Liquid chromatography–electrospray mass spectrometry determination of a bis-thiazolium compound with potent antimalarial activity and its neutral bioprecursor in human plasma, whole blood and red blood cells

Liquid chromatography–electrospray ionization mass spectrometry methods are described for the simultaneous quantification of a bis-thiazolium compound (T3), its related prodrug (TE3) and an intermediate compound (mTE3) that appeared during the prodrug/drug conversion process, in human plasma, whole blood and red blood cells (RBCs). The methods involve solid phase extraction (SPE) of the compounds and the internal standard (verapamil) from the three different matrices using Oasis®HLB columns with an elution solvent of 2 × 1 ml of acetonitrile containing 1 ml/l trifluoroacetic acid (TFA). HPLC separation was performed on a C18 encapped Xterra® column packed with 3.5 μm particles. The mobile phase used a 8 min gradient, from water containing 1 ml/l TFA to acetonitrile containing 1 ml/l TFA, at a flow rate of 400 μl/min. Verapamil and the TE3 compound were characterized by the protonated molecules at m/z 455 and m/z 541, respectively. The mTE3 species was detected through the (M)+ ion at m/z 497. The T3 compound was detected by use of two ions, the quaternary ammonium salt (M2+/2) at m/z 227.3 and by the adduct with TFA (M + TFA)+ at m/z 567.3. The drug/internal standard peak area ratios were linked via a quadratic relationship to plasma (or whole blood) concentrations in the tested range of 6.4–1282 μg/l (12.8–2564 μg/kg) for T3, 20–2000 μg/l (40–4000 μg/kg) for mTE3 and 10–2000 μg/l (40–4000 μg/kg) for TE3, and to T3 concentrations in RBCs ranging from 12.8 to 2564 μg/kg. Inter-assay precision (in terms of R.S.D.) was below 13.5% and accuracy ranged from 95.4 to 107%. The dilution of the samples (plasma or whole blood) has no influence on the performance of the methods. The extraction recoveries averaged 87% for T3, 53% for mTE3 and 79% for TE3 in plasma; 79% for T3, 57% for mTE3 and 65% for TE3 in blood; and 93% for T3 in RBCs, and was constant across the calibration range. The lower limits of quantitation were 6.4 μg/l for T3, 20 μg/l for mTE3 and 10 μg/l for TE3 in plasma; 12.8 μg/kg for T3 and 40 μg/kg for mTE3 and TE3 in blood; and 12.8 μg/kg for T3 in RBCs. Stability tests under various conditions were also investigated. The three-step SPE procedure (loading, clean-up, and elution) described in this paper to quantify these new anti-malarial compounds in plasma, whole blood and RBCs, can easily be automated by using either robotisation or an automated sample preparation system.