UDC 661.718.1:547.76:541.697

Chemistry

Academician B. A. ARBUZOV, A. O. VIZEL, I. V. ZAIKONNIKOVA,
I. A. STUDENTSOVA, V. G. DUNAEV, M. A. ZVEREVA, K. M. IVANOVSKAYA

ORGANOPHOSPHORUS COMPOUNDS

WITH LOW TOXICITY

Since the discovery of the toxic properties of organic phosphorus derivatives, a large number of investigators have been engaged in the synthesis and study of the biological action of compounds of this class. The opinion arose that the physiological activity of organophosphorus compounds is based on their ability to interact with cholinesterase, and the aim of most work was to obtain toxic agents, insecticides, and medicinal substances whose mechanism of action would be based on the inactivation of this enzyme. And although in some studies a discrepancy was noted between the level of toxicity and the degree of cholinesterase inhibition (^1), organophosphorus compounds with low anticholinesterase activity long failed to attract the attention of investigators.

By the present time a sufficient body of information has accumulated indicating that the biological activity of organophosphorus compounds is not exhausted by their ability to inactivate cholinesterase. Among derivatives of this class, preparations have been found that possess antimicrobial (^2, ^3) and antiblastomic action (^4, ^5), as well as insecticides that do not exhibit anticholinesterase activity (^6). The ability of organic phosphorus derivatives to exert both excitatory and blocking action directly on cholinoreactive systems has been established (^1, ^7, ^8). Organophosphorus compounds have been synthesized (^9) that are antagonists of the strong organophosphorus cholinesterase inhibitors—armin and nibufin (^10). The narcotic action of weak anticholinesterase agents from the class of organophosphorus compounds is mentioned in the reports of I. A. Frankov (^11, ^12). In 1957, Vandekar (^13), describing the anesthetic action of sublethal doses of low-toxic esters of phosphoric and thiophosphoric acids that do not exhibit anticholinesterase activity, suggested that in stronger cholinesterase inhibitors the anesthetic effect is not revealed because of their high toxicity.

It may be assumed that cholinesterase inhibition, which causes the rapid death of animals, masks the diverse mechanisms by which synthetic organophosphorus compounds act on the organism. From this point of view, it seems expedient to us to synthesize and study the biological activity of organic phosphorus derivatives with low anticholinesterase activity and low toxicity.

In a previous work it was reported (^14) that certain low-toxic derivatives of phosphacyclopentane induce in animals a state resembling narcosis. To study the relationships between structure and biological action in compounds of this series, new derivatives of phosphacyclopentane were synthesized, information on which is given in Table 1. All the substances obtained are colorless liquids, soluble in water and in ordinary organic solvents. All esters were obtained from 1-oxo-1-bromo-3-methylphosphacyclopentane (^15) and the corresponding alcohols in ether solution in the presence of triethylamine. Amide XIII was obtained from the same bromoanhydride and a twofold amount of diethylamine.

In the biological studies, both newly synthesized compounds and compounds described earlier (^14) were used. As a

The criteria of physiological activity adopted were the mean lethal dose (LD\(_{50}\)), the duration of the lateral position of mice upon administration of the maximum tolerated doses (MTD) of the preparations, and their effect on the action of an absolutely lethal dose of Corazol, a typical antagonist of narcotic substances.

Table 1

* Upper figures are found values; lower figures are calculated.
* Found, %: C 37.04, H 9.01; calculated, %: C 37.44, H 9.10.
** Found, %: Cl 18.18; calculated, %: Cl 18.25.

The results of the experiments presented in Table 2 permit the following conclusions:

1. All the alkoxy derivatives of phosphacyclopentene studied, in maximum tolerated doses, cause depression of the central nervous system of mice of varying duration—their transition into a lateral position. Administration of amide XIII leads to a certain increase in motor activity and an increase in reflex excitability.

