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

Abstract

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.

Fig. 1

Patients are numbered the same in all inserts (also the same as in Table 1) and are ordered by study arm. a Changes in FLT SUV_mean in vaccine draining left axillary lymph nodes are shown for each patient along with changes in non-draining right axillary lymph nodes. A number of outliers with changes in left axillary lymph node SUV_mean greater than 50% are evident. b Representative FLT PET/CT slice is showing a vaccine draining left axillary lymph with elevated uptake after 12 weeks of therapy (patient #1) c Changes in SUV_mean in bone marrow and spleen. d Representative FLT PET/CT slice is showing increased splenic FLT uptake after 12 weeks (patient #17). e Changes in FLT uptake in patients with soft tissue metastases are shown for SUV_mean and SUV_total. Changes in tumor SUV_mean and tumor SUV_total were significantly correlated (ρ = 0.66, P = 0.04). f Representative FLT PET/CT slice is showing metastatic mediastinal lymph nodes with visually increased FLT uptake after 12 weeks of therapy (patient #9). Following RECIST guidelines this patient had radiographically stable disease at week 12 but had subsequent disease progression upon the next radiographic follow-up at 24 weeks. g Changes in thyroid FLT uptake h Representative PET/CT slice is shown for a patient that experienced grade 2 hyperthyroidism (patient #11). The arrow indicates the position of the right thyroid lobe where visually increased FLT uptake is evident at 12 weeks. Notably, this patient had their first pembrolizumab injection 1 day prior to their 12-week PET scan

Fig. 2

a Changes in tumor FLT SUV_mean after 12 weeks are plotted against changes in tumor size after 12 weeks. Tumor size was measured following RECIST guidelines using a diagnostic CT scan. b Changes in tumor FLT SUV_mean after 12 weeks are plotted against changes in tumor size after 24 weeks. c Changes in tumor FLT SUV_mean after 12 weeks are plotted against changes in PSA after 12 weeks. d Changes in tumor FLT SUV_mean after 12 weeks are plotted against changes in PSA after 24 weeks. Note some patients are not included in these figures because they did not have RECIST measurable soft tissue tumors or were on study for less than 24 weeks

Fig. 3

a Change in tumor SUV_mean at 12 weeks differentiated patients who had progression-free survival less than or equal to the median progression-free survival time (24 weeks) from patients who had progression-free survival greater than the median. b Changes in PSA levels after 12 weeks for the same set of patients as shown in insert (a)

Fig. 4

a Axial CT and PET/CT slices with a metastatic tumor indicated. At week 12 this patient had experienced diminished PSA and RECIST measurements but increased tumor FLT uptake. By week 16, this patient was found to have progressive disease with marked increases in tumor size and PSA. b Immunofluorescence images show representative FFPE sections taken from the week 12 biopsy of the tumor indicated in part (a). The left immunofluorescence image shows proliferating T cells (Ki67 + CD8+; yellow arrows) and the right image shows proliferating tumor cells (Ki67 + PSMA+). c Quantification of the immunofluorescence images from the tumor indicated in part (a). The top row shows changes in the number of proliferating cells per unit area (left) and changes in the percentage of proliferating cells (right). The bottom row shows percent changes in proliferating CD8+ T cells (left) and proliferating PSMA+ tumor cells (right). *P-value less than 0.05

Fig. 5

a Thyroid SUV_mean at baseline and after three months for all patients. Patients that experienced a thyroid related-adverse event of Gr2 or greater are shown in various colors to distinguish them from patients that did not experience a thyroid-related adverse event (black). b Receiver operating characteristic curve showing the value of thyroid SUV_mean at baseline for predicting which patients will go on to experience a thyroid-related adverse event