Fosamax vs Evenity (Romosozumab): Long-Term Durability of Response

How Each Drug Actually Works

Alendronate and romosozumab do not compete on the same biological pathway. Understanding that difference is the foundation for comparing their durability.

Alendronate is a nitrogen-containing bisphosphonate. It binds to hydroxyapatite in bone, is engulfed by osteoclasts, and inhibits farnesyl pyrophosphate synthase, an enzyme in the mevalonate pathway. Osteoclast activity drops sharply, bone resorption slows, and existing bone is preserved. The drug persists in bone mineral for years, which is both its durability advantage and the source of concerns about prolonged suppression.

Romosozumab's Dual Mechanism

Romosozumab is a monoclonal antibody that targets sclerostin, a glycoprotein produced by osteocytes that normally brakes the Wnt signaling pathway. Block sclerostin and two things happen at once: osteoblast-driven bone formation increases, and osteoclast-driven resorption decreases. The American Society for Bone and Mineral Research has described this dual action as "unique among approved osteoporosis agents" because virtually every other approved drug is either purely anabolic or purely antiresorptive. [1]

Why Mechanism Shapes Durability

Because alendronate suppresses resorption continuously as long as it is taken (and for some time after), its effect is steady and cumulative over years. Romosozumab's anabolic signal is time-limited: the bone formation boost wanes after roughly 9 to 12 months of treatment even if dosing continues, and the drug is approved only for a 12-month course. The durability question, then, is not just "which drug lasts longer?" but "which drug creates a structural advantage that can be maintained by follow-on therapy?"

Fracture Efficacy: What the Major Trials Show

FIT Trial: Alendronate's Foundational Evidence

The Fracture Intervention Trial (FIT, JAMA 1998, N=2,027 women with low femoral neck BMD) showed that alendronate 5 to 10 mg daily reduced vertebral fractures by 47% relative to placebo over 3 years (relative risk 0.53, 95% CI 0.41 to 0.68, P<0.001). [2] Hip fracture risk dropped by 51% in the subgroup with a prevalent vertebral fracture at baseline.

Those numbers remained meaningful at 5 years in the FIT Long-Term Extension (FLEX) trial, where women who had taken alendronate for 5 years and then continued for another 5 years showed no additional hip fracture reduction compared with those who stopped, but did show continued protection against clinical vertebral fractures.

ARCH Trial: Romosozumab Then Alendronate

The ARCH trial (NEJM 2017, N=4,093 postmenopausal women with osteoporosis and prior fracture) directly compared 12 months of romosozumab 210 mg monthly followed by alendronate, versus alendronate alone for the entire study period. [3] At 24 months, the romosozumab-to-alendronate sequence reduced:

• New vertebral fractures by 48% relative to alendronate alone (6.2% vs 11.9%, P<0.001)
• Clinical fractures by 27% (9.7% vs 13.0%, P<0.001)
• Hip fractures by 38% (2.0% vs 3.2%, P = 0.02)

The lumbar spine BMD gain at 12 months was 13.7% in the romosozumab group versus 5.0% in the alendronate-only group. At the femoral neck, the difference was 6.2% versus 2.5%.

What "Durability" Means for Each Drug

For alendronate, durability comes from continuous dosing. Stop the drug, and BTMs (bone turnover markers) such as serum CTX begin rising within weeks, though BMD loss is typically slow and fracture risk increases only modestly over several years thanks to the drug's residual effect in bone mineral.

For romosozumab, durability after the 12-month course depends entirely on what comes next. In ARCH, when the romosozumab group transitioned to alendronate, the fracture reduction advantage over alendronate alone persisted through 24 months. The anabolic phase effectively "pre-loaded" a higher bone mass peak, and the follow-on bisphosphonate then protected that gain. This concept, often called "anabolic-first then antiresorptive," is now reflected in the Endocrine Society's 2020 clinical practice guideline, which states: "For patients at very high risk of fracture, we suggest using an anabolic agent first, followed by an antiresorptive agent, rather than starting with an antiresorptive agent." [4]

Bone Mineral Density Trajectories Over Time

BMD numbers tell a story that fracture data alone cannot.

Lumbar Spine Trajectory

At 12 months, romosozumab produces roughly 13% lumbar spine BMD gain. Alendronate produces roughly 5% over the same period, with gains continuing at a slower rate through years 2 to 5. After romosozumab stops and alendronate begins, BMD continues rising (though more slowly) because the bisphosphonate locks in the gains. Patients who receive alendronate alone can match a similar absolute BMD level, but only after several more years of therapy.

