The Journey of Dexamethasone

Dexamethasone is available in multiple formulations reflecting
its diverse clinical applications: oral tablets (0.5-20 mg), IV/IM injection, intravitreal implant,
topical preparations, and intrathecal use. Oral bioavailability is approximately 70-80%, reflecting
modest first-pass hepatic extraction. Peak plasma concentrations after oral dosing occur within
1-2 hours. Food delays but does not significantly reduce absorption. Intravenous administration
achieves immediate peak concentrations with 100% bioavailability. Intramuscular formulations
(dexamethasone sodium phosphate) are rapidly hydrolyzed by plasma phosphatases to the active free
alcohol form. Intraarticular and epidural formulations deliver high local concentrations with reduced
systemic absorption. Dexamethasone acetate (ester prodrug) is formulated as a suspension for
prolonged-release intraarticular or intramuscular injection.

Dexamethasone distributes widely throughout the body with a
volume of distribution of approximately 0.7-1.0 L/kg. Plasma protein binding is approximately
77%, primarily to corticosteroid-binding globulin (CBG/transcortin) and albumin. Unlike cortisol,
which saturates CBG at therapeutic concentrations, dexamethasone binds CBG with lower affinity
and a larger fraction distributes as free drug. Dexamethasone's fluorine substitution at the 9-alpha
position and methyl group at the 16-alpha position greatly increase glucocorticoid receptor binding
affinity (~27× cortisol) and reduce mineralocorticoid receptor binding to near zero. The drug
penetrates well into the CNS, synovial fluid, and tumor interstitium. Placental transfer occurs;
dexamethasone (unlike prednisolone) is not significantly inactivated by 11-beta-HSD2 in the placenta,
making it the preferred glucocorticoid when fetal effects are desired (fetal lung maturation).

Dexamethasone, like all glucocorticoids, exerts its effects primarily
through the glucocorticoid receptor (GR, NR3C1) — a nuclear receptor. Free dexamethasone diffuses
across cell membranes and binds cytoplasmic GR, causing receptor conformational change, dissociation
from heat shock proteins (Hsp90, Hsp70), and nuclear translocation. In the nucleus, the GR dimer
binds glucocorticoid response elements (GREs) in promoter regions: transactivation at positive GREs
induces anti-inflammatory proteins (annexin-1/lipocortin-1, GILZ, IκBα, MKP-1) that suppress
phospholipase A2 activity (reducing arachidonic acid release), NF-κB activity, and MAPK signaling.
Transrepression occurs when the GR monomer directly interacts with and inhibits NF-κB and AP-1
transcription factors, reducing expression of inflammatory genes (COX-2, IL-1β, IL-6, TNF-α,
adhesion molecules). The net effect is broad anti-inflammatory and immunosuppressive action.

Dexamethasone is metabolized in the liver primarily by CYP3A4
via 6-beta-hydroxylation and other oxidative pathways. Metabolites include 6-beta-hydroxydexamethasone
and other inactive products excreted as glucuronide conjugates. Dexamethasone has a unique bidirectional
relationship with CYP3A4: it is both a substrate and a potent inducer of CYP3A4 — through GR-mediated
upregulation of PXR (pregnane X receptor), which transcriptionally activates CYP3A4. This autoinduction
can reduce dexamethasone's own bioavailability and increase metabolism of co-administered CYP3A4
substrates. CYP3A4 inducers (rifampicin) significantly reduce dexamethasone exposure. The 9-alpha
fluorine atom makes dexamethasone significantly less susceptible to 11-beta-HSD2 inactivation
compared to cortisol, contributing to its long duration of action.

Dexamethasone metabolites are excreted primarily in urine as
glucuronide conjugates. Approximately 65% of a dose is excreted in urine within 24 hours, with
a small fraction in feces via biliary excretion. Plasma half-life is 3.5-4.5 hours, but biological
(pharmacodynamic) half-life is 36-72 hours — the longest among commonly used glucocorticoids.
This dissociation between pharmacokinetic and pharmacodynamic half-life reflects the duration of
altered gene transcription rather than plasma drug concentration. Dexamethasone suppression tests
exploit this long biological half-life (0.5-1 mg administered at 11 PM suppresses morning cortisol
to <1.8 µg/dL in normal subjects, diagnosing Cushing's syndrome). No dose adjustment is required
for renal impairment; caution in hepatic impairment due to reduced metabolism.

Dexamethasone is used in cerebral edema (primary or metastatic brain
tumors), anaphylaxis, severe asthma, adrenal insufficiency, croup, bacterial meningitis, chemotherapy-
induced nausea/vomiting (highly effective antiemetic), multiple myeloma (RRMM regimens), COVID-19
(RECOVERY trial: 35% mortality reduction in mechanically ventilated patients), and fetal lung
maturation before preterm delivery. Adverse effects reflect glucocorticoid excess: hyperglycemia,
hypertension, immunosuppression, osteoporosis, avascular necrosis, adrenal suppression (even a
single dose at 10 mg), Cushingoid features, cataracts, and psychiatric effects (insomnia, euphoria,
psychosis). The HPA axis recovery after prolonged therapy follows a gradual timeline requiring
tapering protocols.