By Tim Sandle, Ph.D.
Antibody-drug conjugates (ADCs) are a diverse class of biopharmaceutical drugs that combine highly selective monoclonal antibodies specific to surface antigens present on particular tumor cells.1 ADCs are equipped with a highly potent cytotoxic agent linked via a biodegradable chemical linker, which serves to release the anti-cancer agent so it can be internalized by the cell. The drugs are designed to specifically target certain types of cancers, and they represent a fast-evolving field of drug development. To date, only a small number of ADCs have received market approval, while hundreds are in development as researchers explore structure-function relationships in order to develop new indications against cancer targets.
For novel drug products close to approval, attention needs to shift toward current good manufacturing practices (cGMP) in preparation for scale-up and commercialization. Antibody-drug conjugates need to be prepared according to the general regulations and guidelines required for manufacturers of sterile pharmaceuticals, extending to every aspect of personnel, premises, equipment, and clothing through to cleaning, sterilization, documentation, monitoring, and quality control. There are some more specific elements of cGMP that need to be considered, such as segregation and containment, to protect both the product and the manufacturing personnel.2 ADC-specific GMPs center on purification and containment. This article considers the elements that need to be embedded in a firm’s cGMP strategy.
Containment is especially important in the manufacture of ADCs because they contain highly active ingredients that require special rules for their production, as accidental contamination with other materials ruins the formulation and poses an adverse health risk to operators.3 These risks are best understood and mitigated by adopting ICH Q9 Quality Risk Management guidelines.4
For operator protection, safety compliance requires the manufacturer to establish acceptable daily exposures (ADEs) in the form of cleaning limits for the active substances. This requires an ongoing assessment of residue limits derived from the toxicological evaluation using an appropriately validated analytical method. In practice, operators also need to be highly disciplined in executing their work, both for their own protection and to avoid product-to-product contamination.
Manufacturing risks include cross-contamination and mix-ups, and these concerns need to be addressed through dedicated, or at least partially isolated, areas.5 In many cases, dedicated facilities are required since risk assessment outcomes will indicate that the likelihood of cross-contamination cannot be adequately controlled by operational or technical measures.6 In addition, scientific data from the toxicological evaluation may demonstrate that complete isolation or decontamination cannot be reliably achieved. Even where dedicated facilities are used, care must be taken to avoid the contamination of individual batches. This requires controlling airborne concentrations and contamination of product contact surfaces.
Containment can be optimally achieved using closed system reactor equipment with variable air pressure and engineering controls to allow adjustments to be made for different product types. For example, transferring slurry under pressure into reactors decreases the risk of airborne contamination. Containment operations are further strengthened when processing and filling occurs within isolators (for steps such as filtration, drying, and milling, as well as dispensing, weighing, and filtering materials).
Verification of containment is achieved by maintaining segregation controls, whether this is via dedicated cleanrooms or with isolators. This requires good engineering design, preventive maintenance, and continuous monitoring with audible or visual alarms.
Containment must be supported by using dedicated utilities, such as services to cleanrooms in the form of heating, ventilation, and air conditioning (HVAC); the supply of pharmaceutical grade water; dedicated gases; separate access-controlled entry; and single-use personal protective equipment.
Cleaning And Decontamination
Where possible, manufacturing equipment and materials should be single-use and disposable, as this negates the risk of cross-contamination. However, cleaning is invariably required for reactors and isolators. Cleaning is particularly important when processing between batches, and here the cleaning needs to be effective and reproducible. This is demonstrated through validation. Cleaning validation is essential for all types of pharmaceutical manufacturing, although it takes on a special importance to demonstrate the removal of the highly potent compounds processed within reactors.
It is also necessary to periodically decontaminate the facility, such as by using hydrogen peroxide vapor or chlorine dioxide gas. The purpose is to inactivate any remaining compounds, as well as achieve good microbial control.7 This will require cycle development and qualification in order to achieve the optimal concentrations and exposure lengths.
Manufacturing And Purification
ADCs are composed of highly complex structures consisting of antibodies covalently conjugated with small molecule cytotoxic drugs, and processing requires specialized equipment and stages, as these produce both engineering and chemistry challenges that need to be overcome while maintaining cGMP. The ADC manufacturer must demonstrate, through process validation, both reproducibility and robustness of each manufacturing step.
Controls begin with the identity verification, testing, and release of raw materials and extend to controls around the manufacturing process. In particular, accurate weighing and dispensing represent key process steps prior to dissolving and charging the powder (which sometimes requires the aid of heating). A further important manufacturing control for ADCs is stirring, where speed and time represent control variables to avoid undesired nonspecific conjugation or a lowering of the quality of the proteinaceous material produced.
