John Fountain sent me an email about the first antibiotics, the sulfa drugs. I will quote (slightly edited) what he said:

I found an fascinating example of the way in which competition based on an old (1909) but expired (by 1930's) patent on a sulfanimide used in the dye industry ushered in in the antibiotic revolution in the mid 1930's.

The basics are contained in a wikipedia article.

The interesting facts are that the commercially developed chemical entity (Bayer I think) called Prontosil, for which patents were granted in the 30's, proved to be a flop commercially...because in the human body it broke down into bits and pieces. One piece - the "sulfa" - was the real "active ingredient. I like the authors description here "The result was a sulfa craze"! I guess at that time - 1930's - chemicals naturally occuring in the body weren't themselves patentable!!