Problems with mitochondria -- cells' tiny energy-producing units -- can cause illnesses as diverse as cancer and Parkinson's disease. Volkmar Weissig, a pharmaceutical scientist at Northeastern University, says he's devised the first drug delivery system that can shuttle a drug through a cell to target its mitochondria. Weissig coats drugs in a common antibacterial compound; the positively charged coating is attracted to the mitochondria, which are the most negatively charged parts of the cell. In a recent experiment with mice, Weissig found that tumors treated with the coated version of the cancer drug Taxol grew only half as much as those treated with the uncoated drug. Weissig says he could also use the approach to shuttle DNA to the mitochondria, a possible basis for gene therapy to correct mutations in mitochondrial DNA -- which have been implicated in neurological diseases such as Parkinson's and Alzheimer's. A Boston startup company, MitoVec, plans to couple Weissig's technology with several existing cancer drugs and begin testing it in humans in two to three years.