Thiophene-based compounds as fluorescent tags to study mesenchymal stem cell uptake and release of taxanes

Human mesenchymal stem cells (hMSC) are multipotent cells that display the unique ability to home and engraft in tumor stroma. This remarkable tumor tropic property has generated a great deal of interest in many clinical settings. Recently, we showed that hMSC represent an excellent base for cell-mediated anticancer therapy since they are able to internalize paclitaxel (PTX) and to release it in an amount sufficient to inhibit tumor cell proliferation. In order to shed light on the dynamics of drug uptake and release, in the present paper we describe the synthesis of two novel thiophene-based fluorophore-paclitaxel conjugates, namely PTX-F32 and PTX-F35, as tools for in vitro drug tracking. We aimed to study the ability of these novel derivatives to be efficiently internalized by hMSC and, in a properly engineered coculture assay, to be released in the medium and taken up by tumor cells. In order to ensure better stability of the conjugates toward enzymatic hydrolysis, the selected oligothiophenes were connected to the taxol core at the C7 position through a carbamate linkage between PTX and the diamino linker. Antiproliferative experiments on both tumor cells and stromal cells clearly indicate that, in good correlation with the parent compound, cells are sensitive to nanomolar concentrations of the fluorescent conjugates. Moreover, in the coculture assay we were able to monitor, by fluorescence microscopy, PTX-F32 trafficking from hMSC toward glioblastoma U87 tumor cells. Our work paves the way for novel possibilities to perform extensive and high quality fluorescence-based analysis in order to better understand the cellular mechanisms involved in drug trafficking, such as microvescicle/exosome mediated release, in hMSC vehicle cells.