Showing posts with label PSMA. Show all posts
Showing posts with label PSMA. Show all posts

Tuesday, August 11, 2020

PSMA-targeted radiopharmaceutical clinical trials in the US

Now that the VISION trial of Lu-177-PSMA-617 is no longer recruiting, some patients are wondering if they can still get PSMA-targeted radiopharmaceuticals in the US, without traveling to Germany, Australia, India, etc. Here is a list of trials that are active, still open to recruitment, or will soon be recruiting. 

Unless otherwise noted, they are all for men who are: 

  • metastatic
  • castration-resistant 
  • have had at least one taxane chemotherapy
  • at least one of the advanced androgen receptor therapies (e.g., Zytiga, Xtandi, Erleada, or Nubeqa)
  • no Xofigo
  • PSMA-avid on a PSMA PET/CT scan

Links that have contact information are provided.

Sunday, May 31, 2020

Lu-177-PSMA-617 vs Jevtana (cabazitaxel): which should I do next?

We saw recently (see this link) that of chemo and hormonal medicines for metastatic castration-resistant prostate cancer (mCRPC), Jevtana (cabazitaxel) is the preferred third treatment after Taxotere (docetaxel) and Zytiga (abiraterone) or Xtandi (enzalutamide). But when should radiopharmaceuticals, either approved ones like Xofigo (Ra-223), or prospective ones like Lu-177-PSMA-617, be used in the optimal sequencing?

Michael Hofman reported some early results of the TheraP randomized clinical trial (RCT) at the recent ASCO meeting. They randomized some well-selected patients to receive either Lu-177-PSMA-617 or Jevtana. Patients were selected according to the following criteria;
  • mCRPC (PSA≥20 ng/ml and rising)
  • must have had docetaxel
  • must had either Zytiga or Xtandi or both
  • healthy, with good liver, kidney and blood function
In addition, all patients received both an FDG PET scan and a PSMA PET scan. They were excluded from the trial if either:
  • Their metastases were insufficiently PSMA-avid - (10% excluded)
  • There were many metastases that showed up on FDG but not on PSMA PET scans (as described here) - (18% excluded)
  • 85 patients were treated with Jevtana
  • 98 patients were treated with Lu-177-PSMA-617

The endpoint used was the percent of patients whose PSA declined by at least 50% (PSA50) from baseline after the treatment. After a median follow-up of 13 months:
  • Lu-177-PSMA-617 had a PSA50 of 66% vs 37% for Jevtana
  • The percent who had PSA progression was 31% less in those getting Lu-177-PSMA-617 relative to those getting Jevtana
  • It is too early for data on overall survival
  • Serious/life-threatening adverse events occurred in 35% of those taking Lu-177-PSMA-617 vs. 54% of those taking Jevtana
  • The most common adverse events reported by those taking Lu-177-PSMA-617 were fatigue, dry mouth/eyes, low platelets, nausea, and anemia. Only 1 patient discontinued for toxicity.
  • The most common adverse events reported by those taking Jevtana were fatigue, diarrhea, nausea, loss of taste, neuropathy. dry mouth, and neutropenia, 5 patients discontinued for toxicity
Given the comparatively low toxicity, it seems like Lu-177-PSMA-617 should usually be the preferred third treatment, over Jevtana, although longer follow-up will be needed to see if there will be a survival difference.

This study further highlights the importance of getting both an FDG and a PSMA  PET scan at about the same time.

PSMA expression is highly variable. It is not expressed in low grade cancer in the prostate. Expression increases as metastases develop, reach a peak, and then decrease. PSMA expression also increases when second-line hormonals are first used, but then decreases with continued use. Given this variation over time and treatment, several questions about PSMA-targeted therapy remain unanswered:
  • Should it be used soon after second-line hormonals?
  • Should it be used before or soon after docetaxel?
  • Would the problem of heterogeneity be minimized if Jevtana and Lu-177-PSMA were given simultaneously?
  • Should it be used in minimally metastatic patients?
  • Should it be used in newly diagnosed metastatic patients?
  • Should it be used with immunotherapies (e.g., Provenge, Checkpoint inhibitors)?
  • Will PARP inhibitors enhance the cell-kill rate?
  • Is PSA the best biomarker of effectiveness?
  • What are the best radionuclides to use (e.g., Ac-225, Th-227)?
  • What are the best/most specific ligands to use? (e.g., PSMA-617, PSMA-I&T)
  • Are there better surface proteins to target, perhaps simultaneously (e.g., FAPI)
  • How do they compare to PSMA BiTE therapies?
  • How does it compare to Xofigo for bone metastases?

Sunday, December 15, 2019

Why Lutetium-177-PSMA treatment sometimes may not help, and may even harm

Lu-177-PSMA usually improves survival

We've seen in a couple of small trials in Germany and Australia that Lu-177-PSMA seemed to provide better than expected survival. In Germany, median overall survival was 12.9 months across 104 patients. In Australia, median overall survival was 13.3 months across 50 treated patients. In both trials, all or almost all patients had already received taxane chemotherapy and either enzalutamide or abiraterone. There was no control group in either trial, so we can only guess at what overall survival would have been without the therapy.

In the "ALSYMPCA" trial of Xofigo, among the subgroup of patients who had received docetaxel for their painful mCRPC (see this link),  median overall survival was 14.4 months with Xofigo vs. 11.3 months with placebo. The ALSYMPCA trial was conducted before abiraterone and enzalutamide were approved, so it is impossible to know how prior treatment with one of those might have changed survival.

In a recent trial of Jevtana as a third-line therapy, after docetaxel and either abiraterone or enzalutamide, median overall survival was 13.6 months for Jevtana vs. 11.6 months for the other second-line hormonal.

