Replacing non-human primates
The majority of non-human primates in scientific procedures are used in three areas: drug development, safety testing and neurological research. In many cases researchers would prefer to study human volunteers or other human-based methods in place of monkey experiments, if this could be done safely. Here we discuss some of the advanced techniques increasingly able to replace primates in research and testing.
Drug development
About 70% of primate experiments are in regulatory toxicology, particularly in pharmaceutical testing for safety, efficacy, and absorption, distribution, metabolism and excretion (ADME) characteristics. For ADME studies in primates, there is increasingly an alternative.
Human microdosing studies utilise ultra-sensitive analytical methods to study the ADME characteristics of sub-therapeutic and sub-toxic doses of experimental drugs, in the ultimate target species. Positron emission tomography (PET) and accelerator mass spectrometry (AMS) are sensitive enough to analyse drugs and their metabolites at the picomole (10 -12 M) and even attomole (10 -18 M) levels [1].
Drug candidates that would fail later because of inappropriate ADME characteristics in humans can be identified early, and standard studies with primates and dogs avoided. The microdosing concept has been endorsed by the European Medicines Evaluation Agency and by the US Food and Drug Administration. Most of the world’s top pharmaceutical companies have started to use AMS in microdosing.
The advantages are tremendous: although costly, microdose studies are conducted on humans, not other primates, so there are no species-specificity questions; they are safe, ethical and can be done at a very early stage of drug development. A trial to assess whether human microdose studies reliably predict the ADME properties of drugs at therapeutic doses found a 70% concordance rate even with drugs selected for their difficult pharmacokinetics [2].
Vaccine testing
Primates are used routinely to test for reversion to neurovirulence of batches of polio vaccine. The vaccine is injected into the spinal cord of groups of macaques, causing severe suffering, including paralysis.
MAPREC (Mutant Analysis by Polymerase chain reaction and Restriction Enzyme Cleavage) is a non-animal molecular method of assessing the production consistency of polio vaccine. It detects mutations which could lead to the vaccine virus regaining virulence. MAPREC can be used for all three strains of the polio vaccine and has been validated and accepted by the World Health Organisation (WHO) [3]. However, the WHO currently only uses MAPREC as a screening process rather than a complete replacement of primate tests. Vaccine batches that pass MAPREC are still put through the primate neurovirulence test, while any batches that fail MAPREC are discarded.
The primate neurovirulence assay is not 100% effective as it has failed to detect deliberately-induced test mutations in the polio vaccine [4-5]. MAPREC is considered more sensitive than the primate test [4]. The way is therefore open for the implementation of MAPREC as a full replacement for the primate tests. So, it’s doubly unfortunate that the transgenic mouse assay for neurovirulence, which uses more animals and is arguably more severe than even the primate test, has been accepted so rapidly in comparison with MAPREC.
Neurological research
Primates are used to ‘model’ neurological disorders such as Parkinson’s disease, stroke, epilepsy and multiple sclerosis, as well as in fundamental studies of brain function.
Often these studies involve invasive procedures that could not be performed in human volunteers, but new techniques in neurological research mean that invasive experiments are no longer the only way to achieve results. Neuroimaging and related approaches yield data on human brain structure and function in a variety of novel, non-invasive ways.
Magnetic resonance imaging of human patients has already yielded unique information on the nature of the CNS lesions present in multiple sclerosis, surpassing the information from animal studies [6]. Transcranial magnetic stimulation (TMS) allows researchers to study temporary, reversible ‘brain lesions’ safely in human volunteers. TMS, whose development was supported with a Dr Hadwen Trust grant [7], is now used widely in place of studies involving permanent brain lesions in monkeys.
Magnetoencephalography (MEG) generates functional maps of the cortex, with high spatial and temporal resolutions. Dr Hadwen Trust-funded researchers at Aston University developed MEG for the study of human brain function, and the neuroimaging centre that grew out of this early research is now a world-class research facility [8]. Multi-modal imaging, combining MEG with functional MRI for example, promises to advance even further the scope of human imaging studies.
Replacement potential
The above examples are just three of a range of non-animal techniques which can be used in place of tests on non-human primates. The field of replacements is burgeoning and is seen widely, including by the government, as representing “advanced” science. Prioritising the search for replacements and applying them wherever possible will lead to positive changes for both human and non-human primates.
The Medical Research Council has stated that they will be conducting a review of primate experiments over the last decade to establish their effectiveness and impact [9]. The Dr Hadwen Trust considers it essential that this research is subjected to independently conducted and published systematic reviews. Further, the British government’s strategy on primate research, at present being developed, should focus strongly on replacing experiments on primates.
References
1. Combes RD, Berridge T, Connelly J et al (2003). Early microdose drug studies in human volunteers can minimise animal testing: Proceedings of a workshop organised by Volunteers in Research and Testing. Eur J Pharm Sci 19:1-11.
2. Lappin G, Kuhnz W, Jochemsen R et al (2006). Use of microdosing to predict pharmacokinetics at the therapeutic dose: experience with 5 drugs. Clin Pharmacol Ther 80:203-215.
3. WHO Expert Committee on Biological Standardisation (1999). WHO Technical Report Series 889, forty-eighth report:13.
4. Rezapkin GV et al (1998). Genetic stability of Sabin 1 strain of poliovirus: Implications for quality control of oral poliovirus vaccine. Virology 245:183-187.
5. Rezapkin GV et al (1999). Mutations in Sabin 2 strain of poliovirus and stability of attenuation phenotype. Virology 258:152-160.
6. Langley G et al (2000). Volunteer studies replacing animal experiments in brain research: Report and recommendations of a Volunteers in Research and Testing Workshop. ATLA 28:315-331.
7. Rushworth MF, Ellison A & Walsh V (2001). Complementary localization and lateralization of orienting and motor attention. Nature Neurosci 4:656-661.
8. Hall SD, Holliday IE, Hillebrand A et al (2005). Distinct contrast response functions in striate and extra-striate regions of visual cortex revealed with magnetoencephalography (MEG). Clin Neurophysiol 116:1716-1722.
9. MRC response to the Weatherall report on research using primates. 12 June 2007. www.mrc.ac.uk/NewsViewsAndEvents/News/MRC003778