Hip and Femoral Neck

At the total hip, romosozumab's advantage is approximately 7.1% vs 3.4% for alendronate at 12 months in ARCH. [3] This matters because hip fracture is the most clinically consequential osteoporotic fracture, carrying a 1-year mortality of roughly 20 to 30% in older adults. [5]

What Happens If Romosozumab Is Not Followed by Antiresorptive Therapy

Data from the FRAME trial (romosozumab vs placebo, N=7,180) show that when romosozumab-treated patients transitioned to denosumab rather than alendronate, BMD gains were even better preserved. [6] Patients who received placebo during the first 12 months and then switched to romosozumab gained BMD but never fully closed the gap. This asymmetry argues strongly that romosozumab should be used before, not after, antiresorptive agents when both are planned.

Safety Profiles and Long-Term Tolerability

Alendronate's Long-Term Safety Concerns

Alendronate has a well-documented safety record across more than 25 years of use. The main concerns with prolonged therapy (typically beyond 5 years) are:

• Atypical femoral fractures (AFF): rare subtrochanteric or femoral shaft fractures. The absolute risk is approximately 3.2 to 50 per 100,000 person-years depending on duration. [7]
• Osteonecrosis of the jaw (ONJ): very rare in patients on oral bisphosphonates for osteoporosis (estimated at 1 in 10,000 to 1 in 100,000 patient-years). [7]
• Esophageal irritation: requires upright posture for 30 minutes post-dose and adequate fluid intake.
• Renal restriction: contraindicated if creatinine clearance <35 mL/min.

Romosozumab's Cardiovascular Signal

Romosozumab carries an FDA-mandated boxed warning for increased risk of myocardial infarction, stroke, and cardiovascular death. In ARCH, serious cardiovascular events occurred in 2.5% of the romosozumab group versus 1.9% of the alendronate group during the first 12 months. [3] The imbalance was not seen in the FRAME trial (placebo comparator), but the FDA required the warning based on the ARCH data. Romosozumab is contraindicated in patients who have had an MI or stroke within the past year.

Shared Monitoring Requirements

Both drugs require baseline 25-hydroxyvitamin D and calcium levels. Hypocalcemia must be corrected before starting either agent. Romosozumab may cause a transient increase in PTH, though clinical hypocalcemia is uncommon when calcium and vitamin D are adequate.

Switching From Fosamax to Evenity: When It Makes Sense

Switching from alendronate to romosozumab is a documented clinical strategy, though the evidence base is smaller than for the anabolic-first approach.

The Rationale for Switching

Patients who fracture while taking alendronate represent a clear treatment failure. The 2022 American Association of Clinical Endocrinology (AACE) osteoporosis guidelines identify this group as candidates for escalation to anabolic therapy. [8] Romosozumab may offer a meaningful jump in BMD and fracture protection that continued or alternative antiresorptive therapy cannot provide within a comparable time frame.

Evidence for the Switch

A 2019 study by Cosman et al. (N=436) compared romosozumab with teriparatide (another anabolic agent) in women previously treated with bisphosphonates. [9] Romosozumab produced significantly greater lumbar spine BMD gains at 12 months (9.8% vs 5.4%, P<0.001) and total hip gains (2.9% vs 0%, P<0.001). This suggests that even in a bisphosphonate-pretreated skeleton, romosozumab's anabolic signal gets through.

Practical Transition Protocol

The standard protocol when switching from alendronate to romosozumab:

1. Document indication: fracture on therapy, very high baseline fracture risk, or inadequate BMD response after at least 1 year of alendronate.
2. Confirm no cardiovascular contraindications (no MI or stroke in the past 12 months).
3. Correct any vitamin D deficiency (target 25-OH-D ≥30 ng/mL) and ensure calcium intake of 1,000 to 1,200 mg/day.
4. Administer romosozumab 210 mg subcutaneously once monthly for exactly 12 months.
5. Transition immediately to an antiresorptive agent (alendronate, risedronate, or denosumab) after month 12. Do not leave a gap.

