Quinapril. Molecular model of the drug quinapril (C25.H30.N2.O5), an angiotensin-converting enzyme (ACE) inhibitor drug used to treat hypertension (high blood pressure) and congestive heart failure. It is marketed commercially as Accupril. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue) and oxygen (red).