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PROBLEMS OF SMALLPOX SPECIFIC CHEMIPROPHYLAXIS

V. N. Pautov

Research Institution of Epidemiology and Microbiology N.F. Gamaleya of RAMS

Though more than 25 years smallpox are not registered in the world (excluding one case of laboratory challenge in Great Britain), the potential danger of its reemerging is present, since there are variola virus repositories, from which "outflow" of the virus is not completely excluded. Also there are orthopoxviruses of animals (monkeypox, camelpox etc.), related to variola virus, which under certain conditions can be a source of occurrence of a virus such as a variola. It is possible, also, preservation of a variola virus in corpses of people who died in the epidemics of viral genesis at the end of the last century and were buried in the permafrost region of Russia.

The population of Russia, as well as the population of all countries of the world also were vaccinated against smallpox more than 25 years ago, and the persons who are younger than 25 have never been vaccinated. One should take into account that the level of smallpox immunity is considerably reduced after 5 years since the vaccination has been accomplished, and after 10 years immunity disappears completely. Hence the population of Russia and all countries of the world has no immunity against smallpox now, leaving the world at risk of catastrophic consequences if variola virus somehow reemerges . The beginning of disease will require immediate specific protection of all primary infected patients and all persons contacting them.

The realization of specific immunization requires a lot of time both for its implementation and for the achievement of high level of immunity. Moreover, the specific immunization is perspective as to the persons, not yet infected by a variola virus. The use of specific gamma - globulin being effective for prophilaxis of disease in the infecting person, is unreal for protection of large groups of people. In these conditions the decisive significance for protection of people acquires an officinal prophilaxis. The role of officinal prophilaxis is considered recently in detail in the monograph by Karkischenko N.N.

A great number of medications were evaluated for the therapy and prophylactic treatment of smallpox in many laboratories, including those of the USSR, in the 1950-1960-s.

A polyfactorial principle of estimation of preparations efficiency was applied in the researches, using various infecting dozes of virus, time of the beginning of treatment and dozes of preparation. Similar conditions were created in control tests, which either did not infect by the virus or did not give treated preparation.

Hydrazincarbotiamid C10H10N4OS, (methisazon), having structure:

was selected for a more detailed study from several dozes of tested preparations in the USSR.

These studies have established that thiosemicarbazones provided a marked delay of reproduction of variola virus in cell line of man embryo lung (MEL). Methisazon suppressed reproduction of variola virus in concentration 10-6 g\ml fully. Electron microscope study have given evidence that methisazon hindered reproduction and maturation of virus and provoked its degradation. The inhibitory effect of methisazon was insignificant or was fully absent in cases when the preparation was added after the cells had been infected.

Early tested thiosemicarbazones of izathyn group had low toxicity for 12-days chicken embryos. Methisazon, -thiosemicarbazone N-ethylizathyn and -thiosemicarbazone izathyn suppressed reproduction of variola virus in chorioallantois of chicken embryos essentially, manifesting maximal effect at concentration of 1,5 mg\embryo. Therefore methisazon had the best effect.

Application of thiosemicarbazones to mice 7 hours prior to variola virus challenge provided significant protection to animals. At the same time methisazon had the most protective effect. However methisazon had insignificant effect when it is used after the beginning of infection. A more pronounced protective effect of methisazon was noted in cases of earlier treatment and higher dozes. Thus, single application of methisazon in dozes 50 and 100 mg\kg weight of animals 7 hours prior to variola virus challenge provided survival of all animals, whereas all animals was died in control group without treatment with methisazon.

Macaca rhesus monkey (49 monkeys) were inoculated by dozes 6,2x103 PFU or 4,2x104 PFU intravenously, and Cercopithecus aethiops monkey (12 monkeys) were inoculated in dozes 4,5x104 PFU by inhaling. Methisazon was introduced in oesophagus in the form of 20% suspension in sugar syrup or in the form of 0,2g tablet. Methisazon exerted precise influence upon infection in Macaca rhesus monkey. The effect of methisazon was higher in cases of earlier treatment, higher dozes and longer treatment. A more pronounced prophylactic effect of methisazon was noted in case of treatment shortly after intravenous inoculation in monkeys, and then twice a day for 4 days. Methisazon tablets provided protection in case of treatment shortly after inoculation by inhaling.

Comparison of efficiency of methisazon and variola virus gamma - globulin in experience in mice has shown a more expressed preventive action of methisazon, than variola virus gamma - globulin in case of application of preparations 7 hours before challenge of animals. The efficiency of methisazon and variola virus gamma - globulin did not differ in cases of application of these preparations 24 hours before challenge of animals.

A combined application of methisazon and variola virus gamma - globulin was more effective than the individual application of each preparation in variola virus experimental infection in mice and monkeys.

The summary of results of application of methisazon in cases of variola virus experimental infection, illustrated in table 1, allow to admit, that the early application of methisazon for medicinal preventive application in cases of smallpox can provide rather reliable protection against the development of the disease.

Table 1. Efficiency of Methisazon in Cases of Experimental Smallpox

The time of application of methisazon / Model for human smallpox disease

Before challenge
In a moment of challenge
Soon after challenge
After progress of infection
Cell line of man embryo lung
+ + + +
-
Chicken embryos
+ + + +
+ + + +
+ + +
-
White mice
+ + + +
+ + + +
+ + +
-
Monkeys
+ + + +
+ + +
-

Notations:
++++ significant influence
+++ moderate influence
- lack of influence

In conclusion it is possible to note, that the timely introduction of methisazon as a preventive measure for cases of smallpox, supplementing specific vaccination and the whole system of antiepidemic measures directed against smallpox interferes with the development the infection.

The considerable studies of antiviral drugs done at the present time makes it possible to obtain highly efficient medicinal substances for smallpox therapy that will enhance protection of humanity from the disease.

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Proceedings of First Russian Workshop on Biological Security
Copyright © Committee of Scientists for Global Security and Arms Control