Internationalization of read-across as a validated new approach method (NAM) for regulatory toxicology

Read-across (RAx) translates available information from well-characterized chemicals to a substance for which there is a toxicological data gap. The OECD is working on case studies to probe general applicability of RAx, and several regulations (e.g., EU-REACH) already allow this procedure to be used to waive new in vivo tests. The decision to prepare a review on the state of the art of RAx as a tool for risk assessment for regulatory purposes was taken during a workshop with international experts in Ranco, Italy in July 2018. Three major issues were identified that need optimization to allow a higher regulatory acceptance rate of the RAx procedure: (i) the definition of similarity of source and target, (ii) the translation of biological/toxicological activity of source to target in the RAx procedure, and (iii) how to deal with issues of ADME that may differ between source and target. The use of new approach methodologies (NAM) was discussed as one of the most important innovations to improve the acceptability of RAx. At present, NAM data may be used to confirm chemical and toxicological similarity. In the future, the use of NAM may be broadened to fully characterize the hazard and toxicokinetic properties of RAx compounds. Concerning available guidance, documents on Good Read-Across Practice (GRAP) and on best practices to perform and evaluate the RAx process were identified. Here, in particular, the RAx guidance, being worked out by the European Commission’s H2020 project EU-ToxRisk together with many external partners with regulatory experience, is given.

21st Century Approaches for Evaluating Exposures, Biological Activity, and Risks of Complex Substances: Workshop highlights

The June 2019 workshop 21st Century Approaches for Evaluating Exposures, Biological Activity, and Risks of Complex Substances, co-organised by the International Council of Chemical Association's Long-Range Research Initiative and the European Commission's Joint Research Centre, is summarised. Focus was the need for improved approaches to evaluate the safety of complex substances. Approximately 10% and 20% of substances registered under the EU chemicals legislation are 'multi-constituent substances' and 'substances of unknown or variable compositions, complex reaction products and biological substances' (UVCBs), respectively, and UVCBs comprise approximately 25% of the U.S. Toxic Substances Control Act Inventory. Workshop participants were asked to consider how the full promise of new approach methodologies (NAMs) could be brought to bear to evaluate complex substances. Sessions focused on using NAMs for screening, biological profiling, and in complex risk evaluations; improving read-across approaches employing new data streams; and methods to evaluate exposure and dosimetry. The workshop concluded with facilitated discussions to explore actionable steps forward. Given the diversity of complex substances, no single 'correct' approach was seen as workable. The path forward should focus on 'learning by doing' by developing and openly sharing NAM-based fit-for-purpose case examples for evaluating biological activity, exposures and risks of complex substances.

Applying 'omics technologies in chemicals risk assessment: Report of an ECETOC workshop

Prevailing knowledge gaps in linking specific molecular changes to apical outcomes and methodological uncertainties in the generation, storage, processing, and interpretation of 'omics data limit the application of 'omics technologies in regulatory toxicology. Against this background, the European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) convened a workshop Applying 'omics technologies in chemicals risk assessment that is reported herein. Ahead of the workshop, multi-expert teams drafted frameworks on best practices for (i) a Good-Laboratory Practice-like context for collecting, storing and curating 'omics data; (ii) the processing of 'omics data; and (iii) weight-of-evidence approaches for integrating 'omics data. The workshop participants confirmed the relevance of these Frameworks to facilitate the regulatory applicability and use of 'omics data, and the workshop discussions provided input for their further elaboration. Additionally, the key objective (iv) to establish approaches to connect 'omics perturbations to phenotypic alterations was addressed. Generally, it was considered promising to strive to link gene expression changes and pathway perturbations to the phenotype by mapping them to specific adverse outcome pathways. While further work is necessary before gene expression changes can be used to establish safe levels of substance exposure, the ECETOC workshop provided important incentives towards achieving this goal.

Integrated Testing Strategies (ITS) for safety assessment

Integrated testing strategies (ITS), as opposed to single definitive tests or fixed batteries of tests, are expected to efficiently combine different information sources in a quantifiable fashion to satisfy an information need, in this case for regulatory safety assessments. With increasing awareness of the limitations of each individual tool and the development of highly targeted tests and predictions, the need for combining pieces of evidence increases. The discussions that took place during this workshop, which brought together a group of experts coming from different related areas, illustrate the current state of the art of ITS, as well as promising developments and identifiable challenges. The case of skin sensitization was taken as an example to understand how possible ITS can be constructed, optimized and validated. This will require embracing and developing new concepts such as adverse outcome pathways (AOP), advanced statistical learning algorithms and machine learning, mechanistic validation and "Good ITS Practices".

