ADC Payloads Process Development and Quality Control: Compliance Management of Highly Potent Intermediates

Specificity in process development for highly active payloads

Payloads in ADCs are typically highly cytotoxic compounds, with occupational exposure limits (OELs) as low as the nanogram per cubic meter level. During synthesis and purification, enclosed isolators and clean-in-place systems are required. In early process development, safe and low-toxicity building blocks should be designed, and highly active steps should be placed as far downstream in the synthetic route as possible to reduce the risk of operators being exposed to the finished payload. The synthetic route needs to be evaluated for step economy, intermediate stability, and final product purity. For example, construction of the PBD dimer skeleton usually employs stepwise condensation and selective deprotection strategies; its synthetic intermediates are prone to oxidation in air and must be handled under inert gas protection. Maytansine and auristatin derivatives involve multiple chiral centers, making diastereomer control a difficult point in process development.

Purification and separation strategies

Purification of highly active payloads requires balancing separation efficiency and product loss. Conventional silica gel column chromatography can be used at milligram to gram scale, but attention must be paid to residues of highly active compounds caused by adsorption; reverse-phase preparative HPLC (using a water/acetonitrile system) is a commonly used fine purification method, but solvent consumption and lyophilization efficiency need to be optimized. For payloads with poor thermal stability (such as certain camptothecin derivatives), column temperature and residence time must be controlled. In recent years, the use of simulated moving bed chromatography (SMB) and supercritical fluid chromatography (SFC) in chiral separation has gradually increased, which can improve yield and reduce organic solvent use. After purification, the payload structure must be confirmed by LC-HRMS, NMR, and elemental analysis, with strict control of residual solvents, heavy metals, and genotoxic impurities.

Quality control of payloads before conjugation

As the starting material to be conjugated, the quality attributes of the payload directly affect ADC conjugation homogeneity and stability. Key quality indicators include: chemical purity (generally required to be >=95%), single unknown impurity <=0.5%, known impurities <=1.0%; moisture content must be controlled below 0.5% (because the reaction of payload with linker is sensitive to water); assay after response factor correction often uses quantitative nuclear magnetic resonance (qNMR) or differential scanning calorimetry. For payload-linker complexes containing cleavable linkers (such as MC-VC-PAB-MMAE), enzymatic cleavage release efficiency must additionally be verified (using cathepsin B to simulate in vivo cleavage). In addition, enantiomeric purity is also an important control item; for example, the (R)-configuration of DM1 has much higher activity than the (S)-configuration, so a chiral analytical method must be established for monitoring.

ADC conjugation process and drug substance quality control

Among the drug substance quality attributes of ADCs, payload-related indicators include drug-to-antibody ratio (DAR) and its distribution, free payload content, and conjugation site specificity. High-performance liquid chromatography-mass spectrometry (HPLC-MS) and size exclusion chromatography (SEC) are commonly used tools for DAR determination, while hydrophobic interaction chromatography (HIC) is used to characterize DAR distribution. Free payload (unconjugated or prematurely released) needs to be controlled at a low level (such as