Today, I thought I’d share another one of my essays I had to do recently. This one looks at animal testing, problems concerning species differences and what we can do to avoid them. This essay is a little more sciency than my other one on living forever, so I’ll include the references this time. Here goes:

The use of non-human animals in the drug development process can attract criticism due to the issue of species differences. How significant is this problem and what strategies can be employed to minimise the impact of species differences?

Animal testing is a major tool in the drug development process, required by law before any new drug can enter the market. Animal models are set up to not only test the efficacy of a compound for its intended effect, but also to observe any potential side effects, to calculate a safe dosage for humans and to check for any addiction potential. Although animal testing is a legal requirement, implemented for our own safety, it is still only a model; a substitute for human physiology, whose results could be completely erroneous if they were derived from a poorly planned experiment. Differences between species are always a concern when setting up an appropriate animal model, and a lot of time is spent agonising over them to ensure any results obtained are both accurate and applicable to humans. When it comes to experimental design, species differences can be broadly classified into the following categories: anatomical/physiological differences, differences in metabolism and subsequent toxicity, pharmacological differences and behaviour.

Anatomical/physiological Differences

This is perhaps the most obvious class of species difference. It is no good testing a drug on an animal and looking for effects that are physically impossible for the animal to manifest. Any tests carried out on one species with implications for another must only test parts of the physiology common to both species, or identify an analogous symptom that corresponds to the effect you are looking for.

A prime example of this kind of difference crops up when investigating the emetogenic potential of a drug – unfortunately, evolution has not provided rats with a vomiting reflex, so an different model would have to be devised looking for an alternative behaviour or using another species with a physiology closer to ours.

Metabolism & Toxicity Differences

Different species also metabolise drugs differently – either via different metabolic pathways or with different kinetics. As such, a drug toxic to one species may have little effect on another, which is particularly important when trying to determine the toxicity in humans. A drug’s LD50, the amount required to kill 50% of subjects in a particular sample, is usually given in mg/kg of body mass, scaled up from animal experiments. If a drug’s toxicity or pharmokinetics are only determined from one animal species and extrapolated for the average human, the data would not take into account any differences in metabolism that may be present, resulting in potentially extreme inaccuracies.

For example, dogs should never be given coffee or chocolate, as they are poor metabolisers of theobromine¹, a xanthine alkaloid occurring naturally in both, as well as being a metabolite of caffeine. As little as 50g of chocolate can result in theobromine poisoning for small dogs, while humans can metabolise it fast enough without issue.
Similarly, metabolism of NSAIDs shows a huge variation across different species. The plasma half-life of aspirin ranges from 1 hour in ponies up to 37 hours in cats², due to their poor glucuronidation ability, while dogs are more susceptible to aspirin’s gastrointestinal side effects³.

One final example would be the varying MPTP toxicity between species. MPTP can be formed as an unintended byproduct in the manufacture of MPPP, a synthetic opioid with great potential for abuse. MPTP on its own is not harmful, but MPP+, the natural metabolite of MPTP, is a potent neurotoxin. MPP+ is produced via MonoAmine Oxidase B in neuroglia and the capillary endothelia comprising the blood-brain barrier, and results in rapid-onset Parkinsonian symptoms barely indistinguishable from typical Parkinson’s disease⁴. These symptoms are also reduced by L-DOPA, a drug commonly used in Parkinson’s disease. Rats, however, are almost entirely immune to MPTP toxicity, most likely due to a different level of expression of MAO B⁵. Mice, on the other hand, do produce MPP+, but clear it from their brain in a matter of hours, unlike the primate brain, in which clearance can take days.

Pharmacological Differences

The chemical pathways and their associated protein machinery will not necessarily be structurally identical, or indeed act in the same way. Pathways may be more or less complex, depending on the species, with more or less scope for modulation by other factors. Receptors too may also differ in structure, ligand affinity and the type of G proteins they may couple with. All of these factors may be of huge importance when designing a drug with a particular molecular target in mind.

