Adrenocorticosteroid Pharmacology · Module 1 of 4

Steroid Biochemistry, Receptor Mechanisms, and Pharmacokinetics

Biosynthesis zones, GR signaling modes, HPA physiology, and comparative pharmacokinetics

Zone specificity in the adrenal cortex is determined by enzyme expression, not by the shared precursor cholesterol — zona glomerulosa expresses CYP11B2 (aldosterone synthase) but lacks CYP17A1 17-alpha-hydroxylase activity, so it can make aldosterone but not cortisol; zona fasciculata expresses CYP17A1 and CYP11B1 but not CYP11B2, so it makes cortisol but not aldosterone; this enzymatic segregation is why standard glucocorticoid drugs do not affect aldosterone output, and why the HPA axis (which controls cortisol) is distinct from the RAAS (which controls aldosterone).
The rate-limiting step in cortisol synthesis is StAR protein-mediated cholesterol transport from outer to inner mitochondrial membrane — ACTH acutely upregulates StAR via cyclic AMP, explaining how rapid ACTH stimulation rapidly increases cortisol output; CYP21A2 (21-hydroxylase) deficiency is the most common form of congenital adrenal hyperplasia, blocking 17-hydroxyprogesterone to 11-deoxycortisol conversion, causing cortisol deficiency and substrate shunting into androgen synthesis; elevated 17-hydroxyprogesterone is the diagnostic marker; CYP11B1 is the target of metyrapone; CYP17A1 is targeted by abiraterone in prostate cancer.
Glucocorticoid receptor signaling has two mechanistically distinct modes with different clinical consequences — transactivation (GR homodimer binds GRE in gene promoters) drives most adverse metabolic effects: gluconeogenic enzymes (hyperglycemia), ubiquitin ligases (muscle atrophy), suppressed osteocalcin (osteoporosis); transrepression (GR monomer tethers to NF-κB p65 and AP-1 without DNA binding) drives most anti-inflammatory benefit by blocking COX-2, iNOS, interleukins, and adhesion molecule transcription; no available drug achieves selective transrepression in patients, though budesonide achieves a partial pharmacokinetic separation via high first-pass hepatic metabolism.
HPA axis suppression is minimized by morning dosing — the cortisol awakening response peaks 30–60 minutes after waking and falls through the day; morning exogenous glucocorticoid overlaps with an already partially suppressed axis, whereas evening dosing suppresses the nighttime ACTH surge that drives the next morning peak, producing substantially greater cumulative HPA suppression; prednisone <5 mg/day rarely suppresses; >20 mg/day for >3 weeks causes substantial suppression; biologic duration substantially exceeds plasma half-life for all glucocorticoids because gene transcription changes persist after plasma levels fall.
Biologic duration always exceeds plasma half-life because receptor-mediated gene expression changes outlast plasma drug levels — hydrocortisone: plasma t½ 60–90 min, biologic duration 8–12 h; prednisone: plasma t½ 2–3 h, biologic duration 18–36 h; dexamethasone: biologic duration 36–54 h, zero mineralocorticoid activity, preferred for cerebral edema, fetal lung maturation, RECOVERY trial ARDS, and dexamethasone suppression testing; prednisone is a prodrug requiring hepatic 11-beta-HSD1 activation to prednisolone (use prednisolone in severe liver disease); beclomethasone preferred over fluticasone in ritonavir-treated patients (not a CYP3A4 substrate).

Abbreviations: StAR = steroidogenic acute regulatory protein · ACTH = adrenocorticotropic hormone · CRH = corticotropin-releasing hormone · HPA = hypothalamic-pituitary-adrenal · GR = glucocorticoid receptor · GRE = glucocorticoid response element · NF-κB = nuclear factor kappa-B · AP-1 = activator protein-1 · HSP90 = heat shock protein 90 · MC = mineralocorticoid · RAAS = renin-angiotensin-aldosterone system · CYP = cytochrome P450 · 11β-HSD1 = 11-beta-hydroxysteroid dehydrogenase type 1 · DHEA = dehydroepiandrosterone · CAH = congenital adrenal hyperplasia

Adrenal Cortex Zones and Steroidogenesis

Zona Glomerulosa (outermost)

Aldosterone

Regulated by RAAS (angiotensin II) and serum K⁺ — not ACTH
Expresses CYP11B2 (aldosterone synthase)
Lacks CYP17A1 17α-hydroxylase activity → cannot make cortisol
Standard glucocorticoid drugs do NOT affect aldosterone output

Zona Fasciculata (middle, largest)

Cortisol

Regulated by ACTH from anterior pituitary → cyclic AMP → StAR upregulation → cholesterol transport (rate-limiting)
Expresses CYP17A1 and CYP11B1; lacks CYP11B2 → no aldosterone
Pathway: cholesterol → (StAR) → (CYP11A1) pregnenolone → (CYP17A1) → 17-OH-progesterone → (CYP21A2) 11-deoxycortisol → (CYP11B1) cortisol
CYP21A2 deficiency = most common CAH; CYP11B1 targeted by metyrapone

Zona Reticularis (innermost)

Adrenal Androgens

Regulated by ACTH; expresses CYP17A1 with high lyase activity
Produces DHEA and DHEA-S
CYP17A1 targeted by abiraterone in prostate cancer (inhibits both 17α-hydroxylase and lyase activities)

