Chemistry and Development
of ATABRINE1 and PLASMOCHIN8 By ALFRED E. SHERNDAL Winthrop Chemical Company, Inc. Λ LARGE amount of descriptive matter "^^ has been made public in the past few years, covering the nature and importance of malaria as a disease and efforts t o com bat it effectively by various means, b u t there is enough material of interest in t h e chemistry and development of Atabrine and Plasmochin to limit this discussion exclusively to those two drugs and their development. The promotion of their manufacture in the United States is of paramount importance now, in order t o meet the demands created b y the situa tion of the Allied Nations in the war. The development of these drugs t o their present status covers a period of about 2 2 years, dating back to 1920, when the s y s tematic search for synthetic antimalarials 1154
began in theresearchlaboratoriesof the I . G. in Germany. During the following 10 years of laboratory research and prelimi nary clinical examination, Plasmochin in 1924, w a s the first promising synthetic to be produced, followed b y Atabrine in 1930. Since then no other products of equal importance have appeared. In the period between 1930 and 1939, the two drugs were being used clinically in wide areas, and in those areas they be came recognized and established for whatever value they possessed in malarial therapy. Reports on their u s e were cfuite favorable. Late in 1939 w e were faced with the problem of manufacturing them, in the United States independently of any C H E M ICwAL
foreign supplies. We were poorly equipped t o meet t h e suddenly increasing de mands* and we were further handicapped b y a lack of definite ideas a s to how far tfciose demands might eventually increase. Oertain it is, that no one could imagine requirements ever reaching their present msagnitude. But since 1939 there has been considerable development in the tech nique o f the manufacture of these products in. the United States, and a development in. the scale of required production which could n o t have been imagined a t that time. The structural formulas and chemical reactions involved in the synthesis of Afccbrine and Plasmochin, have been la*ely s o frequently shown and described t h a t a repetition might seem superfluous. A number of different procedures for pre paring the products and their interme diates have been suggested and described in the original patents and in other litera ture. Passing over these, it might be of more pointed interest to describe in rough ourtline the procedures which are now a c tually being used for their manufacture in th«e United States. These now start from materials which are exclusively products of American chemical industry and resomirces. "The starting material for the acridine niEcleus of Atabrine is 2,4-dichlorobenzoic acid. T o obtain this material was one of t h e chief difficulties in the early stages of ouïr present manufacturing development. Tt "was finally made and is still being made from t h e 2,4-chloroaminotoluenes, which were available in sufficient quantities out of the dyestuff industry. Both isomers axe usable for t h e purpose. They are converted b y t h e Sandmeyer reaction to 2,4-dichlorotoluene, which by subsequent oxidation yields the required acid. Practically a l l of the 2,4-dichlorobenzoic iicld produced so far in the United States lias been manufactured by this method. Some o f the acid has been made by a sinailar procedure starting from 2,4diaminotoluene. A third method is now &nïng into production by which parachlorotoluene is further chlorinated, the iso»mers separated, the 2,4-dichlorotoluene converted t o the corresponding benzotrichloride which on hydrolysis yields t h e acid. A considerable part of United States 1943 requirements is expected t o be manufactured b y this process. T h e dichlorobenzoic acid is condensed in alkaline solution with para-anisidine, also available from the dyestuff industry, through its amino group yielding a carboxylic acid in -which the cyclization is effected by means of phosphorus oxychBoride. T h e reaction product is a inesthoxydichloroacridine in which one of the chlorines i s readily replaceable by other groups. F o r t h e aliphatic basic side chain intermediate the starting material is diethyl1 See footnote 1 at end of article. * See footnote 2 at end of article.
