We additionally examined drug pairs that have similar chemical structures and act on the same molecular target but differ in their potential for DILI. Again, the rule-of-two predicted hepatotoxicity reliably. Finally, the rule-of-two was applied to clinical case studies

to identify hepatotoxic drugs in complex comedication regimes to further demonstrate its use. Conclusion: Apart from dose, lipophilicity contributes significantly to risk Gefitinib clinical trial for hepatotoxicity. Applying the rule-of-two is an appropriate means of estimating risk for DILI compared with dose alone. (HEPATOLOGY 2013) See Editorial on Page 15 Hepatotoxicity is a major reason for drugs failing clinical trials, being withdrawn from the market after approval, and being labeled with a black box warning by the US Food and Drug Administration (FDA).1 About 1,000 drugs are suspected to be hepatotoxic,2

and drug-induced liver injury (DILI) accounts for more than 50% of acute liver failure cases in the United States alone.3 While animal studies remain the gold standard testing strategy,4, 5 a retrospective analysis revealed that such Torin 1 tests fail in about 45% of DILI cases in clinical trials.6 There is an unmet need to predict risk for DILI more reliably, and to overcome current limitations, the concept of a daily dose was developed.7 Specifically, many drugs prescribed at daily doses of ≥100 mg have either been withdrawn from the market or have received a black box warning due to hepatotoxicity8 Resveratrol to imply a significant relationship between daily dose and risk for DILI.9 In order to safeguard patients, it was recommended to avoid drug development programs that require high doses.7, 10, 11 However, many drugs given at high doses are safe with little or no risk of hepatotoxicity, suggesting that daily dose alone is not a reliable means of guiding the drug development process, regulatory application, and clinical practice. To better predict risk for DILI, the combined factors of daily dose and lipophilicity was examined in two

independent data repositories of 164 and 179 drugs labeled for their liver liabilities. Next to dose, lipophilicity is an important physicochemical property12 to affect cellular uptakes and ADMET (absorption, distribution, metabolism, excretion, toxicity) behavior13 and can be determined by the portioning of drugs between octanol and water (i.e. the logP value). Hughes et al.14 analyzed 245 preclinical compounds and observed an increased likelihood of toxic events in animal studies with highly lipophilic compounds, while Peters et al.15 reported for 213 Roche drug candidates with increased logP and increased off-target activity and found these drugs to be more toxic in in vivo studies.16 Several lines of evidence therefore suggest lipophilicity to be linked to drug toxicity, nonetheless is frequently modulated to improve bioavailability and pharmacological activity.