1614P - PSA responses and PSMA scan changes after immunotherapy for biochemically recurrent prostate cancer (BCR) without androgen deprivation therapy (ADT)

Background

Patients (pts) with BCR have a rising PSA after definitive therapy but negative CT/Tc99 scans. Regardless of PSMA scan findings, no treatments have extended survival in BCR. Biologically, BCR has less tumor burden, less bone-based disease and pts have normal testosterone (T) relative to late stage disease. These key differences could lead to different clinical outcomes of immunotherapy in BCR relative to previous immunotherapy trials in metastatic pts.

Methods

This study (NCT03315871) enrolled pts with negative CT/Tc99 scans, T>100, PSA> 0.8 ng/ml after surgery or 2.0 ng/ml after radiation. Pts had a PSA doubling time (PSADT) of 5-15 months. Treatment was 2 pox viral-based therapeutic cancer vaccines targeting PSA and MUC1/CEA respectively for 7 months. Pts were monitored thereafter. PSA declines were noted based on multiple confirmed PSA declines from an intra-study apex PSA (ISAP; Madan ASCO GU 2018). Serial PSMA imaging was done in most pts.

Results

27 pts enrolled and 23 were treated/evaluated for response. Baseline medians include age of 69.5 years (58-84), PSA 5.1 ng/ml (0.9-32.9) and PSA DT 8.1 months (5-14.4). The treatment was well tolerated with grade 1/2 injection site reactions and rare transient flu-like symptoms. 11/23 (48%) pts had >30% slowing of the PSADT while on treatment, 7/23 (30%) were stable. 8/23 (35%) evaluable pts had confirmed ISAP PSA declines after treatment with a median decline of 19% (11-66%) lasting a median of 140 days (56-375). 21 pts had baseline and follow-up PSMA imaging with 9 having declines in PSMA tumor volume (PSMA-TV). 5/9 pts with declines in PSMA-TV also had PSA declines. The other 4 had stable PSA values prior to PSMA. Of interest, a pt with rising PSA for 553 days before a 66% confirmed PSA decline had a PSMA-TV decline that mirrored the PSA trend.

Conclusions

This is the first study in BCR to show PSA declines after immunotherapy without ADT that are associated with decreased PSMA-TV. The data further support the potential of immunotherapy in BCR without ADT despite negative immunotherapy trials in metastatic prostate cancer. Additional BCR immunotherapy studies with serial PET imaging are ongoing/planned at the NCI.

Clinical trial identification

NCT03315871.

Editorial acknowledgement

Legal entity responsible for the study

Center for Cancer Research, National Cancer Institute.

Funding

Bavarian Nordic.

Disclosure

All authors have declared no conflicts of interest.