The application of fluorine chemistry in the pharmaceutical field can be traced back to the 1950s. In 1953, Dr. Josef Fried and Dr. Emily Sabo prepared a series of cortisone acetate derivatives and discovered that as a glucocorticoid, the 9-fluoro-substituted cortisone acetate exhibited anti-inflammatory activity more than 10 times higher than that of the corresponding parent compound. This first publicly demonstrated that introducing fluorine atoms into specific positions of drug molecules can improve their biological activity.
In 1957, Dr. Robert Duschinsky and others completed a series of studies on the synthesis, characterization, and clinical testing of the nucleic acid antagonist 5-fluorouracil, contributing to breakthroughs in cancer treatment. Introducing fluorine atoms or fluorine-containing groups into drug molecules can alter the permeability and metabolic stability of drug molecules, regulate their pKa and lipophilicity, and affect the absorption, distribution, and interactions with biological targets of drug molecules. This has gradually become a common method in drug screening.
Among the 38 small-molecule drugs approved by the U.S. FDA in 2018, 18 were fluorinated drugs, such as Biktarvy for the treatment of human immunodeficiency virus type 1 (HIV-1) infection and Erleada (Apalutamide) for non-metastatic castration-resistant prostate cancer. However, with the development and popularization of fluorinated drugs, it is necessary to consider their chemical stability in the human body and the effects of metabolites produced under the action of enzymes on the human body. Safety issues during drug treatment also need to be taken seriously. The C-F bond has a high bond dissociation energy (BDE), indicating that it is not prone to homolytic cleavage. However, under the action of nucleophiles and drug-metabolizing enzymes in the human body, certain types of C-F bonds can easily generate fluoride anion species through heterolytic pathways. Early clinical data showed that the broad-spectrum antifungal drug voriconazole can increase plasma fluoride levels in humans. Fluoride has a strong affinity for Ca²⁺ in bones, which can lead to reduced bone strength, bone metabolism disorders, and diseases such as periostitis and osteochondroma. Recently, Dr. Yue Pan from the Novartis Institute for Biomedical Research (NIBR) summarized the possible metabolic pathways of fluorinated drugs with different structures in the human body in the ACS medicinal chemistry journal ACS Medicinal Chemistry Letters. Some drug molecules may decompose to produce fluoride and toxic fluorinated metabolites, and feasible suggestions were proposed for structural improvements in some cases. This serves as a reminder to medicinal chemistry researchers to think carefully when designing drugs with related structures.
For monofluoroalkyl drugs with nucleophilic sites within the molecule, the monofluoroalkyl group is prone to participate in SN2 nucleophilic substitution reactions, leading to C-F bond cleavage. The stability of several 2-(monofluoromethyl)pyrrolidine compounds (1) was investigated, and it was found that they can gradually decompose by 60%-90% at 50 °C and within the pH range of human blood, releasing fluoride anions. In contrast, the non-fluorinated parent compounds decomposed by less than 2% under the same conditions. In 4-fluoroleucine (5), the carboxylate anion can nucleophilically attack the fluorine-substituted tertiary carbon site, leading to C-F bond cleavage. The driving force for the defluorination and decomposition of such compounds is the thermodynamic stability of forming a five-membered lactone ring. Increasing the number of carbon atoms between the carboxylate anion and the fluorine atom can greatly reduce the tendency toward defluorination. In addition to the influence of intramolecular nucleophilic sites, monofluoro substitution at active sites (such as benzylic, allylic, and α-carbonyl positions) can also interact with nucleophiles in living organisms. For example, drug molecule 6 has a half-life of only 20 minutes in rat brain homogenate due to its reaction with glutathione.
Defluorination of monofluoroalkyl drugs (Image source: Reference [1])
Drugs with lone pairs of electrons or partial negative charges at the β-position of fluorine atoms β-fluoro-substituted carbonyl compounds have more acidic α-H atoms, which can undergo HF elimination. Fluoro-substituted methylamines can also undergo defluorination due to the presence of lone pairs of electrons on the nitrogen atom at the β-position, such as drugs with general structural formulas. This electronic effect can also be transmitted remotely through conjugate systems, such as in difluoromethyl imidazole and 6-difluoromethyl indole. The chemical stability of such drugs can be appropriately improved by converting them into corresponding amides.
Defluorination of drugs with lone pairs of electrons or partial negative charges at the β-position of fluorine atoms (Image source: Reference [1])
Fluoroalkyl drugs that produce Michael acceptors after metabolic elimination of HF In addition to inherent chemical instability, some fluorinated compounds can decompose through various metabolic mechanisms to generate fluoride anion species. For example, some fluoroalkyl compounds can produce Brønsted acidic functional groups (-NH2, -OH) during metabolism, followed by deprotonation and fluorine elimination to form Michael acceptors.
Problems such as these can arise from O-demethylation in 12, N-dealkylation in 13, and oxidation of NH2 or OH in 14. The authors noted that α-methylation of the NH2 group in 14 can effectively inhibit its oxidation.
Defluorination of fluoroalkyl drugs that produce Michael acceptors after metabolic elimination of HF (Image source: Reference [1])
Fluoroalkyl, fluoroalkenyl, and fluoroaryl drugs that undergo defluorination after oxidation Fluorocyclohexane 16 can undergo α-hydroxylation under the oxidative action of cytochrome P450 enzymes (CYP), followed by defluorination. CRF-R1 inhibitor 17 can also undergo defluorination under the oxidative action of human liver microsomal enzymes. Alkenyl fluoride 19 can undergo alkenyl double bond epoxidation under CYP oxidation, in which the carboxylate anion formed after amino carboxylation can act as a nucleophile to attack the oxiranyl group, leading to ring opening and defluorination. In addition, some aryl fluorides can also undergo oxidative defluorination under the action of different enzymes, such as drug molecules 21 and 22.
Defluorination of fluoroalkyl, fluoroalkenyl, and fluoroaryl drugs after oxidation (Image source: Reference [1])
Drugs containing N-, O-2-fluoroethyl or 1,3-difluoro-2-propyl substituents N-2-fluoroethyl-substituted compound 23 can undergo N-dealkylation and oxidation during metabolism to form fluoroacetic acid 24, which can block the tricarboxylic acid cycle in the human body and is highly toxic. Drugs containing the 1,3-difluoro-2-propyl structural unit, such as compound 26, can also undergo dealkylation and oxidation after metabolism to form 24.
Defluorination of drugs containing N-, O-2-fluoroethyl or 1,3-difluoro-2-propyl substituents
The above describes the molecular structures of fluorinated drugs that may decompose in the human body and produce toxic fluorinated metabolites. The purpose of this article is not to advise against the use of fluorinated drugs but to provide a clearer understanding of the molecular structures of fluorinated drugs that may pose safety risks. This will help avoid such structures during future drug molecular design and screening, saving time and costs while improving research efficiency. At the same time, it is a reminder that when designing novel drug molecules, in addition to focusing on the effects of drug derivatization on biological activity, it is also necessary to deeply understand the drug's mechanism of action, analyze its possible metabolic pathways in the human body, and comprehensively evaluate its effects.
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