Abaloparatide (Tymlos)

1. Overview

Abaloparatide is a synthetic 34-amino acid peptide analog of human parathyroid hormone-related protein (PTHrP(1-34)) developed as an anabolic therapy for osteoporosis [1][12]. Its amino acid sequence (Ala-Val-Ser-Glu-His-Gln-Leu-Leu-His-Asp-Lys-Gly-Lys-Ser-Ile-Gln-Asp-Leu-Arg-Arg-Arg-Glu-Leu-Leu-Glu-Lys-Leu-Leu-Aib-Lys-Leu-His-Thr-Ala-NH2) differs from native PTHrP(1-34) at eight positions, most notably the incorporation of alpha-aminoisobutyric acid (Aib) at position 29 and a C-terminal amidation [5][13]. The molecular formula is C174H300N56O49 with a molecular weight of 3961.47 Da.

Abaloparatide was developed by Radius Health, Inc. and received its first global approval from the U.S. Food and Drug Administration in April 2017 under the brand name Tymlos for the treatment of postmenopausal women with osteoporosis at high risk for fracture [12]. In January 2023, the FDA expanded the indication to include men with osteoporosis at high risk for fracture based on the ATOM trial [6]. It has since been approved in the European Union as Eladynos (December 2022) and in Japan as Ostabaro.

The drug is administered as a once-daily subcutaneous injection of 80 mcg using a prefilled pen device, delivered to the periumbilical region of the abdomen. Following subcutaneous injection, abaloparatide reaches peak plasma concentration at approximately 0.51 hours, with an absolute bioavailability of 36%, plasma protein binding of approximately 70%, and a mean elimination half-life of 1.7 hours [12]. It is degraded into smaller peptide fragments by non-specific proteolytic enzymes, followed by renal excretion. The volume of distribution is approximately 50 liters.

Molecular Weight

3961.47 g/mol

Sequence

34 amino acids (PTHrP analog with Aib at position 29)

Half-life

~1.7 hours (subcutaneous)

Bioavailability

36% (subcutaneous)

Routes

Subcutaneous injection (approved)

FDA Status

Approved (Tymlos, April 2017; expanded to men Jan 2023)

Approved Indication

Osteoporosis in postmenopausal women and men at high fracture risk

2. Mechanism of Action

Abaloparatide is a selective activator of the parathyroid hormone type 1 receptor (PTH1R), a class B G protein-coupled receptor expressed on osteoblasts and osteocytes [5][13]. Its mechanism of action is fundamentally defined by its preferential binding to specific conformational states of PTH1R, which distinguishes it from both native PTH and teriparatide (PTH(1-34)).

Receptor conformation selectivity. PTH1R exists in two high-affinity conformational states: RG, a G protein-dependent conformation that produces transient cAMP signaling upon ligand binding, and R0, a G protein-independent conformation that supports sustained cAMP signaling even after ligand dissociation from the cell surface [5]. Hattersley et al. (2016) demonstrated that abaloparatide binds with high affinity to the RG conformation but only weakly to the R0 conformation. In contrast, PTH(1-34) (teriparatide) binds efficiently to both RG and R0, with approximately four-fold greater affinity for R0 compared to PTHrP(1-34) [5]. This RG selectivity of abaloparatide produces rapid, transient increases in intracellular cAMP that are sufficient to activate osteoblast-mediated bone formation but quickly subside, avoiding the prolonged receptor activation that promotes osteoclast-mediated bone resorption.

Anabolic window. The transient signaling profile of abaloparatide creates a wider "anabolic window" -- a temporal separation between the stimulation of bone formation and bone resorption [13]. While both abaloparatide and teriparatide stimulate osteoblast activity through the cAMP-PKA signaling cascade, abaloparatide's shorter duration of receptor activation results in relatively less stimulation of RANKL expression and osteoclastogenesis. This translates clinically into robust increases in bone mineral density with a lower incidence of hypercalcemia compared to teriparatide [1].

Differential effects on bone turnover markers. In clinical studies, abaloparatide rapidly increases the bone formation marker procollagen type I N-terminal propeptide (P1NP) while producing a smaller and more delayed increase in the bone resorption marker C-terminal telopeptide of type I collagen (CTX) [4][13]. This early and preferential stimulation of formation over resorption supports the concept of an anabolic window.

