High Potency Molecules: The Importance of Excellent Chemistry in their Successful Delivery

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INTRODUCTION
During this short article we will highlight some of the challenges brought by the design of routes for high potency molecules. Our experience with these molecules has shown that due to low volume requirements of the APIs, alongside the difficulties and lack of facilities available to carry out the chemistry, the optimisation of routes is often not undertaken. Before getting into some of the more interesting technical discussion we feel it is first important to highlight the development of this market and the increasing need for high quality facilities to carry out route selection and process development.

 

The high potency drug market is an exciting place right now. There are a growing number of approved drugs, plus an increasing pipeline of drug candidates vying to be future blockbusters.

 

In 2022, the current market for high potency medicines was $25 billion. What’s more, it is expected that by the end of the decade, this figure will grow at a CAGR (Compound Annual Growth Rate) of between 6 and 8%. This will lead to a market value for high potency drugs of >$40 billion by 2030 (1).

 

The current market is dominated by oncology medicines, which account for >75% of the market. Hormonal and glaucoma drugs account for 20%, with the remainder covered by other indications.

This growth over the next decade will be fuelled by the increasing sales of in-line products, and by the maturation of compounds in current biopharmaceutical company pipelines.

This article will explore Antibody Drug Conjugates (ADCs), paying particular attention to the exciting chemistry required to manufacture the ‘warheads’.

In addition to the approved ADCs, there are also over 150 ADCs in various stages of clinical trials, with over 50 companies engaged in research (2-4). Many of the early ADCs focused on similar structures, resulting in low diversity. However, as the number of compounds has increased, so has the structural diversity. Table 1 shows how ADCs have already evolved:
The payloads of currently-approved ADCs fall mainly into 6 classes: Auristatins, Maytansinoids, Tubulysins, Calicheamycins, Duocamycins, Exetecans, and Pyrrolobenzodiazepines. Further information of the structures of these classes can be found in an existing publication (2).

 

DEVELOPMENT OF CHEMISTRY TO WARHEADS/PAYLOADS
There are additional challenges when it comes to the development of high potency drugs over the more standard small molecules. In this article, we are focussing on the chemical synthesis of payloads. Therefore, the handling of the linker and antibody conjugation are out of scope.

Route Selection for the Synthesis of a High Potency Drug
When it comes to the development of a synthetic route for a traditional small molecule drug, there tend to be a few criteria which aid decision-making:

 

1. Safety
2. Cost of the drug substance, ensuring a viable commercial cost of goods
3. Manufacturing strategy, including technology considerations, geographical locations, etc.
4. Environmental impact and ability of a route to minimise waste type and quantity
5. Supply chain robustness, starting material supply

 

Due to the additional challenges, we feel it is important to add an extra criterion when working with these compounds. This criterion is the point of the synthesis at which the molecule becomes highly potent.

 

Understanding the biology, activity, and toxicity of the molecule and intermediates shapes the strategy that should be followed. Ideally, the synthesis should be designed to introduce or unmask the potency as close to the end of the synthesis as possible. Taking this approach goes hand-in-hand with the other criteria; limiting the number of steps and unit operations will open more supply chain options. It also aids in reducing costs, as manufacture of materials in high containment comes at a significantly increased price.

 

Focus on the Synthesis
High potency molecules, driven by low therapeutic doses, tend to have very low bulk requirements. Where a traditional small molecule will likely have an annual bulk requirement measure in tonnes, a highly potent drug may only require low tens of kilogrammes. In some cases, such as niche drugs, it can be even less. The increase in marketed highly potent drugs, alongside the explosion in the sizes of development pipelines – particularly in the ADC space – led many CDMOs (Contract Development and Manufacturing Organisation) to make notable investment in manufacturing capacity. This helped to significantly close the gap in capacity for the manufacture of both clinical and commercial material to support the development of highly potent APIs. However, one of the remaining challenges is that the capacity for process development work on high potency molecules did not increase at the same rate. Much of the added lab capacity was in support of manufacturing and scale-up; also, the skills required to undertake significant rerouting packages did not fall in the bandwidth of many CDMOs. This has led to risks of under capacity, thus causing a long lead-time on development work for highly potent molecules. In some cases, companies have chosen to continue to manufacture with a sub-optimal route, which can increase both cost and supply chain length.

