Simultaneous monitoring of pazopanib and its metabolites by UPLC-MS/MS.

Pazopanib is a multi-targeted tyrosine kinase inhibitor (TKI) approved as first-line treatment for patients with advanced renal cell carcinoma (RCC) and as second-line treatment for patients with advanced soft tissue sarcoma (STS) previously treated with chemotherapy. The most common adverse events, observed during the RCC and STS trials, were gastrointestinal disorders, hypertension, fatigue, elevated ALAT and ASAT, but the molecular mechanisms explaining pazopanib toxicity remain unclear. Therapeutic activity is considered to be mainly dependent on pazopanib exposure as the primary metabolites are inactive or display low plasma concentrations, but metabolites may be involved in toxicity as relationships between metabolite profiles and toxicity have not been evaluated. We report here, for the first time, the validation of a method for the simultaneous quantification of pazopanib and semi-quantification of its metabolites (relative determination). As there are no standards available, pazopanib metabolites were generated with human liver microsomes (HLM) to provide controls in the development of an UPLC-MS/MS method for monitoring both pazopanib and metabolites. The optimised method was validated for specificity, linearity, sensitivity, precision, accuracy, matrix effect and stability. The coefficient of variation (CV%) for intra-day and inter-day precision varied from 2.1% to 7.9% and 5.6% to 13.1% respectively. The biases varied from -12% to 2.3% (intra-day) and 3.8% to 13.1% (inter-day) for accuracy evaluation. Intra-day and inter-day precision CV were respectively 20.1% and 19.6% and accuracy biases were between 20.7% (intra-day) and 3.8% (inter-day) at the limit of quantification. The recoveries from matrix samples spiked with pazopanib were respectively 102.6 ± 12.9% and 102.5 ± 1.2% at low and high levels of calibration range. No matrix effect was evidenced as demonstrated by the normalised matrix factor values: 1.3 ± 0.1 and 1.2 ± 0.2 respectively measured at low and high part of calibration range. A good stability of pazopanib was observed during short term, long term and in process storage conditions and after three freeze/thaw cycles. The method was applied to clinical samples from three patients treated with pazopanib to establish the metabolite profiles (semi-quantitative data) during treatment. The assessment of metabolite profiles could be useful to improve our understanding of the occurrence of adverse events and to improve pazopanib pharmacokinetic-pharmacodynamic relationships.