Chemotherapy

CHEMOTHERAPY (Cheraotherapie, Chimiotherapie), the doctrine of the regular therapeutic effects on the infectious (invasive, oncological) process by means of chemically defined substances possessing specific affinity for the causative agent of the disease (or for cells of the organism that have become to some degree foreign and altered). The practical task of chemotherapy is to find and create such specific preparations for different causative agents and to develop a rational methodology for their application. A chemotherapeutic preparation achieves its goal if the effect it produces ultimately leads to the death of the pathogenic agent and to the cure of the patient organism (proper chemotherapy). In other cases, the goal is considered achieved if the timely administration of the substance prevents infection or hinders the development of severe forms of the disease (chemioprophy-laxis). The fundamental possibility of chemotherapy is based on the existence of differences between the cells of the macroorganism on the one hand, and the causative agent of the disease or the cells of the tumor on the other. These differences concern the degree of sensitivity and the degree of affinity to the preparations used. The goal of chemotherapy is to free the organism of parasites by means of substances that do not cause substantial harm to the organism as a whole. In this connection, it should be kept in mind that the infected organism itself may react differently to certain chemical substances than a healthy one. Thus, certain gold preparations prove to be considerably more toxic for trypanosomiasis-infected animals than for healthy ones. Malaria creates increased lability of erythrocytes with respect to quinine, which in a normal organism does not cause hemolysis. The distribution of preparations in the tissues and organs of the infected organism may also be different compared to a healthy animal. The infectious process as such may affect the transformations of the preparation introduced into the organism, modifying it, which may also substantially influence its role in the affected organism. On the other hand, the state of the microorganism (or parasite in general) in the affected macroorganism is under the strong influence of its interaction with the macroorganism. Accordingly, its relation in vivo to different substances may differ considerably from its relation to them in vitro. In the latter circumstance, the conditions of the medium, which in this case are the juices, tissues, and cells of the macroorganism, may also play a significant role. From what has been said, the differences between chemotherapy and pharmacology follow. If pharmacology deals with one organism, chemotherapy studies the effect of specific preparations in the complex unity of two organisms established in the infectious process. Chemotherapy thus has before it a very complex problem of three bodies: the macroorganism, the causative agent (foreign organism or pathologically altered cell), and the specific preparation in their interaction. Already Ehrlich noted that the pharmacological analysis of the effect of mercury or potassium iodide cannot for us decode the effect of these preparations in syphilis. Chemotherapy is often designated as "internal disinfection" of the organism. But between chemotherapy and disinfection there is essentially a huge difference. A disinfecting substance can in principle be any substance possessing the ability to kill a living cell. From it, in most cases, a non-specific and necessarily lethal action is required. In contrast to this, a chemotherapeutic substance, on the one hand, does not always itself have to kill the microorganism (or foreign cell), on the other hand, it must possess specific selective affinity for the given microorganism and an action that ultimately leads to its death with the least harm to all other cells of the macroorganism. Finally, the main feature of a chemotherapeutic substance is its relation to the infectious process, its ability to develop its specific action in the affected organism. At the same time, a chemotherapeutic agent has in common with a disinfecting agent that the basis of its action is the ability to bind with certain elements of the cell (or cells) of the parasite. The main idea that guided Ehrlich, the creator of chemotherapy as a science, was the concept that the specific effect of a substance on a cell is the result of a material interaction with its elements. This principle, put forward by Ehrlich in the eighties, runs like a red thread through the entire vast diversity of areas of his scientific creativity. The phenomena of acquired chemoresistance (drug resistance) of parasites, discovered and deciphered by Ehrlich and his students twenty years later, confirmed the correctness of the initial position. The basis of chemotherapy, even in