Hydrophobic Interaction Chromatography: Your Best Teammate in the Analysis of ADCs

Antibody-Drug Conjugates (ADCs) is a complex class of targeted cancer therapeutics. They consist of three integral components: an antibody, a chemical linker, and a potent cytotoxic payload. By leveraging the specificity of the antibody to recognize tumor-associated antigens, the ADC delivers the toxic payload directly into the malignant cells, minimizing systemic toxicity and maximizing therapeutic efficacy.

ADCs are often described as "biological bombers", but how many missiles does one ADC carry? This critical quality attribute is called Drug-to-Antibody Ratio (DAR). The DAR represents the number of cytotoxic drug molecules attached to each antibody. For each conjugate the value is an even number (0, 2, 4, or more), and for one ADC drug we care about the average number of each ADC.

The DAR is a primary determinant of the drug's "firepower". If the DAR is too low, the potency may be insufficient to achieve clinical results; if it is too high, the ADC may become unstable, cleared too rapidly from systemic circulation, or lead to increased toxicity. Precision in DAR determination is therefore paramount for both the research and development and the quality control of these complex biologics.

High-Performance Liquid Chromatography (HPLC) is the standard tool for DAR analysis. However, traditional reversed-phase HPLC (RP-HPLC) can be "violent" toward the protein's structure, potentially leading to the denaturation or dissociation of the ADC's native conformation.

In contrast, Hydrophobic Interaction Chromatography (HIC) is a "gentle" approach which separates species based on the varying degrees of hydrophobicity. Its mechanism is elegant: while the naked antibody is relatively hydrophilic, the conjugated small-molecule drugs are typically highly hydrophobic. Consequently, as the number of drug molecules attached to the antibody increases, the overall hydrophobicity of the ADC molecule increases proportionally.

HIC acts as a meticulous organizer, separating the ADC species from low to high DAR values based on this increasing hydrophobicity, allowing for the determination of the DAR distribution and the calculation of average DAR value simultaneously in a single analysis.

Column selection has a major impact on separation quality. For example, the Advanchrom Bio HIC-Butyl column from Welch Materials is an excellent general-purpose choice for most antibodies and ADCs. It typically utilizes a standard ammonium sulfate buffer system to achieve robust separation.

For antibodies, proteins, and ADCs with lower hydrophobicity, the Advanchrom HIC-Butyl column offers a strong option. It is notably compatible with volatile, mass spectrometry-friendly mobile phase systems (such as ammonium acetate and acetonitrile). This allows for a more accurate and rapid correlation between LC-UV and LC-MS results, facilitating easier identification of specific DAR species.

The core principle of HIC operation is high-salt loading and low-salt elution. The analysis begins with a high concentration of salt as the initial mobile phase so that all ADC species bind to the stationary phase. Then the salt concentration of the mobile phase is gradually lowered, forcing the species to "surrender" (elute) in order of their hydrophobicity—the species with the lowest DAR elute first, followed sequentially by species with increasingly higher DAR.