One of the challenges of chemotherapy is delivery of sufficient quantities of drug to tumor cells to kill them while sparing healthy cells and tissues. One strategy for achieving this balance is to use small particles, only nanometers in size, to deliver drugs to tumors. Each nanoparticle—which can be an organic molecule, such as a lipid or protein, or a metal like gold or iron oxide—can be loaded with hundreds of drug molecules, allowing more drug to get inside the tumor. So far, a handful of nanoparticle-drug combinations have been approved by the FDA or are in clinical trials for use in humans. Approved agents include Doxil (Centocor Ortho Biotech), which consists of the drug doxorubicin bound to liposomes; abraxane, paclitaxel bound to the protein albumin; and oncaspar (pegaspargase), the form of L-asparaginase that has been covalently attached to polyethylene glycol (PEG). In many patients, these agents are more effective and have fewer side effects than their conventional drug counterparts. Among agents currently in clinical trials, the drug preparation Aurimune (CytImmune Sciences), which consists of 27-nm gold particles coated with human TNF-α and PEG, has been shown to deliver 20-times-higher doses of TNF-α to patients' tumors than would have been possible with the drug alone.

Given these promising results, it is not surprising that new types of nanomaterials continue to come on the scene. One of the latest additions is nanodiamonds—carbon-based particles 2 to 8 nm in size with a truncated octahedral structure that gives each particle multiple facets, not unlike those of a diamond. The investigators from a recently published study (Chow et al. Sci Transl Med 2011;3:73ra21) reported that they had attached doxorubicin to nanodiamonds and used the resulting particles to treat mice with either breast or liver tumors. They found that nanodiamond-bound doxorubicin was retained in tumor cells in greater amounts, compared with doxorubicin alone, leading to increased tumor cell death and lower toxicity. This increased retention of the drug in tumor cells was due to the fact that the drug attached to the nanodiamonds remains in tumor cells longer than the drug on its own, according to the authors.

It remains to be seen how nanodiamonds will fare in clinical trials and whether they will have significant benefits over other nanoparticle-based delivery systems. For now, one promising trait of nanodiamonds appears to be their versatility. These particles can be outfitted with a variety of different surface functionalities that allow for binding to a wide range of drugs.