How WuXi AppTec Helps Clients Overcome Macrocycle Synthesis Challenges?

To address the industry-wide challenge of macrocycle synthesis, WuXi AppTec has developed a systematic solution through years of continuous effort. The team has assembled a technical toolbox of over 20 representative cyclization methods to address the key hurdles in macrocycle synthesis, enabling flexible selection of the optimal synthetic route based on molecular structural characteristics, and thereby effectively improving synthesis success rates and overall efficiency. Meanwhile, the team continuously refines its integrated capabilities—from molecular design and synthesis optimization to scalable preparation, helping partners efficiently transform complex macrocyclic compounds into viable drug candidates.

Several years ago, WuXi AppTec’s Research Chemistry Services (RCS) team took on a new assignment. A client project required the construction of a macrolide compound. In the project’s early phase, the route provided by the client was able to construct the target molecule, but the reaction was complex, and the main byproduct—a dimer—accounted for a significant fraction of the product mixture, resulting in an isolated yield of just 13% of the desired product. To compound the issue, the product and byproduct had similar polarity, making purification inefficient and scale-up even more difficult. For a molecule at a critical stage of clinical advancement, this synthesis clearly could not meet the supply requirements as the program progressed from gram-scale to hundred-gram batches and beyond.

Upon taking over the project, the RCS team analyzed the byproduct formation pathway based on their mechanistic understanding and extensive project experience, and systematically optimized the reaction conditions. Through multiple rounds of design and screening, they identified key parameter combinations that significantly improved selectivity for the desired product. What was once a complex reaction system became well-controlled and predictable. Purification steps were simplified, and the yield was boosted from 13% to 45%.

The yield improvement was only the first step. Scale-up presented an even greater challenge to the team’s process development capabilities. Leveraging their deep expertise in macrocycle chemistry, the team successfully achieved reproducible synthesis of the target macrocycle at a scale exceeding 80 grams.

This case is underpinned by years of accumulated expertise in macrocycle chemistry at WuXi AppTec. Today, the team combines the ability to solve complex macrocycle synthesis challenges with a solid foundation for scalable process development—built upon its integrated platform.

Image source: 123RF

Macrocyclic Drugs: Breaking the “Undruggable” Barrier, but Synthesis Remains Daunting

Macrocycles are generally defined as organic molecules containing a ring of at least 12 heavy atoms (atoms other than hydrogen). Historically, macrocyclic drugs originated from natural sources, with classic examples including erythromycin, rifamycin, and cyclosporine A. Inspired by these natural frameworks, medicinal chemists have attempted to explore new macrocyclic chemical space through chemical synthesis, but have long been constrained by a core challenge: notably low cyclization efficiency.

A milestone breakthrough occurred at the beginning of this century. In 2005, olefin metathesis, particularly ring-closing metathesis (RCM) was recognized with the Nobel Prize in Chemistry. This technology enables the exchange and rearrangement of carbon–carbon double bonds through metal catalysts, making previously multi-step, low-yield macrocyclization reactions more feasible. This technology quickly demonstrated its value in drug synthesis: the key macrocyclic scaffolds of several FDA-approved hepatitis C virus (HCV) protease inhibitors were constructed via RCM. Moreover, it provides an efficient and reliable technical pathway for building the core structures of various macrocyclic drugs, including antiviral agents, anticancer drugs, and immunosuppressants.

According to an editorial in the Journal of Medicinal Chemistry (February 2026), the FDA has approved more than 60 macrocyclic drugs for indications including infectious diseases, oncology, and autoimmune disorders. These include NS3/4A protease inhibitors for hepatitis C, melanocortin-4 receptor inhibitors for obesity, JAK2 inhibitors for myelofibrosis, and ROS1/TRK inhibitors for non-small cell lung cancer.

This number represents approximately 4% of all FDA-approved drugs (excluding biologics). Due to the inherent difficulty of synthesizing macrocycles, they have historically been underdeveloped as a drug class.

In recent years, beyond RCM, emerging methods such as intramolecular C–H arylation and template-directed cyclization have been continuously streamlining the design of synthetic routes for macrocyclic drugs. These ongoing innovations in synthetic technology, combined with growing R&D demand for novel targets, are driving a resurgence of industry interest in macrocyclic drugs.

More importantly, macrocyclic drugs are emerging as key players in tackling historically “undruggable” targets. Studies show that macrocycles can effectively bind to secreted proteins, cell surface receptors, and intracellular proteins lacking druggable binding pockets. With molecular weights that fall between those of small molecules and biologics (roughly 500–2000 Da), they functionally combine the advantages of both: achieving high binding affinity that small molecules often cannot, while overcoming the inability of biologics to reach intracellular targets. A February 2026 report in the Journal of Medicinal Chemistry vividly described macrocycles as “molecular diplomats.”

The ability of macrocycles to integrate the advantages of small molecules and biologics stems from their unique cyclic scaffold. They can selectively adopt different dominant conformations depending on the polarity of their environment—exposing polar groups in aqueous environments to maintain solubility, and shielding polarity through intramolecular hydrogen bonds in hydrophobic membrane environments to facilitate transmembrane transport. For this reason, they are also colloquially called “molecular chameleons.”

This characteristic gives macrocycles unique potential in oral drug delivery—they can simultaneously breach membrane barriers and maintain systemic exposure, expanding the boundaries of traditional oral drug design. Moreover, this environment-responsive property also endows macrocycles with high binding affinity, with some optimized molecules achieving nanomolar to sub-nanomolar potencies, comparable to those of antibodies.

