Analysis of PSMA expression and outcome in patients with advanced Prostate Cancer receiving 177 Lu-PSMA-617 Radioligand Therapy

Rationale: PSMA-PET-CT enables measuring molecular expression of prostate-specific membrane antigen (PSMA) in vivo , which is the target molecule of 177 Lu-PSMA-617 (Lu-PSMA) therapy. However, the correlation of PSMA expression and overall survival (OS) in patients treated with Lu-PSMA therapy is currently unclear; especially with regard to coexistence of high and low PSMA expressing metastases. To this end, this retrospective single arm study elucidates the correlation of PSMA expression and overall survival in patients treated with Lu-PSMA therapy. Additionally, PET based criteria to define low PSMA expression were explored. Methods: Eighty-five patients referred to Lu-PSMA therapy were included in the analysis. Pretherapeutic 68 Ga-PSMA-PET-CT scans were available for all patients. SUV max of the highest PSMA expressing metastasis (PSMA max), SUV max of the lowest PSMA expressing metastasis (PSMA min), and average SUV max of all metastases (PSMA average) amongst other PET parameters were measured for each patient. A log-rank cutoff-finder was used to determine low (lowPSMA average) and high (highPSMA average) average PSMA expression as well as low (lowPSMA min) and high (highPSMA min) minimal PSMA expression. Results: PSMA average was a significant prognosticator of overall survival in contrast to PSMA max (HR: 0.959; p = 0.047 vs. HR: 0.992; p = 0.231). Optimal log rank cut-offs were: PSMA average = 14.3; PSMA min = 10.2. Patients with low average PSMA expression (lowPSMA average) had significantly shorter survival compared to those with high average expression (highPSMA average) (5.3 vs. 15.1 months; p < 0.001; HR: 3.738, 95%CI = 1.953-7.154; p < 0.001). Patients with low PSMA expressing metastases (lowPSMA min) had shorter survival compared to those without a low PSMA expressing metastasis (highPSMA min) (p = 0.003; 7.9 months vs. 21.3; HR: 4.303, 95%CI = 1.521-12.178; p = 0.006). Patients that were classified as highPSMA average but with lowPSMA min had an intermediate overall survival (11.4 months; longer compared to lowPSMA average , 5.3 months, p = 0.002; but shorter compared to highPSMA min , 21.3 months, p = 0.02). Conclusion: Low average PSMA expression is a negative prognosticator of overall survival. Absence of low PSMA expressing metastases is associated with best overall survival and the maximum PSMA expression seems not suited to prognosticate overall survival. Low PSMA expression might therefore be a negative prognosticator for the outcome of patients treated with Lu-PSMA therapy. Future studies are warranted to elucidate the degree of low PSMA expression tolerable for Lu-PSMA therapy.