What is the mechanism of action of Doxycycline in the context of periodontal therapy?

Correct answer: D. Inhibits matrix metalloproteinases (MMPs)

• A. INCORRECT — DNA gyrase inhibition is the mechanism of fluoroquinolones (ciprofloxacin), not tetracyclines.
• B. INCORRECT — While doxycycline may have some indirect effects on bone, its primary identified periodontal mechanism is MMP inhibition, not direct osteoclast inhibition.
• C. INCORRECT — Tetracyclines (including doxycycline) inhibit 30S ribosomal subunit (protein synthesis), NOT cell wall synthesis. Cell wall synthesis inhibition is the mechanism of beta-lactams.
• D. CORRECT — Sub-antimicrobial dose doxycycline (SDD) is used in periodontal therapy specifically because it inhibits host-derived matrix metalloproteinases (MMPs), particularly collagenase. This prevents excessive collagen breakdown in the periodontal tissues, independent of its antibiotic activity.