Quantitative determination of Selinexor concentrations in plasma samples from children with non-rhabdomyosarcoma soft-tissue sarcomas: Troubleshooting plasma instability issues

Selinexor (KPT-330), a first-in-class, CNS-penetrant oral inhibitor of Exportin-1, disrupts the nuclear export of tumor suppressor proteins, promoting their accumulation and inducing cancer cell death. In this study, a reliable and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) was developed and validated to quantify selinexor concentrations in human plasma. A standard solid-phase extraction method using an Oasis HLB μElution plate was utilized to isolate selinexor and its internal standard, selinexor-d₃, from human plasma. The chromatographic separation was executed on a reversed-phase analytical column with a binary gradient of water and acetonitrile, both containing 0.1 % formic acid, at a flow rate of 0.5 mL/min. Mass spectrometry detection was performed in positive ion mode by tracking the mass transitions of 444.0 > 334.0 for selinexor and 447.0 > 333.9 for selinexor-d₃. The developed LC-MS/MS assay for selinexor was rigorously validated over a wide range of clinically relevant concentrations (1–1000 ng/mL, r² ≥ 0.99) in accordance with FDA bioanalytical method validation guidelines. The method exhibited inter-day accuracy, expressed as relative error (R.E.), ranging from 2.28 % to 4.38 %, with precision values not exceeding 5.92 %. Intra-day accuracy showed R.E. values between 0.24 % and 7.30 %, accompanied by precision values ≤4.81 %. Additionally, the method demonstrated high extraction recovery, ranging from 82.80 % to 87.87 %, and a negligible matrix effect. The pH adjustments applied to the plasma prior to storage and processing maintained the stability of selinexor under several experimental conditions, including multiple freeze-thaw cycles and long-term storage at −80 °C. As proof of principle, the LC-MS/MS assay was successfully applied to a phase I clinical pharmacokinetic study of selinexor in pediatric patients with non-rhabdomyosarcoma soft tissue sarcomas, yielding reliable and reproducible measurements of selinexor concentrations in plasma.