Fosamax vs Evenity (Romosozumab): Switching Between Them

How These Two Drugs Actually Work

Alendronate and romosozumab target bone biology through completely different pathways. Alendronate binds to hydroxyapatite on bone surfaces and inhibits farnesyl pyrophosphate synthase inside osteoclasts, which kills those bone-resorbing cells and slows skeletal loss 1. Romosozumab blocks sclerostin, a protein produced by osteocytes that ordinarily suppresses both bone formation and bone resorption simultaneously 2.

Alendronate: Antiresorptive Only

Alendronate works by eliminating osteoclast activity. It does not stimulate new bone formation. Daily 10 mg or weekly 70 mg oral dosing is FDA-approved for postmenopausal osteoporosis, glucocorticoid-induced osteoporosis, and Paget disease 3. Patients must take it fasting, with 8 oz of plain water, and remain upright for 30 minutes because of esophageal irritation risk.

The Fracture Intervention Trial (FIT, JAMA 1998, N=2,027) showed alendronate reduced morphometric vertebral fractures by 47% over 3 years compared with placebo (relative risk 0.53, 95% CI 0.41 to 0.68) 4. Non-vertebral fracture reduction in higher-risk subgroups reached 20 to 30% 4.

Romosozumab: Dual Mechanism

Romosozumab (Evenity) is a humanized monoclonal antibody that binds and inhibits sclerostin. Blocking sclerostin simultaneously activates the Wnt signaling pathway to stimulate osteoblasts and reduces RANKL-mediated osteoclast activity 5. The net effect is rapid, large BMD gains that exceed anything seen with bisphosphonates alone.

The approved regimen is 210 mg subcutaneous injection once monthly for exactly 12 months. The FDA granted approval in April 2019 6. After 12 months, treatment must transition to an antiresorptive agent to preserve gained bone mass 6.

What the Key Trials Actually Show

Comparing FIT and ARCH directly requires understanding that they asked different questions. FIT tested alendronate against placebo. ARCH tested a romosozumab-then-alendronate sequence against alendronate alone from the start 2. No trial has yet randomized patients to alendronate alone versus romosozumab alone with fracture outcomes as the primary endpoint.

FIT: The Alendronate Evidence Base

FIT enrolled 2,027 postmenopausal women aged 55 to 81 with low femoral neck BMD 4. Women received alendronate 5 mg daily for 2 years then 10 mg daily, or placebo, over 3 years. The 47% relative reduction in radiographic vertebral fractures was statistically strong (P<0.001) 4. Hip fracture reduction reached 51% in women with existing vertebral fractures at baseline 4.

ARCH: Romosozumab Then Alendronate vs Alendronate Alone

ARCH enrolled 4,093 postmenopausal women with osteoporosis and high fracture risk 2. Participants received either romosozumab 210 mg monthly for 12 months followed by open-label alendronate, or alendronate alone for 24 months total.

At 24 months, the romosozumab-to-alendronate sequence produced:

• 48% fewer new vertebral fractures (relative risk 0.52, 95% CI 0.40 to 0.66, P<0.001) 2
• 19% fewer clinical fractures (HR 0.81, 95% CI 0.71 to 0.94, P=0.004) 2
• 27% fewer non-vertebral fractures (HR 0.73, 95% CI 0.61 to 0.88, P<0.001) 2

The ARCH trial also detected a cardiovascular safety signal. Serious cardiac events occurred in 2.5% of the romosozumab group versus 1.9% in the alendronate group during the 12-month treatment phase 2. That imbalance led directly to the FDA boxed warning 6.

BMD Gains: What the Numbers Look Like

In ARCH, romosozumab increased lumbar spine BMD by 13.7% and total hip BMD by 6.2% over 12 months 2. Alendronate over the same 12 months increased lumbar spine BMD by 5.0% and total hip BMD by 2.5% 2. The roughly 2.5-fold BMD advantage for romosozumab at the spine is clinically significant for patients who need rapid improvement before an elective surgery or whose T-score is severely negative.