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aminoethanol, which has been commer cially available for some years and is also an intermediate for the local anesthetic procaine. This alkamine is converted into its chloride by means of thionyl chloride. The diethylaminoethyl chloride is condensed with sodium ethylacetoacetate forming an intermediate ester which is not isolated a s such, but is hydrolyzed and decarboxylated in the reaction mix ture by heat. When the solution is made strongly alkaline the resulting ketone separates out as an oil which is rectified by distillation in vacuum. In the next step the ketone is catalytically hydrogenated in the presence of ammonia whereby the oxygen of the ke tone group is replaced by NH2> forming the aliphatic side chain intermediate for Atabrine. The diamine and the chloracridine are heated together in anhydrous phenol. There is formed an intermediate phenoxy compound of the acridine which reacts readily with the diamine t o form Atabrine. The molten reaction mixture is poured into a large volume of acetone which holds the phenol in solution, while the Atabrine i s thrown out in the form of its dihydrochloride by adding excess hydro chloric acid. For purification, the prod uct is dissolved in water and again thrown out as dihydrochloride b y excess hydro chloric acid. From this brief outline of our present manufacturing process in the United States, it can be noted that the materials and the reactions are all well known and straight forward. The over-all yields are fairly good. There is no resemblance t o a "jig-saw puzzle", and there are no "secret ingredients" involved. Anyone can manufacture Atabrine, and it could almost be said that everyone does or is about to do so. The reason is to be found in the present effort to meet the enormous demands which are being made on the United States to supply this anti malarial drug t o a large and for us an im portant part of the world's human popu lation. The extent of this demand is even now straining the facilities and capacities of our already overburdened chemical industry. Under war condi tions the strain may increase still further. Plasmochin has rated lower than Ata brine in importance, because its action is principally against the gamete or sexual form of t h e parasite, which is responsible for the transmission, but not directly for the symptoms of the disease. There are signs of increasing interest in the product as an adjunct to quinine or Atabrine therapy. The starting point for its present manu facture in the United States is nitroaminoanisole, available out of the dyestuff in dustry, from which the quinoline nucleus of Plasmochin is formed by the Skraup reaction. The nitro group of the result ing quinoline is reduced by iron to the amino group. The basic side chain which V O L U M E
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The author, Dr- Alfred E. Sherndal, photographed in the laboratory at Rensselaer, Ν . Υ.
is then required, to form Plasmochin is the same as in Atabrine. However, it must be introduced through the amino group of the quinoline, and therefore the procedure differs from that used in Atabrine. The same intermediate ketone is used in both cases, but in t h e Plasmochin manufacture, this ketone i s hydrogenated without ammonia, yielding the alcohol which is subsequently converted into its bromide by concentrated hydrobromic acid. Con densation of this bromide with the aminoquinoline forms Plasmochin base, which is isolated as a n oil, rectified by distillation in a high vacuuim, and is then converted into an insoluble stable salt suitable for compressing into tablets. Before the development of these manu facturing procedures, and preceding the first tentative introduction of Atabrine and Plasmochin as aids in malarial ther apy, was the period of laboratory re search. That background of develop ment will be of particular interest to those who are engaged in research in medicinal chemistry. Here we can attempt to trace the origins, the sequence of ideas and observations, the laboratory investi »
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2 5,
1943
gations out of which emerged these first important synthetic antimalarials- The high points of this development are familiar enough, but it is interesting to attempt piecing them together into a logical and step-by-step sequence. The two products are definitely the re sult of a systematic and deliberate search. Essential to such a search is a test method by which a series of substances can be evaluated and compared with respect to their curative action. The basically im portant contribution of the pharmacologist to chemotherapeutic research is the dis covery and utilization of such test meth ods. The tests cannot exactly duplicate the conditions found in human disease; nevertheless, they supply the necessary guides and directives for the chemical development of new products. Without them the synthetic chemist would be working in the dark. Such a test for antimalarial action was discovered by the pharmacologist, Roehl, about 1920. He found that certain birds could be infected with avian malaria and so be used to evaluate curative action of drugs against 1155
V i e w of the apparatus in which the manufacture of side chain intermediates are produced by Winthrop Chemical.
that parasite. His successor, Kikuth, refined and further developed the test s o that both quantitative and qualitative comparisons could be made of the action of drugs on different sectors of the complicated cycle of malarial infection. It m a y fairly b e said that the period of systematic research which resulted in Atabrine and Plasmochin began with the discovery of those test methods. However, many experiments and observations had been made long before, which played a part in origins and which became of value to later and controlled research. Back in 1891 Ehrlich observed that methylene blue excellently stained the malaria Plasmodium. At that time i t w a s not a far-fetched idea that the dye might consequently also influence the parasites in the human blood. By clinical trials some faint effect was shown, b u t the results were doubtful and the possibility of methylene blue as an effective antimalarial drug was soon abandoned. Much later, Ehriich's observations on methylene blue led to experiments which gave valuable indications o f the lines along which synthetic research could b e conducted.
