Vanadium (III,IV, V)-dipicolinate complexes with different redox properties were selected to investigate the structure-property relationship of insulin-mimetic vanadium complexes for membrane permeability and gastrointestinal (GI) stress-related toxicity using the Caco-2 cell monolayer model. The cytotoxicity of the vanadium complexes was assayed with 3-(4,5-dimethylthiazoyl-2-yl) 2,5-diphenyl-tetrazolium bromide (MTT) assays and the effect on monolayer integrity was measured by the trans-epithelial electric resistance (TEER). The three vanadium complexes exhibited intermediate membrane permeability (Papp=1.4-3.6x10 6 cm/s) with low cellular accumulation level (<1%). The permeability of all compounds was independent of the concentration of vanadium complexes and excess picolinate ligands. Both V(III) and V(V)-dipicolinate complexes induced 3M-fold greater reactive oxygen and nitrogen species (RONS) production than the V(IV)-dipicolinate complex; while the vanadium (III)-dipicolinate was 3-fold less damaging to tight junction of the Caco-2 cell monolayer. Despite the differences in apparent permeability, cellular accumulation, and capacity to induce eactive oxygen and nitrogen species (RONS) levels, the three vanadium complexes exhibited similar cytotoxicity (IC50=1.7-1.9 mM). An ion pair reagent, tetrabu-tylammonium, increased the membrane apparent permeability by 4-fold for vanadium (III and IV)-dipicolinate complexes and 16-fold for vanadium (V)-dipicolinate as measured by decrease in TEER values. In addition, the ion pair reagent prevented damage to monolayer integrity. The three vanadium (III, IV, V)-dipicolinate complexes may pass through caco-2 monolayer via a passive diffusion mechanism. Our results suggest that formation of ion pairs may influence compound permeation and significantly reduce the required dose, and hence the GI toxicity of vanadium-dipicolinate complexes.

Purpose The aim of this study was to investigate the mechafusm of permeation and cytotoxicity of vanadium compounds, [VO(acac)2], [VO(ma)2], and vanadate. Methods. Absorptive transport were carried out in Caco-2 monolayers grown on transwell inserts. Vanadium was quantified using inductively coupled plasma atomic emission spectrometry (ICP-AES). The change of Caco-2 cells in the microvilli morphology and F-acfm structure was visualized by transmission electron microscopy and confocal laser scanning microscopy. Results. The three vanadium compounds were taken up by Caco-2 cells via simple passive diffusion. [VO(acac)2] were mainly transcel-Marly transported and exhibited the highest apparent permeability coefficients (8.2×10-6 cm-1). The cell accumulation of [VO(acac)2] was found to be greater than that of [VO(ma)2], and vanadate caused much less accumulation than the other two compounds. Vanadium compounds induced intracellular reactive oxygen species, reduced the transepithelial electric resistance, caused morphological change in microvilli, and led to different perturbation of F-actin structure. Conclusions. The three compounds exhibited different permeability due to different diffusion process and cellular uptake. The toxicity of vanadium complexes on Caco-2 monolayer involved F-actin related change of tight junction and impairment of microvilli. The toxicity was also related to elevated intracellular reactive oxygen species (ROS) and their cellular accumulation.