its modern aspect, remains the concept of direct affinity between the parasite and the specific preparation. Only on the basis of this principle, confirmed by the most recent research (see below), can chemotherapy exist as a rational and experimental field of knowledge, because only on its basis can we seek regular dependencies between the structure of the preparation and its specific properties. It goes without saying that in certain cases there is the possibility of affecting the infected organism by means of substances that, without entering into direct interaction with the parasite, influence the course of infection by strengthening or changing certain functions of the macroorganism. Such kinds of effects, however, go beyond the proper framework of chemotherapy and are subject to other regularities. Corresponding to the specificity of its regularities, chemotherapy also has its specific methodology for the experimental study of the problems before it. Basically, the methodology of chemotherapeutic experiment, developed by Ehrlich, comes down to the serial reproduction in laboratory animals of diseases caused by a specific causative agent, and to the study of the effect of preparations administered to animals under different conditions and dosages at different stages of infection. The study of the fate of the introduced preparation in the affected organism, its relation to the cells and tissues of the macroorganism, the changes undergone by the parasites under the influence of the introduction of the preparation, allows one to look into the mechanism of action of the preparation. The selection of preparations for experiment is conducted according to the so-called principle of chemical variations. Having found a chemical compound of a certain type possessing expressed specific properties, a series of chemical variants close to it are created, trying to find that one which shows the greatest chemotherapeutic activity with respect to the given infection at the least toxicity for the macroorganism. Between the corresponding properties of preparations of close and distant chemical series, parallels and divergences are established. The relations of different groups of preparations to infections caused by different types of causative agents are studied. The comparison of data from the chemical systematics of preparations and the biological systematics of causative agents and the comparison of the effect of different preparations in different infections makes it possible to establish regular correspondences between the chemical structure of a substance and its chemotherapeutic properties and gives indications for the synthesis of increasingly perfect preparations. Thus step by step the research was conducted that led Ehrlich from atoxyl to salvarsan. Thus later were created germanin, plasmoquine and plasmodium, atebrin, antimosan and other most valuable preparations, each of which created a revolution in the therapy of the corresponding diseases. History of Chemotherapy. Experimental chemotherapy arose at the turn of the 20th century. The prerequisites for its development were, on the one hand, the high level of microbiology and parasitology achieved by that time, the deepening of knowledge about the nature of the infectious process, and on the other hand, the high development of chemical technology, which made it possible to obtain a large number of variants in diverse groups of chemical compounds and to carry out systematic syntheses in certain directions on the indication of biologists. The first attempts, however, gave negative results, because they were based on the mechanical transfer of phenomena taking place in a test tube to the more complex conditions observed in the infected organism. Thus, for example, Koch and Behring tried to carry out so-called "internal disinfection" of the organism. Koch tested the effect of mercuric chloride, phenol, compounds of gold in anthrax, diphtheria, tuberculosis, but without success. Behring tested with a similar purpose dyes and came to the conclusion about the impossibility of affecting the infectious principle located in the organism of an animal or human being, because the cells of the latter are much more sensitive to the action of disinfecting substances than the causative agents of infectious diseases. Individual successes in this direction were nevertheless achieved. Thus, Stilling in 1890 obtained good results by applying pyoctanin in purulent processes in the eye. Medical thought persistently sought ways of affecting the causative agents of infection. Among Russian researchers, already S. P. Botkin pointed to the necessity of seeking parasitotropic means. D. D. Romanovsky points to the "specificity of action on the very essence of the disease," manifesting itself "in the destructive effect on the parasite" (the action of quinine in malaria).

