A High Sensitivity UPLC-MS/MS Method for the Analysis of Clopidogrel and Clopidogrel Carboxylic Acid Metabolite in Human K₂EDTA Plasma

In this application note, we present a high-sensitivity method for the analysis of clopidogrel and clopidogrel carboxylic acid metabolite from human plasma using UPLC-MS/MS.

Benefits

A high-sensitivity method for the analysis of clopidogrel and its carboxylic metabolite was developed using solid-phase extraction along with UPLC-MS/MS that achieved an LLOQ of 1 pg/mL in human plasma.

Clopidogrel (trade name Plavix, Figure 1), is a thienopyridine derivative antiplatelet prodrug used in the prevention of artherosclerotic events. It is dosed in an inactive form, and requires a hepatic biotransformation to yield the active thiol-metabolite which binds to cell receptor P2Y12, irreversibly inhibiting the platelet activation process.¹

In addition to the active metabolite, an inactive carboxylic acid metabolite is also formed. This acid metabolite accounts for the majority of circulating clopidogrel related material with very low levels of the active metabolite and unchanged prodrug being present.^2,3 Due to the reactivity of the thiol metabolite, coupled with the low levels of the unchanged prodrug, most quantitative studies are based on the circulating levels of the inactive metabolite.

The ability to detect the low levels of unchanged prodrug will provide more accurate data on the pharmacokinetics of clopidogrel, allowing improved evaluation of the bioavailability of new formulations.

Most published methods for the analysis of clopidogrel/metabolite employ a liquid-liquid extraction (LLE) technique, often times requiring a double LLE prior to LC-MS.^3,4 This method is both time consuming and tedious and often requires large volumes of harmful chemicals, such as hexane and diethyl ether.

To address this issue the use of solid-phase μElution technology was investigated, with the aim to increase throughput while decreasing solvent consumption. A typical LLE will consume anywhere from 2 to 8 mL of organic solvent per sample, in contrast to μElution methods that require less than 0.5 mL of organic solvent. The use of μElution plates also allows the sample to be concentrated during the extraction process which facilitates lower detection limits.

In this application note, we present a high-sensitivity method for the analysis of clopidogrel and clopidogrel carboxylic acid metabolite from human plasma using UPLC-MS/MS.

Data integration and calculation software

Waters UNIFI Scientific Information System

Sample Preparation

Solid phase extraction

Plasma samples were extracted by diluting 350 μL of plasma sample, containing 10 μL of internal standard solution at a concentration of 250 pg/mL, with 350 μL of aqueous 2% formic acid. Samples were added to an Oasis HLB μElution plate, after pre-conditioning the plate with methanol (200 μL) followed by water (200 μL). Samples were drawn through under vacuum, and then washed with 2% formic acid (200 μL) and 5% methanol/water (200 μL). The sample was eluted with 2 x 25 μL of methanol, and then diluted with an equal volume of water prior to injection. Samples were prepared at the following concentrations: 1.00, 2.50, 5.00, 10.0, 25.0, 50.0, 100, 250, and 500 pg/mL.

The resulting chromatograms obtained for standards clopidogrel and clopidogrel carboxylic acid, as well as the internal standards d₄-clopidgrel and d₄-clopidogrel carboxylic acid are shown in Figure 2.

The metabolite and deuterated internal standard elute at 1.7 min, while clopidogrel and its internal standard elute at 2.8 min. The calibration curve was linear over the range of 1 to 500 pg/mL, with no carryover present (Figures 3 to 5).

The use of μElution technology along with a 1.0-mm I.D. column gave a lower limit of quantification (LLOQ) of 1 pg/mL for both clopidogrel and its metabolite. The use of microbore chromatography is known to give increases in sensitivity of up to four-fold over that obtained with a standard 2.1-mm column. Clopidogrel showed a signal-to-noise value of 7:1 while the metabolite showed signal-to-noise value of 5:1 (Figure 6). Quality control (QC) samples were injected in replicates of five at four different levels spanning the range of the calibration curve. All QCs met acceptance criteria of accuracy/precision ± 20% for the LLOQ and accuracy precision ± 15% for all QC levels (Table 1).

Another essential part of method reliability and robustness is the ability to separate the analyte(s) of interest from any background interferences. The most common interferences encountered in bioanalysis are phospholipids, which can be especially problematic for late eluting compounds, such as clopidogrel. To determine if phospholipids would coelute with the drug and metabolite, the generic 184 >184 transition was monitored (Figure 7). As can be seen from the chromatogram, clopidogrel elutes in between two major regions of interferences while the metabolite elutes over a minute before any of the phospholipids.

• A high-sensitivity method for the analysis of clopidogrel and its carboxylic metabolite was developed with an LLOQ of 1 pg/mL
  
• Quality Control (QC) samples meet required acceptance criteria put forth by the U.S. FDA
  
• Both clopidogrel and the carboxylic acid metabolite were successfully separated from possible coeluting phospholipids
  
• There was no detectable carryover present

1. J-M. Pereillo, M. Maftouh, A. Andrieu, M-F. Uzabiaga, O. Fedeli, P. Savi, M. Pascal, J-M. Herbert, J-P. Maffrand, C. Picard, Drug Metabolism and Disposition 30 (2002) 1288-1295.
   
2. A. Robinson, J. Hillis, C. Neal, A.C. Leary, Journal of Chromatogr. B 848 (2007) 344-354.
   
3. J-J. Zou, H-W Fan, D-Q Guo, Y-B. Li, S. Lin, Y-B Zhu, C-X. Yu, J. Zhou, J-H. Liu, Y-F Hu, Chromatographia 70 (2009) 1581-1586.
   
4. M. El Sadek, S. Moustafa, H. Kadi, A. Moneim, A. Al-Hakami, American Journal of Analytical Chemistry 2 (2011) 447-455.