Evaluating the utility of hepatic in vitro models for the assessment of the multi-mechanistic toxicity of drug-induced liver injury

Adverse drug reactions (ADRs) are a major burden to the National Health Services (NHS), accounting for 6.5 % of hospital admissions in 2004. It is estimated that ADRs cost the NHS £466 million annually due to them being the 7th leading cause of death. However, ADRs are not just a hospital problem; they represent a major pharmaceutical concern. In 2015, it was estimated that the cost of getting a drug from bench to market cost $2.6 billion. During 1975 - 1999, of the 548 new drugs that were approved, 10.2 % acquired black box warnings or were withdrawn from the market due to ADRs. For this reason, there is a great pharmaceutical concern as millions of pounds and time could be wasted.

Whilst ADRs can affect different organs in the body, the liver is one of the two most reported cases. Drug-induced liver injury (DILI) presents itself as a variety of pathological conditions, with 20 - 40 % of reported cases causing cholestasis. Drug-induced mitochondrial toxicity (DIMT) has been reported as a determinant of DILI with 50 % of drugs with black box warnings for DILI also having mitochondrial burdens. DILI is associated with multiple mechanisms of toxicity. For this reason, a 'one test fits all' approach cannot be used, which has contributed to the difficulty in detecting DILI preclinically. Nevertheless, mitochondrial toxicity and biliary transporter implications are recognised as major mechanisms of hepatotoxicity.

The mitochondria have a wealth of structural and functional features which can be targeted by a compound and lead to toxicity. These can include electron transport chain inhibition, oxidative phosphorylation uncoupling, opening of the mitochondrial permeability transition (MPT) pore, alterations in mitochondrial dynamics and the depletion of the mitochondrial genome. Due to the mitochondria having their own genome, the effects of mitochondrial DNA (mtDNA) variation upon susceptibility to DILI is another potential factor requiring investigation.

The aim of the project is to develop and validate screening models for the assessment of mitochondrial toxicity and transporter dysfunction in DILI. HepaRG cells are terminally differentiated hepatic cells derived from a human hepatic progenitor cell line that retains many characteristics of primary human hepatocytes (PHH), including xenobiotic metabolism enzymes, drug transporters and functional biliary structures. HepaRG cells can be manipulated to circumvent the Crabtree effect, allowing investigations of mitochondrial toxicity. Therefore, the ability of HepaRG cells to connect the interplay between mitochondria and transporter dysfunction will be evaluated. Additionally, the development of the first personalised, liver-specific in vitro model for mitochondrial genetic variation, coined transmitochondrial HepG2 cybrids will allow novel investigations of the effects of

individual susceptibility to DILI. More predictive and sensitive models for DILI and their links with mitochondrial toxicity and transporter alterations would prove invaluable in the prevention of late-stage drug attrition and the development of safer drugs.