Pharmacokinetics explain in vivo/in vitro discrepancies of carcinogen-induced gene expression alterations in rat liver and cultivated hepatocytes

In Vitro-In Vivo Extrapolation by Physiologically Based Kinetic Modeling: Experience With Three Case Studies and Lessons Learned

Physiologically based kinetic (PBK) modeling has been increasingly used since the beginning of the 21st century to support dose selection to be used in preclinical and clinical safety studies in the pharmaceutical sector. For chemical safety assessment, the use of PBK has also found interest, however, to a smaller extent, although an internationally agreed document was published already in 2010 (IPCS/WHO), but at that time, PBK modeling was based mostly on in vivo data as the example in the IPCS/WHO document indicates. Recently, the OECD has published a guidance document which set standards on how to characterize, validate, and report PBK models for regulatory purposes. In the past few years, we gained experience on using in vitro data for performing quantitative in vitro–in vivo extrapolation (QIVIVE), in which biokinetic data play a crucial role to obtain a realistic estimation of human exposure. In addition, pharmaco-/toxicodynamic aspects have been introduced into the approach. Here, three examples with different drugs/chemicals are described, in which different approaches have been applied. The lessons we learned from the exercise are as follows: 1) in vitro conditions should be considered and compared to the in vivo situation, particularly for protein binding; 2) in vitro inhibition of metabolizing enzymes by the formed metabolites should be taken into consideration; and 3) it is important to extrapolate from the in vitro measured intracellular concentration and not from the nominal concentration to the tissue/organ concentration to come up with an appropriate QIVIVE for the relevant adverse effects.

Recapitulation of First Pass Metabolism Using 3D Printed Microfluidic Chip and Organoid

The low bioavailability of oral drugs due to first pass metabolism is a major obstacle in drug development. With significant developments in the field of in vitro organ modeling and microfluidic chip three-dimensional (3D) printing, the challenge is to apply these for the production and evaluation of new drug candidates. This study aimed to produce a microfluidic chip to recapitulate and assess the feasibility of the first pass metabolism. The infill condition of the polycarbonate transparent filament and layer height was optimized to visualize and maintain the organoid or spheroid on the chip. Next, the chip was fabricated using a 3D printer after a computer-aided design (CAD). The chip consisted of three wells of different heights. The small intestinal (SI) organoid and colorectal adenocarcinoma spheroids were placed on the second and third wells, respectively. No additional equipment was assembled, and the tilted tunnel was connected to each well to transport the material by gradient force. The chip was fabricated using 50% and 0.1 um thickness. Among the three different prototypes of chip (chips 1, 2, and 3), the highest distribution of plasmids in the Matrigel of the second well was observed in Chip 2 at 48 h. The effect of first pass metabolism was analyzed using docetaxel. In the chip without an SI organoid, there was a marked decrease in the viability of colorectal adenocarcinoma spheroids due to drug efficacy. However, in the chip with the SI organoid, no significant change in viability was observed because of first pass metabolism. In conclusion, we presented a simple, fast, and low-cost microfluidic chip to analyze the efficacy change of candidate drug by the first pass metabolism.

Restoring circadian synchrony in vitro facilitates physiological responses to environmental chemicals

BACKGROUND

The growing requirement of hazard and risk assessment of environmental chemicals and the efforts to minimize animal testing, increases the demand for innovative and predictive in vitro test systems in toxicology, reflecting the physiological conditions of human nature. Here, an elemental factor regulating a variety of physiological processes is the day-night rhythm. This circadian rhythm, describing a biological oscillation with a 24-h period is hardly acknowledged in toxicology and test method development. Whilst, in animals or humans the entire organism exhibits a rigorous cellular circadian synchrony, in conventional in vitro systems each cell follows its own rhythm, due to the absence of appropriate synchronizing signals.

OBJECTIVE

Here we investigated whether circadian synchronization of human cells in an in vitro system improves the cellular response and, thus, increases the sensitivity of the test system. Since the circadian regulation of metabolism is particularly well understood, and dioxin and dioxin-like compounds are of major concern for environmental health we focused on the ubiquitous drug metabolizing detoxification system mediated by the aryl hydrocarbon receptor (AHR).