Despite these promising prospects, the development of macrocyclic drugs is not without significant challenges. One core challenge lies in synthesis: low cyclization efficiency, lengthy synthetic routes, and scale-up difficulties—factors that collectively limit the pace of advancement for such molecules.

WuXi AppTec’s Integrated Platform Addresses Macrocyclic Drug Synthesis Challenges

WuXi AppTec’s RCS team has built a mature macrocycle chemistry capability, drawing on years of experience in macrocyclic drug research to provide partners with integrated support—from molecular design and synthesis optimization to scalable preparation. 

To address challenges such as low cyclization efficiency, complex conformational control, and scale-up difficulties in macrocycle synthesis, WuXi AppTec has assembled a technical toolbox of over 20 representative cyclization methods. These include Mitsunobu cyclization, RCM cyclization, lactam cyclization, sulfonamide cyclization, click reaction, reductive amination, C–H activation, and disulfide bond formation. This allows the team to flexibly select the optimal route based on the molecular characteristics of each target, improving synthesis success rates and overall efficiency. This capability works in deep synergy with the company’s integrated platform, helping customers efficiently advance complex molecules into drug candidates with real development potential.

To date, the WuXi AppTec RCS team has synthesized over 10,000 macrocyclic compounds, covering structural classes such as macrolides, lactams, sulfonamides/sulfonimides, and cyclic ethers. The team has accumulated solid hands-on experience in the reproducible scale-up of complex synthetic routes.

Leveraging WuXi AppTec’s integrated platform, the team has also applied flow chemistry into the scale-up production of macrocyclic compounds, providing clients with a more stable and efficient scale-up pathway to better enable the advancement of their projects.

This integrated technical system for macrocyclic compounds has demonstrated its enabling value in drug research projects across fields including anti-infectives, immunology, and oncology. Consider an anti-infective program: in response to a client’s SAR (structure–activity relationship) study requirements, the RCS team optimized the synthetic route and, leveraging efficient cyclization methods, rapidly delivered a range of structurally diverse macrocycle analogs, helping the client accelerate the drug discovery timeline.

In another client project, the synthesis of the target macrocycle faced cyclization challenges. The existing esterification cyclization strategy reported in the literature proved ineffective for this project, mainly generating dimer byproducts and failing to yield the desired product. Rather than defaulting to existing methods, the RCS team conducted an in-depth analysis of the molecular structure and reaction mechanism. Ultimately, they re‑routed the synthetic strategy toward a coupling reaction, successfully achieving high-yielding synthesis of the target macrocycle.

Image source: 123RF

Currently, both technological development and industry demands continue to evolve, the boundaries of macrocycle chemistry are constantly expanding. For example, how to efficiently introduce chirality into macrocycles via catalytic methods remains a frontier question in the field. In February 2026, the journal Science reported a technological breakthrough: building on the asymmetric organocatalysis principles recognized by the 2021 Nobel Prize in Chemistry, a research team used bifunctional peptide catalysts to pre-organize the terminal functional groups of linear precursors, successfully synthesizing the core structure of the natural product robotnikinin. This method provides a practical route to chiral macrocycles with predictable stereochemistry, highlighting the critical role of asymmetric catalysis in such syntheses.

At the same time, drug discovery is increasingly guiding the industry toward frontier molecules with more complex structures and better-defined targeting; macrocycles are a prime example. With innovations in synthetic methods, a deeper understanding of medicinal chemistry, and the rapid emergence of cutting-edge technologies, macrocyclic molecules are poised to play an increasingly important role in future therapeutic development. 

As an enabler of innovation, WuXi AppTec stays attuned to both scientific progress and client needs, continuously expanding and refining its capabilities and technologies in macrocycle chemistry. Alongside this, the company will continue to strengthen its capabilities across various complex molecule modalities, including macrocycles, to better serve the industry and its clients, helping global clients accelerate the delivery of new and better medicines to patients in need.

Key Takeaways: 

• WuXi AppTec’s RCS team has built a mature macrocycle chemistry capability, drawing on years of experience in macrocyclic drug research to provide partners with integrated support—from molecular design and synthesis optimization to scalable preparation. 
• To address challenges such as low cyclization efficiency, complex conformational control, and scale-up difficulties in macrocycle synthesis, WuXi AppTec RCS has assembled a technical toolbox of over 20 representative cyclization methods. These include Mitsunobu cyclization, RCM, lactam cyclization, sulfonamide cyclization, click reaction, reductive amination, C–H activation, and disulfide bond formation. This allows the team to flexibly select the optimal route based on the molecular characteristics of each target, improving synthesis success rates and overall efficiency.
• WuXi AppTec’s macrocycle synthesis capabilities work in close synergy with the company’s integrated platform, enabling projects to strike a balance between innovative design and process feasibility early in development, and helping partners efficiently advance complex molecules into drug candidates with real development potential.
• To date, the WuXi AppTec RCS team has synthesized over 10,000 macrocyclic compounds, covering structural classes such as macrolides, lactams, sulfonamides/sulfonimides, and cyclic ethers. The team has accumulated solid hands-on experience in the reproducible scale-up of complex synthetic routes.
• Leveraging WuXi AppTec’s integrated platform, the WuXi AppTec RCS team has also applied flow chemistry to the scale-up production of macrocyclic compounds, providing clients with a more stable and efficient scale-up pathway to better enable the advancement of their projects.