Drug-induced liver injury (DILI) is one of the most frequent adverse clinical reactions and a relevant cause of morbidity and mortality. Hepatotoxicity is among the major reasons for drug withdrawal during post-market and late development stages, representing a major concern to the pharmaceutical industry. The current biochemical parameters for the detection of DILI are based on enzymes (alanine aminotransferase (ALT), aspartate aminotransferase (AST), gammaglutamyl transpeptidase (GGT), alkaline phosphatase (ALP)) and bilirubin serum levels that are not specific of DILI and therefore there is an increasing interest on novel, specific, DILI biomarkers discovery. Metabolomics has emerged as a tool with a great potential for biomarker discovery, especially in disease diagnosis, and assessment of drug toxicity or efficacy. This review summarizes the multistep approaches in DILI biomarker research and discovery based on metabolomics and the principal outcomes from the research performed in this field.
The use of omics techniques for the identification of the biochemical changes caused by hepatotoxic drugs, and hence, uncovering new DILI biomarkers is an active field of research. In the past decade, transcriptomics and proteomics have been widely used for the study of hepatotoxicity [33][34]. These techniques help to visualise effects of hepatotoxins on gene activation and protein expression, but cannot reveal subtle changes in the cell’s metabolism, the end step of any disturbing action elicited by a toxicant. The liver plays a key role in the homeostasis of the whole organism, with many active metabolic processes susceptible to being altered by the noxious effects of a drug. Thus, it is reasonable to assume that the effects of a drug ultimately causing DILI are reflected in changes in the metabolome [26][35]. While genomics and proteomics describe what might be potentially happening, given that metabolites are the end products of the metabolic reaction pathways, metabolite alterations, and the metabolome provide the best comprehensive insight into what is actually happening as a consequence of a noxious stimulus [36]. They better represent the actual metabolic status of a cell or tissue being a relevant source of information to identify the molecular initiating events in the context of the Adverse Outcome Pathway (AOP) approach [37]. Metabolites are commonly present in many cell types, tissues, and organisms and are part of many conserved metabolic pathways; thus, the analysis and translation of results to humans are affordable [38].
Metabolomics also represents an interesting approach regarding toxicity response prediction, based on the identification of preclinical indicators of susceptibility to hepatotoxins that can be anticipated by assessing the basal pattern of endogenous metabolites prior to drug administration [39][40]. Indeed, several studies have evidenced that humans display characteristic and individual metabolic phenotypes or “metabotypes” [41][42][42] which are intended to be used to anticipate the distinct responses to drugs in drug metabolism, and therefore susceptibility to DILI, advocating for individualized drug therapies.
Considerable advances have recently been made in metabolomics in the discovery of useful biomarkers. Here, we review the major outcomes of metabolomics studies performed for DILI evaluation. A literature search was conducted using the terms “metabolomics”, “DILI” and “humans” in the PUBMED, Web of Science, PubTator, and Embase databases. In vitro and animal studies were excluded from the revision. Despite some initial studies being performed with NMR, MS was the prominent methodology used in the vast majority of research since it provides different ionisation techniques, increasing the number of metabolites that can potentially be detected with enormous sensitivity and selectivity. Being an emerging area of research, the number of scientific reports is reduced. Table S1 (Supplementary Material) summarises metabolic alterations associated with DILI and provides a list of metabolites consistently reported between 2009 and 2021 in 25 clinical and experimental studies, considered as emerging circulating DILI biomarkers. Attention was paid principally to their links to the DILI and not to the drug causing the event. Among them, few studies addressed the identification of specific markers for DILI prediction or individual susceptibility determination [39][43][44][45][46] while the vast majority were focused on DILI diagnosis and follow-up [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66].
Detailed information with the results, outcomes and conclusions of the review can be found at: doi: 10.3390/metabo12060564.
Great advances have been made in the applicability of MS-metabolomics for the diagnosis of DILI by examining urine, plasma, and serum human samples of patients. These biological samples carry over parts of the hepatocyte exometabolome that allows the identification of DILI-specific biomarkers. These research papers support the great potential of metabolomics as a major source of novel biomarkers for DILI diagnosis and susceptibility, type and severity of the response, and effects caused by a given drug therapy. Altogether, it can contribute to a better understanding of the DILI phenomena and increase safety scrutinization of drugs under development. Several metabolites appeared as statistically significant discriminants in those studies, demonstrating their potential as putative DILI biomarkers; however, up to today, there is not a unanimous agreement on a specific metabolic fingerprint that can be recommended for the routine diagnosis and prognosis of DILI in the clinics. Nevertheless, very promising steps forward have been undertaken and point at future clinical routine use. Inconsistencies among studies can be explained by several factors. First, the different methods applied for metabolomic analysis may influence the type and number of analytes detected and identified. Second, the majority of the studies clustered DILI and non-DILI patients without considering the DILI phenotypes and the causing agent that can belong to a wide range of pharmacological categories and, hence, influence distinct metabolic pathways. Third, the prevailing limitation for clinical verification of a novel biomarker is the requirement to test them in a meaningfully large and well-defined patient population, and here, both the number of individuals included in each cohort study and the number of studies performed so far are still insufficient to provide biomarker candidates with enough statistical discriminative power. Even more, not all studies claiming new biomarkers have proved the sensitivity and specificity of their candidate metabolites as diagnostic biomarkers of DILI. Fourth, the scope of the research differs among the studies. While some of them are focused on the identification of discriminating metabolites between DILI and non-DILI patients, others included distinct grades of DILI severity and even other types of liver diseases which difficults the identification of commonly agreed biomarkers. Therefore, there is still a need for standardised and multicentre studies to address these challenges in the field of DILI metabolomics. All this has prevented up to now, the widespread use of metabolomics-derived biomarkers in routine clinical practice. No less relevant is the fact that data analysis still requires technical expertise and highly experienced personnel. This will probably be made simpler when user-friendly analytical software for data processing will be developed and standardised. Notwithstanding the current limitations, some studies have made very significant advances in the way DILI can be approached from the metabolome analysis, not only to discriminate the phenotype, severity, and evolution of the disease but also to bring new insights to numerically describe the behaviour of a given drug in different patients. An important outcome is that focus should be put on the comparisons of the metabolic pathways altered in DILI rather than on a combination of individual metabolites. The influences of the methodology in biassing data having been retrieved from patients in different moments and with different analysers that tend to be minimised, reaching a comparable diagnosis for a given patient’s sample. The increasing applicability of metabolomics in biomedical research has led to the development of computational and visual tools such as the mummichog algorithm and pathway analysis, as well as tools for the comparison and meta-analysis of metabolomics results, which permit extracting meaningful and more reproducible information in this type of research than with the analysis of individual metabolite levels per se. Given the great possibilities that offer to combine physiological and metabolic pathway information, together with further technological innovation and larger cohorts of patients, the application of metabolomics will provide new findings of early specific markers of diagnostic, treatment, and prognosis of DILI.
This entry is adapted from the peer-reviewed paper 10.3390/metabo12060564