" [...] German scientist Paul Ehrlich was the first to exploit the properties of dyes for the development of chemotherapies. In 1901, Ehrlich became interested in the chemotherapy of trypanosomiasis and tested more than 100 synthetic dyes against Trypanosoma equinum, a species that causes a disease in equids, and T. brucei brucei, which is responsible for Nagana, a disease of cattle. The only dye displaying trypanocidal activity was a benzopurpurine compound named Nagana Red. When trypanosome-infected mice were treated with Nagana Red, parasites became undetectable in the animals for a short period and the treated mice survived 2 days longer than control mice (5/6 days versus 3/4 days). Ehrlich thought that the poor efficacy of Nagana Red was due to its low solubility which impaired the absorption of the drug into the bloodstream from the subcutaneous inoculation site. In 1903, Ludwig Benda working for Cassella Farbwerke near Frankfurt synthesised a derivative of Nagana Red, called Trypan Red, with an extra sulphonic acid function and enhanced water solubility. Trypan Red proved to be both curative and prophylactic for T. equinum infections in mice [...]"
And so it was that modern drug discovery was born!