Increased pH and inorganic phosphate in temporal seizure foci demonstrated by [31P]MRS

To investigate alterations of brain metabolism associated with temporal lobe epilepsy, [31P]MRS studies were performed on the anterotemporal lobes of patients with medically refractory complex partial seizures. Interictally, the pH was significantly more alkaline in the temporal lobe ipsilateral to the seizure focus (7.25 vs. 7.08, p less than 0.05), and the inorganic phosphorous concentration was greater on the side of the epileptogenic focus (1.9 vs. 1.1 mM, p less than 0.05). These changes in pH and inorganic phosphate may represent metabolic alterations secondary to seizures. Alternatively, because alkalosis enhances neural excitability and may enhance seizure activity, the increased pH of the seizure focus may provide insight into the pathophysiologic mechanism of epileptic seizures.

Alterations in brain phosphorus metabolite concentrations associated with areas of high signal intensity in white matter at MR imaging

Areas of high signal intensity in white matter are identified on brain magnetic resonance (MR) imaging studies in 25%-50% of elderly subjects. The authors used phosphorus-31 MR spectroscopy to characterize the metabolic status of hemispheric white matter brain volumes in 30 elderly subjects with white matter areas of high signal intensity at MR imaging. Compared with white matter volumes with no or minimal areas of high intensity, white matter volumes with extensive areas of high intensity evidenced a 26% decrease in the adenosine triphosphate (ATP)/inorganic phosphate (Pi) ratio (P = .03) and a 21% decrease in the ATP concentration (P = .05), with the Pi level unchanged. A pilot P-31 spectroscopic imaging study in a subject with a large, coalescing white matter area of high signal intensity demonstrated large reductions in metabolite concentrations in the high-signal-intensity area. These results suggest that extensive white matter areas of high signal intensity indicate a process that affects white matter cellular energy metabolism.

Decreased phosphorus metabolite concentrations and alkalosis in chronic cerebral infarction

A study was performed to determine quantitatively the alterations in phosphorus metabolite concentrations and pH in regions of the human brain damaged by chronic stroke. Image-guided phosphorus-31 magnetic resonance spectroscopy was performed on the brains of eight healthy subjects and six patients with cerebral infarction of more than 3 months duration. Phosphorus metabolite concentrations in infarcted regions were reduced 8%-67%. Significant decreases occurred in phosphomonoester (PME), phosphodiester (PDE), and adenosine triphosphate (ATP) concentrations, while inorganic phosphate (Pi) and phosphocreatine (PCr) concentrations showed smaller, nonsignificant decreases. The PCr/ATP ratio was significantly increased, while the ATP/Pi ratio was somewhat lower. The phospholipid ratio PDE/PME was also significantly increased, while the ratios of phospholipid (PME, PDE) to phosphate (PCR, Pi) metabolites were significantly decreased. The pH of the infarcted region indicated significantly more alkalinity than in the normal brain. The results suggest that chronic stroke is associated with significant changes in brain metabolite concentrations and pH that are different from those reported for other brain diseases.

Characterization of prostate cancer, benign prostatic hyperplasia and normal prostates using transrectal 31phosphorus magnetic resonance spectroscopy: a preliminary report

