1. Classification of Tetracyclines
Tetracyclines are classified based on duration of action:
| Type | Drugs | Half-life |
|---|---|---|
| Short-acting | Tetracycline, Oxytetracycline | 6–8 hours |
| Intermediate-acting | Demeclocycline | 12–16 hours |
| Long-acting | Doxycycline, Minocycline | 16–24 hours |
2. Mechanism of Action
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Inhibit protein synthesis by binding to the 30S ribosomal subunit.
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Bacteriostatic action (inhibit bacterial growth).
3. Spectrum of Activity
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Broad-spectrum: effective against Gram-positive, Gram-negative, intracellular organisms.
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Common organisms:
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Rickettsia, Chlamydia, Mycoplasma
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Helicobacter pylori (part of triple therapy)
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Acne-causing bacteria (Propionibacterium acnes)
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4. Resistance Mechanisms
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Efflux pumps (bacterial mechanisms to pump drug out).
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Ribosomal protection proteins.
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Enzymatic inactivation of tetracycline.
5. Pharmacokinetics
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Absorption: Oral absorption reduced by food (except Doxycycline and Minocycline).
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Distribution: Good tissue penetration, including into bones and teeth.
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Excretion:
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Renal (except Doxycycline — mainly fecal excretion).
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Safe in renal failure: Doxycycline preferred.
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6. Adverse Effects
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Gastrointestinal upset (nausea, vomiting, diarrhea).
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Photosensitivity (sunburn easily).
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Teeth discoloration (if given in children <8 years).
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Hepatotoxicity (especially in pregnancy).
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Superinfections (e.g., Candidiasis).
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7. Contraindications
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Pregnancy
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Children <8 years (due to teeth and bone effects)
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Renal impairment (except Doxycycline)
MCQs on Tetracyclines
1. Which ribosomal subunit is targeted by tetracyclines?
A) 50S
B) 30S
C) 40S
D) 60S
➔ Answer: B) 30S
2. Which of the following tetracyclines is preferred in renal failure?
A) Tetracycline
B) Oxytetracycline
C) Doxycycline
D) Demeclocycline
➔ Answer: C) Doxycycline
3. Which of the following is NOT an adverse effect of tetracyclines?
A) Teeth discoloration
B) Photosensitivity
C) Tendon rupture
D) Gastrointestinal upset
➔ Answer: C) Tendon rupture (seen with fluoroquinolones)
4. Tetracyclines are contraindicated in which of the following?
A) Adults over 40
B) Pregnancy
C) Hypertension
D) Diabetes mellitus
➔ Answer: B) Pregnancy
5. Which of the following organisms is commonly treated with tetracyclines?
A) Plasmodium falciparum
B) Rickettsia rickettsii
C) Mycobacterium tuberculosis
D) Candida albicans
➔ Answer: B) Rickettsia rickettsii
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