The Isomer Challenge of Ursodeoxycholic Acid: From API Impurities to Bioequivalence Analysis

I. What Is Ursodeoxycholic Acid? Where Do Isomeric Impurities Come From?

Ursodeoxycholic acid (UDCA) is a natural bile acid found in high concentrations in the bile of hibernating animals such as bears, while constituting only a small fraction of the human bile acid pool.It is more hydrophilic than cholic acid and chenodeoxycholic acid (CDCA), allowing it to replace more hydrophobic and toxic bile acids in bile, thereby reducing cholestatic liver injury;clinically, it is used to dissolve cholesterol gallstones and to treat cholestatic liver diseases such as primary biliary cholangitis (PBC)[1].

Ursodeoxycholic acid  CAS No.: 128-13-2

Structurally, the chemical name of UDCA is 3α,7β-dihydroxy-5β-cholan-24-oic acid (C24H40O4, CAS 128-13-2)[2,3], and its 7α-epimer is CDCA.Whether using chemical synthesis (starting from bovine bile-derived cholic acid, first removing the 12-OH to yield CDCA) or enzymatic transformation (7α-HSDH/7β-HSDH dual-enzyme conversion), the 7α-OH must first be oxidized to 7-ketolithocholic acid (7-KLCA) and then reduced to 7β-OH[4]. Consequently, residual 7-KLCA and residual CDCA are the most direct process-related impurities.

II. Which Impurities Are Controlled by Pharmacopoeias?

Ph. Eur. monograph 1275 lists impurities A–I, among which A (CDCA) and C (lithocholic acid) are specified impurities[2]:

EP-Designated Impurities: Structural Deviation and Probable Origin

Compared against ursodeoxycholic acid (3α,7β-dihydroxy-5β-cholan-24-oic acid). Probable origins are process-based inferences for troubleshooting, not regulatory conclusions.

Source of names and structures: Ph. Eur. 7.0, 1275[2]; "Possible Source" column is inferred based on synthetic routes.

Ph. Eur. specifies impurity A ≤ 1.0%, any other individual impurity ≤ 0.10%, and total impurities ≤ 1.5%; lithocholic acid is separately controlled by TLC, ≤ 0.1%[2]. USP relaxes the limit for chenodeoxycholic acid to ≤ 1.5% and tightens lithocholic acid to ≤ 0.05%[3].

III. Without Chromophores, How Are Isomers Separated and Determined?

Bile acids possess a saturated steroid skeleton and exhibit very weak UV absorption. The Ph. Eur. method for related substances employs reversed-phase HPLC with refractive index detection, with an injection volume of up to 150 µL; system suitability requires a resolution between impurity H and UDCA of ≥ 1.5. The relative retention time of H is about 0.9, closely eluting before the main peak; CDCA is about 2.8, well separated from the main peak[2]. The USP method for related substances uses silica gel TLC with phosphomolybdic acid visualization[3].

Refractive index detection does not permit gradient elution, and TLC offers limited quantitative precision. Previous studies have established an HPLC-CAD (charged aerosol detection) method to simultaneously quantify 9 related substances in UDCA, and identified 2 unknown impurities using HPLC-Q-TOF-MS[5]. Regardless of the detector chosen, peak identification, resolution confirmation, and correction factor determination all require individual impurity reference standards.

IV. Why Is the Isomer Issue More Prominent in BE Studies?

UDCA is an endogenous bile acid, generated by gut microbiota 7-HSDH via isomerizing the 7α-OH of CDCA into 7β-OH. It exists in the body mainly in the form of glycoursodeoxycholic acid (GUDCA) and tauroursodeoxycholic acid (TUDCA), with GUDCA predominating in adults[6]. Therefore, the EMA, FDA, and CDE all require baseline correction, and the FDA and CDE also require the simultaneous determination of unconjugated UDCA and total UDCA (including glycine and taurine conjugates)[8–10].

