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Osteoporosis - Therapeutic Notes

Treatment Thresholds & Indications

• Postmenopausal women and men ≥50 years with a prior osteoporotic fracture
• BMD T-score ≤ -2.5 (diagnostic threshold for osteoporosis)
• Osteopenia + high fracture probability: >20% 10-year major fracture risk or >3% 10-year hip fracture risk (FRAX, US thresholds)
• Minimal-trauma fracture with BMD in osteopenia range, or multiple risk factors even if BMD is not in osteoporotic range

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Drug Classification Overview

Because bone remodeling is a coupled process, antiresorptive drugs ultimately also reduce bone formation - they do not promote substantial BMD gains. BMD increases seen in the first years of antiresorptive therapy represent a contraction of the remodeling space to a new steady-state plateau. (Goodman & Gilman's, p. 575)

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Relative Efficacy on Lumbar Spine BMD

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1. Bisphosphonates (First-Line)

Mechanism of Action

• Bind to hydroxyapatite crystals in bone at sites of active remodeling
• Inhibit osteoclast function and reduce osteoclast number by inducing apoptosis
• Nitrogen-containing bisphosphonates additionally inhibit farnesyl pyrophosphate synthase (mevalonic acid pathway), disrupting intracellular protein trafficking
• Zoledronic acid has very high affinity for mineralized bone; a single IV infusion suppresses resorption for up to 1 year

Relative Antiresorptive Potency

Effect on Lumbar Spine BMD (Alendronate)

Dosing & Administration

• Take with 6-8 oz of plain water only (risedronate delayed-release: at least 4 oz)
• Take ≥30 min before any food, drink, or medications (≥60 min for ibandronate)
• Remain upright and do not lie down for ≥30 min after (≥60 min for ibandronate)
• Risedronate delayed-release: take immediately after breakfast

Adverse Effects

• GI: Diarrhea, abdominal pain; esophagitis and esophageal ulcers (oral agents) - minimized by upright posture
• Musculoskeletal pain
• Osteonecrosis of the jaw (ONJ): Risk factors include longer/higher-dose therapy, IV route, dental procedures, glucocorticoids, diabetes, smoking
• Atypical femur fractures: Risk increases with long-term use
• Drug holiday: Recommended after 5 years of oral bisphosphonate or 3 years of zoledronic acid in patients whose fracture risk has decreased; do NOT discontinue in high-risk patients
• Avoid in severe renal impairment (eliminated renally)

Fracture Efficacy

• Alendronate: reduces hip and vertebral fractures; the beneficial effect persists for several years
• Bisphosphonates are preferred first-line agents for postmenopausal, male, and glucocorticoid-induced osteoporosis

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2. Denosumab (RANKL Inhibitor)

Mechanism of Action

• Inhibits osteoclast formation, activity, and survival
• Produces a potent and rapid antiresorptive effect

Key Pharmacology

• Route/Frequency: Subcutaneous injection every 6 months
• Unlike bisphosphonates (where BMD plateaus at 4-5 years), BMD continues to increase as long as denosumab is continued
• 10-year extension data: +21.7% spine BMD, +9.2% hip BMD
• Fracture reduction (FREEDOM trial, 3 years): Vertebral fractures -68%, Hip fractures -40%, Nonvertebral fractures -20%

Indications

• Postmenopausal osteoporosis at high fracture risk
• Male osteoporosis
• Glucocorticoid-induced osteoporosis
• Women on aromatase inhibitors (breast cancer)
• Men on androgen deprivation therapy (prostate cancer)
• Considered an alternative first-line agent, particularly for higher-risk patients

Adverse Effects

• Hypocalcemia (especially in CKD with eGFR <30 mL/min / hemodialysis - monitor serum Ca²⁺)
• Hypersensitivity reactions, dermatitis, rash, eczema
• Increased risk of infections
• ONJ (~5/10,000 patient-years), atypical femur fractures (~1/10,000 patient-years)

Critical Warning: Rebound Effect on Discontinuation

• Single IV infusion of zoledronic acid at time of missed dose maintains BMD for 1-2 years
• Oral bisphosphonates for 12-24 months are an alternative
• Monitor bone turnover markers and BMD after transition

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3. Estrogen / Hormone Replacement Therapy (HRT)

Efficacy

• Prevents bone loss and induces small increases in BMD at spine, hip, and total body
• Epidemiologic data: ~50% reduction in osteoporosis-related fractures (including hip) in women who start early and continue treatment
• Benefit declines after discontinuation; no residual fracture protection by 10 years post-cessation
• Effective via oral, transdermal, or subcutaneous implant routes

