Transcriptomic Profiling of the Tumor Immune Microenvironment Reveals Prognostic Markers in mCRPC Patients Treated with LuPSMA Therapy

<b>Rationale:</b> The mode of action of [¹⁷⁷Lu]Lu-PSMA-617 (LuPSMA) therapy is not fully understood and a relevant fraction of patients show treatment failure. Therefore, this study aimed to investigate the prognostic significance of immune suppression in the tumor immune microenvironment (TME) of LuPSMA therapy patients. <b>Methods:</b> Tumor tissue samples from 61 patients, who were referred for LuPSMA from March 2018 until March 2022, were retrieved. Among these, 40 patients fulfilled all criteria and were therefore included in the analysis. Of these, 3 patients had two biopsies; prior and under systemic treatment, which is why we analyzed 43 samples: 29 (67%) with treatment-naïve tissues samples (cohort 1) and 14 (33%) during systemic treatment. Patients were followed up to assess overall survival. We examined gene expression and immune cell counts (derived from gene expression data) in the two sub-cohorts through transcriptome profiling with the Decipher prostate assay (Veracyte, San Diego, CA), a subset of these patients has been described previously. <b>Results:</b> In the total cohort, the ratio of activated (M1)/naive (M0) macrophages (HR = 0.90 [0.84-0.98]; p = 0.009) was a significant prognosticator of OS. In cohort 1, PD-L2 expression (HR = 1.07 [1.02 - 1.11]; p = 0.003)) and the M1/M0 ratio signature (HR = 0.89 [0.81-0.99]; p = 0.026) were significant independent prognostic factors of OS when analyzed together in a multivariate analysis. AR gene expression was significantly elevated in cohort 2 compared to 1 (p < 0.001). Several DNA repair signatures analyzed were significantly higher in cohort 2 than in cohort 1 (p < 0.05). In cohort 2, PD-L2 expression (HR = 0.87 [0.77 - 0.98]; p = 0.017) emerged as an independent prognostic factor associated with improved OS when included in a multivariate model with the immune 190 score, a negative prognosticator in this analysis (HR = 1.25 [1.02 - 1.53]; p = 0.028). <b>Conclusions:</b> The ratio of M1/M0 macrophages was associated with favorable outcome of LuPSMA in the total cohort of patients. In treatment-naive patient samples, <i>PD-L2</i> expression was associated with unfavorable, whereas M1/M0 macrophages with favorable outcomes, which might indicate that immune checkpoint inhibition could be a combination partner of LuPSMA therapy. In patient biopsy samples acquired after the start of systemic treatment, AR gene expression and DNA repair signatures appear to be significantly altered and <i>PD-L2</i> became a protective marker.