What does ARCH stand for?

ARCH is an acronym for Active-contRolled fraCture study in postmenopausal women with osteoporosis at High risk. The name reflects that it used an active comparator (alendronate) rather than placebo.

How is romosozumab different from bisphosphonates?

Bisphosphonates like alendronate slow bone breakdown (resorption). Romosozumab actively builds new bone by blocking sclerostin, a protein that inhibits bone formation. It also reduces resorption to a lesser degree, giving it a dual mechanism.

Why was the ARCH trial significant compared to the FRAME trial?

FRAME compared romosozumab to placebo. ARCH compared it head-to-head against alendronate, the most commonly used osteoporosis drug. Beating an active standard-of-care treatment is a much higher bar and carries more weight in clinical decision-making.

What was the cardiovascular concern with romosozumab in ARCH?

There were more serious cardiovascular events (heart attack, stroke) in the romosozumab group than the alendronate group during the first 12 months. The difference was not statistically significant, and it remains debated whether romosozumab increases risk or alendronate provides protection. The FDA added a boxed warning as a precaution.

Who is a candidate for romosozumab based on ARCH?

Postmenopausal women with osteoporosis who are at very high fracture risk, particularly those with a prior fragility fracture and very low bone density. Patients with recent heart attack or stroke within the past year are not candidates due to the cardiovascular warning.

How long is a course of romosozumab?

Twelve months. The drug is given as a monthly subcutaneous injection (two injections per visit). After 12 months, patients transition to a bisphosphonate or denosumab to maintain the bone gains.

Did romosozumab reduce hip fractures in ARCH?

Yes. The romosozumab-first sequence reduced hip fractures by 38% compared to alendronate alone (2.0% vs 3.2%), a statistically significant result that strengthened the case for the drug in high-risk populations.

What happens if you stop romosozumab without starting another drug?

Bone density gains can be lost. The ARCH protocol transitioned all romosozumab patients to alendronate after 12 months. Current guidelines strongly recommend following any anabolic agent with an antiresorptive to preserve the newly formed bone.

Is romosozumab better than teriparatide?

ARCH did not compare romosozumab to teriparatide. A separate study (STRUCTURE) compared the two on BMD endpoints and found greater hip BMD gains with romosozumab. Head-to-head fracture data between these two anabolic agents do not yet exist.

How much does romosozumab cost?

The list price for a 12-month course of Evenity is approximately $22,000 to $26 to 000 in the United States, though actual out-of-pocket costs vary by insurance coverage. Many commercial and Medicare plans cover it with prior authorization for patients meeting high-risk criteria.

ARCH Trial: A Plain-English Overview of What It Established

By HealthRX Medical Team

Published May 25, 2026Updated May 25, 2026Last reviewed May 25, 2026

At a glance

| Field | Detail | |---|---| | Trial name | ARCH (Active-contRolled fraCture study in postmenopausal women with osteoporosis at High risk) | | N | 4,093 postmenopausal women | | Intervention | Romosozumab 210 mg subcutaneous monthly for 12 months, then alendronate 70 mg oral weekly | | Comparator | Alendronate 70 mg oral weekly for the full study period | | Duration | 24 months (primary analysis); median follow-up ~33 months | | Primary endpoint | Incidence of new vertebral fractures at 24 months | | Key result | 48% relative risk reduction in new vertebral fractures (6.2% vs 11.9%; p <0.001) | | Registration | NCT01631214 |

The Question ARCH Set Out to Answer

Before ARCH, romosozumab had already beaten placebo in the FRAME trial. Beating placebo, though, is a low bar. Clinicians wanted to know whether romosozumab could outperform an active comparator, specifically a bisphosphonate that was already standard of care. ARCH was designed to answer that question in the patients who need the answer most: postmenopausal women who had already sustained a fragility fracture and remained at high risk of another.

The comparator was alendronate (brand name Fosamax), the most widely prescribed bisphosphonate worldwide. If romosozumab could beat alendronate on hard fracture endpoints, it would justify a fundamentally different treatment sequence for severe osteoporosis: build bone first with an anabolic agent, then maintain gains with an antiresorptive.

Who Was Enrolled

Eligible women were postmenopausal, aged 55 to 90, with a T-score of −2.5 or lower at the total hip or femoral neck, and at least one moderate or severe vertebral fracture (or two or more mild vertebral fractures). This was a genuinely high-risk population. The mean age was 74, and roughly 96% of participants had prevalent vertebral fractures at baseline.