Switching From Romosozumab to Alendronate

This is the standard, evidence-supported sequence. All major guidelines and the FDA label for Evenity specify that romosozumab must be followed by an antiresorptive agent 6. Alendronate is the most commonly used option for this transition.

Why the Sequence Matters

Romosozumab's anabolic effect is transient. Sclerostin rebounds after the drug clears, and without antiresorptive coverage, BMD gains erode within months 7. A 2019 analysis in the Journal of Bone and Mineral Research showed that patients who stopped romosozumab without transitioning to an antiresorptive lost 4.8% of lumbar spine BMD within 12 months 7.

Alendronate after romosozumab locks in the new bone by suppressing osteoclast-driven resorption. The ARCH data confirm this: gains achieved during the romosozumab phase were preserved through month 24 with alendronate follow-on therapy 2.

Practical Transition Timing

Clinicians typically start alendronate 70 mg weekly within 4 weeks of the last romosozumab injection. There is no required washout period 6. Monitoring DXA at 12 to 24 months after transitioning is standard practice per American Association of Clinical Endocrinology (AACE) 2020 guidelines 8.

Switching From Alendronate to Romosozumab

This direction is more clinically complex. Evidence from the STRUCTURE trial (N=436) demonstrated that patients previously on alendronate gained significantly less total hip BMD on romosozumab than treatment-naive patients 9. At the hip, alendronate-pretreated patients gained 2.9% versus 4.1% in bisphosphonate-naive patients over 12 months 9.

The Bisphosphonate Carryover Effect

Alendronate deposits in bone mineral for years after discontinuation 10. Residual bisphosphonate continues suppressing osteoclast activity, which also partially blunts the coupling signal needed for romosozumab-driven osteoblast activation at the hip 9. The lumbar spine shows less attenuation because trabecular bone turns over faster than cortical hip bone 9.

BMD gains at the lumbar spine in alendronate-pretreated patients on romosozumab in STRUCTURE were 9.8%, which still substantially exceeds what continued alendronate or denosumab achieves 9. For patients who break a hip while on long-term alendronate, switching to romosozumab remains clinically reasonable 8.

How Long to Wait After Stopping Alendronate

No consensus washout period exists. Some clinicians wait 6 to 12 months after stopping alendronate before starting romosozumab to allow partial bisphosphonate dissociation from bone surfaces, but supporting data for this practice are limited 10. The AACE 2020 guidelines do not specify a mandatory gap 8.

Who Should Start With Romosozumab vs Alendronate

Patient risk stratification drives this decision more than any other single factor. The Endocrine Society 2020 Clinical Practice Guideline on Osteoporosis states: "For women with postmenopausal osteoporosis at very high fracture risk, we suggest initial treatment with an anabolic agent (teriparatide, abaloparatide, or romosozumab) over bisphosphonate therapy" 11.

Criteria That Favor Starting Romosozumab

• T-score at or below negative 2.5 at hip or spine with a prior fragility fracture
• T-score at or below negative 3.0 regardless of fracture history
• High 10-year fracture probability on FRAX (hip fracture probability above 3%, major osteoporotic fracture above 20%) 11
• Need for rapid BMD gain before scheduled joint replacement or spinal surgery
• Fracture while on antiresorptive therapy ("treatment failure")

Patients who meet very-high-risk criteria but have a history of myocardial infarction or stroke within the past 12 months should not receive romosozumab 6. For those patients, teriparatide or abaloparatide are anabolic alternatives without the cardiovascular warning 12.

Criteria That Favor Starting Alendronate

• T-score between negative 1.0 and negative 2.5 (osteopenia range) with elevated FRAX score
• Moderate fracture risk without prior fragility fracture
• Cost barriers or lack of injection administration support
• Cardiovascular contraindication to romosozumab
• Preference for oral daily or weekly dosing

Alendronate costs under $20 per month as a generic 13. Romosozumab approaches $1,800 per monthly injection before insurance or assistance programs. Cost is a real barrier for many patients and should be addressed in shared decision-making.