The most common method for the purification of ADCs is tangential flow filtration (TFF), which acts as a rapid and efficient method for separation and purification of biomolecules.8 Selecting a filter with the most compatible materials and pore size is essential. Adopting this technology is not straightforward; the requirements for higher pressures, recirculating flow paths, pumps, valves, and sensors, present a quality by design challenge.
Careful selection and assessment of construction materials to meet flow and pressure requirements and to enable the attachment and connectivity of the sensors essential for process control represent examples of these design considerations. Sensors fit well with process analytical technology (PAT) expectations, which will form part of the cGMP strategy based on in-process controls. In addition, cleaning a TFF cassette and system is essential to maintain flux rates and minimize cross-contamination risk in a multiproduct system.
The above measures must demonstrate that the process can consistently produce ADCs of the required efficacy and quality. This is verified through in-process and end product quality control testing based on identity, strength, purity, and composition, together with other characteristics specific to the individual ADC. The assessment of purity requires analysis through hydrophobic interaction chromatography and quadrupole time-of-flight mass spectrometry, among the other methods necessary to derive the accurate drug-antibody ratio.9 The laboratory methods for analysis need to be qualified, with an overview of any lot-by-lot variation assessed by trend analysis to demonstrate uniform product properties.10 As novel products, ADCs require careful and controlled monitoring. In turn, this requires analytical methods to have appropriate levels of specificity and accuracy.
Across all stages of ADC manufacturing, the regulatory requirements pertaining to good documentation practice and data integrity need to be applied. These considerations fit into the wider cGMP framework.
ADCs represent emerging biotechnologies with the promise of treating patients with hitherto incurable forms of cancer. Specific aspects of cGMP of relevance to ADC producers relate to containment and purification. It is also the case that many of the startups operating in the ADC space lack established and mature quality systems and, hence, a careful approach to operating in accordance with cGMP requirements in general is required. In particular, care needs to be taken when bridging from manufacture for clinical studies, where phase-appropriate cGMP will be operating, to commercial operations, where strict adherence to cGMP is required.
- De Cecco, M., Galbraith, D. and McDermott, L. (2021) What makes a good antibody–drug conjugate?, Expert Opinion on Biological Therapy, 21:7, 841-847
- Petrelli F, Caraffa A, Scuri S, et al. (2019) The requirements for manufacturing highly active or sensitising drugs comparing Good Manufacturing Practices. Acta Biomed. 23; 90(2):288-299
- Nguyen C, Petrelli F, Scuri S. et al (2019) A systematic review of pharmacoeconomic evaluations of erlotinib in the first-line treatment of advanced non-small cell lung cancer. Eur J Health Econ. doi: 10.1007/s10198-019-01040-7
- ICH Expert Working Group, International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use. 2005. ICH Harmonized Tripartite Guideline: Quality Risk Management Q9
- Sargent EV, Flueckiger A, Barle EL, et al. (2016) The regulatory framework for preventing cross-contamination of pharmaceutical products: History and considerations for the future. Regul Toxicol Pharmacol. 79 (Suppl 1): S3–S10
- Sussman R. (2016) Identifying and assessing highly hazardous drugs within quality risk management programs, Regulatory Toxicology and Pharmacology. 79:S11–S18
- Lorcheim. K. (2011) Chlorine Dioxide Gas Inactivation of Beta-Lactams Applied Biosafety. 16 (1): 34-43
- Matsuda, Y. (2021) Current approaches for the purification of antibody–drug conjugates. Journal of Separation Science 21 https://doi.org/10.1002/jssc.202100575
- Nowak, T., Shchurik, V., Adpressa, D. et al (2022) Rapid antibody conformational screening by matrix‐assisted laser desorption/ionization hydrogen‐deuterium exchange mass spectrometry, Journal of Separation Science, 45 (12): 2055-2063
- Stump, B. and Steinmann, J. (2013) Conjugation process development and scale-up. Methods Mol. Biol. 1045, 235– 248
About The Author:
Tim Sandle, Ph.D., is a pharmaceutical professional with wide experience in microbiology and quality assurance. He is the author of more than 30 books relating to pharmaceuticals, healthcare, and life sciences, as well as over 170 peer-reviewed papers and some 500 technical articles. Sandle has presented at over 200 events and he currently works at Bio Products Laboratory Ltd. (BPL), and he is a visiting professor at the University of Manchester and University College London, as well as a consultant to the pharmaceutical industry. Visit his microbiology website at https://www.pharmamicroresources.com.