Use of adult human hepatocytes in primary culture for the study of clometacin-induced immunoallergic hepatitis

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Structural alert/reactive metabolite concept as applied in medicinal chemistry to mitigate the risk of idiosyncratic drug toxicity: a perspective based on the critical examination of trends in the top 200 drugs marketed in the United States

Because of a preconceived notion that eliminating reactive metabolite (RM) formation with new drug candidates could mitigate the risk of idiosyncratic drug toxicity, the potential for RM formation is routinely examined as part of lead optimization efforts in drug discovery. Likewise, avoidance of "structural alerts" is almost a norm in drug design. However, there is a growing concern that the perceived safety hazards associated with structural alerts and/or RM screening tools as standalone predictors of toxicity risks may be over exaggerated. In addition, the multifactorial nature of idiosyncratic toxicity is now well recognized based upon observations that mechanisms other than RM formation (e.g., mitochondrial toxicity and inhibition of bile salt export pump (BSEP)) also can account for certain target organ toxicities. Hence, fundamental questions arise such as: When is a molecule that contains a structural alert (RM positive or negative) a cause for concern? Could the molecule in its parent form exert toxicity? Can a low dose drug candidate truly mitigate metabolism-dependent and -independent idiosyncratic toxicity risks? In an effort to address these questions, we have retrospectively examined 68 drugs (recalled or associated with a black box warning due to idiosyncratic toxicity) and the top 200 drugs (prescription and sales) in the United States in 2009 for trends in physiochemical characteristics, daily doses, presence of structural alerts, evidence for RM formation as well as toxicity mechanism(s) potentially mediated by parent drugs. Collectively, our analysis revealed that a significant proportion (∼78-86%) of drugs associated with toxicity contained structural alerts and evidence indicating that RM formation as a causative factor for toxicity has been presented in 62-69% of these molecules. In several cases, mitochondrial toxicity and BSEP inhibition mediated by parent drugs were also noted as potential causative factors. Most drugs were administered at daily doses exceeding several hundred milligrams. There was no obvious link between idiosyncratic toxicity and physicochemical properties such as molecular weight, lipophilicity, etc. Approximately half of the top 200 drugs for 2009 (prescription and sales) also contained one or more alerts in their chemical architecture, and many were found to be RM-positive. Several instances of BSEP and mitochondrial liabilities were also noted with agents in the top 200 category. However, with relatively few exceptions, the vast majority of these drugs are rarely associated with idiosyncratic toxicity, despite years of patient use. The major differentiating factor appeared to be the daily dose; most of the drugs in the top 200 list are administered at low daily doses. In addition, competing detoxication pathways and/or alternate nonmetabolic clearance routes provided suitable justifications for the safety records of RM-positive drugs in the top 200 category. Thus, while RM elimination may be a useful and pragmatic starting point in mitigating idiosyncratic toxicity risks, our analysis suggests a need for a more integrated screening paradigm for chemical hazard identification in drug discovery. Thus, in addition to a detailed assessment of RM formation potential (in relationship to the overall elimination mechanisms of the compound(s)) for lead compounds, effects on cellular health (e.g., cytotoxicity assays), BSEP inhibition, and mitochondrial toxicity are the recommended suite of assays to characterize compound liabilities. However, the prospective use of such data in compound selection will require further validation of the cellular assays using marketed agents. Until we gain a better understanding of the pathophysiological mechanisms associated with idiosyncratic toxicities, improving pharmacokinetics and intrinsic potency as means of decreasing the dose size and the associated "body burden" of the parent drug and its metabolites will remain an overarching goal in drug discovery.

Use of hepatocyte cultures for the study of hepatotoxic compounds

Human and animal hepatocytes in primary culture are widely used in pharmacotoxicological research. They represent a unique in vitro model since they retain both phase I and phase II enzyme activities as well as their inducibility by xenobiotics. Hepatocyte cultures are used for drug screening, identification of the lesions induced by toxic compounds and determination of mechanisms by which xenobiotics exert liver injury.

