Quantitative determination of the atypical antipsychotic ziprasidone in human plasma by means of HPLC with fluorescence detection for therapeutic drug monitoring purposes

Ziprasidone (5-[2-[4-(1,2-benzisothiazol-3-yl)-1-piperazinyl]ethyl]-6-chloro-1,3-dihydro-2H-indol-2-one) is one of the most recent atypical antipsychotics introduced onto the market. It probably is the most selective antipsychotic to date, with respect to affinity for 5-HT2A receptors as compared to that for D2 and 5-HT2C receptors; its 5-HT2A selectivity is even greater than that of risperidone. The main clinical advantage of ziprasidone with respect to other atypical antipsychotics is its low incidence of metabolic side effects. However, the drug can still cause several undesired effects, the most important of whom are QTc prolongation, sedation and orthostatic hypotension. Thus, accurate therapeutic drug monitoring (TDM) of patients is clearly an advantage during therapy with this drug. In fact, TDM can contribute to the individuation of chemical-clinical relationships: for example, the correlations between daily doses and plasma levels, or between plasma levels and therapeutic and toxic effects. For ziprasidone in particular, TDM could be useful to prevent side effects related to QTc prolongation, which are probably the most important problems associated with therapy with this drug. Thus, aim of this study is the development of an original analytical method for the determination of ziprasidone in psychotic patients’ plasma, based on HPLC with spectrofluorimetric detection. Since the compound is natively fluorescent, this kind of detection confers high sensitivity and selectivity to the method without the need for expensive and time-consuming derivatisation procedures. The analysis is carried out on a Zorbax C8 reversed phase column, using a pH 6.5 phosphate buffer / acetonitrile mixture as the mobile phase. Citalopram was chosen as the internal standard (IS) since it has physical-chemical characteristics and a retention time similar to those of ziprasidone. Under the reported experimental conditions, a chromatographic run lasts about 6 minutes. An original sample pre-treatment procedure, based on solid phase extraction (SPE), has been developed for this assay. The procedure uses hydrophilic-lipophilic balance (HLB, 30 mg, 1 mL) cartridges and 200 µL of plasma are sufficient for a complete analysis. Particular attention has been paid to the development of the washing and elution steps, in order to obtain a good purification from endogenous and exogenous matrix components, while retaining high extraction yields of the analyte and the IS. The resulting SPE procedure is fast and feasible. Method validation is currently under way; the results obtained until now are satisfactory in terms of precision (R.S.D. < 6%), extraction yield (> 90%) and selectivity (no interference form the biological matrix). Moreover, assays are in progress in order to apply the method to samples of other biological fluids, such as saliva. In this way, the TDM of patients undergoing therapy with ziprasidone could provide more complete and meaningful information regarding chemical-clinical correlations.