Thursday, August 8, 2019

PSMA PET finds more cancer than Axumin

A PSMA PET scan (Ga-68-PSMA-11) detected more sites of cancer than an Axumin PET scan in the same recurrent patients. This prospective clinical trial was  conducted among 50 men at UCLA in 2018. All men had post-prostatectomy PSA from 0.2- 2.0 ng/ml.  The Calais et al. findings are summarized in the following table:




Ga-68-PSMA-11
Axumin
Detected - % of patients
56%
26%
Prostate bed
14%
18%
Pelvic lymph nodes
38%
8%
Extra-pelvic lesions
16%
0%

The two scans performed equally well at detecting recurrence in the prostate bed, but the PSMA PET scan was able to detect more cancerous pelvic lymph nodes and non-regional metastases. The surprising result is that more recurrences are attributable to pelvic lymph nodes (stage N1) or to extra-pelvic metastases (stage M) than to cancer in the prostate bed. If this is true of all recurrent men, it indicates that salvage whole pelvic radiation is usually preferred over salvage prostate bed radiation. We saw (see this link) that salvage whole pelvic radiation improved progression-free survival compared to salvage prostate bed-only radiation. But in that SPPORT trial, the authors noted that the improvement did not hold up at low PSAs. Even the best PSMA PET/MRI has a tumor size detection limit of about 4 mm. If cancer in the pelvic lymph nodes is still curable, it may be necessary to treat cancer while it is still undetectable.

The detection rate by PSA was as follows, but is based on small numbers of patients in each PSA group. The differences in the detection rates are statistically significant for PSAs over 0.5:


PSA (ng/ml)
Ga-68-PSMA-11
Axumin
0.2-0.5 (n=26)
46%
27%
0.51-1.00 (n=18)
67%
28%
1.01-2.00 (n=6)
67%
17%


The other PSMA-based PET scan, DCFPyL, has completed recruiting.

They will be submitting a new drug application to the FDA within the next few days, and if all goes well, FDA approval can be expected in about 9 months.

2 comments:

  1. I don't recall seeing such poor results for axumin in other trials. My impression was that at 2.0 they were both pretty efficient. Maybe the six subjects were all just a bit over 1.0?

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    1. You are right that these are very small numbers, especially at higher PSAs. It wasn't designed to show the detection rate for higher PSAs. Because there is a pressing need to decide on SRT at the lowest PSA possible, the goal is to find the imaging technique that works at the lowest PSA possible.

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