Representative of fully synthetic fluorocyclines: Eravacycline – Structure, synthesis process, and key points of quality research

Introduction

Therapeutic options for multidrug-resistant Gram-negative bacterial infections are becoming increasingly scarce, yet the tetracycline "old" skeleton has been rejuvenated through total synthesis technology.Eravacycline is the first approved fully synthetic tetracycline antibacterial drug, and its molecular design, process scale-up, and impurity control all deserve the attention of drug R&D and quality research personnel.This article outlines its key technical information.

Marketing Overview

Eravacyclinewas developed by Tetraphase Pharmaceuticals, approved by the US FDA in August 2018 for the treatment of complicated intra-abdominal infections (cIAI) in adults, and received a positive opinion from the European CHMP around the same time. Two Phase III studies, IGNITE1 and IGNITE4, using ertapenem and meropenem as controls respectively, both met the non-inferiority endpoints. In March 2023, Yijia® (eravacycline) was approved by the NMPA for marketing in China.

Eravacycline dihydrochloride

Common forms of clinical drugs and APIs

The marketed formulation is a lyophilized powder for injection, containing 50 mg of eravacycline per vial, which is equivalent to 63.5 mg of eravacycline dihydrochloride. The active pharmaceutical ingredient (API) exists in the form of dihydrochloride, a characteristic that runs through its process and quality research.

Chemical Structure and Structure-Activity Significance

Eravacycline (TP-434)has a molecular formula of C₂₇H₃₁FN₄O₈ and a chemical name of (4S,4aS,5aR,12aS)-4-(dimethylamino)-7-fluoro-3,10,12,12a-tetrahydroxy-1,11-dioxo-9-[2-(pyrrolidin-1-yl)acetamido]-1,4,4a,5,5a,6,11,12a-octahydrotetracene-2-carboxamide.

Compared with tigecycline, it features two modifications on the D-ring:the dimethylamino group at the C-7 position is replaced by a fluorine atom, and the t-butylglycylamido group at the C-9 position is replaced by a pyrrolidine acetamido group, hence the name "fluorocycline".Like other tetracyclines, it binds to the bacterial 30S ribosomal subunit to inhibit protein synthesis; modifications on the D-ring allow it to maintain activity against strains expressing certain common tetracycline-specific resistance mechanisms. In vitro studies have shown that it exhibits activity against ESBL-producing and carbapenem-resistant Enterobacteriaceae, Acinetobacter, etc. (except Pseudomonas aeruginosa).

From the perspective of quality research, structural features that require attention include: 4 chiral centers (4S, 4aS, 5aR, 12aS), the dimethylamino group at the C-4 position (epimerization site), oxidation-sensitive regions composed of polyphenolic hydroxyl/enol groups, and the salt form of the dihydrochloride.

Why Total Synthesis: Process Route Analysis

Traditional tetracycline drugs are mostly semisynthetic starting from fermentation products, with limited modifiable sites.Although semisynthetic methods starting from 7-fluorosancycline and others have been reported in patents, literature suggests that traditional semisynthetic processes are not suitable for the large-scale production of eravacycline. In contrast, the establishment of a total synthesis route enables more diverse chemical modifications at the C-7, C-8, and C-9 positions of the D-ring.

Several key literature references in the research context:

• Platform Methodology:The Myers research group established a total synthesis platform for tetracyclines based on the Michael–Dieckmann (also known as Michael–Claisen) reaction, constructing the C-ring via "AB-ring + D-ring" coupling, which provided a universal route for novel tetracyclines with structural diversity.

• Medicinal Chemistry Discovery:Xiao et al. reported in J. Med. Chem. a series of 7-fluoro-9-substituted-6-demethyl-6-deoxytetracyclines—namely "fluorocyclines"—designed, synthesized, and evaluated based on this total synthesis platform, with TP-434 being compound 17j.

