Therapeutic monitoring of chlorpromazine II: Pitfalls in whole blood analysis.

G McKay, J K Cooper, E M Hawes, J W Hubbard, M Martin, K K Midha

Index: Ther. Drug Monit. 7(4) , 472-7, (1985)

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Abstract

Pooled whole blood from healthy volunteers was spiked with pure, synthetic chlorpromazine, chlorpromazine sulfoxide, or chlorpromazine N-oxide and then made alkaline with either sodium hydroxide or sodium carbonate. The addition of alkali causes lysis of red cells, the contents of which spill into the plasma. The lysed samples were allowed to stand at room temperature for various timed intervals before extraction with organic solvents and analysis by high performance liquid chromatography. It was found that a portion (10-14%) of the chlorpromazine spike was oxidised to chlorpromazine sulfoxide, whether the blood was made alkaline with sodium hydroxide or sodium carbonate. Chlorpromazine N-oxide added to whole blood was entirely destroyed in the presence of alkali. The chlorpromazine N-oxide was rapidly reduced to chlorpromazine, a portion of which subsequently underwent oxidation to chlorpromazine sulfoxide. We have found that chlorpromazine N-oxide resides almost entirely in the plasma with only a small portion (less than 4%) distributed into the red cells. Hence, it is essential that red cells and plasma be separated before analysis. Chlorpromazine and chlorpromazine N-oxide can then be extracted from plasma by a method that does not lead to reduction of chlorpromazine N-oxide. Alkaline extraction methods must be avoided in the analysis of chlorpromazine in the red cell fraction.