Mice Are Not Small Rats


One theme of this blog is that both intra- and inter-species differences make prediction of drug and disease response problematic. Genes differ depending on their backgrounds and modifier genes, receptors differ, and outcomes differ. The following is from Setola and Roth 2003:

The now-classic approach for validating 5-HT6 receptors as molecular targets for therapeutics is to construct a 5-HT6 knockout mouse and to characterize its phenotype. As Hirst et al. (Hirst et al. 2003) discovered, however, it is unlikely that 5-HT6 knockout mice will be useful for validating the 5-HT6 receptor as a therapeutic target because of pronounced and unexpected species differences in both receptor regional distribution and pharmacology. It is now widely appreciated that slight differences in rodent and human amino acid sequences can lead to unexpectedly large differences in the pharmacology of the receptors, with potentially disastrous effects for drug-discovery efforts. What has not been clearly documented until the Hirst et al. study (2003), however, is that mouse receptors could be significantly different from rat receptors.

In the article published in this issue of Molecular Pharmacology, Hirst et al. (2003) elegantly demonstrate that the mouse 5-HT6 receptor is, in nearly every respect, distinct from rat and human 5-HT6 receptors . . . Thus, quantitative polymerase chain reaction studies demonstrated that the mouse 5-HT6 receptor mRNA was at least 10-fold less abundant than the rat or human 5-HT6 receptor mRNAs in every brain region examined. Surprisingly, whereas 5-HT6 receptor mRNA and radioligand binding activity was enriched in the basal ganglia of rat and human brain, there was no such enrichment in the mouse brain.

Additionally, via a combination of site-directed mutagenesis and molecular modeling studies, Hirst et al. (2003) describe the presumed molecular and atomic reasons for the peculiar mouse 5-HT6 pharmacology. Two amino acids—Tyr188 (in helix 5, which is Phe188 in rats and humans) and Ser290 (in helix 6 which is Asn290 in rats and humans)— were found to account for the bulk of the differences in pharmacology. A nice feature of the study is the parallel inclusion of elegant modeling studies of the various ligands used. Hirst et al. (2003) use this model to present a plausible molecular rationale for the differential interactions of various 5-HT6 receptor-selective ligands with human, rat, and mouse 5-HT6 receptors.

The findings described by Hirst et al. (2003) have important implications for drug discovery. Because the mouse 5-HT6 receptor is distinct in nearly every way from the human (and rat) 5-HT6 receptor, the results force us to question the use of knockout mice in a wholesale fashion to provide validated molecular targets for drug discovery. Their studies strongly imply that before a knockout mouse is accepted as a validated model for a particular human disease, the molecular target needs to be demonstrated to have a pharmacology, regional tissue distribution, and abundance similar to the human homolog. Therefore, this study stands as an important reminder to us all that mice are not miniature humans and, sometimes, not even small rats. (Setola and Roth 2003)

Studies like the above explain why what a drug does in a mouse is not predictive for what it will do in a rat much less a human.

After drugs pass animal testing (and other tests) they go on to be tested on humans but this is no assurance of safety or efficacy. Drugs are tested on a population of humans but then work on only a minority of patients. (Gabler et al. 2009) This is why personalized medicine and pharmacogenomics are so promising and need more funding. Breckenridge et al.:

The current model of drug research and development (R&D) is under intense scrutiny, as the increased investment in recent decades has not led to a corresponding increase in new marketed products. The underlying challenges are diverse, but innovations in clinical development and in the application of regulatory science have been identified as key factors in improving R&D productivity by the Food and Drug Administration (FDA) Critical Path Initiative in the United States and the Innovative Medicines Initiative in Europe. (Breckenridge et al. 2011)

Mattes et al. 2010 sum up the situation:

A medicine is defined as 'a substance or preparation used in treating disease'. Society expects that the benefits of medicines should substantially exceed their risks, and this expectation has been translated into governmental policy around the world. Part of the mission of the US Food and Drug Administration (FDA) is to protect the public health by assuring the safety and efficacy of medicines. The FDA has carried out its mission by relying upon the best current scientific knowledge and practice. By definition, gaps in current scientific knowledge and practice limit the ability of regulatory agencies, such as the FDA and the European Medicines Agency (EMEA; London), to carry out their mission. Current gaps include a limited ability to extrapolate animal data to humans (Collins 2001) (Peters 2005) (Voisin et al. 1990) the difficulty of evaluating genetic and carcinogenic risks and our poor understanding of gender-specific responses. It is hoped that new knowledge, technologies and tools can address these and other gaps and improve the evaluation of new drugs and medicines. (Mattes et al. 2010)

If you understand why personalized medicine and pharmacogenomics are needed then you should also understand why animals are not predictive for human response to drugs and disease.


Breckenridge, Alasdair, Peter Feldschreiber, Simon Gregor, June Raine, and Leigh-Ann Mulcahy. 2011. Evolution of regulatory frameworks. Nat Rev Drug Discov 10 (1):3-4.

Collins, J. M. 2001. Inter-species differences in drug properties. Chem Biol Interact 134 (3):237-42.

Gabler, N. B., N. Duan, D. Liao, J. G. Elmore, T. G. Ganiats, and R. L. Kravitz. 2009. Dealing with heterogeneity of treatment effects: is the literature up to the challenge? Trials 10:43.

Hirst, W. D., B. Abrahamsen, F. E. Blaney, A. R. Calver, L. Aloj, G. W. Price, and A. D. Medhurst. 2003. Differences in the central nervous system distribution and pharmacology of the mouse 5-hydroxytryptamine-6 receptor compared with rat and human receptors investigated by radioligand binding, site-directed mutagenesis, and molecular modeling. Mol Pharmacol 64 (6):1295-308.

Mattes, William B., Elizabeth Gribble Walker, Eric Abadie, Frank D. Sistare, Jacky Vonderscher, Janet Woodcock, and Raymond L. Woosley. 2010. Research at the interface of industry, academia and regulatory science. Nat Biotech 28 (5):432-433.

Peters, T. S. 2005. Do preclinical testing strategies help predict human hepatotoxic potentials? Toxicol Pathol 33 (1):146-54.

Setola, Vincent, and Bryan L. Roth. 2003. Why Mice Are Neither Miniature Humans nor Small Rats: A Cautionary Tale Involving 5-Hydroxytryptamine-6 Serotonin Receptor Species Variants. Molecular Pharmacology 64 (6):1277-1278.

Voisin, E. M., M. Ruthsatz, J. M. Collins, and P. C. Hoyle. 1990. Extrapolation of animal toxicity to humans: interspecies comparisons in drug development. Regul Toxicol Pharmacol 12 (2):107-16.


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