Significance of higher drug concentration in erythrocytes of mice infected with Schistosoma japonicum and treated orally with mefloquine at single doses.

The purpose of the present study is to understand the pharmacokinetic feature of mefloquine measured by erythrocytes and plasma in Schistosoma japonicum (S. j.)-infected mice and non-infected mice after oral administration of the drug at single doses. A high-performance liquid chromatography (HPLC) method was used to measure the plasma and erythrocyte concentrations of mefloquine at varying intervals posttreatment. Our results demonstrated that in non-infected mice treated orally with mefloquine at an ineffective dose of 50 mg/kg or effective dose of 200 mg/kg for 2-72 h, the erythrocyte-to-plasma ratios of mefloquine were 5.8-11.2 or 2-14.2. On the other hand, in S. j.-infected mice treated with the same single doses of the drug, the erythrocyte and plasma drug concentration ratios were 3.1-4.6 or 2.9-8.5, manifesting that either in infected mice or in non-infected mice that received oral mefloquine resulted in higher concentration of mefloquine in erythrocytes than that in plasma. Unexpectedly, under oral administration of mefloquine at a higher single dose of 200 mg/kg, the pharmacokinetic parameter C max values for plasma from S. j.-infected and non-infected mice were 1.6 ± 0.3 and 2.0 ± 0.4 μg/mL, respectively, which were below the determined in vitro LC50 (50 % lethal concentration) value of 4.93 μg/mL. Therefore, the plasma concentration of mefloquine may display a little effect against schistosomes during the treatment. Although the values of T 1/2 and AUC0-∞ for erythrocytes were significantly longer and higher in infected mice than those of corresponding non-infect mice that received the same single mefloqine dose of 50 mg/kg, the C max value was only 2.6 ± 0.4 μg/mL lower than the determined in vitro LC50, which may explain why this low single dose is ineffective against schistosomes in vivo. After administration of higher mefloquine dose of 200 mg/kg, the C max value for erythrocytes in infected mice was 30 % (7.4 ± 0.7 versus 10.7 ± 2.7 μg/mL) lower than that in the corresponding non-infected mice, but its level was above the determined in vitro LC95 (95 % lethal concentration) value of 6.12 μg/mL. Meanwhile, longer T 1/2 value of 159.2 ± 129.3 h in infected mice led to significant increase in AUC0-∞ value (1969.3 ± 1057.7 vs 486.4 ± 53.0 μg/mL·h), relative to corresponding non-infected mice. In addition, the mean residence time (MRT0-∞) in infected mice was also significantly longer than that in non-infected mice. All these results may beneficial for the treatment. According to the results, we suggest that higher ratios of mefloquine concentration in erythrocytes to plasma may offer a way to transport mefloquine to the worm gut through ingestion of erythrocytes by the worms, where the gut is the site for displaying the effect by mefloquine.