Rather than relying on trial-and-error, Elion and Hitchings discovered new drugs using rational drug design, which used the differences in biochemistry and metabolism between normal human cells and pathogens (disease-causing agents such as cancer cells, protozoa, bacteria, and viruses) to design drugs that could kill or inhibit the reproduction of particular pathogens without harming human cells. The drugs they developed are used to treat a variety of maladies, such as leukemia, malaria, lupus, hepatitis, arthritis, gout, organ transplant rejection (azathioprine), as well as herpes (acyclovir, which was the first selective and effective drug of its kind). Most of Elion's early work came from the use and development of purine derivatives.