The Journey of Metoprolol

Metoprolol tartrate (immediate-release) and metoprolol succinate
(extended-release, XR) are both rapidly and completely absorbed from the small intestine after
oral administration. Bioavailability of immediate-release formulations is 40-50% due to first-pass
hepatic metabolism (primarily CYP2D6 and CYP3A4). The extended-release matrix of metoprolol
succinate (Toprol XL) releases drug gradually over 20 hours, reducing peak-to-trough fluctuations
and improving once-daily tolerability. Peak plasma concentrations of immediate-release formulations
occur at 1-2 hours. Food increases bioavailability of the immediate-release form by approximately
40% by reducing first-pass effect, though the clinical significance is modest. Bioavailability is
profoundly affected by CYP2D6 genotype — CYP2D6 poor metabolizers achieve 3-5 times higher
steady-state plasma concentrations than extensive metabolizers at identical doses.

Metoprolol distributes widely with a volume of distribution of
approximately 3.2-5.6 L/kg. Plasma protein binding is relatively low (12%), meaning free drug
concentrations closely track total plasma concentrations. The drug crosses the blood-brain barrier
(logP 1.68) and is detectable in the CNS, potentially contributing to CNS side effects (fatigue,
vivid dreams, depression). It crosses the placenta and is present in breast milk. Metoprolol
reaches its pharmacological target — beta-1 adrenergic receptors (ADRB1) in the sinoatrial node,
atrioventricular node, and ventricular myocardium — through systemic circulation. Its beta-1
selectivity (cardioselectivity index ~75x over beta-2) makes it safer than non-selective beta-
blockers in mild asthma or COPD, though selectivity is dose-dependent and lost at high concentrations.

Metoprolol competitively antagonizes catecholamines (primarily
adrenaline/epinephrine and noradrenaline/norepinephrine) at beta-1 adrenergic receptors (ADRB1),
which are Gs-coupled GPCRs. In the sinoatrial node, ADRB1 blockade reduces If (funny current)
and ICaL (L-type calcium current), slowing spontaneous depolarization and heart rate (negative
chronotropy). In the atrioventricular node, conduction velocity and refractory period are prolonged
(negative dromotropy). In the ventricular myocardium, contractility is reduced (negative inotropy)
by blocking cAMP-mediated phosphorylation of L-type calcium channels, troponin I, and
phospholamban. The net hemodynamic effects — reduced heart rate, contractility, and cardiac output
— lower myocardial oxygen demand, making metoprolol beneficial in angina and post-MI settings.
In heart failure, paradoxically, chronic ADRB1 blockade allows receptor upregulation and
reversal of adverse catecholamine-mediated cardiac remodeling (β-blocker reverse remodeling),
improving ejection fraction and survival.

Metoprolol undergoes extensive first-pass and systemic hepatic
metabolism primarily by CYP2D6, and to a lesser extent CYP3A4. Alpha-hydroxymetoprolol and
O-demethylmetoprolol are the major metabolites, both pharmacologically inactive. CYP2D6 is
highly polymorphic — approximately 7-10% of Caucasians and 1-2% of East Asians are poor
metabolizers (PM), lacking functional CYP2D6. PMs have 3-5-fold higher plasma metoprolol
concentrations at standard doses, with exaggerated bradycardia and hypotension risk. Ultrarapid
metabolizers (CYP2D6 gene duplication) have lower plasma levels and may have attenuated response.
Drugs inhibiting CYP2D6 (fluoxetine, paroxetine, quinidine) effectively convert extensive
metabolizers to phenotypic poor metabolizers, dramatically raising metoprolol exposure.

Metoprolol metabolites are excreted primarily by the kidneys
(95%), with less than 5% excreted as unchanged drug in urine. Because renal excretion is
predominantly of inactive metabolites, dose adjustment is not required in renal impairment.
However, in hepatic impairment, where CYP2D6 activity may be reduced, plasma concentrations
increase and dose reduction is appropriate. Plasma half-life is 3-7 hours in CYP2D6 extensive
metabolizers and can extend to 7-15 hours in poor metabolizers.

Metoprolol succinate (extended-release) reduces cardiovascular
mortality by 34% in heart failure (MERIT-HF). Cardioselectivity provides relative safety in mild
reversible airway disease. Abrupt discontinuation can precipitate rebound angina or myocardial
infarction due to beta-receptor upregulation during therapy — always taper over 1-2 weeks. CYP2D6
drug interactions with antidepressants (SSRIs, especially paroxetine) are frequent and clinically
significant. Metoprolol is a CYP2D6 substrate but not an inhibitor.