So, in heavily pre-treated patients, Lu-177-PSMA seems to improve survival about as well as Xofigo or Jevtana when used as a third-line therapy. We will get a better handle on the actual survival benefit when we get the results of the VISION trial next year.

PSA is not always a good indicator of effectiveness, as has been found for Xofigo and Provenge. Lu-177-PSMA reduced PSA in about 2/3 of treated patients in most studies. That leaves about 1/3 of patients who derived no benefit (even though they had PSMA-avid tumors), and waterfall plots showed that a few patients had large increases in PSA following PSMA-targeted therapy.

It is worth noting that the PSMA protein contributes to the survival of the cancer, and just the PSMA ligand that attaches to it has some activity in delaying progression, even without a radioactive component (similar to the way an anti-androgen attaches to the androgen receptor, delaying progression). It is also worth noting that ADT initially increases PSMA expression, but decreases its expression with continued use.

The opportunities are:
  • to select patients who are likely to benefit
  • give alternative therapies (like Jevtana) to patients who are unlikely to benefit
  • provide adjuvant therapies that may increase survival

PSMA avidity - optimal point in time

It has long been known that PSMA is a moving target. The advent of PSMA PET scans has enabled us to track PSMA expression. Cancers that express a lot of PSMA (called PSMA-avid tumors) can be distinguished from cancers that express very little. Radiologists determine avidity by comparing the uptake of the tracer in cells that express PSMA to the uptake of the tracer in cells known to not express PSMA. Early low-grade prostate cancer does not express PSMA at all. Higher grade prostate cancer may express some PSMA. PSMA expression really starts to take off when the cancer metastasizes, although it is highly variable between patients. About 90-95% of metastatic men express at least some PSMA on their prostate cancer cells. At some point, however, as genomic breakdown continues, PSMA is no longer expressed by metastases. Thus, there is an optimal point for treating each patient with PSMA-targeted therapy. Treatment too early or too late, may exert selective pressure on the predominant non-PSMA-types, allowing them to take over.

Michael Hofman and others at the Peter MacCallum Cancer Center in Melbourne (see this presentation and this link) have initiated several clinical trials using Lu-177-PSMA at earlier stages of disease progression:

  • #lutectomy trial (Declan Murphy,  PI) is treating PSMA-avid high-risk patients with Lu-177-PSMA, followed by prostatectomy and pelvic lymph node dissection
  • #upfrontPSMA (Arun Asad, PI) is treating patients first diagnosed with high volume metastases with Lu-177-PSMA + ADT + docetaxel vs ADT + docetaxel.

Other opportunities for early use include Lu-177-PSMA treatment for those in the following settings:
active surveillance

  • persistent PSA after prostatectomy
  • salvage treatment after first recurrence
  • salvage treatment after second recurrence
  • metastatic CRPC before docetaxel or advanced hormonal therapies
  • non-metastastic (on bone scan/CT) CRPC before docetaxel or advanced hormonal therapies

Centers in Germany may be willing to treat patients per protocol (i.e., outside of a clinical trial) in some of those situations.


In radiobiology, one of the ways in which radiation can fail to destroy cancer is called repopulation. It means that when radiation kills some cancer cells but leaves many behind, the remaining ones now have access to space in which to expand and access to nutrients and oxygen that the other cancer cells had deprived them of. Paradoxically, the tumor can then grow faster than it ever would have before the treatment. This is sometimes seen with rapidly growing tumors, as some head and neck cancers. They sometimes irradiate those cancers multiple times a day to prevent repopulation.

Repopulation is never seen with X-ray (or proton) treatment of relatively slow-growing prostate cancers. X-rays penetrate throughout the prostate and kill all the cancer there. If there is any survival of an oxygen-deprived tumor core, it will be killed by the next fraction of X-rays in a day or two. However, Lu-177 emits beta rays that may only penetrate to about 125 cells around each target. Ac-225 (also sometimes used in PSMA therapy) only kills about 8 cells around each target. With such short-range killing, there is a real danger of repopulation if there are insufficient PSMA targets within the tumor. Multiple treatments are usually not given for several weeks, and the tumors may have changed by then.

PSMA heterogeneity

What we have learned recently is that not only does PSMA expression change over time, but in a given patient, some tumors may express PSMA and some may not. Moreover, even within a single tumor, some cells may express PSMA and some may not.

Paschalis et al. looked at the degree of PSMA expression of 60 patients with metastatic castration-resistant prostate cancer (mCRPC). They also looked at tissue samples of 38 of them taken when they were diagnosed with hormone-sensitive prostate cancer (HSPC). To detect the amount of PSMA expressed, they used an antibody stain that attaches to the part of the PSMA protein that lies above the cellular membrane. They rated the tumors "0" if there was no PSMA up to "300" if all cells expressed PSMA. They also performed a genomic analysis, looking for mutations in over 100 genes associated with DNA-repair defects.

