Editor's Highlight: Transgenic Zebrafish Reporter Lines as Alternative In Vivo Organ Toxicity Models.

Organ toxicity, particularly liver toxicity, remains one of the major reasons for the termination of drug candidates in the development pipeline as well as withdrawal or restrictions of marketed drugs. A screening-amenable alternative in vivo model such as zebrafish would, therefore, find immediate application in the early prediction of unacceptable organ toxicity. To identify highly upregulated genes as biomarkers of toxic responses in the zebrafish model, a set of well-characterized reference drugs that cause drug-induced liver injury (DILI) in the clinic were applied to zebrafish larvae and adults. Transcriptome microarray analysis was performed on whole larvae or dissected adult livers. Integration of data sets from different drug treatments at different stages identified common upregulated detoxification pathways. Within these were candidate biomarkers which recurred in multiple treatments. We prioritized 4 highly upregulated genes encoding enzymes acting in distinct phases of the drug metabolism pathway. Through promoter isolation and fosmid recombineering, eGFP reporter transgenic zebrafish lines were generated and evaluated for their response to DILI drugs. Three of the 4 generated reporter lines showed a dose and time-dependent induction in endodermal organs to reference drugs and an expanded drug set. In conclusion, through integrated transcriptomics and transgenic approaches, we have developed parallel independent zebrafish in vivo screening platforms able to predict organ toxicities of preclinical drugs.

High-content micronucleus assay in genotoxicity profiling: initial-stage development and some applications in the investigative/lead-finding studies in drug discovery.

This article describes the first step toward full (that includes conditions for both absence and presence of metabolic activation) validation and drug discovery application of a 96-well, automated, high-content micronucleus (HCMN) assay. The current validation tests were performed using Chinese hamster ovary cells, in the absence of metabolic activation, against three distinct sets of drug-like compounds that represent all stages of a drug discovery pipeline. A compound categorization scheme was created based on quantitative relationships between micronucleus (MN) signals, cytotoxicity, and compound solubility. Results from initial validation compounds (n = 38) set the stage for differentiating overall positive and negative MN inducers. To delve deeper into the compound categorization process, a more extensive validation set, consisting of a larger set (n = 370) of "drug-like but less optimized" early-stage compounds, was used for further refinement of positive and negative compound categories. The predictivity and applicability of the assay for clinical stage compounds was ascertained using (n = 168) clinically developed marketed drugs or well-studied compounds. Upon full validation, a detailed analysis of results established five compound categories--NEG (negative), NEG/xx µM (negative up to the solubility limit of xx µM), WPOS (weak positive), POS (positive), and INCON (inconclusive). Furthermore, examples of lead-finding applications and ongoing investigative HCMN activities are described. A proposal is offered on how the HCMN assay can be positioned in parallel to the overall stage gates (e.g., scaffold selection, lead optimization, late-stage preclinical development) of drug discovery programs. Because of its greater throughput, 1-week turnaround time, and a substantially reduced (1-2 mg) requirement for compound consumption, the HCMN assay is appropriate for developing structure-genotoxicity relationships and for mechanistic genotoxicity studies. The assay does not replace the Organization for Economic Cooperation and Development-compliant, non-good laboratory practice in vitro MN test (e.g., slide-based MN test in TK6 lymphoblastoid cells) that is used for full characterization of lead candidates.