Bone-targeting parathyroid hormone conjugates outperform unmodified PTH in the anabolic treatment of osteoporosis in rats

Abstract

Synthetic parathyroid hormone (PTH) is clinically indicated for the treatment of osteoporosis, through its anabolic effects on parathyroid hormone receptors (PTHRs), located on osteoblast cells. However, the bioavailability of PTH for bone cells is restricted by the short half-life of PTH and the widespread distribution of PTHRs in non-skeletal tissues. To impart affinity for mineralized bone surfaces, bisphosphonate (BP)-mediated PTH analogues were synthesized, characterized, and evaluated in vitro and in vivo. The successful synthesis of PTH-PEG-BP was identified on MALDI-ToF mass spectra; bone-targeting potential was evaluated by hydroxyapatite binding test; and receptor bioactivity was assessed in UMR-106 (rat osteosarcoma) cells that constitutively express PTHRs. Therapeutic efficacy was evaluated using ovariectomized rats that remained untreated for 8 weeks to allow development of osteopenia. Those rats then received daily subcutaneous injections of PTH-PEG-BP, thiol-BP vehicle, or unmodified PTH, and compared to sham-operated healthy rats at 0, 4, 8, 12, and 16 weeks. In vivo micro-CT was conducted on the proximal tibial metaphysis to measure microstructural bone parameters, and new bone formation was detected using dynamic labeling. Bone strength was assessed using three-point bending mechanical testing. Our study determined that PTH-PEG-BP conjugates significantly enhanced PTH targeting to the bone matrix while retaining full PTH bioactivity. Moreover, PTH-PEG-BP conjugates significantly increased trabecular bone quality, anabolic bone formation, and improved bone strength over systemically administered PTH alone. We highlight the promise of a novel class of bone-targeting anabolic compound for the treatment of osteoporosis and related bone disorders.

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Acknowledgments

This research was funded by the Mitacs Elevate Strategic Fellowship Program (SFP), the Osteoarthritis Alberta Team Grant from Alberta Innovates—Health Solutions (AIHS), and the Canadian Institutes for Health Research (CIHR). We thank Madhuri Newa and Kathy Tang for their technical support in completion of this study.

Authors’ role

Study design and conduct: MRD and YY; preparation of PTH-PEG-BP analogues: YY and KHB; rat dosing and gavaging: YY, AP, and YW; micro-CT scanning: YY and YW; EPMA: YY, AP, and YW; histology: YY and AP; bone biomechanics: YY and YW; data analysis: YY, AP, YW, and MRD; data interpretation: YY, AAH, AP, YW, and MRD; drafting manuscript: YY, AAH, and AP; revising manuscript content: YY, AAH, AP, and MRD; approving final version of manuscript: YY, AAH, AP, and MRD. MRD takes responsibility for the integrity of the data analysis.

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Correspondence to Michael R. Doschak.

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All animal procedures were approved by the University of Alberta Animal Care and Use Committee.

Disclosures

This study was presented in part as an abstract at the 2014 Annual Meeting & Exposition of the Controlled Release Society (CRS) in Chicago USA. The contents of this article are solely the responsibility of the authors, and all the authors state that they have no conflicts of interest.

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Yang, Y., Aghazadeh-Habashi, A., Panahifar, A. et al. Bone-targeting parathyroid hormone conjugates outperform unmodified PTH in the anabolic treatment of osteoporosis in rats. Drug Deliv. and Transl. Res. 7, 482–496 (2017). https://doi.org/10.1007/s13346-017-0407-2

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Keywords

  • PTH
  • Bone drug delivery
  • Anabolic therapy
  • Osteoporosis
  • Micro-CT