KLK3 and prostate cancer: This study plans to enroll 40 patients with progressive metastatic castration-resistant prostate cancer who previously completed 4-6 cycles of [<sup>177</sup>Lu]Lu-PSMA-617 with a favorable response (i.e., ≥50% decrease in prostate-specific antigen [PSA] level at any point during the first [<sup>177</sup>Lu]Lu-PSMA-617 regimen).