HGH (human growth hormone), discovered and chemically characterized and synthesized decades ago. But that's where we learned that peptide (chains of amino acids) are just too fragile to make it to the areas in the brain where they are (or are not) produced naturally.
My guess is that there just wasn't anyone with the expertice at Amgen to REALLY address this decades old question of delivery. Remember what new pharmacologists are taught; "ADME" Absorption, Delivery, Metabolism, Excretion. If only we could find a genuinely workable method to deliver GDNF to exactly where it's needed, then we could better study it to see it's real efects and promise as a PD therapy. But...............