Protein Quantitation: Finding the Right Assay for Your Needs

The detection and quantitation of proteins, whether therapeutic agents or endogenous biomarkers, plays a central role in drug development and biopharmaceutical research. While small molecules have historically dominated the pharmaceutical landscape, large molecule therapeutics such as monoclonal antibodies have grown substantially over the past two decades and are projected to continue expanding in the years ahead. As with all pharmaceutical products, understanding the stability, bioavailability, and metabolism of these molecules in-vivo is essential.

Large molecule drugs are subject to proteolysis and other biological processes that can alter their structure and function over time. In addition, the route of administration (e.g., intravenous or intramuscular) can influence distribution, half-life, and overall bioactivity. These factors are key components of pharmacokinetics (PK), which describes how a drug moves through the body.

Equally important is pharmacodynamics (PD), which describes the biological effects of a drug. Many biologics are designed to modulate specific pathways, such as reducing inflammation or inhibiting abnormal cell growth. Evaluating efficacy often involves measuring downstream biomarkers, for example, changes in cytokine levels following treatment. At the same time, biologics may trigger immunogenic responses, making it important to monitor both therapeutic proteins and host-response biomarkers throughout development.

Bioanalytical Challenges in Protein Quantitation

Protein quantitation in biological matrices presents several challenges. Human blood and other biofluids contain a large and diverse population of proteins, and this complexity can increase further in disease states. For well-characterized conditions, known biomarkers can guide assay development. In other cases, particularly for rare or less-studied diseases, broader discovery-based approaches may be needed before targeted quantitation can be established.

Multiple analytical platforms are available to support both discovery and targeted measurement, each offering distinct advantages depending on the stage of development, analyte characteristics, and study requirements.

Mass Spectrometry-Based Approaches (LC-MS)

Liquid chromatography–mass spectrometry (LC-MS) is widely used for both biomarker discovery and targeted protein quantitation. High-resolution mass spectrometry (HRMS) systems, such as Orbitrap-based instruments, enable detailed molecular characterization, including accurate mass measurement and structural analysis. These capabilities are particularly useful in early-stage studies where identifying and characterizing analytes is a priority.

For targeted quantitation, triple quadrupole mass spectrometers are commonly used with techniques such as multiple reaction monitoring (MRM). These methods can provide high specificity and are well-suited for measuring analytes in complex biological matrices.

LC-MS workflows can support multiplexing and provide detailed molecular information, making them applicable across a range of PK/PD studies. At the same time, method development, sample preparation, and instrument operation can be technically complex and may require specialized expertise and infrastructure. As with all bioanalytical methods, LC-MS assays must be developed and validated in accordance with regulatory guidelines (e.g., ICH M10, FDA guidance).

Figure 1: Comparison of HRMS versus Triple Quad MS.

Ligand Binding Assays (LBA): ELISA

Ligand binding assays (LBAs) are another widely used approach for protein quantitation. Among these, the Enzyme-Linked Immunosorbent Assay (ELISA) is one of the most established formats.

In an ELISA, a capture antibody immobilized on a microplate binds the target analyte. A detection antibody, typically linked to an enzyme such as horseradish peroxidase (HRP), generates a measurable signal following substrate addition. The signal is then related to analyte concentration using a calibration curve.

ELISAs are broadly used due to their accessibility, cost-effectiveness, and compatibility with high-throughput workflows. A wide range of commercial kits and validated antibodies are available, which can simplify assay development. ELISA methods are also well-established in regulated environments.

Considerations for ELISA include the potential for matrix effects, dependence on antibody specificity, and assay format constraints such as incubation times and wash steps. Dynamic range and multiplexing capability may vary depending on assay design.

Electrochemiluminescence-Based Assays: MSD Platform

Electrochemiluminescence assays, such as those offered by the Meso Scale Discovery (MSD) platform, represent another LBA-based approach. These assays use a similar antibody-based capture format but rely on electrochemiluminescent detection. A detection antibody labeled with a ruthenium complex emits light upon electrical stimulation at the plate surface, which is then measured by the instrument.

Figure 2. ELISA signals are generated by the activity of an enzyme tag (e.g., HRP) on a substrate, resulting in a color change in the microplate well. In MSD, the Ru tag is excited by the electricity passing through the electrode and produces a light signal.

MSD assays can support multiplexing and are often used in applications requiring measurement of multiple analytes within a single sample. The platform is designed to operate over a relatively broad dynamic range and can accommodate small sample volumes.

As with other methods, there are practical considerations, including instrument platform requirements, assay setup, and cost structure. MSD assays are used across various stages of drug development and can be validated for regulated studies.

Choosing the Right Approach

Each of these analytical approaches, LC-MS, ELISA, and MSD, offers a different combination of capabilities. The most appropriate method depends on several factors, including:

• Study stage (discovery vs. targeted analysis)
• Number and type of analytes
• Required sensitivity and dynamic range
• Sample matrix and volume
• Throughput needs
• Availability of reagents (e.g., antibodies)
• Regulatory considerations

In many programs, multiple techniques are used in a complementary manner, for example, mass spectrometry for initial characterization followed by ligand binding assays for routine sample analysis.

Table 1. Comparison of three protein quantification methods based on performance and costs.

All of these protein quantitation methods: HRMS, triple quad MS, ELISA, and MSD, are available at Emery Pharma. With our extensive expertise, we can help select the method best suited to your biopharmaceutical research, whether in early-stage discovery or clinical trials.

If you are developing a novel large molecule drug or producing a biosimilar, contact us online or call +1 (510) 899-8814 to see how we can accelerate your drug development project!

About the Author

Authored by Dr. Janet Liu, Director of Biology, and Dr. Prajita Pandey, Associate Director of Chemistry.