Fluorimetric quantitation of citalopram and escitalopram in plasma: Developing an express method to monitor compliance in clinical trials

Background: Selective serotonin reuptake inhibitors (SSRIs) in general, and citalopram/escitalopram in particular, are widely used to treat clinical depression. However, SSRI bioavailability and non-compliance represent major issues, especially in the clinical trials setting. In this context, frequent drug-level measurements for compliance monitoring would be a desirable tool to improve clinical outcomes with SSRIs. However, the liquid chromatography techniques available are expensive, requiring excessive sample preparation, and suffer from high complexity. We sought to develop a rapid method for the measurement of citalopram/escitalopram levels in human plasma by fluorimetry. Methods: A total of 34 frozen human plasma samples were thawed at room temperature and repeatedly centrifuged in cellulose to remove aggregates, proteins and solids. Fluorescence spectra were measured in the range 270-450 nm with excitation at 240 nm on a FluoroMax 3 spectrofluorimeter. Control samples contained known concentrations of SSRIs. Results: SSRI absorbance spectra were recorded in the range 230-320 nm. The shape of the spectra and the absorbance of citalopram and escitalopram were very similar, with UV maximum absorbance at 239 nm. The maximum extinction coefficient was ε₂₃₉=15,930 M^-1 cm^-1 for citalopram and ε₂₃₉= 13,630 M^-1 cm^-1 for escitalopram. The fluorescence spectra of SSRIs are unique and are characterized by the presence of two well-defined conjugated spectra with maxima at 300 and 382 nm. Conclusions: Fluorimetry is very suitable for assessment of plasma SSRI levels. This inexpensive and efficient technique can objectively and reliably quantify drug levels in biological fluids, thereby directly determining the level of patient adherence to the prescribed drug regimen. This method will be useful in a broad spectrum of applications, from compliance/bioavailability assessments in animal and human experiments to utilization in large-scale clinical trials.