Captopril, the first ACE inhibitor, is a functional and structural analog of a peptide derived from the venom of the jararaca, a Brazilian pit viper (Bothrops jararaca). Enalapril is a derivative, designed by scientists at Merck to overcome the rash and bad taste caused by captopril.
Enalapril is actually a prodrug; the active metabolite is enalaprilat.
The di-acid metabolite of enalapril, enalaprilat, and its lysine analogue lisinopril are potent inhibitors of angiotensin converting enzyme (ACE); they do not contain sulphydryl groups. Both drugs can be assayed by high pressure liquid chromatography and by radioimmunoassay and plasma ACE inhibition remains stable under normal storage conditions. It is therefore possible to study their pharmacokinetics as well as their pharmacodynamic effects in humans. Enalaprilat and lisinopril as well as ACE activity have been measured in blood taken during the course of two studies of the effects of these drugs on blood pressure and autonomic responsiveness.
Lisinopril is a synthetic peptide derivative of captopril. Scientists at Merck created lisinopril by systematically altering each structural unit of enalaprilat, substituting various amino acids. Adding lysine at one end of the drug turned out to have strong activity and had adequate bioavailability when given orally; analogs of that compound resulted in lisinopril, which takes its name from the discovery with lysine.  Merck conducted clinical trials, and the drug was approved for hypertension in 1987 and congestive heart failure in 1993.
The discovery posed a problem, since sales of enalapril were strong for Merck, and the company did not want to diminish those sales. Merck ended up entering into an agreement with Zeneca under which Zeneca received the right to co-market lisinopril, and Merck received the exclusive rights to an earlier stage aldose reductase inhibitor drug candidate, a potential treatment for diabetes.  Zenecas marketing and brand name, "Zestril", turned out to be stronger than Mercks effort. The drug became a blockbuster for AstraZeneca (formed in 1998), with annual sales in 1999 of $1.2B.
The US patents expired in 2002. Since then, lisinopril has been available under many brand names worldwide; it is also available in combination drugs with diuretic hydrochlorothiazide (as lisinopril/hydrochlorothiazide), and with calcium channel blocker amlodipine (as lisinopril/amlodipine).