• Process Research:Rönn et al. reported a kilogram-scale scale-up process in Org. Process Res. Dev. The route adopts a convergent strategy: the D-ring precursor (left-hand fragment) is deprotonated by a strong base to form a benzyl carbanion, which undergoes diastereoselective Michael addition to the AB-ring enone, followed by Dieckmann condensation with a phenyl ester to construct the C-ring, yielding the protected tetracycline skeleton. This reaction is condition-sensitive and must be performed at approximately -70°C. The final product is precipitated in the form of dihydrochloride in an ethanol/methanol solution using ethyl acetate as an antisolvent.

Additionally, patent literature indicates that existing processes face challenges in obtaining suitable solid forms, process scale-up, and separation/drying steps, which is also the background for the emergence of subsequent crystal form patents for eravacycline dihydrochloride.

Quality Research Perspective: Key Points of Impurity Control

Combining the structural characteristics of eravacycline and the commonalities of tetracycline drugs, the following types of impurities usually require focal attention:

1. Epimeric impurity: The C-4 dimethylamino group of tetracyclines readily undergoes reversible epimerization in weakly acidic solutions (approx. pH 2–6) to form 4-epimers, a process accelerated by elevated temperatures. It is a routinely monitored target in stability studies and release testing for this class of drugs, and may also be generated during various synthetic steps and post-processing.

2. Process-related impurities: The total synthesis route involves multiple steps, including left-right fragment coupling, cyclization, deprotection, and C-9 amidation, which may introduce residual intermediates, side-reaction products, and side-chain-related impurities.

3. Degradation impurities: The polyphenol hydroxyl/enol structure is sensitive to oxidation. Studies on similar structures indicate that oxidative degradation is more pronounced under neutral-to-alkaline conditions in the presence of oxygen, and solutions are less stable than the solid state. Additionally, the effects of light and metal ions must be considered, and degradation pathways should be systematically identified through forced degradation studies. Notably, eravacycline features a 6-demethyl-6-deoxy structure without a C-6 hydroxyl group, meaning it lacks the classic degradation pathway of tetracyclines involving dehydration to form anhydrotetracycline.

4. Solid-state and salt-formation aspects: The stoichiometry, hydration state, and crystalline form of the dihydrochloride salt affect its stability and are also critical points in process research.

High-quality reference standards with well-elucidated structures form the foundation for method development, method validation, impurity identification and quantification, and stability studies.

CATO Related Products (In Stock)

CATO can provide eravacycline-related impurity reference standards. The following products are available in stock:

CATO Eravacycline Impurity Series · Catalog Number List

The above products can be used for method development, method validation, and stability studies in eravacycline impurity research. Please feel free to contact us for more structural information, specifications, and COAs.

CATO holds dual ISO 17034 accreditations (CNAS RM0030 / ANAB AR-2832), providing reliable reference material support for drug R&D and quality control.

References

1. U.S. FDA. XERAVA (eravacycline) for injection, Prescribing Information. NDA 211109, 2018.

2. Zhanel G.G., Cheung D., Adam H., et al. Review of Eravacycline, a Novel Fluorocycline Antibacterial Agent. Drugs 2016, 76(5): 567–588.

3. Sun C., Wang Q., Brubaker J.D., et al. A Robust Platform for the Synthesis of New Tetracycline Antibiotics. J. Am. Chem. Soc. 2008, 130(52): 17913–17927.

4. Xiao X.-Y., et al. Fluorocyclines. 1. 7-Fluoro-9-pyrrolidinoacetamido-6-demethyl-6-deoxytetracycline: A Potent, Broad Spectrum Antibacterial Agent. J. Med. Chem. 2012, 55: 597–605.

5. Rönn M., et al. Process R&D of Eravacycline: The First Fully Synthetic Fluorocycline in Clinical Development. Org. Process Res. Dev. 2013, 17(5): 838–845.

6. Solomkin J., et al. JAMA Surg. 2017, 152(3): 224–232 (IGNITE1); Solomkin J.S., et al. Clin. Infect. Dis. 2019, 69(6): 921–929 (IGNITE4).

7. Everest Medicines. Announcement on the Approval of XERAVA® (eravacycline) for Marketing by the National Medical Products Administration, 2023.