The development of an antibody drug conjugate (ADC) represents a marriage between the disciplines of biology and chemistry. The creation of an ADC involves a combination of bioprocess manufacturing techniques with traditional synthetic chemistry skills to make the small molecule part, then conjugating it to the biologic.
SAFC has positioned its ADC manufacturing within its large molecule plant in St Louis, rather than housing it in a small molecule facility. This gives the advantage of having the knowledge in handling biologics, as well ready access to the expertise and extensive tool kit required for analytical characterisation capabilities to manufacture and test a complex biologic.
Apart from the manufacturing challenges, ADCs pose interesting questions in the regulatory arena. As the biologic and the HPAPI can both be considered to be active ingredients, they both need to be characterised and produced according to GMP rules before they are brought together in the final, conjugated molecule that is delivered as a therapeutic.
Apart from the manufacturing challenges, ADCs pose interesting questions in the regulatory arena
In addition to specifications for release testing, a more extensive set of analytical tests is needed to characterise the conjugate. This involves looking at the purity and potency of the small molecule component, linker, and the large molecule – typically a monoclonal antibody, although some novel drug conjugates are using alternative scaffolds. It also means looking at any process or product-related impurities that might be brought into the process from either of these components, or created during the conjugation process itself. Clearly, a multidisciplinary team will be required to assemble all the necessary analytical and other data and documentation for filing and review.
Current ADC manufacturing technology relies on creating an active site on the antibody, and an active site on the small molecule drug via a linker, before the two are covalently coupled. There are two well-developed platforms for covalently coupling active sites. Seattle Genetics uses cysteine-based linker chemistry to create ADCs. This was used to create brentuximab vedotin (Adcetris), which is marketed in collaboration with Takeda to treat lymphoma. ImmunoGen’s linker chemistry is based on lysine; this is the technology behind trastuzumab emtansine (Kadcyla), which was developed and is sold by Roche.
Seattle Genetics and ImmunoGen both have numerous other products in internal development, as well as through collaboration or licences with partners.
However, neither method is perfect. Each of these chemistries gives a heterogeneous population of ADCs; in other words, the number of HPAPI molecules and their location on the biologic varies from one conjugate to the next. So a product may have a drug antibody ratio of 4, which is a key attribute in terms of potency, but there will actually be a Gaussian distribution of the amount of payload drug that is loaded onto the antibody molecules across the entire population of ADCs. This is a concern for the regulatory authorities, as they prefer well-characterised drug substances. So while the overall manufacturing processes may be reproducible when they are well controlled, the result will still always be a heterogeneous population of ADCs.
Newer technologies that give more reproducible results are now starting to enter development
Newer technologies that give more reproducible results are now starting to enter development. Current pipelines largely rely on the SeaGen and ImmunoGen technologies that have already been validated in the clinic and by commercial approval, and this is likely to continue for the next decade as those products move through the pipeline. But in the future, conjugation techniques that are more site-specific will become increasingly important because of their ability to produce a more tightly controlled product distribution.
However, with the two current technologies, there are moves to improve reproducibility. In particular, it is important to control the stoichiometry of the reactions. Drug-antibody ratio is, clearly, one key aspect that needs to be controlled, representing a measure of the potency of the molecule. This can be achieved, to some extent, by keeping a tight rein on the stoichiometry of the initial reaction that is used to develop reactive sites on the antibody, and subsequently driving complete coupling of the drug-plus-linker moiety to the antibody. Controlling the availability and accessibility of these conjugation sites on the antibody is key to reducing variability.
Once the ADC has been formed, it is important to be able to characterise it carefully. Analytical techniques have evolved significantly in recent years, and it is now possible to characterise the site of the attachment of the antibody to the payload using mass spectrometry. A fully characterised distribution and heterogeneity of the ADC population is something the regulators are increasingly looking for in terms of proof that the process delivers the correct product in a reproducible manner.
Product-related impurities can also be minimised through careful control of the reaction conditions, which also helps ensure controlled activation of the protein scaffold for a well-defined range of accessible sites. Aggregation can be a particular problem, as aggregated proteins appear to be related to immunogenicity and other potentially adverse clinical reactions. They therefore represent a particular focus for the regulators. The tendency towards aggregation can be minimised by careful control of process conditions such as ionic strength, temperature and pH. Chromatographic techniques are increasingly being used to remove any aggregates that form.
In terms of the design of the manufacturing facility, there are two conflicting requirements that must be balanced. One is the containment of the HPAPI in terms of maintaining the safety of the personnel handling the materials; the other is ensuring appropriate environmental controls are in place to guarantee the quality and safety of the product being manufactured. Yet requirements for environmental classifications and room pressurisations are the exact opposite of each other.
When handling potent materials, it is crucial that they are contained within that space, so operating within a negative pressure environment is preferable. But in terms of handling the bulk drug substance – the ADC – a higher level of cleanliness is necessary to protect the product, and particles must be prevented from coming into the room. This implies a need for positive pressure.
These two conflicting requirements can be balanced to a great extent by segregating some of the unit operations. In SAFC’s new commercial facility, the design includes a separate room where the weighing and dissolution of the HPAPI takes place. This enables highly hazardous steps being contained within an isolator in a room that is under negative pressure, minimising the risk of cross contamination. In contrast, the filling of the drug substance into bulk packaging is carried out in a separate space under positive pressure to provide the necessary protection to the product itself. In addition to the appropriate room classifications, the facility has a single-pass, unidirectional airflow.
Containment requirements are set to become ever more stringent in future, with novel payloads being developed that have an even higher potency than the HPAPIs that are used to make ADCs today. Engineering design and control must be able to cope with the future direction of manufacturing demands, meeting increasingly stringent safety and containment requirements so that more potent payloads can be handled safely.
There is also a growing interest in alternative scaffolds. Those ADCs that are on the market and in late-stage clinical trials today rely on monoclonal antibodies as the targeting portion, but Fab fragments, bi-specific antibodies and other types of protein scaffold are all garnering interest. As additional product types enter the clinical pipeline, there is the potential for more widespread use of ADCs to deliver active molecules other than cytotoxics in diseases and conditions other than cancer, or even as diagnostics.
ADCs as a field of therapeutics is still very much in its infancy. Regulatory agencies are demanding further information and documentation to support product filings, and a good deal of learning is still underway, both among the regulators and at the pharma companies and manufacturers that are pushing these products forward.
Regulatory agencies are demanding further information and documentation to support product filings
There is a concerted effort to consider all quality and control aspects of every single component of the ADC – the biologic, the HPAPI and even the linker. The linker is more than just the ‘glue’ that holds the ADC together – it has significant impact in terms of the drug’s mechanism of action and the delivery of the payload into the cell, and quality must not be ignored here, either.
Fundamentally, regulators want to ensure that the company making the ADC fully understands its manufacturing process. Gone are the days when all drugs were simple, small molecules, whose quality was straightforward to establish. With biological products, especially those that are as complex as ADCs, the manufacturing process is a critical part of the product itself. With the implementation of the ICH guidelines for biologics manufacturing, there is a push for full-process understanding much earlier in a product’s development life cycle. Adequate information and control of all aspects of the production process are essential if the demands of the regulators are to be met.