Clofibrate metabolism

Pharmaceuticals) Ethinyl oestradiol, paracetamol, aspirin, dextropropoxyphene, clofibrate and oxytetracycline Aire river (UK), Lambro river (Italy) and Rur river (Germany) Surface water - Product consumption - Worst-case influent - Excreted post-metabolism - WWTP removal - River water die-away [50]... 

Another therapeutic class to be briefly discussed is that of the lipid-lowering agents known as fibrates, e.g., clofibrate and fenofibrate (8.5). Here also, the acidic metabolite is the active form clofibrate (an ethyl ester) is rapidly hydrolyzed to clofibric acid by liver carboxylesterases and blood esterases [11], Human metabolic studies of fenofibrate (8.5), the isopropyl ester of fenofibric acid, showed incomplete absorption after oral administration, while hydrolysis of the absorbed fraction was quantitative [12], This was followed by other reactions of biotransformation, mainly glucuronidation of the carboxylic acid group. 

The improvement in insulin resistance seen with thiazolidinediones is thought to be mediated dtrough their interaction with PPARy. Clofibrate binds PPARa affecting different aspects of lipid metabolism than the thiazolidinediones. 

Rifampin is known to induce the hepatic microsomal enzymes that metabolize various drugs such as acetaminophen, oral anticoagulants, barbiturates, benzodiazepines, beta blockers, chloramphenicol, clofibrate, oral contraceptives, corticosteroids, cyclosporine, disopyramide, estrogens, hydantoins, mexiletine, quinidine, sulfones, sulfonylureas, theophyllines, tocainide, verapamil, digoxin, enalapril, morphine, nifedipine, ondansetron, progestins, protease inhibitors, buspirone, delavirdine, doxycycline, fluoroquinolones, losartan, macrolides, sulfonylureas, tacrolimus, thyroid hormones, TCAs, zolpidem, zidovudine, and ketoconazole. The therapeutic effects of these drugs may be decreased. 

Lautier J., J.L. Chanal, and A. DeUion (1986). Comparative hydrolipidemic effects of clofibric acid and itanoxone on the metabolism and distribution of lipids in the crab Pachygraspus marmoratus (Decopoda, Brachyura). Comparative Biochemistry and Physiology C 85 269-274. 

Mizuguchi, H., Kudo, N., Ohya, T. Kawashima, Y. (1999) Effects of tiadenol and di-(2-ethyl-hexyl)phthalate on the metabolism of phosphatidylcholine and phosphatidylethanolamine in the liver of rats comparison with clofibric acid. Biochem. Pharmacol, 57, 869-876 Mocchiutti, N.O. Bernal, C.A. (1997) Effects of chronic di(2-ethylhexyl) phthalate intake on the secretion and removal rate of triglyceride-rich lipoproteins in rats. Food chem. Toxicol, 35, 1017-1021... 

With the clofibrate type of inducer, other changes are also apparent. Thus, there is a proliferation in the number of peroxisomes (an intracellular organelle) as well as induction of a particular form of cytochrome P-450 involved in fatty acid metabolism. A number of other enzymes associated with the role of this organelle in fatty acid metabolism are also increased, such as carnitine acyltransferase and catalase. This phenomenon is discussed in more detail in chapter 6. 

Figure 5.36 Mechanism of the receptor-mediated induction of CYP4A by a chemical such as the drug clofibrate. The inducer-receptor (PPAR) complex enters the nucleus, binds with RXR, and the complex binds to the receptor response elements in the CYP gene. This induces the production of CYP4A mRNA, which leads to the production of CYP4A protein and functional enzyme. Alternatively, the drug may perturb lipid metabolism leading to increases in a lipid(s), which will bind to the receptor and cause the same response. Abbreviations PPAR, peroxisome proliterator-activated receptor RXR, retinoid X receptor.

Oral contraceptives increase the plasma clearance of clofibric acid, which is mainly metabolized by glucuronidation (341,342). 

The antipsychotic chlorpromazine is a prototype heptotoxicant for production of cholestasis. Pleiotropic effects of chlorpromazine on membrane permeability and associated ion gradients and microfilament-mediated canalicular contraction have been attributed to detergent effects. Valproic acid, an anticonvulsant, is associated with microvesicular steatosis. Inhibition of mitochondial fatty acid (S-oxidation is an important component of this toxic effect and is apparently related to carnitine availability as evidenced by the protection afforded by L-carnitine supplements. The hypolipidemic drugs clofibrate, fenofibrate, and gemfibrozil are peroxisome prolif-erators in rodent liver, but not in humans. Isoniazid, an antibiotic used to treat tuberculosis, exhibits an approximately 1 % incidence of hepatotoxicity. Although toxicity is known to be metabolism-dependent and protein adduction has been well-... 

The interaction of rifampicin with clofibrate has been reviewed (105). Rifampicin induces the metabolism of clofibrate, and in five subjects rifampicin 600 mg/day caused a 35% fall in steady-state serum clofibrate concentrations (106). 

Fibric adds Clofibrate Gemfibrozil Fenofibrate Increase VLtt-catabolism PPAR , agonist LDLi 5-20% HDL t 10%-20% TG i 20%-S0% Gl upset, dyspepsia, gallstones, T LFTs, myopathy Nonmetabolism Warfarin Cyclosporine Statins Liver or severe renal disease primary biliary cirrhosis preexisting gallbladder disease... 

Clofibrate at a concentration of 0.5 mmol in culture medium maintained the cytochrome P-450 content of rat hepatocytes for up to 96 h. This effect was associated with a marked induction of lauric acid hydroxylation whereas little effect was observed on the metabolism of three other cytochrome p450 dependent mixed function oxidase substrates. 

---