Fosinopril-d5 (SQ28555-d5 (free acid)) is deuterium labeled Fosinopril. Fosinopril (SQ28555 free acid) is the ester proagent of angiotensin-converting enzyme (ACE) inhibitor with an IC50 value of 0.18 µM. Fosinopril demonstrates a non-competitive inhibition effect on ACE activity with an Ki value of 1.675 µM[1][2].