Cytotoxic activity of T cells and non-T cells from diclofenac-immunized mice against cultured syngeneic hepatocytes exposed to diclofenac

To evaluate whether hepatocellular protein adducts of the nonsteroidal antiinflammatory drug diclofenac could elicit a specific cell-mediated or antibody-dependent immune response that eventually results in liver cell destruction, we developed a murine ex vivo/in vitro mixed lymphocyte hepatocyte culture (MLHC) model. C57BL/6 mice were immunized either with diclofenac conjugated to keyhole limpet hemocyanin (KLH) or with KLH alone. Splenocytes from mice exhibiting hgih antidiclofenac antibody titers were isolated and co-cultured at an effector/target cell ratio of 100:1 with syngeneic murine hepatocytes preexposed to diclofenac. By 48 and 72 hours, extracellular alanine transaminase (ALT) activity had increased 6.4- and 7.6-fold, respectively, versus the 24-hour value. In contrast, there was no significant cytotoxic response after either drug treatment alone or immunization alone. Furthermore, those cellular populations capable of inducing ALT release also showed lymphocyte stimulation as determined by interleukin-2 (IL-2) receptor expression and lymphocyte proliferation analysis. The extent of cell injury was highest in the presence of lymphocytes highly enriched in T cells and was reduced by 40% in the presence of anti-MHC I antibodies. Similarly, albeit to a lesser extent, non-T cell-enriched lymphocyte fractions also induced hepatocyte injury. The addition of co-culture supernatants to hepatocytes had no effect, thus ruling out the possibility that soluble factors alone mediated the cell injury. However, supernatants from diclofenac-stimulated lymphocytes, combined with nonstimulated splenocytes, triggered an immediate (< 1 hour) cytotoxic response, suggesting antibody-dependent cell-mediated mechanisms of target cell injury. These results indicate that diclofenac-treated hepatocytes carried antigenic determinants that were recognized by T cells and non-T cells derived from diclofenac/KLH-immunized mice, resulting in cell-mediated destruction of the target hepatocytes.

Idiosyncratic liver toxicity of nonsteroidal antiinflammatory drugs: molecular mechanisms and pathology

This review explores the clinical hepatic pathology associated with the use of nonsteroidal antiinflammatory drugs (NSAIDs), possible cellular and molecular mechanisms of injury, and future challenges. NSAIDs comprise a group of widely used compounds that have been associated with rare adverse reactions in the liver, including fulminant hepatitis and cholestasis. These reactions are idiosyncratic, mostly independent of the dose administered, and host-dependent. The mechanisms responsible for the initiation and perpetuation of NSAID-induced hepatotoxicity remain poorly understood and have been largely inferred from clinical manifestation. A mounting body of evidence, however, indicates that many acidic NSAIDs are metabolized to reactive acyl glucuronides that can form covalent adducts with plasma proteins and hepatocellular proteins. In hepatocytes cocultured with lymphocytes, these NSAID-altered proteins can become antigenic. Thus, long-lived, drug-altered proteins may act as immunogens and produce cytotoxic T-cell-mediated or antibody-dependent, cell-mediated toxicity in susceptible patients. Alternatively, individual abnormalities in metabolism or disposition of some NSAIDs may lead to the formation or accumulation of toxic metabolites. Additional work with transgenic animal models is needed to permit better understanding of the general and specific risk factors involved in the pathogenesis of the idiosyncratic liver injuries related to NSAIDs and other drugs.

Formation of trifluoroacetylated protein antigens in cultured rat hepatocytes exposed to halothane in vitro

Immune responses to novel, halothane metabolite-modified protein antigens (tri-fluoroacetylated proteins; TFA-proteins) have been implicated in the pathogenesis of halothane hepatitis. The aim of the present study was to investigate and characterize expression of TFA-proteins in cultures of rat hepatocytes which were exposed to halothane in vitro. Following exposure to halothane, the hepatocytes were harvested, then subcellular fractions were prepared and were analysed by immunoblotting for expression of antigens recognized by a rabbit anti-TFA antiserum, and by antibodies in sera from two patients with halothane hepatitis. Hepatocytes exposed to halothane in vitro were shown to express novel microsomal protein antigens, which exhibited molecular masses that were identical to the molecular masses of the major TFA-protein antigens expressed in vivo, in livers of halothane-treated rats (100, 80 and 60 kDa). Experiments in which hepatocytes were exposed to halothane in the presence of SKF-525A, or were exposed to deuterated halothane in place of halothane, confirmed that these novel antigens were TFA-modified proteins whose generation required cytochrome P450-mediated metabolism of halothane. The maximal levels of TFA-antigens expressed in vitro were about 30% of the levels expressed in halothane-treated rats in vivo. Maximal expression of the TFA-antigens in vitro occurred when hepatocytes were exposed to halothane at doses which yielded concentrations of the drug in culture medium of about 13 microM. Expression of the antigens in vitro occurred slowly, with an apparent half-time of about 8 hr. Overall, these results demonstrate that the properties of the TFA-antigens expressed in cultured hepatocytes in vitro closely resemble the properties exhibited by the antigens expressed in vivo, in livers of halothane-treated rats.