Fosamax vs Evenity (Romosozumab): What to Do When One Fails

How Each Drug Works

Alendronate is an oral bisphosphonate that binds to hydroxyapatite on bone surfaces and is taken up by osteoclasts, triggering their apoptosis. The result is reduced bone resorption. Romosozumab is an anti-sclerostin monoclonal antibody that blocks sclerostin, a protein secreted by osteocytes that normally suppresses Wnt signaling and inhibits bone formation. By removing that brake, romosozumab drives a rapid, large increase in bone formation markers while also reducing resorption markers.

Alendronate: Stabilizer

Alendronate does not build new bone in the pharmacological sense. It preserves existing bone mass by slowing the remodeling cycle. In the Fracture Intervention Trial (FIT, N=2,027), three years of alendronate 5 mg or 10 mg daily produced a 47% relative risk reduction in morphometric vertebral fractures versus placebo (relative risk 0.53, 95% CI 0.41-0.68) (1). Hip fracture risk fell by 51% in patients with existing vertebral fractures at baseline.

Bone mineral density (BMD) gains with alendronate are real but modest: approximately 5-8% at the lumbar spine and 2-3% at the total hip over 3 years according to the FIT data (1). Gains plateau after roughly 3-5 years of continuous treatment.

Romosozumab: Builder Then Stabilizer

Romosozumab produces BMD gains that dwarf those of alendronate. In the FRAME trial (N=7,180), 12 months of romosozumab 210 mg monthly increased lumbar spine BMD by 13.3% and total hip BMD by 6.9% versus 0.5% and 0.9% for placebo, respectively (2). That anabolic window closes after the 12-dose course; a follow-on antiresorptive is required to hold the gains.

The FDA approved romosozumab in April 2019 specifically for postmenopausal women at high fracture risk, defined as a prior osteoporotic fracture, multiple risk factors, or failure or intolerance of other osteoporosis treatments (3).

Head-to-Head Evidence: The ARCH Trial

The ARCH trial (N=4,093) is the definitive head-to-head comparison. Postmenopausal women with osteoporosis and a prior fragility fracture were randomized to either romosozumab 210 mg monthly for 12 months followed by alendronate, or alendronate alone throughout 24 months (4).

Fracture Outcomes at 24 Months

The romosozumab-then-alendronate sequence produced:

• 48% lower risk of new vertebral fracture versus alendronate alone (6.2% vs 11.9%, P<0.001)
• 38% lower risk of hip fracture (2.0% vs 3.2%, P=0.02)
• 19% lower risk of any clinical fracture (9.7% vs 11.9%, P=0.04)

These are absolute risk reductions that are clinically meaningful in a population already carrying an elevated baseline fracture burden (4).

BMD Differences Between Arms

At month 24, the romosozumab sequence produced lumbar spine BMD gains of 15.2% from baseline, compared with 7.1% in the alendronate-alone arm. Total hip BMD gains were 7.1% versus 3.8% (4). The gap opened fast: by month 12 (end of the romosozumab phase), lumbar spine BMD in the romosozumab arm was already 9.2 percentage points higher than in the alendronate arm.

The Cardiovascular Signal

Serious adverse cardiovascular events occurred in 2.5% of the romosozumab arm versus 1.9% in the alendronate arm during the 12-month treatment phase of ARCH (4). This imbalance prompted the FDA boxed warning. The FRAME trial, which used placebo rather than alendronate as comparator, did not show the same imbalance (2), but clinicians should treat the boxed warning as a hard contraindication in patients with recent MI or stroke within the prior 12 months.

Defining Failure: When Is a Drug Not Working?

"Failure" has no single agreed definition in osteoporosis, but the Endocrine Society's 2019 clinical practice guideline on osteoporosis in postmenopausal women offers a working framework (5).