The emergence of rational chemotherapy and its successful development became possible only on the basis of the work of Ehrlich, who, during the last two decades of the 19th century, studied the degree of affinity of different cells for substances introduced into the organism as the basis for the distribution of these substances in the body and their effect on its various elements. In 1892, Ehrlich, based on his developed doctrine of vital staining and the distribution of substances in the body, proposed to use methylene blue for specific action on the malarial parasite, to which it has a specific affinity. This year can be considered the date of the origin of chemotherapy. After Laveran and Mesnil had inoculated laboratory animals with trypanosomes, which later became a favorite object of chemotherapy research, these authors in 1903 attempted to treat trypanosomiasis with arsenious acid. Soon Ehrlich, proceeding from his theoretical positions on the relationship between the chemical structure of substances and the degree of their affinity for different cellular elements, synthesized the dye trypanrot, which proved to be a very effective remedy against Trypanosoma equinum and some other species of trypanosomes. Following this, Nicolle and Mesnil introduced trypanblau, Wendelstadt and Felner-malachite green and brilliant green, Ehrlich introduced parafuchsin, and his student Rell obtained even better results with a chloro derivative of the latter drug - triparosan. Thus, the fundamental possibility of acting with the help of different compounds on pathogens located in the affected organism was proven. Soon atoxyl (Thomas, Breuil, Koch, etc.) was introduced into the therapy of trypanosomiases, and Uhlenhuth showed its effectiveness against spirochetoses. In particular, experimental chemotherapy of syphilis became possible after it was possible to infect rabbits with syphilis. In 1906, Ehrlich, who had been working with atoxyl since 1902, came to the conclusion that the properties of this drug (comparatively low toxicity and high effectiveness) are incompatible with the formula attributed to it of anilide of arsenious acid (formulas see below, in the section on the chemistry of chemotherapy compounds), and in the same year together with Bertheim he could prove that this drug has a different structure, namely para-aminophenylarsenic acid. In 1907 and 1908, compounds of antimony (Mesnil and Brimon, Plimmer, Uhlenhuth, etc.) were introduced into chemotherapy, first in the form of tartar emetic, after which a number of compounds of the aromatic series were created, such as stibosan, stibinyl, etc., which received enormous importance for the fight against leishmaniasis, trypanosomiases, schistosomiasis and other diseases. Approximately at the same time, Ehrlich discovered the trypanocidal properties of bismuth compounds. The chemotherapy properties of bismuth with respect to syphilis were discovered later by Soton and Levaditi, later it proved to be effective against leptospirosis (infectious jaundice). After the end of the world war, the drug germanin (Bayer 205-naganol) was released in Germany, a drug that is not a dye and does not contain metal atoms, possessing powerful trypanocidal properties (see Bayer 205). Experimental chemotherapy of malaria became possible after a suitable laboratory model was found in the form of malaria of songbirds. Vasilevsky in 1908 showed the sensitivity of the bird parasite Plasmodium praecox to quinine. After a series of works by various researchers in 1926, extensive studies by Guzman and his collaborators were published, systematically testing a number of antimalarial drugs on birds. The same period includes the research of Ehrlich's student-Rell, Schuhleman, Schenhefer and Winkler, which led to the creation of the first synthetic antimalarial drug-plasmochine. Derivatives of quinoline, to which plasmochine and the Soviet antimalarial drug plasmo-cide belong, have the remarkable property of damaging the sexual forms of the parasite, as a result of which patients taking these drugs become non-infectious to mosquitoes in the shortest possible time, which has enormous epidemiological importance. A new synthetic antimalarial drug atabrine has very high therapeutic properties. Chemotherapy of bacterial infections was developed mainly by Morgenroth and his school and Neufeld in Germany, Browning (a student of Ehrlich) in England. Morgenroth studied a number of quinine derivatives and in optochine found a substance very strongly acting on pneumococci. Eucupin and vucin from the same series proved to be very effective against pyogenic cocci and the causative agents of gas gangrene. The drug tripaflavin, synthesized by Ehrlich from the series of acridine dyes, and the similar drug rivanol also proved to be effective against some causative agents of bacterial origin. Various authors proposed derivatives of gold for the treatment of tuberculosis, but without noticeable results. Against Calymmatobacterium granulomatis, the causative agent of venereal granuloma, compounds of antimony proved to be very specific. The work of Meyer and Kikuta (1927) laid the foundations for chemotherapy of bartonelloses (arsenic compounds). Significant successes have also been achieved in chemotherapy of helminthiases. Neoantimozan (fuadin) proved to be very effective against schistosomiasis. A number of drugs acting against intestinal and liver parasites (carbon tetrachloride, hexylresorcinol, etc.), against amebiasis (yatrene) have been found and synthesized. Chemotherapy of plant diseases has also made a number of successes. In the very recent time, promising results have been obtained in the field of chemotherapy of cancer (Collier-Rothman-a pyridine derivative containing lead and sulfur) and sarcoma in experimental animals.--In tsarist Russia, corresponding to the weakness of the chemical industry and its almost complete subordination to foreign firms, chemotherapy drugs were not produced at all. Only during the imperialist war was the production of drugs of the salvarsan series organized. Under Soviet power, along with the chemicalization of the country, the possibilities for the development of chemotherapy also grew. The first steps consisted in the synthesis of drugs of the neosalvarsan type, stovarsol, then naganiin (=naganol), trypanblau were synthesized, the structure of plasmochine and atabrine was deciphered and similar drugs were synthesized. Recently, a drug similar to plasmochine, the antimalarial drug plasmo-cide, and a drug identical to atabrine, acrichine, have been put into circulation. Chemistry of chemotherapeutic compounds. In chemotherapy, drugs belonging to the most diverse groups of chemical compounds are used, starting from elementary bodies and their simplest compounds, such as, for example, trioxide of antimony, and ending with the most complex organic compounds. Guzman was one of the first to draw attention to the special significance for chemistry of the fifth group of Mendeleev's periodic system, containing a number of elements possessing pronounced chemotherapeutic properties: arsenic, antimony, bismuth, tantalum, vanadium, while among the other 75 chemical elements chemotherapeutic properties have been established only for mercury and silver. Levaditi with colleagues subjected 45 chemical elements to systematic research. Of the others, some could not be studied due to their gaseous state, others because they are present in significant quantities in the body. High chemotherapeutic properties with respect to trypanosomes and spirochetes were established by him for the following elements: vanadium, arsenic, antimony, tellurium, gold, mercury, bismuth. Of medium strength are gallium and indium, weak action is shown by platinum. With the exception of gallium, all elements effective in chemotherapy are precipitated by hydrogen sulfide. This property can be related to the mechanism of action of these substances, the effect of which on parasites is probably associated with an effect on their respiratory function through the binding of sulfhydryl groups. Of the elements of the fifth group, arsenic in the form of aromatic derivatives having in their basis a phenyl radical to which the arsenic atom is attached in the side chain has acquired the greatest importance in chemistry to the present time. The first of these compounds was atoxyl. Bechamp, who discovered it in 1861, attributed to this compound the structure of anilide of arsenious acid O II /OH NH-As(I

