Pharmacological Management of Metabolic Bone Diseases

Bisphosphonates - Skeleton Savers

• MoA: Pyrophosphate analogs; ↓ osteoclast activity & bone resorption.
• Administration & Examples:
  • Oral: Alendronate, Risedronate (weekly); Ibandronate (monthly).
    • Bioavailability <1%.
    • 📌 Stay UPRIGHT 30-60 min post-dose (empty stomach, water) to prevent esophagitis.
  • IV: Zoledronic acid (yearly for osteoporosis), Pamidronate. Higher potency.
• Key Uses:
  • Osteoporosis (postmenopausal, steroid-induced).
  • Paget's disease.
  • Malignancy-related hypercalcemia.
• Side Effects:
  • Oral: Esophagitis, GI issues.
  • IV: Acute phase reaction.
  • Hypocalcemia.

  ⭐ Osteonecrosis of the Jaw (ONJ) & Atypical Femoral Fractures (AFF) are rare with long-term use (>5 yrs).

• Drug Holiday: Consider after 3-5 yrs (oral) / 3 yrs (IV Zoledronate) if stable.

Beyond Bisphosphonates - Diverse Defenders

• Key alternatives to bisphosphonates with distinct mechanisms and uses.

⭐ Teriparatide (recombinant PTH) is an anabolic agent that stimulates osteoblasts more than osteoclasts, leading to a net increase in bone formation. This "anabolic window" effect is crucial for its bone-building capacity.

Vitamin D & Calcium - Foundational Fuel

• Vitamin D
  • Forms:
    • D2 (Ergocalciferol): From plants, fortified foods.
    • D3 (Cholecalciferol): Skin (UVB), animal sources (e.g., oily fish).
  • Activation: Skin (UVB) $\rightarrow$ D3 $\xrightarrow{Liver}$ $25(OH)D \xrightarrow{Kidney}$ $1,25(OH)_2D$ (active Calcitriol). , kidney hydroxylation to 1,25-dihydroxyvitamin D (calcitriol))
  • Levels ($25(OH)D$):
    • Sufficiency: >30 ng/mL
    • Insufficiency: 20-29 ng/mL
    • Deficiency: <20 ng/mL
  • 📌 Deficiency: Bone pain, myopathy, fractures (Adults); Rickets (Children).
• Calcium (Ca)
  • Requirement (Adults): 1000-1300 mg elemental Ca/day.
  • Supplements (% elemental Ca):
    • Ca Carbonate: 40% (with food).
    • Ca Citrate: 21% (good for PPI users).

⭐ Cholecalciferol (Vitamin D3) is generally preferred over ergocalciferol (Vitamin D2) for supplementation due to better efficacy in raising and maintaining serum $25(OH)D$ levels.

Special Cases - Paget's & Renal Rx

• Paget's Disease of Bone:
  • Goal: ↓ bone turnover, pain relief.
  • Rx: Bisphosphonates (IV Zoledronic acid 5mg single dose - 1st line; Oral Alendronate). Calcitonin (2nd line).
• Renal Osteodystrophy (CKD-MBD):
  • Goals: Normalize PO₄, Ca, PTH. Target Ca x P < 55 $mg^2/dL^2$.
  • Phosphate Binders:
    • Ca-based (Acetate): If PO₄ ↑ & Ca normal/↓.
    • Non-Ca based (Sevelamer): If PO₄ ↑ & Ca ↑.
  • Vitamin D Sterols (e.g., Calcitriol): If PTH ↑ & Ca/PO₄ controlled.
  • Calcimimetics (Cinacalcet): If PTH ↑ despite Vit D or if Ca/PO₄ high.

  ⭐ Cinacalcet, a calcimimetic, reduces PTH in secondary hyperparathyroidism (CKD-MBD) by increasing the sensitivity of calcium-sensing receptors on the parathyroid gland to extracellular calcium.

High‑Yield Points - ⚡ Biggest Takeaways

• Bisphosphonates (e.g., Alendronate) are first-line for osteoporosis; inhibit osteoclasts.
• Denosumab, a RANKL inhibitor, offers potent antiresorption; SC every 6 months.
• Teriparatide (recombinant PTH) is an anabolic agent for severe osteoporosis; max 2 years use.
• Raloxifene (SERM) for postmenopausal osteoporosis; also reduces invasive breast cancer risk.
• Calcium & Vitamin D supplementation is crucial with all osteoporosis treatments.
• Key side effects: Bisphosphonates - ONJ, atypical fractures; Teriparatide - hypercalcemia.