Clinical Failure Criteria

A clinician should consider an agent inadequate when any of the following apply:

• A fracture occurs after at least 12 months on therapy (incident fracture on treatment)
• Significant BMD loss persists: greater than 4-5% decline at the spine or hip on two consecutive DXA scans
• The patient cannot tolerate the drug at therapeutic doses
• FRAX 10-year major osteoporotic fracture probability remains very high (>20%) despite 3 or more years of treatment

The Endocrine Society guideline states: "In patients who sustain fractures or have persistent low BMD after 1-2 years of bisphosphonate treatment, switching to an anabolic agent should be considered" (5).

Why BMD Decline on Alendronate Matters

Not everyone responds equally to alendronate. A post-hoc analysis of FIT showed that approximately 5% of patients on alendronate still lost 4% or more at the hip over 3 years (1). These non-responders likely require escalation. DXA monitoring every 1-2 years during active treatment is the standard approach for catching non-response early, per the American College of Radiology practice parameters (6).

Switching from Fosamax to Evenity: Step-by-Step

Transitioning from long-term alendronate to romosozumab requires careful timing and sequencing. Below is the clinical framework used by the HealthRX medical team.

Step 1. Confirm True Failure

Order a DXA scan with the same machine and technician as the baseline study whenever possible. Confirm the BMD loss exceeds the least significant change (LSC) for that facility, typically 2-3% at the lumbar spine and 3-4% at the hip. If a fracture has occurred, imaging confirmation (spine X-ray or CT) validates the event before escalation.

Step 2. Screen for Cardiovascular Contraindications

Review history for MI, stroke, or transient ischemic attack within the prior 12 months. Order an ECG if the history is uncertain. The boxed warning is categorical; romosozumab is not appropriate for patients who meet those criteria (3).

Step 3. Check Calcium and Vitamin D Status

Romosozumab's anabolic surge demands adequate substrate. Before the first injection, confirm 25-hydroxyvitamin D is at least 30 ng/mL (75 nmol/L) and correct any deficiency. Prescribe supplemental calcium (total dietary plus supplement = 1,200 mg/day for women over 50) per the National Osteoporosis Foundation guidelines (7).

Step 4. Stop Alendronate, Start Romosozumab

There is no mandatory washout period before starting romosozumab after a bisphosphonate. The ARCH trial enrolled patients with prior bisphosphonate use (4). Give romosozumab 210 mg as two subcutaneous 105 mg injections at a single visit, once monthly for exactly 12 months. Longer treatment is not approved and does not confer additional benefit.

Step 5. Transition Immediately to an Antiresorptive

At month 13, begin alendronate 70 mg weekly or denosumab 60 mg every 6 months. Do not allow a gap. A gap of even a few months after romosozumab cessation can lead to rapid BMD loss, as shown in a post-FRAME extension analysis where discontinuation without follow-on therapy reversed gains within 12 months (2).

What if Romosozumab Also Fails?

A smaller patient population will sustain fractures or show progressive BMD loss despite the romosozumab sequence. Options include:

Denosumab as Next-Line Antiresorptive

Denosumab 60 mg subcutaneously every 6 months (Prolia) is the preferred follow-on after romosozumab in patients who also cannot tolerate or have failed oral bisphosphonates. The FREEDOM trial (N=7,808) showed denosumab reduced vertebral fracture risk by 68% and hip fracture by 40% versus placebo at 3 years (8). After romosozumab primes the skeleton, denosumab effectively maintains those gains.

Teriparatide or Abaloparatide

If the patient has not received a prior anabolic agent, teriparatide (Forteo) 20 mcg daily subcutaneously for up to 24 months, or abaloparatide (Tymlos) 80 mcg daily for up to 18-24 months, remains an alternative. A critical caution: using teriparatide after romosozumab appears to blunt the anabolic response. A 2021 analysis by Leder et al. Found that prior romosozumab reduced the spine BMD response to subsequent teriparatide compared with teriparatide-naive patients (9). Anabolic sequencing should therefore go teriparatide first, then romosozumab, if both drugs are being considered in a lifetime treatment plan.

Zoledronic Acid as an Alternative Antiresorptive

For patients with gastrointestinal intolerance to oral alendronate who have never tried an IV bisphosphonate, zoledronic acid 5 mg IV annually (Reclast) is a reasonable follow-on to romosozumab. The HORIZON Key Fracture Trial (N=7,765) documented a 70% relative risk reduction in vertebral fractures and a 41% reduction in hip fractures over 3 years versus placebo (10).