 

The Value
As mentioned, there is significant value to be gained from delivering better quality and innovative chemistry. Investing at the right time in developing the right chemistry will pay significant dividends during the product’s lifecycle. This route development chemistry can be completed internally or externally, such as with a CRO (Contract Research Organisation) or CDMO. A well-thought-out route design strategy will likely improve safety, increase supply chain efficiency, reduce cost, and minimise the environmental impact of your manufacture.

 

Below, there are some examples where good process chemistry has made a significant improvement to the synthesis of two highly potent molecules:

 

Duocarmycins
The development of a synthesis for high potency chemicals possesses several challenges:

 

1. The payload is often of a natural product-derived origin with a high molecular complexity, which can result in long, complex routes
2. The installation for a handle to connect a linker that bridges the payload to the antibody part
3. The high toxicity of the molecules can lead to several operations being performed in specialised facilities for highly potent materials

 

These strategies can be exemplified in the total synthesis of (+)-Duocarmycin SA, published by Eastgate and co-workers (BMS) in 2018 (5).

 

The careful design of the synthesis is inspired by the long history of synthesis of Duormycin spanning more than 40 years (6). The strategy relies on installing the core pharmacophore stereoselectively in the initial steps, while masking the key cytotoxic motif (in blue in Figure 1) until the end of the synthesis. This limits the number of steps requiring special containment.

Simplification of the strategy:

• Early installation of the stereocentre
• New key reaction to form the indole 2-carboxy ester, avoiding previous challenges and use of transition metals, no indoline C4 functionalisation as in previous syntheses, reducing the step count
• Use of a different protecting group for the indoline, enabling easy crystallisation
• Late stage amidation for easy diversification and allows for a potential handle towards conjugation (similar to MDX-1203)
• Only one step in high containment facilities

 

Enantioselective hydrogenation
To reach the desired precursor for the key enantioselective indole hydrogenation Eastgate and co-workers started with commercially available 6-benzyloxyindole (1). In four efficient steps, focusing on crystallisation or precipitation for isolation of the pure intermediates, key protected indole 2 was obtained on multigram scale in 86% yield. This enabled the investigation of the enantioselective hydrogenation reaction. An extensive screen gave [Rh(COD)(acac)] with ligand L1 in isopropanol as the best conditions and gave very high ee and excellent conversion, however, the precatalyst suffered from being highly sensitive to air and impurities in 2. This justified an extra purification step through a silica plug for 2 to allow for lower amount of L1 to be used.

Innovative VNS reaction
After an additional 4 steps notably changing the indoline protecting group to the more practical 4-cyanobenzensulfonamide (4-Cs), intermediate 4 was obtained. This laid the ground for a three-step Vicarious Nucleophilic Substitution (VNS): reduction of the nitro group, formation of the corresponding imine with methyl 2-hydroxy-2-methoxyacetate, and nucleophilic attack followed by re-aromatisation/olefination. Extensive investigation of this key transformation was required to reach the optimal conditions, particularly as there was no precedent on this class of compound in the literature. Under optimised conditions, it could be scaled up to 50g scale and gave the desired indole 5 in high yield (73% over three steps) after an aqueous work-up and subsequent crystallisation.

Unmasking the payload
In this strategy, it was important to minimise the high-containment steps. Thus, the only high-containment step was the last step, where through a dehydration of 7 under Mitsunobu-like condition, the payload was finally unmasked. The choice of reagents was carefully chosen for ease of removal: azodicarbonyl dimorpholide (ADDM) derivatives were removed through an aqueous wash and phosphine byproducts were removed via a hexane wash. A final column afforded the pure (+)-Duocarmycin SA.