Among the tumor samples from men with HSPC they found:
  • 42% of the 38 men with HSPC  had no PSMA at diagnosis - it only emerged later
  • 5 of the 6 HSPC men diagnosed with Gleason score 6 or 7 had little or no PSMA expression at that time
  • About half of 30 HSPC men diagnosed with Gleason score 8-10 had little or no PSMA expression at that time
  • Those who expressed PSMA had a worse prognosis
  • Expression of PSMA varied greatly (heterogeneous) between patients
  • Expression of PSMA varied greatly between biopsy samples from the same patient
  • The higher the PSMA expression in a patient, the greater the amount of PSMA heterogeneity
Among the tumor samples from the 60 men with mCRPC they found:
  • PSMA expression had increased from when they were diagnosed with HSPC
  • Half of the tumors with no PSMA at HSPC diagnosis continued to have no PSMA
  • 73% expressed PSMA; 27% did not - only 1 of whom had neuroendocrine prostate cancer
  • 84% of those expressing PSMA exhibited marked PSMA heterogeneity
  • Heterogeneous patterns were identified:
    • PSMA positive and negative cells interspersed in a single area
    • PSMA-positive islands in a sea of PSMA-negative cells
    • PSMA-positive regions separated by >2 mm from PSMA-negative regions
    • Some metastases wholly PSMA-positive, some wholly PSMA-negative in the same patient
  • Bone and lymph node metastases had similar PSMA expression; liver metastases (none neuroendocrine) had lower PSMA expression
Analysis of DNA-repair defects revealed:

  • mCRPC patients with DNA-repair defects had higher PSMA expression
  • HSPC patients without DNA-repair defects were less likely to become PSMA-positive
  • Patients treated with PARP inhibitors were more likely to respond if they were PSMA-positive
  • For validation, in a separate sample of tumors, those with DNA-repair defects were found to have much higher PSMA expression than those without such defects. This was especially true for somatic mutations in BRCA2, ATM, and dMMR.
  • PSMA was downregulated in androgen-independent basal cancer cells (resistant to advanced anti-androgens) and neuroendocrine cells.

The significance of this study is that it may explain why about a third of PSMA-avid patients do not respond to Lu-177-PSMA therapy. The emitted beta particles may kill cells within about 125 cells from where they are attached at the PSMA site. Thus cells that do not express PSMA that are more than 2 mm from a PSMA-avid site will not be killed (see "Repopulation" above).

The authors hypothesize that DNA-damage repair defects cause PSMA to proliferate. If they are right, a PARP inhibitor (like olaparib), which has also been found to be effective when there are DNA-repair defects (see this link), may be able to increase the efficacy of PSMA treatment. This is the subject of an ongoing clinical trial.

Practical detection of heterogeneity/ clinical trials

Now that we know that heterogeneity can impact Lu-177-PSMA effectiveness, it behooves us to find a way of determining the degree of heterogeneity without doing a biopsy of every single metastatic site. One way is to give each patient two PET scans, so they could see the sites that exhibited PSMA expression as well as the sites that exhibited high uptake on an FDG PET scan.

It is futile to offer PSMA-targeted therapy if there are many sites that show up only on an FDG PET scan but few sites that display uptake of PSMA. It also may be futile to treat patients that show some sites where PSMA and FDG sites do not overlap - "discordant." On the other hand, where there is a high degree of overlap between FDG and PSMA - "concordant" - the PSMA radiotherapy will kill both cancers simultaneously. Of course, the ideal candidate would display only highly PSMA-avid sites.  Thang et al. reported on the survival of 30 patients who were treated with Lu-177-PSMA (who were either high PSMA/low FDG or concordant, compared to 16 patients who were excluded based on lack of PSMA (8 patients) or a high degree of discordant sites (8 patients). All patients were heavily pretreated.

  • Treated patients survived 13.3 months (median)
  • Untreated patients survived 2.5 months (median)

It is unknown whether the survival of the excluded patients might have been longer or shorter had they received treatment. It is possible that discordant patients may benefit from sequenced (before or after) or concomitant treatment with:
It is possible that such adjuvant treatment may decrease the population of discordant sites, and minimize repopulation effects.

Based on this new knowledge, it is recommended that patients who are good candidates for Lu-177-PSMA therapy have both a PSMA PET/CT scan and an FDG PET/CT at around the same time. FDG PET scans are generally covered by insurance; PSMA PET scans are not covered by insurance yet.

Thursday, July 26, 2018

F18-PSMA-1007 - the latest PSMA-based PET indicator

The development of new PET indicators for prostate cancer continues. As we've seen, the Ga-68-PSMA-11 indicator is already making a difference in clinical practice. Many of the new PET indicators have been developed in Germany, although the best one so far before this, F18-DCFPyL was developed at Johns Hopkins.

Researchers in Germany have developed a new PSMA-based PET indicator, F18-PSMA-1007, that seems to be even better. They tested it on 251 biochemically recurrent (after prostatectomy) patients at 3 academic centers.

  • 81% had a recurrence detected
  • 44% had a local (prostate bed) recurrence
  • 41% had a pelvic lymph node recurrence
  • 20% had a retroperitoneal lymph node recurrence
  • 12% in lymph nodes above the diaphagm
  • 40% had bone metastases
  • 4% had visceral organ metastases

Detection rates varied by PSA:

  • 62% in those with PSAs from 0.2-<0.5
  • 75%  in those with PSAs from 0.5-<1.0
  • 90%  in those with PSAs from 1.0-<2.0
  • 94%  in those with PSAs >2.0

Interestingly, those who had ADT in the last 6 months had higher detection rates (92%) compared to those who'd had no ADT recently (78%). This may be because those who had ADT recently had more advanced tumors. There was some early evidence in mice and lab studies (like this one and this one) that ADT upregulated PSMA. One clinical study indicated that ADT improved detection of PSMA. Two studies  (this one and this one) showed no effect of ADT on PSMA detection. More recent evidence indicates use of ADT negatively impacts detection rates. The patient should avoid ADT before getting a PSMA-based PET scan, if possible.

The detection rate among those with PSAs between 0.2-2.0 was 78%, which is comparable to the 88% detection rate reported for men with PSAs between 0.2-3.5 for F18-DCFPyL and much better than the detection rate of 66% reported for Ga-68-PSMA-11 in that PSA range. F18 has an advantage over Ga-68 in having a longer half-life (118 minutes vs 68 minutes) and is more tightly bound to the ligand. Because it is not appreciably excreted through the urinary tract, it can be seen more easily around the prostate - important when the recurrence is near the site of the anastomosis, as most recurrences are. In a mouse study, it was superior to F18-DCFPyL. In a clinical pilot study, they both detected the same tumors.