Glucocorticoid Receptor Signaling — Two Modes

Transactivation — GR Homodimer Binds GRE

Drives Most Adverse Metabolic Effects

Ligand binds cytoplasmic GR → HSP90 dissociates → GR-ligand complex translocates to nucleus → GR homodimerizes → binds glucocorticoid response elements (GREs) in promoters
Upregulates gluconeogenic enzymes → hyperglycemia
Upregulates muscle ubiquitin ligases → muscle atrophy
Suppresses osteocalcin synthesis → osteoporosis
Skin fibroblast inhibition → skin atrophy, striae
Also upregulates I-κB (second layer of NF-κB suppression via transactivation)

Transrepression — GR Monomer Tethers to TF

Drives Most Anti-Inflammatory Benefit

GR monomer physically tethers to p65 subunit of NF-κB and to AP-1 (c-Fos/c-Jun heterodimer) without binding DNA directly
Blocks NF-κB target transcription: COX-2, iNOS, IL-1, IL-6, IL-8, adhesion molecules (ICAM-1, VCAM-1)
Blocks AP-1 target transcription: matrix metalloproteinases, additional cytokines
The therapeutic goal: maximizing transrepression while minimizing transactivation; no available drug achieves this in patients
Budesonide: partial pharmacokinetic separation via ~85–90% first-pass hepatic inactivation → local transrepression with minimal systemic transactivation

Comparative Pharmacokinetics of Clinical Glucocorticoids

Agent	GC Potency	MC Potency	Equiv. Dose	Biologic Duration / Key Notes
Hydrocortisone	1	1	20 mg	8–12 h; preferred for physiological replacement (replaces GC + MC); IV for adrenal crisis
Prednisone / Prednisolone	4	0.8	5 mg	18–36 h; prednisone = prodrug → hepatic 11β-HSD1 → prednisolone; use prednisolone in severe liver disease; most oral anti-inflammatory indications
Methylprednisolone	5	0.5	4 mg	18–36 h; negligible MC activity; preferred IV for acute indications (sodium succinate); 1:1 oral-to-IV conversion
Dexamethasone	25–30	0	0.75 mg	36–54 h; zero MC activity; greatest HPA suppression per dose (avoid chronic); uses: cerebral edema, fetal lung maturation, RECOVERY trial ARDS, anti-emesis, dexamethasone suppression testing
Budesonide	High topical	Low	Local only	~85–90% first-pass hepatic inactivation → <15% systemic bioavailability; use: inhaled (asthma, COPD), oral CR (Crohn, microscopic colitis), intranasal (allergic rhinitis); preferred when pharmacokinetic steroid-sparing is feasible

HPA Axis Rules, Morning Dosing, and CYP3A4 Interactions

HPA axis suppression is minimized by morning dosing (7:00–8:00 AM). The cortisol awakening response peaks 30–60 minutes after waking and falls through the day; morning exogenous glucocorticoid adds to an already partially suppressed axis. Evening dosing suppresses the nighttime ACTH surge driving the next morning peak, producing substantially greater cumulative HPA suppression. Patients must be counseled to take once-daily glucocorticoids specifically in the morning, not simply once daily. Alternate-day dosing further reduces HPA suppression while maintaining anti-inflammatory efficacy in many chronic indications. Suppression thresholds: prednisone <5 mg/day for any duration rarely suppresses clinically; >20 mg/day for >3 weeks = substantial suppression; duration matters as much as dose.

Adrenal crisis management: if suspected, give hydrocortisone 100 mg IV bolus immediately before awaiting laboratory confirmation; draw cortisol and ACTH before the dose for retrospective confirmation; then 200 mg hydrocortisone over 24 hours by infusion; aggressive IV saline; treat precipitating cause. Sick-day rules for all chronically suppressed patients: double or triple oral glucocorticoid dose during febrile illness; use IM hydrocortisone 100 mg if vomiting prevents oral administration.

CYP3A4 interactions: all systemic glucocorticoids are CYP3A4 substrates. Inducers (rifampin, phenytoin, carbamazepine, phenobarbital) accelerate metabolism → therapeutic failure or adrenal crisis in dependent patients when started without dose adjustment. Inhibitors (ketoconazole, ritonavir, clarithromycin) reduce metabolism → iatrogenic Cushing syndrome at standard doses; ritonavir-boosted antiretroviral regimens + fluticasone-containing ICS = particularly dangerous combination; use beclomethasone instead (not a CYP3A4 substrate).

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 40: Estrogens, Progestins, and the Female Reproductive Tract	McGraw-Hill; 2021
Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 44: Estrogens and Progestins	McGraw-Hill; 2023
Miller WL, Auchus RJ.	The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders	Endocr Rev. 2011;32(1):81–151
Stocco DM.	StAR protein and the regulation of steroid hormone biosynthesis	Annu Rev Physiol. 2001;63:193–213
Chrousos GP.	Adrenocorticosteroids and adrenocortical antagonists. In: Katzung BG, ed. Basic and Clinical Pharmacology, 14th ed.	McGraw-Hill; 2018:687–710
Barnes PJ.	Glucocorticosteroids: current and future directions	Br J Pharmacol. 2011;163(1):29–43
Stahn C, Buttgereit F.	Genomic and nongenomic effects of glucocorticoids	Nat Clin Pract Rheumatol. 2008;4(10):525–533
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Czock D et al.	Pharmacokinetics and pharmacodynamics of systemically administered glucocorticoids	Clin Pharmacokinet. 2005;44(1):61–98
Henzen C et al.	Suppression and recovery of adrenal response after short-term, high-dose glucocorticoid treatment	Lancet. 2000;355(9203):542–545