Detection of Prostate Metastases with a new Radiotracer

Medscape Medical News from the

Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2015 Annual Meeting

Medscape Medical News > Conference News
New Radiotracer Ups Detection of Prostate Metastases

Kate Johnson June 12, 2015



BALTIMORE, MD — A radioactive "minibody" that targets prostate-specific membrane antigen (PSMA) can detect metastatic prostate disease better than conventional imaging, new research suggests.

The engineered radiotracer "can assist in guiding biopsies to improve yield and can help identify occult sites of disease," reported Neeta Pandit-Taskar, MD, from Memorial Sloane Kettering Cancer Center in New York City, during a press conference here at the Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2015 Annual Meeting.

The minibody (IAB2M) is labeled with radioactive zirconium-89 to create a radiotracer known as Zr-89 Df-IAB2M, and is used with positron emission tomography–computed tomography (PET/CT).

"This offers the potential for direct imaging of metastatic prostate cancer," said Dr Pandit-Taskar. "To date, conventional imaging is limited in detecting prostate cancer metastases accurately and measurably. Using this agent, we can detect the prostate cancer cells that have metastasized to bone — one of the most difficult areas to evaluate using standard methods. We hope that this research will help us develop earlier and more effective detection of disease and assist in clinical decision-making."

The study used a variety of conventional imaging techniques as well as the experimental.

Researchers identified 393 metastatic prostate lesions (279 in bone and 114 in soft tissue) with a combination of CT, magnetic resonance imaging, and molecular bone scans, as well as PET, using the common radiotracer fluorodeoxyglucose radiotracer (FDG-PET) in addition to the new agent Zr-89 IAB2M (IAB2M PET/CT).

The new approach of using Zr-89 Df-IAB2M PET/CT imaging identified 25% more lesions than other imaging modalities, Dr Pandit-Taskar reported.

Specifically, it detected 82% of bone lesions compared with 57.7% detected by bone scans, 53% by CT, and 26.2% by FDG-PET.

Zr-89 Df-IAB2M PET/CT imaging also detected 86% of soft tissue lesions compared with the 65.8% identified with CT and the 48.2% identified with FDG.

Overall, 65 bone and 32 soft tissue sites were visualized exclusively with minibody imaging.

Working closely with interventional radiologists, the team used Zr-89 Df-IAB2M PET/CT imaging to improve prostate biopsy yield, explained Dr Pandit-Taskar.

"One of the advantages of using this minibody is that once you give the injection, you can image the patient several days later, even 10 days later, and the interventional radiologist can actually direct the biopsy needle to the site of the suspicious lesion," she said.

Of 19 sites biopsied, overall concordance for true-positive and true-negative disease was highest for Zr-89 Df-IAB2M PET/CT imaging (89.5%) vs FDG/PET imaging (84%) and bone scans (84%), she said.

This is a phase 1/2 trial, she added. "For the next few years, we will be exploring IAB2M in additional clinical trials. More data are needed to understand its clinical value, but if results are favorable, this imaging agent could play a critical role in the standard of care for prostate cancer."

Asked to comment on the findings, Steve Y. Cho, MD, associate professor, University of Wisconsin School of Medicine and Public Health, told Medscape Medical News that there are several classes of PSMA imaging radiotracers currently under clinical translation "demonstrating great promise for improved prostate cancer imaging."

In fact, his group has investigated 89Zr-Df-IAB2M, along with another new fluorine-18 labeled low-molecular weight inhibitor of PSMA (18F-DCFPyL).

Both radiotracers "show good PSMA PET targeting for bone and soft tissue lesions, and with markedly higher PSMA PET tumor detection rate compared to standard CT or bone scan," he said. The new study from Dr Pandit-Taskar and colleagues "was able to further demonstrate a high concordance with pathologic proof that sites of PSMA PET-positive metastatic disease showed true metastatic disease, which is an important finding."

"Overall, both agents validate PSMA-targeted PET imaging as a viable functional imaging strategy to improve the detection of metastatic prostate cancer for clinical management of prostate cancer patients," he said.

"What is even more exciting is that as robust functional PET agents, they also open the door for improved therapy response assessment and quantitation in prostate cancer. However, please note that there are now suddenly a plethora of prostate cancer PET and [single photon emission CT] imaging agents, and PSMA functional imaging of prostate cancer needs to be compared with and validated with these other emerging prostate PET radiotracers such as choline, [1 amino 3 18F-fluorocyclobutane-1-carboxylic acid], and gastrin-releasing peptide receptor (GRPR or bombesin) in systematic larger clinical trials for specific prostate cancer indications."

Dr Pandit-Taskar and Dr Cho have disclosed no relevant financial relationships.

Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2015 Annual Meeting: Abstract 400. Presented June 9, 2015.