We assessed the ability of 31phosphorus (31P) transrectal magnetic resonance spectroscopy to characterize normal human prostates as well as prostates with benign and malignant neoplasms. With a transrectal probe that we devised for surface coil spectroscopy we studied 15 individuals with normal (5), benign hyperplastic (4) and malignant (6) prostates. Digital rectal examination, transrectal ultrasonography and magnetic resonance imaging were used to aid in accurate positioning of the transrectal probe against the region of interest within the prostate. The major findings of the in vivo studies were that normal prostates had phosphocreatine-to-adenosine triphosphate (ATP) ratios of 1.2 +/- 0.2, phosphomonoester-to-beta-ATP ratios of 1.1 +/- 0.1 and phosphomonoester-to-phosphocreatine ratios of 0.9 +/- 0.1. Malignant prostates had phosphocreatine-to-beta-ATP ratios that were lower (0.7 +/- 0.1) than those of normal prostates (p less than 0.02) or prostates with benign hyperplasia (1.1 +/- 0.2, p less than 0.01). Malignant prostates had phosphomonoester-to-beta-ATP ratios (1.8 +/- 0.2) that were higher than that of normal prostates (p less than 0.02). Using the phosphomonoester-to-phosphocreatine ratio, it was possible to differentiate metabolically malignant (2.7 +/- 0.3) from normal prostates (p less than 0.001), with no overlap of individual ratios. The mean phosphomonoester-to-phosphocreatine ratio (1.5 +/- 0.5) of prostates with benign hyperplasia was midway between the normal and malignant ratios, and there was overlap between individual phosphomonoester-to-phosphocreatine ratios of benign prostatic hyperplasia glands with that of normal and malignant glands. To verify the in vivo results, we performed high resolution magnetic resonance spectroscopy on perchloric acid extracts of benign prostatic hyperplasia tissue obtained at operation and on a human prostatic cancer cell line DU145. The extract results confirmed the differences in metabolite ratios observed in vivo. We conclude that transrectal 31P magnetic resonance spectroscopy can characterize metabolic differences between the normal and malignant prostate.

P-31 spectroscopy study of response of superficial human tumors to therapy

Studies were performed to characterize phosphorus-31 magnetic resonance (MR) spectra obtained from 10 superficial human tumors outside the brain and to determine whether P-31 MR spectroscopy could allow detection of a response to therapy before a change in tumor size was measured. The ratio of phosphomonoester to adenosine triphosphate peak intensities (PME/ATP) was unusually large in all tumors studied. The average PME/ATP in lymphomas (1.8 +/- 0.5) was greater than in nonlymphoma cancers (1.1 +/- 0.15). The average PME/ATP for all tumors studied (1.4 +/- 0.5) was much greater than that of underlying skeletal muscle (0.23 +/- .09). Eight of the tumors were studied before and after therapy. Responders were distinguished from nonresponders on the basis of changes in tumor size. PME/ATP decreased during therapy in three lymphomas that responded to therapy. In an adenocarcinoma and Ewing sarcoma that did not respond to therapy, PME/ATP increased. PME/ATP remained constant in two squamous cell carcinomas that responded to therapy and decreased in one squamous cell carcinoma that decreased in size by 40% but was classified as a nonresponder. Changes in PME/ATP did not always parallel changes in tumor size during therapy. In two patients, a decrease in PME/ATP preceded a decrease in tumor size. In four patients, PME/ATP increased transiently during periods when tumor size remained constant.

Alterations in brain phosphate metabolite concentrations in patients with human immunodeficiency virus infection

Human immunodeficiency virus (HIV)-infected individuals often demonstrate neuropsychiatric impairment; however, it is unclear how brain metabolism may be altered in such patients. We used in vivo phosphorus 31 magnetic resonance spectroscopy to noninvasively assess brain energy and phospholipid metabolism by measuring brain concentrations of adenosine triphosphate (ATP), phosphocreatine (PCr), and inorganic phosphate (Pi), as well as phospholipid compounds and intracellular pH. In study 1, 17 HIV-seropositive men with varying degrees of neuropsychiatric impairment and six control subjects were studied. Localized spectra were obtained from a heterogeneous 5 x 5 x 5-cm volume of interest (VOI). Patients with HIV infection had a significantly lower ATP/Pi ratio and a trend for a lower PCr/Pi ratio than did the control group. In addition, the ATP/Pi and PCr/Pi ratios were both significantly negatively correlated with overall severity of neuropsychiatric impairment. In study 2, three HIV-seropositive men with neuropsychiatric impairment were compared with 11 HIV-seronegative men. Localized phosphorus 31 magnetic resonance spectra were obtained from two relatively homogeneous VOIs: (1) a predominantly white matter VOI, and (2) a predominantly subcortical gray matter VOI. The three HIV-infected patients demonstrated significantly decreased ATP and PCr concentrations in the white matter VOI. These results suggest that HIV infection of the brain may impair brain cellular oxidative metabolism and that the degree of metabolic compromise may be related to the severity of neuropsychiatric impairment.

