LC–MS/MS measurements of serum zoledronate in children and young people receiving treatment – findings from the Moving Towards Individualised BisphosphonatE Therapy (TIBET) study

Background: Zoledronate is a nitrogen-containing bisphosphonate (BP) recognised for its antiresorptive potency. The pharmacokinetic/pharmacodynamic of zoledronate remains unclear. Although adverse events from overuse of BP is rare, there have been reports of over-treatment resulting in pathophysiological consequences. The current administration regimen is not tailored to the individual’s therapeutic response to zoledronate. Quantifying post-dose serum concentrations could improve drug monitoring and reduce over-treatment.

Method: A high-throughput liquid chromatography tandem mass spectrometry (LC–MS/MS) method for the quantification of zoledronate in human serum was developed, validated, and applied to samples collected for the TIBET study (IRAS:292768). The study recruited pediatric patients who had received zoledronate doses ranging between 0.02–0.05 mg/kg 4–6 months ago. Serum samples (n=16) were obtained with parental consent and stored at −20 °C until analysis. Participants age ranged between 3–17 yrs consisted of 9 males and 7 females with various diagnoses including OI and Duchenne muscular dystrophy. Sample analysis was performed using a triple quadrupole mass spectrometer system (Sciex API4000, Macclesfield, UK). Solid phase extraction using 96-wells weak anion exchange (WAX) plates were processed using an automated platform (Biotage Extrahera, Sweden) to remove sample matrix components.

Results: The zoledronate assay achieved adequate linearity across the range of 35–900 nmol/L and showed intra/inter-assay precision(CV%) of <9.0% and <12.3%. The lower limit of quantification (LLoQ) was 35.0 nmol/L and recovery was 99.3%. Zoledronate was detected in all 16 patient samples; three samples had quantifiable concentrations above the LLoQ between 67.8 to 114.4 nmol/L. The highest zoledronate concentration was found in a sample collected 4-months post infusion compared to an average of 6-months. Our findings showed residual amounts of zoledronate were present in circulation long after the expectant half-life, indicating variable responses in absorption into the bone and that differences in excretion may result in zoledronate remaining in the systemic circulation.

Conclusion: We have shown that our LC–MS/MS method can be used to determine zoledronate concentrations in patients post intravenous infusion. We report variable concentrations of zoledronate likely due to individual differences in absorption/excretion. A better understanding of zolendronate pharmacokinetics could enable personalised treatment plans, reducing the risk of under/over-treatment.