Early promising results are emerging for ADI-PEG 20, a new cancer drug that breaks down arginine. Arginine is an amino acid—a building block for proteins—that is necessary for cell function. Normal cells can produce their own arginine, but many tumor cells cannot. By depriving the body of arginine, ADI-PEG selectively disrupts cancer cells. In a phase II clinical trial, ADI-PEG prolonged the time without cancer progression in patients with malignant pleural mesothelioma, a type of lung cancer. Almost half of the patients had a partial response to the drug, defined as their tumors shrinking at least 30%.