That has the U.S. at 47% of drugs developed. Keep in mind those are based on drugs approved by the FDA, so it skews towards the U.S., and especially away from countries like Japan. Adjusted for population (I'm not entirely sure this is fair), that puts us neck-and-neck with the U.K., behind Israel, and far behind Switzerland.
That being said, the only source I can find for your claim is a throwaway sentence in an op-ed on Forbes (https://www.forbes.com/sites/paulroderickgregory/2012/07/01/...).
I found some more realistic (and sourced) statistics here: http://blogs.sciencemag.org/pipeline/archives/2010/11/09/whe...
That has the U.S. at 47% of drugs developed. Keep in mind those are based on drugs approved by the FDA, so it skews towards the U.S., and especially away from countries like Japan. Adjusted for population (I'm not entirely sure this is fair), that puts us neck-and-neck with the U.K., behind Israel, and far behind Switzerland.