Radium 223-Mediated Zonal Cytotoxicity of Prostate Cancer in Bone

J Natl Cancer Inst. 2019 Oct 1;111(10):1042-1050. doi: 10.1093/jnci/djz007.

Abstract

Background: Bone-targeting radiotherapy with Radium-223 (Rad-223), a radioisotope emitting genotoxic alpha-radiation with limited tissue penetrance (∼100 µm), prolongs the survival of patients with metastatic prostate cancer (PCa). Confoundingly, the clinical response to Rad-223 is often followed by detrimental relapse and progression, and whether Rad-223 causes tumor-cell directed cytotoxicity in vivo remains unclear. We hypothesized that limited radiation penetrance in situ defines outcome.

Methods: We tested Rad-223 overall response by PC3 and C4-2B human PCa cell lines in mouse bones (n = 5-18 tibiae per group). Rad-223 efficacy at subcellular resolution was determined by intravital microscopy analysis of dual-color fluorescent PC3 cells (n = 3-4 mice per group) in tissue-engineered bone constructs. In vivo data were fed into an in silico model to predict Rad-223 effectiveness in lesions of different sizes (1-27, 306 initial cells; n = 10-100 simulations) and the predictions validated in vivo by treating PCa tumors of varying sizes in bones (n = 10-14 tibiae per group). Statistical tests were performed by two-sided Student t test or by one-way ANOVA followed by Tukey's post-hoc test.

Results: Rad-223 (385 kBq/kg) delayed the growth (means [SD]; comparison with control-treated mice) of PC3 (6.7 × 105[4.2 × 105] vs 2.8 × 106 [2.2 × 106], P = .01) and C4-2B tumors in bone (7.7 × 105 [4.0 × 105] vs 3.5 × 106 [1.3 × 106], P < .001). Cancer cell lethality in response to Rad-223 (385 kBq/kg) was profound but zonally confined along the bone interface compared with the more distant tumor core, which remained unperturbed (day 4; 13.1 [2.3%] apoptotic cells, 0-100 µm distance from bone vs 3.6 [0.2%], >300 µm distance; P = .01).In silico simulations predicted greater efficacy of Rad-223 on single-cell lesions (eradication rate: 88.0%) and minimal effects on larger tumors (no eradication, 16.2% growth reduction in tumors of 27 306 cells), as further confirmed in vivo for PC3 and C4-2B tumors.

Conclusions: Micro-tumors showed severe growth delay or eradication in response to Rad-223, whereas macro-tumors persisted and expanded. The relative inefficacy in controlling large tumors points to application of Rad-223 in secondary prevention of early bone-metastatic disease and regimens co-targeting the tumor core.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Bone Neoplasms / diagnosis
  • Bone Neoplasms / radiotherapy*
  • Bone Neoplasms / secondary*
  • Cell Line, Tumor
  • Cell Proliferation / radiation effects
  • Disease Models, Animal
  • Dose-Response Relationship, Radiation
  • Humans
  • Male
  • Mice
  • Microscopy, Fluorescence, Multiphoton
  • Prostatic Neoplasms / pathology*
  • Radium / adverse effects*
  • Radium / therapeutic use
  • Tumor Burden / radiation effects
  • Tumor Cells, Cultured
  • Xenograft Model Antitumor Assays

Substances

  • Radium-223
  • Radium