Antibody-drug conjugates pose particular problems for analytical scientists. Developing new methods for small molecules is generally fairly straightforward, while for biologics there will usually be a platform method that can be applied, removing the requirement for validation. But ADCs combine both a small molecule and a biologic component, and this makes life significantly more complicated. Each individual case is different.
Take, for example, the way that the two components are connected. It might be via the random and unselective technique of lysine conjugation; it could be using cysteine conjugation, which is still random but there is some regioselectivity; or perhaps a site-specific technique is used. All of these will impact the nature of the conjugation, and therefore the method.
The number of small molecules per biologic will also vary. It is commonly anywhere between one and eight for both regioselective and site-specific conjugation which will, again, affect the method. And then there are the chemical properties of the drug linker that connects them – is it hydrophobic? How big is it? What charge does it have?
And as the creativity of ADC designers and developers expands, new protein modalities are breaking into the pipeline on top of standard IgG antibodies. Bispecific antibodies, VHH domain Fc fusions, Fab2/Fab-fragments, single chain variable fragments and even VHH nanobodies are now being investigated, adding to the challenge facing method developers.
This huge diversity of components and chemistries makes it impossible to develop a platform approach. Although there are multiple standard methods that can provide useful starting points, each individual ADC will still require a specific method to be developed, optimised and validated.
These standard methods cover a significant number of the specific modalities, and it is often possible to adapt them in some way to make them more product-specific. Alternatively, they can provide a starting point for developing a new method. Either way, it will require validating for each new product.
This provides a faster and cheaper alternative to either implementing a customer-developed method, which may well use different analytical equipment and therefore require a lot of additional work, or developing a completely new method from scratch.
In the past few years, here at Lonza we have greatly expanded our analytical toolbox. This now contains standard methods for the majority of the critical quality attributes, across a wide range of different modalities. Some methods, such as those covering drug-antibody ratios and their distribution will likely require different strategies, however. The skill lies in determining whether, and how, the standard methods can be applied.
A great example is our Ibex® Design ADC Program, which integrates speed, quality and efficiency to facilitate short timelines to IND. The methods included within the DNA-to-IND programme are a key factor in achieving fast timelines and help mitigating development risks for cytotoxic bioconjugates.
We are continuously checking the list of available standard methods, and evaluating alternatives. As equipment manufacturers improve their products, and new technologies and better media are implemented, we will always look to see whether these could lead to advances in the accuracy or speed of the standard methods.
The breadth of knowledge we have in the different teams here at Lonza means we can share best practice with our colleagues working on both proteins and small molecules. There are significant overlaps, despite the added complexity of ADCs.
It’s not the same as simply plugging in new variables into a platform method, and every method we develop will still require validation. But either way, there is no substitute for experience – something we have a huge amount of here at Lonza, working in the bioconjugates market since 2006, bringing over 70 projects from preclinical development to commercial scale manufacturing.