Cell-killing potential of a water-soluble radical initiator

The diazo compound, 2,2′-azobis [2-(2-imidazolin-2-yl) propane] dihydrochloride (AIPC), is a water-soluble radical initiator that can be activated at mild temperatures (37°-40°C). Potential biomedical applications of this compound include the fabrication of hydrogels by radical polymerization (e.g., cell encapsulation or drug delivery) and the thermal sensitization of cancerous cells to induce localized cell death. In this study we evaluated whether this compound could induce cell death at 37°C in vitro and in vivo using a tumor animal model. Cytotoxicity was quantitated with a sulforhodamine B colorimetric assay by monitoring growth inhibition of human glioma cells in vitro. AIPC was entrapped in fibrin gel and exposed to cells in culture as a potential way to localize the compound in a controlled release environment. The mechanism of action for cell death was evaluated by quantitating caspase-3 activity in cells. In vivo studies included human glioma tumors that were grown subcutaneously in rats to study the effect of intra-tumor injections of AIPC. AIPC was also injected subcutaneously into normal tissue. Concentrations of 0.2% and 0.02% (w/v in RPMI medium) showed 93% and 84% inhibition of cell growth in vitro, respectively. Cell-growth inhibition using gel-entrapped AIPC was comparable to that obtained with AIPC in solution after 48 hr (86% inhibition at 0.2% w/v). Exposure to AIPC resulted in a significant increase of caspase activity (up to 163 units after 20 min), suggesting induced apoptosis as a possible mechanism of action of the AIPC. Histological pictures showed that, relative to normal tissue, cancerous tissue was more sensitive to the effects of AIPC.