Tamsulosin (-)-(R)-5-{2-[[2-(O-ethoxyphenoxy)ethyl]amino]propyl}-2-methoxybenzene-
sulfonamide - is a selective α _1A adrenoreceptor antagonists, which is used clinically as an oral medication to ameliorate the dysuria associated with prostatic hypertrophy [1, 2]. The aim of the study was to adapt and validate HPLC-MS method for determination of tamsulosin in human plasma.

Detection was performed on a quadrupole mass spectrometer LCMS-2010 (Shimadzu). The electrospray ionization (ESI) with capillary voltage at 4.9 kV, nitrogen flow rate at 4.5 L/min, CDL (Curved Desolvation Line) voltage at 40 V, temperature of CDL and block set at 190 °C and 260 °C, respectively, were used. Target ions were monitored in positive mode at m/z 409.05 for tamsulosin [M+H]⁺ and at m/z 573.20 for I.S. [M+H]⁺.

Sample preparation was based on liquid-liquid extraction with hexane-ethyl acetate (1:1) after addition of saturated sodium bicarbonate solution to 0.5 mL of plasma. Organic layer was evaporated and reconstituted in mobile phase.

The separation from endogenous compounds was performed on a Nucleosil C18, 5 m m (125 mm x 4.0 mm i.d.) column with oven temperature at 40 °C. Mobile phase consisted of methanol and 0.05 M ammonium acetate buffer pH 3.7 (6:4, v/v, flow rate at 0.5 mL/min). Internal standard (IS) was (R)-5-{2-[Bis[2-(2-ethoxyphenoxy)ethyl]amino]propyl}- 2-methoxybenzenesulfonamide.

The limits of detection and quantification were determined at 0.1 ng/mL and 0.7 ng/mL, respectively. The calibration curves were obtained by weighted (1/x) linear regression analysis and were linear up to 35.0 ng/mL.

Validation parameters calculation was based on assaying three different concentrations of tamsulosin (2.1, 14.0 and 28.0 ng/mL). The recovery of both tamsulosin and I.S. was ca. 70-80%. No matrix effect influencing selectivity, sensitivity and precision of the method was observed. Accuracy during the day and within three days ranged 91-103% and 98-108%, respectively. Precision of the method within one day and within three days was determined at 2.1-3.1% and 2.4-5.9%, respectively.

Stability of stock and working solutions of tamsulosin and I.S. was confirmed. No influence of storing plasma samples neither for 4 months at -20 °C, nor for 4 hours at room temperature, on stability of tamsulosin was observed. Three freeze-thaw cycles did not effect quantitation of tamsulosin. Obtained results confirmed stability of processed samples in autosampler (18 hrs).

The method was validated according to regulatory authority requirements (EMEA, FDA) and was subsequently applied to bioavailability studies of 0.4 mg tamsulosin modified release capsules.

References:

[1] M. I. Wilde, D. McTavish, Drugs 52 (1996) 883.

[2] K. Koiso et al., J. Clin. Pharmacol. 36 (1996) 1029.

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