Key exclusion criteria included metabolic bone disorders other than osteoporosis, recent bisphosphonate or parathyroid hormone analog use, and conditions that would confound fracture assessment (such as severe spinal deformity). Calcium and vitamin D supplementation were provided to all participants throughout the study.

The trial enrolled patients across 40 countries, making the cohort broad enough for generalizability but also introducing variability in background fracture rates and standards of care.

What Each Group Received

Randomization was 1:1, double-blind, and double-dummy (each patient received both a monthly injection and a daily oral pill, one active and one placebo, so neither patient nor investigator could tell the assignment).

Romosozumab arm (n = 2,046): 210 mg romosozumab subcutaneously once monthly for 12 months, plus oral placebo matching alendronate. After month 12, all patients transitioned to open-label alendronate 70 mg weekly.
Alendronate arm (n = 2,047): Alendronate 70 mg orally once weekly for the full study, plus monthly placebo injections during the first 12 months.

This design tested a treatment sequence, not just a single drug. The clinical question was whether leading with a bone-building phase before switching to a bone-protecting phase would produce better outcomes than standard antiresorptive therapy from the start.

How Fractures Were Measured

Vertebral fractures were assessed by lateral spine radiographs at baseline, 12 months, and 24 months. A central radiology lab graded each vertebra using the Genant semiquantitative method (grades 0 through 3, based on height loss). A new fracture required a change from grade 0 to grade 1 or higher, or an increase in grade from a previously fractured vertebra.

Clinical fractures (nonvertebral and clinical vertebral) were tracked as they occurred and adjudicated by an independent committee. Hip fracture was a separate secondary endpoint.

Bone mineral density (BMD) at the lumbar spine, total hip, and femoral neck was measured by dual-energy X-ray absorptiometry (DXA) at multiple time points. Bone turnover markers, including P1NP (formation) and beta-CTX (resorption), were collected in a subset.

The Results in Detail

The HealthRX ARCH Results Framework

The results below are organized by endpoint tier and time point to show both the early bone-building phase and the sustained fracture protection after transition to alendronate.

Primary Endpoint: New Vertebral Fractures at 24 Months

| Outcome | Romosozumab → Alendronate | Alendronate alone | Relative risk reduction | p-value | |---|---|---|---|---| | New vertebral fracture at 24 months | 6.2% (127 of 2,046) | 11.9% (243 of 2,047) | 48% | <0.001 |

The absolute risk difference was 5.7 percentage points, meaning roughly 18 women needed to be treated with the romosozumab-first sequence (instead of alendronate alone) to prevent one additional vertebral fracture over two years.

Secondary Endpoints

| Outcome | Romosozumab → Alendronate | Alendronate alone | Risk reduction | p-value | |---|---|---|---|---| | Clinical fracture (to primary analysis) | 9.7% | 13.0% | 27% (HR 0.73) | <0.001 | | Nonvertebral fracture (to primary analysis) | 8.7% | 10.6% | 19% (HR 0.81) | 0.04 | | Hip fracture (to primary analysis) | 2.0% | 3.2% | 38% (HR 0.62) | 0.02 | | New vertebral fracture at 12 months | 4.0% | 8.0% | 50% | <0.001 |

The 12-month vertebral fracture data are worth noting: the separation between arms was already significant before the romosozumab group even switched to alendronate. This suggests that the bone-building phase itself drove a large part of the benefit.

BMD Changes

At 12 months (end of the anabolic phase), romosozumab-treated patients had gained approximately 13.7% at the lumbar spine and 6.2% at the total hip, compared to 5.0% and 2.8% with alendronate. After both groups were on alendronate, BMD gains in the romosozumab arm were largely maintained, while the alendronate-only arm continued its slower climb. By 24 months, the gap had narrowed slightly but remained clinically meaningful.

The Cardiovascular Safety Signal

ARCH identified a numerical imbalance in adjudicated serious cardiovascular adverse events. During the first 12 months, the romosozumab group recorded 50 positively adjudicated major adverse cardiac events (MACE) compared to 38 in the alendronate group. The difference was not statistically significant, but it drew scrutiny from regulators and the medical community.