As of now, the F18 PSMA-based PET indicators seem to be superior, but others are working on ligands that detect other prostate cancer proteins more sensitively and more specifically. Leading candidates are hK2, FMAU, Citrate, Prostate-Stem-Cell-Antigen, , DHT/androgen receptor, uPAR receptor, VPAC receptor, or multiple ligands.

Also see:

Monday, October 16, 2017

Does Lu-177-PSMA-617 increase survival?

We have enthusiastically reported the encouraging outcomes of the early clinical trials of the radiopharmaceutical Lu-177-PSMA, most recently at this link. Based on reduction in PSA, it performs well. But medicines have no real benefit if all they do is treat PSA. We want medicines that increase survival.

Rahbar et al. reported the outcomes of 104 patients treated with Lu-177-PSMA-617 at University Hospital Muenster, Germany. All patients had metastatic castration-resistant prostate cancer (mCRPC) and had already received docetaxel and at least one of abiraterone or enzalutamide. After the first of an average of 3.5 cycles, they had the following outcomes:
  • 67% of patients had some PSA decline
  • 33% of patients had a PSA decline of at least 50%
  • Median overall survival was 56 weeks (13 months)
The authors conclude:
177Lu-PSMA-617 RLT is a new effective therapeutic and seems to prolong survival in patients with advanced mCRPC pretreated with chemotherapy, abiraterone and/or enzalutamide. 
But is this conclusion justified? It's hard to know without a prospective clinical trial where patients are randomized to receive the radiopharmaceutical or standard-of-care. The best we can do is look at the overall survival from clinical trials involving patients with symptomatic mCRPC. In the "ALSYMPCA" trial of Xofigo, among the subgroup of patients who had received docetaxel for their painful mCRPC (see this link), overall survival was:
  • 14 months with Xofigo
  • 11 months with placebo
The ALSYMPCA trial was conducted before abiraterone and enzalutamide were approved, so it is impossible to know how prior treatment with one of those might have changed survival. There have been a couple of small trials of "third-line" medicines after docetaxel and abiraterone were used.

In a non-randomized trial among 24 mCRPC patients after treatment with docetaxel and abiraterone, overall survival was:
  • 9 months with cabazitaxel
In a Danish study among 24 mCRPC patients after treatment with docetaxel and abiraterone, overall survival was:
  • 5 months with enzalutamide
So these data suggest that Lu-177-PSMA-617 may have prolonged life more than third-line treatment with another taxane or another hormonal agent. However, we expect much cross-resistance between abiraterone and enzalutamide, and resistance building up with prolonged use of taxanes. It is always hazardous to compare patient outcomes or declare success when they have not been randomized. Certainly there is enough suggestive data to warrant a Phase 3 randomized clinical trial.

Tuesday, September 5, 2017

A new Lu-177-PSMA ligand has good results in a new study

Targeted nuclear medicine has shown some impressive outcomes in several small studies, mostly conducted in Germany. Most of the studies have used a radioactive beta-particle emitter, Lutetium 177, attached to a ligand that has high and specific affinity for prostate cancer cells. Most medicines developed for this purpose have a ligand that attaches to Prostate-Specific Membrane Antigen (PSMA), a protein found on 90% of all prostate cancer cells. The ligand for Lu-177-PSMA has to have a "grappling hook" on one end (called a chelator) that holds onto the Lu-177. On the other end is a "magnet" of sorts that binds tightly to the PSMA. The beta particles then kill the cell that the ligand attaches to and some nearby cells as well.

There are also ligands that attach to prostate cancer proteins other than PSMA, and radioactive elements other than Lu-177 that are in clinical trials. This is a rapidly developing field.

The new ligand is called PSMA-I&T (imaging and therapy) or sometimes PSMA-DOTAGA. The ligand used in most of the other studies was PSMA-617 (also known as PSMA-DKFZ) or PSMA-J591. The ideal ligand attaches strongly to PSMA in prostate cancer tumors and to nothing else. Importantly, it should not accumulate in the kidneys to a great extent because it could damage them.

Last year, the Central Clinic of Bad Berka, Germany reported on 56 patients treated with Lu-177-PSMA-I&T (see this link). 80% of treated patients had a PSA response and toxicity was minor. Heck et al.  at the Technical University of Munich reported on 19 metastatic castration-resistant patients who were treated with 7.4 GBq per cycle and up to 4 cycles.
  • In 56%, PSA decreased by at least 30%
  • In 33%, PSA decreased by at least 50%
  • In 11%, PSA decreased by at least 90%
  • Complete remission of metastases in 5%
  • Metastases stayed stable in 63%
  • Metastases progressed in 32%
  • Performance status was stable or improved in 74%
  • In those with bone pain, it was reduced partially or completely in 58%
  • Mild (Grade 1 or 2) toxicities included dry mouth (37%), anemia (32%), and platelet loss (25%)
  • There were no severe (Grade 3 or 4) toxicities.
  • There was no kidney toxicity up to 40 GBq (see this link)
(Update 11/2018) Heck et al. updated the above with information on 100 patients. They were heavily pre-treated with a median of 3 pre-treatments. In fact, they were required to have had Zytiga or Xtandi, and at least one cycle of taxane chemo. They were all mCRPC and 35% had visceral metastases. They may have had up to 6 cycles of Lu-177-PSMA-617 (average was 3.2 cycles).
  • In 38%, PSA decreased by at least 50%
  • Median clinical progression-free survival was 4.1 months
  • Median overall survival was 12.9 months
  • Treatment-emergent hematologic grade 3/4 toxicities were anemia (9%), thrombocytopenia (4%), and neutropenia (6%)
A meta-analysis looked at the PSMA-I&T and PSMA-617 ligands in relation to the PSMA-J591 ligand. With a combined sample size of 369 patients across 10 studies, Calopedos et al. reported that:

