Abstract 5881
Background
Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancers, making it an imaging & therapeutic target of interest. The utility of the PSMA-targeted imaging agent 99mTc-EC0652 is being evaluated, along with biomarker analysis of circulating tumor cells (CTCs), in pts with mCRPC in a completed PSMA-targeted chemotherapeutic study. We now report the PSMA heterogeneity via CTC vs. imaging in the pt population treated to date.
Methods
A total of 51 pts evaluated at the time of the data cut had baseline CT & bone scans performed in addition to a 99mTc-EC0652 SPECT/CT as a measure of imaging-based PSMA expression. 41 of 51 pts provided pre-treatment blood samples evaluable for CTC biomarker analysis, which included PSMA expression & predicted genomic instability status (pGI). Images collected from pts were analyzed for both their PSMA+ status as well as sensitivity relative to conventional imaging.
Results
41 of 51 pts (80.4%) had detectable CTCs in their samples prior to therapy while 32 of 38 (84.2%) of the analyzed paired on-therapy samples had CTCs after treatment. At baseline PSMA+ CTCs were found in 19 of 51 (37.2%) pts and overall was expressed on 33% (IQR 11.3% - 40.1%) of CTCs within each pt sample. 15 of 17 (88%) PSMA+ pt samples were also pGI+. 31 of the 51 pts (60.7%) of the pts had bone scan (BS) data available to analyze. 27 of the 51 pts had CT scan data available to analyze for nodal lesions. Concordance between Conventional Imaging & 99mTc-EC0652.Table: 842P
| 99mTc- EC0652+ | 99mTc- EC0652- | BS+ | BS- | CT+ | CT - | |
|---|---|---|---|---|---|---|
| Bone Lesion (770) | 770 (100%) | 0 | 587 (76.2%) | 183 (23.8%) | N/A | N/A |
| Nodes >1.5cm (54) | 48 (88.9%) | 6 (11.1%) | N/A | N/A | 54 | N/A |
| Nodes <1.5cm (69) | 69 | N/A | N/A | N/A | N/A | N/A |
Conclusions
In this cohort, most pts had CTCs for assessment. The majority of pts with CTCs had PSMA-CTCs. The pts CTCs expressing PSMA accounted for a small percentage of the total CTCs observed. These PSMA+ CTCs had high pGI. Pts with CTCs had PSMA imagable disease, especially in bone. The discordance between CTCs & imaging illustrate the disease heterogeneity in regards to PSMA expression. These data should be considered in designing future PSMA-directed regimens.
Clinical trial identification
NCT02202447.
Legal entity responsible for the study
Endocyte Inc.
Funding
Endocyte, Inc.
Editorial Acknowledgement
Disclosure
M.J. Morris, O. Sartor: Consultant: Endocyte. A. Armour, R. Messmann, M. Groaning: Employee: Endocyte. All other authors have declared no conflicts of interest.
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