Ostabolin-C

Summary

Ostabolin‑C (also called PTH(1‑31) cyclic lactam or RS‑66271) is an investigational cyclic fragment of human parathyroid hormone. It is being studied primarily for its ability to stimulate bone formation in osteoporosis and, experimentally, to enhance bone‑marrow recovery after chemotherapy or radiation. The peptide is administered by subcutaneous injection and remains in the research‑only category pending further clinical evaluation.

Mechanism of Action

Ostabolin‑C binds to the type‑1 parathyroid hormone receptor (PTHR1) on osteoblastic lineage cells, activating the G‑protein‑linked cAMP pathway. This leads to protein kinase A activation, CREB phosphorylation, and β‑catenin stabilization, which together drive osteoblast differentiation and proliferation. By expanding the osteoblastic niche, the peptide also indirectly enlarges the hematopoietic stem‑cell pool, supporting marrow engraftment in pre‑clinical models.

What the Research Shows

Cell‑culture work showed that Ostabolin‑C, together with other PTH fragments, stimulates alkaline phosphatase activity and proliferation of human marrow stromal cells, although the response diminishes with donor age (Aging Cell 2011). Modified bacterial mutagenicity assays found no genotoxic signal for Ostabolin‑C when using wash‑out and overlay methods (Mutagenesis 2005). A 2006 review noted that the peptide had entered phase‑II trials for osteoporosis, demonstrating potent bone‑building activity comparable to approved PTH analogues. The same review and a separate cancer‑research article discussed pre‑clinical evidence that PTH analogues, including Ostabolin‑C, can expand the hematopoietic niche and improve bone‑marrow engraftment after irradiation, though human data are lacking.

Reported Benefits

Available evidence indicates that Ostabolin‑C can promote osteoblast differentiation and bone matrix formation in vitro and has progressed to phase‑II clinical testing for osteoporosis, suggesting potential to increase bone density and reduce fracture risk. Pre‑clinical studies also propose a role in expanding the hematopoietic stem‑cell pool, which could aid marrow recovery after cytotoxic therapy.

Limitations of the Evidence

The osteogenic response to Ostabolin‑C appears reduced in cells from older donors, highlighting possible age‑related efficacy limits. Human efficacy and safety data are limited to early‑phase trials; no regulatory approval has been granted. The marrow‑engraftment benefits are based on animal and cell models, with no clinical confirmation. Long‑term outcomes and comparative effectiveness versus existing therapies remain uncertain.

Safety Considerations

Genotoxicity testing using adapted bacterial assays reported no mutagenic activity for Ostabolin‑C. However, the abstracts provide no detailed human safety or adverse‑event information, and the peptide remains investigational. Caution is warranted until comprehensive clinical safety data are available.

How It Is Administered

Ostabolin‑C is formulated for subcutaneous injection. As an investigational peptide, it is supplied in a sterile solution intended for research or clinical‑trial use only; no oral or alternative delivery routes have been described in the literature.

Routes of Administration

Goals & Uses

• Osteoporosis treatmentBone MetabolismModerate
• Fracture risk reductionBone HealthLow
• Reduced hypercalcemia risk vs full PTHSafety ProfileModerate
• Increase bone mineral densityBone HealthModerate

Contraindications

• HypercalcemiaMetabolic DisorderHigh
• Severe renal impairmentOrganModerateKidney function concerns
• Paget's disease of boneBone DisorderModerate
• Prior radiation therapy to skeletonOncologic HistoryModerate
• Active malignancy or history of bone metastasesOncologicHigh

Adverse Effects

• Dizziness/orthostatic hypotensionCardiovascularUncommon
• HypercalcemiaMetabolicUncommon
• Injection site reactionsLocalCommon
• HypercalciuriaMetabolicUncommon
• NauseaGastrointestinalUncommonFeeling of sickness or urge to vomit

Drug Interactions

• BisphosphonatesLow
• DigoxinHigh
• Calcium supplementsModerate
• Vitamin D analogsModerate

Population Constraints

• Pediatric patientsAgeRelative
• Patients with hyperparathyroidismEndocrineAbsolute
• Pregnant womenReproductiveRelative
• Severe renal impairment (CrCl <30 mL/min)Organ ImpairmentRelative

Regulatory Status

• European UnionInvestigationalNo EMA marketing authorization obtained.
• United StatesInvestigationalNever received FDA approval; clinical development did not advance to NDA submission.
• United KingdomInvestigationalNo MHRA approval; remains a research compound.

Has not received regulatory approval in any major jurisdiction. Investigated primarily in preclinical and early clinical studies. Development was largely conducted by Allelix Biopharmaceuticals/NPS Pharmaceuticals in the 1990s–2000s.

Evidence & Sources

• Effects of age on parathyroid hormone signaling in human marrow stromal cells
  Journal ArticleLowZhou S, et al.2011-01-01T00:00:00.000000Z
• Modified bacterial mutation test procedures for evaluation of peptides and amino acid-containing material
  Journal ArticleLowThompson C, et al.2005-01-01T00:00:00.000000Z
• Osteoporosis-treating parathyroid hormone peptides: What are they? What do they do? How might they do it?
  Journal ArticleModerateWhitfield JF2006-01-01T00:00:00.000000Z
• Parathyroid hormone: a novel tool for treating bone marrow depletion in cancer patients caused by chemotherapeutic drugs and ionizing radiation
  Journal ArticleModerateWhitfield JF2006-01-01T00:00:00.000000Z

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