Downstream signaling pathways. Activation of PTH1R by abaloparatide leads to stimulation of adenylyl cyclase via Gs-alpha, increasing intracellular cAMP and activating protein kinase A (PKA). PKA phosphorylates the transcription factor CREB (cAMP response element-binding protein), which drives expression of genes critical for osteoblast differentiation and function. Additionally, PTH1R activation can engage Gq/11-mediated phospholipase C/protein kinase C signaling and beta-arrestin-mediated pathways, though the relative contribution of these pathways to the net effects of abaloparatide versus teriparatide remains an area of active investigation [5].

3. Researched Applications

Postmenopausal Osteoporosis (Strong Evidence -- FDA Approved)

The ACTIVE trial (Abaloparatide Comparator Trial In Vertebral Endpoints), a pivotal Phase 3 study, randomized 2,463 postmenopausal women with osteoporosis to abaloparatide 80 mcg SC daily, open-label teriparatide 20 mcg SC daily, or placebo for 18 months [1]. Abaloparatide reduced new morphometric vertebral fractures by 86% compared to placebo (0.58% vs 4.22%; RR 0.14; 95% CI, 0.05-0.39; p<0.001) and nonvertebral fractures by 43% (2.7% vs 4.7%; HR 0.57; 95% CI, 0.32-1.00; p=0.049). Notably, abaloparatide significantly reduced major osteoporotic fractures compared to both placebo (HR 0.30; p=0.0004) and teriparatide (HR 0.45; p=0.03) [1].

BMD increases at 18 months were substantial: 9.2% at the lumbar spine, 3.4% at the total hip, and 2.9% at the femoral neck (all p<0.001 vs placebo) [1]. Abaloparatide produced significantly greater increases in total hip and femoral neck BMD compared to teriparatide [1][11]. BMD response rate analysis showed that at 18 months, 44.5% of abaloparatide patients achieved more than 3% total hip BMD gain compared to 32.0% with teriparatide and 1.9% with placebo [11].

Sequential Therapy: Abaloparatide Followed by Antiresorptive (Strong Evidence)

The ACTIVExtend study enrolled 1,139 women who completed ACTIVE to receive open-label alendronate 70 mg weekly for up to 24 months [2][3]. The initial 6-month report demonstrated that BMD gains achieved during abaloparatide treatment were maintained and further increased with sequential alendronate [3]. The full 24-month report showed that over the entire 43-month period, the abaloparatide-to-alendronate sequence reduced new vertebral fractures by 84% compared to placebo-to-alendronate (0.9% vs 5.6%) [2]. Risk reductions were sustained for all fracture types, including nonvertebral, clinical, and major osteoporotic fractures.

Male Osteoporosis (Strong Evidence -- FDA Approved)

The ATOM trial (Abaloparatide for the Treatment of Men with Osteoporosis) randomized 228 men aged 40 to 85 years with osteoporosis 2:1 to abaloparatide 80 mcg SC daily or placebo for 12 months [6]. BMD increases were significant at all measured sites: lumbar spine (8.48% vs 1.17%), total hip (2.14% vs 0.01%), and femoral neck (2.98% vs 0.15%), all p<0.0001 [6]. Based on these results, the FDA expanded the Tymlos indication to include men with osteoporosis at high risk for fracture in January 2023.

Elderly Patients (Moderate Evidence)

Post hoc analysis of the ACTIVE trial in women aged 80 years or older (n=94) demonstrated that abaloparatide was effective in increasing BMD in this very elderly subgroup, with a safety profile similar to the overall study population [9].

Patients with Renal Impairment (Moderate Evidence)

Analysis of ACTIVE data stratified by baseline renal function showed no meaningful differences in abaloparatide efficacy or safety among patients with normal, mild, or moderate renal impairment, supporting use without dose adjustment in these populations [10].

