Evenity (Romosozumab) Safety Signals and FDA Actions

How Romosozumab Works: The Sclerostin Pathway

Romosozumab is a humanized monoclonal antibody that binds and inhibits sclerostin, a glycoprotein secreted primarily by osteocytes [1]. Sclerostin normally acts as a brake on the Wnt signaling pathway, suppressing osteoblast differentiation and bone formation. By blocking sclerostin, romosozumab releases that brake.

The result is a dual mechanism unique among osteoporosis therapies. Bone formation markers such as procollagen type I N-terminal propeptide (P1NP) increase rapidly, peaking within the first month of treatment. Simultaneously, bone resorption markers like C-terminal telopeptide (CTX) decrease [2]. This "anabolic window" of simultaneous formation increase and resorption decrease produces rapid gains in bone mineral density (BMD). In the FRAME trial (N=7,180), romosozumab increased lumbar spine BMD by 13.3% at 12 months compared with 0.0% for placebo [3].

That speed matters clinically. A 13.3% BMD gain in one year exceeds what teriparatide achieves in two years. The bone-forming effect wanes after approximately 6 to 9 months, which is why treatment is capped at 12 monthly doses. Once the anabolic course ends, patients must transition to an antiresorptive agent to preserve gains [4]. Without sequential therapy, the BMD improvements reverse within 12 months of discontinuation.

The Wnt pathway is not confined to bone. Sclerostin is expressed at low levels in vascular smooth muscle and in calcified atherosclerotic plaques [5]. This observation became the biological starting point for the cardiovascular safety debate that would define romosozumab's regulatory history.

The ARCH Trial: Where the Cardiovascular Signal Emerged

The ARCH trial was a Phase III, randomized, double-blind study comparing romosozumab 210 mg monthly for 12 months followed by alendronate versus alendronate alone in 4,093 postmenopausal women with osteoporosis and a prior fragility fracture [1]. Its primary endpoint was new vertebral fracture incidence.

On efficacy, the results were compelling. Romosozumab reduced new vertebral fractures by 48% compared with alendronate at 24 months, and hip fractures by 38% [1]. No prior osteoporosis drug had demonstrated superiority over an active comparator of this magnitude in a fracture endpoint trial.

The cardiovascular safety data told a different story. Positively adjudicated major adverse cardiovascular events (MACE) occurred in 50 of 2,046 patients (2.5%) in the romosozumab-to-alendronate group versus 38 of 2,047 patients (1.9%) in the alendronate-only group during the 12-month romosozumab treatment period [1]. Cardiac ischemic events specifically were reported in 16 romosozumab patients versus 6 alendronate patients. Cerebrovascular events occurred in 16 versus 7.

The imbalance was not statistically significant for the overall MACE composite (hazard ratio 1.31, 95% CI 0.85 to 2.00). That wide confidence interval left room for both no effect and a meaningful hazard. The FDA's Arthritis Advisory Committee voted 18 to 1 in January 2019 that the cardiovascular signal was clinically concerning enough to warrant a boxed warning [6].

The FDA Boxed Warning: What It Says and Why It Exists

The FDA approved romosozumab on April 9, 2019, with a boxed warning stating that Evenity "may increase the risk of myocardial infarction, stroke, and cardiovascular death" [7]. The label instructs prescribers: "Do not initiate Evenity in patients who have had a myocardial infarction or stroke within the preceding year. Consider whether the benefits outweigh the risks in patients with other cardiovascular risk factors."

This makes romosozumab the only anabolic osteoporosis agent with a cardiovascular boxed warning. Teriparatide (Forteo) and abaloparatide (Tymlos) carry different warnings related to osteosarcoma risk observed in rodent studies, but neither has a cardiac signal.

The European Medicines Agency (EMA) took a slightly different path. The EMA approved romosozumab in December 2019 but restricted it to postmenopausal women "at high risk of fracture and with no history of myocardial infarction or stroke" [8]. The EMA's Committee for Medicinal Products for Human Use (CHMP) stated in its assessment report that "the cardiovascular risk cannot be excluded based on the available data, particularly in view of the ARCH study results." Both agencies arrived at the same clinical message through different regulatory language.

The Japanese regulatory authority (PMDA) had approved romosozumab earlier, in January 2019, without a cardiovascular restriction. Japan later updated its prescribing information after reviewing the ARCH data and international regulatory actions [9].