A few interesting cases have resulted from these types of differences. For a while, Leptin was theorised to suppress hunger, as knockout mice that did not express leptin or its associated receptor got fat. Giving leptin to those that could not express it themselves, but still possessed the appropriate receptor, caused them to lose weight⁶ – a potential gold mine if the results were also applicable to humans. Unfortunately, they were not. Leptin showed little effect in humans, as weight problems tended to concern signal transduction rather than a lack of leptin⁷, in much the same way as insulin-resistant diabetes.

Another, rather more serious example is that of TGN1412, a monoclonal antibody with not only a high affinity for the human CD28 receptor, but a strong agonist ability too. Originally intended to help patients with rheumatoid arthritis and B cell chronic lymphocytic leukaemia, TGN1412 was initially tested on animals and an apparently safe dosage calculated. Of the 6 volunteers hospitalised, each given a dose 500 times smaller than that given to their animal counterparts, 4 developed multiple organ failure as a result of cytokine storm⁸. Hopefully, this example highlights the importance of species difference; that it is a real issue and not just a theoretical concern.

Behavioural Differences

The final category, and perhaps least obvious, is that concerning animal behaviour. Unfortunately for us, animals are not able to clearly express their feelings, so we are left to try and interpret that behaviour, which can be particularly difficult. Humans seem to have an intrinsic penchant for anthropomorphism – we are always unconsciously trying to attribute characteristics that are uniquely human, such as complex emotions or intention, onto animals and even non-living objects. Children are especially guilty of this, smacking a rock, perhaps, as a punishment because it tripped them up. It is only as we grow older and put in a little more thought that we realise that perhaps the rock was not to blame. With animal models, we must also put in that extra thought when it comes to interpreting an animal’s behaviour, instead of opting for the instinctive, humanised interpretation.

Other problems are encountered when we assume a particular behaviour is a result of a particular effect. For example, in the tail flick assay, designed to measure effects on nociception, analgesia is associated with an increased latency in moving the tail away from a heat source. Approving a new drug as an analgesic based on only this interpretation could be disastrous if the increased tail flick latency was instead due to a loss of muscle control or paralysis.

One final thought concerning animal behaviour, is that some behavioural responses may be unique to the species in question. For example, a hedgehog might curl up into a ball as a typical fear response. While this may be easy to interpret, other idiosyncratic responses may not.

Strategies

A number of strategies have been devised for combating the issues species difference brings up, ranging from simple common sense to the rather more complex. An in-depth knowledge of the species under investigation is a good start. Experience and familiarity with a particular species will naturally lead to a better ability to read an animal’s behaviour, just as we become better at reading the people around us the longer we spend in their company. Someone new to animal work will be more likely to anthropomorphise, drawing instead from their experience with other people, whereas someone with ample experience could make a more accurate judgement. Another benefit from experience is that any of the more subtle differences between that species and us is more likely to spring to mind, reducing the risk of something important being overlooked. For example, rat models are a useful tool when studying the intestinal bioavailability of drugs, but are a poor choice when it comes to intestinal metabolism⁹.

Another strategy to reduce the risks imposed by any unknown or overlooked differences, and one that is required by law, is to test on more than one species. Doing so greatly reduces the chances that any observed response is unique to one species in particular, and is therefore likely to be exhibited by humans too.

Although there are an incredible number of individual species, some proteins remain relatively conserved. Working with these specific proteins that share a great deal of similarity between their human counterparts will likely lead to more reliable results. For example, the muscarinic receptor family has remained much the same throughout evolution such that the human and rat receptors share a very similar agonist/antagonist profile¹⁰. It is very likely that something acting on rat muscarinic receptors will elicit the same response in humans, making this an accurate model.

