The number of new drugs and chemical
products which appear every year on the world market is enormous.
A variety of factors stimulate this flood: competition between
the manufacturers; the feeling of the general public that for
every ill there's a pill; the promotion through advertising of
patent medicines, cleaning products, cosmetics, pesticides (there
are 35,000 pesticide formulations alone).
When it comes to drugs a vast amount of careful testing is required both to protect the public and the pharmaceutical manufacturer. Dr. W.G. Richards describes the process roughly as follows: "A research group finds a compound which will produce a biological response of a useful sort. They then get chemists to synthesize compounds and these are tested on animals until a better potential drug is found." He calls it "little more than inspired hit or miss" and, since thousands of drugs are tested for the one that eventually reaches the market, "wasteful of time, money and a great cause of suffering." (Richards,W., 1975).
As an alternative to this, there is the application of what has become known as the science of "drug design," an attempt to correlate the observed biological response with the molecular structure of the drug. The goal is to "predict the biological activity of the molecule prior to evaluation or even synthesis in order to reduce the costly and time-consuming synthetic work" - and to minimize screening with animals. Why, asks Dr. Richards, kill 2,000 rats or monkeys trying to understand the action of a molecule or two in the whole effect of a drug when mathematical calculations can tell us what are all the possible effects in advance? Then, if animals are still needed to confirm the results, no more than 50 may be required to produce the desired information. Since Dr. Richards, a chemistry lecturer at Oxford, is a leader in theoretical pharmacology, he can take us far enough into this difficult subject to open it up for us, even though we shall have to part company before the bristling mathematical equations appear.
"The conventional wisdom," says Richards, "is that drug action at the molecular level involves the interaction of the drug, a small molecule, with a macromolecular receptor." On the latter is a binding site waiting for a drug with molecular structure and an electrical charge which will fit. The small molecule is flexible, however, changing its shape (or conformation of its atomic nuclei) and the distribution of its electrons to suit different receptors. Richards says: "A drug receptor will experience the effects of the electron clouds of the approaching small molecule," and theoretical calculations, supported by experimental studies such as nuclear magnetic resonance spectroscopy, are capable of indicating the range of possible conformations of a drug's active molecules which might be suitable for binding. (Richards, W., 1977).
The story of how the drug azetomicin, which is a cancer-cell killer in mice, was developed, illustrates the above. As described in an Apr. 3, 1978, news item in The New York Times, the scientists who synthesized the drug - Dr. Martin A. Apple and associates - did so by looking at a range of drugs which used the basic genetic material of cancer cells - DNA - as receptors. They obtained basic information with the aid of PROPHET, a nationwide computer which has a vast store of chemical and biological data. They then simulated molecules in three dimensions on television screens and moved them around until they got conformations which they knew would adhere for comparatively long periods to the receptors on the surface of the cancer cells. While not yet safe to test on humans, azetomicin in mice has proved to be one of the most potent anti-cancer drugs ever made. (Anon., 1978c).
R. and M. Harrison describe the value of computers in drug design. "Computers have been applied in pharmacology in several areas. They have been used together with models of drug actions in simulating and inhibiting physiological responses to predict effects of individual drugs and combinations of drugs in various doses. There is even a science of quantum pharmacology which makes quantum chemical calculations and correlates factors such as the strength and configuration of various chemical bonds within a drug molecule with the pharmacologic actions." (Harrison, R., 1978).
Another experiment in drug design relies on knowledge of the structure of the receptor site in human hemoglobin. A small molecule with a long name, 2, 3-diphosphoglycerate (DPG), interacts with this receptor site and causes the hemoglobin in human blood cells to lose some of the oxygen it has an affinity for. C.R. Beddell and associates at Wellcome Research Laboratories in England used a scale model of the receptor site and designed three new compounds with atoms arranged to bind with those in the site. These compounds predictably promoted oxygen liberation, like DPG. This method was radically different from the old-fashioned practice of starting with a substance which has a biological property, say that of liberating oxygen, then synthesizing similar compounds with this property and testing them on animals to see which one works the best. Beddell's group, on the other hand, concentrated on compounds which fitted the receptor Site. These in fact turned out not to be closely related in structure to the natural starting substance DPG, even though they shared its oxygen liberating property. (Beddell, C., 1976).
