By Marian Turner

Attaching chemotherapy drugs to small particles called nanodiamonds can make the drugs more effective, according to a study published this week in Science Translational Medicine.

Anticancer drugs tend to become ineffective because cancer cells quickly pump them out before they have had time to do their work. This kind of drug resistance accounts for 90% of treatment failure in malignant cancer.

Nanodiamonds -- carbon-based particles 2-8 nanometres in diameter with a truncated octahedral structure that gives it multiple facets not unlike a diamond's -- overcome this problem because the cellular transport proteins that usually pump the drug out of the cell can't carry them. The drug therefore stays inside the cell.

Dean Ho, a biomedical engineer from Northwestern University in Evanston, Illinois, who led the study, says that the surface chemistry of nanodiamonds is what makes them special. The diamonds' facet surfaces possess differing properties, such as electrical charge. So a drug could be attached to one neutral surface, for example, while another facet retains an electrostatic charge, allowing the nanodiamond to disperse in fluids. Theoretically, nanodiamonds could be loaded with both a drug and an agent to target cancer cells, although the team have not yet done this.

Other nanoparticles, such as synthetic polymers made from PLGA (poly(lactic-co-glycolic acid)), are already in clinical use for drug delivery, but they do not have this inbuilt surface versatility.

Cheap and non-toxic

Nanodiamonds are non-toxic and don't cause inflammation. They are also cheap to produce in large quantities, says Ho. "The first idea for nanodiamonds was to use them as friction agents in the car industry."

Scientists from Ho's group attached the anticancer drug doxorubicin to nanodiamonds. They treated mice with liver tumours with either this compound or with doxorubicin alone, and checked levels of the drug in the tumours two days later. They found that doxorubicin levels were ten times higher in mice treated with the nanodiamond compound compared with mice given doxorubicin alone, and remained high for seven days. The tumours of mice receiving nanodiamond-doxirubicin also shrank more and the mice survived longer.

Ho's group then put the nanodiamonds to a tougher test -- using a model of breast cancer which is highly resistant to doxorubicin. The nanodiamond-doxirubicin compound worked better there too. Strikingly, the nanodiamonds also reduced the toxicity of the drug by releasing it more slowly. Doses that would have killed mice if given as free drug did not even cause them to lose weight when the drug was carried on nanodiamonds.

But the concept of using nanodiamonds for drug delivery is still in its infancy. "No one has used them in humans yet," cautions Robert Langer, a drug-delivery biochemist at Massachusetts Institute of Technology in Cambridge. "I'd like to see a really significant advantage over materials already in use that are approved by the Food and Drug Administration."

Bioengineer Tim Deming from the California NanoSystems Institute at the University of California, Santa Barbara, thinks that the concept might need refining. "Synthetic polymers have reproducible properties and composition, and we can fine-tune the structures," he points out.

The process of generating nanodiamonds is more crude. "It's like detonating TNT and then doing a lot of purification," says Deming. Further polishing of the nanodiamond production process might be necessary if they are to be used for human therapeutics, he says.