More recently, the latest tools and techniques of the genetic engineer promise to make animal models even more relevant. Genetic manipulation has already delivered knockout animals, not expressing particular genes, and transgenic animals, expressing genes belonging to another species, but in 2008 a chimeric mouse with 90% human hepatocytes (liver cells) was produced¹¹. Until now, the best tool for studying the effects of drugs on the liver would be to use actual human liver (another strategy for overcoming species differences is to use human cells if possible), but the chimeric mouse has already shown great potential. The liver is mainly responsible for the pharmacokinetics of a drug, as it is the primary place that drugs are metabolised, which has subsequent effects on the toxicity and efficacy of that drug. The chimeric mouse has shown a similar pharmacokinetic profile to the human donor, as well as human-specific metabolites not ordinarily found in mice, making this an excellent model with which to study pharmacokinetics and toxicity. This advancement brings with it all the benefits of testing drugs on an actual human target, without any of the ethical considerations raised with human testing.

We humans are an animal species like any other, and we may have our own species-specific responses that are impossible to capture or anticipate with any animal model. It is important to remember that an animal model is just that – a model. Species differences will always be an issue; there are even idiosyncratic reactions to drugs within the same species, such as some humans being allergic to penicillin, so we can never eliminate these differences completely. Increasing research, awareness, criticisms from the animal rights campaigners and new genetic techniques will continue to help us reduce the severity of these issues until they can be reduced no further.

References

1. Kahn CM, editor. The Merck Veterinary Manual. 9th Ed. New Jersey: Merck & Co., Inc; 2008.
2. Boothe DM. The Analgesic, Antipyretic and Anti-inflammatory Drugs. In: Adams HR, editor. Veterinary Pharmacology and Therapeutics. 8th Ed. Iowa: Iwoa University Press; 2001. p. 433-454
3. Crosby JT. Veterinary Questions and Answers – Can you give a dog or cat aspirin? [cited: 2008 Sept 02] About.com: Veterinary Medicine. Available from: http://vetmedicine.about.com/cs/altvetmedgeneral/a/dogcataspirin.htm
4. Langston JW, Ballard P. Parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP): implications for treatment and the pathogenesis of Parkinson’s disease. Can J Neurol Sci. 1984 Feb;11(1 Suppl):160-165.
5. William Langston JW. The Impact of MPTP on Parkinson’s Disease Research: Past, Present, and Future. In: Factor SA, Weiner WJ, editors. Parkinson’s Disease: Diagnosis and Clinical Management, New York: Demos Medical Publishing, 2002. p. 407-436
6. Pelleymounter MA, Cullen MJ, Baker MB, et al. Effects of the obese gene product on body weight regulation in ob/ob mice. Science. 1995 Jul 28;269(5223):540-543
7. Considine RV, Sinha MK, Heiman ML, et al. Serum immunoreactive-leptin concentrations in normal-weight and obese humans. N Engl J Med. 1996 Feb 1;334(5):292-295
8. Suntharalingam G, Perry MR, Ward S, et al. Cytokine storm in a phase 1 trial of the anti-CD28 monoclonal antibody TGN1412. N Engl J Med. 2006 Sep 7;355(10):1018-1028
9. Hurst S, Loi CM, Brodfuehrer J, El-Kattan A. Impact of physiological, physicochemical and biopharmaceutical factors in absorption and metabolism mechanisms on the drug oralbioavailability of rats and humans. Expert Opin Drug Metab Toxicol. 2007 Aug;3(4):469-489
10. Venter JC, Eddy B, Hall LM, Fraser CM. Monoclonal antibodies detect the conservation of muscarinic cholinergic receptor structure from Drosophila to human brain and detect possible structural homology with alpha 1-adrenergic receptors. Proc Natl Acad Sci USA. 1984 Jan;81(1):272-276
11. Katoh M, Tateno C, Yoshizato K, Yokoi T. Chimeric mouse with humanized liver. Toxicology. 2008 Apr 3;246(1):9-17

This was an essay I wrote last year about the ethics involved with curing the ageing process. A worthy topic of discussion, I hope you’ll agree. I thought it was alright, so here it is, sans references.

“O brave new world that hath such people in’t!”

