Safety testing of pharmaceuticals
In safety testing, regulatory requirements and scientific considerations may almost mandate the use of NHPs if NHPs represent the non-rodent species resembling humans most closely regarding pharmacodynamics and pharmacokinetics. It needs to be noted that testing of new pharmaceuticals in NHPs represents only a very small part of the total safety and efficacy testing. Results obtained in NHPs are introduced into the risk assessment process, which integrates all information from safety testing based on a weight of evidence approach. The total replacement of animals, including NHPs in testing for safety, is not possible based on present knowledge. Arguments against phasing out NHPs in safety testing of pharmaceuticals are therefore identical to those regarding using rodents for toxicity testing, i.e. incomplete knowledge of integrated body systems and pathophysiology, poor representation of pharmacokinetics by in vitro systems, and the absence of NOAEL or benchmark doses vital for human risk assessment (SCHER, 2005).
Regarding safety testing of the highly specific monoclonal antibodies and the other biotechnology derived products, NHPs are often the only relevant model for humans. In certain cases, genetically modified rodents, carrying the human pharmacological target, may replace NHPs. This requires, however, that downstream signalling is relevant for humans and that the alternatives are sufficiently well characterised. At present, genetically modified rodents as well as testing of the homologous protein in rodent species are usually considered as supportive data and not as replacements for the use of NHPs by regulators (Anonymous, 2008).
Micro-dosing is sometimes postulated to be able to replace some animal testing. Microdose studies in humans are considered to be clinical trials in accordance with the EU Clinical Trials Directive and, therefore, have to be supported by animal toxicity studies Therefore, micro-dosing cannot replace animal testing, and administration of chemicals or pharmaceuticals to humans in low doses to study pharmacokinetics and toxicokinetics (biokinetics) (Amberg et al., 1999; Monster et al., 1976) has been used for a long time in research. Recent developments in analytical chemistry such as LC/MS-MS or accelerator mass spectrometry have only refined microdose studies due to more simple sample workup and higher sensitivity. Micro-dosing in early human studies only investigates pharmacokinetics and is performed after administration of very low single doses (max. of 100-fold below the pharmacologically active dose in animals). As a prerequisite for performing microdose experiments in humans, single dose toxicity data in an appropriate animal model are needed to ensure that the microdose given to humans can be considered a safe dose. Thus, toxic effects are not expected in humans and a toxicity profile cannot be established. Toxicity in animals is the relevant endpoint in all safety testing and this can thus not be studied with micro-dosing. However, compounds with an unfavourable human pharmacokinetic profile are not further developed and in that sense, the use of animals in toxicity testing may be reduced due to earlier termination of an unpromising compound. On the other hand, if a compound shows a favourable human pharmacokinetic profile in micro-dosing, all standard animal safety tests are needed for further clinical development, so that micro-dosing in humans can also result in an increase in the number of animals used for a specific compound (single dose toxicity study plus standard tests) (EMEA, 1994).
The US National Academy of Sciences has recently issued a report on “Toxicity testing in the 21st century”. The report discusses a “vision” to reduce the need for animal testing based on a combination of in vitro testing, “omics”-technologies applied to in vitro systems, and physiologically-based pharmacokinetic modelling within the next decades. Animal testing should only be used when unclear results are obtained or specific concerns are present. However, it needs to be noted that the mandate of the NAS committee was restricted to environmental chemicals where daily human doses are much lower then those used in therapy with pharmaceuticals. Therefore, the conclusions of this report cannot be applied to pharmaceutical safety testing at the present time.