Spin echo 31P spectroscopic imaging in the human brain

Spectroscopic imaging of phosphorus metabolites in the human brain has been carried out with two data acquisition methods: by observation of the free induction decay (FID) signal and by a short spin echo sequence. The resultant spectral images and spatially resolved spectra are compared. Spin echo observation is found to provide spectra of superior quality, and by suitably selecting the sequence timing, no significant increase in T2 losses, as compared with the FID method, is encountered. 31P images with approximately 3.5 cm spatial resolution are obtained within times of 37 min at 2.0 T field strength.

Comparison of 31P MRS and 1H MRI at 1.5 and 2.0 T

The goals of this study were to compare 31P magnetic resonance spectroscopy (MRS) and 1H magnetic resonance imaging (MRI) of human subjects and phantoms at 1.5 and 2.0 T. The 31P signal-to-noise (S/N) ratios in phantom standards and in localized volumes in human brain and liver were compared at 1.5 and 2.0 T. In addition, T1 values for 31P resonances in human brain, 31P linewidths of metabolites in human brain and liver, 1H S/N in a phantom standard, and MR image quality in human head and body were compared at the two field strengths. The results of our study showed that at the higher strength field, (1) in vivo 31P MRS studies benefited from up to 32% improvement in S/N; (2) in vivo 31P MRS studies also benefited from increased spectral dispersion; (3) the quality of MR head images remained comparable; and (4) body images showed some decrease in image quality due to increased chemical shift, and flow and motion artifacts.

P-31 MR spectroscopy of normal human brain and brain tumors

Image-guided phosphorus-31 magnetic resonance (MR)-localized image-selected in vivo spectroscopy was performed on normal human brain and brain tumors. Peak area ratios, absolute molar concentrations of metabolites, and pH were determined. T1 values in normal brain were measured. The most important finding was that the metabolite concentrations detectable with MR spectroscopy in brain tumors were reduced from 20% to 70%. Phosphomonoesters, phosphodiesters, and phosphocreatine (PCr) showed the greatest decreases, while inorganic phosphate (Pi) showed the least change. The PCr-Pi ratio was significantly reduced in tumors. The pH of brain tumors (7.12 +/- 0.03) was more alkaline than that of normal brain (6.99 +/- 0.01). The authors conclude that the metabolite concentrations and pH in human brain tumors differ significantly from those in normal brain. These differences may be ultimately useful in characterizing tumors in man.

Phosphorus-31 magnetic resonance spectroscopy in humans by spectroscopic imaging: localized spectroscopy and metabolite imaging

In in vivo phosphorus magnetic resonance spectroscopy (MRS), spectroscopic imaging (SI) can be used as a flexible localization technique, producing spectra from multiple volumes in a single examination. Presented here are phosphorus SI studies of human organs in which a selective-volume SI reconstruction was used rather than the usual array-format SI reconstruction. A linear predictor technique was used to estimate the initial points of the free induction decay missing because of the delay needed for phase-encoding gradients, significantly reducing the baseline artifacts which commonly complicate interpretation of SI spectra. In studies of heart, brain, liver, and kidney, the performance of SI was found to compare favorably with that of ISIS. SI phosphorus metabolite intensity images from a brain tumor patient were obtained at 2 X 2-cm in-plane resolution (with "slice" thickness of roughly 16 cm, determined by coil sensitivity) in 34 min, demonstrating the feasibility of obtaining clinically useful metabolite images in clinically reasonable examination times.

Clinical MRS studies of the brain

Image-guided 31P and 1H magnetic resonance localized spectroscopy was performed on patients with brain tumors, temporal lobe epilepsy, chronic brain stroke, and deep white matter lesions. Absolute molar concentrations of metabolites, peak area ratios, and pH were obtained. The important findings were that 31P metabolite concentrations were significantly reduced in tumors, infarcts, and deep white matter lesions. Similarly, 1H metabolite intensities were reduced in chronic stroke. In the seizure foci of epilepsy patients, in tumors, and in chronic stroke, the pH was more alkaline than the normal pH. Peak area ratios were altered in tumors (reduction of phosphocreatine/inorganic phosphate (PCr/Pi) and in chronic stroke (large increases in Cr/NAA and Cho/NAA). Finally, the spectroscopic imaging technique offers a versatile alternative to the "single point" techniques, producing spectra or images of the spatial distribution of individual 31P metabolites.