Pharmacology · Antibacterial Agents

Tetracyclines & Glycylcyclines

Mechanism, generations, indications, adverse effects, and resistance

Tetracyclines inhibit bacterial protein synthesis by binding reversibly to the 30S ribosomal subunit, blocking aminoacyl-tRNA entry at the A site — activity is bacteriostatic and depends on active intracellular drug accumulation.
Doxycycline is the workhorse of the class: once-daily dosing, safe in renal failure (biliary/fecal elimination), acceptable with food, and the drug of choice for RMSF, ehrlichiosis, and Lyme disease.
Tetracycline (first-generation) accumulates in renal failure and is contraindicated — doxycycline or minocycline should always be used instead in patients with kidney disease.
Tigecycline (glycylcycline) overcomes the two main tetracycline resistance mechanisms and covers MRSA, VRE, CRE, and anaerobes — but has no Pseudomonas activity and achieves low plasma concentrations, making it unsuitable for bacteremia.
Tetracyclines bind polyvalent cations and form insoluble chelates — take oral doses at least 2 hours before or 4–6 hours after antacids, calcium, iron, zinc, or dairy products.

Abbreviations: tRNA = transfer RNA · MDR = multidrug-resistant · CRE = carbapenem-resistant Enterobacteriaceae · MRSA = methicillin-resistant Staphylococcus aureus · VRE = vancomycin-resistant Enterococcus · RMSF = Rocky Mountain spotted fever · FDA = US Food and Drug Administration · CAP = community-acquired pneumonia · ESBL = extended-spectrum beta-lactamase

Tetracycline Generations — Pharmacokinetic Comparison

Agent	Oral Bioavailability	Half-Life / Dosing	Renal Failure	Key Limitation
Tetracycline (1st gen)	60–80% fasting; food ↓ significantly	6–12 hr; four times daily	Contraindicated — accumulates, worsens azotemia	Frequent dosing; food/cation interactions; renal contraindication
Doxycycline (2nd gen)	~93%; food acceptable	18–22 hr; once-daily	Safe — biliary/fecal elimination	Photosensitivity; contraindicated in pregnancy and under age 8 (except RMSF)
Minocycline (2nd gen)	~95–100%	~16 hr; twice-daily	Safe — hepatic elimination	Vestibular toxicity (dizziness, ataxia — reversible); pigmentation
Tigecycline (glycylcycline)	IV only — no oral formulation	~42 hr IV; once-daily	Safe (reduce dose in severe hepatic impairment)	No Pseudomonas coverage; low plasma levels — avoid for bacteremia

Key Clinical Indications

Doxycycline — Workhorse Agent

Intracellular, Vector-Borne, and Respiratory

RMSF, ehrlichiosis, anaplasmosis — drug of choice at any age (mortality risk outweighs dental risk in children)
Lyme disease (adults and children ≥8 years)
Chlamydia, Mycoplasma pneumoniae, Legionella
Brucellosis (+ rifampin), Q fever, anthrax (post-exposure prophylaxis)
CAP (outpatient, in combination or alone)
Malaria prophylaxis (chloroquine-resistant areas)
Acne and rosacea (anti-inflammatory doses)

Tigecycline — MDR Reserve Agent

Multidrug-Resistant Infections (Non-Bacteremic)

CRE (non-bacteremic infections)
MRSA (skin/soft tissue, pneumonia — non-bacteremic)
VRE
ESBL-producing organisms
Polymicrobial infections including anaerobes
Not for bacteremia — low plasma levels insufficient for bloodstream clearance
Not for Pseudomonas aeruginosa — intrinsically resistant

Key Adverse Effects

Teeth and Bone

Developmental Contraindication

Chelates calcium in developing teeth and bone — permanent yellow-brown discoloration and enamel hypoplasia
Contraindicated in pregnancy and children under 8 years
Exception: doxycycline for RMSF at any age — short courses have minimal dental risk and the infection is fatal without treatment

GI and Hepatic

Esophageal and Hepatotoxicity

Nausea, vomiting, esophageal irritation — take with full glass of water, remain upright 30 minutes
Esophageal ulceration with doxycycline if taken supine or without water
Hepatotoxicity: rare with oral doses; reported with high-dose IV tetracycline (now avoided)
Tetracycline worsens azotemia in renal failure — anti-anabolic effect increases blood urea nitrogen

Photosensitivity and Vestibular

Skin and CNS Effects

Photosensitivity: doxycycline > others — sunscreen and protective clothing required
Vestibular toxicity: minocycline-specific — dizziness, ataxia, nausea; reversible on discontinuation
Pseudotumor cerebri (benign intracranial hypertension) — rare; presents as headache and visual changes
Pigmentation: minocycline causes blue-gray skin, teeth, and mucous membrane discoloration

Resistance Mechanisms

Mechanism	How It Works	Tigecycline Overcomes?
Efflux pumps (tet A/B/C/E; tet K/L)	Actively export the tetracycline–Mg²⁺ chelate complex from the cell before it reaches the ribosome	Yes — bulky C-9 substituent prevents recognition by these pumps
Ribosomal protection (tet M/O/Q)	GTPase proteins dislodge tetracycline from the 30S ribosomal binding site	Yes — ~5× higher ribosomal affinity outcompetes protective proteins
Enzymatic inactivation (tet X variants)	Flavoprotein monooxygenase hydroxylates tetracycline at C-11a, destroying activity	Emerging threat — tet X3/X4 variants can inactivate tigecycline; co-located with other MDR genes

Clinical Rules: Renal Failure, RMSF Exception, and Tigecycline Limits

Doxycycline is safe in renal failure — tetracycline (first-generation) is not. Never use first-generation tetracycline in patients with significant renal impairment; it accumulates and exacerbates azotemia through an anti-anabolic effect on protein metabolism. Always use doxycycline or minocycline instead.

The age restriction for tetracyclines does not apply to doxycycline in RMSF: doxycycline is the drug of choice for Rocky Mountain spotted fever, ehrlichiosis, and anaplasmosis at any age, including children under 8. The infection carries significant mortality without treatment, and short courses of doxycycline produce minimal dental staining.

Tigecycline FDA black box warning (2013): higher all-cause mortality compared to comparators across multiple indications in clinical trials. Reserve for infections where no alternative exists. Do not use as monotherapy for bacteremia — plasma concentrations are insufficient for bloodstream infections. Do not use for Pseudomonas.

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 44: Tetracyclines, Macrolides, Clindamycin, Chloramphenicol, Streptogramins, and Oxazolidinones	McGraw-Hill, 2021
Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 51: Protein Synthesis Inhibitors and Miscellaneous Antibacterial Agents	McGraw-Hill, 2023
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Petersen PJ, Jacobus NV, Weiss WJ, Sum PE, Testa RT	In vitro and in vivo antibacterial activities of a novel glycylcycline, the 9-t-butylglycylamido derivative of minocycline (GAR-936)	Antimicrob Agents Chemother. 1999;43(4):738–744
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FDA Drug Safety Communication	FDA warns of increased risk of death with IV antibacterial Tygacil (tigecycline) and approves new boxed warning	US Food and Drug Administration; September 27, 2013
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