Targeting PSMA has been a historically difficult task. Researchers are now focusing on a group of urea-based small molecules that are tagged with 68Ga or 18F that appear to have a much stronger affinity to the PSMA protein. Ga-tagged PSMA agents have been more extensively used and studied, and much of what we know about PSMA scanning comes from use of Ga-based imaging. However, a number of 18F agents such as 18F-DCFBC & 18F-DCFPyL have more recently come into use. A 3rd-generation agent, 18F-PSMA-1007, is now available that appears to have the most promising imaging characteristics to date.
There are several reasons that 18F compounds may be superior to 68Ga compounds:
1. 68Ga requires on-site synthesis due to its short half-life, and this synthesis only produces 2-5 doses/day.
2. 18F has a 109 min half-life that allows for central production and national distribution, making it more economically viable.
3. Some data now exists to suggest 18F has superior image quality and sensitivity profiles.
4. The higher 68Ga positron energy makes PET images somewhat less clear/precise. This is because the path length for positrons is longer from 68Ga, translating to worse image quality.
5. 18F appears to be better at PSA ranges of 0.5 – 3.5. Above and below that interval, it doesn’t appear to matter if you use 68Ga vs. 18F.
6. 18F-PSMA-1007 appears to have the best imaging characteristics of all PSMA agents thus far, partly because of its low urinary tract uptake. A lot more data is needed on this agent, however.
Interestingly, a commercial entity has recently backed 18F-DCFPyL and is pursuing clinical trials with this imaging tracer. This likely means that it is bound for FDA approval in the next few years, which may open the door for other PSMA modalities down the road.
PSMA-PET imaging is a well-known and easily utilized clinical tool. Increasing the armamentarium of useful tracers will only increase this utility, and will help the field move forward with more precise tools for detection.
Presented by: Michael Gorin, MD. Johns Hopkins
Written by: Shreyas Joshi, M.D. @ssjoshimd, Fox Chase Cancer Center, Philadelphia, PA at the 18th Annual Meeting of the Society of Urologic Oncology, November 20-December 1, 2017 – Washington, DC