A research to develop a predicting system of mammalian subacute toxicity. I. Prediction of subacute toxicity using the biological parameters of acute toxicities

A No-Observed-Adverse-Effect Level of 1000 Mg/Kg in A 28-Day Repeated-Dose Study as a Limit Value for Acute Toxicity Testing

Following the guidelines of the European Community (EC), chemicals are not classified according to acute toxicity if the estimated LD50 exceeds 2000 mg kg-1. Instead of an LD50 test, a limit test is often performed for relatively nontoxic chemicals. Since a short-term repeated-dose study is required in addition to an acute toxicity test, our investigation was aimed at finding out if acute toxicity testing could be omitted under certain circumstances. The latter would contribute to the reduction of (unnecessary) animal use. Data on LD50 and the no-observed-adverse-effect level values of 28-day repeated-dose studies (NO AEL 28-d) were retrieved from EC notifications for new substances. We statistically evaluated the LD50 and the NOAEL28-d values of 766 compounds (rat data only). At a NO AEL28-d of 1000 mg kg-1 there was a 4% probability of the associated LD50 being lower than 2000 mg kg-1. The probability of the LD50 being lower than 200 mg kg-1 was estimated at 0.04%. If such small probabilities of misclassification are considered acceptable, acute toxicity testing could be omitted at a NOAEL28-d of 1000 mg kg-1 or higher. A NOAEL28-d of 1000 mg kg-1 could thus function as a limit value for the acute toxicity testing requirement. Application of this limit value would have made acute toxicity testing redundant for 286 out of 766 compounds reviewed. The use of the limit value may thus form an important contribution to the reduction of animal use.

Nucleic acid microarray technology for toxicology: promise and practicalities

Much ongoing research in toxicology focuses on a hypothesis-driven mechanism of action approach aimed at understanding the molecular events mediating the actions of the chemicals of interest. Using this approach, investigators develop hypotheses based on observations, which may be derived from a host of resources but most frequently have been made within their own laboratories, or uncovered by others and reported in the scientific literature. Although the bulk of current understanding ofbiochemical toxicology emerged using studies based on observations derived in this way, this process, which is essentially based on existing information, may often limit the expansion knowledge. More simply expressed, one only finds that which one seeks. Without a clear understanding of the processes targeted by a specific toxin the problem of making observations that globally and accurately reflect the events mediating pathology which have been induced by the toxic agent is challenging. Recently, the development of high-throughput technologies for biochemical analysis of gene expression has led to innovative approaches in addressing the problem of making broad-based observations that more accurately reflect the entire spectrum of molecular lesions induced by specific toxins. These strategies include the use of new techniques in analysis of gene expression to convey information on alterations in mRNA levels, one of the earliest cellular signs initiated in response to a potential toxin. Prior to this time studies on toxicant-induced altered gene expression were limited to single, or small numbers of identified genes chosen by an investigator who reasoned, based on an existing observations, that levels of the proteins encoded by these genes were likely to be altered during toxic injury. Now, using cDNA or oligonucleotide genome-wide arrays, toxin-induced alterations in gene expression of thousands of genes can be examined simultaneously. Using these tools, molecular toxicologists can for the first time employ reasoned strategies to make observations, and then formulate hypotheses based on these observations.

Research to develop a predicting system of mammalian subacute toxicity, (3). Construction of a predictive toxicokinetics model

A new predictive toxicokinetics model was developed to estimate subacute toxicity (target organs, severity, etc.) of non-congeneric industrial chemicals, where the chemical structures and physico-chemical properties are only available. Thus, a physiological pharmacokinetics model, which consists of blood, liver, kidney (these were experimentally found as major toxicological targets), muscle and fat compartments, was established to simulate the chemical concentrations in organs/tissues with pharmacokinetic parameters by means of Runge-Kutta-Gill algorithm. The pharmacokinetic parameters, i.e. absorption rate, absorption ratio, hepatic extraction ratio of metabolism and renal clearance were calculated by using separately established Quantitative Structure-Pharmacokinetics Relationship equations. The developed predictive model was then applied to simulations of 43 non-congeneric industrial chemicals. The chemical concentrations in organs/tissues after single oral administration were simulated, and their maximum concentrations (Cmax's) and area under the concentration-time curves (AUC's) were calculated. Fast Inverse Laplace Transform was newly applied for the purpose of simulation of 28-day repeated dose toxicity. Simulated concentrations of 28 days repeated dose were, however, found to be the same as those of simple repetitions of a single administration per day because of the short half-lives of non-congeneric industrial chemicals. A comparison of subacute toxicity data with Cmax's and AUC's in a single dose scenario suggested that the organs/tissues with relatively high concentrations of tested chemical substances were the most sensitive targets within a chemical. Chemical concentrations in liver, for instance, were correlated with the severity of hepatotoxicity among the chemicals. It was also suggested that to improve and widen the present approach, data of metabolite and reactivity of non-congeneric industrial chemicals to organs/tissues, receptors, etc. should be incorporated into the model.

A research to develop a predicting system of mammalian subacute toxicity. II. Single dose detailed toxicity studies

"Single dose detailed toxicity study" in rats was conducted with 6 chemical substances to establish a predicting system of subacute toxicity of non-congeneric industrial chemicals. In the detailed examinations, new and previously reported biological parameters were developed and utilized for the establishment and confirmation of the correlations for both qualitative and quantitative prediction of target organs and effects in repeated dose toxicity studies of non-congeners; the new parameters were concerned with liver, kidneys and blood and the established relating equations for prediction had a correlation coefficient of 0.83 (liver), 0.49 (kidneys) and 0.94 (blood), while the old parameters effectively confirmed the previous correlations. The serial changes in the study gave a biological explanation to the established relationships between acute and subacute toxicities in terms of extrapolation.