The Endocrine Society guideline specifically warns against stopping antiresorptive therapy after romosozumab without a clear plan, noting that BMD losses can occur relatively quickly in that scenario. [4]

Who Should Not Switch

Switching is not appropriate for:

• Patients with a history of MI or stroke in the last 12 months.
• Patients with active hypocalcemia.
• Pregnant or premenopausal women without documented osteoporosis.
• Patients whose primary concern is cost without insurance coverage for romosozumab (a 12-month course can exceed $20,000 without assistance programs).

Comparing Total Duration and the "Drug Holiday" Question

Alendronate Drug Holidays

After 3 to 5 years of alendronate, guidelines from the American Society for Bone and Mineral Research and the AACE support considering a drug holiday of 1 to 2 years in lower-risk patients (T-score above -2.5 at hip, no prior hip or vertebral fracture). [8] During the holiday, bone turnover markers and BMD should be monitored, and the drug is restarted if BMD declines or fracture occurs.

High-risk patients (T-score at or below -2.5, prior fracture, or history of vertebral fracture) generally should not take a holiday from antiresorptive therapy. For this group, the fracture cost of stopping outweighs the AFF risk of continuing.

Romosozumab Has No Holiday Option

Romosozumab is not a long-term drug in the conventional sense. Its 12-month course is the treatment, not a starting dose that continues indefinitely. The durability challenge is therefore about what follows romosozumab, not about how long to take romosozumab itself. Prescribers must plan sequential therapy before writing the first prescription.

Cost, Access, and Real-World Considerations

Alendronate is available as a generic. The 70 mg weekly tablet typically costs under $10 per month with common discount cards. Adherence at 1 year is approximately 50 to 60% in real-world pharmacy data, a number that substantially limits the drug's real-world fracture protection compared with clinical trial results. [10]

Romosozumab (brand: Evenity, manufactured by Amgen) requires two subcutaneous injections per month administered in a clinical setting or by a trained patient. Prior authorization is typically required by commercial insurers and Medicare Part B. Amgen's patient assistance program (Amgen SupportPlus) may cover costs for eligible patients. The 12-month fixed course means the total cost is bounded, unlike a drug taken indefinitely.

From a real-world adherence standpoint, romosozumab may actually have an advantage for the 12-month window: injections in a clinical setting are directly observed, removing the compliance variability that affects oral bisphosphonate outcomes.

Head-to-Head Summary Table

| Parameter | Alendronate (Fosamax) | Romosozumab (Evenity) | |---|---|---| | Mechanism | Antiresorptive only | Dual (anabolic + antiresorptive) | | Route | Oral weekly | Subcutaneous monthly | | Duration | Years to decades | 12 months, then transition | | Lumbar spine BMD at 12 months | ~5% | ~13.7% | | Vertebral fracture RRR vs placebo | 47% (FIT) | 73% at 12 months (FRAME) | | Vertebral RRR vs alendronate (ARCH) | Reference | 48% better than alen. Alone | | Key safety concern | AFF, ONJ (long-term) | CV events (boxed warning) | | Generic available | Yes | No | | Approximate monthly cost | <$10 generic | Varies, often $1,500+ brand | | Requires follow-on therapy | No (continuous) | Yes (mandatory) |

Clinical Decision Framework: Which Drug for Which Patient?

The choice between alendronate and romosozumab is not binary for most patients. It is a sequencing question.

Start With Alendronate When:

• T-score is between -2.5 and -3.0 with no prior fracture (moderate risk).
• Cardiovascular history contraindicates romosozumab.
• Cost or access is a barrier.
• Patient strongly prefers oral over injectable therapy.
• Prognosis or life expectancy makes a 12-month anabolic investment less practical.

Start With Romosozumab When:

• T-score is below -3.0 or there is a prior fragility fracture (very high risk per AACE 2022 criteria). [8]
• Patient is fracturing on current antiresorptive therapy.
• Rapid BMD gain is needed (e.g., very low femoral neck T-score, imminent fracture risk).
• No cardiovascular contraindications are present.
• Sequential therapy (romosozumab then antiresorptive) can be reliably planned and executed.

Switch From Alendronate to Romosozumab When:

• Patient sustains a fracture while adherent to alendronate.
• BMD continues to decline on alendronate after 1 to 2 years.
• Patient is reclassified as very high risk based on updated FRAX score or imaging findings.

A practical rule from the HealthRX clinical advisory panel: if the 10-year FRAX hip fracture probability exceeds 4.5% on current antiresorptive therapy, the case for escalating to romosozumab is strong enough to prompt a specialist referral even if no fracture has occurred.