METHODS

To this end, we applied various prototypical AHR activators onto different human cell lines under non-synchronized or circadian synchronized conditions and determined the dose response on representative endogenous target genes.

RESULTS

Remarkably, the cellular response dynamic upon chemical treatment was substantially enhanced in circadian synchronized cells and followed a rhythmic expression pattern. This broader dynamic range was associated with a strikingly higher induction of AHR target genes and the corresponding enzymatic activity, thereby rather mimicking the in vivo situation.

CONCLUSION

Our findings indicate that a synchronized circadian rhythm in a cell culture based test system can improve the physiological relevance of an appropriate in vitro method by reflecting the biological in vivo situation more closely. Accordingly, it is a promising tool to facilitate the wide acceptance of in vitro methods in the field of regulatory toxicology and to further optimize the toxicological assessment of environmental chemicals.

Prediction of human drug-induced liver injury (DILI) in relation to oral doses and blood concentrations

Drug-induced liver injury (DILI) cannot be accurately predicted by animal models. In addition, currently available in vitro methods do not allow for the estimation of hepatotoxic doses or the determination of an acceptable daily intake (ADI). To overcome this limitation, an in vitro/in silico method was established that predicts the risk of human DILI in relation to oral doses and blood concentrations. This method can be used to estimate DILI risk if the maximal blood concentration (C_max) of the test compound is known. Moreover, an ADI can be estimated even for compounds without information on blood concentrations. To systematically optimize the in vitro system, two novel test performance metrics were introduced, the toxicity separation index (TSI) which quantifies how well a test differentiates between hepatotoxic and non-hepatotoxic compounds, and the toxicity estimation index (TEI) which measures how well hepatotoxic blood concentrations in vivo can be estimated. In vitro test performance was optimized for a training set of 28 compounds, based on TSI and TEI, demonstrating that (1) concentrations where cytotoxicity first becomes evident in vitro (EC₁₀) yielded better metrics than higher toxicity thresholds (EC₅₀); (2) compound incubation for 48 h was better than 24 h, with no further improvement of TSI after 7 days incubation; (3) metrics were moderately improved by adding gene expression to the test battery; (4) evaluation of pharmacokinetic parameters demonstrated that total blood compound concentrations and the 95%-population-based percentile of C_max were best suited to estimate human toxicity. With a support vector machine-based classifier, using EC₁₀ and C_max as variables, the cross-validated sensitivity, specificity and accuracy for hepatotoxicity prediction were 100, 88 and 93%, respectively. Concentrations in the culture medium allowed extrapolation to blood concentrations in vivo that are associated with a specific probability of hepatotoxicity and the corresponding oral doses were obtained by reverse modeling. Application of this in vitro/in silico method to the rat hepatotoxicant pulegone resulted in an ADI that was similar to values previously established based on animal experiments. In conclusion, the proposed method links oral doses and blood concentrations of test compounds to the probability of hepatotoxicity.

Toxicogenomics - What added Value Do These Approaches Provide for Carcinogen Risk Assessment?

It is still a major challenge to protect humans at workplaces and in the environment. To cope with this task, it is a prerequisite to obtain detailed information on the extent of chemical perturbations of biological pathways, in particular, adaptive vs. adverse effects and the dose-response relationships. This knowledge serves as the basis for the classification of non-carcinogens and carcinogens and for further distinguishing carcinogens in genotoxic (DNA damaging) or non-genotoxic compounds. Basing on quantitative dose-response relationships, points of departures can be derived for chemical risk assessment. In recent years, new methods have shown their capability to support the established rodent models of carcinogenicity testing. In vitro high throughput screening assays assess more comprehensively cell response. In addition, omics technologies were applied to study the mode of action of chemicals whereby the term "toxicogenomics" comprises various technologies such as transcriptomics, epigenomics, or metabolomics. This review aims to summarize the current state of toxicogenomic approaches in risk science and to compare them with established ones. For example, measurement of global transcriptional changes generates meaningful information for toxicological risk assessment such as accurate classification of genotoxic/non-genotoxic carcinogens. Alteration in mRNA expression offers previously unknown insights in the mode of action and enables the definition of key events. Based on these, benchmark doses can be calculated for the transition from an adaptive to an adverse state. In short, this review assesses the potential and challenges of transcriptomics and addresses the impact of other omics technologies on risk assessment in terms of hazard identification and dose-response assessment.

