⁶⁸Ga-PSMA-11 PET Imaging of Response to Androgen Receptor Inhibition: First Human Experience

⁶⁸Ga-PSMA-11 Labeling Procedure

⁶⁸Ga was obtained by eluting a ⁶⁸Ge/⁶⁸Ga generator (iTG) yielding radioactivity in the range of 555–1,110 MBq (15–30 mCi) during synthesis. The syntheses were conducting using an iQS ⁶⁸Ga fluidic labeling module (RadioMedix Inc.). Precursor (DKFZ-PSMA-11) or Glu-NH-CO-NH-Lyx(Ahx)-HBED-CC, where HBED-CC is N,N′-bis[2-hydroxy-5-(carboxyethyl)-benzyl]ethylenediamine-N,N′-diacetic acid, was purchased from ABX Advanced Biochemical Compounds. All reagents for formulating the final product were provided in a kit from ABX.

The precursor was dispensed into 5-μg aliquots dissolved in 100 μL of 0.25 M sodium acetate buffer and stored at −20°C until use. The precursor (made up to a total of 1 mL with sodium acetate buffer) was added to the preheated reaction vessel (100°C–110°C), and the generator was immediately eluted into the reaction vessel. The contents were heated for 5 min, after which they were pushed through an activated C18 Sep-Pak Light cartridge (Waters) and directed to a waste vial. The reaction vessel was further washed with 5 mL of saline, which was directed to a waste vial. The product was then eluted with 1 mL of 60% ethanol in water followed by 10 mL of saline and passed through a sterilizing filter. Typical yields are based on the total ⁶⁸Ga eluted and range from 85% to 90%.

Animal Experiment

Our prior data showing that upregulation of PSMA can be quantified with PET were acquired with ⁶⁴Cu-labeled J591. Because J591 is a full-sized monoclonal antibody with a long residence time in blood, it was not obvious whether changes in PSMA could be measured with a more rapidly clearing agent such as ⁶⁸Ga-PSMA-11. Therefore, we measured relative changes in PSMA expression with ⁶⁸Ga-PSMA-11 in mice bearing LNCaP-AR xenografts. Twelve 3- to 5-wk-old male nu/nu mice obtained from Charles River were studied: 4 treated with vehicle, 4 with orchiectomy, and 4 with apalutamide (ARN-509), which is a nonsteroidal competitive AR inhibitor. LnCAP-AR cells (5 × 10⁷) in a 1:1 mixture (v/v) of medium and Matrigel (Corning) were subcutaneously inoculated into one flank. Tumors were palpable within 20–30 d after injection. Surgical castration was performed according to our Institutional Animal Care and Use Committee–approved protocol under anesthesia with isoflurane. Separate arms were treated with daily oral gavage of vehicle or ARN-509 (10 mg/kg/d) for 7 d.

PET imaging was conducted using an Inveon small-animal PET/CT scanner (Siemens) at 2 time points, once before treatment and once 7 d after treatment. The mice were injected with approximately 7.4 MBq of ⁶⁸Ga-PSMA via the tail vein. One hour after injection, they were anesthetized with 2% isoflurane and imaged for 10–20 min to acquire approximately 20 million coincident events. In the ARN-509–treated cohort, the PET acquisitions took place approximately 6 h after the final gavage within an energy window of 350–650 keV and a coincidence-timing window of 3.432 ns. The data were converted into 2-dimensional histograms, and images were reconstructed by filtered backprojection. After the PET acquisition, a coregistered CT scan was acquired within approximately 10 min. SUV_mean was calculated for each xenograft using manually segmented regions of interest, and the ratio of SUV_mean after treatment to SUV_mean before treatment was calculated for each animal.

Human Experiment

The local institutional review board approved the study, and written informed consent was obtained from the patient, a 51-y-old man who had castration-sensitive metastatic prostate cancer with a Gleason score of 10. He had received no treatment of any kind before the first PSMA PET study. For ADT, he received a single 7.5-mg intragluteal injection of leuprolide acetate (Lupron; AbbVie). He also received 50 mg of bicalutamide (Casodex; AstraZeneca) daily for 30 d.

The patient was imaged on a 3.0-T Signa PET/MRI scanner (GE Healthcare) at two time points, once before initiation of ADT and again 4 wk (29 d) after initiation. Imaging began an average of 97 minutes after injection of 207 ± 22 MBq (mean ± SD) of ⁶⁸Ga-PSMA-11. A 6-bed-position whole-body acquisition was performed from the mid thighs to the vertex for 3 min at each bed position. The PET data were reconstructed using time-of-flight information, ordered-subset expectation maximization with 2 iterations and 28 subsets, and a matrix size of 256 × 256. The PET transaxial and z-axis fields of view were 600 and 250 mm, resulting in a voxel size of 2.3 × 2.3 mm. Axial slices were reconstructed at a 2.78-mm thickness. Attenuation was corrected using a 2-echo Dixon fat–water separation algorithm for the body, whereas the lung was segmented using a region-growing algorithm provided with the scanner (16).

All lesions capable of segmentation at both time points were segmented using a semiautomated method. SUV_max was calculated for each lesion. If a lesion was seen on only the pretherapy images or only the posttherapy images, the volume of interest was placed in the same location on both sets of images and the SUV_max was recorded. SUV_peak and SUV_mean were not calculated, because many lesions were seen at only one time point, and therefore comparing SUV_peak and SUV_mean across two imaging studies was not feasible. All measurements were performed using an Advantage Workstation (version 5.0; GE Healthcare).

Statistical Analysis

A paired Student t test was used to compare SUV_max before and after ADT in the patient and in the mouse xenografts. An unpaired Student t test was used to compare the change in mouse xenograft SUV_mean between the treatment groups and the vehicle group.