Abaloparatide
High EvidenceAbaloparatide (Tymlos) is a synthetic 34-amino-acid analog of parathyroid hormone-related protein, PTHrP(1-34), engineered to bind the PTH1 receptor with a strong preference for its transient RG conformation rather than the long-lived R0 conformation favored by teriparatide. That receptor bias produces a shorter burst of signaling per injection - enough to drive osteoblast activity, but short enough to limit the bone resorption and calcium mobilization that follow a prolonged signal. In the 18-month Phase 3 ACTIVE trial in 2,463 postmenopausal women, abaloparatide 80 mcg daily reduced new morphometric vertebral fractures by 86% versus placebo, alongside significant reductions in nonvertebral, major osteoporotic and clinical fractures. The FDA approved Tymlos in April 2017; in December 2021 the osteosarcoma boxed warning and the two-year cumulative lifetime limit were both removed from the label.
What It Is
Abaloparatide (brand name Tymlos, developed by Radius Health) is an osteoanabolic peptide - a drug that builds new bone rather than merely slowing its loss. It is a synthetic 34-amino-acid analog of the N-terminal fragment of parathyroid hormone-related protein, PTHrP(1-34), modified at several positions from the native human sequence to improve stability and receptor behavior. PTHrP is not the same molecule as parathyroid hormone, but the two share enough N-terminal homology that both engage the same target, the PTH1 receptor. Understanding abaloparatide requires understanding why that receptor is unusual. The PTH1 receptor exists in at least two high-affinity conformational states. The R0 state is G-protein-independent and long-lived: a ligand that stabilizes R0 stays bound and keeps signaling long after the peptide has cleared the plasma. The RG state is G-protein-coupled and transient: a ligand that prefers RG produces a shorter pulse of cAMP signaling that terminates quickly. Teriparatide, PTH(1-34), binds R0 with relatively high affinity. Abaloparatide is selective for RG. In practice this means the two drugs deliver signal to the same receptor with different durations per dose, and duration is what determines whether PTH1R signaling is net anabolic or net catabolic. A short daily pulse recruits and activates osteoblasts and expands the anabolic window - the interval during which bone formation outpaces the resorption that inevitably follows. A longer signal narrows that window, because the coupled osteoclastic response has time to catch up, and it mobilizes more calcium out of the skeleton into the blood. This is the same principle that governs palopegteriparatide, which takes the opposite route: it uses continuous physiologic exposure to make PTH(1-34) a replacement hormone instead of an osteoporosis drug. Three peptides, one receptor family, three completely different clinical identities determined by the shape of the signal. The pivotal evidence for abaloparatide comes from ACTIVE (Abaloparatide Comparator Trial In Vertebral Endpoints), an 18-month randomized, double-blind, placebo-controlled, multicenter Phase 3 study in 2,463 postmenopausal women with osteoporosis, which also included an open-label teriparatide arm. Abaloparatide 80 mcg subcutaneously daily reduced the risk of new morphometric vertebral fractures by 86% versus placebo; teriparatide reduced it by 80%. Abaloparatide also significantly reduced nonvertebral, major osteoporotic and clinical fractures versus placebo, and increased bone mineral density at the lumbar spine, total hip, femoral neck and ultradistal radius. The results were published in JAMA in August 2016 and supported FDA approval in April 2017. Because anabolic therapy is a limited-duration intervention - stop it and the newly built bone is lost without follow-on treatment - the ACTIVExtend study followed ACTIVE participants through 24 months of alendronate after abaloparatide, and showed that the fracture-risk reduction was maintained and BMD gains extended. That sequence, anabolic first and antiresorptive second, is now the standard architecture for treating high-risk osteoporosis, and abaloparatide is one of the drugs that established it. Two later developments changed how the drug is used. In December 2021, following a class-wide regulatory review of long-term post-marketing data across PTH-analog drugs, the FDA approved removal of the osteosarcoma boxed warning from the Tymlos label along with the two-year cumulative lifetime limit on use - the rodent osteosarcoma signal that had constrained the whole class since teriparatide's approval did not reproduce in humans. Separately, the Phase 3 wearABLe study tested a transdermal microstructured patch delivering 300 mcg against the 80 mcg injection in roughly 500 postmenopausal women. It missed its non-inferiority margin of 2.0% for 12-month lumbar spine BMD: the patch produced a 7.1% increase versus 10.9% for the injection. Both were real bone-building effects, but the patch was measurably weaker, and the needle-free version did not go on to approval on that basis.