^OH Ehrlich together with Bertheim proved that atoxyl has a different structure O II /OH As(|^OHNH₂ and represents para-aminophenylarsinic acid. Atoxyl belongs to the group of phenylarsinic acids with pentavalent arsenic. These include O

0 II /OH

|| /OH

AsC

As< °\ NH-CO-CH₃ OH ^vONa NH-CO-CH₃ arsacetin \ NH-CH₃-CO-NH₂ triparsamide stovarsol-arsenol-spirtsid Differences in the side chains cause differences both in the toxicity of the preparations and in the nature of their action in one or another infection. According to Ehrlich, pentavalent arsenic compounds in the body are reduced to trivalent derivatives, which are much more active against pathogenic agents. Thus, atoxyl is reduced to para-aminophenylarsenic oxide o И As^-" NH₂, which is very poisonous for trypanosomes. Correspondingly, Ehrlich developed a series of preparations with trivalent arsenic, starting from arseno-benzol. These include arseno-phenylglycine CH₂ :OONa CH_г COONa salvarsan, neosalvarsan, neoarsenobenzol. Of the same type are arsalitin, sulfoxylsalvarsan, myosalvarsan, and others. Preparations of this kind have a very strong effect in syphilis, relapsing fever, yaws, spirochetosis in birds, glanders in horses, anthrax, tertian malaria, sodo (see).

Antimony finds application in chemistry in the form of its trioxide Sb₂O₃ (the emulsion in oil is called trioxidine), in the form of tartar emetic, and in the form of aromatic derivatives. The latter, similar to analogous arsenic derivatives, break down into a group of pentavalent and a group of trivalent compounds. The pentavalent ones include navlivayutsya in the body into much more active trivalent compounds.

Bismuth finds application in chemistry mainly in the form of salts with tartaric acid (trepol, bismoverol), compounds with iodine and quinine (biochinol), with thioglycolic acid (thiobismol). Silver is applied in chemistry in the form of argoflavin (compound with trypaflavin), argochrom (with methylene blue), ichthargan (with ichthyol), protargol. Silver is part of zilbersalvarsan.

Gold is applied in the form of sodium thiosulfate-sanocrysin and aromatic derivatives: NH-CH₂-SO₃Na -S-Au SO₃Na solganal Similar preparations are solganal B and crysolgan. Of the metalloids, iodine in the form of pyridine or quinoline derivatives has become most widespread in chemistry: SO₃H ONa OHN₂ N₂ "yatren", "selektan

Of organic dyes in chemotherapy, representatives of benzidine and azo dyes, triphenylmethane, acridine, and thiazine groups are mainly used. Trypanblau and trypanrot are derivatives of benzidine, but also contain azo groups: NH₂OH₂ CH₃ CH₃ OH NH₂ ^N-O-0-^N-^N-f SO₃Na /чААя, SO₃Na SO₃Na SO₃Na Sb₃ ONa NH-CO-CH₃ stibenyl Sb .ONa Cl NH-CO-CH₃ stibozan possessing trypanocidal and spirocheticidal action. Acts well in sodo. Neostibozan o II .OH (C₂H₅)HN . . . Sb ( '^xOH NH₂ has a specific effect in visceral leishmaniasis, inguinal lymphogranulomatosis.

To the derivatives of trivalent antimony belong antimosan, neoantimosan, or fuadin NaO₃SAO₄ /O/NsO₃Na ,o><oU

-^,H,° SO₃Na Na SO₃Na Pentavalent derivatives of antimony, like the corresponding arsenic compounds, are reduced in the body to trivalent compounds, which are more active against trypanosomes. Trypanblau

To the acridine dyes belong trypaflavin, rivanol, and the antimalarial preparation atabrine: /\/^CH^^ j4^\ N ^\/ /ч CH₃ Cl trypaflavin ⁴NH₃ ch₃-ch_a-ch_гch₃ ЩС^ЩЬ CH; ' U\_N^UCl atabrine

To the group of quinoline derivatives, besides the mentioned yatren, belong the natural alkaloids-quinine, cinchonine, etc.-and synthetic antimalarial preparations: ch₃ '°00 ^ch-°00 | N₂ | N NH

