SOLID-PHASE EXTRACTION AND HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC DETERMINATION OF TAMOXIFEN AND ITS MAJOR METABOLITES IN PLASMA

Tamoxifen (TAM) is a triphenylethylene anti-oestrogen, commonly used i n the treatment of breast cancer. Patients receiving tamoxifen therapy may experience both de novo and acquired resistance. As one of the me chanisms for this may be extensive peripheral bio-transformation of ta moxifen, there has been considerable interest in the pharmacokinetics and metabolism of tamoxifen. A reversed-phase high-performance liquid chromatography separation has been developed to determine the levels o f tamoxifen and its major metabolites in human plasma. The method is h ighly sensitive (2 ng/ml) and selective for tamoxifen, cis-tamoxifen ( CIS), 4-hydroxytamoxifen (4-OH) and desmethyltamoxifen (DMT). A mu Bon dapak C-18 10 mu m column (30 cm x 3.9 mm I.D.) was used, with a mobil e phase of methanol-1% triethylamine at pH 8 (89:11, v/v). Sample prep aration was carried out using a C-2 (500 mg sorbent, 3 ml reservoirs) solid phase extraction method, and extraction efficiencies were approx imately 60% for TAM and its metabolites. Accuracy and precision, as de termined by spiking plasma samples with a mixture of tamoxifen and its metabolites, ranged from 85-110% (+/- 5-10%) at 1 mu g/ml, 101-118% ( +/- 8-20%) at 0.1 mu g/ml and 111-168% (+/- 43-63%) at 0.01 mu g/ml. R esults from 59 patients show mean values of 54 ng/ml for 4-OH; 190 ng/ ml for DMT; 93 ng/ml for TAM and 30 ng/ml for CIS (detected in three p atients only). This methodology can be applied routinely to the determ ination of TAM and its metabolites in plasma from patients undergoing therapy.