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About 1908 the chemical structure of quinine was established. T h e laboratory" investigations involved in determining that structure, the recognition of the quinoline nucleus and the aliphatic side chain, obviously gave later workers in synthetic research a valuable basis of information as to how to proceed i n their investigations. So did the attempts to duplicate the quinine molecule b y synthesis, and to improve on its activity by the replacement of its characteristic groups. I t can be assumed, that when synthetic antimalarial research was placed on a systematic basis, the voluminous chemical literature on quinine w a s carefully studied and gave valuable aids to the work. An important observation was made early in this period o f systematic research, when Wingler, o n the basis of the then well-developed biological test methods, was able to show that a definite increase in activity and a more favorable therapeutic index could be produced in the methylene blue molecule b y modifying its substituent groups. T h e favorable indication was obtained by replacing the methyl radicals of an amino group by cer-
CHEMICAL
tain basic alkyls. Here was another aid in developing a working hypothesis in the search for active synthetic antimalarials— namely, a demonstration of the effect of certain aliphatic basic side chains. To combine this observation with the knowledge of t h e quinoline nucleus in quinine seems a logical step. Perhaps Plasmochin owed its preparation to that line of thought. Continuing research after Plasmochin resulted in the acridine derivative Atabrine. To explain on a logical basis, this transition in research from the rather successfully used quinoline nucleus of Plasmochin, to t h e acridines, we might s a y that the acridines could b e thought of a s quinoline derivatives, like quinine and Plasmochin, but with a benzene ring fused t o the nucleus. However, going back to origins, there is another possible explanation, which is based on a story about the discovery of acridines a s useful medicinal substances. Some acridine derivatives already had achieved a useful place in medicine, notably the acriflavines and Rivanol, although these are devoid of antimalarial activity. To explain the original introduction of acridines into medicine it is told that Paul Ehrlich and h i s group were led into that field b y accident. They were studying the action of t h e tripbenylmethane derivatives o n Ehrlich's pet trypanosomes. One product in t h e series was dichloroparafuchsin. T h e original laboratory preparation was so outstanding among other members of the series that larger amounts were ordered for f tudy. The production of these large amounts was relegated t o a manufacturing department, which as so often happens, changed the laboratory procedure to one which was better adapted t o their equipm e n t and manner of working. Their product looked good, but it b y n o means compared in activity with the original laboratory preparation. I t just fitted in with the other members of t h e series, and no more. Examination disclosed that the laboratory preparation w a s contaminated by a n acridine derivative, a by-product of the process used, and t h a t the outstanding activity of t h e laboratory product was due t o this contaminant. A study of acridine derivatives immediately followed. If we accept this story about the introduction of acridines into the field of medicine, then t h e development of Atabrine is based in part on that accidental impurity in Ehrlich's parachlorofuchsin. At any rate, we know that Rivanol was a well known product during the development of t h e antimalarial research. T o produce it, an intermediate acridine derivative had long since been synthesized, which is a close analog of the acridine used in t h e manufacture of Atabrine. T h i s intermediate, like the one for A t a · brine, is substituted in t h e same positions a s is Atabrine, w i t h a replaceable chlorine
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is our only available dependable curative agent. Early in 1940, it was estimated that 240,000,000 tablets per year would meet requirements. In the fall of last year, it was estimated that the requirements of the United States for Atabrine could be met by 450,000,000 tablets per year, or 90 times the prewar production. This goal had then already been met and was. promptly exceeded. In December of last year, it was announced that Atabrine was being manufactured in the United States at the rate of 800,000,000 tablets per year, sufficient to treat 53,000,000 cases of malaria, which would be the equivalent of the total prewar world supply of quinine. Today we are producing Atabrine at the rate of 1,200,000,000 tablets per year.
in the 9 position. It appears quite natural, therefore, that such available intermediates should be included in the antimalarial research, and furthermore, that the same basic side chains which produced effectiveness in Plasmochin and methylene blue, should also be applied to the acridines. The hoped for but probably unexpected result was Atabrine. This-is the apparent thread of sequences in the research development of Atabrine and Plasmochin. That work is now proceeding at an accelerated pace in the United States i n the search for new and improved antimalarial drugs. Finally, in tracing the development of Atabrine and Plasmochin, the enormous increase in production and demand for these products i n the United States since 1939 will be of interest and suggestive value to all who are engaged in research in medicinal chemistry. It demonstrates strikingly the potential value of work in that field.
Astronomical Production
Total production in the United States for the year 1943 is now scheduled to reach the astronomical rate of 2,500,000,000 tablets, or almost 7,000,000 tablets for every day of the year. There are in-
M a l a r i a A n O l d Enemy
Four years ago none of us would have considered malaria or synthetic antimalarial drugs a s of sufficient interest to warrant a symposium at a meeting of the AMERICAN CHEMICAL SOCIETY.
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In Plasmochin
Increased inter-est in Plasmochin as an adjumct to Atabrâne or quinine treatment is raising anoth-er production problem* The United Stafees is already producing this synthetic at a rate of over one hundred times prcw^ar level. If clinical reports of its use continue favorable, we may expect greatly increased demands.
