Diagnostic performance of 18F-rhPSMA-7.3 PET in newly diagnosed unfavourable intermediate to very high-risk prostate cancer

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Published: 16 Mar 2023
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Prof Tobias Maurer - Martini-Klinik Prostate Cancer Center, Hamburg, Germany

Prof Tobias Maurer speaks to ecancer about the results from the phase 3 LIGHTHOUSE study. The LIGHTHOUSE study evaluated 18F-rhPSMA-7.3’s diagnostic performance in men with newly diagnosed unfavourable intermediate to very high-risk prostate cancer who were scheduled for radical prostatectomy and pelvic lymph node dissection.

The study found that 18F-rhPSMA-7.3 PET provides high specificity and clinically useful information regarding the presence of N1 disease prior to surgery in this specific patient population.

Prof Maurer says that the trend of lower sensitivity is consistent within the class of PSMA-PET ligands.

He concludes by discussing the clinical impact of these results.

 

The LIGHTHOUSE study was a phase III prospective study from Blue Earth Diagnostics evaluating a special PSMA tracer, fluorinated PSMA radio-hybrid PSMA 7.3, that was the name of the tracer. It evaluated in patients and primary prostate cancer patients who were scheduled for radical prostatectomy and that had to have unfavourable intermediate risk features, at least, or high risk prostate cancer features. The primary endpoint, it was a split endpoint between sensitivity and specificity of this fluorinated radio-hybrid PCMA-7.3 tracer for detection of lymph node metastasis in the patient. Histology served, of course, as the gold standard.

I also have to say, with other PSMA tracers that are reported in the literature already, this tracer basically performs as good as the other tracers but increases significantly the detection when you look at conventional imaging. This was not really looked at in the study but when you look at the histopathology, the lymph nodes that you detect and the specificity of PSMA-PET and especially this tracer was also very high – 96% specificity for detection of lymph nodes. For the sensitivity the majority read was 24% which also is not bad because mostly these are small lymph node metastases.

So it confirms a little bit also the proPSMA study by Michael Hofman that also showed a very good high specificity. However, in this study by Michael Hofman only a little bit over 80% received radical prostatectomy and had histopathology as gold standard, also it included roughly 300 patients. In our study, in the LIGHTHOUSE study, we actually have the full dataset for 296 patients who after PSMA PET-CT underwent radical prostatectomy and extended pelvic lymph node dissection. So the quality of the data here is really robust. It, again, confirms the value of PSMA-PET for detection of lymph node metastases in primary prostate cancer. 

When we talk more specifically about this specific tracer, it’s very interesting, it’s a very interesting tracer as it can be labelled with different radiometals. In the study it was a fluorinated PSMA tracer which is really nice, gives you nice images compared to gallium-based tracers because of the positron range giving you the signal for the PET imaging. But also it can be labelled with, for example, radiometals that are used then for therapy. So you’re having the same specificity of the molecule detecting PSMA but on the other hand you can label it with different radiometals.

That is one point. The second point which is also very interesting specifically for this tracer, that it shows low renal excretion. So you have not a lot of bladder background or background in the pelvis because there is no activity. With several other tracers you do have renal excretion. This is, of course, more important for when you image biochemical recurrent prostate cancer patients where you might want to detect a local recurrence or rule it out. This was also looked at with another study, the SPOTLIGHT study, which is not presented now here. But generally these are two features that are very interesting for this tracer. Furthermore, fluorinating also enables to produce the tracer and bring it to other institutions for imaging. With gallium you have to produce it, basically, on site because of the low half-life.

How can these results impact the treatment of prostate cancer?

Now we do have advanced imaging, we detect disease earlier, we detect metastatic spread earlier. Also in this cohort roughly 20%, depending on the reader, there were three blinded independent readers in this study, also a good sign of a good study, I would say. But in 20% metastatic disease was detected which would not have shown up on conventional imaging most likely. So what do we do now with this new imaging showing us metastatic disease? For now we have to stick to the knowledge we have obtained from old conventional imaging and should not modify our treatment on these novel advanced imaging results too much. But we should also take it into consideration. We don’t know if we’re now treating better, but I think we can target the disease better. If this translates, then, into better outcomes, I would assume so however this data will not be available for the next 5-10 years or so. But it will impact definitely prostate cancer treatment.