  • 68% of patients had some PSA decline
  • 37% of patients had a PSA decline of at least 50%
  • More patients had a PSA decline with the PSMA-I&T and PSMA-617 ligands, but there was a wide range of outcomes

These early indicators look good. Even if it just stabilizes performance status and mitigates bone pain in these end-stage patients, there is an important benefit. Of course, what we really want to see is evidence that it increases overall survival

While PSMA-I&T was developed to be a good ligand for imaging purposes as well as therapeutic purposes, a recent study found that, when used with Ga-68 (a positron emitter), PSMA-HBED-CC (also known as PSMA-11) was slightly better at detecting metastases (see this link). Another PSMA ligand, DCFPyL, that incorporates the positron emitter F18 into the ligand more tightly (avoiding chelation, which can easily be reversed), seems to be superior to the Ga-68-PSMA-HBED-CC PET tracer (see this link). Both DCFPyL PET and Ga-68-HBED-CC PET are in numerous clinical trials in the US and Canada. Lu-177 is a gamma emitter that can be seen by a gamma camera or via SPECT. However, it is usually used in conjunction with a positron-emitter in order to obtain a superior image.

Readers may wish to read these other articles on this subject:

Will Lutetium-177-anti-PSMA be the next Xofigo?
Lu-177-PSMA update
Lu-177-PSMA: another update
First in-human trial of Actinium-225-PSMA-617
Ac-225-PSMA-617 extends survival (update)
Ac-225-PSMA-617 (update)
I-131-MIP-135, a new radiopharmaceutical, in clinical trial at Memorial Sloan Kettering

Friday, January 27, 2017

I-131-MIP-1095, a new radiopharmaceutical, in clinical trials at Memorial Sloan Kettering

There are few radiopharmaceuticals in clinical trials in the US (there are several in use in Germany), so when a new one is announced, we take notice. I-131-MIP-1095 has had a very limited clinical trial in Germany in 28 patients, and will now be tried in the US.

Like Lutetium 177, Iodine 131 is a beta particle emitter (see this link). It's beta particle energy is somewhat higher, so that it can penetrate greater distances through tissue - up to 3.6 mm, compared to 1.9 mm for Lu-177. This is an advantage in that it can destroy larger tumors, but it is a disadvantage in that it may destroy more healthy tissue, causing hematological and renal side effects. It is also similar to Lu-177 in that its uptake in human tissues can be detected using a gamma ray camera or SPECT detector. Because gamma ray detection does not afford the image quality that PET/CT does, it may be combined with a positron emitter, I-124. Lu-177 is sometimes combined with Ga-68 for the same purpose. This combination of therapeutic and diagnostic (sometimes called theranostic) may be useful in tailoring the dose to the patient based on individual uptake characteristics.

The molecule (or ligand) that the I-131 is attached to is MIP-1095. MIP-1095 is attracted to the PSMA protein on the surface of 95% of prostate cancer cells. Although it is highly specific for prostate cancer, there are other tissues that express PSMA, especially the salivary glands and lacrimal glands. It is excreted by the liver and kidneys, and may show up in the intestines, and the lower urinary tract. The dose to the kidneys may limit the amount of the pharmaceutical that may be given to the patient.

A group from the University Hospital Heidelberg, Zechman et al., treated 28 metastatic castration-resistant patients with I-131-MIP-1095 with the following results:

  • In 61%, PSA was reduced by >50%. This is better than the response seen with Lu-177-PSMA-617 in these trials and in this one.
  • PSA decreased in 21 of 25 patients, increased in 4.
  • 85% had complete or moderate reduction of bone pain. 
  • 25% had a transient slight to moderate dry mouth, which resolved in 3-4 weeks.
  • White blood cell count, red blood cell count and platelets declined during treatment, but there were only 3 cases of grade 3 hematologic toxicity, often in patients with low blood counts at baseline.
  • No renal toxicity was observed.
  • The effective dose to cancer cells was higher than for Lu-177-PSMA-617, red marrow and kidney doses were similar, and liver dose was lower.

The clinical trial that is now recruiting at Memorial Sloan Kettering, is a Phase 1 trial to find the best dose of I-131-MIP-1095 among patients with metastatic castration-resistant prostate cancer. Doses will be administered 12 weeks apart for up to 5 cycles or until dose-limiting toxicity is observed (monthly assessments). Interested patients in the New York City metropolitan area should call the contacts listed on the bottom of this trial description.

Sunday, December 11, 2016

PET scans for prostate cancer

In the last few years there has been an explosion in the number of new PET scan indicators. I thought it a good idea to provide some background and an update.

Bone scans

PET scans may be understood as an improvement over bone scans. The traditional way of finding distant metastases is to use a technetium bone scan and CT. There are several problems with bone scans:
  • they show bone overgrowth, which may be bone metastases, but may just be arthritis or old injuries
  • only a bone biopsy can tell for sure, and it's not often feasible when the suspected mets are small or inaccessible
  • they reveal few mets when PSA is below 10-20 ng/ml or when PSA is stable
  • they only show bone mets, not soft tissue
The main advantage is that they are relatively inexpensive.

The principal uses are: 
  • to rule out bone mets in high risk patients prior to curative treatment
  • to diagnose metastases that may respond to chemo, Xofigo, or spot radiation
  • to track response to treatment among metastatic patients.