To the group of quinoline derivatives, besides the mentioned yatren, belong the natural alkaloids-quinine, cinchonine, etc.-and synthetic antimalarial preparations: ch₃ '°00 ^ch-°00 | N₂ | N NH

NH CH₃-CH Cll-i CH₂ N(0,115)2 plasmochin N11 ! CH_a CHg I CH_a N(C_aH₅)₂ plasmoïd Pronounced trypanocidal properties are also possessed by synthetic amino-anilquinolines and aminostyrilquinolines: ^NH2~00-^Gri=N-O^NHa N aminoanilquinoline N aminostyrilquinoline In amebic dysentery and helminth invasions, synthetic derivatives of hydroquinone and resorcinol are used, such as dihydro-nol, hexylresorcinol. Mechanism of action of chemotherapeutic substances. The main link in the chain of phenomena developing after the introduction of a chemotherapeutic agent into the affected organism is the fixation of the agent or its transformation products on the parasite. But the implementation of this fixation (and in some cases the preceding transformations of the agent) and the further unfolding of events are determined by the environmental conditions, which for the microorganism in this case are the juices and tissues of the macroorganism, the nature of the relationship between the parasite and the host, and the state of each of them at the given stage of the infectious or invasive process. Evidence that the chemotherapeutic effect is associated with the fixation of the agent (or its transformation products) on the parasite was obtained by Ehrlich and Honder. Trypanosomes sensitive to certain dyes were quickly stained and died in solutions of these dyes, while resistant strains of trypanosomes in the same solutions remained motile and unstained for a much longer period. In recent times, York with collaborators was able to show that trypanosomes sensitive to triparasamide extracted the reduced triparasamide from the solution, while resistant trypanosomes did not bind this compound. Yanchu was also able to confirm this position using other methods: colorimetric determination of the amount of dye bound by trypanosomes and determination of the photodynamic effect of the dye bound by elements of the microorganism cell. In many cases, it is possible to establish a known parallelism between the chemical and physicochemical properties of agents, their ability to bind to certain ingredients of the parasite cell, and the chemotherapeutic action of the corresponding agents. Thus, the destruction of the blepharoplast in trypanosomes occurs under the influence of basic dyes that have an affinity for the 'acoids' of this organ, in which these dyes can be detected under the microscope (Yanchu). Rell established a certain correspondence between the ability of substances containing sulfogroups to combine with bases and the chemotherapeutic properties of these substances. He put forward on this basis the 'fixation theory', according to which 'one complex sulfonic acid forms insoluble salts with the cellular bases of trypanosomes, another with the bases of spirochetes, and a third possibly with both'. Recent work has shown that a number of agents, such as neosalvarsan, germanin, etc., with respect to which it was previously assumed that they do not directly act on the corresponding parasites (spirochetes, trypanosomes), when using more sophisticated techniques, turn out to be lethal to them not only in vivo but also in vitro. The fixation of the agent (or its transformation products, for example, trivalent arsenic derivatives formed in the body due to the reduction of pentavalent compounds) on the parasite may be lethal to it both directly and indirectly. The direct lethal action may be due to the effect on the respiratory functions of the microorganism, blockade of oxygen receptors, paralyzing action on enzymes, disintegration of certain cell elements, etc. The lethal action of the agent may also manifest itself in the disruption of the parasite's reproduction mechanisms or in preventing the formation of relapsing strains, allowing the parasite to continue development in the body that has developed antibodies against the original strain of the parasite (antimutative action, characteristic according to Morgenroth and Freund of germanin). The indirect action of the agent may be expressed in the fact that the fixation of the agent on the parasite, in itself, may be quite indifferent to the latter, makes it accessible to the action of certain factors of the macroorganism. In some cases, this indirect action of the agent (or its derivatives) is opsonizing, facilitating the phagocytosis of the parasite by the cells of the macroorganism. In other cases, the agent may play the role