|
Are animal models on target?
Animal models are indispensible to drug testing, but are
the results generated from small animals conclusive for humans? Katya Naidu
finds out
 New
formulations were once tried on war prisoners. As civilisation progressed and
wars became fewer, mankind sought other alternatives. The next best thing to
humans, animals play a vital role in virtually every major medical advance.
Regulations mandate that every new drug has to go through three basic phases
of testing to prove its efficacy and more importantly, its safety, before it
reaches human trials-in-vitro (test-tube) tests, in-silico (computer) modelling
and animal testing. “After a drug passes through in-vitro systems, you
would want to know what would happen in the milieu of a living organism. That’s
where animal testing plays a part,” explains Dr Surekha M Zingde, Deputy
Director of Cancer Research Institute, Advanced Centre for Treatment, Research
& Education in Cancer (ACTREC).
Advantage Animalia
Animal models provide persuasive evidence of the safety of a drug. Moreover,
animal testing is a researcher’s tool, which offers an insight into the
way a disease progresses, and factors important to the disease process, as well
as, disease treatment. Testing in animals also provides the analogues of several
disease pathological conditions.
Animal models are also of great importance for the analysis of genes. It’s
possible to set up models for the examination of gene expression and gene regulation.
“Genes may be specifically inactivated within animals (knock-out model),
and the effect on the organism can be studied. By inserting an additional recombinant
gene (transgene animal model), its expression and influence on the metabolism
and organism can be examined,” says Prof Harish Padh, Director, PERD Centre.
Other than toxicological evaluations, animal testing furthermore
tests the side-effects of a drug. “When I am studying bacterial cell kill,
there are several other effects, like haemoglobin level, liver function, kidney
function and cell numbers. All this information is mandatory for drug safety
evaluations,” says Dr Girish Matu, Head of Tobacco Carcinogenesis Group,
ACTREC.
|
Even though animal models serve as strong models
to predict gross toxicological effects, it is estimated that only about
5-25 percent of toxic effects found in animals occur in humans. Some examples
where drugs passed safe during animal testing but proved tragic consequences
in humans are:
Opren: 3500 people suffered serious
side effects including damage to skin, eyes, liver, and kidneys.
Thalidomide: Caused about 10,000
birth defects worldwide.
Clioquinol: Caused 30,000 cases of
blindness and/or paralysis and thousands of deaths.
Milrinone: increased survival of
rats with artificially induced heart failure, but humans experienced a
30 percent increase in mortality.
Fialuridine: Appeared safe in animal
tests, but it caused liver failure in seven of 15 humans taking
the drug, five of whom died and two required liver transplantation.
Conversely, many drugs that are beneficial
to humans are dangerous or even fatal to animals:
Penicillin: An antibiotic to humans,
but kill’s guinea pigs.
Aspirin: Caused birth defects in
rats, mice, monkeys, guinea pigs, cats and dogs, but not humans.
|
Different strokes
The foremost question that anyone would ask is—how well can a rat represent
the physiochemical reactions that occur in humans? “Mice, considered the
prime model of inherited human disease, are genetically tractable and share
99 percent of their genes with humans. But animals differ from humans with respect
to their biochemical pathways, mechanisms of absorption, distribution, metabolism
and excretion,” says Padh.
For example, the end product of the human excretory pathway is uric acid, as
compared to ammonia being released as the end product of rat excretory mechanism.
Similarly, experimental guinea pigs lack the ability to synthesise endogenous
ascorbic acid.
Rodents fail to demonstrate emetic properties because they have a powerful barrier
between the stomach and the oesophagus. They don’t have the oesophageal
muscle strength to overcome and open this barrier by force, which is necessary
for vomiting. Anatomically, rats lack a gall bladder.
The process
The challenges in animal testing are handled by changing the delivery routes
in animals to simulate the effect of the drug in the humans. Matu says that
in order to maximise the effect of a drug in animals, as it occurs in humans,
a novel exposure route should be determined.
There are various routes through which a chemical can be administered to animals:
dermal, intra-peritoneal, inhalation, oral and intra-muscular. “We generally
try to simulate the route of exposure as it occurs in humans in animal experiments.
But most times, when you try to follow the same route, the circulating levels
of the chemicals obtained are not comparable to that of humans,” says Matu.
The chemical given in a diet to a rat will not follow the same route as it does
in humans. To achieve similar concentrations, the drug should be given by another
route to produce the same effect. For example, if a drug is given via the oral
route, the concentration of the drug that reaches the lung is different to that
of the humans. Hence, the drug is injected directly to the intra-peritoneal
cavity. “For each target site in humans, there are experimental models
in animals,” says Matu.
|
A controlled birth, conditioned
life, induced diseases and a pre-decided death is all these lab animals
get. It is estimated that 50-100 million animals worldwide are used annually
and subsequently killed in scientific procedures.
In an attempt to give a better direction to the usage
of animals which like humans are live forms and experience pain, the Committee
for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA)
has come up with guidelines for animal testing. Every institution and
animal facility has to get registration from this committee.
