Complementary inhibition of synoviocyte, smooth muscle cell or mouse lymphoma cell proliferation by a vanadyl curcumin complex compared to curcumin alone

Abstract

A novel vanadyl curcumin complex (VO(cur)₂) has been synthesized and and its physicochemical properties characterized. Biological characterization included in vitro testing for anti-rheumatic activity in synoviocytes, angiogenesis inhibition in smooth muscle cells and anti-cancer potential in mouse lymphoma cells; as well as in vivo testing for hypoglycemic activity by oral gavage in streptozotocin (STZ)-diabetic rats. VO(cur)₂ was more effective as an anti-cancer agent, compared to uncomplexed curcumin or vanadyl ion alone, was more than twice as effective as curcumin alone as an anti-arthritic agent, and was more than four times as effective as curcumin alone in inhibiting smooth muscle cell proliferation. In both acute and chronic screening tests, VO(cur)₂ was ineffective as an insulin mimetic agent; however, it also proved to be exceptionally non-toxic, with no evidence of negative symptomatology during a month-long treatment period, at doses up to and including 2.0 mmol kg⁻¹ day⁻¹.

Introduction

Curcumin, an extract of turmuric, Curcuma longa L., has been used for centuries in a variety of pharmaceutical applications [1], [2], [3], including as a treatment for arthritis [4], as an anti-inflammatory agent [5], [6] and as an orally available treatment for diabetes [7]. Curcumin is well absorbed, both in vitro [8] and in vivo [9], and has an exceedingly low toxicity index [6], [10]. Administration of curcumin, 80 mg kg⁻¹ day⁻¹ by oral gavage for 21 days, reduced blood sugar, glycosylated hemoglobin and thiobarbituric reactive substances (a measure of oxidative stress) in liver and plasma in alloxan-induced diabetic albino rats [7]. The anti-diabetic potential of curcumin itself has also been shown by amelioration of kidney lesions associated commonly with streptozotocin (STZ)-induced diabetes in the rat [11] and decreased oxidative stress, including prevention of advanced glycosylation end products (AGEs) in diabetic rats [12]. The curcuminoids (1,7-diaryl-1,6-heptadiene-3,5-diones), of which curcumin is a naturally occurring phytochemical, are potent antioxidants [13], inhibiting lipid peroxidation [14], reducing oxidative damage in an Alzheimer transgenic mouse [15] and effectively scavenging superoxide, the hydroxyl radical and nitrogen dioxide [16], [17], among other reactive oxygen species (ROS) [17], [18]. They also readily bind to divalent metal ions, through keto-enol tautomerization of the β-diketonate moiety [14], [17], [18], [19].

A variety of metallocomplexes of curcumin have been previously synthesized and characterized, usually with biological studies in mind. These include a five-coordinate curcumin–gold complex, Au(cur)₂Cl, in which curcuminate is bidentate [20]; its anti-arthritic properties were assessed in an adjuvant-induced rat polyarthritis model. Greatly reduced paw swelling was seen after 3 weeks of Au(cur)₂Cl, 30 mg kg⁻¹ day⁻¹ by injection. Three manganese complexes, prepared by reaction of manganese acetate with curcumin or one of two related compounds, diacetylcurcumin and 4-(4-hydroxy-3-methoxy-phenyl)-1-[7-(4-hydroxy-3-methoxy-phenyl)-[1,4]diazepan-5-ylidene]but-3-en-2-one [21], were isolated, characterized and tested in vitro for antioxidant properties and superoxide dismutase activity, with IC₅₀ values for the former in the range 6.3–26.3 μM, and for the latter 8.9–29.9 μM. All three were also tested in vivo for their potential as neuroprotective agents in vascular dementia; Mn(curcumin)(OAc) showed significant protective effects in a transient ischemia/reperfusion mouse model of neuronal damage [21]. Cu(II) complexes of curcumin and analogues have been synthesized and tested for anti-tumour effects, both in vivo and in vitro, with considerable success [22], [23]. Of a series of metal curcuminoids, the CuL₂ complexes were most cytotoxic in cultured L929 cells [22], and also showed significant reduction in solid tumour volume in ascites tumour-bearing mice. Recently, syntheses of a series of 1,7-diarylheptanoids and their VO(IV), Co(II), Ni(II) and Cu(II) complexes were reported [23]. The Cu(II) coordination complexes of these ligands had IC₅₀ values of 6.6–11.1 μM in Erlich ascites tumour cells [23].

Coordination complexes of vanadyl ion and peroxovanadate [24] are also of interest as candidate chemotherapeutic agents [25], [26], [27], [28], [29], [30]. Vanadium is an ubiquitous trace element (cellular concentrations ≅ 10⁻⁸ M in mammals), which is able to influence signal transduction cascades with extraordinary sensitivity at several regulatory points [31]. VO(IV) coordination complexes have been extensively investigated for their insulin mimetic or enhancing potential [32], [33], [34]. In particular, bis(maltolato)oxovanadium(IV) (BMOV) and bis(ethylmaltolato)oxovanadium(IV) (BEOV), VO(IV) complexes with maltol or ethylmaltol, both of which are approved food additives, have proven effective as anti-diabetic agents in a variety of animal models of diabetes mellitus [32], [34], [35]. Vanadium’s potential in vanadium-containing chemotherapeutic agents, and in anti-diabetic agents, may be linked to potent inhibition of protein tyrosine phosphatase enzymes [31], [36], [37]. Vanadium is also known to have both pro- and antioxidant properties, depending on dose and chelation [37], [38], [39], [40]. The combined effects of protein tyrosine phosphatase inhibition and antioxidant properties of vanadyl ion (for both) and curcumin (for the latter) present an ideal opportunity to test the potential of synergistic enhancement of ligand effects by combination with an appropriate metal ion [41].

Previous studies have explored the potential for complementarity in pharmacological potency when metal ions are complexed to functional ligands, e.g. in vanadyl biguanides [42], vanadyl complexes of thiazolidinediones [43] (both for insulin mimetic effect), and Au(I) and Cu(II) clotrimazole and ketoconazole as anti-parasitic infection drugs [44]. In this study, we synthesized a bis(ligand)oxovanadium(IV) complex, with the curcuminate anion as the ligand, in order to take advantage of the pharmacological properties of both curcumin and vanadyl, with potential complementary efficacy.