Assessing Concordance of Drug-Induced Transcriptional Response in Rodent Liver and Cultured Hepatocytes

The effect of drugs, disease and other perturbations on mRNA levels are studied using gene expression microarrays or RNA-seq, with the goal of understanding molecular effects arising from the perturbation. Previous comparisons of reproducibility across laboratories have been limited in scale and focused on a single model. The use of model systems, such as cultured primary cells or cancer cell lines, assumes that mechanistic insights derived from the models would have been observed via in vivo studies. We examined the concordance of compound-induced transcriptional changes using data from several sources: rat liver and rat primary hepatocytes (RPH) from Drug Matrix (DM) and open TG-GATEs (TG), human primary hepatocytes (HPH) from TG, and mouse liver / HepG2 results from the Gene Expression Omnibus (GEO) repository. Gene expression changes for treatments were normalized to controls and analyzed with three methods: 1) gene level for 9071 high expression genes in rat liver, 2) gene set analysis (GSA) using canonical pathways and gene ontology sets, 3) weighted gene co-expression network analysis (WGCNA). Co-expression networks performed better than genes or GSA when comparing treatment effects within rat liver and rat vs. mouse liver. Genes and modules performed similarly at Connectivity Map-style analyses, where success at identifying similar treatments among a collection of reference profiles is the goal. Comparisons between rat liver and RPH, and those between RPH, HPH and HepG2 cells reveal lower concordance for all methods. We observe that the baseline state of untreated cultured cells relative to untreated rat liver shows striking similarity with toxicant-exposed cells in vivo, indicating that gross systems level perturbation in the underlying networks in culture may contribute to the low concordance.

Gene expression studies in model systems are widely used for understanding the mechanism of drugs and other perturbations in biological systems. Other researchers have examined the reproducibility of microarray studies between laboratories, or comparing microarrays and/or RNA sequencing. However, no large scale studies have compared results from protocols which differ in minor details, or results generated in vivo vs. in vitro culture systems thought to serve as useful models. The rat liver is by far the most extensively studied model evaluating effects of drugs and other perturbations, and existing data allowed us to assess the level of concordance between rat liver and rat primary hepatocytes cultured in collagen-coated plates (i.e. “flat” culture) for hundreds of drugs. We found that the mouse liver serves as a better model of the rat liver than do rat primary hepatocytes, even after allowing for differences due to pharmacokinetics. The low concordance observed between rat liver and rat hepatocytes suggests that validating the utility of ‘omics data generated on emerging cell culture approaches (e.g. “organ-on-a-chip”, 3D-printed tissues) using rat cells and comparison to the rat liver may be necessary in order to gain confidence these approaches substantially improve on traditional culture models of human cells.

Toxicology: a discipline in need of academic anchoring--the point of view of the German Society of Toxicology

The paper describes the importance of toxicology as a discipline, its past achievements, current scientific challenges, and future development. Toxicological expertise is instrumental in the reduction of human health risks arising from chemicals and drugs. Toxicological assessment is needed to evaluate evidence and arguments, whether or not there is a scientific base for concern. The immense success already achieved by toxicological work is exemplified by reduced pollution of air, soil, water, and safer working places. Predominantly predictive toxicological testing is derived from the findings to assess risks to humans and the environment. Assessment of the adversity of molecular effects (including epigenetic effects), the effects of mixtures, and integration of exposure and biokinetics into in vitro testing are emerging challenges for toxicology. Toxicology is a translational science with its base in fundamental science. Academic institutions play an essential part by providing scientific innovation and education of young scientists.

