From what I understand PSCA is highly expressed in prostate cancer cells but also in other kinds of cancer and also in other tissues (in small amounts), and its production increases as the disease progresses, making it a better target for later stages (but this is my speculation!).
What is important is that we have another good target for therapies!!
About the other part, the CAR T cells part, I will copy/paste here:
"In a 73-year-old male patient with PSCA-positive mCRPC who was treated with MB-105 and lymphodepletion (a standard CAR T pre-conditioning regimen) after failing eight prior therapies, MB-105 demonstrated on day 28 a 94 percent reduction in prostate-specific antigen (PSA), near complete reduction of measurable soft tissue metastasis by computerized tomography, and improvement in bone metastases by magnetic resonance imaging."