In this study, the complexation behavior of β‑CD, mono‑subetadex (monoSBX), and subetadex (SBX) was investigated using citalopram as a model guest molecule. Stability constants were determined with four analytical techniques—phase‑solubility analysis, Hummel-Dreyer (HD) method, affinity capillary electrophoresis (ACE), and isothermal titration calorimetry (ITC)—and the results were critically compared.
The phase‑solubility method produced stability constants that deviated from the other techniques, with monoSBX – CIT complex showing the highest stability constant. This discrepancy likely arises from the long equilibration time (24 h), which allows additional interactions to occur. As a consequence, phase‑solubility remains valuable for assessing solubility enhancement, but may not always yield accurate stability constants. Importantly, the data highlight that solubility and complex stability are not directly proportional: a highly stable complex may still exhibit limited solubility.
The HD method proved fast, reproducible, and material‑efficient, requiring only small amounts of CD. Although the absolute stability constants differed from ACE and ITC, the trend in CD affinity was consistent across methods (β‑CD < monoSBX < SBX). Additional advantages include high reproducibility and low standard deviation, making it a reliable alternative, especially in laboratories equipped with HPLC but lacking specialized instruments.
The ACE and ITC methods yielded highly similar stability constants for β‑CD and monoSBX, with only minor deviation for SBX. Both methods confirmed that stability constants increase with the degree of substitution of CDs. The ACE and ITC appear to be the most reliable techniques for determining CD-guest stability constants. However, their availability is more limited than HPLC-based methods.
In summary, while each method has its advantages and limitations, ACE and ITC provide the most accurate stability constants, HD offers a reliable and resource-efficient alternative, and phase solubility remains useful for solubility enhancement studies.
Dóra V. Ujj, Petr Kasal, Ida Fejős, Szabolcs Béni, József Kardos, Gábor Benkovics, Erika Bálint, Béla Mátravölgyi; Complexation of Citalopram with β‑Cyclodextrin, Mono-subetadex, and Subetadex: Phase Solubility, Hummel–Dreyer, Affinity Capillary Electrophoresis, ITC, and NMR Studies. ACS Omega 28 April 2026; 11 (16): 24323–24333. https://doi.org/10.1021/acsomega.5c13544