of a unique ambceptor, sensitizing the parasite to the humoral effects of the macroorganism. The effect of the introduction of a chemotherapeutic agent may differently affect individual forms and stages of parasite development. A brilliant example are the plasmodia of malaria, with respect to which some agents have schizotropic action (cessation of development of asexual forms), others mainly gamotropic action (cessation of the sexual cycle). The schizotropic action of agents also turns out to be different. The course of disappearance, the sequence of death of individual age groups of the parasite when using different chemotherapeutic agents differ significantly (Moshkovsky). The latter circumstance speaks in favor of the fact that the action of agents is specific and does not reduce, as some authors believe (Ulenhut, Seifert), only to the mobilization of the macroorganism's mechanisms. If the introduction of a chemotherapeutic agent does not lead to the death of the parasite, then it may affect the parasite in another sense, may lead to a change in its properties and its relationship with the macroorganism. The most important from a practical point of view is the possibility of the parasite acquiring so-called drug resistance to the agent to which it was exposed. This phenomenon was discovered by Ehrlich and his students: repeated administration to animals infected with trypanosomes or spirochetes of doses of the agent insufficient for complete sterilization of the sick animal, whether in the same organism or during successive passages, leads to a significant decrease in the sensitivity of the microorganism to this agent. With some agents, it is possible to achieve that the parasites continue to develop in the animal's body despite the administration of doses tens of times exceeding the dose necessary to sterilize an animal infected with a normal strain of the same trypanosome. Acquired drug resistance is maintained by trypanosomes for a very long time, practically in an infinite chain of passages from animal to animal. Recently, York proved the possibility of trypanosomes acquiring drug resistance under the influence of the agent in vitro. The basis of drug resistance is a change in the properties of the microorganism, loss of the ability to bind the agent or its transformation products. Drug-resistant strains may differ from normal ones also in morphological respects (absence of blepharoplast in trypanosomes-Verbichsky) and biological (decrease in virulence). Drug resistance to a particular agent can often arise under the influence of the action of another agent of similar composition, and sometimes very distant. Thus, Ehrlich showed that orthoquinoid dyes and arsenic derivatives can create cross-resistance in trypanosomes. In other cases, one agent can cause resistance to another, but not vice versa. It is very important that the administration to an animal infected with microorganisms resistant to a particular agent of an agent from another group can return the parasite its original sensitivity to the original agent. Thus, Morgenroth and Rosenthal were able to destroy the acquired resistance of trypanosomes to quinine with the help of salvarsan. In practice, it is necessary to strictly distinguish between drug resistance of the parasite and drug resistance of the clinical case. The latter is not always caused by resistance of the parasite; it may be due to the state of the macroorganism and the peculiarities of the infectious process. Thus, Hoffman and Armuzzi and other authors were able to show that in so-called salvarsan-resistant cases of syphilis, the spirochetes themselves possess normal sensitivity to salvarsan, which could be verified by transplanting them to rabbits. Müllens and others were able to verify that in so-called quinone-resistant cases of malaria, the transfusion of infected blood to other persons leads to infections that are perfectly susceptible to the action of quinine. Under experimental conditions, it is sometimes possible to observe a sharp decrease in the sensitivity of the parasite to the agent already in the first hours after its administration. This phenomenon, noted by Morgenroth, received the name chemoflexia. For the most part, the resistance discovered in the course of chemoflexia is quickly lost in subsequent passages. The point of application of the chemotherapeutic agent is the microorganism, the parasite, the foreign cell, but the macroorganism in this case is not only the environment in which, as in an incubator or test tube, the process of reproduction and death of parasites takes place.

