Determination of clozapine and norclozapine in dried plasma spot and dried blood spot by liquid chromatography-tandem mass spectrometry.

The use of alternative blood sampling strategies in clozapine (CLZ) therapeutic drug monitoring (TDM) aims to facilitate collection and improve drug therapy and adherence. This study aimed to develop and validate two methods for the determination CLZ and norclozapine (NOR) in dried blood spots (DBS) and dried plasma spots (DPS) by high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). The analytes were extracted from one 10 microliter volumetric DBS disc punch and from one 6 mm DPS disc punch with methyl tert-butyl ether: methanol (1:9, v/v) and injected into the HPLC-MS/MS with Atmospheric pressure chemical ionization (APCI) source. Separation was performed in a phenyl column, with mobile phase ammonium formate 1 mM pH 4.0 with methanol in gradient mode. The method was linear from 50 to 1500 ng/ml (r > 0.99), with accuracy between 98% and 105% in DBS and 91-101% in DPS, and intra- and inter-assay CV% from 5.23% to 9.35% in DBS and 2.22-11.36% in DPS for both analytes. The matrix effect was compensated by the internal standard, between - 5.1-6.89% in DBS and - 2.45-5.74% in DPS. The average extraction efficiency was 63-67% for CLZ and 58-69% for NOR with no significant impact of hematocrit (HCT). The analytes were stable in the dried matrices stored up to 42 °C for 26 days. The method was applied in the evaluation of clozapine therapy in 13 schizophrenic patients with mean serum levels of 401 ng/ml (43-914 ng/ml). Only 38% were within the therapeutic range, 46% below and 23% above. CLZ and NOR concentrations in dried samples were highly correlated to serum levels, with greater accuracy for DPS compared to DBS (97 versus 89%, and 99 versus 131%, for CLZ and NOR, respectively). Our data support the use of DBS and DPS as alternative sampling strategies in CLZ therapeutic drug monitoring, with satisfactory performance and logistics advantages.