Bioanalytical Method Development and Validation of Letrozole by LC-ESI-MS/MS in Human Plasma

Letrozole (CAS Number-112809-51-5) is widely used as an oral non-steroidal aromatase inhibitor for the treatment of hormonally-responsive breast cancer after surgery. For quantitation of letrozole, negative polarity was used to achieve adequate response because this was highly sensitive for compounds with high electron affinity. It was applied to fragment the analytes and to obtain intense and consistent product ions. The deprotonated precursor ions [M-H]−at m/z 284.1 was observed in Q1(MS) for letrozole. Characteristic product ions found in Q3(MS) was at m/z 242.0, 102.1, 126.7, 140.7, 215.1, 240.3 for letrozole. Most stable and consistent fragment ion selected m/z 242.0 having the triazole ring. For the internal standard (tolbutamide) the fragment at Q1 and Q3 having m/z 269.1 and 170.1 was responsible for possessing the methoxyethyl-phenoxy group. In the present study LC-MS/MS-ESI method was attempted to develop for detection and quantification of letrozole in human plasma. Therefore, it can be concluded from the present study findings that developed method by using LC-MS/MS API 2000 and liquid-liquid extraction technique for letrozole in human plasma was found to be simple, specific, sensitive, reproducible and fully validated in according to EMA guidelines. The present method was found superior enough to be applied to comparative pharmacokinetic studies in future.