Significance of the chemotherapeutic index in the treatment of schistosomiasis with antimony compounds.

No tes Significance of the Chemotherapeutic Index in the Treatment of Schistosomiasis with Antimony Compounds * by NICOLo ERCOLI 1 The discovery of the decreased toxicity and of the increased therapeutic index of antimony bound to pyrocatechol disulfonates (Uhlenhuth, Kuhn & Schmidt, 1924, 1925) led to many hopes for improvements in the treatment of schistosomiasis and for the development of less toxic antimonial compounds. Yet, we are still faced with the problem of explaining why modem antimonial compounds such as antimony sodium gluconate, stibocaptate, and stibophen, which give a high therapeutic index in experimental work, do not show a definite superiority over antimony potassium tartrate (tartar emetic). The purpose of the present study was to establish the toxicity of trivalent antimony and variations in activity, using different antimonial compounds, and to correlate the findings with clinical results and tolerance studies. This problem held a special interest since, by preparing a chelate of the antimony tartrates with 3-mercaptovaline, the cytotoxicity has been eliminated and general toxicity has been decreased, while activity in animals (Ercoli, 1965, 1967, 1968) and man (Ron Pedrique et al., 1967; Ron Pedrique & Ercoli, 1969; Ron Pedrique, Barbera, Ercoli, 1970) has been maintained. The meaning of the therapeutic index from the standpoint of clinical utility is analysed here in the light of those results. Materials and methods Antimony sodium tartrate (39.4% Sb), antimony potassium tartrate (36.6% Sb), stibophen (16% Sb), stibocaptate (25 % Sb), and a chelate (NAP) of sodium antimony tartrate and 3-mercaptovaline (penicillamine) (14.5 % Sb) were the drugs included in this study. The subcutaneous LD50 in albino mice weighing 20-25 g was determined by injecting aqueous solutions of the drug (0.5 ml/20 g of body * The substance of this paper was presented at the 4th Latin-American Congress of Zoology held in Caracas, Venezuela, in November 1968. 1 Institute of Tropical Zoology, Central University of Venezuela, Caracas, Venezuela. 2734A weight) into the mice. In the same experiment, different drugs were tested in 35-45 mice; the mortality results were added and used to calculate the LD50 values shown in Table 1. Effectiveness in infections of Trypanosoma equiperdum was determined in mice infected 2 days previously and showing in blood samples an average of 5 parasites per microscopic field at a magnification of x 400. The animals were observed 3 hours after treatment, then daily for relapses; if samples remained clear for 23 days, the mice were reinoculated; positive reinfection was considered to indicate a cure. The minimum dose of drug required to cause temporary disappearance of the parasites from the blood was also determined but, for comparisons of activity, the average dose effecting a permanent cure (CD50) was used. In Schistosoma mansoni infections, the dose required to cause migration of the parasites from the mesenteric and portal veins to the liver (" shift ") was determined in mice infected percutaneously 60 days previously with 160 cercariae. The mice were treated daily by subcutaneous inoculation and they were autopsied 3 days after the last (sixth) treatment. The parasites were extracted by means of the perfusion technique of Yolles et al. (1947). A total of 220 mice were used in 9 experiments. For in vitro studies the worms collected from the portal vein of mice infected 56-70 days previously were suspended in Seitz sterilized sheep serum. The drug was dissolved in saline and added to 20 parts of serum containing the parasites to the concentration desired. Untreated worms remained motile for 3 days. Cercariae were exposed in aerated distilled water to the drugs and observed for shape and motility at intervals of 1/4 hour. Results The in vitro studies revealed that the potassium and sodium antimony tartrates, NAP, and stibocaptate, when compared in terms of trivalent antimony content, have a similar parasiticidal effect on the adult and larval forms of S. mansoni. Correlations were noted between the time elapsing before the death -371 8 372 NOTES Table 1. Toxicity (LD5o) and curative activity of antimonial compounds in T. equiperdum infections of mice by single subcutaneous inoculations (CD5o) Toxicity Drug No. of mice used Curative activity LD5o (mg/kg) Trivalent Drug antimony No. of mice used CD5o (mg/kg) Trivalent Drug antimony antimony sodium tartrate antimony potassium tartrate 105 48 19 80 27 11 95 55 20 60 30 11 270 390 57 80 96 14 stibocaptate 80 2 000 500 40 500 125 stibophen 80 670 110 40 230 37 NAP of the parasites and the concentration of antimony employed. Exposure of the worms to 18-25 jug of antimony per ml as NAP or antimony tartrates reduced the survival period to 8-10 hours, and larger concentrations of antimony (72 jug/m) to 5 hours. The effect of stibocaptate in low concentrations (18-25 ,tg of antimony/ml) was also evident, although data for precise comparison with the other antimonial compounds were lacking. Stibophen did not show any noticeable effect in concentrations of 25-50 ,ug of antimony/ml. This lack of activity was also seen in tests on larvae; while 30,ug of antimony/ml administered as antimony sodium tartrate, NAP, or stibocaptate immobilized all the cercariae within 2 hours, 150kug of antimony/ml as stibophen affected only 15% of the lavae after 22 hours. The lethal (LD50) and the curative (CD50) doses in T. equiperdum infections by single administration are given in Table 1. These results permit an accurate evaluation of the chemotherapeutic index (LD50/CD50) to be made. In the hepatic shift of S. mansoni there was some variation between experiments, and the values obtained are approximate within a range of 20%. The values for the single dose, repeated for 6 consecutive days, are as follows: 8 mg of antimony/kg for both potassium and sodium antimony tartrate, 10 mg of antimony/kg for NAP, 30 mg of antimony/ kg for stibophen, and 80 mg of antimony/kg for stibocaptate. In the antischistosome tests, the activity has been established by repeated administration, which for some drugs leads to cumulative toxicity, and the toxicity by single injections; therefore, we do not obtain a real index, i.e., the results do not indicate the margin between the toxic and therapeutic doses but provide a formula representing this ratio in an indirect way. The indices obtained for the two infections are given in Table 2. Discussion and conclusions The considerable increase of the therapeutic index of NAP, stibocaptate, and stibophen over that of sodium and potassium antimony tartrates is evident. However, from an examination of the absolute dose variation of trivalent antimony, it appears that the increase has been obtained in different ways and that the antimonial compounds can be divided into three groups, as follows: (1) those that are effective and toxic in small doses, which (...truncated)