Preclinical toxicity testing

Such sentinel workflow uses a prediction to select compounds for a more expensive screen that can confirm predicted hazards (liabilities, such as toxicity). It is, provably, the best workflow in contexts where a low prevalence of the hazard is anticipated, and where there is a backstop means further downstream (e.g., preclinical toxicity testing) for detecting hazards before humans are exposed. This workflow then allows the compounds predicted as safe to bypass the expensive hazards screen, without unacceptable risk, and can add significant value in terms of external screening costs or avoiding use of what may be a bottleneck resource. 

Gad, S.C. (1996b). Preclinical toxicity testing in the development of new therapeutic agents. Scand. J. Lab. Anim. Sci. 

Drug discovery requires a host of inputs, new ideas, design and synthesis of substances, evaluation of preclinical toxicity tests in animals, clinical studies in human volunteers, permission to market the drug, postmarketing studies of safety, and comparison with other medicines. Drug development is highly technical and enormously expensive, with a success rate of 1 in 10,000 compounds. 

Animal tissues are collected at necropsy of humanely euthanized, purpose-bred laboratory animals immediately following death. Some nonhuman primates used for tissue acquisition may be wild caught. The animals used are preferably matched as closely as possible to that of the strain or origin of the animals to be used for preclinical toxicity testing. 

By the time Phase III testing is completed, some additional preclinical safety tests must also generally be in hand. These include the three separate reproductive and developmental toxicity studies (Segments I and III in the rat, and Segment II in the rat and rabbit) and carcinogenicity studies in both rats and mice (unless the period of therapeutic usage is intended to be very short). Some assessment of genetic toxicity will also be expected. 

Some biotechnologically derived pharmaceuticals will cross-react with species that can be evaluated toxicologically, while others cross-react only with nonhuman primates such as the chimpanzee, a protected species. In this case, a well-designed safety, or Phase 0 study at doses higher than the proposed clinical dose may provide valuable safety information. However, a lack of cross-reactivity with any nonhuman species does not necessarily make preclinical safety evaluation impossible, not does it limit toxicity testing to species in which the protein lacks relevant pharmacological activity. Some alternative possibilities are summarized in Table 12.9. 

Host-resistance assays can be used to assess the overall immunocompetence of the humoral or cell-mediated immune systems of the test animal (host) to fend off infection with pathogenic microbes, or to resist tumorigenesis and metastasis. These assays are performed entirely in vivo and are dependent on all of the various components of the immune system to be functioning properly. Thus, these assays may be considered to be more biologically relevant than in vitro tests that only assess the function of cells from one source and of one type. Since these assays require that the animal be inoculated with a pathogen or exogenous tumor cell, they cannot be performed as part of a general preclinical toxicity assessment, and are thus classified as Type 2 tests in the revised Redbook. These assays are also included as Tier II tests by the NTP. 

It is therefore essential that before pivotal (repeat dose) preclinical studies are initiated, bioanalytical assay development must be completed. This has to cover potential test species, normal and diseased humans. The assays must be validated in the sampling matrix of the toxicity test species, and one should also develop suitable assays for antibodies to the test article. 

Differences in preclinical requirements for developmental toxicity testing between vaccines and medicinal agents... 

The goals of preclinical toxicity studies include identifying potential human toxicities, designing tests to further define the toxic mechanisms, and predicting the specific and the most relevant toxicities to be monitored in clinical trials. In addition to the studies shown in Table 5-1, several quantitative estimates are desirable. These include the no-effect dose—the maximum dose at which a specified toxic effect is not seen the minimum lethal dose—the smallest dose that is observed to kill any experimental animal and, if necessary, the median lethal dose (LD50)—the dose that kills... 

For a drug that has never been used in humans previously, the initial step that a pharmaceutical company must take is to perform preclinical toxicity studies involving appropriate in vitro systems or whole animals. The FDA usually requires that dose-related toxicity be determined in at least two mammalian species (routinely rodents). The toxicity information obtained from these studies can then be used to make risk/benefit assessments and help determine the acceptability of the drug for testing in humans, and to estimate a safe starting dose. 

---