The effect of introducing a chemotherapeutic agent is undoubtedly influenced by its fate in the macroorganism, the ability of the latter's cells and tissues to fix the agent and subject it to a series of transformations. When introduced into an infected organism, the agent is distributed between the host's and parasite's cells according to their relative affinity for the given agent. In this* sense, the increased affinity of the macroorganism's cells for the chemotherapeutic agent is generally unfavorable for the agent's effect. In a number of cases, however, a certain sufficient degree of affinity of certain elements of the macroorganism for the chemotherapeutic agent is a condition for the therapeutic effect. This affinity is necessary in cases where the parasite develops in certain cells of the host (malaria - affinity of erythrocytes for quinine). On the other hand, for many agents it has been established that, when deposited in reticulo-endothelial and other elements of the body, they are subsequently gradually released back into the bloodstream and develop an effect of sufficient duration. Thus, in these cases, the affinity of the body's elements for the agent prevents its rapid excretion and ensures its delivery to parasites. In this connection, it should be borne in mind that both the affinity of the macroorganism's cells for chemotherapeutic agents and the sensitivity of its cells to the toxic effect of agents undergo significant changes during the infectious process itself (see above). On the other hand, the influence of the introduced agent on the various functions of the macroorganism, and in particular on those special functions and apparatuses which play a role both in the formation of the infectious process and in the destruction of the microorganism, is extremely important. Effects of the latter kind are particularly essential for the final effect of chemotherapeutic intervention; they can unfold differently in different phases and stages of the infectious process. This circumstance, among others, accounts for the differences in the result of introducing the same substance to animals of different species infected with the same parasite, or to animals of the same species but at different stages of infection. These differences are also connected with the different state of the microorganism at different stages of infection. Generally speaking, the chemotherapeutic effect is closely connected with the processes of interaction between the macroorganism and the microorganism. Recovery, complete sterilization of the organism, even in cases where rapid destruction of parasites occurs soon after the introduction of the agent, takes place with the participation of a number of mechanisms of the macroorganism, in particular immunoglobulins, both those arising in response to antigenic stimulation that began even before the introduction of the agent, and those that develop in response to the massive flooding of the body with products of microorganism destruction caused by the introduction of the agent. This immunizing stimulation of the body, arising in connection with the destruction of parasites caused by the introduction of a chemotherapeutic agent, was designated by Ehrlich as ictus immunisatorius. In accordance with the works of a number of authors (Pfeiffer and Marx, Billing and Isaak), who proved the enormous role of the reticulo-histiocytic system in the production of antibodies, recent research (Kroo and Jancso and others) have established that the production of immunoglobulins participating in processes related to chemotherapeutic intervention also proceeds with the participation of the reticulo-endothelium. From this point of view, the data established by Krichevsky, Feldt, Jungbluth and others concerning the importance of the spleen and reticulo-histiocytic elements for the chemotherapeutic effect find their explanation. The participation of the reticulo-histiocytic system in this effect is threefold: on the one hand, it can play the role of a depot, retaining in the body for a longer period significant amounts of chemotherapeutic substances; on the other hand, it participates in the production of immunoglobulins that complete the effect of the application of the chemotherapeutic substance. Finally, in the destruction of parasites, macrophages play a very essential role, whose phagocytic activity may also be connected both with chemotherapeutic action (the opsonizing role of the chemotherapeutic substance bound to the parasite or corresponding antibodies) and with immunizing stimulation. The possibility cannot be excluded that in some cases elements of the reticulo-endothelial system participate in the transformation of substances introduced into the body that are little active into significantly more active ones. The different intensity and character of interaction between the macro- and microorganism at different phases of infection require the use of different doses and different agents. In syphilis, the probability of achieving sterilization of the body is significantly higher in the initial stage of the disease than in later stages. In sleeping sickness, the use of different agents is indicated depending on whether the trypanosomes have already passed into the central nervous system; in the latter case, the use of agents that, for example, tryparsamide, pass through the hemato-encephalic barrier is appropriate. The intensity and tension of the infectious process, characterized by the virulence of the microorganism on the one hand and the specific reactivity of the macroorganism on the other, are of enormous importance for the chemotherapeutic effect. Thus, Kroo noted that infection caused by a more virulent strain of 'spirochete of relapsing fever gives in mice a higher percentage of non-relapse cure compared to infection caused by a less virulent strain. If, thus, the character of the infectious process caused in a given animal by a given microorganism or the stage of this process largely determines the result of chemotherapeutic intervention, then, on the other hand, the introduction of a chemotherapeutic agent, if it does not lead to the complete elimination of infection, can significantly modify the course of infection. The most striking example of such an effect is the phenomenon of mitigation (Browning, Riehl), consisting in the fact that the acute fatal course of infection turns into a prolonged chronic one. In laboratory animals, this phenomenon can be detected when using parafuchsin and triparosan (Riehl), bismuth (Gimsa), germanin (Leipold, Moschowski). When introducing germanin in doses insufficient for cure, surra, which in horses runs as an acute disease with fatal outcome, took the character of a prolonged chronic infection, similar in course to the natural disease caused by the same parasite (Trypanosoma evansi) in cattle. The observations of Serge and brothers also belong here, who by the introduction of quinine shortened or even completely suppressed the acute period of malaria in birds, transferring it to a state of latent benign infection. The basis of the phenomenon of mitigation is not simply a change in the properties of the microorganism, as the first authors who observed it (Ehrlich) believed, but a change in the relationship between the microorganism and the macroorganism. Chemotherapeutic intervention, by reducing the number of parasites and prolonging the course of infection, gives the animal the opportunity to live long enough to acquire partial resistance to this virus, after which the animal itself begins to cope with the parasites; thus, relationships are created similar to those that exist in animals in which this infection spontaneously has a chronic recurrent course. Thus, chemotherapeutic intervention is an effect on the infectious process (or on the processes of interaction between the macroorganism and the parasite) by means of substances which, when introduced into the affected organism, are fixed on the parasite in unchanged or transformed form, and the chemotherapeutic effect is the combined result of changes undergone by the parasite under the influence of the agent and the related changes in the relationship between the macro- and microorganism. Technique of application of chemotherapeutic substances. Combined X. The effect of introducing a chemotherapeutic agent depends to a large extent on the technique, routes, and methods of administration of the agent. Thus, for example, salvarsan gives a better effect when introduced in more diluted solutions compared to concentrated ones, which may be connected with a change in the degree of dispersion of the agent. The single administration of large doses leading to sterilization of the body is called therapia magna sterilisans. The high toxicity of many chemotherapeutic agents forces the use of fractional doses, of so-called staged treatment - therapia sterilisans fractionata. The introduction of a large dose of a poorly soluble agent, forming in the body a kind of depot from which small amounts of the active substance gradually enter the bloodstream, is called therapia mite curans. The combined application of chemotherapeutic agents is of considerable practical interest. In principle, the simultaneous or sequential application of agents of different composition for acting on the same infection presents a number of advantages. By acting on different functions of the parasite or leading to its death by other indirect means, agents of different chemical nature sum up their effect with respect to the parasite.