Several points of context matter here:

The FRAME trial (romosozumab vs placebo) did not show the same imbalance. The MACE rates in FRAME were similar between arms.
Some researchers have argued that the imbalance in ARCH could partly reflect a cardiovascular protective effect of alendronate rather than a harmful effect of romosozumab. Bisphosphonates have been associated with reduced vascular calcification in observational data.
The FDA approved romosozumab (Evenity) in April 2019 with a boxed warning: the drug should not be used in patients who have had a myocardial infarction or stroke within the preceding year. Prescribers are advised to consider whether the benefits outweigh the risks in patients with other cardiovascular risk factors.
The European Medicines Agency took a similar position, approving romosozumab in late 2019 with cardiovascular contraindications.

This remains an area of active investigation. Clinicians treating severe osteoporosis now weigh fracture severity against baseline cardiovascular risk when deciding whether romosozumab is the right first-line anabolic.

Limitations the Authors Acknowledged

The ARCH investigators and subsequent editorials flagged several limitations:

No placebo arm. Because the enrolled population was at very high fracture risk, a placebo arm was considered unethical. This means absolute efficacy of each drug cannot be isolated, only the relative comparison.
Open-label transition. After 12 months, the switch to alendronate was open-label. While vertebral fracture assessment was performed by a blinded central lab, reporting of clinical fractures could theoretically be influenced by knowledge of treatment assignment.
Cardiovascular event adjudication. The cardiovascular signal was unexpected and the trial was not powered to detect a difference in MACE. Post-hoc subgroup analyses could not definitively attribute the imbalance to romosozumab.
Generalizability. The enrolled population, women aged 55 to 90 with prior fracture, represents the highest-risk segment. Results may not directly apply to patients with osteoporosis who have not yet fractured.
Duration of anabolic therapy. Romosozumab was given for only 12 months, consistent with its mechanism (sclerostin inhibition produces a time-limited bone formation window). Whether extending the anabolic phase or repeating it would add benefit is unknown.

What ARCH Changed in Clinical Practice

Before ARCH, the typical osteoporosis treatment sequence began with a bisphosphonate. Anabolic agents like teriparatide (Forteo) were reserved for patients who fractured on bisphosphonates or who had very severe disease. ARCH provided Level 1 evidence that starting with a bone-building agent and then transitioning to an antiresorptive produces better fracture outcomes than starting with an antiresorptive alone.

This "anabolic-first" concept has since been endorsed by multiple professional societies. The American Association of Clinical Endocrinology (AACE) 2020 guidelines recommend considering romosozumab or teriparatide as initial therapy in patients at very high fracture risk, followed by transition to a bisphosphonate or denosumab. The Endocrine Society's 2020 updated guidelines similarly support anabolic-first strategies in appropriate patients.

In practical terms, this means a postmenopausal woman with a T-score of −3.0 and a prior vertebral fracture might now receive 12 months of romosozumab injections before transitioning to oral alendronate or denosumab injections, rather than starting on alendronate from day one.

The Bigger Picture for Romosozumab

Romosozumab works by inhibiting sclerostin, a protein produced by osteocytes that normally puts the brakes on bone formation. Blocking sclerostin simultaneously increases bone formation (measured by P1NP) and decreases bone resorption (measured by beta-CTX), a dual mechanism no other approved osteoporosis drug achieves. This "modeling-based" bone formation is distinct from the remodeling-based action of teriparatide and appears to produce faster, more substantial gains in cortical bone.

The catch is that the anabolic window is self-limiting. After roughly 12 months, the formation markers return toward baseline even with continued dosing, likely because the body upregulates sclerostin production in response to antibody blockade. This is why ARCH and the Evenity prescribing information specify a 12-month treatment course followed by antiresorptive transition.

Frequently asked questions

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References

Saag KG, Petersen J, Brandi ML, et al. Romosozumab or alendronate for fracture prevention in women with osteoporosis. N Engl J Med. 2017;377(15):1417-1427. PubMed
Cosman F, Crittenden DB, Adachi JD, et al. Romosozumab treatment in postmenopausal women with osteoporosis (FRAME). N Engl J Med. 2016;375(16):1532-1543. PubMed
Evenity (romosozumab-aqqg) prescribing information. Amgen Inc. Revised 2019. FDA Label
Camacho PM, Petak SM, Binkley N, et al. American Association of Clinical Endocrinologists/American College of Endocrinology clinical practice guidelines for the diagnosis and treatment of postmenopausal osteoporosis, 2020 update. Endocr Pract. 2020;26(Suppl 1):1-46. PubMed
Shoback D, Rosen CJ, Black DM, et al. Pharmacological management of osteoporosis in postmenopausal women: an Endocrine Society guideline update. J Clin Endocrinol Metab. 2020;105(3):587-594. PubMed