One way of studying the molecular structure of receptors is with the electron microscope, which can visualize objects of the order of 5 - 10 angstroms. This is within the range of molecular dimensions. Walton and Bucley observe in their 1975 paper:
"As interactions of cells with molecules having known structures are studied and understood, it will become increasingly possible to predict on the basis of its molecular structure the changes that a molecule or newly designed drug will induce in cells. This vast but largely unexplored area of toxicology promises important developments which are basic to the understanding of cell toxicity and to the design of effective therapeutic agents." (Walton, A.,1975).
Recombinant DNA techniques or "gene
splicing" have made it possible for scientists to move genes
between species; for example, to take one of these biologic units
of heredity from a disease-causing virus - actually to cut it
out with a specific enzyme - and to insert it, again with the
use of an enzyme (the microbiologist's scalpel) into a bacterial
cell. By this process, the nature of the gene and the deoxyribonucleic
acid (DNA) of which it is composed can be studied, both as to
structure and function. Meanwhile the bacteria, like a little
factory, will start replicating the foreign gene in abundance,
and via RNA (the other nucleic acid molecule) the amino acid code
of the gene can be "translated" and expressed in the
form of protein. If the particular gene selected is the one that
expresses the surface antigenic protein - the one that elicits
a human immune response in the form of antibodies - a vaccine
against the disease has been synthesized.
If the long coils of DNA molecules are merely extracted from the animal cells and not reinserted in the bacterial cells, the DNA will break up into fragments and the genes cannot be isolated without contamination. But if they are introduced into certain components of the bacterial cell, which can be either plasmids (rings of extra-chromosomal DNA) or bacteriophage (viruses that live inside the bacteria), then the inserted material can be cloned without impurities, since plasmids and bacteriophage have very few genes compared with the whole bacterium which has several thousand. (Gilbert, W., 1980).
Researchers at several European centers
and at Stanford University in California have actually cloned
a surface protein of the hepatitis B gene in E. coli bacteria,
and the production of the antigen is now possible. (Anon.,1979b).
This does not mean that a commercial vaccine is available - there
is much testing and refining still to he done - but it is clear
that this will be accomplished sooner or later. Such a vaccine
would be far superior to one produced from whole virus (as in
the present polio virus technique) for the following reasons:
1. Using clones from highly purified genetic material, no animals
with the variables and contaminating gems they invariably introduce
would be involved in the production of the vaccine. Thus it would
be scientifically superior. 2. No animals' lives would have to
be sacrificed in the production phase, thus it would be humane.
3. When the whole virus, even if inactivated or attenuated, is
present in the vaccine, there is always a potential of virulence
and the transmission of the disease.
As explained in the discussion of polio vaccine production (cf.p.195 ff.), much of the testing of such products - testing inevitably associated with suffering of test animals - is to guard against the disease-producing elements of the virus. But these are absent when only the gene for the antigenic surface protein has been extracted from the DNA. Thus there would be no need to test the final product, the antigenic vaccine, for virulence, since it would lack the capacity to cause the disease although retaining the power to elicit immunity in the individual receiving it. The potency of the vaccine can be tested immunologically - in vitro - by radioimmunoassay techniques against a radiolabeled standard vaccine of known potency. The unlabeled vaccine will compete with the radiolabeled standard for binding sites on their antibody. The lower the percentage of radioactivity bound to the antibody (by precipitate count) the higher the potency of the unlabeled vaccine.
One of the unpleasant complications of persistent infection with hepatitis B is cancer of the liver (hepatoma). This, however, has led to an interesting discovery. Arie Zuckerman at the London School of Hygiene and Tropical Medicine has found that a viral-type protein on the surface of liver cancer cells in people who are also carriers of hepatitis B is very similar to the viral protein of the hepatitis antigen in their blood.
Both of these antigens will stimulate resistance to the disease, but cell cultures can be made from the cancer, stored in a deep freeze, and there they will go on producing the viral antigens indefinitely. From these, carefully extracted to avoid any risk of contamination from the cancer, large-scale vaccine production may get under way even before the gene-splicing method is generally available. (Anon.,1980b).
Man's effort to dominate and to exploit the higher animals for his own benefit has brought much suffering to his fellow creatures. Those who have striven to relieve the animals have looked everywhere for substitutes and, as this book testifies, have found not a few, starting with humans themselves and running the gamut of technology. But help now comes from a most unexpected quarter - from spliced genes, pathogenic bacteria and malignant cancer cells. Thanks, ingenious scientists, for these humane and truly elegant experiments! Here at last are investigations which one can read about with admiration rather than despair. They are recorded with enthusiasm - and with new hope for the animals.