Any kind of animal research has to go through two
tiers of regulations. First, the scientific query should be institutionally
acceptable i.e. the study should be scientifically peer-reviewed by experts
in the area. Once it is cleared, the animals ethics committee decides
if the number of animals meets the study guidelines and decides the statistical
proportions. “The ethics committee looks at whether we are treating
them with a human touch. Even if we are performing surgery we have to
take prior permission,” informs Matu.
|
Controlled conditions
As a part of the scientific enquiry, animal testing mandates that all the organisms
tested should be of similar characteristics. Animal testing allows the researcher
to exercise maximum control of the experiment by making the testing material
‘constant’.
One can keep the animal factor constant by maintaining absolute similarity in
rats so that the only variable is the drug concentration. This way, any change
observed in the animal can be attributed exclusively to the drug.
The animals have to be randomly divided into the control and experimental groups.
Every small detail should be observed to maintain extreme similarity between
the two groups. “If you are giving injections, similar injections with
either saline or solvent have also to be given to control animals to ensure
that such repeated injections are not responsible for the observed cancer or
change,” informs Matu.
To attain the constant factor, some institutes start off from scratch by breeding
the animals themselves. “We are particular about our animal maintenance,
hygiene, molecular biology, genetics, bio-chemical parameters,” says Zingde.
To maintain consistence in diet, the food that is given to animals is pelleted
by ACTREC itself. Temperature and humidity are maintained constant throughout
the year. The animals are subjected to subsequent 12 hour dark and light phases.
In addition, the animals are closely monitored to avoid infections.
Since genetics affect the responses of an animal to a drug
or disease, animals are also kept genetically close to each other
by controlled breeding to produce pure strains. “To eliminate
the heterogeneity of the genetic matter, we practice brother-sister
mating at least for 20 consecutive generations,” says Aravind
Ingle, Officer-in-Charge, Animal house, ACTREC.
|
Drug testing is done on specific species of specific
phyla of adaptable animals. The animals that come closest to representing
humans are the monkeys. Some of the monkeys used are baboons, macaques,
marmosets and chimpanzees. The number of genetic differences between humans
and chimps is about 60 times less than that between humans and mice, and
about 10 times less than that between mice and rats. “Our genetic
makeup is indisputably similar to the great apes, our closest relatives.
Estimates suggest a 98 percent genetic overlap with chimpanzees, with
gorillas and orangutans following close behind, and the monkeys our next
closest cousins,” says Padh. But the use of great apes, also known
as, Hominidae i.e. gorillas, chimpanzees and orangutans, is prohibited
in some countries.
Invertebrates, like Drosophila melanogaster and
Caenorhabditis elegans, are very popular, the obvious advantage being
the very short generation time of under a week. Among amphibians, the
major species used are the zebrafish, Danio rerio, and the African frog,
Xenopus laevis. Albino rabbits are used in eye irritancy tests because
they have less tear flow than other animals. Amongst dogs, beagles are
used because they are friendly and gentle in toxicity tests, surgery,
and dental experiments. Some cats and horses are also used for such studies.
Other commonly-used vertebrates are rats, mice, guinea
pigs and hamsters. They are used in large proportions because they are
small, cheap, easy to handle and care for, and can produce 100 babies
a year. Many models have been established in mice for human diseases such
as diabetes, degeneration of horizontally striped muscles (muscle dystrophy)
and various kinds of cancer. For other diseases, like high blood pressure
and cardiovascular diseases, rats are used. “The generation time
of rats is longer than with mice, so they are more suitable for physiological,
pharmacological and behavioural research,” says Padh.
|
Limiting factors
There are certain thera-peutic areas where animals cannot mimic the results
in humans. Only humans develop AIDS from HIV, and all animal “models”
of AIDS differ fundamentally from the human condition. They fail to address
the essential issue of co-factors such as diet, exercise, lifestyle and drug
use in the development of the disease. Progress against AIDS has derived from
human clinical investigation and in-vitro studies of the virus itself. Indeed,
some human vaccine trials have been performed without encouraging animal data
because it is recognised that animals cannot reliably predict which AIDS vaccines
will work.
Similarly, only humans develop Alzheimer’s. The superficial similarity
in animal models does not constitute a valid model from which extrapolations
can be made. Another telling example is animal models of stroke, in which artificially
induced brain blood vessel occlusions have resulted in conditions, which do
not meaningfully resemble human strokes. “While all animal models are problematic,
animal models of mental disorders are particularly compromised because interspecies
communications difficulties undermine attempts to determine the animal’s
mental state. Not surprisingly, all major drugs affecting mental states and
cognition have been discovered through clinical investigation, usually serendipitous
observations of the side effects of existing drugs,” says Padh. Other than
therapeutic areas, animal models cannot be used to estimate the absolute safety
of a drug. “If you find that a given carcinogen is inducing tumours, can
we say that this is going to be carcinogenic in human? The answer is no. If
we understand the limitations, we will not make sweeping conclusions, while
interpreting,” concludes Matu.
editorial@expresspharmaonline.com
|