Inherent limitations

The new PET scans are better than previous ones in terms of the size of the metastases they can detect, but they do not detect all metastases.

  • A cancer cell is many times smaller than the resolution of the CT or MRI. 
  • The activity of the cancer cell seems to influence whether it is detectable on any of the scans. 
  • There is "noise" in even the most specific tracer, with no sharp delineation between signal and background.

Salvage Radiation

The most important use of these new PET scans is to rule out salvage treatment when it would be futile. For men who have persistently elevated PSA after prostatectomy, or who have had a recurrence (nadir+2) after primary radiation treatment, a PET scan showing distant metastases can spare the man the ordeal and side effects of salvage treatment.

The FDA has approved Ga-68-PSMA-11 PET/CT for detection of recurrences after prostatectomy or radiation. They have also approved Axumin PET scans and C-11 Choline PET scans (at Mayo) for this purpose.

For salvage after primary radiation failure, it is necessary to locate areas within the prostate where the cancer may still be localized.  Memorial Sloan Kettering and the Mayo Clinic have effectively used PET scans to target areas within the prostate for salvage focal ablation or brachytherapy.

For salvage radiation after prostatectomy, it may be possible to identify areas of the prostate bed where spread is evident. While the entire prostate bed must be treated (most of the prostate cancer is below the limit of detection of even the most accurate PET/MRI scan), some radiation oncologists like to provide an extra boost of radiation to the detected cancer foci.

There has been accumulating evidence in the last few years (see this link) that very early salvage radiation treatment may improve salvage radiation outcomes over waiting until the PSA has risen above 0.2 ng/ml. Unfortunately, none of our PET indicators are any good at detecting metastases when PSA is below 0.2 ng/ml. It is unlikely that there are any distant metastases when PSA is that low, but there are some relatively rare forms of prostate cancer that metastasize without putting out much PSA. This leaves the patient without any assurance that salvage radiation will be successful. Perhaps the new PORTOS genetic test will be able to detect distant metastases biochemically, but this remains to be proven.

Pelvic Lymph Node (LN) Treatment

For men diagnosed with high risk prostate cancer, a difficult question is whether the pelvic lymph nodes ought to be treated, either with radiation or with pelvic lymph node dissection. Nomograms based on disease characteristics are used to determine whether the pelvic LNs merit treatment, but such nomograms are often inaccurate. A CT scan can sometimes identify lymph nodes enlarged (>1.2 cm) due to cancer. However, some LNs are only slightly enlarged (0.8-1.2 cm), and some cancerous LNs are not enlarged at all (<0.8 cm). LNs are usually enlarged by infection, so size alone is not a good indicator of cancer. An advanced PET scan can sometimes detect cancerous LNs. This may aid the decision on whether to have whole pelvic treatment for men with high risk cancer. Men who have already had radical prostate therapy that may have included radiation (primary or salvage) to other areas (i.e., the prostate or prostate bed) may face a similar decision as to whether to treat the pelvic LNs with radiation.

Just as it is necessary to irradiate the entire prostate bed and not just the detected foci when giving salvage radiation after prostatectomy, it is probably necessary to treat the entire pelvic LN area, and not just individual LNs, when cancer is detected anywhere in the pelvic LN area. Of course, such a decision must be balanced against the risk of side effects. There are ongoing clinical trials (RTOG 0534 for salvage therapy and RTOG 0924 for primary therapy) to determine whether such treatment provides any survival advantage when LN involvement is suspected. The STAMPEDE trial included an arm where patients were node positive (but negative for distant metastases) and were treated with radiation. Short term follow-up demonstrated an improvement in failure-free survival of 52% among those who had treatment.

Oligometastatic radiation of distant metastases

Although some have theorized that there is a stage in prostate cancer metastatic progression where the cancer is still curable, or where it can be delayed by removal of 1-3 detectable metastases, this theory has never been proven. In fact, a meta-analysis this year (see this link) showed that metastatic progression continues in almost all men despite such treatment. The possibility remains that progression may be slowed by spot treatment, although this remains uncertain as well. The natural history of metastatic progression is often very slow in early stages, with years between the first few metastases. The reason for this may be because the tissue in metastatic sites must first be biochemically transformed by signals from micrometastases in order to accommodate the growth of larger metastases. This preparation of fertile "soil" in which metastatic "seeds" can grow may take some time. PET scans for detection undoubtedly introduce lead-time bias into the calculation; i.e., the time between the first and second detected metastasis is certainly longer because a more sensitive PET scan was used, and not necessarily because the first detected metastasis was spot-treated.

In spite of the uncertainty concerning efficacy of spot treatment, patients often want to treat whatever can be detected. When such metastases are detected with sensitive PET scans and are in locations amenable to spot treatment with SBRT, and there is minimal risk of radiation damage to nearby organs, it is hard to argue against such use. However, the patient should understand that there is so far no evidence that such treatment will provide any benefit. He should also understand that detectable metastases in distant sites means that his cancer is systemic. There are thousands of circulating cancer cells and undetectable cancer cells already lodged in tissues. For this reason, it is never a good idea to delay systemic therapy (e.g., hormone therapy) in order to wait for PSA to increase to a point where metastases become detectable on a PET scan. The TOAD randomized clinical trial suggested that immediate hormone therapy at the first sign of recurrence after curative options were exhausted cut 5-year mortality in half compared to waiting for PSA to rise and metastases to become detectable.

Palliative treatment of metastases

Metastases can cause pain and interference with organ function. Bone scans can find larger bone metastases, and they are the ones most apt to cause pain, fracture, or spinal compression. Metastases in weight-bearing bones may be spot-radiated with SBRT to prevent such problems and to relieve pain. In the unusual event that a bone scan can't locate them accurately enough for SBRT treatment, a PET scan may be used.

