Automated, Kit-Based Sample Preparation Strategy for LC-MS Quantification of Cetuximab in Rat Plasma

This application describes a fully automated kit-based approach using ProteinWorks Auto-eXpress Digest and μElution SPE Clean-up Kits, performed on the Hamilton Microlab  STAR  liquid handler (STAR) for the accurate quantification of the monoclonal antibody, cetuximab.

Benefits

• Automated and standardized approach for high throughput protein quantification; broadly applicable optimized digest kit streamlines method development; high sensitivity cetuximab quantification (100 ng/mL) from plasma.

In 2015, the global biologics market was valued at $276.6 billion (USD).¹ Monoclonal antibodies (mAbs) have grown to make up over 30%² of that market, with no signs of slowing down. What is it that makes mAbs so popular and successful? MAbs work by attaching to specific antigens found endogenously. This bond blocks communication between cells or elicits an attack from the immune system. With longer circulating half lives, greater target specificity, and higher efficacy, mAbs offer a great advantage over other conventional therapies. This highly targeted approach to treatment is being used as a single agent or combination therapy for oncology, immunological disorders, and infectious diseases.¹

With the vast increase in large molecule therapeutics, the demand for protein quantification in support of drug research and development has also increased. Traditionally, ligand binding assays (LBAs) have been used to quantify proteins in matrix. However, use of LC-MS/MS has steadily increased due to the many limitations of the LBA approach (e.g., lack of standardization, lot-to-lot reproducibility, issues with specificity, and limited dynamic range). While LC-MS protein quantification offers several advantages, it has presented great challenges for traditional “small molecule” scientists, as these complex molecules require intricate analytical workflows for their quantification. The surrogate peptide, or bottom-up, approach is the most common sample preparation technique employed for LC-MS protein quantification. With many reagents and a multitude of sample processing steps required for this approach, method development can be complex, often requiring highly skilled scientists for their successful execution and transfer. Additionally, to reach the desired low-level detection limits, sample enrichment techniques, such as peptide level solid-phase extraction (SPE) or protein level immunoaffinity, are frequently required. This increases sample preparation complexity and can introduce variability to the method.

An automated and standardized approach to protein bioanalysis, including the necessary reagents and protocols, would enable novice large molecule bioanalysts to successfully and reproducibly quantify proteins with ease, increasing throughput, minimizing human error, and streamlining the process. This application describes a fully automated kit-based approach using ProteinWorks Auto-eXpress Digest and μElution SPE Clean-up Kits, performed on the Hamilton Microlab STAR liquid handler (STAR) for the accurate quantification of the monoclonal antibody, cetuximab (Figure 1). Using this fast and simple sample preparation approach, lower limits of quantification (LLOQ) of 100 ng/mL were easily achieved.

Sample description

Rat plasma samples were spiked with cetuximab (0.1–250 μg/mL) and SILu MAB (16 μg/mL) prior to digestion. SILuMAB, a humanized, stable isotopically labeled protein with ¹³C₆, ¹⁵N₄-labeled arginine and ¹³C₆, ¹⁵N₂-labeled lysine was used as the internal standard (IS) to minimize pre-analytical variability. Since the label occurs where the enzyme, trypsin, cleaves, subsequent labeled tryptic peptides can be used for IS corrections. Calibration curve standards were prepared in duplicate, while QC and blank (non-spiked) plasma samples were prepared in triplicate. The prepared plasma samples (40 μL) were digested using the ProteinWorks Auto-eXpress High 5 Digest Kit reagents and low volume protocol, which includes the reduction and alkylation of the proteins. Post digestion purification of the subsequent tryptic peptides was performed using the ProteinWorks μElution SPE Clean-up Kit and protocol. Specifically, 100 μL of the digested sample was loaded onto the Oasis MCX μElution plate, processed according to the described protocol, then peptides were eluted using 50 μL of elution solution and diluted with 50 μL of water. Both digestion and SPE sample preparation were carried out on the STAR. The ProteinWorks automation deck layout and protocol includes 2 Hamilton Heater Shakers for incubation and vortexing, a plate sealer, a hold down module for piercing through seals, and a dark box for alkylation. The automated SPE was performed using a CVS Vacuum System (115 V) on the STAR. Digested and purified samples were then subjected to LC-MS/MS analysis.

