Elsevier

Journal of Molecular Structure

Volume 1032, 30 January 2013, Pages 69-77
Journal of Molecular Structure

X-ray crystallographic, FT-IR and NMR studies as well as anticancer and antibacterial activity of the salt formed between ionophore antibiotic Lasalocid acid and amines

https://doi.org/10.1016/j.molstruc.2012.07.036Get rights and content

Abstract

Two new complexes of the ionophore antibiotic Lasalocid acid (LAS) with phenylamine (PhA) and butylamine (BuA) were synthesized and their molecular structures were studied using single crystal X-ray diffraction and spectroscopic methods. In the solid state both amines are protonated and all NH3+ protons are hydrogen bonded to etheric, hydroxyl and carboxylic oxygen atoms of the LAS anion. In chloroform solutions the structure observed in the crystal of LAS–BuA complex is preserved and an equilibrium between the LAS–PhA complex and dissociated Lasalocid acid and phenylamine is observed. In vitro antimicrobial tests of the complexes showed a significant activity towards some strains of Gram-positive bacteria. For the first time Lasalocid acid and its complexes with amines were tested in vitro for cytotoxic activity against human cancer cell lines: A-549 (lung), MCF-7 (breast), HT-29 (colon) and mouse cancer cell line P-388 (leukemia). We found that LAS and its complexes are strong cytotoxic agents towards all tested cell lines. The cytostatic activity of the compounds studied is greater than that of cisplatin, indicating that Lasalocid and its complexes are promising candidates for new anticancer drugs.

Highlights

Ionophore antibiotic complexes with aromatic and aliphatic amines were obtained. ► Antimicrobial tests demonstrated activity of all compounds studied against Gram-positive bacteria. ► Lasalocid acid and its complexes can be recognized as potential anticancer drug candidates. ► A pseudo-cyclic structure was found to be strictly requested for cytotoxic activities.

Introduction

Natural products have proven to be the most reliable single source of new and effective anticancer agents. Nearly 60% of anticancer and anti-infective agents that are commercially available or in the late stages of clinical trials originate from natural products [1]. Available literature data suggest that the utility of natural products as sources of novel structures, but not necessarily the final drug entity, is still topical [2], [3]. Thus, in the field of anticancer drugs, in the time range from the 1940s to date, over 70% of the agents are other than synthetic, with almost 50% actually being either natural products or directly derived from them [4].

Lasalocid acid is a representative of naturally occurring polyether ionophore antibiotics [5], [6]. Ionophores are lipophilic chelating agents that transport cations across lipid bilayer membranes, such as the plasma and subcellular membranes of cells. This unregulated membrane transports of various ions leads to mitochondrial injury and cell swelling, vacuolisation, and finally death [5], [6], [7]. To date, over 120 polyether ionophore structures have been characterized [8]. Ionophore antibiotics show a broad spectrum of bioactivity ranging from antibacterial, antifungal, antiparasitic, antiviral, and recently discovered anti-tumour cell cytotoxicity [9]. The discovery of strong anticancer properties of one of them – salinomycin has received much attention in recent years [10], [11], [12], [13], [14], [15]. It has been shown that salinomycin is 100 times more effective against breast cancer stem-like cells than Taxol (paclitaxel), a commonly chemo-therapeutic drug used against breast cancer [15]. After discovery of the anticancer properties of salinomycin, which has been so far the top candidate for a chemotherapeutic agent among the ionophore antibiotics, other ionophore antibiotics have been more thoroughly explored [14]. It has been also demonstrated that several candidates for chemotherapeutic drugs can be found among other naturally occurring ionophore antibiotics such as monensin A, inostamycin and nigericin [9], [16], [17], [18], [19], [20].

Lasalocid acid (Scheme 1), isolated from Streptomyces lasaliensis, is able to form pseudomacrocyclic complexes with monovalent and divalent cations and to transport these cations across cell membranes [21]. It is commercially used as a coccidiostat for poultry and as growth promoter for ruminants [22].

In previous publications, we have shown that Lasalocid acid forms complexes with allylamine [23] and tetramethylguanidine [24]. The first complex shows higher antibacterial activity than pure Lasalocid and the second complex has antibacterial activity comparable to that of Lasalocid. As an extension of these studies we synthesized and investigated two new hydrogen-bonded complexes of Lasalocid acid (LAS) with aromatic amine (phenylamine, PhA) and aliphatic amine (butylamine, BuA) by X-ray and FT-IR, and NMR spectroscopy. We determined the antimicrobial and cytotoxic activity of the Lasalocid complexes for the first time and compared them to the respective activity of uncomplexed Lasalocid.

Section snippets

General

Lasalocid sodium salt was extracted from Avatec. Phenylamine (PhA) and n-butylamine (BuA) and solvents were obtained from Sigma–Aldrich or Fluka and used without any further purification.

Preparation of Lasalocid–Phenylamine (LAS–PhA) and Lasalocid–Butylamine (LAS–BuA) complexes

Lasalocid sodium salt (1.0 g) was dissolved in dichloromethane (150 ml) and stirred vigorously with a layer of diluted aqueous sulphuric acid (pH = 1.5) (100 ml). The organic layer containing Lasalocid acid (LAS) was washed three times with distilled water. Subsequently dichloromethane was evaporated under reduced

Crystal structure of the Lasalocid–Phenylamine (LAS–PhA) and Lasalocid–Butylamine (LAS–BuA) complexes

The crystals of Lasalocid–Phenylamine (LAS–PhA) and Lasalocid–Butylamine (LAS–BuA) complexes are isostructural. Both complexes crystallise in the same space group and the crystals have very similar unit-cell dimensions (Table 1). The unit-cell dimension a is longer by about 0.5 Å in LAS–PhA, but parameter c is more than 1 Å longer in LAS–BuA, and the unit-cell volumes of both compounds differ in only 5 Å3. It is characteristic that LAS anions assume very similar pseudo-cyclic conformations, with

Conclusions

Two new complexes of ionophore antibiotic Lasalocid acid (LAS) and amines such as phenylamine (PhA) and butylamine (BuA) have been synthesised and their molecular structures have been fully characterized using crystallographic and spectroscopic methods.

In the solid state both complexes of Lasalocid acid are completely deprotonated and the amine groups are protonated. The structures of the complexes are stabilized by the intra- and intermolecular hydrogen bonds formed between the Lasalocid anion

Acknowledgments

Financial support from budget funds for science in years 2012-2013 – Grant ”Iuventus Plus” of the Polish Ministry of Science and Higher Education-No. 0179/IP3/2011/71, is gratefully acknowledged by Adam Huczyński.

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