Research Funding
None
Background
The recent introduction of Lu-177 PSMA for the treatment of castration-resistant prostate cancer (CRPC) has been met with much excitement. Initial reports of clinical response are promising, despite known inter- and intra-patient molecular heterogeneity. In this study, we examined the utility of PSMA protein expression in metastatic tumor tissues as a predictor of lesion-specific response to Lu-177 PSMA therapy in men with CRPC.
Methods
Between 2015-2020, 19 patients with metastases at multiple sites underwent metastatic lesion biopsy, Ga-68 PSMA PET imaging, and subsequent treatment with three cycles of Lu-177 PSMA. A monoclonal anti-PSMA antibody (EPITOMICS (USA), 1:50) was used to semi-quantitatively assess PSMA protein expression in the biopsy specimen. The histoscore (range 0-300) was derived from intensity and extent of the immunohistochemistry staining and was determined by experienced genitourinary pathologists. Imaging evaluation was performed according to the Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST) criteria. We assessed the association of the PSMA protein expression in metastatic tumor tissues and the lesion-specific response to Lu-177 PSMA therapy.
Results
In 12 patients with biopsy specimens available for staining, PSMA expression correlated with enhancement (SUVmax) of the biopsy site on Ga-68 PSMA PET imaging (rs = 0.63). Of the nine patients with repeat imaging after Lu-177 PSMA therapy, five (55.6%) had a lesion-specific response at the site of biopsy. PSMA expression on immunohistochemistry was unable to accurately predict lesion-specific response in univariable analysis (p = 0.81, 95% CI 94.6-76.6). Among the five men with a lesion-specific response, three (60%) experienced overall progression based on PERCIST. There was no association between lesion-specific response and overall progression (p = 0.64).
Conclusions
In patients with multiple metastases, PSMA protein expression from a single site biopsy was not predictive of site-specific Lu-177 PSMA response based on PERCIST. Additional studies are necessary to further interrogate the clinical consequence of PSMA expression heterogeneity in metastatic sites as well as the mechanisms underpinning resistance to Lu-177 PSMA in patients with CRPC.