Introduction

One way to define ageing is an increased chance of dying as time progresses as a result of cumulative natural changes and degradation of the body. Therefore a cure for ageing wouldn’t simply be a cure for all of the most common diseases associated with old age, such as cancer, heart disease and so on, but rather a cure for the underlying cause of the body being more susceptible to those diseases. Even if we could cure cancer or heart disease, the disease itself may not kill you, but something else would, as the body would still have accumulated years of stress and damage making it increasingly more likely to fail. Instead, a cure for ageing itself would mean prevention (and even reversal) of the ageing process, ensuring a state of perpetual youth for those that partake.

As such, the incredibly complex ethical considerations for such a cure are echoed throughout a number of social and political issues, calling into question the rights of the current generation over future generations, the rights of the individual versus the rights of the society and the purpose of life itself.

Overpopulation

The primary concern that springs to the mind of most people when the topic of curing old age is discussed is overpopulation. Already, the population is growing exponentially, even when the majority of people are dying before they reach 100. If people are living for double that amount of time and reproduction continues at its current rate, surely we will run out of room sooner than if people were dying before 100? It follows then, that we would exhaust that same amount of habitable space even quicker should life expectancy be increased further, to say 500 or in the thousands, provided that the rate of childbirth remained the same.
This idea of cramped living conditions conjures up an image of Victorian style slums or today’s “High Density Living” solution to the same problem in Hong Kong, where the concept of your own space outside has almost disappeared. Not only does that sound uncomfortable with a diminished sense of privacy, but the more people there are in any given area, the more easily and more likely it is that infectious diseases will spread. So how can this problem be resolved? By drastically reducing the birth rate.

Controlling Birth Rate

It would appear that the only option besides killing a large proportion of the population every so often is to place a limit on the rate of child birth for society as a whole. On the surface, this suggests that the generation that decides not to have children so they can extend their own life are making an immoral selfish choice, but let us first take a look at how society handles this issue today.

In 2004, the average number of children per married couple in the UK is approximately 1.8. It is important to realise that this is not a physical limit imposed by the human body, but an amount which is convenient. With the use of birth control and abortions, we can decide when it would be appropriate for us to have a child and how many children we have overall. The point here is that by choosing when to have a child based on factors such as financial stability, we already are being selfish when it comes to reproduction. The world at present is rife with examples of people putting their career (and hence their own satisfaction and financial gain) ahead of their future children, which we do not tend to see a problem with. This hypocrisy extends even further when it comes to the stigma of underage pregnancy – if we as a society should selflessly put our potential offspring before ourselves, surely we should be reproducing as early as possible, no matter what the cost to ourselves? Apparently not. What may seem selfish to some may be perfectly acceptable to others. Unfortunately, it is never clear where the line should be drawn with most ethical dilemmas, and this is no exception.

Selfish or otherwise, there are other pressing matters relating to this kind of population control that must also be discussed. For example, who decides who should reproduce and when? Even China’s notorious “One Child Policy” is not enough to curb population growth. According the British Medical Journal in 2006, “China still has one million more births than deaths every five weeks”, so to prevent overpopulation, the average number of children per family would have to be reduced to far less than one to even keep the population growing at the same rate as China’s is now. Since it’s not possible for every family to give birth to a rather low percentage of one child, the responsibility of deciding who could reproduce and when would have to fall to someone, or some specified group of people, leaving the potential system open to all manner of imperfections. This could include bribery, blackmail, human error and any other form of corruption, which is particularly important with matters as fundamental as this. My lack of faith in humanity being able to think up and implement the perfect system for this situation is still not the most important concern, however.

Assuming that some method of control was necessary and in place, some people would simply not be permitted to reproduce for the interests of society. Not just limited to one child, but not at all. Currently, though, people who decide not to have children, or limit the number they have, retain their right to choose, no matter who may think it immoral; but if society decided the majority wanted to live forever, and the right to reproduce was something worth sacrificing, the choice would then belong to the society and not the individual. Many people see the point of life as having children, and could imagine life as worthless and hollow in hindsight should they not have had their children. The idea of potentially removing what point a lot of people saw in life from those people is one big step up from allowing people to choose when they have children themselves.