 

Overall, Eastgate and the team designed an elegant synthesis crystallising decades of efforts. They overcame the challenging C4-indoline functionalisation by bringing an innovative vicarious nucleophilic substitution and bypassed several difficult purifications by using a strategic protecting group to increase crystallinity (4-cyanobenzensulfonamide, 4-Cs). This facilitated the isolation and purity for most intermediates. By strategically unmasking the potency of the payload in the final step, only one step in the synthesis was undertaken in high containment facilities, dramatically reducing the overall cost.

 

CONCLUSION
The importance of the synthetic design of a future ADC cannot be understated. Often, the molecular complexity and high toxicity of these compounds cause unique challenges that ultimately result in extra-cost and hazards. An ideal approach will focus on the simplification of the synthesis, often through innovative chemistry, as less steps equates to lower costs. Another crucial element is to limit the number of high-containment steps. This could be done by either unmasking the cytotoxic functionality at the end of the synthesis and installing it in the later stage, or by circumventing it altogether by the synthesis of prodrugs, unveiling the core pharmacophore in vivo through enzymatic pathways.

THE FUTURE
Overall, the future of ADCs and high potency molecules looks secure. Pfizer’s agreement to purchase ADC specialist Seagen for $43B shows the value that the industry puts on ADCs and their future potential as treatments. It is likely their design and development will continue to improve with more potent and targeted payloads. The need for innovative chemistry due to the inner complexities, plus exceptional route design coupled with full SAR(Structure–activity relationship) understanding to craft a perfect synthesis around the high containment steps, will be critical to successful development. Companies who can deliver this stand to succeed in this rapidly evolving market.

 

Table 1. Evolution of ADCs.

 

Figure 1. (+)-Duocarmycin SA strategy.

 

Scheme 1. Enantioselective indole – 3-bond reduction.

Scheme 2. Key indole 2-carboxy ester formation – Vicarious Nucleophilic Substitution.

Scheme 3. Unmasking the payload – Only high containment step.

REFERENCES AND NOTES

1. High Potency Active Pharmaceutical Ingredients Market Size, Share & Trends Analysis Report By Product (Synthetic, Biotech), By Manufacturer Type (In-House), By Drug Type, By Application, By Regio n, And Segment Forecasts, 2023 – 2030; 978-1-68038-563–2; Grand View Research; p 191. https://www.grandviewresearch.com/industry-analysis/high-potency-active-pharmaceutical-ingredients-hpapi-market (accessed 2023-09-14).
2. Fu, Z.; Li, S.; Han, S.; Shi, C.; Zhang, Y. Antibody Drug Conjugate: The “Biological Missile” for Targeted Cancer Therapy. Signal Transduct. Target. Ther. 2022, 7 (1), 93. https://doi.org/10.1038/s41392-022-00947-7.
3. Goundry, W. R. F.; Parker, J. S. Payloads for Antibody–Drug Conjugates. Org. Process Res. Dev. 2022, 26 (8), 2121–2123. https://doi.org/10.1021/acs.oprd.2c00227.
4. Tang, H.; Liu, Y.; Yu, Z.; Sun, M.; Lin, L.; Liu, W.; Han, Q.; Wei, M.; Jin, Y. The Analysis of Key Factors Related to ADCs Structural Design. Front. Pharmacol. 2019, 10, 373. https://doi.org/10.3389/fphar.2019.00373.
5. Schmidt, M. A.; Simmons, E. M.; Wei, C. S.; Park, H.; Eastgate, M. D. An Enantioselective Total Synthesis of (+)-Duocarmycin SA. J. Org. Chem. 2018, 83 (7), 3928–3940. https://doi.org/10.1021/acs.joc.8b00285.
6. Felber, J. G.; Thorn-Seshold, O. 40 Years of Duocarmycins: A Graphical Structure/Function Review of Their Chemical Evolution, from SAR to Prodrugs and ADCs. JACS Au 2022, 2 (12), 2636–2644. https://doi.org/10.1021/jacsau.2c00448.