Cell-Proliferation Imaging for Monitoring Response to CDK4/6 Inhibition Combined with Endocrine-Therapy in Breast Cancer: Comparison of [18F]FLT and [18F]ISO-1 PET/CT

Clin Cancer Res. 2019 May 15;25(10):3063-3073. doi: 10.1158/1078-0432.CCR-18-2769. Epub 2019 Jan 28.


Purpose: Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors in combination with endocrine-therapy have emerged as an important regimen of care for estrogen receptor (ER)-positive metastatic breast cancer, although identifying predictive biomarkers remains a challenge. We assessed the ability of two PET-proliferation tracers, [18F]FLT and [18F]ISO-1, for evaluating response to CDK4/6-inhibitor (palbociclib) and ER-antagonist (fulvestrant).

Experimental design: To determine the effect of CDK4/6 inhibition combined with estrogen-blockade, we assessed cell proliferation in six breast cancer cell lines after 1, 3, and 6 days of treatment with palbociclib and/or fulvestrant. These data were correlated to in vitro radiotracer assays and results were verified by longitudinal [18F]FLT and [18F]ISO-1 micro-PET imaging performed in MCF7 tumor-bearing mice.

Results: All palbociclib-sensitive cell lines showed decreased [18F]FLT accumulation and S-phase depletion after treatment, with both measures augmented by combination therapy. In contrast, these cells showed changes in [18F]ISO-1 analogue-binding and G0 arrest only after prolonged treatment. MicroPET imaging of MCF7 xenografts showed a significant decrease in [18F]FLT but no changes in [18F]ISO-1 uptake in all treated mice on day 3. On day 14, however, mice treated with combination therapy showed a significant decrease in [18F]ISO-1, corresponding to G0 arrest, while maintaining reduced [18F]FLT uptake, which corresponded to S-phase depletion.

Conclusions: Our data suggest complementary roles of [18F]FLT and [18F]ISO-1 PET in evaluating tumor-proliferation after combined CDK4/6 inhibitor and endocrine therapy in breast cancer. [18F]FLT is more sensitive to immediate changes in S-phase, whereas [18F]ISO-1 can assess more delayed changes related to cell-cycle arrest and transition to G0 quiescence from combination therapy. These data suggest a potential role for early prediction of long-term response using these imaging biomarkers.

Publication types

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

MeSH terms

  • Animals
  • Antineoplastic Combined Chemotherapy Protocols / pharmacology*
  • Breast Neoplasms / diagnostic imaging
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / enzymology
  • Breast Neoplasms / pathology
  • Cell Proliferation / drug effects
  • Cyclin-Dependent Kinase 4 / antagonists & inhibitors*
  • Cyclin-Dependent Kinase 6 / antagonists & inhibitors*
  • Dideoxynucleosides
  • Estrogen Receptor Antagonists / administration & dosage
  • Female
  • Fluorine Radioisotopes
  • Fulvestrant / administration & dosage
  • Humans
  • Longitudinal Studies
  • MCF-7 Cells
  • Mice
  • Mice, SCID
  • Piperazines / administration & dosage
  • Positron Emission Tomography Computed Tomography / methods
  • Pyridines / administration & dosage
  • Radiopharmaceuticals
  • Random Allocation
  • Receptors, Estrogen / antagonists & inhibitors
  • Tumor Cells, Cultured
  • Xenograft Model Antitumor Assays


  • Dideoxynucleosides
  • Estrogen Receptor Antagonists
  • Fluorine Radioisotopes
  • Piperazines
  • Pyridines
  • Radiopharmaceuticals
  • Receptors, Estrogen
  • 3',3'-difluoro-3'-deoxythymidine
  • Fulvestrant
  • CDK4 protein, human
  • CDK6 protein, human
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinase 6
  • palbociclib
  • Fluorine-18