For API development teams, considering radiolabelling during synthetic route development can reduce the need to rework established chemistry when material is required for an absorption, distribution, metabolism and excretion (ADME) study.
Identifying this later in development, can mean revisiting completed development work or creating a separate synthesis, adding cost and putting timelines under pressure. Assessing isotope incorporation while the route is still being developed gives teams time to resolve these issues and retain common chemistry where practical.
What are radiolabelled mass balance studies?
Human radiolabelled mass balance studies help development teams establish what happens to an administered dose: how much is absorbed, which metabolites are formed and how drug-derived material is excreted.
To do this, they incorporate an isotope, typically carbon-14 (¹⁴C) or tritium (³H), into the drug molecule. Measuring radioactivity in blood, urine and faeces allows researchers to account for the dose and investigate the pathways by which the drug and its metabolites leave the body.
These findings help teams decide which metabolites require further investigation and whether additional drug–drug interaction or renal and hepatic impairment studies are needed. To allow the results to inform the development programme, the FDA’s 2024 guidance recommends generally completing mass balance studies before late-phase clinical trials begin .
For API development teams, establishing the study requirement early also means allowing time to make the radiolabelled material. Isotope position and the chemistry used to introduce it need to be considered alongside the conventional synthetic route, before that route becomes difficult or costly to change.
Why timing matters: avoiding two separate development pathways
When radiolabelled material is needed for an ADME study, an established API route may not offer a practical point for isotope incorporation if this has not been considered during development. The problem can be compounded if the original chemistry is difficult to reproduce or the technical package is incomplete.
This can leave innovators having to redevelop existing chemistry or establish a separate synthetic route for the radiolabelled API, adding time, cost and complexity to an already advancing programme.
Considering both requirements during route development can help identify these issues early. At Sterling, we assess the conventional synthetic route to identify where ¹⁴C or ³H can appropriately be incorporated. Wherever practical, the aim is to retain a common underlying route rather than develop two independent approaches.
This can help:
- Reduce duplicated route-development work
- Maintain greater consistency between conventional and radiolabelled material
- Simplify chemistry and process knowledge transfer
- Avoid unnecessary complexity from fundamentally different synthetic pathways
Using a common synthetic route does not mean using identical manufacturing processes. Radiolabelled synthesis may require changes to reaction scale, handling, purification and analysis, while retaining the underlying chemistry.
Achieving this requires more than specialist radiochemistry expertise. It requires an understanding of route design and process development alongside knowledge of how and where a radioisotope can successfully be introduced.
More complex molecules create new radiolabelling challenges
Radiolabelling becomes more challenging as molecules increase in size and complexity. Established approaches for small molecules may not translate easily to peptides, oligonucleotides, antibody drug conjugates (ADCs) and other novel modalities, where molecular structure and biological behaviour can place additional constraints on isotope selection and positioning.
For these molecules, developers need to consider:
- Where should the isotope sit? The position needs to provide meaningful information while remaining appropriate for the molecule and intended study.
- How can it be incorporated? The synthetic strategy needs a practical route to introduce the labelled starting material.
- Can the material be purified and characterised? Complexity does not end with synthesis. Appropriate purification and analytical methods are also required.
- How does it fit the wider programme? The radiolabelled route should be considered alongside the conventional API strategy, rather than in isolation.
This is where combining radiochemistry with broader route, process and modality expertise becomes particularly valuable.
At Sterling, our radiolabelling specialists can draw on peptide and bioconjugation expertise across our network to support this assessment. Peptides provide a good example: our team can use ¹⁴C-labelled amino acids as building blocks in peptide synthesis, providing a defined point for isotope incorporation. The selected building block must also be compatible with the subsequent synthesis, purification and analytical methods.
Considering these steps together helps teams choose a labelling strategy that can deliver suitable material for the intended study, while making use of the wider programme’s development work.
Radiolabelling requires more than synthetic chemistry
Radiolabelling also brings practical requirements that conventional API programmes may not encounter.
Radioactive materials require appropriate licences, specialist facilities and safe-handling procedures. Analytical capabilities are needed to assess chemical and radiochemical purity, while storage, packaging and shipping require additional controls.
There are practical considerations beyond the laboratory too. Radiolabelled starting materials can have limited availability, making sourcing and supply-chain planning particularly important. Appropriate systems are also required for their receipt, storage and inventory management, as well as the packaging and shipping of the resulting radiolabelled material.
Sterling’s North Carolina, US, facility is licensed to handle 14C and 3H and provides specialist capabilities for radiolabelling programmes. Established relationships with US suppliers and access to key radiolabelled starting materials can also help manage supply risks in a market where sourcing options may be limited.
Together, these capabilities mean that isotope incorporation can be considered alongside the practical requirements needed to deliver material successfully for the intended study.
Connecting radiolabelling with the wider development programme
Considering future radiolabelling requirements during conventional route development allows teams to build on the same chemistry and process knowledge when study material is needed. This is the main benefit of planning the two together: identifying necessary changes early and reducing repeat development work.
Sterling’s Extended Bench™ model brings specialist radiolabelling expertise together with broader route development, process chemistry, analytical and manufacturing capabilities across its network. These teams can assess labelling options in parallel whilst the conventional route is being developed, then use the findings to support radiolabelled synthesis.
As the conventional API moves towards larger-scale requirements, that knowledge can also transfer with the programme into Sterling’s wider manufacturing network.
For innovators, this provides continuity between early route development, radiolabelled study supply and later API manufacture, with access to the expertise and process understanding developed along the way.
Build radiolabelling into the development plan
The quantities needed for an ADME study may be small but producing them can require substantial chemistry and preparation. Assessing radiolabelling while the API route is still being designed gives teams time to investigate isotope incorporation, source starting materials and establish the necessary testing and handling procedures.
With more than 50 years’ experience in API development and manufacturing, Sterling combines synthetic and organic chemistry expertise with specialist 14C and 3H radiolabelling capabilities. This allows us to support bespoke GMP and non-GMP programmes while connecting radiolabelling with wider route development, process development, scale-up and manufacturing requirements.
Planning an ADME programme? Speak to Sterling about integrating radiolabelling into your API development strategy.