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that time, malaria was well enough known as an ancient and malignant disease, important particularly because of its economic aspects. Plasmochin and Atabrine were known to malariologists, and quite favorable reports about the value of the drugs had been extensively published from various parts of the world, including carefully controlled tests in the United States. The final step in the synthesis of Atabrine and Plasmochin had been carried out in the United States for a period of about six years from intermediates imported from Germany. But the demands for Atabrine in the United States during the years 1933—39 were completely met by an annual production of about 1,200 pounds or 5,000,000 tablets. Demands for Plasmochin were so small as to be negligible. The amount o f quinine used throughout the world for malaria therapy was estimated at some 2,000,000 pounds or, with the usual dosages, sufficient to treat about 50,000,000 cases per year. Such was tr*e status of malaria and antimalarial drugs in the United States up to the end of 1939 when the picture started to change rapidly. The change was due, of course, first to the outbreak of the war, and later to the complete stoppage of quinine supplies by enemy action, and still later to the widening field of military activity and the dependence of so many populations of the world on the United States for supplies of antimalarial drugs. The present importance of Atabrine is due to the fact that we are faced with a lot of malaria, and Atabrine V O L U M E
dications thatcvem these production levels may foe insufficieoit to meet total demands. In order to accomplish this scheduled 1943 production, American pharmaceutical manufacturers must produce six different intermediates to an aggregate amouoit of 3,00O,COO pounds, whose present use is limited entirely to the manufacture of Atabrisne, and they must draw on American cliestnical industry for some 25,000,000 poundls of other chemical materials as well as an array of special processing equipmesnt. Chemists, laboratories;, and a large group of manufacturers in the United Stages and England are now joined and collaborating in order to meet these extraordinary demands for Atabrine.
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Original manufacturing set-up for Atabrine. »
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25,
1943
Photograph» showsfinalstep. 1157
whicfi could be asked of an ideal anti malarial drug. They h a v e been criticized by s o m e o n the score of total efficiency and foy-effects.
Improvements W i l l Come Β i x t we m a y expect t h a t the present intcrnse activity in our university and in dustrial research laboratories, and our rudely awakened recognition of the i m portance o f malaria as a world disease, and ^the constant refinement of synthetic and biological methods, will eventually resul* in improvements in miliaria ther apy. Future Dependence A n a l y z e d
Above.
Atabrine tablet
machine operator. Below, Part of t h e present manufacturing facilities a t Winthrop Chemical.
It appears that even if the world's entire prewar annual supply of 2,000,000 pounds of quinine were available t o us, it would meet less tlian one fifth of t h e present demands o f t h e United States for antimalarial drugs. Such is t h e •striking picture of t h e development of Ata brine and Plasmochin from, the stand point of demonstrated usefulness, demand, and volume of production.
Malaria i s a name given to a complex disease, complicated b y the types of para sites a n d by the different forms which Ikhcy assume during their life cycle. The term "antimalarial" i s far t o o general to dedSiie the specific role of any of our known remedies against the disease, and taolwithstanding their enormous value t o us at this time, the present synthetics appar ently d o n o t meet a l l t h e requirements
Thu3 synthetic medicinale from our elicmmical laboratories do not depend for tlieir production on restricted conditions. To manufacture them, it is not necessary to p o s s e s s or control restricted areas of the earth's territory where soil and climatic conditions favor the cultivation a n d growth of any certain plantations. They can "be manufacture i wherever supplies such as air, water, oil 6elds, 'and s a l t miness are available t o the chemist. Those requirements for manufacture are n o t readüly controlled or restricted by n a tional policy, war, or monopolies. I t may well be that our future dependence for onedicinai supplies will rest primarily with the chemical laboratory, and that drugs of plant origin m a y disappear or occupy a subordinate position in t h e treatment of disease. W e have already found a number o f synthetic drugs for ccrtauin purposes which surpass in medicinal "value those obtained from natural sources. T o discover a n d develop such aids in t h e fight against disease is t h e long—range task to which so many of our research laboratories h a v e now devoted themselves. 'A-tabrine is 3-chloro-7-methoxy-9-(l-methyl4-diefchylanainobutylamino)-acridine dinydrochloride, which, has been given t h e non-proprietary name? of qiiinacrine hydrochloride. T h e n a m e Atabirine is registered i n the XJ. S. P a t e n t Office. » P-lasmochin is 6-methoxy-8-(l-methyl-4-diethylsaniino"butylamino)-quinoline which has been givem the non-proprietary n a m e of pamaquine. The aname Plasmochin is registered in t h e Ü. S. Patent Office.
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