The blood schizontocidal activity of lumefantrine, monodesbutyl-benflumetol (DBB) and a 999:1 combination of both compounds has been investigated in 26 fresh isolates of Plasmodium falciparum from northwestern Thailand, using the WHO standard protocol Mark II for determining the inhibition of schizont maturation. The geometric mean cut-off concentrations of schizont maturation (GMCOC) were 943.2 nM for lumefantrine, 146.3 nM for DBB and 182.2 nM for the 999:1 combination of lumefantrine and DBB. The EC(50) values were 27.3 nM for lumefantrine, 5.7 nM for DBB, and 16.5 nM for the combination, and the EC(90) values 163.1 nM for lumefantrine, 44.1 nM for DBB, and 78.3 nM for the combination. Despite the very low concentration in the combination, DBB exerted significant synergistic activity with lumefantrine that was strongest at the EC(90) and EC(99) levels. Correlation analysis indicates that DBB is the leading determinant for the activity of the combination.

The blood schizontocidal activity of monodesbutyl-benflumetol (DBB), retinol (RET) and combinations (DBB-RET) at retinol concentrations corresponding to the 50th, 65th and 80th percentile of physiological retinol concentrations in healthy adults has been investigated in Plasmodium falciparum. Parallel in vitro tests with DBB, RET and the 3 DBB-RET combinations were carried out with 26 fresh parasite isolates from northwestern Thailand, following the WHO standard protocol Mark II for determining the inhibition of schizont maturation. The EC(50) values for DBB, RET and for DBB in DBB-RET low, medium and high were 5.72 nM, 561.83 nM, 1.68 nM, 0.60 nM and 0.07 nM, respectively, the EC(90) values 44.14 nM, 9338.60 nM, 49.00 nM, 28.48 nM and 8.94 nM. The geometric mean cut-off concentrations of schizont maturation for DBB alone and for DBB in DBB-RET low, medium and high were 153.20 nM, 62.93nM, 34.00 nM and 13.74 nM, respectively, indicating significant synergistic interaction between DBB and retinol. The degree of synergism increases with the retinol concentration in the combination and is highest at the EC(99) level for DBB.