FRAME Trial: A Cleaner Cardiovascular Profile Against Placebo

The FRAME trial offers a counterpoint to ARCH. This study randomized 7,180 postmenopausal women with osteoporosis (but not necessarily prior fracture) to romosozumab 210 mg or placebo monthly for 12 months, followed by denosumab in both groups [3].

MACE events were balanced. Cardiovascular serious adverse events occurred in 1.3% of romosozumab patients and 1.2% of placebo patients during the 12-month blinded period [3]. No signal emerged. The key difference between FRAME and ARCH populations: FRAME enrolled lower-risk patients with a mean age of 70.9 years and no fracture history requirement, while ARCH enrolled higher-risk patients (mean age 74.3 years) with prior fragility fractures and, consequently, higher baseline cardiovascular risk.

This distinction raises a question that remains unresolved. Did romosozumab cause cardiovascular events in ARCH, or did alendronate protect against them, making romosozumab look worse by comparison? A meta-analysis published in the Journal of Bone and Mineral Research pooling data from ARCH, FRAME, and two smaller Phase III studies found a pooled cardiovascular hazard ratio of 1.18 (95% CI 0.82 to 1.69) for romosozumab versus all comparators, which was not statistically significant [10]. The signal is real enough to warrant the boxed warning but imprecise enough to leave the mechanism debated.

Post-Marketing Surveillance and FAERS Data

Since approval, the FDA Adverse Event Reporting System (FAERS) has accumulated reports of cardiovascular events in romosozumab-treated patients [11]. Post-marketing pharmacovigilance data must be interpreted cautiously. Reporting is voluntary, denominators are uncertain, and the Weber effect (increased reporting for newer drugs) inflates early signal counts.

A 2023 analysis of FAERS disproportionality data found that romosozumab had elevated reporting odds ratios for myocardial infarction (ROR 2.14) and cerebrovascular accident (ROR 1.87) compared with other osteoporosis drugs in the database [11]. These disproportionality signals are hypothesis-generating, not causal. They reflect the same population-level pattern seen in ARCH without adding mechanistic clarity.

Amgen's periodic safety update reports submitted to the FDA and EMA have not identified new cardiovascular risks beyond those described in the original labeling [12]. The 12-month treatment duration limits cumulative exposure, and long-term cardiovascular follow-up data beyond 24 months remain limited. The ongoing FRAME Extension study has provided BMD data out to 36 months but was not powered for cardiovascular endpoints.

No FDA-mandated post-marketing cardiovascular outcomes trial (similar to what the FDA required for rosiglitazone or certain GLP-1 agonists) has been imposed on romosozumab. The limited 12-dose treatment course and the drug's positioning as a last-line therapy for severe osteoporosis may have factored into that decision.

Biological Plausibility: Sclerostin, Wnt, and Vascular Calcification

The cardiovascular concern is not purely statistical. Preclinical and observational data provide a biological rationale. Sclerostin is expressed in calcified atherosclerotic plaques, and low circulating sclerostin levels have been associated with increased cardiovascular mortality in some cohort studies [5].

A 2020 Mendelian randomization study published in the Journal of Bone and Mineral Research used genetic variants in the SOST gene region (which encodes sclerostin) to estimate the cardiovascular effects of pharmacological sclerostin inhibition [13]. The analysis found that genetically predicted lower sclerostin levels were associated with higher BMD, consistent with the drug's mechanism, but also with a trend toward increased risk of major cardiac events (OR 1.18 per standard deviation decrease in sclerostin). The confidence intervals overlapped 1.0, so this result was suggestive rather than definitive.

The Wnt pathway regulates vascular smooth muscle cell differentiation. Excessive Wnt activation could theoretically promote osteogenic transformation of vascular smooth muscle cells, accelerating arterial calcification [14]. Animal studies using sclerostin-knockout mice have shown increased aortic calcification in models fed high-phosphate diets, though these conditions do not directly replicate the 12-month pharmacological inhibition in humans.

A competing hypothesis holds that alendronate, the ARCH comparator, has a mild cardioprotective effect. A Danish registry study of over 60,000 bisphosphonate users found a 28% reduction in myocardial infarction risk during active bisphosphonate use [15]. If alendronate reduced MACE in the comparator arm rather than romosozumab increasing it, the ARCH imbalance would not reflect true romosozumab cardiotoxicity.

Who Should and Should Not Receive Romosozumab

The American Association of Clinical Endocrinologists (AACE) 2020 guidelines position romosozumab as a first-line option for patients at "very high" fracture risk, defined as a recent fracture within the past 2 years, a T-score of <-3.0, or a history of fractures while on approved osteoporosis therapy [16]. The Endocrine Society's 2020 guideline similarly recommends romosozumab for patients who meet very-high-risk criteria [17].

