Ethics of Research involving animals
Principal types of animal-based toxicity tests - continuation II
9.24 In the development of pharmaceutical products, additional tests are needed to detect exaggerated intended or unintended pharmacological responses. Pharmacology studies to evaluate safety are generally conducted in the dog (for cardiovascular endpoints) and in the rodent (for assessment of the effect on the whole body). Some examples of the types of studies performed in animals are described briefly below (see Box 9.4).
- Dog telemetry: dogs are implanted with radio-transmitters for continuous monitoring of blood pressure, heart rate, body temperature and electrocardiogram (ECG, see paragraph 4.56). These parameters can be monitored from the conscious dog, and allow the measurement of the effects of test compounds on the cardiovascular system in vivo. Dogs are usually reused in multiple studies, subject to veterinary and regulatory approval by the Home Office. They are euthanised at the end of the studies.
- Haemodynamics of anaesthetised dogs: this type of research is undertaken as a followup to dog telemetry. Under terminal anaesthesia, multiple systems may be investigated including, for example, blood pressure, heart rate, ECG and peripheral blood flows (also coronary and renal blood flows).
- Absorption, distribution, metabolism and excretion studies (ADME): although not strictly toxicity studies, these investigations (typically undertaken in rodents and dogs) are used to assess the amount of chemical or pharmaceutical that is absorbed into the animal, where it is distributed within the body, how it is changed by metabolism, the time-course for these events and how, and at what rate, the material is eliminated from the body (see paragraph 9.31). This information is used to select dose levels for toxicity studies and clinical trials, to identify compounds for further development, to interpret toxicity data, and in risk assessment (see paragraphs 8.10–8.11).
- ‘Balance’ studies: in these studies radiolabelled doses are given to intact or surgically prepared animals and samples including blood, bile, urine, faeces and expired air are collected to determine the processing of the drug-related material, and to investigate its absorption and possible retention.
- Pharmacokinetic studies: studies are conducted for pharmacological and toxicological evaluation of candidate drugs to characterise their pharmacokinetic behaviour, usually after intravenous and oral administration, although other routes may also be used (see paragraphs 8.20–8.26). This information is used to support the more limited sampling performed in toxicity studies, to fully characterise the pharmacokinetics in animals and to predict the pharmacokinetics in humans, which assists in estimating the likely human dose.
| Box 9.4: Example of research – testing species differences in the toxicity profile of an approved herbicide (currently in use) Lappin GJ, Hardwick TD, Stow R et al. (2002) Absorption, metabolism and excretion of 4-chloro-2- methylphenoxyacetic acid (MCPA) in rat and dog Xenobiotica 32(2): 153–63.* This research investigated differences between rats and dogs in the toxicity of a herbicide, MCPA. This chemical is used to control a wide variety of broad-leaved weeds in many crops as well as non-crop areas. A radioactive version of the herbicide was fed to the rats and dogs. Twenty rats between six to eight weeks old and four beagle dogs between six and 12 months of age were used, obtained from suppliers of laboratory animals in the UK. Two groups of rats were administered single doses of the herbicide at different levels by gavage (feeding by means of a stomach tube, see paragraph 9.28). Half the rats were group-housed in the period following dosing and a sample of their blood was taken on ten occasions. The remaining rats were housed individually, and their urine and faeces were collected for seven days. For all four dogs a single dose was administered by capsule, followed by a second single dose at a higher concentration four weeks later. All four dogs were housed individually for five days following dosing, during which time their blood was sampled at 11 time points, and samples of urine and faeces were collected. Signs of toxicological response to this compound had previously been shown to include reduced weight gain, increased kidney weight and altered clinical chemistry in the rat. The effects in the dog were more severe with clear hepatotoxicity (having a damaging effect on the liver), anaemia and severe renal toxicity. The highest dose given in this procedure resulted in mild toxicological effects in the rats. The responses in dogs were described as being beyond the MTD if repeated exposures at this level had occurred. The researchers found that MCPA did not accumulate in rat tissue. The results were less clear in the case of the dog as this species is more sensitive to the effects of MCPA. The authors reached the most probable physiological explanation for the species differences. They also investigated previous evidence that this type of compound may reach higher blood concentrations in males than females, and found that there were in fact no differences. For this reason the researchers went on to use only male dogs, rather than increase the number of dogs used. The authors state that the data add to a growing body of evidence showing that the dog is deficient in the excretion of weak organic acids, and that therefore this species is not appropriate for assessing the toxicological significance of this class of compound in humans. * This is an example of animal research that has been carried out in the UK and published in a peer-reviewed journal. Details relate to this specific example and should not be taken to represent a ‘typical’ animal experiment. It is important to note that individually published experiments usually form one part of a continuing area of research, and the significance of the results may therefore be difficult to interpret. |
Ecotoxicity
9.25 All of the tests described above are carried out to assess the possible adverse effects of a substance on human health, but an increasing amount of testing is being done to investigate potential effects on the environment and wildlife. For example, large numbers of fish, and smaller numbers of birds and amphibians, are used to test industrial and agrochemicals for their toxicity to wildlife populations (see also Box 9.4).