Standard Doses

• Conjugated equine estrogens: 0.625 mg/day (lower doses of 0.3-0.45 mg also effective)
• Esterified estrogens: 0.3 mg/day
• Ethinyl estradiol: 5 μg/day
• Transdermal estradiol: 50 μg/day patch

Mode of Action

• Two estrogen receptor subtypes (ERα, ERβ) are expressed on osteoblasts and cells of monocyte lineage
• ERα is the dominant subtype in bone (ERα knockout → reduced bone mass)
• Estrogen mainly acts indirectly by reducing pro-osteoclastic cytokines (IL-1, IL-6, TNF) from osteoblasts and T-cells, rather than by direct inhibition of osteoclasts

WHI Findings - Risk-Benefit Profile

• Estrogen-only arm (women post-hysterectomy): no increased MI risk, and decreased breast cancer risk - suggesting some detrimental effects are progestin-related
• Benefit/risk ratio improved if commenced before age 60 or within 10 years of menopause
• Current guidelines: not recommended for osteoporosis prevention as primary strategy (USPSTF)

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4. SERMs - Selective Estrogen Receptor Modulators

• Acts as an estrogen agonist in bone (antiresorptive) and a partial antagonist in breast and uterus
• Increases BMD by ~2-3% at lumbar spine (less than estrogen or bisphosphonates - see efficacy graph above)
• Fracture reduction: Reduces vertebral fracture risk; no proven effect on non-vertebral or hip fractures
• Additional benefit: reduces invasive breast cancer risk by ~76% in high-risk postmenopausal women
• Adverse effects: Hot flashes, leg cramps, increased venous thromboembolism risk (similar to estrogen)
• Does not cause endometrial hyperplasia

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5. Calcitonin

• Polypeptide hormone produced in the thyroid; suppresses osteoclast activity by direct action on the osteoclast calcitonin receptor (osteoclasts cannot maintain their active ruffled border)
• Preparations: nasal spray (200 IU/day) or injectable
• Small increments in spinal BMD; small reduction in new vertebral fractures
• No proven effect on non-vertebral or hip fractures
• Not indicated for prevention; not potent enough to prevent bone loss in early postmenopausal women
• Possible analgesic effect on bone pain
• Safety concerns: possible increased cancer risk (European Medicines Agency removed osteoporosis indication; FDA Advisory Committee voted similarly)
• Injectable form: frequent reactions including nausea and facial flushing

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6. Anabolic Agents

A. Teriparatide [PTH(1-34)]

• Recombinant human PTH fragment (hPTH 1-34)
• Acts as a PTH receptor agonist: once-daily SC administration leads to stimulation of osteoblastic activity and net bone formation (intermittent PTH is anabolic; continuous PTH is catabolic)
• Most potent BMD gain of all agents (see efficacy graph - ~14% spine BMD increase at 18 months)
• Indications: High fracture risk, failure or intolerance of antiresorptive therapy
• Dose: 20 μg SC daily (approved); max 2 years cumulative lifetime use
• Adverse effects: Injection site reactions, hypercalcemia, orthostatic hypotension
• Contraindication: Patients at risk for osteosarcoma (Paget disease, prior radiation to bone, unexplained elevated ALP, open epiphyses); increased osteosarcoma risk in rats
• After completion: Must transition to an antiresorptive agent to maintain gains

B. Abaloparatide [PTHrP(1-34)]

• Analog of parathyroid hormone-related peptide (hPTHrP 1-34)
• Similar mechanism and indications to teriparatide
• Additional adverse effect: Hyperuricemia (vs. teriparatide)
• Same 2-year lifetime limit and same post-treatment antiresorptive transition requirement

C. Romosozumab (Sclerostin Inhibitor)

• Monoclonal antibody against sclerostin (a Wnt pathway inhibitor secreted by osteocytes that normally inhibits bone formation)
• Blocking sclerostin → promotes osteoblast activity (anabolic) + decreases bone resorption (antiresorptive) = dual mechanism (unique among osteoporosis drugs)
• Indication: Postmenopausal women with osteoporosis at high fracture risk
• Administration: Monthly SC injection for 12 months (then transition to antiresorptive)
• Key safety concern: Increased risk of cardiovascular events (MI, stroke) - contraindicated in patients with prior MI or stroke within the last year
• Must follow with antiresorptive therapy after 12-month course

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Sequential Therapy Strategy

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Non-Pharmacologic Foundations

• Calcium: Dietary increase or supplementation (adequate intake essential for all drug therapies)
• Vitamin D: Supplementation; a 2025 meta-analysis (Cong & Zhang, BMC Musculoskel Disord) confirms combined Ca²⁺ + Vit D significantly improves BMD and reduces fracture risk in postmenopausal women
• Exercise: Muscle-strengthening and weight-bearing
• Lifestyle: Avoid smoking; limit alcohol

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Drug Comparison Summary

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