Both guidelines echo the FDA labeling: avoid romosozumab in patients with a recent (within 1 year) myocardial infarction or stroke. The AACE guideline adds that "careful cardiovascular risk assessment should be performed before initiating romosozumab, and the potential benefits of reduced fracture risk should be weighed against the potential cardiovascular risk" [16].

Practical clinical application narrows the eligible population. A patient with severe osteoporosis and multiple vertebral compression fractures who has no cardiovascular disease history is an ideal candidate. A patient with the same skeletal profile plus a prior transient ischemic attack 8 months ago is not. Between those poles lies a gray zone that requires individualized risk-benefit discussion.

Age is a complicating factor. The patients at highest fracture risk (women over 75 with prior fractures) are also those at highest cardiovascular risk. In the ARCH trial, the MACE imbalance was numerically more pronounced in patients over 75, though subgroup analyses were not powered for interaction testing [1].

Monitoring During and After Treatment

No FDA-mandated cardiac monitoring protocol accompanies romosozumab treatment. The prescribing information recommends monitoring for signs and symptoms of myocardial infarction and stroke and discontinuing the drug if a cardiovascular event occurs [7].

Standard practice in many clinics includes baseline cardiovascular risk assessment using the ACC/AHA Pooled Cohort Equations or similar tools before initiating romosozumab. Serial cardiac troponin or imaging is not indicated based on current evidence. The 12-month treatment course limits the monitoring window.

During treatment, P1NP (a bone formation marker) peaks within the first month and can confirm pharmacodynamic response. BMD should be assessed by DXA at baseline and after completing the 12-dose course. After the final dose, immediate transition to denosumab 60 mg every 6 months or a bisphosphonate is recommended. Delaying antiresorptive sequencing by even 6 months results in measurable BMD loss [4].

The ARCH trial protocol transitioned all patients to alendronate after the romosozumab phase. In the romosozumab-to-alendronate group, the MACE imbalance observed during the first 12 months did not widen during the subsequent alendronate phase, suggesting that the cardiovascular risk, if real, is confined to the active treatment window [1].

Romosozumab Versus Other Anabolic Agents: A Safety Comparison

Teriparatide (Forteo) was the first anabolic agent approved for osteoporosis in 2002. Its boxed warning relates to osteosarcoma observed in Fischer 344 rats treated with near-lifetime high-dose exposure [18]. After 20 years of human use and long-term registry data, no confirmed increase in human osteosarcoma has been identified. Teriparatide has no cardiovascular signal.

Abaloparatide (Tymlos), approved in 2017, carries a similar osteosarcoma precaution based on rodent data. The ACTIVExtend trial showed no cardiovascular imbalance in the abaloparatide group [19]. Neither parathyroid hormone analog affects the Wnt/sclerostin pathway, which may explain the absence of vascular signals.

The choice between romosozumab and teriparatide or abaloparatide often hinges on fracture severity and cardiovascular status. For a patient with very high fracture risk and no cardiovascular history, romosozumab's superior BMD gains and fracture reduction data (the 48% vertebral fracture reduction in ARCH exceeds any published teriparatide comparator trial result) favor its use [1]. For a patient with equivalent fracture risk and a cardiovascular history, teriparatide or abaloparatide avoids the MACE signal entirely.

Open Questions and Ongoing Research

Several gaps persist. No dedicated cardiovascular outcomes trial for romosozumab exists or is currently registered on ClinicalTrials.gov. The mechanism behind the ARCH signal remains unconfirmed. Whether the risk is a direct sclerostin-inhibition effect, an alendronate cardioprotective artifact, or a chance finding in a population with high baseline cardiovascular risk has not been resolved.

Real-world evidence studies using insurance claims databases have begun to appear. A 2024 cohort study using the MarketScan database found no significant increase in MACE among romosozumab initiators compared with denosumab initiators over 12 months of follow-up (adjusted HR 1.08, 95% CI 0.71 to 1.64), though the sample size was modest [20]. Larger, longer-duration observational studies are needed.

The FDA has not signaled any intent to revise the boxed warning based on post-marketing data to date. For now, the clinical directive is clear: assess cardiovascular risk before prescribing, avoid the drug in patients with recent MI or stroke, and complete the 12-dose course followed by prompt antiresorptive transition.