MEDICINAL PLANTS AS SOURCES OF ANTIVIRALS

MEDICINAL PLANTS AS SOURCES OF ANTIVIRALS

Prof. Jim Hudson

University of British Columbia, Vancouver, Department of Pathology, University of Bc, Vancouver, Canada



Most countries have traditional uses for medicinal plant preparations in the treatment of infectious diseases, including many that we know are due to virus infections. We do not know how any of these materials work; in theory mechanisms of action could be

I) direct antiviral activities of phytochemicals;

ii) indirect actions mediated by stimulation of appropriate host defences;

iii) combinations of these by different compounds.



My long-range research objectives are to answer several relevant questions:

i) Do medicinal plant extracts contain identifiable phytochemicals with direct antiviral activities? The answer to this is...Yes, many such compounds can be detected and some have been chemically characterized. Studies on their mechanisms are underway.

ii) Are the antiviral effects of these compounds modified by other plant constituents? Yes... such effects have been documented.

iii) Do extracts contain other phytochemicals (or even the same compounds) with host-stimulatory activities, e.g. the ability to influence the levels of specific cytokines, chemokines, and other physiological factors involved in host defense mechanisms against pathogenic organisms. Recent evidence indicates that the answer to this question is also...Yes.

iv) Can these individual activities explain, or partly explain, the apparent efficacy of the medicinal plant preparations. This is a crucial question that must be answered, because it is important not only in deciding whether or not this research approach, as indicated in points i to iii, is useful and valid (as an indicator of authenticity and as a possible quality control device), but also it is essential to satisfy the criticisms of those researchers and medical practitioners who question the real value of medicinal plants.



Needles to say, if a traditional preparation can be shown by relatively simple laboratory tests to meet these demands, as imposed by traditional western concepts of scientific explanation, then this would help to justify the cultivation and processing of such plants as local resources.



In carrying out these kinds of laboratory investigations, we have begun to understand the roles of a number of variables and factors, in the plant extracts, that influence the activities that we are testing for, and which might be relevant to their activities in vivo.



i.) The source and condition of the plant itself, the parts of the plant used for extraction, the nature of the solvent used, as well as the precise techniques used to test for antiviral activity, i.e. the bioassay conditions, are all factors that can affect the test results. Some of these variables have been known for a long time; yet it is surprising that no attention has been given to standardizing the conditions, or to ensure that the extracts used in the test are actually relevant to the traditional methodology used in the application of these materials.



ii) It is a fact, often ignored by phytochemists, that medicinal plants from many parts of the world are rich in photosensitizes, compounds whose biological activities require are enhanced by light, particularly those wavelengths prevalent in sunlight. These substances are typically secondary metabolites that are thought to play important roles in protection against insect predation. It is also possible that, by virtue of their common antiviral and antimicrobial activities, they may have been found serendipitously to possess useful medicinal properties. Thus their presence could explain the role of sunlight in the application of some traditional medicines. Apart from this, laboratory tests need to be modified to accommodate these photosensitizer, and we have spent some time working out simple techniques for doing this in the context of laboratories with limited facilities.



Some studies with antiviral phytochemicals were flawed because of inattention to the presence of photosensitizer. Furthermore, some promising activities have been missed in large-scale screening programs of plant extracts through ignorance of this factor.



iii) Factors that influence apparent activities. Others and we have often found that the apparent potency of a plant extract, for example in terms of antiviral activity, sometimes diminished as the "active ingredient" is progressively purified. Possible explanations for this property include: removal of synergistic factors; removal of one or more other compounds, chemically distinct, which contribute to the overall activity. Sometimes the extracts also contain inhibitory compounds. These factors are relevant to the issue of what kind of material should be used medicinally, i.e. should we think of plant extracts simply as sources of individual "drugs" that can be isolated and used as single compounds, or is it preferable to continue the practice of using crude extracts in order to obtain maximum benefit.



iv) Chemical structure of active compounds. Chemical alteration of the structure of a known phytochemical often results in a loss or decrease in activity. This consideration is relevant to "drug discovery" programs, in which the aim is to produce a synthetic derivative of the natural compound that can then be patented. It is also relevant to the processing of crude materials, since chemical alteration can occur during handling or storage. Improper processing could result in loss of activity.



In spite of all these factors that have to be taken into account, I think that it is feasible that investigations of this kind can be carried out in laboratories without much sophisticated equipment.