Enhancing the solubility and antibacterial activity of novel molecular salts of enrofloxacin drug with isomeric pyridinedicarboxylic acids

Abstract

Enrofloxacin (EFX) is a third-generation synthetic fluoroquinolone with a broad spectrum of antibacterial activity but suffers from low water solubility, affecting its bioavailability. This study attempts to enhance the physicochemical and biological properties of enrofloxacin by converting it into multicomponent forms using crystal engineering concepts. Cocrystallization of enrofloxacin with isomeric pyridine-2,n-dicarboxylic acids (n = 3,4,5,6) resulted in four new crystalline salts (1:1): EFX·Py2,3DCA, EFX·Py2,4DCA, EFX·Py2,5DCA·H₂O and EFX·Py2,6DCA·H₂O; two of these are monohydrates. The protonation of the nitrogen atom of the piperazine moiety and the presence of crystallization water molecules were confirmed by single-crystal X-ray diffraction and Fourier transform infrared spectroscopy. Thermogravimetric analysis provided information on the thermal behaviour of multicomponent forms. The biological studies showed that the obtained salts are characterized by high antibacterial activity against Gram-positive and Gram-negative bacteria, and their haemolytic activity is low. The new salts demonstrate significantly greater solubility in water compared to the parent drug, along with enhanced antibacterial activity; hence, pyridinedicarboxylic acids appear to be efficient cocrystallizing agents for improving the efficacy of pharmaceutical ingredients.

Keywords: Antibacterial activity; Crystal engineering; Crystal structure; Enrofloxacin; Fluoroquinolones; Solubility.

Fig. 1

Structures of enrofloxacin and isomeric pyridine-2,n-dicarboxylic acids (n = 3,4,5,6).

Fig. 2

View of the asymmetric unit of EFX·Py2,3DCA (a), EFX·Py2,4DCA (b), EFX·Py2,5DCA·H₂O (c) and EFX·Py2,6DCA·H₂O (d), with atom-numbering schemes. Displacement ellipsoids are drawn at the 50% (a) and 30% (b–d) probability level. H-atoms are shown as spheres of arbitrary radii.

Fig. 3

A scheme of intermolecular interactions for EFX·Py2,3DCA (a) EFX·Py2,4DCA (b) EFX·Py2,5DCA·H₂O (c) and EFX·Py2,6DCA·H₂O (d).

Fig. 4

Fourier-transform infrared spectra of EFX·Py2,3DCA (red), EFX·Py2,4DCA (violet) EFX·Py2,5DCA·H₂O (green) and EFX·Py2,6DCA·H₂O (cyan).

Fig. 5

Thermoanalytical curves for the analysed enrofloxacin salts with pyridinedicarboxylic acids and QMID (quasi multiple ion detection) for H₂O⁺ ion current (m/z = 18) at 25–500 °C. The TG (thermogravimetric) curves are plotted in green, DTG (derivative thermogravimetric) in red, DTA (differential thermal analysis) in blue and QMID in pink.

Fig. 6

The haemolysis of erythrocytes in the presence of the tested compounds. The obtained results are expressed as the mean ± SD. * - represents statistically-significant results (p ≤ 0.05) determined via one-way analysis of variance (ANOVA).