drugs, his problem has been in each case to find a protoplasmic poison of such nature that it will not injure the patient's tissues, but will sterilize his body against the parasites in one or two injections. Success in finding such drugs must obviously depend upon an intimate knowledge of the relation between chemical structure and pharmacodynamic action and, in this obscure matter, Ehrlich has had at his fingers' ends a fund of practical information that is almost unprecedented. It is known that the physiological action of the organic radical in a drug molecule is the same, no matter what combination it enters into, while the inert parts of the molecule may alter the degree, but not the kind of action. Thus the anesthetic effect of cocaine or its derivatives is due to the amido-benzoic acid group in the cocaine molecule. Again the most toxic compounds are those which most rapidly liberate the active atom group in the molecule by decomposition, as in the case of many coal-tar products. Building upon facts of this kind, Ehrlich has in a surprisingly short time turned out definite effective remedies like methylene blue for quartan fever, trypan red in bovine piroplasmosis (Texas fever), arsenophenylglycine for the trypanosomiases (sleeping sickness in man, surra and mal de caderas in horses), dioxydiamidoarsenobenzol or "606" for the spirilloses (syphilis and relapsing fever). The technical and structural details of this wonderful piece of chemical research have been very thoroughly and ably described in a recent number of Science by Dr. H. Schweitzer[1] to which our readers may be referred. One instance of the extreme specialization of Ehrlich's chemotherapeutic knowledge may be quoted, his theorem that effective remedies for sleeping sickness must be "tetrazo colors derived from naphthalen disulpho-acids with the sulpho-groups in the 3.6 position."[2] The labors involved in building up and trying out several hundred of these new compounds was enormous, and in order to facilitate a system of exclusion, Ehrlich utilized his discovery of parasitic immunity against drugs in his device of a "cribrum therapeuticum" or therapeutic sieve, which will immediately classify any new chemotherapeutic substance in regard to its destructive effects upon pathogenic parasites. This is accomplished by rendering different parasites resistant to various drugs (e. g., fuchsine or atoxyl) through many generations, until finally a "strain" or breed is produced that is definitely fuchsine-fast, atoxyl-fast, etc. When a new drug is tried upon these different resistant strains, its pharmacodynamic status can be ascertained at once. If it destroys all the resistant strains it clearly belongs to a new and untried group. Ehrlich has even succeeded in cultivating strains of trypanosomes each of them re-
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