FLT PET/CT imaging of metastatic prostate cancer patients treated with pTVG-HP DNA vaccine and pembrolizumab

J Immunother Cancer. 2019 Jan 30;7(1):23. doi: 10.1186/s40425-019-0516-1.


Background: Immunotherapy has demonstrated remarkable success in treating different cancers. Nonetheless, a large number of patients do not respond, many respond without immediate changes detectable with conventional imaging, and many have unusual immune-related adverse events that cannot be predicted in advance. In this exploratory study, we investigate how 3'-Deoxy-3'-18F-fluorothymidine (FLT) positron emission tomography (PET) measurements of tumor and immune cell proliferation might be utilized as biomarkers in immunotherapy.

Methods: Seventeen patients with metastatic castrate resistant prostate cancer were treated with combination pTVG-HP DNA vaccine and pembrolizumab. Patients underwent baseline and 12-week FLT PET/CT scans. FLT PET standardized uptake values (SUVs) were extracted from tumors, non-metastatic lymph nodes, spleen, bone marrow, pancreas, and thyroid to quantify cell proliferation in these tissues. Regional immune cell responses to pTVG-HP DNA vaccine were assessed by comparing FLT uptake changes in vaccine draining and non-draining lymph nodes. Cox proportional hazards regression was utilized to relate FLT uptake and other clinical markers (PSA and tumor size) to progression-free survival. Area under receiver operating characteristic (AUC) curves and concordance indices were used to assess the predictive capabilities of FLT uptake.

Results: Changes in FLT uptake in vaccine draining lymph nodes were significantly greater than changes in non-draining lymph nodes (P = 0.02), suggesting a regional immune response to vaccination. However, the changes in FLT uptake in lymph nodes were not significantly predictive of progression-free survival. Increases in tumor FLT uptake were significantly predictive of shorter progression-free survival (concordance index = 0.83, P < 0.01). Baseline FLT uptake in the thyroid was significantly predictive of whether or not a patient would subsequently experience a thyroid-related adverse event (AUC = 0.97, P < 0.01).

Conclusions: FLT PET uptake was significantly predictive of progression-free survival and the occurrence of adverse events relating to thyroid function. The results suggest FLT PET imaging has potential as a biomarker in immunotherapy, providing a marker of tumor and immune responses, and as a possible means of anticipating specific immune-related adverse events.

Trial registration: NCT02499835 .

Keywords: Adverse events; Cell proliferation; Clinical trial; DNA vaccine; FLT PET; Imaging; Pembrolizumab; Prostate cancer; Response assessment.

Publication types

  • Clinical Trial, Phase I
  • Clinical Trial, Phase II
  • Randomized Controlled Trial
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antibodies, Monoclonal, Humanized / therapeutic use*
  • Antineoplastic Agents, Immunological / therapeutic use*
  • Dideoxynucleosides
  • Humans
  • Male
  • Positron Emission Tomography Computed Tomography
  • Prostatic Neoplasms, Castration-Resistant / diagnostic imaging*
  • Prostatic Neoplasms, Castration-Resistant / therapy*
  • Vaccines, DNA / therapeutic use*


  • Antibodies, Monoclonal, Humanized
  • Antineoplastic Agents, Immunological
  • Dideoxynucleosides
  • Vaccines, DNA
  • pembrolizumab
  • alovudine

Associated data

  • ClinicalTrials.gov/NCT02499835