The Journey of Lithium
The Simplest Drug with Complex Actions
Lithium, the lightest solid element and simplest possible psychiatric drug, is absorbed from the gut as the Li+ ion, distributes throughout total body water without protein binding, and exerts diverse CNS effects through multiple proposed mechanisms — including inhibition of inositol monophosphatase (depleting second messenger inositol) and glycogen synthase kinase-3 (GSK-3beta inhibition) — stabilizing mood in bipolar disorder with a narrower therapeutic window than almost any other drug in clinical use.
Absorção
Lithium carbonate (Li₂CO₃) is the most commonly prescribed
lithium salt, though lithium citrate and lithium chloride are also used. Following oral ingestion,
lithium carbonate dissociates to release Li+ ions, which are absorbed from the GI tract — primarily
the small intestine — by passive diffusion and potentially via sodium transport pathways (lithium
mimics sodium at some transporters). Immediate-release lithium carbonate achieves peak plasma
concentrations within 1-2 hours. Extended-release formulations (Lithobid, Eskalith CR) have peak
concentrations at 4-6 hours and reduce peak-to-trough fluctuations. Oral bioavailability of lithium
is essentially 100% — no first-pass hepatic metabolism occurs because the element undergoes no
biotransformation. Food has minimal effect on total absorption but slows the rate. The narrow
therapeutic index (0.6-1.2 mEq/L) and the severe toxicity at plasma concentrations >2.0 mEq/L
require therapeutic drug monitoring (12-hour post-dose trough levels) for all patients.
Distribuição
Lithium distributes throughout total body water (Vd approximately
0.7-1.0 L/kg), reflecting its behavior as a sodium analogue. There is no plasma protein binding —
lithium circulates entirely as the free Li+ ion. Distribution to the CNS is slow: blood-brain barrier
transfer takes approximately 24 hours to achieve equilibrium, explaining why acute ingestion causes
GI toxicity before neurological symptoms, and why CNS toxicity persists after plasma levels have
been lowered by dialysis (the therapeutic modality for severe lithium toxicity). Intracellular lithium
concentrations are lower than plasma (ratio approximately 0.5) but accumulate in brain to a greater
degree. Red blood cell lithium concentrations (measured as the erythrocyte:plasma lithium ratio)
may provide a marker of CNS exposure. Thyroid and bone accumulate lithium to higher concentrations
than plasma over long-term treatment.
Mecanismo de Ação
Despite 75+ years of clinical use, lithium's precise mechanism
in bipolar disorder remains incompletely understood. Two major targets are established at therapeutic
concentrations: (1) Inositol monophosphatase (IMPase) inhibition: lithium is a non-competitive
inhibitor of IMPase (IMPA1, IMPA2), blocking dephosphorylation of inositol monophosphate back to
free inositol. In neurons with high phospholipase C-mediated signaling activity, reduced inositol
availability limits the resynthesis of phosphatidylinositol bisphosphate (PIP2), attenuating
second messenger (IP3, DAG) generation — a selective damping of hyperactive pathways (inositol
depletion hypothesis). (2) GSK-3beta inhibition: lithium directly inhibits glycogen synthase kinase-3
beta (GSK-3β) by competing with magnesium as an enzyme cofactor and by promoting inhibitory
phosphorylation of GSK-3β at Ser9. GSK-3β regulates multiple signaling cascades affecting neuronal
plasticity (BDNF/AKT signaling), circadian rhythm (Per2 phosphorylation), and apoptosis (beta-
catenin stabilization). Lithium also inhibits inositol polyphosphate 1-phosphatase (INPP1) and
may modulate monoamine neurotransmission.
Metabolismo
Lithium is not metabolized. The element lithium undergoes no
biotransformation in the body — no oxidation, reduction, conjugation, or any other metabolic process.
There are no metabolites, no CYP450 enzyme interactions, and no protein conjugation. Lithium's
entire pharmacokinetic journey from gut to kidney involves only the Li+ ion. This extreme metabolic
simplicity stands in stark contrast to its complex pharmacodynamics. The only drug interactions
with lithium that matter pharmacokinetically are those affecting renal handling — particularly
sodium balance: thiazide diuretics reduce lithium excretion (increasing levels by 25-50%), loop
diuretics have less effect, ACE inhibitors/ARBs reduce lithium excretion (20-40% level increase),
and NSAIDs reduce renal blood flow reducing lithium clearance (10-60% increase). Salt depletion
and dehydration also markedly increase lithium reabsorption and toxicity risk.
Excreção
Lithium is excreted entirely and unchanged by the kidney — 100%
renal excretion. At the glomerulus, Li+ is freely filtered (no protein binding). In the proximal
tubule, approximately 80% is reabsorbed along with sodium via NHE3 (sodium-hydrogen exchanger 3)
and other sodium transporters — lithium mimics sodium sufficiently to be reabsorbed. The remaining
20% is excreted in urine. Factors reducing sodium reabsorption (volume depletion, low-sodium diet,
thiazides) increase Li+ reabsorption proportionally — potentially leading to toxic accumulation.
The elimination half-life is 18-36 hours in adults, longer in the elderly (up to 48+ hours)
and in renal impairment. Hemodialysis efficiently removes lithium and is the treatment of choice
for severe lithium toxicity (plasma level >4 mEq/L or severe neurological toxicity).
Significância Clínica
Lithium is the gold-standard mood stabilizer for bipolar I disorder,
with superior evidence for suicide prevention compared to valproate and antipsychotics. Long-term
prophylaxis reduces relapse frequency by approximately 50%. Lithium toxicity symptoms are dose-
dependent: fine tremor and polyuria/polydipsia at therapeutic levels; coarse tremor, ataxia,
confusion, and nausea at mild toxicity (1.5-2.5 mEq/L); coma, seizures, arrhythmia at severe
toxicity (>3.5 mEq/L). Hypothyroidism (20-40% after 10+ years) and nephrogenic diabetes insipidus
(polyuria in 20-40%, due to downregulation of aquaporin-2 by lithium) are major long-term effects.
Chronic nephropathy (tubular damage, gradual CKD) is a concern with decades of use, though
recent evidence suggests the incidence may be lower than historically reported.