3.2 Acetaminophen
Acetaminophen (APAP) (paracetamol) is an over‐the‐counter (OTC) analgesic drug (NSAID) used extensively by the US population for the treatment of pain and fever. It is often used in combination with other OTC products for the relief of pain and cold symptoms [1–7]. It is also one of the oldest drugs (>50 years) that remains on the US market in spite of its association with up to 50% of drug‐induced liver failures in the US, leading to deaths or a need for liver transplantation [8–10]. APAP hepatotoxicity is often due to intentional or unintentional overdosage, with liver toxicity reported to be observed in patients administered chronic doses of >5 g per day or acute doses of >7 g [11]. However, there are also cases of APAP‐induced liver failure in patients who used the recommended therapeutic doses [10], with alcohol intake as a major exacerbating factor [12–20]. Hepatotoxicity can be attenuated upon intravenous treatment with N‐acetyl cysteine (NAC) within eight hours of APAP ingestion. The effectiveness of NAC treatment is a function of plasma APAP level as well as the time interval between NAC administration and APAP ingestion [21, 22].
3.2.1 Drug Metabolism and Toxicity
APAP represents a prototypical protoxicant in which metabolic activation is a key mechanism of its hepatotoxicity. Its metabolic activation pathways have been extensively evaluated in both human in vitro systems as well as in vivo in laboratory animals. The generally accepted metabolic activation scheme is that APAP undergoes oxidative metabolism by P450 to N‐acetyl‐p‐benzoquinone imine (NAPQI) which is a highly reactive metabolite that is either detoxified via glutathione (GSH) conjugation or undergoes covalent binding with cellular macromolecules, leading to hepatocellular necrosis. Inflammatory response elicited by cellular injuries and the subsequent Increased oxygen/nitrogen stress mediated by Kupffer cell activation [23] and recruitment of neutrophils [24] may also be involved in APAP hepatotoxicity.
In vitro studies using recombinant human P450 isoforms show that, in comparison to the other human P450 isoforms (CYP1A2, CYP2E1, and CYP2D6), CYP3A4 is the most effective in the metabolism of APAP to NAPQI, with Km equal to plasma drug concentration at therapeutic doses [25]. The association of alcohol consumption with APAP hepatotoxicity has led to the postulation that alcohol enhancement of CYP2E1 activity via increased gene expression [26] and/or enzyme stabilization [27], may be responsible for the increased toxicity [28, 29]. An alternative explanation is that APAP is metabolically activated by CYP3A4 while alcohol consumption depletion of cellular GSH [30–32], leading to compromised detoxification of NAPQI and thereby enhanced toxicity.
3.2.2 Transporters and Toxicity
APAP undergoes direct glucuronidation and sulfation, and the reactive metabolite NAPQI subjected to GSH conjugation. The various conjugates are excreted via transporter‐mediated efflux into the bile duct. Recently, using membrane vesicles, APAP–GSH was found to be substrates of MRP1, MRP2, and MRP [33]. Hepatobiliary efflux transporters MRP1 and MRP4 mRNA levels were elevated in livers from patients after toxic APAP ingestion [34]. This upregulation of transporter expression by APAP is also observed in rodent models of APAP hepatotoxicity [34–36]. Treatment of animals with clodronate liposomes to deplete Kupffer cells abolished Mrp4 upregulation by APAP as well as increasing its hepatotoxicity, suggesting that transporter upregulation is protective, and that the upregulation may be a result of Kupffer cell activation [37].
3.2.3 Risk Factors
As APAP is metabolically activated by the inducible P450 isoforms, especially CYP1A2 and CYP3A4, to NAPQI which in turn is detoxified by GSH conjugation, genetic or environmental factors leading to increased CYP3A4 activity and depletion of hepatocellular GSH concentration are likely risk factors of its hepatotoxicity [38]. Omeprazole induction of CYP1A2 in CYP2C19 poor metabolizers have been postulated to be a risk factor of APAP toxicity [39]. Induction of CYP3A4 as a risk factor has been substantiated in a clinical study where a patient coadministered with phenytoin, a CYP3A4 inducer, experienced APAP hepatotoxicity at therapeutic doses [40, 41]. There is also evidence that concurrent inflammatory events may enhance APAP toxicity, suggesting that bacterial or viral infection may be risk factors [23, 42, 43]. As efflux transporters may have protective roles toward APAP toxicity, environmental and genetic factors leading to compromised efflux transporter activities may also be risk factors.