The analytical difficulty lies in: CDCA, isoursodeoxycholic acid, hyodeoxycholic acid, and UDCA share the same quantitative ion transitions, and the method must be able to distinguish these substances and evaluate specificity during validation[7]; free bile acids, except those containing a 12-hydroxy group, lack characteristic fragment ions suitable for MRM, making selectivity highly dependent on chromatographic separation[6]. Published methods for human plasma utilize negative-ion ESI and quantify using UDCA-d4, GUDCA-d4, and TUDCA-d4 as internal standards[11].

V. CATO Ursodeoxycholic Acid Related Impurity Reference Standards

CATO offers over a hundred reference standards for the ursodeoxycholic acid series. Below are representative products directly relevant to the issues discussed above:

Reference Standards Catalogue

Note: EP Impurities D, E, G, I and Ursodeoxycholic Acid API itself (Chenodeoxycholic Acid EP Impurity A, C4X-1422) are also available; please contact CATO for the full list. Click the Cat. No. to view product details (specifications, purity, etc.) on the CATO official website; Data source: CATO Research Chemicals (www.cato-chem.com).

CATO (Guangzhou CATO Research Chemicals Inc.) is dually accredited under ISO 17034 by CNAS (RM0030) and ANAB (AR-2832), and products are accompanied by a Certificate of Analysis (COA).

Frequently Asked Questions

Q1: In bioequivalence (BE) method validation, how can it be proven that UDCA can be distinguished from its isomers?
Isomers that share quantitative ion transitions with UDCA must be included in the specificity evaluation to confirm chromatographic separation, and blank matrices from different sources should be evaluated for potential interference. The chromatographic method can also be combined with peak purity assessment[7]. Isomer reference standards such as isoursodeoxycholic acid (EP Impurity H) and taurochenodeoxycholic acid can be obtained from CATO.

Q2: Should d4 or d5 be selected as the deuterated internal standard?
The key lies in whether the mass difference can avoid interference from the analyte's isotopic peaks, whether the labeling position is stable, and the internal standard's contribution to the analyte channel needs to be evaluated during validation. CATO provides both UDCA-d4/d5 and deuterated derivatives of GUDCA and TUDCA, which can be selected based on method requirements.

References

[1] LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. Ursodiol (Ursodeoxycholic Acid). Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; updated 2017-09-25. NCBI Bookshelf ID: NBK548309.

[2] European Pharmacopoeia 7.0. Ursodeoxycholic acid, monograph 04/2010:1275.

[3] USP 35–NF 30. Ursodiol (official monograph).

[4] Tonin F, Arends IWCE. Latest development in the synthesis of ursodeoxycholic acid (UDCA): a critical review. Beilstein J Org Chem, 2018, 14: 470–483. doi:10.3762/bjoc.14.33

[5] Development of HPLC-CAD method for simultaneous quantification of nine related substances in ursodeoxycholic acid and identification of two unknown impurities by HPLC-Q-TOF-MS. J Pharm Biomed Anal, 2023. PMID: 36966621; PMCID: PMC10033147

[6] Ding JS, Wang AN, Huang L, Jia W, Liu CX, Lan K. Technical challenges in the study of human ursodeoxycholic acid metabolism and bioequivalence. Acta Pharmaceutica Sinica, 2020, 55(9): 2070–2079. doi:10.16438/j.0513-4870.2020-0447

[7] Liu SJ, Yan F, He RR. Technical considerations and case studies on bioequivalence research of endogenous drugs. Drug Evaluation Research, 2025, 48(3): 787–792. doi:10.7501/j.issn.1674-6376.2025.03.025

[8] EMA. Ursodeoxycholic acid capsule, film-coated tablet and suspension product-specific bioequivalence guidance. EMA/CHMP/559890/2021, effective 1 January 2023.

[9] U.S. FDA. Draft Guidance on Ursodiol (Tablet; revised Oct 2024) and Draft Guidance on Ursodiol (Capsule; revised May 2026).

[10] Center for Drug Evaluation, NMPA. Technical Guideline for Bioequivalence Studies of Ursodeoxycholic Acid Capsules, September 2021.

[11] Pinto MC, Berton DC, de Oliveira AC, Lazaro CM, Carandina SAC. Method development and validation of ursodiol and its major metabolites in human plasma by HPLC-tandem mass spectrometry. Clin Pharmacol, 2019, 11: 1–13. doi:10.2147/CPAA.S187519