Bone Microarchitecture (Moderate Evidence)

Post hoc analyses using trabecular bone score (TBS) and DXA-based 3D modeling have demonstrated that abaloparatide improves trabecular bone microarchitecture and produces greater increases in hip cortical volumetric BMD compared to teriparatide [14]. In the Phase 2 study, TBS increased 4.21% with abaloparatide 80 mcg at 24 weeks.

4. Clinical Evidence Summary

5. Dosing in Research

The FDA-approved dose of abaloparatide is 80 mcg administered once daily by subcutaneous injection into the periumbilical region of the abdomen, using a prefilled pen device [12]. The cumulative use of abaloparatide is limited to no more than 2 years during a patient's lifetime.

In the Phase 2 dose-finding study, three doses were evaluated (20, 40, and 80 mcg daily for 24 weeks), with the 80 mcg dose demonstrating the most robust BMD increases across all skeletal sites and being selected for Phase 3 development [4]. The dose-response relationship was clear: lumbar spine BMD increased by 2.9%, 5.2%, and 6.7% at the 20, 40, and 80 mcg doses respectively.

In both the ACTIVE trial (postmenopausal women) and the ATOM trial (men), the 80 mcg once-daily dose was used [1][6]. Clinical guidelines recommend sequential treatment with an antiresorptive agent (such as alendronate, denosumab, or zoledronic acid) after completing abaloparatide therapy to maintain and extend BMD gains [2][3].

A transdermal microneedle patch delivery system was also investigated (Phase 2 at 50, 100, and 150 mcg daily; Phase 3 wearABLe trial), but the Phase 3 study did not meet its primary endpoint of non-inferiority to subcutaneous injection, and development of the transdermal formulation was subsequently discontinued.

6. Safety and Side Effects

The safety profile of abaloparatide has been characterized in clinical trials involving over 3,600 patients across the Phase 2, ACTIVE, ACTIVExtend, and ATOM programs [1][6][7][13].

Common adverse reactions. The most frequently reported adverse events in clinical trials include hypercalcemia (3.4%), dizziness (10%), nausea (8%), headache (8%), palpitations (5%), fatigue (3%), upper abdominal pain (3%), and vertigo (2%) [1][12]. Injection site reactions (redness, edema, pain) occurred in approximately 15% of abaloparatide-treated patients.

Hypercalcemia. Abaloparatide produces significantly less hypercalcemia than teriparatide. In the ACTIVE trial, the incidence of hypercalcemia (albumin-corrected serum calcium above the upper limit of normal at any time during the 4-hour post-dose period) was 3.4% with abaloparatide versus 6.4% with teriparatide [1]. This reduced hypercalcemia is attributed to abaloparatide's more transient PTH1R activation profile.

Cardiovascular effects. Detailed cardiovascular safety analysis from the ACTIVE trial demonstrated that abaloparatide was associated with transient increases in heart rate (mean increase of 7.9 bpm from pre- to 1-hour post-dose after first injection vs 5.3 bpm for teriparatide and 1.2 bpm for placebo) and small decreases in blood pressure [7]. Heart rate increases resolved within approximately 4 hours. Major adverse cardiovascular events (MACE) occurred in 0.5% of the abaloparatide group, 0.6% of the teriparatide group, and 1.2% of the placebo group, demonstrating no increased cardiovascular risk [7].

Orthostatic hypotension. Due to its transient blood pressure-lowering effect, abaloparatide carries a warning regarding orthostatic hypotension. Patients should be advised to sit or lie down if symptoms of lightheadedness occur after injection.

Osteosarcoma (preclinical). In a 2-year carcinogenicity study in Fischer 344 rats, daily subcutaneous administration of abaloparatide produced dose-dependent increases in the incidence of osteosarcoma, osteoblastoma, and focal osteoblast hyperplasia [12][13]. This is a class effect observed with all PTH and PTHrP analogs tested in rats and is attributed to the exaggerated bone remodeling response in rodents due to their open growth plates and different skeletal biology. The abaloparatide prescribing label initially carried a boxed warning regarding osteosarcoma risk; however, this warning was subsequently removed based on post-marketing surveillance data and clinical experience showing no signal of increased osteosarcoma risk in human patients. The cumulative 2-year lifetime treatment limitation remains in place as a precautionary measure.

