Tetracyclines and Glycylcyclines

Classic tetracycline has oral bioavailability of approximately 60–80% under fasting conditions but drops substantially when taken with food, dairy, or polyvalent cation-containing products. Doxycycline achieves oral bioavailability of approximately 93% and, unlike tetracycline, is not significantly impaired by food—making it far more reliably absorbed in clinical practice. Minocycline approaches 95–100% oral bioavailability. Tigecycline cannot be administered orally and is available only as an intravenous formulation.

Calcium in dairy and supplements, magnesium and aluminum in antacids, iron in supplements, bismuth, and zinc all form insoluble chelate complexes with tetracyclines in the gastrointestinal lumen, reducing absorption. The standard management is to take oral tetracyclines at least two hours before or four to six hours after any polyvalent cation-containing product. Doxycycline can be taken with food but should still be separated in time from antacids, calcium supplements, and iron preparations.

Tetracycline is predominantly eliminated by renal excretion and accumulates dangerously in patients with renal failure, worsening azotemia through an anti-anabolic effect on protein metabolism. It is contraindicated in significant renal impairment. Doxycycline, by contrast, is eliminated primarily through biliary and intestinal secretion and does not accumulate in renal failure. It can be prescribed at standard doses in patients with advanced chronic kidney disease or on dialysis without dose adjustment. When a tetracycline is indicated in a patient with renal impairment, doxycycline is the agent of choice. Minocycline is also hepatically metabolized and does not require renal dose adjustment. Tigecycline requires dose reduction in severe hepatic impairment but not in renal impairment.

All tetracyclines achieve excellent tissue penetration. Tetracyclines concentrate in phagocytic cells (macrophages, neutrophils), which is pharmacokinetically essential for treating intracellular pathogens such as Rickettsia, Ehrlichia, and Chlamydophila . Minocycline has superior central nervous system penetration compared to doxycycline due to greater lipophilicity. Tigecycline has an exceptionally large volume of distribution (far exceeding total body water), with plasma concentrations after intravenous dosing being relatively low despite high tissue levels—which contributes to the inadequacy of tigecycline monotherapy for bacteremia.

Tetracyclines can antagonize bactericidal cell-wall-active agents (penicillins, cephalosporins, vancomycin) by halting bacterial growth, which these bactericidal drugs require to exert their lethal effects. This antagonism is most clinically relevant in infections requiring bactericidal therapy—bacterial meningitis, infective endocarditis—where tetracyclines should not be substituted for or combined with bactericidal antibiotics. Rifampin induces cytochrome P450 enzymes and reduces doxycycline plasma concentrations by approximately 50%; patients on rifampin-containing regimens who require doxycycline may need an alternative agent.

Doxycycline is the drug of choice for rickettsial infections including Rocky Mountain spotted fever, epidemic typhus, ehrlichiosis, and anaplasmosis—infections where prompt initiation is life-saving. It is first-line for Lyme disease in adults, for Chlamydia trachomatis and nongonococcal urethritis, for Brucella infections (typically combined with rifampin), Coxiella burnetii (Q fever), Vibrio cholerae, and for prophylaxis and treatment of anthrax. In community-acquired pneumonia, doxycycline is a first-line alternative for outpatient management in patients without comorbidities, covering both typical and atypical organisms.

Malaria prophylaxis is a major global indication: doxycycline is recommended by the Centers for Disease Control and Prevention for prophylaxis in areas with chloroquine-resistant Plasmodium falciparum. It must not be used as monotherapy for acute malaria treatment but is used in combination with quinine or artesunate for multidrug-resistant falciparum malaria. Doxycycline is also widely used for acne vulgaris and rosacea, exploiting its anti-inflammatory properties at sub-antimicrobial doses.

Tigecycline is approved for complicated skin and soft tissue infections, complicated intra-abdominal infections, and community-acquired pneumonia. Its most valuable clinical role is in polymicrobial infections and infections caused by multidrug-resistant organisms including carbapenem-resistant Enterobacteriaceae, methicillin-resistant Staphylococcus aureus in non-bacteremic contexts, vancomycin-resistant Enterococcus, and extended-spectrum beta-lactamase-producing organisms resistant to most beta-lactams. Tigecycline should not be used as monotherapy for bacteremia given its low plasma concentrations, and combination therapy with another active agent is typically employed for serious multidrug-resistant infections.

Efflux is the most widespread tetracycline resistance mechanism, mediated by membrane transport proteins encoded predominantly by tet genes on plasmids and transposons. These pumps actively export tetracycline-magnesium chelate complexes against their concentration gradient, reducing intracellular drug to sub-inhibitory levels. These efflux pumps are highly effective against tetracycline and doxycycline. Tigecycline evades these classical efflux pumps because its structural modification prevents recognition by these pumps—the primary structural basis for its activity against tetracycline-resistant organisms.

Ribosomal protection proteins bind to the ribosome and dislodge tetracycline from its binding site, restoring ribosomal function even in the presence of drug. They are widespread in streptococci, enterococci, and Bacteroides species. Tigecycline overcomes ribosomal protection resistance because its greater ribosomal binding affinity allows it to successfully compete with and displace these protective proteins.