# Bone mineral (hydroxyapatite)

Source: https://nuclide.cc/targets/bone-hydroxyapatite/  
Nuclide record `bone-hydroxyapatite` (Target). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

The hard mineral that makes up bone. Certain radioactive drugs stick to this mineral wherever bone is being actively rebuilt, which is how bone scans find fractures, infection and cancer that has spread to bone, and how some treatments deliver radiation straight to bone metastases.

## Summary

Bone is a composite of collagen protein and hydroxyapatite, a crystalline calcium phosphate mineral (Ca5(PO4)3OH). Bisphosphonates - and the older diphosphonates such as methylene diphosphonate (MDP) and hydroxymethylene diphosphonate (HDP/HMDP) - bind hydroxyapatite with high affinity through a P-C-P backbone that chelates surface calcium ions in a bidentate arrangement; when labelled with technetium-99m, the resulting Tc-99m-MDP or Tc-99m-HDP complex accumulates on bone in proportion to local blood flow and osteoblastic activity (new bone formation provides more, smaller hydroxyapatite crystals with greater surface area for binding), which is the basis of conventional bone scintigraphy.

A related but chemically distinct mechanism is direct ionic exchange: 18F-fluoride ions substitute for hydroxyl groups in the hydroxyapatite lattice to form fluoroapatite, giving even faster, higher-contrast bone-turnover imaging on PET than Tc-99m bone scans. On the therapy side, the same hydroxyapatite affinity is exploited by bone-seeking radionuclide therapies: samarium-153 lexidronam (a Sm-153-EDTMP bisphosphonate complex) and radium-223 dichloride (a calcium mimetic incorporated directly into the mineral at sites of osteoblastic activity) both concentrate in areas of high bone turnover, including osteoblastic bone metastases, to deliver localised radiation for pain palliation or, for radium-223, a survival benefit in castration-resistant prostate cancer with bone metastases.

As a target, hydroxyapatite's strength is that bone-turnover tracers and therapies go wherever remodelling is happening, with no receptor saturation or internalisation step required (bone-seeking agents work by chemisorption or ionic exchange, not receptor binding). Its central weakness is a lack of disease specificity: fractures, arthritis, Paget's disease and infection all increase osteoblastic activity and light up alongside malignancy, so bone scans are sensitive but not specific, and specificity depends entirely on the clinical and radiological context.

## Fields

- Kind: Target
- Last checked: 2026-09-22
- Also known as: Hydroxyapatite; bone mineral matrix
- Tags: bone; mineral; palliation
- Class: other
- Biology: The crystalline calcium phosphate mineral phase of bone, bound to a collagen matrix. Bisphosphonates and fluoride ions bind or exchange with this mineral at sites of active bone turnover through chemisorption or ionic substitution respectively, concentrating radiotracers or radionuclide therapy wherever bone remodelling, including reactive new bone at metastases, is occurring.
- Where found: Sites of active bone remodelling generally: fractures, degenerative and inflammatory bone disease, Paget's disease, infection; Osteoblastic bone metastases (prostate, breast and other cancers): increased uptake reflecting reactive new bone formation; Normal growing skeleton: high uptake at epiphyseal growth plates and osteochondral junctions

## Sources

- StatPearls: bone scan (mechanism of Tc-99m MDP/HDP binding to hydroxyapatite): https://www.ncbi.nlm.nih.gov/books/NBK531486/
- MDPI: bisphosphonates as radiopharmaceuticals - DOTAZOL review of bone-targeting chemistry: https://www.mdpi.com/1422-0067/25/1/462
- Applied Radiation and Isotopes review: targeted delivery to bone using bisphosphonate ligands: https://ame.nd.edu/research-legacy/faculty-research-labs/rroeder/publications/publication-pdf/2016%20ADDR%20Cole.pdf
- JNM: kinetics of bone-seeking radiotracers (Tc-99m-diphosphonate vs 18F-fluoride): https://jnm.snmjournals.org/content/jnumed/early/2013/03/11/jnumed.112.114298.full.pdf

## Connected records

- drugs: [Phosphorus-32 (radiophosphorus)](https://nuclide.cc/drugs/phosphorus-32/), [Radium-223 dichloride](https://nuclide.cc/drugs/radium-223/), [Samarium-153 lexidronam](https://nuclide.cc/drugs/samarium-153-lexidronam/)
- technologies: [SPECT & bone scan](https://nuclide.cc/technologies/spect/)
- terms: [Bone metastases and skeletal-related events](https://nuclide.cc/terms/bone-metastases/)
- isotopes: [Fluorine-18](https://nuclide.cc/isotopes/f-18/), [Radium-223](https://nuclide.cc/isotopes/ra-223/), [Samarium-153](https://nuclide.cc/isotopes/sm-153/), [Technetium-99m](https://nuclide.cc/isotopes/tc-99m/)

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