes in the gut epithelium produce S(-). Different species produce the R(+) and S(-) enantiomers in different