ADC Purification

ADC purification is a multi-step downstream process that removes free payload, unreacted small molecules, aggregates, and DAR heterogeneity to yield a stable, homogeneous product, typically combining tangential flow filtration (TFF) with one or more chromatography steps.

Purification Objectives

Post-conjugation reaction mixtures contain several types of impurities that must be cleared: unreacted linker-payload, residual organic solvents used to dissolve the hydrophobic payload, quenching reagents, aggregated ADC, and a heterogeneous distribution of DAR species. No single technique addresses all of these simultaneously, so purification trains typically combine complementary methods based on molecular size, hydrophobicity, and charge.

Tangential Flow Filtration (TFF/UF-DF)

TFF is the workhorse first-line technique because of the large size difference between the ADC (~150 kDa) and free payload/linker (<2 kDa), making it highly scalable, cost-effective, and higher-yielding than chromatography. It serves three functions: removing membrane-permeable small molecules (free payload, solvents, salts) via diafiltration, exchanging buffer into the final formulation, and concentrating the product. A typical protocol concentrates the crude ADC to ~25–30 g/L, then performs constant-volume diafiltration with 5–7 diavolumes to clear residual buffer components and organic solvent, followed by final concentration. Single-pass TFF (SPTFF) is an emerging variant that avoids recirculation, reducing shear stress and aggregation risk while simplifying process design. Importantly, TFF cannot resolve species that differ mainly by DAR, charge, conformation, or aggregation state — those all remain retained together by the membrane, so a property-selective chromatography step is needed when those impurities are present.

Size Exclusion Chromatography (SEC)

SEC separates by molecular size and is often called "the gold method" for ADC purification because it simultaneously performs buffer exchange, removes free payload/small-molecule impurities, and clears aggregates — all under mild, stability-friendly conditions (neutral pH, room temperature). In practice, larger aggregates elute first, followed by the monomeric ADC, then any small fragments or free drug, and it's frequently used both as the primary clean-up step after conjugation and as a final polishing step for aggregate removal.

Hydrophobic Interaction Chromatography (HIC)

HIC is the standard technique for resolving and controlling the DAR distribution, since each additional hydrophobic drug-linker increases the ADC's overall hydrophobicity. Using a decreasing (or increasing, depending on setup) salt gradient — commonly starting near 1–1.5 M ammonium sulfate — unconjugated antibody (DAR0) elutes first, followed by progressively higher-DAR species (DAR2, DAR4, DAR6, DAR8) in order of increasing hydrophobicity. Beyond simple purification, HIC can be run preparatively to isolate ADC fractions with a specific, narrower drug-load range, directly improving product homogeneity.

Ion Exchange and Multimodal Chromatography

Ion exchange chromatography (IEX), particularly cation exchange (CEX), separates based on surface charge differences (which can shift with conjugation) and is effective for removing aggregates and free payload — one case study showed CEX-style bind-and-elute chromatography reducing free payload from 52.2% down to 5.0% via flow-through and wash steps. Multimodal (mixed-mode) chromatography combines ion-exchange and hydrophobic interaction selectivity in a single resin, offering unique separation power for challenging impurities like aggregates that single-mode methods struggle to resolve. Hydroxyapatite (HA) chromatography is another polishing option, shown capable of reducing antibody aggregate content from 60% down to 0.1% when used after initial Protein A capture.

Typical Purification Workflow

A representative downstream process sequence looks like this: Protein A affinity chromatography first captures and purifies the antibody intermediate before conjugation; after the conjugation reaction, an initial SEC or TFF step removes bulk free payload/solvent; if the DAR distribution or aggregate level isn't yet acceptable, a polishing chromatography step (HIC for DAR control, CEX or multimodal for charge/aggregate control) is added; and a final TFF step concentrates the product and exchanges it into formulation buffer.

Choosing a Purification Strategy

For quality control, four measurements are recommended regardless of chosen method: quantifying material balance across feed/retentate/permeate/fractions (not just measuring residual payload in isolation), tracking free payload/solvent/salt clearance against diavolume count, using stability-indicating LC-MS to distinguish parent linker-payload from degradation products, and pairing average DAR results with a distribution-sensitive method like HIC or native MS.