Safety Profiles Side by Side

Both drugs carry distinct risk profiles that affect patient selection and monitoring plans.

Alendronate Safety

Common adverse effects include esophageal irritation, gastroesophageal reflux, and upper abdominal pain. Rare but serious risks are osteonecrosis of the jaw (ONJ, reported at roughly 1 in 10,000 to 1 in 100,000 patients in the oral treatment setting) 14 and atypical femoral fracture (AFF). AFF incidence rises with duration of use, reaching approximately 78 per 100,000 patient-years after more than 8 years of therapy 15. The American Society for Bone and Mineral Research task force recommends drug holidays after 5 years of oral bisphosphonate therapy in lower-risk patients 15.

Romosozumab Safety

The FDA boxed warning for romosozumab states: "Romosozumab-aqqg may increase the risk of myocardial infarction, stroke, and cardiovascular death. Romosozumab-aqqg should not be initiated in patients who have had a myocardial infarction or stroke within the preceding year" 6. Injection-site reactions occur in roughly 12% of patients 6. Hypocalcemia is a risk, particularly in patients with vitamin D deficiency, and calcium and vitamin D adequacy must be confirmed before starting therapy 6. ONJ and AFF have also been reported with romosozumab, though rates appear lower than with long-term bisphosphonate use 16.

Dosing, Administration, and Monitoring

Alendronate Dosing

• Treatment of postmenopausal osteoporosis: 10 mg orally daily or 70 mg orally once weekly 3
• Prevention of postmenopausal osteoporosis: 5 mg daily or 35 mg weekly 3
• Glucocorticoid-induced osteoporosis: 5 mg daily (10 mg daily in postmenopausal women not on estrogen) 3
• Renal caution: avoid if creatinine clearance <35 mL/min 3

Romosozumab Dosing

• 210 mg subcutaneous injection once monthly for 12 months, then mandatory transition to antiresorptive 6
• Two 105 mg injections given sequentially at each monthly visit 6
• Serum calcium, 25-hydroxyvitamin D, and renal function should be checked before initiation 6

Monitoring After Either Drug

DXA scanning every 1 to 2 years is standard when treatment response is uncertain 8. Bone turnover markers (serum CTX for resorption, P1NP for formation) can confirm biochemical response within 3 months of starting therapy 17. A rising P1NP in the first 1 to 3 months of romosozumab therapy and a falling CTX confirm that both anabolic and antiresorptive effects are active 17.

Male Osteoporosis: Does the Comparison Change?

Alendronate is FDA-approved for osteoporosis in men at 10 mg daily or 70 mg weekly 3. Romosozumab is approved for osteoporosis in men at high risk of fracture based on data from the BRIDGE trial (N=245), which showed lumbar spine BMD gains of 12.1% versus 1.2% for placebo at 12 months 18. The cardiovascular caution applies equally in men.

Practical Prescribing Summary

| Feature | Alendronate (Fosamax) | Romosozumab (Evenity) | |---|---|---| | Mechanism | Antiresorptive only | Dual: anabolic plus antiresorptive | | Route | Oral daily or weekly | Subcutaneous monthly | | Duration | Typically 3 to 5 years then reassess | Exactly 12 months, then transition | | Lumbar spine BMD gain (12 mo) | Approx. 5% | Approx. 13.7% | | Vertebral fracture reduction | 47% vs placebo (FIT) | 48% vs alendronate (ARCH) | | Cardiovascular warning | No | Yes, FDA boxed warning | | ONJ / AFF risk | Yes (duration-dependent) | Low, reported rarely | | Approximate monthly cost | <$20 generic | Approx. $1,800 | | Approved in men | Yes | Yes (BRIDGE trial) | | After discontinuation | Drug holiday possible | Must transition to antiresorptive |