The cells of the macroorganism, however, suffer significantly less with such combined treatment, since each drug is used in doses smaller than those required for a cure by any single one, and thus are even further from the doses toxic to the macroorganism. Nevertheless, the possibility of mutual potentiation of the toxic effects of different drugs on the macroorganism is not always excluded and must always be taken into account in practice. The combined use of drugs from different groups can produce an effect significantly exceeding the maximum possible therapeutic effect of each individually, as for example in three-day malaria with combined treatment by plasmoquine and osarsol. Under certain conditions, the combined use of specific drugs can also give an unexpected negative result (the so-called phenomenon of interference-Browning and Gulbransen): the administration to an infected animal of two drugs specifically acting on a given pathogen one after the other (such as combinations of parafuchsin and trypaflavin, pyoctanine and salvarsan, etc.) not only fails to give a summation of effects, but on the contrary proves ineffective. If in some cases the phenomenon of interference can be due to the mutual binding of the substances introduced, other cases require further clarification. In any case, this phenomenon indicates that the combined use of chemotherapeutic drugs does not always lead to the desired effect, and therefore its possible results must be preliminarily tested in experiment. Testing of chemotherapeutic drugs. Laboratory testing of chemotherapeutic drugs is carried out: a) for finding effective drugs among a large series of related chemical products, b) for comparing different drugs in respect to their partial properties, c) for standardizing already known drugs before their release to the clinic. Along with studying the effect of the drug on the course of infection, experimental chemotherapy studies its effect on the corresponding pathogens in vitro, the effect of the drug on their motility, morphology, virulence in vitro and in vivo, the sequence of disappearance of individual age, asexual and sexual forms, the effect on individual functions of the parasite, for example on the ability to divide, to reproduce sexually, etc. Of helminthological objects, the action in vitro is usually studied on cultures of nematode larvae, on adult nematodes, on tapeworm cysticerci. The main chemotherapeutic experiment is conducted in different variants: as a curative experiment, when the substance is introduced in a developed infection; as a simultaneous experiment, when the drug is administered simultaneously with infection of the animal; and as a preventive experiment, when the chemotherapeutic drug is administered prior to infection. The effectiveness of the drug is expressed as a figure indicating the smallest dose capable of producing a certain effect on the course of the infectious process in a given animal or on the condition of the parasite developing in it. These doses are compared with the largest dose tolerated by the given experimental animal without noticeable harm (dosis maxima bene tolerata=DT). The dose required to sterilize the animal is usually calculated by weight (for 20 g mouse, 100 g rat, etc.), and is designated as dosis curativa minima=DC. The ratio DC/DT is designated as the chemotherapeutic index of the drug (Ehrlich). In recent years, with the increasing complexity of chemotherapeutic experiments, a number of other indices have been introduced, based on the determination of the smallest dose causing temporary disappearance of parasites-dosis parasitocida, causing delay in the appearance of parasites-dosis retardans, etc. When comparing indices obtained on laboratory animals for different drugs, it should be borne in mind that indices derived for one species of animal are by no means obligatory for others. This circumstance is due to differences both in the numerator of the index and in its denominator. On the one hand, the sensitivity of different animal species to the same substance, and consequently the corresponding value of DT, are not the same; on the other, the doses required to eliminate the same infection may differ sharply in different animal species due to differences in the environmental conditions provided by the two organisms for the action of the drug, and especially due to differences in the nature of the infectious process caused by the same parasite in different animal species. The limitations mentioned do not diminish the great importance of the chemotherapeutic experiment in finding new etiotropic drugs. They only require that its results be critically evaluated. The proof of the correctness of the paths of experimental chemotherapy outlined by Ehrlich are the enormous successes achieved by this science in respect to syphilis, yaws, relapsing fever and other spirochetoses, trypanosomiases, leishmaniasis, malaria, schistosomiasis and a number other infectious and invasive diseases.