Use of dried blood spots for the determination of serum concentrations of tamoxifen and endoxifen.

Abstract

The anti-estrogenic effect of tamoxifen is suggested to be mainly attributable to its metabolite (Z)-endoxifen, and a minimum therapeutic threshold for (Z)-endoxifen in serum has been proposed. The objective of this research was to establish the relationship between dried blood spot (DBS) and serum concentrations of tamoxifen and (Z)-endoxifen to allow the use of DBS sampling, a simple and patient-friendly alternative to venous sampling, in clinical practice. Paired DBS and serum samples were obtained from 50 patients using tamoxifen and analyzed using HPLC-MS/MS. Serum concentrations were calculated from DBS concentrations using the formula calculated serum concentration=DBS concentration/([1-haematocrit (Hct)]+blood cell-to-serum ratio×Hct). The blood cell-to-serum ratio was determined ex vivo by incubating a batch of whole blood spiked with both analytes. The average Hct for female adults was imputed as a fixed value.Calculated and analyzed serum concentrations were compared using weighted Deming regression. Weighted Deming regression analysis comparing 44 matching pairs of DBS and serum samples showed a proportional bias for both analytes. Serum concentrations were calculated using [Tamoxifen] serum, calculated =[Tamoxifen] DBS /0.779 and [(Z)-Endoxifen] serum, calculated =[(Z)-Endoxifen] DBS /0.663. Calculated serum concentrations were within 20% of analyzed serum concentrations in 84 and 100% of patient samples for tamoxifen and (Z)-endoxifen, respectively.In conclusion, DBS concentrations of tamoxifen and (Z)-endoxifen were equal to serum concentrations after correction for Hct and blood cell-to-serum ratio. DBS sampling can be used in clinical practice.