The Journey of Lisinopril

Lisinopril is unique among ACE inhibitors in being administered
as the active diacid form, not as a prodrug ester (unlike enalapril, ramipril, perindopril).
This simplifies its pharmacokinetics because no hepatic bioactivation is required. However,
lisinopril's oral bioavailability is variable (6-60%, mean ~25%), partly due to its highly
hydrophilic nature (logP -1.6) and reliance on peptide transporters (PEPT1, SLC15A1) for
intestinal absorption. Absorption occurs primarily in the small intestine. Peak plasma
concentration is reached at 6-8 hours — later than esterified ACE inhibitors (2-4 hours for
enalaprilat). Food does not significantly affect bioavailability. The 25% bioavailability is
sufficient for therapeutic effect because lisinopril binds with very high affinity (Ki ~0.27 nM)
to ACE in the vascular endothelium.

Lisinopril does not bind to plasma proteins to any significant
degree, which means free drug levels equal total plasma drug levels and it is not subject to
protein-binding displacement interactions. Its volume of distribution is approximately 1 L/kg.
The drug distributes to the vascular endothelium throughout the body where ACE is abundant —
particularly in the lung (which contains 40% of total body ACE), kidney, heart, and brain vasculature.
Tissue-bound lisinopril in the vascular endothelium is pharmacologically active. The drug
penetrates the blood-brain barrier minimally but inhibits central ACE. It does not penetrate
adipose tissue or cross the placenta readily, though ACE inhibitors carry significant fetal risk
in the 2nd and 3rd trimesters (fetal renal dysfunction, oligohydramnios).

ACE (angiotensin-converting enzyme, kininase II, CD143) is a
zinc-containing dipeptidyl carboxypeptidase that cleaves the C-terminal dipeptide (His-Leu)
from angiotensin I to produce angiotensin II, a potent vasoconstrictor. Lisinopril mimics the
structure of the C-terminal dipeptide of angiotensin I, fitting into the ACE active site and
coordinating with the zinc atom via its carboxylate and amine groups. This competitive inhibition
(with very slow off-rate — effectively tight-binding) blocks angiotensin II generation from
angiotensin I. The resulting decrease in angiotensin II reduces: (1) direct vasoconstriction;
(2) aldosterone secretion (reducing sodium retention and potassium excretion); (3) sympathetic
nervous system activation; (4) renal efferent arteriolar constriction (improving GFR in heart
failure). ACE also degrades bradykinin; inhibition of this secondary reaction increases bradykinin
levels, causing vasodilation via B2 receptor/nitric oxide but also the signature ACE inhibitor
dry cough (10-20% of patients, especially in East Asians) and the rare but serious angioedema.

Lisinopril is not metabolized by hepatic enzymes. It is absorbed
unchanged and eliminated unchanged by the kidney. No cytochrome P450, UGT, or other metabolic
enzymes are involved. This pharmacokinetic simplicity makes it resistant to drug-drug interactions
mediated by metabolic enzyme induction or inhibition and means hepatic impairment does not
alter its kinetics.

Lisinopril is excreted entirely by the kidney unchanged via
glomerular filtration. Its renal clearance approximates GFR, indicating no significant tubular
secretion or reabsorption. Plasma half-life is approximately 12 hours under normal renal function.
In renal impairment, lisinopril accumulates proportionally to the fall in GFR, requiring dose
reduction when eGFR is below 30 mL/min/1.73m². Lisinopril is removed by hemodialysis.

ACE inhibitors reduce mortality by 20-25% in heart failure and
25% in high-risk cardiovascular patients (HOPE trial). They slow progression of diabetic nephropathy
and non-diabetic CKD via intraglomerular pressure reduction. Hyperkalemia risk is increased when
combined with potassium-sparing diuretics, potassium supplements, ARBs, or renin inhibitors.
RAAS dual blockade (ACEi + ARB) increases adverse events without additional cardiovascular benefit
(ONTARGET) and is contraindicated. ACE inhibitors are absolutely contraindicated in pregnancy.