image: Computational model of stereoselective fluoride incorporation.
Credit: Scripps Research
LA JOLLA, CA—Fluorine is critical for biomedicine. This element can help drug compounds be more potent and last longer in the body, and its radioactive isotope, fluorine-18, powers medical imaging techniques such as positron emission tomography (PET). But scientists have long struggled with adding fluorine to the most common chemical bonds—carbon–hydrogen (C–H) bonds—in a way that’s precise, efficient and compatible with the molecules used to create many modern medicines. There’s been particular interest in constructing carbon–fluorine bonds stereoselectively—that is, attaching fluorine from a specific direction in space to create the needed fluorinated stereoisomer (“mirror image” form) of the target molecule. Stereoselective C–H fluorination has remained one of the most challenging synthetic transformations, and the limited approaches developed to date have relied on expensive specialty chemicals or complicated, multi-step procedures.
Now, chemists at Scripps Research have developed a long-sought method to stereoselectively attach fluorine atoms to complex, drug-like molecules in a single step using cheap, readily available fluoride salts. The method, published in Nature Catalysis, enables new ways of molecular editing to accelerate the development of more effective therapeutics and more accessible radioactive tracers for medical imaging.
“This discovery changes how we think about C–H fluorination,” says corresponding author Jin-Quan Yu, the Bristol Myers Squibb Endowed Chair in Chemistry and the Frank and Bertha Hupp Professor of Chemistry at Scripps Research. “It not only solves a decades-old problem but also points to new strategies for designing catalysts, which are substances that are vital to speeding up chemical reactions.”
Over the past decade, some years saw as many as half of new FDA-approved drugs contain fluorine. However, directly incorporating fluorine into the stable C–H bonds of drug-like molecules in a stereoselective fashion has long been a challenge. The new approach solves this by combining a palladium-based catalyst with a specially designed chiral “helper” molecule called a ligand.
“Our approach employed interaction-driven ligand design and computational modeling to guide the catalyst to a specific spot on the target molecule, and to position it precisely in 3D space in order to form the required fluorinated stereoisomer,” says first author Nikita Chekshin, a former doctoral student at Scripps Research who’s now a scientist at Bristol Myers Squibb.
This method successfully fluorinated a range of medicinally relevant drug-like compounds, including derivatives of flutamide (a prostate cancer drug) and fentanyl (an opioid used for pain relief) with high yields and enantioselectivity—meaning it created only the desired stereoisomer of the product, the version most likely to work as intended in the body. The team also tested numerous fluoride salts to refine the chemistry. Their protocol allowed using cheap potassium fluoride, which is essential for the incorporation of fluorine-18, to access radioactive tracers used in PET imaging.
“Fluoride is not only the most sustainable and readily available source of fluorine; it’s also the way fluorine-18 is synthesized and used in radiochemical applications,” notes Chekshin.
Importantly, the reaction can be run quickly—in only 45 minutes. That’s critical because fluorine-18 has a half-life of about 110 minutes, meaning its amount halves in less than two hours. The reaction’s speed renders the method useful for preparing PET tracers on clinical timelines. Until now, that half-life made it challenging to introduce fluorine-18 with older, slower approaches. Radiochemists often had to take longer synthetic routes, which limited the kinds of molecules they could work with.
“As a PET tracer, fluorine-18 has to be injected into the studied tissue almost immediately because it decays so quickly,” explains Chekshin. “This new method simplifies the process of attaching fluorine-18 to drug-like molecules, making it far more practical for medical imaging.”
Thanks to the technique, it’s now possible to take fluorine-18 directly from a cyclotron—a particle accelerator that produces radioactive compounds for medical imaging—and incorporate it into drug-like molecules in a single step before it decays. In collaboration with Bristol Myers Squibb, the team demonstrated that the method could radiolabel bioactive compounds—in other words, tag drug-like molecules with fluorine-18, which allows tracking their path through the body with medical scans.
“This platform will enable late-stage functionalization of medicines,” says Yu. “That means attaching radioactive tracers to drug molecules at the final step so they can be tracked in imaging studies, without needing to remake the entire compound from scratch.”
“Because this fluorine platform has been applied in industry, we’re optimistic that it will accelerate both drug discovery and diagnostic imaging,” adds Chekshin.
And the team has already extended their platform to other elements besides fluorine—including oxygen, halogens and nitrogen—allowing scientists to even more easily fine-tune drug compounds.
In addition to Chekshin and Yu, authors of the study “Enantioselective Pd-catalysed nucleophilic C(sp3)–H (radio)fluorination,” include Luo-Yan Liu, D. Quang Phan and Yuxin Ouyang of Scripps Research; and David J. Donnelly, Kap-Sun Yeung and Jennifer X. Qiao of Bristol Myers Squibb Research and Development.
This work was supported by Scripps Research; the National Institutes of Health (the National Institute of General Medical Sciences, grant 2R01GM084019); and Bristol Myers Squibb.
About Scripps Research
Scripps Research is an independent, nonprofit biomedical research institute ranked one of the most influential in the world for its impact on innovation by Nature Index. We are advancing human health through profound discoveries that address pressing medical concerns around the globe. Our drug discovery and development division, Calibr-Skaggs, works hand-in-hand with scientists across disciplines to bring new medicines to patients as quickly and efficiently as possible, while teams at Scripps Research Translational Institute harness genomics, digital medicine and cutting-edge informatics to understand individual health and render more effective healthcare. Scripps Research also trains the next generation of leading scientists at our Skaggs Graduate School, consistently named among the top 10 US programs for chemistry and biological sciences. Learn more at www.scripps.edu.
Journal
Nature Catalysis