Rapid automated spectrophotometric competitive complexation studies of drugs with cyclodextrins using the flow injection gradient technique: tricyclic antidepressant drugs with α-cyclodextrin

Abstract

An automated gradient flow injection (FI) spectrophotometric technique for competitive complexation studies of tricyclic antidepressant drugs with α-cyclodextrin (α-CD) using Methyl Orange (MO) as a probe is described. Formation constants for the complexes were determined utilising the total information contained in transient FI signals at 510 nm, the λ_max of the MO probe. The study was carried out rapidly in three steps: (a) calibration of the FI concentration gradient by injection of p-nitrophenol in a buffered carrier stream, (b) determination of the probe–α-CD complex formation constant by injecting α-CD into the probe carrier stream and (c) determination of the drug–α-CD complex formation constant by injecting α-CD into the probe–drug carrier stream. In this way the preparation and measurement of a great number of mixed drug–probe–α-CD solutions was automated, resulting in the fast and precise determination of complexation constants of ligands (drugs), that are optically transparent, with α-CD. Methyl Orange was found to be an excellent probe for α-CD in acidic solutions (pH 1.0) with a complexation constant of (5.3 ± 0.1) × 10² mol^–1 l. Complex formation constants determined at 26.5 ± 0.5 °C and pH 1.0 (HCl) utilising transient α-CD concentrations in the range (1–1000) × 10^–4 mol l^–1, were (1.2 ± 0.1) × 10² mol^–1 l for protriptyline, (0.7 ± 0.1) × 10² mol^–1 l for nortriptyline, (2.4 ± 0.1) × 10² mol^–1 l for maprotiline, (1.4 ± 0.1) × 10² mol^–1 l for doxepin, (1.13 ± 0.07) × 10² mol^–1 l for amitriptyline and (1.3 ± 0.2) × 10² mol^–1 l for imipramine. A 1:1 stoichiometry was found in all cases. The within- and between-run precisions ranged from 1.2 to 6.4% and from 4.2 to 15.4% (RSD), respectively. The determined complex formation constants were in good agreement with those in the literature. Using the proposed FI gradient technique, the tedious manual preparation of a great number of mixed solutions with variable α-CD to drug ratios and the subsequent spectrophotometric measurements are replaced by a single FI measurement lasting 80 s.