Regional Pharmacokinetics of [68Ga]Ga-PSMA-11 in Intermediate-to-High-Risk Prostate Cancer Patients Imaged Prior to Prostatectomy

Introduction: To characterize the tumor kinetics of [⁶⁸Ga]Ga-PSMA-11 in primary prostate cancer using a cohort of patients with intermediate-to-high-risk disease who were undergoing research PET/CT imaging (IND 141706) to assess [⁶⁸Ga]Ga-PSMA-11 utility in surgical planning.

Methods: Dynamic single-bed-position [⁶⁸Ga]Ga-PSMA-11 PET/CT imaging was performed on 27 patients (89 ± 16 kg) with biopsy-proven intermediate-to-high risk prostate cancer. All patients were already scheduled to undergo robotic prostatectomy with subsequent capture of whole-mount pathology. PET data were collected in list-mode from 0-55-minutes following i.v. [⁶⁸Ga]Ga-PSMA-11 administration (182 ± 4 MBq) with the patient’s pelvis and lower abdomen in the field-of-view. [⁶⁸Ga]Ga-PSMA-11 radiochemical purity averaged 99.4 ± 0.7%, and the total co-administered mass of PSMA-11 was estimated to be 3.5 ± 0.7 µg. Images were reconstructed by filtered-back-projection for quantitative analysis of regional radiopharmaceutical uptake and pharmacokinetics. Analysis included regions placed on 37 tumors within the 27 prostates, as well as regions for normal prostate, muscle, and blood (at bifurcation of the abdominal aorta).

Results: High tumor/background contrast was routinely obtained by 15-minutes post-injection. Regional uptake was quantified as a percentage of the injected dose per liter (%ID/L), rather than SUV, since there is no evidence of muscle or fat being significant reservoirs of radiopharmaceutical retention. Tumors exhibited two distinct patterns of uptake, either nearly plateauing by ~15-minutes with only modest further uptake (28 tumors; uptake values of 5.4 ± 1.4 and 6.2 ± 1.8 %ID/L at 15-minutes and 55-minutes, respectively), or exhibiting uptake that more dramatically increased throughout the imaging period (9 tumors; uptake values of 7.8 ± 2.1 and 11.9 ± 2.2 %ID/L at 15-minutes and 55-minutes, respectively) (Figure 1). (Factoring body mass into the quantification increases the observed variance in the tumor uptake values. Tumor SUV values for the low slope group averaged 5.0 ± 1.7 and 5.8 ± 2.3 at 15- and 55-minutes, respectively, while SUV values for the high slope tumors averaged 6.9 ± 2.4 and 10.7 ± 3.6 at 15- and 55-minutes, respectively.) The differing tumor uptake patterns were not associated with Gleason grade group.

Conclusions: In these primary prostate tumors, good tumor/background contrast was consistently obtained by 15-minutes following [⁶⁸Ga]Ga-PSMA-11 administration. There was significant heterogeneity in the observed tumor uptake patterns over the 15-to-55-minute timeframe. Most tumors showed only modest increases in tracer accumulation beyond 15-minutes; however, a subset of the tumors were observed to exhibit significant further radiopharmaceutical accumulation throughout the imaging period.

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