Equality & Prejudice

A further ethical topic in need of discussion is just how widespread this cure for ageing would be. The two factors that determine just how far we can expect this cure to reach are choice and availability. The former addresses the question of whether or not the choice would be left up to the individual or decided by the majority.

If it is a majority that decide the fate of quite possibly all of human kind, this decision and all of its implications as outlined here could have a profoundly negative impact upon that minority, however small in number they may be. That minority that would have normally refused treatment if the decision was up to the individual could still be forced not to reproduce by the government for example, as mentioned previously. If the majority voted against it, there would no doubt be ways that particularly rich and powerful people could still acquire the treatment.

If the decision was left up to the individual, some people opting for extended life and others not, it is easy to see how society as we know it may be torn in two in a fashion not too dissimilar to Aldous Huxley’s Brave New World: a completely state-controlled “utopia” on one side, and the “savages” on the other, who opt out of the apparent benefits that such advances may bring. It is not too far fetched to imagine health care for the elderly refused with treatment being the only option, or perhaps a lack of work or housing. We already fear prejudice and ill treatment as a result of genomic sequencing, something that can be kept a secret, but whether or not you’ve taken a cure for ageing could not be hidden. There is an incentive for companies to hire employees who have taken the treatment over people who have not – no pension plans, a reduction of staff turnover, a continued increase in skills without the need to retrain new people. One person doing one job for 150 years will likely be a lot better at that job than someone who has done it for only 50 years, so why wouldn’t companies discriminate against those that opt?

The second factor, availability, needs to be thought about at an international level. Already, the availability of drugs in industrialised nations far exceeds that of developing countries, with over a third of the world’s population having no access to essential drugs. There is no reason to suggest availability of a cure for ageing would be any different, driving the wedge between the rich and poor even further. A possible result could be war for land or resources between both sides of this divide once the need for population control and limited space become a factor for those with the cure.

Dying Peacefully

One topic we’ve not touched on so far is death. If we remove the natural cap that the aging process forces upon us, then there won’t necessarily be a maximum age we can live to. However, death from anything not related to age would still occur. Currently, we think of death as an inevitable natural process although the causes of death can be many and varied. When asked to think about death and how they would like to die, the majority of people hope for a peaceful death during their sleep, at the end of a long and fulfilling life, and without pain. As we’ve already discussed, how fulfilled your life may be could already be compromised by denying you the right to bear children, so what about the rest of our ideal scenario? A long life? Yes. But pain-free in your sleep? That’s another story.

After dying of old age, the only causes of death that remain involve accidents, murder and other diseases that can affect anyone, not just the elderly. Discounting instantaneous (but still gruesome) death, any other situation in which a life is about to be ended will undoubtedly be accompanied by fear and pain. This is not to suggest that fear and pain are not part of dying of old age, but any hopes of peacefully dying in your sleep would be shattered. As people get older, the thought of death becomes more and more a factor in their life as something they have to come to terms with, but this will no longer be the case. Death will only be associated with terror and pain; with lying in hospitals fed through a tube; certainly without peace.

Legalising and actively supporting euthanasia, on the other hand, would be the only acceptable solution to this problem. Only then would the problem of the perception of death being necessarily negative be alleviated, but this raises yet more ethical problems, particularly among religious communities.

Conclusion

A cure for old age may bring with it the promise of an undefined limit to humanity’s lifespan, allowing us to do more than we ever thought possible; read more books, watch more films, and finally build that shed you’ve been talking about, among other things. On the surface, this seems idyllic, but only when you begin to scratch the surface do you reveal a swamp of ethical concerns that muddies this picturesque vision of the future. Living forever may require our lives to change so significantly that life might not be worth living in the first place.

It would seem that having our cake and eating it is simply not feasible. Would you really want to live forever if you could never eat cake again?

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I’d love to hear your comments and opinions.