Bone scans do not detect metastases in soft tissue, while most PET scans (other than the NAF18 PET) can. A PET scan may locate metastases in organs that may be biopsied or treated with radiation or other therapies, like embolization or ablation.

Multiple metastases

The CHAARTED study has taught us that prostate cancer with multiple distant metastases behaves in a different way and reacts to different therapies compared to prostate cancer with a low metastatic burden. Although the metastatic burden in the CHAARTED study was based on bone scan and CT, there may be a potential to identify patients who may respond to earlier systemic therapy if a PET scan were to be used. This use has yet to be explored.

Tracking success of treatments (radiographic progression)

PSA is not always the best measure of whether a treatment is successful and ought to be continued. Because they destroy cancer cells, some therapies may actually raise the PSA level for some time immediately following treatment. Chemotherapy does not always immediately reduce PSA, but the patient wants to know whether the potentially toxic treatment should be continued. Most of the time, serial bone scans can provide an adequate radiographic assessment. However, in patients with low PSA or low metastatic burden, serial PET scans may sometimes provide a more accurate assessment.

Initial detection, active surveillance, focal therapy, dose painting

Just as multiparametric MRIs can be used to detect significant prostate cancer when suspicion remains after a first negative biopsy, a PET scan can conceivably be used for such a purpose (as in this clinical trial). PET scans can also be used to track progression of prostatic foci in patients on active surveillance. It is hard to justify the cost for such purposes, and there is as yet no evidence that it is any better than a multiparametric MRI. In light of the recent evidence that multiparametric MRI may fail to delineate up to 80% of detected prostatic index tumors, they may find future use of PET/MRI in contouring treatment areas for focal ablation and for dose painting (see this link).

The FDA has approved Ga-68-PSMA-11 PET/CT at UCLA and UCSF for unfavorable risk prostate cancer.

How PET scans work

Positron Emission Tomography (PET) is a way of creating a 3D anatomical image. Instead of using X-rays, as a Computerized Tomography (CT) scan does, it detects positrons, which are positively charged electrons (which do not exist in nature). When a positron encounters a normal negatively-charged electron, they annihilate each other and release 2 gamma rays in opposite directions. When the machine detects such a pair of gamma rays, it extrapolates their source position, putting an image together. The PET scanner is combined with a CT scanner in the same device in order to provide anatomic detail.

As a cautionary note, PET scans do expose the patient to significant amounts of ionizing radiation from both the PET indicators and the simultaneous CT scan. It is not something one wants to do frequently.

PET emitters are short-lived radioisotopes that are created in a nearby cyclotron. Commonly used ones include carbon 11 (C11), fluorine 18 (F18), gallium 68 (Ga68), copper 64 (Cu64), zirconium 89 (Zr 89), and iodine 124 (I124). The choice of which one to use is based on cost, access, half-life, strength of signal, and ease of integration with the ligand. C11, for example, has an extremely short half life of only 21 minutes. This means it has to be manufactured very nearby where it will be incorporated into a ligand (like acetate or choline), and must be used immediately. F18 has a longer half-life (118 minutes) and has excellent detectability, but interferes somewhat with metabolism of acetate or choline. I124 has a long half-life (4.2 days) which may be too long for a patient who may have to remain isolated for the duration.

PET emitters are chemically attached (chelated) to molecules, called ligands, that have particular affinity for prostate cancer cells. Some ligands are metabolically active, meaning they are food for the cancer cell, but not as much for healthy cells. Other ligands are created to attach to specific binding sites on the surface or inside prostate cancer cells. Sodium fluoride (NaF18) replaces hydrogen with positronic fluoride when hydroxyapatite, the mineral that constitutes our bones, is actively accumulating in bone metastases.

Metabolic ligands

Because rapidly growing cancer cells metabolize a lot of glucose, fluoro-deoxy-glucose (FDG) has long been used in PET scans for cancer. Prostate cancer in its early stages does not metabolize glucose readily, so FDG can't be used until later stages.

Prostate cancer cells do metabolize fats, consuming choline and acetate. C-11 choline and acetate overcomes the interference problem of F-18 choline and acetate, but is very difficult to work with. Although the FDA has approved the C-11 Choline PET, only the Mayo Clinic offers it in the US. A few sites offer the C-11 Acetate PET, but it is expensive. It also requires a fairly high PSA, ideally ≥ 2.0 ng/ml, or fairly large metastases, to detect anything.

Fluciclovine has recently been FDA approved. It is incorporated into prostate cancer cells as part of amino acid metabolism. It can detect somewhat smaller metastases at lower PSAs.

PSMA ligands

95% of prostate cancers express a protein called Prostate-Specific Membrane Antigen (PSMA) on the surface of cells. There are a variety of ligands that are attracted to it. Some of the ligands are antibodies (like J591), some are shortened antibodies (called minibodies, like Df-IAb2M), and some are small molecules (peptides, like PSMA-HBED-CC or DCFPyL). PSMA-targeted ligands may accumulate in salivary glands, tear glands and kidneys, and urinary excretion may interfere with readings in the prostate area. PSMA has also been found on the cell surface of some other kinds of cancer. New PSMA ligands are still being developed and tried. So far, the one that seems to have the highest specific affinity for PSMA are the F18-based ligands (F18-DCFPyL, F18-PSMA-1007, and F18-rh-PSMA-7). They detect more metastases at lower PSA than the others. 

