Tag: Synthetic PTHrP 1-34 analog

Abaloparatide (BA058) is a synthetic 34-amino-acid peptide analog of human parathyroid hormone-related protein (PTHrP) (1-34) that acts as a selective agonist at the parathyroid hormone type 1 receptor (PTH1R), a class B G protein-coupled receptor expressed on osteoblasts and osteocytes. The compound was rationally designed to exploit conformational selectivity at PTH1R: abaloparatide binds the G protein-coupled RG conformation with an IC50 of approximately 0.20 nM and the G protein-independent R0 conformation with an IC50 of approximately 316 nM, yielding a 1,600-fold RG/R0 selectivity ratio that exceeds native PTHrP(1-36) (110-fold), native PTH(1-34) (12-fold), and the long-acting analog LA-PTH (2.2-fold). This selectivity produces a transient downstream cAMP signaling response following receptor activation, which, when delivered as a once-daily intermittent subcutaneous injection, preferentially stimulates osteoblast-mediated bone formation with lesser stimulation of osteoclast-mediated bone resorption and a lower incidence of hypercalcemia than teriparatide (recombinant PTH(1-34)), the first-in-class parathyroid hormone receptor agonist for osteoporosis.

The compound shares 76% amino acid sequence homology with native PTHrP(1-34) and 41% homology with native PTH(1-34). The first 22 amino acids are identical to PTHrP(1-22); positions 23 through 34 carry eight substitutions, including an alpha-aminoisobutyric acid (Aib) residue at position 29, introduced to constrain the C-terminal alpha-helix and enhance peptide stability and receptor binding selectivity. The molecular formula is C174H300N56O49 with a molecular weight of approximately 3,960 daltons. The compound is administered as a once-daily 80 microgram subcutaneous injection into the periumbilical abdominal region. Pharmacokinetics after subcutaneous dosing are characterized by rapid absorption (median Tmax approximately 0.51 hours), an absolute bioavailability of approximately 36%, a volume of distribution of approximately 50 liters, plasma protein binding of approximately 70%, and an elimination half-life of approximately 1 hour, with clearance proceeding through nonspecific proteolytic degradation to smaller peptide fragments followed by renal excretion.

Clinical efficacy was established in the pivotal ACTIVE trial (Abaloparatide Comparator Trial In Vertebral Endpoints), an 18-month, international, randomized, double-blind, placebo- and active-controlled Phase 3 study in 2,463 postmenopausal women with osteoporosis, in which abaloparatide 80 microgram daily reduced the risk of new morphometric vertebral fractures by 86% relative to placebo (0.58% versus 4.22%) and nonvertebral fractures by 43% (2.7% versus 4.7%). The subsequent ACTIVExtend trial demonstrated that sequential treatment with abaloparatide followed by alendronate maintained fracture risk reduction and further increased bone mineral density over a combined 43-month observation period. The ATOM trial extended efficacy to men with osteoporosis, demonstrating lumbar spine bone mineral density gains of 8.48% versus 1.17% on placebo at 12 months. The compound received United States Food and Drug Administration approval on April 28, 2017, for postmenopausal women with osteoporosis at high fracture risk (marketed as Tymlos), with an expanded indication for men with osteoporosis approved in December 2022. European Medicines Agency approval was granted in December 2022 under the trade name Eladynos. A transdermal solid microstructured transdermal system (sMTS) formulation is in clinical development as an alternative to subcutaneous injection. Treatment duration is limited to a cumulative lifetime maximum of 2 years owing to a dose- and time-dependent increase in osteosarcoma incidence observed in preclinical rodent carcinogenicity studies, a class effect shared with teriparatide.

This monograph reviews the chemistry, synthesis, and structural pharmacology of abaloparatide; the molecular mechanism of PTH1R conformational selectivity and downstream signaling; comprehensive pharmacokinetics; preclinical pharmacology in ovariectomized and orchiectomized rodent models; the clinical evidence base across the ACTIVE, ACTIVExtend, and ATOM trials; sourcing and quality verification for research applications; reconstitution and handling; stack-interaction considerations; the adverse-event and safety profile including the osteosarcoma signal and cardiovascular safety analysis; and a structured comparative assessment of five alternative bone-active agents (teriparatide, romosozumab, denosumab, zoledronic acid, and raloxifene) against abaloparatide on five competency standards.