NEW YORK Modern cancer drugs supercharge immune systems, target specific gene mutations and pack modified viruses into vaccines. Amid the increasing sophistication, one investigational treatment stands out for its simplicity.Rose Bengal, a cheap industrial chemical that turns yarn and food bright red, has been used as a diagnostic staining agent for some time. Now, some scientists are looking at its potential to fight various forms of cancer.At the forefront is Provectus Biopharmaceuticals Inc, which is testing a reformulated version of the industrial dye on melanoma, the deadliest form of skin cancer. The Knoxville, Tennessee, company reported promising results in a small melanoma study. While some doctors are encouraged by the research, government approval is years off and not guaranteed. The company must replicate its early results on a bigger scale, and a U.S. Food and Drug Administration decision is not expected before 2019.Rose Bengal's potential against cancer was discovered by accident. The salt was first patented in 1882 as a wool dye and has been used for years as a diagnostic stain in tests for jaundice in newborns and to detect eye damage.In 1998, scientists who later founded Provectus were looking for a safe photoreactive agent to use in an investigation of lasers against cancer. Rose Bengal fit the bill. As it turned out, the Rose Bengal solution appeared to work on its own to dissolve tumors when directed injectly into them, recalled Provectus Chief Technology Officer Eric Wachter, a former scientist from Oak Ridge National Lab who co-founded the company. "It made the lasers obsolete."In a study of 80 people with advanced melanoma, half of the patients who had all of their lesions injected appeared cancer free after an average of two months. A year later, 11 percent continued to show no signs of cancer, according to a report published the Annals of Surgical Oncology. The lesions were destroyed from the inside with no apparent harm to healthy tissue, researchers said. Reported side effects included injection site pain and blistering.Final results from an ongoing 225-patient melanoma trial of the experimental drug compared to chemotherapy are expected in early 2018. The hope is that the drug, known as PV-10, will prevent melanoma from progressing beyond Stage III, in which the disease has spread but not yet to other organs, and allow patients with more advanced cancer to live longer."This is one of the really neat examples of what we call repurposing, taking drugs that been around for years ... and suddenly realizing that they may have an oncologic value," said Dr. Vernon Sondak, head of cutaneous oncology at the Moffitt Cancer Center in Tampa, Florida. Sondak has been running clinical trials for Provectus.IMMUNE SYSTEM RESPONSE The company also has begun a melanoma trial of the drug with Merck's Keytruda, a treatment that works by helping the immune system fight cancer, to see if the combination has a more pronounced effect on slowing disease than either alone. Doctors said they hoped PV-10 combinations would not create the additional toxicity seen with some other combination therapies because of its mild side effect profile.    In addition to PV-10 destroying injected skin lesions, researchers noticed shrinkage in untreated lesions and tumors as far away as the lungs, suggesting it may also provoke an immune system response.    "We've come to the conclusion that it is immune based," said Dr. Sanjiv Agarwala, chief of medical oncology and hematology at St. Luke's Cancer Center in Bethlehem, Pa. Agarwala, who has been conducting PV-10 trials funded by Provectus, also has run several cancer immunotherapy studies, including on Keytruda. Dr. Patrick Hwu, an immunotherapy expert from MD Anderson Cancer Center, said PV-10 is one of several interesting tumor ablation techniques in the works. "The biggest value will be if it can affect distant disease," said Hwu, who is not involved in PV-10 testing.Pfizer holds a co-patent on PV-10 for use in combination therapies, and Boehringer Ingelheim has secured right of first refusal on use of the drug against liver cancer in China, the companies told Reuters.Provectus estimates the cost of PV-10 from laboratory to approval will be about $100 million, a fraction of the more than $1 billion in research and development for a new drug that the industry often cites to defend against criticism that medicine prices are too high.Provectus executives say the small development tab - along with relatively low manufacturing costs and easy handling requirements - could make PV-10 a less expensive new treatment. But the final decision on price is likely to be made by a bigger drugmaker, as Provectus plans to put itself up for sale once its drug is approved.The $95 million company trades at about 49 cents per share, off a high of 98 cents last April, on very small volumes, possibly reflecting investor uncertainty over the drug's prospects. (Reporting by Bill Berkrot; Editing by Michele Gershberg and Lisa Girion)