Optimizing drug discovery by Investigative Toxicology: Current and future trends

Investigative Toxicology describes the de-risking and mechanistic elucidation of toxicities, supporting early safety decisions in the pharmaceutical industry. Recently, Investigative Toxicology has contributed to a shift in pharmaceutical toxicology, from a descriptive to an evidence-based, mechanistic discipline. This was triggered by high costs and low throughput of Good Laboratory Practice in vivo studies, and increasing demands for adhering to the 3R (Replacement, Reduction and Refinement) principles of animal welfare. Outside the boundaries of regulatory toxicology, Investigative Toxicology has the flexibility to embrace new technologies, enhancing translational steps from in silico, in vitro to in vivo mechanistic understanding to eventually predict human response. One major goal of Investigative Toxicology is improving preclinical decisions, which coincides with the concept of animal-free safety testing. Currently, compounds under preclinical development are being discarded due to the use of inappropriate animal models. Progress in Investigative Toxicology could lead to humanized in vitro test systems and the development of medicines less reliant on animal tests. To advance this field a group of 14 European-based leaders from the pharmaceutical industry founded the Investigative Toxicology Leaders Forum (ITLF), an open, non-exclusive and pre-competitive group that shares knowledge and experience. The ITLF collaborated with the Centre for Alternatives to Animal Testing Europe (CAAT-Europe) to organize an "Investigative Toxicology Think-Tank", which aimed to enhance the interaction with experts from academia and regulatory bodies in the field. Summarizing the topics and discussion of the workshop, this article highlights Investigative Toxicology's position by identifying key challenges and perspectives.

Validation of a photobeam system for assessment of motor activity in rats

To evaluate the validity and sensitivity of test procedures for routine assessment of chemically induced changes in motor activity in rats, studies were carried out with a commercially available photocell cage system. Testing of single, untreated rats showed that motor activity decreased within 10 min to a steady level. Overall activity was the same when testing was repeated at weekly intervals for 6 weeks. In groups of each 10 rats tested immediately after i.p. injection, chlorpromazine HCl (6 or 2 mg/kg) and physostigmine salicylate (0.5 mg/kg) decreased activity. d-Amphetamine sulfate (1.0 mg/kg) caused an increase of virtually all parameters, whereas scopolamine HCl (1.0 mg/kg) and physostigmine salicylate (0.02 mg/kg) led to an increase of particular aspects of motor activity only. Data generated with a mechanical calibrator varied 1-5% between the cages tested. It is concluded that the above procedures of recording of selected parameters of motor activity in groups of 10 rats for periods of up to 20 min would comply with the guidelines recommended by EPA TOSCA.

Validation of a radial maze test for assessing learning and memory in rats

Choice behavior of rats in a radially symmetrical 6-arm maze, without food reward, was validated for assessing changes in learning and memory following treatment with 4 psychoactive agents. The test is designed for future use in routine toxicity studies with laboratory rodents. Each radial main arm of the maze leads to a T-shaped choice-point with a blind alley on the left and a long angled alley on the right. Order of choice of the radial main arms served to score within-session working memory, by evaluating relative recency of arm reentries. The choice between blind alley and long-angled alley at the T-intersections provided a measure of between-session reference memory. Maze behavior as an indicator for impairment in learning and memory was validated by testing rats treated with d-amphetamine, chlorpromazine, scopolamine and physostigmine. Based on the above evaluations, working memory was found to be severely impaired by 0.3 and 1.0 mg/kg scopolamine, and reference memory to be improved by 0.02 mg/kg physostigmine and 1.0 mg/kg amphetamine. Locomotor activity, in terms of the total number of arm choices per test session, was altered by all substances as expected from previous reports in the literature. The test appears to be a valid and sensitive method for assessing learning and memory in the rat without the use of food reward, and thus well suited for implementation in routine toxicity studies with rodents.

The pattern of pyridoxine-induced lesion: difference between the high and the low toxic level

A fundamental prerequisite for the design of neurotoxicological safety studies is the determination of the target areas in the nervous system which are specific for particular kinds of toxicants. The target areas should be identified in special "lesion-pattern-finding" experiments. The pyridoxine-induced lesion is a prototype of the pattern characteristic of a peripheral, primary sensory neuropathy. Depending on the conditions of treatment such as the dosage level and the timing of examination, the structural changes may be more obvious either in the proximal or the distal portion of the neuron, while the functional signs may vary from severe sensory paralysis to quite unremarkable behavior.

Automated recording of rat activity in a 6-arm radial maze for routine evaluation of behaviour in subchronic toxicity studies

Using the 6-arm radial maze constructed by Bättig et al. and FitzGerald at el., the spatial locomotor activity of rats can be characterized during five-minute-sessions. Due to the complexity of the maze the rats keep patrolling the gangways without being rewarded for it. The optimal behaviour is reached by visiting all six arms successively until a visit is repeated. Hippocampal lesioned rats, with the according behavioural disturbances, show more reenterings than the control animals. Each arm of the maze is provided with a short blind alley and a long main gangway. After several trials, blind alley entries reach a low and stable level: the rat enters the main arm rather than the blind alley. The decision of entering the main alley depends on the "reference memory", of entering the alleys in the proper sequence, depends on the "working memory". Thus, correlates of long- and short-term memory can be studied. Rats learn to achieve the proper maze behaviour during five successive daily sessions each of a five minute term. By then they have reached a stable behaviour. The monitoring by photobeams of the rat-behaviour is reconstructed by a microprocessor and automatically evaluated.

Reduction of nicotine-induced hyperactivity by pCPA

Spontaneous locomotion of female Wistar rats was measured in six to ten minute sessions in an automated tunnel maze consisting of a central arena and six radially symmetrical angled arms. Nicotine (0.2 mg/kg subcutaneous, 20-30 minutes pretest) increased total arm visit frequency, but intrasession habituation and number of repetitive arm visits in the first six choices were not affected. pCPA (300 mg/kg IP three days pretest) reduced arm-visit frequency in nicotine-, but not in saline-treated rats; it had no effect on intrasession habituation or number of repetitions in either treatment group. 5-HTP (50 mg/kg IP 90 minutes pretest) reduced arm entry frequency in saline-, nicotine-, and pCPA-treated groups. Possible reasons for this discrepancy are discussed.

The corticomedial amygdala and learning in an agonistic situation in the rat

Social agonistic behaviour of intact male rats is strongly reduced by the experience of defeat by a dominant male conspecific. Small electrolytic lesions in the corticomedial amygdala strongly affected this behavioural change due to defeat. No effects of the lesions were observed before and during the defeat. Some learning is still possible in corticomedial amygdala lesioned animals. A comparison of the effects of lesions made before the defeat with lesions made after the defeat revealed that the lesions primarily produce a retention deficit in social learning.

Neophobia and conditioned taste aversion deficits in the rat produced by undercutting temporal cortex

Adult male hooded rats with parasagittal knife-cuts between the amygdala and temporal cortex (n = 8), with electrolytic basolateral amygdala lesions (n = 8), and sham-operated controls (n = 8), were tested for neophobia and LiCl-induced aversion to a 0.1% saccharin solution in a one-bottle forced choice paradigm. Both types of lesion produced equal deficits in neophobia and conditioned aversion. It was concluded that severing the connections between the amygdala and the temporal cortex produces the same deficits as basolateral amygdala damage. Possible anatomical substrates for these effects are discussed.