Drug-loaded carbon nanohorns: adsorption and release of dexamethasone in vitro

Single-wall carbon nanohorns (SWNHs) are recently discovered nanostructured spherical aggregates of graphitic tubes. The unique physicochemical properties of SWNHs, including their large surface area, suggest their possible utility as carriers in drug delivery systems. Here we investigated the in vitro binding and release of the antiinflammatory glucocorticoid dexamethasone (DEX) by as-grown SWNHs and their oxidized form, oxSWNHs. Adsorption analyses using [3H]-DEX determined the amount of DEX adsorbed by oxSWNHs to be 200 mg for each gram of oxSWNHs in 0.5 mg/mL of DEX solution, which was approximately 6 times larger than that obtained for as-grown SWNHs. Adsorption kinetics indicated that oxSWNHs had higher affinity for DEX than as-grown SWNHs. Treatment of oxSWNHs at 1200 degrees C under H2, which removed the oxygen-containing functional groups on oxSWNHs, did not diminish the high affinity for DEX, suggesting that oxygen-containing functional groups have little contribution for the affinity. DEX-oxSWNH complexes exhibited sustained release of DEX into phosphate-buffered saline (pH 7.4) at 37 degrees C and more rapid biphasic release into culture medium. The biological integrity of the released DEX form was confirmed by activation of glucocorticoid response element-driven transcription in mouse bone marrow stromal ST2 cells and induction of alkaline phosphatase in mouse osteoblastic MC3T3-E1 cells. Notably, synthesis of SWNHs does not require a metal catalyst, the toxicity of which could become problematical in clinical use, and no cytotoxicity was observed in cells cultured in the presence of oxSWNHs under our conditions. Taken together, these observations highlight the potential utility of SWNHs in drug delivery systems.