Bioanalysis to support PK-PD studies and clinical trials can be challenging especially when quantifying highly potent compounds. In this study, quantitation attributes were determined for a set of drug-like compounds in human plasma using microflow LC and high resolution mass spectrometry (HRMS) with ionKey/MS and the Xevo G2-XS Time of Flight (Tof) mass spectrometer. The iKey is a micro-flow separation device which is compatible with Waters Tandem and QTof platforms. The Xevo-G2XS QTof is the latest QTof instrument with enhanced resolution and sensitivity compared to its predecessor. The combined use of both technologies yields an extremely sensitive LC/HRMS instrument. Addition of a trap-and-elute configuration enables the system to handle analytical scale sample volume injection while maintaining excellent peak shape, which further enhances the system's sensitivity.
Test compounds, including buspirone, propranolol, verapamil, and clopidogrel were prepared in human plasma. The LC/MS system used was the AQUITY M-Class UPLC system, coupled with the Xevo G2-XS QTof mass spectrometer and the ionKey/MS source. The iKey tile used was the HSS T3 1.8 u 150 um x 50 mm at 45°C. The trap column used was the HSS T3 1.8 um 300 um x 50 mm. A generic, linear gradient from 5-60 B% in 3.5 minutes at flow rate of 3 uL/min was used (mobile phase A was water + 0.1% formic acid; mobile phase B was acetonitrile + 0.1% formic acid (v/v)).
Serially diluted samples of test compounds in human plasma were quantified via direct injection onto the LC/MS system using Tof-MRM mode of acquisition. Results showed excellent linearity ranging from log = 3.6 to 4.2 for verapamil and clopidogrel, respectively. LLOQs ranged from 0.8 fg to 3.0 fg on column. The signal/noise ratio at LLOQ ranged from 9 to 79. These attributes suggest the system is well-suited to meet the needs of routine bioanalysis. In the second set of experi-ments, a trap valve manager was installed, and the system was config-ured for trap-and-elution. Increasing injection volumes from 1 to 5, 10, and 20 uL showed excellent peak shape and peak resolution. A linear response with R2 = 0.9987 was observed, indicating complete sample recovery using the trapping column. For a 20 uL injection of samples containing 20% acetonitrile, which is equivalent to a 4 mL injection at analytical scale using a 50 x 2.1 mm column, polar compounds such as buspirone and propranolol were well-retained and showed no peak distortion. These data suggest the microflow LC/MS system is well-suited to support routine bioanalytical sensitivity requirements. Additional benefits for using the system include ease-of-use and a 90% reduction in solvent usage compared with analytical LC.
Novel aspects of this paper include the use of microflow LC/HRMS and dual pump trap-and-elute for high sensitivity small molecule quantitation in human plasma at analytical scale injection volumes.