738 



6. ARSENICALS 



phases: (1) fixation of tlie arsenical, (2) secondary chemical reactions due 

 to the fixation, and (3) the death process within the cells. The fixation oc- 

 curs rapidly with the potent arsenicals, being complete within a few min- 

 utes, and is reversible. Neuschlosz (1919) used the same equation to describe 

 the action of arsenite on paramecia, and determined the following values: 

 Co = 1 milf, n = 1.38, and k = 1.21. 



A lag period for the pentavalent arsenicals is also observed in vivo. 

 This and the reason are well illustrated by Murgatroyd et al. (1934), who 

 injected tryparsamide and tryparsamide oxide intravenously into rabbits 

 and determined the trypanocidal activity in the serum after various times 



24 



HOURS 



Fig. 6-9. Trypanocidal activity of rabbit serum following intra- 

 venous injection of arsenicals. (From Murgatroyd et al., 1934.) 



(Fig. 6-9). The oxide is immediately active whereas tryparsamide exerts 

 a maximal activity only after 6-8 hr indicating that tryparsamide must 

 be reduced before it is effective. When animals with trypanosomiasis are 

 treated with oxophenarsine, the trypanosomes rapidly disappear from the 

 blood; e. g., 5 mg/kg intravenously brings about disappearance in 14-15 min 

 (Pfeiffer and Tatum, 1935). That this is not entirely due to the killing and 

 lysis of the trypanosomes w^as shown by removing a sample of the blood 

 5 min after the administration of the oxophenarsine, in which the organisms 

 lived for 70-90 min. The disappearance of the cells from the blood stream 

 in vivo must be in part due to the participation of the defense mechanisms of 

 the host, particularly the reticuloendothelial system. 