Regulatory Status
Tymlos (abaloparatide) injection was approved by the FDA on April 28, 2017 for the treatment of postmenopausal women with osteoporosis at high risk for fracture, and the indication was subsequently expanded to include men with osteoporosis at high risk for fracture. It is supplied as a once-daily 80 mcg subcutaneous injection in a multiple-dose prefilled pen. In December 2021 the FDA approved removal of the osteosarcoma boxed warning and of the two-year cumulative lifetime limit on use. It is a prescription osteoanabolic therapy requiring clinical supervision and is not a research chemical, supplement or over-the-counter product.
Effective: April 2017
View FDA SourceWhy Researchers Study It
Abaloparatide is the cleanest available demonstration that receptor conformational selectivity is a drugable design parameter, not just a pharmacology curiosity. Two peptides hitting the same receptor - teriparatide biased toward the persistent R0 state, abaloparatide toward the transient RG state - produce measurably different clinical profiles from the same target. The consequences researchers care about are practical: a shorter signaling pulse should widen the anabolic window, produce earlier hip and femoral neck BMD gains, and mobilize less calcium from bone into serum. ACTIVE's head-to-head design, with an open-label teriparatide arm running alongside, is one of the few osteoporosis trials that allows those predictions to be examined directly rather than inferred across studies. Beyond the mechanism, abaloparatide matters for sequencing. Anabolic agents are not maintenance therapy - the bone they build is provisional until it is locked in by an antiresorptive - and ACTIVExtend is one of the trials that made the anabolic-then-alendronate sequence an evidence-based standard rather than an expert opinion. Third, the 2021 label change is a case study in how a preclinical safety signal can outlive its evidence: the rodent osteosarcoma finding that had capped PTH-analog treatment at two lifetime years was retired after long-term human post-marketing data failed to reproduce it, which reopened the question of how long anabolic therapy can reasonably be continued. Finally, the failed wearABLe transdermal bridging study is a useful negative result for anyone studying peptide delivery: a patch achieved real bone building but not equivalence, a reminder that alternative routes for peptides are usually a bioavailability problem before they are a convenience win.
Proposed Mechanisms
- Synthetic 34-amino-acid analog of parathyroid hormone-related protein, PTHrP(1-34), modified from the native human sequence at several positions for stability and receptor behavior
- Agonist at the PTH1 receptor with selectivity for the transient, G-protein-coupled RG conformation rather than the long-lived, G-protein-independent R0 conformation preferred by teriparatide
- RG selectivity shortens the duration of cAMP signaling per injection, producing a brief anabolic pulse rather than a prolonged one
- The short pulse widens the anabolic window - the interval in which osteoblastic bone formation outpaces the coupled osteoclastic resorption that follows PTH1R activation
- Stimulates osteoblast recruitment, differentiation and survival, increasing bone formation markers and building new trabecular and cortical bone
- Less prolonged receptor engagement is associated with less mobilization of calcium out of the skeleton, and in the ACTIVE head-to-head arm with a lower rate of hypercalcemia than teriparatide
- Effect is duration-limited: after discontinuation, newly formed bone is lost unless an antiresorptive such as alendronate is started, which is why sequential therapy is standard
Evidence Snapshot
| Study Type | Model | Outcome | Link |
|---|---|---|---|
| Phase 3 (ACTIVE, NCT01343004) - randomized, double-blind, placebo-controlled with open-label active comparator | 2,463 postmenopausal women with osteoporosis randomized to abaloparatide 80 mcg subcutaneously daily, placebo, or open-label teriparatide 20 mcg daily for 18 months | New morphometric vertebral fracture risk reduced by 86% with abaloparatide and 80% with teriparatide versus placebo. Abaloparatide also significantly reduced nonvertebral, major osteoporotic and clinical fractures versus placebo. Published in JAMA, August 2016 | Source |
| Phase 3 (ACTIVE) - bone mineral density endpoints | Postmenopausal women with osteoporosis on abaloparatide 80 mcg daily for 18 months | Significant increases in bone mineral density at the lumbar spine, total hip, femoral neck and ultradistal radius versus placebo, consistent in direction and magnitude with the observed fracture-risk reduction | Source |
