Assessment of Safety and Efficacy of 64Cu-SAR-bisPSMA in Patients with Biochemical Recurrence of Prostate Cancer Following Definitive Therapy

(UroToday.com) The 2024 American Society of Clinical Oncology (ASCO) annual meeting featured a session on prostate cancer, and a presentation by Dr. Luke Nordquist discussing results of COBRA, an assessment of the safety and efficacy of ⁶⁴Cu-SAR-bisPSMA in patients with biochemical recurrence of prostate cancer following definitive therapy.

Between 20-40% of patients with prostate cancer will relapse within 10 years of their primary prostate cancer treatment, as identified by increasing PSA levels. Most relapses will occur within 5 years after definitive therapy, thus early diagnosis of biochemical recurrence with accurate staging is essential to inform the best treatment strategy. Over the last several years, PSMA has been used as an imaging target in prostate cancer. 

 ⁶⁴Cu-SAR-bisPSMA may offer several advantages over the currently approved PSMA PET agents due to the bivalent structure of SAR-bisPSMA and longer half-life (t_1/2) of ⁶⁴Cu (12.7 hours), compared to the monovalent agents utilizing ¹⁸F and ⁶⁸Ga (t_1/2
Clinical/translational evidence has demonstrated higher tumor uptake (2-3x), prolonged retent, ion, and detection of additional prostate cancer lesions using ⁶⁴Cu-SAR-bisPSMA compared to approved PSMA agents.

The COBRA study was a phase I/II study assessing the safety and efficacy of ⁶⁴Cu-SAR-bisPSMA (200 MBq) in prostate cancer patients with biochemical recurrence and negative or equivocal standard of care imaging. Patients underwent PET/CT on Day 0 and Day 1 (1-4 hours and 24 ± 6 hours post-dose, respectively), interpreted by 3 blinded central readers. The study design is as follows:
The PET/CT results were assessed against a reference standard (histopathology, standard of care imaging, PSA response) that was determined by an independent, blinded, central expert panel. Efficacy endpoints included detection rate and positive predictive value. Additionally, the intended change in prostate cancer treatment due to the ⁶⁴Cu-SAR-bisPSMA results was recorded.

There were 52 patients that received ⁶⁴Cu-SAR-bisPSMA (safety set), two replacements (protocol deviations), 50 patients that proceeded to follow-up, 8 patients without a reference standard, and 42 patients with a reference standard (efficacy set). Only one adverse event was related to ⁶⁴Cu-SAR-bisPSMA (Grade 2 worsening of type II diabetes, resolved). The total number of lesions identified increased from Day 0 (70) to day 1 (129) across three readers, representing an 82% increase in detection:
The Day 0 detection rate range across the readers was 44–58% (95% CI 30.0–71.8), increasing on Day 1 to 58–80% (95% CI 43.2–90.0), representing a 34% increase in patients having a positive ⁶⁴Cu-SAR-bisPSMA scan on Day 1 (71%) versus Day 0 (53%) across the average of 3 readers. Pelvic lymph nodes had a Day 0 positive predictive value range of 71.4–87.5% (95% CI 29.0–99.7) and Day 1 of 50.0–61.5% (95% CI 15.7–86.1). The relative decrease in Day 1 positive predictive value was related to the challenges to obtain the reference standard for additional lesions identified on Day 1, where biopsy of all lesions was not feasible and due to the low sensitivity of current standard of care imaging that were used for co-localization:
Of note, ⁶⁴Cu-SAR-bisPSMA detects lesions in the 2 mm range:
Importantly, the ⁶⁴Cu-SAR-bisPSMA imaging led to clinicians changing their intended treatment plans in 48% of patients.

Written by: Zachary Klaassen, MD, MSc – Urologic Oncologist, Associate Professor of Urology, Georgia Cancer Center, Wellstar MCG Health, @zklaassen_md on Twitter during the 2024 American Society of Clinical Oncology (ASCO) Annual Meeting, Chicago, IL, Fri, May 31 – Tues, June 4, 2024.

Related Content: COBRA Trial Shows Promise for 64Cu-SAR-bisPSMA PET in Biochemical Recurrence – Neal Shore