Thrombopoietin (TPO) development was difficult because it caused platelets to rise late, go too high, and increase the risk of blood clots.

“so thrombopolitans um uh EPO and Nupagen are as if you design them to to make recominant forms and give them for cancer patients or other patients who need them uh thrombopoin not so much it it's it's a just to make it simple if you said "Okay your platelets are going way down from your chemotherapy and I'm going to give you thrombopetin to make them go up." They they um come back up really late and they go too high so yeah so I'm making you at risk for a blood clot by giving you a million platelets but later than you need to and you're recovering on your own already”