3.3 Osteoporosis Drug Therapy: Antiresorptive, Anabolic, and Hormonal Agents

3.3 Osteoporosis Drug Therapy: Antiresorptive, Anabolic, and Hormonal Agents

Quick Answer: ARRT groups osteoporosis drug therapy into antiresorptive, formation (anabolic), and hormonal and others. Antiresorptives — bisphosphonates and denosumab — slow osteoclastic resorption and raise spine BMD roughly 2–4% in the first year. Anabolics — teriparatide, abaloparatide, romosozumab — stimulate formation and produce larger spine gains. Hormonal agents include estrogen therapy, the SERM raloxifene, and calcitonin. Technologists do not prescribe, but they must recognize what each class does to a serial BMD trajectory.

Why Drug Class Matters at the Scanner

Serial DXA interpretation is drug-aware. A 1.5% spine gain after a year of alendronate is an expected therapeutic response that may still fall below the facility least significant change. The same 1.5% after a year of teriparatide is an underperformance worth flagging. And a patient whose denosumab lapsed eight months ago may present with a spine BMD that has fallen below her pre-treatment baseline — a pattern that looks like a technical error and is not. Knowing the class tells you whether the number is plausible.

Antiresorptive Agents

Antiresorptives reduce the number and activity of osteoclasts. Remodeling slows, the remodeling space fills in, existing bone mineralizes more completely, and BMD rises for one to several years before plateauing.

Bisphosphonates

Bisphosphonates bind avidly to hydroxyapatite at sites of active remodeling and are internalized by osteoclasts, disrupting their function and survival. They persist in bone for years, which is the basis of the drug holiday concept.

Oral administration rules patients ask about constantly: take first thing in the morning on an empty stomach with a full glass of plain water, remain upright and take nothing else by mouth for at least 30 minutes (60 for ibandronate). The rules exist because oral bioavailability is under 1% and is abolished by food, coffee, juice, and — notably — calcium supplements, and because reclining risks pill esophagitis.

Drug holidays. Because bisphosphonate persists in the skeleton, patients at lower ongoing risk may pause therapy after roughly 5 years of oral or 3 years of intravenous treatment, with periodic reassessment. During a holiday, DXA monitoring is what detects resumption of loss. Denosumab has no comparable holiday.

Atypical femoral fracture and osteonecrosis of the jaw are rare long-duration adverse effects. Their relevance to densitometry is that many DXA protocols now include a long femur view to screen for the lateral cortical thickening or beaking that precedes atypical fracture in long-term users.

Denosumab

Denosumab is a monoclonal antibody against RANK ligand (RANKL). By binding RANKL it prevents osteoclast formation, function, and survival — the same pathway estrogen protects. It is given as a subcutaneous injection every 6 months.

The densitometrically critical fact: denosumab is not retained in bone. When it is stopped or delayed, the suppressed remodeling rebounds sharply, producing rapid BMD loss back toward or below baseline and a documented risk of multiple vertebral fractures. Guidance is unambiguous that denosumab must not simply be discontinued; patients are transitioned to a bisphosphonate. If a patient reports a lapsed or long-delayed injection, that history belongs in the scan record because it explains an otherwise inexplicable decline.

Bone-Formation (Anabolic) Agents

Anabolic agents increase osteoblast number and activity, building new bone rather than preserving existing bone.

Three points recur:

1. The PTH paradox. Continuously elevated endogenous PTH, as in primary hyperparathyroidism, causes cortical bone loss. Intermittent pulsed exposure, as with once-daily teriparatide, is anabolic. Same hormone, opposite skeletal effect, determined by exposure pattern.
2. Anabolic courses are time-limited and must be followed by an antiresorptive agent, or the gains are lost.
3. Sequence matters. Anabolic-then-antiresorptive produces greater BMD gains than the reverse; prior potent antiresorptive therapy blunts the response to a subsequent anabolic. Romosozumab carries a boxed warning regarding cardiovascular events and is avoided in patients with recent myocardial infarction or stroke.

Expect larger spine responses from anabolics — commonly in the range of 5% to 10% over one to two years — with more modest hip responses, because the spine is trabecular-rich and responds fastest.

Hormonal and Other Agents

• Estrogen therapy (with progestin if the uterus is intact) prevents postmenopausal bone loss by restoring RANKL suppression. It is FDA-approved for prevention of postmenopausal osteoporosis and is generally used at the lowest effective dose for the shortest appropriate duration, with vasomotor symptoms as a common co-indication. Bone loss resumes when it stops.
• Raloxifene, a selective estrogen receptor modulator (SERM), is an estrogen agonist in bone and an antagonist in breast tissue. It reduces vertebral fracture risk and breast cancer risk; it has not demonstrated hip fracture reduction and raises venous thromboembolism risk.
• Calcitonin, usually intranasal, inhibits osteoclasts weakly. Its BMD and fracture effects are modest and it is now a later-line option.
• Calcium and vitamin D are adjuncts, not treatments. Every agent above was studied on a background of adequate calcium and vitamin D, and repletion is assumed rather than optional.

Summary Table for Serial Interpretation

Against every one of these numbers stands the same rule: a measured change is reported as real only when it meets or exceeds the facility's least significant change, calculated from that technologist's own precision study.