Biotin-Targeted Nanomicellar Formulation of an Anderson-Type Polyoxomolybdate: Synthesis and In Vitro Cytotoxicity Evaluations

Cancer refers to a wide range of diseases in which cells grow and divide rapidly and migrate into other organs ۱-۲. Breast cancer is the most prevalent cancer among women, with ۲.۱ million mortalities per year ۴. Among all of the various clinical treatments, chemotherapy using anti-cancer drugs such as cisplatin, ۵-fluorouracil (۵-FU), doxorubicin, etc., is still the most common ۵-۶. Most of the chemotherapy drugs have a nonselective function and distribute into the other organs and tissues during therapy, which leads to serious side effects and dramatically reduces the quality of life. Polyoxometalates (POMs) are a group of anionic clusters with metal oxide (MOx) assembled subunits. In their structures, M is metal from d-block transition metals of the periodic table such as W(VI), Mo (V, VI), or V(IV, V)۷. POMs with versatile structures, various shapes, and versatile sizes have attractive applications in various fields such as photochemistry ۸, catalysis۹, biology, and more interestingly in medicine ۱۰ as anti-cancer ۱۱, antibacterial ۱۲ and antiviral agents۱۳. Yamase and et al. are pioneer researchers in this regard that synthesized and evaluated the anti-cancer activity of [NH۳Pri]۶[Mo۷O۲۴] (PM-۸) in vitro and in vivo. Although the precise antitumor activity mechanism is not clear yet, some studies supported that POMs could induce apoptosis and inhibit the ATP generation, phosphatase, and fibroblast growth factor ۴, ۱۶. There are different frameworks of POMs, including Wells–Dawson, keggin, Anderson-Evans, Lindqvist, etc. The Anderson-Evans structure with the general formula [Hy(XO۶)M۶O۱۸]n- is one of the most famous frameworks of POMs, where M and X are addenda atoms (Mo, W) and central heteroatom, respectively which form octahedral building blocks ۱۷. Also the Anderson POM have suitable physicochemical properties including good size (the average dimensions ۸.۶ × ۸.۶ × ۲.۷ Å) and disc shape that provide a proper approach to synthesis versatile organic-inorganic hybrids based POM۱۸-۱۹, but the strong acidity, oxygen-rich surface and poor solubility in water are drawbacks that lead to difficult handling for biological activity۲۰. A number of studies showed that the preparation of nano-formulation from POMs led to the several modifications in the polarity, charge and other properties can either improve the anticancer activity and bioavailability of them ۲۱. Despite the benefits and success of POMs as an anti-cancer agent, they have not yet found clinical applications. The two main limiting factors for developing pure inorganic POMs in clinical are toxicity to normal cells and high values of their IC۵۰ compared to small organic anti-cancer drugs, which should be used in high concentrations ۴, ۱۸. Toward more potency and selectivity besides the targeted delivery, to get fewer side effects on healthy cells, two approaches, including POMs bio-conjugates synthesis and nano-formulation of POMs or entrapment of POMs in a nano-carrier, have been developed ۲۲. Polymeric Micelles (PM) have been interested more specifically as drug carriers due to their unique characteristics such as low toxicity, high colloidal stability, prolonged blood circulation time, high drug loading index, and increased bioavailability ۲۳-۲۶. PMs consist of the hydrophilic and hydrophobic parts that could establish the core-shell structures and could entrap hydrophobic and poorly soluble drugs in their core by self-assembly in an aqueous solution۲۷. These nano-carriers could be targeted with various targeting ligands in the scope of targeted drug delivery. Vitamins, such as biotin, have special overexpression on the surface of fast-growing cancerous cells۲۸-۳۰. So it seems the biotin can be a proper ligand for making targeted drug delivery systems in the scope of chemotherapy. Several studies have excellent results of the anti-cancer effect in the biotin conjugated drug delivery systems ۳۱. Previous evaluations have yielded very good results for the entrapment of POMs in polymeric structures and nanoparticles۳۲, which prompted us to evaluate the cytotoxic effects of nanomicelles containing POM. Furthermore, since the poor solubility of POMs in water makes their applications to some extent challenging in biological environments, it seems that the micellar solubilization could eliminate the worries in this regard. In this study, the tetrabutylammonium (TBA) salt of the TRIS-modified Anderson-type Manganese polyoxomolybdate {MnMo۶O۱۸[(OCH۲)۳CNH۲]۲}۳- (TRIS-MnPOMo) was synthesized, and its micellar formulation was evaluated for the first time in order to enhance the potency and reduce the side effects. In this regard, biotin-targeted stearic acid-poly ethylene glycol (SPB) polymeric micelles were newly established as the nano-carrier to entrap the TRIS-MnPOMo, due to the remarkable benefits of micelles in drug delivery systems in cancer researches (Figure ۱). Herein, two types of breast cancer cell lines (MDA-MB-۲۳۱ and MCF-۷) and HUVEC normal cells were selected for in-vitro investigations.