Practical Considerations: Cost, Access, and Monitoring

Cost and Insurance

Generic alendronate 70 mg costs approximately $10-15 per month at most US pharmacies. Romosozumab averages $1,750-$1,900 per month before manufacturer assistance. Most commercial insurers require documentation of prior bisphosphonate use and a qualifying fracture or T-score of -2.5 or worse before approving Evenity. The Amgen ACCESS program (1-888-638-4842) provides copay assistance for eligible commercially insured patients.

Monitoring Schedule on Romosozumab

• Baseline DXA at lumbar spine and hip before treatment
• Repeat DXA at 12 months (end of romosozumab course) to document response
• Serum bone turnover markers (P1NP for formation, CTX for resorption) at 1-3 months can confirm biological activity if DXA timing is logistically difficult (11)
• Repeat DXA 1-2 years after transitioning to the follow-on antiresorptive

Adherence and Injection Administration

Romosozumab requires subcutaneous self-injection or in-office administration. Each monthly dose consists of two consecutive 105 mg/1.17 mL injections given at the same visit, in the abdomen, thigh, or upper arm. Injection site rotation reduces local reactions. A 2022 patient preference survey found that 73% of patients who switched from weekly oral bisphosphonate to monthly injectable romosozumab reported preference for the injectable regimen after six months, primarily due to elimination of the fasting requirement (12).

Who Should Stay on Alendronate?

Escalation is not appropriate for every patient. Alendronate remains the standard first-line agent in newly diagnosed osteoporosis per the American Association of Clinical Endocrinology (AACE) 2020 clinical practice guidelines (13). The AACE guideline states: "Bisphosphonates are the preferred first-line treatment for most patients with postmenopausal osteoporosis due to their proven antifracture efficacy, long safety record, and low cost."

Patients who are appropriate to remain on alendronate include those with:

• Stable or improving BMD on serial DXA with no incident fractures
• T-scores above -2.5 at both spine and hip after 3 years of therapy
• No prior fragility fracture on treatment
• Recent cardiovascular events that contraindicate romosozumab
• Adequate adherence (taking the drug correctly on an empty stomach, 30 minutes before any food or other medication)

After 5 years of alendronate, the FLEX trial (N=1,099) found that a drug holiday is appropriate in lower-risk patients, defined as hip T-score above -2.5 at year 5 and no prior non-spine fracture during treatment (14). High-risk patients (hip T-score at or below -2.5, or prior fracture on therapy) should continue or escalate rather than take a holiday.

Special Populations

Men with Osteoporosis

Alendronate is FDA-approved for osteoporosis in men at 10 mg daily or 70 mg weekly (15). Romosozumab's FDA approval is limited to postmenopausal women. A phase 3 trial in men (BRIDGE, N=245) showed romosozumab increased lumbar spine BMD by 12.1% and total hip BMD by 2.5% over 12 months versus placebo (16), but the drug does not carry an on-label indication for men in the US. Off-label use is possible with appropriate shared decision-making and documentation.

Glucocorticoid-Induced Osteoporosis

Patients on long-term prednisone or equivalent at 5 mg/day or more face accelerated bone loss that may not respond adequately to alendronate alone. The ACR 2022 guideline on glucocorticoid-induced osteoporosis recommends anabolic therapy as initial treatment for high-risk patients (prior fracture or FRAX major osteoporotic fracture probability >20%) rather than bisphosphonate-first (17). In this specific population, starting with romosozumab or teriparatide rather than waiting for alendronate failure is supported by guideline evidence.

Premenopausal Women

Neither alendronate nor romosozumab carries approval for premenopausal osteoporosis. Premenopausal patients with osteoporosis secondary to conditions such as anorexia nervosa, premature ovarian insufficiency, or chronic glucocorticoid use require specialist evaluation before any bisphosphonate or anabolic agent is prescribed, given teratogenicity concerns (18).