2. For most compounds with unbranched hydrocarbon radicals, the duration of the lateral position decreases with increasing length of this radical and increasing toxicity. The exception is the methyl ether.

3. Upon administration of iso-compounds (III, V, VII), the duration of the lateral position of mice decreases somewhat in comparison with the isomers containing unbranched ether radicals.

4. The duration of the lateral position in most cases is parallel to the ability of the preparations to reduce the toxic action of Corazol (prevention of death or increase in the life span of the animals).

Comparison of the toxicity and the duration of the lateral position of the animals shows that the maximum discrepancy between these quantities is exhibited by 1-ethoxy-1-oxo-3-methylphosphacyclopentene-3 (II), which we selected for further, more detailed study. Of particular interest was the investigation of the anticholinesterase activity of this compound, which showed the greatest selectivity of action. Experiments carried out by Hestrin’s method showed that, to suppress enzyme activity in vitro by 50% (\(Y_{50}\)), very high concentrations of the preparation are required: for cholinesterase of rabbit brain, \(2.50 \cdot 10^{-1}\) mol/l; for cholinesterase of rabbit blood serum, \(1.87 \cdot 10^{-1}\) mol/l.

Table 2

* These data were published earlier (14); they are given here for comparison.

Table 3

Table 3 presents the results of experiments investigating the influence of compound II on the action of absolutely lethal doses of certain analeptics and cholinomimetic agents. All these substances, in toxic doses, cause severe convulsions as a result of excitation of various parts of the central nervous system. An aqueous solution of the preparation was administered to mice subcutaneously 15 min before the subcutaneous injection of the convulsant poisons. From the data in Table 3 it is evident:

1. Preliminary administration of 1-ethoxy-1-oxo-3-methylphosphacyclopentene-3 completely prevents the death of animals upon administration of absolutely lethal doses of corazole, nibufin, strychnine, and picrotoxin, and protects from death 70% of animals poisoned with cordiamine.

2. The average lifespan of mice that received absolutely lethal doses of cordiamine, caffeine, nicotine, armin, and chlorophos, against the background of preparation II, increases 3–8-fold.

1. The preparation completely eliminates or significantly weakens convulsions caused by the administration of all the substances studied, except caffeine.

2. Prophylactic administration of preparation II prevents the salivation that occurs upon administration of lethal doses of corazol, cordiamine, caffeine, nicotine, nibufin, and chlorophos, but does not affect the appearance of this symptom in poisoning with picrotoxin, eserine, and armin.

The results obtained indicate that the compounds studied exhibit a specific depressant action on the central nervous system of animals, not associated with inactivation of cholinesterase.

Institute of Organic Chemistry
Academy of Sciences of the USSR
Kazan

Kazan State
Medical Institute

Received
8 V 1965

LITERATURE CITED

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2. M. Smith, E. Emmott, B. Wesfoll, J. Pharmacol. and Exp. Therap., 74, 163 (1942).
3. S. M. Vyaseleva, O. A. Ignat’eva et al., Proceedings of the Second Conference, Chemistry and Application of Organophosphorus Compounds, Publishing House of the Academy of Sciences of the USSR, 1962, p. 532.
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10. I. V. Pusenkova-Berezovskaya, Proceedings of the Kazan State Medical Institute, Kazan, 14, 265 (1964).
11. I. A. Frankov, Proceedings of the First Conference, Chemistry and Application of Organophosphorus Compounds, Publishing House of the Academy of Sciences of the USSR, 1957, p. 370.
12. I. A. Frankov, Proceedings of the Second Conference, Chemistry and Application of Organophosphorus Compounds, Publishing House of the Academy of Sciences of the USSR, 1962, p. 452.
13. M. Vandecar, Nature, 179, No. 4551, 154 (1957).
14. A. O. Vizel’, M. A. Zvereva et al., DAN, 160, No. 4, 826 (1965).
15. B. A. Arbuzov, A. O. Vizel’, DAN, 158, No. 5, 1105 (1964).