| Phase 3 extension (ACTIVExtend) - sequential therapy | ACTIVE participants from the abaloparatide and placebo arms transitioned to 24 months of open-label alendronate after completing 18 months of blinded treatment | Fracture-risk reduction achieved during abaloparatide treatment was maintained and bone mineral density gains extended through the alendronate period, across baseline fracture-risk strata - the evidence base for anabolic-first, antiresorptive-second sequencing | Source |
| Regulatory - FDA label update (December 2021) | U.S. prescribing information for Tymlos (abaloparatide) injection | FDA approved removal of the osteosarcoma boxed warning and of the two-year cumulative lifetime limit on use, following review of long-term post-marketing data for abaloparatide and the PTH-analog class in which the rodent osteosarcoma signal did not reproduce in humans | Source |
| Phase 3 (wearABLe, NCT04064411) - randomized, open-label, active-controlled BMD bridging study | Approximately 500 postmenopausal women with osteoporosis at high fracture risk randomized to abaloparatide transdermal system 300 mcg or Tymlos 80 mcg injection for 12 months | Missed the primary non-inferiority endpoint (margin 2.0%) for percent change in lumbar spine BMD: transdermal 7.1% (95% CI 6.2-8.0) versus injection 10.9% (95% CI 9.9-11.8). Both arms built bone, but the patch was not equivalent and did not proceed to approval on this result | Source |
| Systematic review and meta-analysis | Pooled randomized controlled trial data on abaloparatide in postmenopausal osteoporosis | Confirms consistent BMD gains and vertebral and nonvertebral fracture-risk reduction versus placebo, with an adverse-event profile dominated by hypercalciuria, dizziness, palpitations, nausea and injection-site reactions | Source |
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Safety & Cautions
- Abaloparatide is a prescription osteoanabolic drug for diagnosed osteoporosis at high fracture risk - it is not a wellness, longevity, recovery or performance peptide and has no legitimate use outside a clinician-directed osteoporosis treatment plan
- Anabolic benefit is not durable on its own: bone gained during abaloparatide treatment is lost after discontinuation unless an antiresorptive such as alendronate is started promptly, which is why sequential therapy rather than monotherapy is the standard of care
- Orthostatic hypotension can occur, typically within four hours of injection and most often with the first several doses - initial injections should be given where the patient can sit or lie down if lightheaded
- Hypercalcemia and hypercalciuria occur, and the drug is not appropriate for patients with pre-existing hypercalcemia; patients with active or prior urolithiasis need individual assessment because increased urinary calcium can worsen stone risk
- Common adverse reactions include hypercalciuria, dizziness, nausea, headache, palpitations, fatigue, upper abdominal pain, vertigo and injection-site reactions
- The osteosarcoma boxed warning and the two-year cumulative lifetime use limit were removed from the U.S. label in December 2021, but the current approved prescribing information remains the authoritative source on duration, warnings and populations in whom use is not recommended - do not rely on secondary summaries, including this one
- Abaloparatide is not interchangeable with teriparatide or with palopegteriparatide. All three act at the PTH1 receptor and all three have different dosing, different signal duration and different indications; substituting one for another is a clinically meaningful error
- The abaloparatide transdermal system is not an approved product. The Phase 3 wearABLe study missed its non-inferiority margin against the injection, and any patch marketed outside an approved channel should be treated as unvalidated
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Citations
- [1] Effect of Abaloparatide vs Placebo on New Vertebral Fractures in Postmenopausal Women With Osteoporosis: The ACTIVE Randomized Clinical Trial - JAMA (2016) PubMed
- [2] Fracture and Bone Mineral Density Response by Baseline Risk in Patients Treated With Abaloparatide Followed by Alendronate: Results From the Phase 3 ACTIVExtend Trial - PMC PubMed
- [3] TYMLOS (abaloparatide) injection - U.S. Prescribing Information, FDA Access Data PubMed
- [4] Radius Announces Update on TYMLOS (abaloparatide) Label - removal of the boxed warning and the two-year cumulative use limit (December 2021) PubMed
- [5] Radius Announces Results from the wearABLe Trial Evaluating Abaloparatide Transdermal System in Postmenopausal Women with Osteoporosis (December 2021) PubMed
- [6] The Safety and Efficacy of Abaloparatide on Postmenopausal Osteoporosis: A Systematic Review and Meta-analysis - Clinical Therapeutics (2024) PubMed
- [7] Teriparatide and Osteosarcoma Risk: History, Science, Elimination of Boxed Warning, and Other Label Updates - PMC PubMed
- [8] FDA Approves Radius Health's TYMLOS (abaloparatide) - Bone Health & Osteoporosis Foundation PubMed
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