Inhibition of protein phosphatase 2A sensitizes pancreatic cancer to chemotherapy by increasing drug perfusion via HIF-1α-VEGF mediated angiogenesis

Pancreatic cancer is a malignant disease without efficient treatment. Improved treatments are urgently needed to enhance or replace chemotherapy. Here we used a small molecular compound LB-100 to assess the effect of pharmacological inhibition of protein phosphatase 2A (PP2A) in combination with doxorubicin on the proliferation of pancreatic cancer in cell lines and a xenograft model. LB-100 moderately reduced PP2A activity and the growth of the cell lines but did not show chemosensitization in vitro. In vivo, however, LB-100 synergistically enhanced the activity of doxorubicin. This effect was associated with increased microvessel density, blood perfusion, and doxorubicin concentrations within the xenografts. Mechanically, LB-100 induced expression of hypoxia-induced factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF). In an umbilical vein endothelial cell monolayer model for measuring changes in vascular permeability, increased VEGF secretion following exposure to LB-100 and doxorubicin was accompanied by increased amounts of doxorubicin penetrating the endothelial barrier. In conclusion, PP2A inhibition by LB-100 enhanced the cytotoxicity of doxorubicin in vivo but not in vitro potentially via HIF-1α-VEGF mediated angiogenesis. Combining inhibition of PP2A with chemotherapeutic regimens may enhance their effectiveness against pancreatic cancer.

Synergistic effects of glycated chitosan with high-intensity focused ultrasound on suppression of metastases in a syngeneic breast tumor model

Stimulation of the host immune system is crucial in cancer treatment. In particular, nonspecific immunotherapies, when combined with other traditional therapies such as radiation and chemotherapy, may induce immunity against primary and metastatic tumors. In this study, we demonstrate that a novel, non-toxic immunoadjuvant, glycated chitosan (GC), decreases the motility and invasion of mammalian breast cancer cells in vitro and in vivo. Lung metastatic ratios were reduced in 4T1 tumor-bearing mice when intratumoral GC injection was combined with local high-intensity focused ultrasound (HIFU) treatment. We postulate that this treatment modality stimulates the host immune system to combat cancer cells, as macrophage accumulation in tumor lesions was detected after GC-HIFU treatment. In addition, plasma collected from GC-HIFU-treated tumor-bearing mice exhibited tumor-specific cytotoxicity. We also investigated the effect of GC on epithelial–mesenchymal transition-related markers. Our results showed that GC decreased the expression of Twist-1 and Slug, proto-oncogenes commonly implicated in metastasis. Epithelial-cadherin, which is regulated by these genes, was also upregulated. Taken together, our current data suggest that GC alone can reduce cancer cell motility and invasion, whereas GC-HIFU treatment can induce immune responses to suppress tumor metastasis in vivo.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Modeling bioavailability to organs protected by biological barriers

Computational pharmacokinetic (PK) modeling gives access to drug concentration vs. time profiles in target organs and allows better interpretation of clinical observations of therapeutic or toxic effects. Physiologically-based PK (PBPK) models in particular, based on mechanistic descriptions of the body anatomy and physiology, may also help to extrapolate in vitro or animal data to human.

Once in the systemic circulation, a chemical has access to the microvasculature of every organ or tissue. However, its penetration in the brain, retina, thymus, spinal cord, testis, placenta,… may be limited or even fully prevented by dynamic physiological blood-tissue barriers. Those barriers are both physical (involving tight junctions between adjacent cells) and biochemical (involving metabolizing enzymes and transporters).

On those cases, correct mechanistic characterization of the passage (or not) of molecules through the barrier can be crucial for improved PBPK modeling and prediction.

In parallel, attempts to understand and quantitatively characterize the processes involved in drug penetration of physiological barriers have led to the development of several in vitro experimental models. Data from such assays are very useful to calibrate PBPK models.

We review here those in vitro and computational models, highlighting the challenges and perspectives for in vitro and computational models to better assess drug availability to target tissues.