Life style risk factors for APAP toxicity include obesity [44] and chronic alcohol consumption [18, 45, 46]. It is interesting to note that alcohol consumption has been reported to lead to GSH depletion in human subjects [47], thereby supporting GSH as a key risk factor for APAP toxicity.
3.3 Cerivastatin
Cerivastatin belongs to a class of 3‐hydroxy‐3‐methylglutaryl coenzyme A (HMG‐CoA) reductase inhibitors commonly known as statins. Statins are one of the most prescribed drugs indicated for the lowering of plasma cholesterol. Cerivastatin was introduced by Bayer into the U.S. market in June 1997 and was withdrawn in August 2001 due to 31 deaths due to rhabdomyolysis [48–53]. Cerivastatin was introduced to compete with an existing statin, atorvastatin. Statins are commonly coadministered with fibrates for the treatment of patients with mixed hyperlipidemia [54]. Pharmacokinetic drug interactions between cerivastatin and fibrates, leading to elevated plasma levels of cerivastatin, is believed to be the major cause of rhabdomyolysis.
3.3.1 Drug Metabolism and Toxicity
Compromised hepatic clearance is believed to be the major mechanism of cerivastatin toxicity. Cerivastatin‐induced rhabdomyolysis occurred mostly in patients coadministered with fibrate, suggesting that the toxicity is a result of interaction between these two administered drugs. Cerivastatin is metabolized by CYP2C8 to hydroxylated metabolites [55], and CYP3A4 to desmethylcerivastatin which is further glucuronidated [56]. Gemfibrozil was found to inhibit the formation of CYP2C8‐mediated metabolite but not the CYP3A4 metabolites in human liver microsomes [57, 58]. Gemfibrozil coadministration was found to greatly enhance plasma cerivastatin concentrations in human patients [59] while potent CYP3A4 inhibitors such as erythromycin and itraconazole had no significant effects [60]. These observations suggest that CYP2C8 plays a more important role than CYP3A4 in the hepatic clearance of cerivastatin.
Rhabdomyolysis is also observed in patients administered cerivastatin alone. Association of CYP2C8 genetic polymorphism and cerivastatin toxicity, however, is not as definitive as that observed with fibrate inhibition of CYP2C8 activity [61]. Patients with rhabdomyolysis administered only cerivastatin and not coadministered with fibrate have been found to have a higher frequency of CYP2C8 polymorphic genes [62], leading to CYP2C8 activity both higher and lower than that for wildtype. Genetic polymorphism leading to lower CYP2C8 activity would lead to slower hepatic clearance, resulting in higher cerivastatin activity, thereby enhancing the likelihood of rhabdomyolysis. Higher CYP2C8 activity should lead to lower plasma cerivastatin concentrations and therefore would not lead to toxicity. It is possible that for these patients, the role of uptake transporter plays a greater role in hepatic clearance than CYP2C8 (see below).
3.3.2 Transporter and Toxicity
Cerivastatin is one of the drugs where toxicity can be a function of both drug metabolism and transport. Besides being a CYP2C8 inhibitor, Gemfibrozil and its glucuronide are inhibitors of cerivastatin uptake by the organic anion transporting polypeptide 2 (OATP2/OATP1B1:SLC21A6) [58, 63]. Inhibition of transporter‐mediated uptake of cerivastatin into hepatocytes leads to lower hepatic metabolic clearance. Physiologically based pharmacokinetic (PBPK) models have been developed to evaluate the complex interaction between cerivastatin and gemfibrozil, taking into account the inhibitory effects of gemfibrozil on cerivastatin uptake and metabolism [64, 65].