Adverse events leading to discontinuation. In the ACTIVE trial, the most common adverse events leading to discontinuation were nausea (1.6%), dizziness (1.2%), headache (1.0%), and palpitations (0.9%) [1]. Discontinuation rates were generally low and comparable across treatment groups.

Contraindications. Abaloparatide is contraindicated in patients with hypersensitivity to abaloparatide or any excipient, and should be avoided in patients with pre-existing hypercalcemia, severe renal impairment, metabolic bone diseases other than osteoporosis (including hyperparathyroidism and Paget's disease), unexplained elevations of alkaline phosphatase, open epiphyses (pediatric and young adult patients), prior external beam or implant radiation therapy involving the skeleton, and bone metastases or pre-existing skeletal malignancies.

7. Regulatory Status

United States (FDA). Abaloparatide (Tymlos) was approved in April 2017 for the treatment of postmenopausal women with osteoporosis at high risk for fracture, defined as a history of osteoporotic fracture, multiple risk factors for fracture, or patients who have failed or are intolerant to other available osteoporosis therapy [12]. In January 2023, the indication was expanded to include treatment of men with osteoporosis at high risk for fracture based on the ATOM trial results [6]. Tymlos is manufactured by Radius Health, Inc. (now part of Ipsen).

European Union (EMA). The regulatory path in Europe was complex. The CHMP initially rejected the marketing application for abaloparatide in 2018, citing concerns about cardiovascular effects (heart rate increases). A second application was also refused. However, following additional cardiovascular safety data and a revised assessment, the EMA approved abaloparatide under the brand name Eladynos in December 2022 for treatment of osteoporosis in postmenopausal women at increased risk of fracture.

Japan. Abaloparatide was approved in Japan under the brand name Ostabaro by partner Teijin Pharma Limited for treatment of male and female osteoporosis patients at high risk of fracture.

Treatment duration limitation. In all jurisdictions, the cumulative lifetime use of abaloparatide is limited to 2 years. This restriction was originally based on the rodent osteosarcoma findings observed in the preclinical carcinogenicity study, although the boxed warning regarding osteosarcoma has since been removed from the US label.

8. Related Peptides

See also: Teriparatide (Forteo)

9. References

1. [1] Miller PD, Hattersley G, Riis BJ, et al. (2016). Effect of Abaloparatide vs Placebo on New Vertebral Fractures in Postmenopausal Women With Osteoporosis. JAMA. DOI PubMed
2. [2] Bone HG, Cosman F, Miller PD, et al. (2018). ACTIVExtend: 24 Months of Alendronate After 18 Months of Abaloparatide or Placebo. J Clin Endocrinol Metab. DOI PubMed
3. [3] Cosman F, Miller PD, Williams GC, et al. (2017). ACTIVExtend Initial 6-Month Results. Mayo Clin Proc. DOI PubMed
4. [4] Leder BZ, O'Dea LSL, Zanchetta JR, et al. (2015). Effects of abaloparatide on BMD in postmenopausal women with osteoporosis. J Clin Endocrinol Metab. DOI PubMed
5. [5] Hattersley G, Dean T, Corbin BA, et al. (2016). Binding Selectivity of Abaloparatide for PTH-Type-1-Receptor Conformations. Endocrinology. DOI PubMed
6. [6] Czerwinski E, Cardona J, Plebanski R, et al. (2022). Efficacy and Safety of Abaloparatide-SC in Men With Osteoporosis (ATOM). J Bone Miner Res. DOI PubMed
7. [7] Cosman F, Peterson LR, Towler DA, et al. (2020). Cardiovascular Safety of Abaloparatide. J Clin Endocrinol Metab. DOI PubMed
8. [8] Cosman F, Crittenden DB, Adachi JD, et al. (2018). BMD Response Rates With Abaloparatide vs Teriparatide vs Placebo. Bone. DOI PubMed
9. [9] Cosman F, McMahon DJ, Dempster DW, Nieves JW. (2023). Effect of Abaloparatide on Trabecular Bone Score. J Bone Miner Res. DOI PubMed
10. [10] Shirley M. (2017). Abaloparatide: First Global Approval. Drugs. DOI PubMed