Other ligands

There are other prostate cancer-specific molecules for which ligands have been developed and to which positron emitters have been attached.  One of the most promising is the bombesin or RM2 ligand that attaches to the gastrin-releasing peptide receptor (GRPR) on the prostate cancer cell. In pre-clinical studies, it outperformed C-11 Choline. Clinical trials have started. Clinical trials have begun on several PET ligands that are designed for other receptor sites: human kallikrein-related peptidase 2 (hK2), FMAU, Ga-68-Citrate, I-124-Prostate-Stem-Cell-Antigen, Ga-68-DOTATATE (Somatostatin receptor), F18-DHT (androgen receptor), Cu-64-DOTA-AE105 (uPAR receptor), Cu-64-TP3805 (VPAC receptor). or multiple radiotracers.

The winner (so far) is...

Based on clinical trials, below are various PET indicators in approximate rank order of their sensitivity to detect recurrent prostate cancer, and their specificity for detecting it exclusively:
  1. F18-PSMA-1007
  2. F18-rhPSMA-7
  3. F18-DCFPyL
  4. F18-DCFBC
  5. Ga68-PSMA-HBED-CC (Ga68-PSMA-11)
  6. Fluciclovine (F18 - FACBC)/ Axumin
  7. C11-Choline/ C-11-Acetate
  8. F18-Choline
  9. NaF18
  10. F18-FDG
The following table shows the percent of patients who had metastases detected at various PSAs. F18-PSMA-1007 and F18-rhPSMA-7, are experimental PET indicators that are cleared quickly from the bladder, and are the best so far. F18-DCFPyL is much better than Ga68-PSMA at low PSA.  At PSAs between 0.5-3.5 ng/ml. it detected prostate cancer in 88% of recurrent patients, while Ga68-PSMA-11 only detected prostate cancer in 66% of the same patients - an improvement in sensitivity by a third. Next in line is fluciclovine, which was recently FDA approved. In 10 patients screened for recurrence at a median PSA of 1.0 with both Ga68-PSMA-11 and fluciclovine, the PSMA scan detected cancer in 5 of the 10 men that were negative on fluciclovine. In addition, positive lymph nodes were detected in 3 of the men using the PSMA scan that were undetected with fluciclovine (see this link). Most other PET indicators, like C-11 Choline or Acetate, or NaF18 are not at all reliable when PSA is less than 2.0.

Percent of patients in whom prostate cancer was detected by the PET indicator, broken down by the PSA of the patients

PSA range


PET Indicator

<0.2 ng/ml

0.2- 0.5 ng/ml

0.5 -2.0 ng/ml

> 2.0 ng/ml


88% (0.5-3.5)



66%  (0.5-3.5)

F18-rh-PSMA-7 (experimental)


86% (0.5-1.0)

86% (1.0-2.0)



F18-PSMA-1007 (experimental)


89% (0.5-1.0)

100% (1.0-2.0)





50% (0.5-1.0)

89% (1.0-2.0)










73% (0.5-1.0)

93% (1.0-2.0)













36% (PSA<1, PSADT>6 months)


(PSADT<6 months)





53.3% (0.5-1.0)

71.4% (1.0-2.0)






69.2% (0.5-1.0)

86.7% (1.0-2.0)


100% (>5.0)



37.5% (0.2-1.0)    77.8% (1.0-2.0)



C-11 Choline


46% (0.5-1.0)

62% (1.0-2.0)




PET scans are usually combined with simultaneous CT scans for image resolution. Siemens has a device that simultaneously provides a PET scan and an MRI. This enables  greater image resolution and the detection of smaller metastases than is possible with a PET/CT. (GE and Philips manufacture dual scanners rather than an integrated single scanner). The PET/MRI exposes the patient to a much lower dose of ionizing radiation than the PET/CT. These devices are expensive, and are only available at a few large tertiary care facilities. In one PET/CT vs. PET/MRI comparison using Ga-68-PSMA, the PET/MRI was able to detect 42% more metastases in recurrent patients, 10% more lymph node metastases, and 21% more bone metastases. In the US, PET/MRIs are in use at Mass General, Johns Hopkins, Stanford, UCSF, Washington University, Cleveland Clinic, and Memorial Sloan Kettering and several others.


So far, only Ga-68-PSMA-11, FDG, C11-Choline, and Fluciclovine are FDA approved for prostate cancer detection. NaF is approved for clinical trials and registries only. FDA approval opens the way for Medicare and private insurance to approve and pay for them. Sometimes, insurance plans will agree to pick up the cost. Otherwise, patients who want them must pay out of pocket, if they are available. Ga68-PSMA-11, while FDA-approved at UCLA and UCSF so far, costs $3,000  out of pocket. Hopefully, Medicare/insurance will reimburse soon.

Clinical trials

All of the newer PET tracers require validation with larger sample sizes. While there are some diagnostic tests that have a "gold standard" against which performance can be evaluated, this is problematic for detecting metastases. A positive finding can often be confirmed with a biopsy, so a PET scan's positive predictive value (true positives and false positives) can be ascertained. But there is no easy way to determine whether negatives were true or false in live patients.

F18-DCFPyL is available for free in a trial at NIH for free among any men who are (1) high risk or (2) recurrent (see this link). It is available as a PET/MRI for specific purposes at the  Northwestern University.

It is also available at Johns Hopkins, where it was first developed, for a wide range of indications if ordered by any of their physicians Contact details are available here, There are several studies in Canada: in BC.

Ga68-PSMA-11 is approved at UCLA and UCSF and is available in a clinical trial in the US, including one with a PET/MRI, at  Cleveland Clinic

Fluciclovine is available almost everywhere in the US, and is covered by Medicare for recurrent patients.

If you are interested in one of those PET scans for an indication outside of the clinical trial, call the contact anyway. Some are planning clinical trials for expanded indications shortly, and some may make the PET scan available for purchase outside of the clinical trial. Inquire about cost and get pre-authorization from your insurance company if you can. These can be very expensive.