The effort to achieve therapeutically effective concentrations as well as the control
of side effects call for therapeutic drug monitoring. By now, 32 different antipsychotics
are marketed in Germany. TDM laboratories need analytical methods and allow the quantitative
determination of as many substances as possible with identical settings. We present
such a method that allows the analysis of olanzapine, clozapine, risperidone and its
main metabolite 9-hydroxyrisperidone, haloperidol and ziprasidone. As internal standard
methylrisperidone is used.
The six compounds are isolated from human serum by solid phase extraction, using Oasis
HLB® cartridges 2 cc (60mg) (Waters®).
2ml of the sample is loaded on the preconditioned solid-phase-extraction-column and
eluted after several washing steps, using alkalised methanol. This precleaning of
the samples takes about 90 minutes, including the evaporation step.
HPLC separation is performed at a flow-rate of 0,5ml/min on a luna® phenyl-hexyl column
(250×3,0mm), (Phenomenex, Aschaffenburg) with Acetonitril/Methanol/HK2O4P (10:35:55, V/V, pH 9) as the mobile phase.
Typical retention times are 9.1min. for 9-hydroxyrisperidone, 9.9min. for olanzapine,
13.2min. for risperidone, about 14.2min. for haloperidol, 15.4min. for clozapine,
18.5min. for methylrisperidone and about 20min. for ziprasidone.
The separation using HPLC takes about 22 minutes.
The compounds are detected on a digital electrochemical amperometric detector (Decade®,
Antec/Leyden) at a potential of 0,8V.
The lower limit of quantification is set at 1.5 ng/ml for risperidone, its 9-hydroxy-metabolite
and haloperidol and below 10ng/ml for olanzapine, clozapine and ziprasidone.
The above method shows sufficient sensitivity to determine the prescribed substances
at serum concentrations below 10ng/ml, in combination with the use of 2ml serum for
the analysis. The maintenance of the electrochemical flowcell is quite simple but
has to be performed continuosly.