Cetuximab was approved by the United States Food and Drug Administration (FDA) in 2004. Since its initial approval, it has been re-approved multiple times for different treatments of colorectal cancer and head and neck cancer.³ Used as single agent or combination therapy, patients have experienced a better quality of life and better survival outcomes when treated with cetuximab.³ With US patent expiry in 2016, biosimilar developers are eager to release new products, putting the pressure on small labs and CROs (contract research organizations) to develop methods for its accurate quantification. This has proved challenging for classically small molecule scientists due to the complexity of the LC-MS/MS quantification workflow, with its multiple steps and reagents in need of optimization. In this application, we have used the ProteinWorks Auto-eXpress Digest and μElution SPE Clean-up Kits, fully automated on the Hamilton STAR, to accurately quantify cetuximab in plasma.

A typical set of standards and QCs, spiked with cetuximab and IS, was prepared in rat plasma. For IS peptide selection, SILuMAB tryptic peptides with corresponding, or similar, retention times to cetuximab tryptic peptides provide the best performance. Labeled generic signature peptides digested from SILuMAB should correspond to generic signature peptides digested from cetuximab.

An example of this unlabeled (cetuximab, A)/labeled (SILuMAB, B) relationship can be seen in Figure 2. Multiple unique and generic human peptides (found in the conserved region of the mAb) were simultaneously monitored for cetuximab quantification. Final MRM conditions for cetuximab and SILuMAB can be found in Table 1. Figure 3 highlights the chromatographic separation of cetuximab tryptic peptides using the ACQUITY Peptide BEH C18, 300Å, 1.7 μm, 2.1 x 150 mm Column. A standard LC gradient of 2% to 45% mobile phase B (MPB) over 7 minutes with the Peptide BEH column provided ample separation of any interfering peaks.

Using the ProteinWorks Auto-eXpress Digest and μElution SPE Clean-up Kits and protocols (Figure 4, Panels A and B, respectively) automated on the Hamilton STAR (Figure 5) with only 40 μL of plasma, sub μg/mL LLOQs and high accuracies for cetuximab were achieved. Figure 6 compares the performance of blank rat plasma against the LLOQ at 100 ng/mL for the cetuximab tryptic peptide, DILL. Calibration curves for each of the tryptic peptides were linear, r²>0.99, with mean accuracies averaging 99%, well within regulatory guidlines.5 Standard curves were linear over 3 orders of magnitude for the various cetuximab signature peptides. A summary of standard curve performance is shown in Table 2. Additionally, QC performance was excellent with average CVs ≤15% across all peptides. This performance is summarized in Table 3. QC chromatographic performance for the SQVFFK cetuximab tryptic peptide is highlighted in Figure 7.

The ProteinWorks Auto-eXpress Digest and μElution SPE Clean-up Kits, fully automated on the Hamilton STAR, were successfully used to quantify cetuximab from rat plasma. This fully automated, kit-based, sample preparation solution facilitates fast digestion and sample enrichment in 6 hours, ensuring sensitive, accurate, and reproducible cetuximab quantification down to 100 ng/mL for any bioanalyst.

1. Grand View Research (2017 Mar) Biologics market size worth $399.5 billion by 2025 | growth rate: 3.9%. Retrieved from: http://www.grandviewresearch.com/press-release/global-biologics-market.
2. Grand View Research (2016 Nov) Monoclonal antibodies (mAbs) market size worth $138.6 billion by 2024. Retrieved from: http://www.grandviewresearch.com/press-release/global-monoclonal-antibodies-market.
3. Marnett Research Laboratory, Department of Biochemistry (Accessed 2017 Nov) Cetuximab. Retrieved from: https://medschool.vanderbilt.edu/marnett-lab/cetuximab.
4. FDA Guidance for Industry for Bioanalytical Method Validation, CDER.