We’ve talked about PSMA-PET before, but now it has gotten FDA approval for use in imaging prostate cancer.  This is just the start: more approvals are expected.  PSMA-targeting is also being used in Europe and Australia, and in clinical trials in the U.S., as a means of treating prostate cancer, not just showing where it’s hiding in the body.  For the Prostate Cancer Foundation (PCF), I recently interviewed Thomas Hope, M.D., part of a team of scientists at UCSF and UCLA whose PCF-funded research led to the FDA approval for PSMA-PET imaging.  The possibilities here are truly exciting:

“If we can see it on PSMA-PET, we can treat it, right?”

 “My PSA is no longer undetectable after surgery, but cancer didn’t show up on a PSMA-PET scan.  Do I still need radiation therapy?”

 “I’m at high risk of cancer recurrence.  A bone scan was negative, but the PSMA-PET scan shows a few spots of cancer outside the prostate.  Do I have metastatic prostate cancer?”

 These are just some of many new questions that men with prostate cancer and their doctors are starting to deal with after recent FDA approval of PSMA-PET, a new kind of scan that can show, for the first time, the needles in the haystack – tiny spots of prostate cancer hiding in the body that are too small to be picked up by standard imaging.

PSMA stands for prostate-specific membrane antigen, a molecule identified in the late 1980s that sits on the surface of prostate cancer cells.  Supported by many years of PCF funding, scientists have managed to link PSMA to radioactive tracers that can home in on this very specific molecule wherever it happens to be:  think of heat-seeking missiles locking onto a target.  Depending on the radioactive molecule linked to PSMA, it can either detect prostate cancer by shining a virtual spotlight on areas as small as a BB – the imaging technique the FDA has just approved – or detonate it with chemotherapy or tiny doses of radiation delivered by radionuclides at the cellular level.  In Europe and Australia, and in clinical trials in the U.S., PSMA-PET is being used to target and kill cancer in just those tiny outposts, leaving nearby cells unscathed.

“The PCF saw the potential of PSMA targeting way back in 1993,” says medical oncologist and molecular biologist Jonathan Simons, M.D., CEO of PCF.  “Over nearly 30 years, we have invested more than $26 million in research on PSMA, with the goal of finding cancer that has escaped the prostate when it is very early and at a very small volume, because we believe that the sooner we can target it, the sooner we will be able to treat it and change the course of metastatic prostate cancer.”

The particular PSMA-targeted contrast agent that just got approved – a remarkable achievement in itself, based on five years of research by investigators Thomas Hope, M.D., at the University of California-San Francisco, and Johannes Czernin, M.D., and Jeremie Calais, M.D., MSc., at the University of California-Los Angeles – is called 68Ga-PSMA-11.  (The “Ga” stands for gallium; other PSMA agents are in various stages of getting FDA approval.)  And this particular FDA approval, for now, is for use on a very small scale:  only in California, at UCSF and UCLA.  But it’s a start – and it marks an important milestone in prostate cancer detection and treatment.  

This FDA approval is for use of PSMA-PET imaging in two main groups of patients (for now), says Hope, who is Director of Molecular Therapy in the Department of Radiology and Biomedical Imaging at UCSF: “in high-risk men before treatment with prostatectomy or radiation therapy, and in men who have already been treated for localized prostate cancer who have a rising PSA.

The strong collaboration among the PCF-funded scientists at UCLA and UCSF undoubtedly helped secure the FDA’s approval – itself a bit of a milestone.  “This is really unusual,” Hope notes.  “The FDA has never approved a drug at two manufacturing centers before, and both centers were approved on the same day.”

Achieving a PSMA-PET scan is more labor-intensive and expensive than patients might realize, Hope adds.  “We have to make the imaging agent ourselves in small batches,” a high-tech process that requires a gallium generator, and the solution can’t be stockpiled for long-term storage, because gallium has a half-life of a little more than an hour.  “For now, there is no commercially available PSMA-PET contrast agent,” but Hope believes this will change soon; two new drug applications for PSMA agents are under review by the FDA, and more are expected.

Note:  Many men won’t ever need PSMA-PET.  If you have a small amount of Gleason 6 prostate cancer and you are enrolled in active surveillance, or you were diagnosed with low- or intermediate-risk cancer that was treated with surgery or radiation and your PSA is undetectable, then PSMA-PET is probably not something you will need to consider.  But for other men – those with a rising PSA after treatment, for instance; men at high risk of cancer recurrence; or some men with metastatic prostate cancer – PSMA-PET can help determine what to do next.  As Hope says, “Now we know where it is.  The question then becomes, what’s the best way to treat it?”

Smarter Treatment

Having this extra insight shouldn’t be a scary prospect, he adds.  “It’s never bad to know; instead, what can we do with this knowledge?” One exciting thing is to treat men with oligometastasis, as oncologist Phuoc Tran, M.D., Ph.D., is doing at Johns Hopkins: and he’s going after a cure!   Another thing is to actually put the treatment where the cancer is, instead of where it is not.  Hope explains:  Many men who have a rising PSA after prostatectomy “get radiation therapy blindly to the prostate bed; 30 percent of those patients have a recurrence of cancer after about two years.  But with PSMA-PET, we know that about 30 percent of these patients have disease outside the radiation field.  Those are the patients who are recurring!  Now we can expand the radiation field to include known sites of cancer.  We assume the patient will benefit – we just haven’t proven it yet.  Do we not want to know where the disease is, and treat them blindly?” No! And this could be a game-changer for some men.

It’s also important to note that PSMA-PET is not the perfect crystal ball; it can’t detect areas of cancer that are really tiny.  Hope says that “some patients take a negative PSMA-PET to mean they don’t need any treatment,” and that’s not always correct.  “If you have biochemical recurrence (a rising PSA), and PSMA-PET doesn’t show any evidence of disease, the cancer is going to continue to progress.  Don’t think you don’t need treatment, particularly if you’re a candidate for salvage radiation therapy.”

These and other issues will become increasingly clear as PSMA-PET becomes incorporated into the standard of care.  As Hope notes, “It’s early days yet.”

In addition to the book, I have written about this story and much more about prostate cancer on the Prostate Cancer Foundation’s website, pcf.org. The stories I’ve written are under the categories, “Understanding Prostate Cancer,” and “For Patients.”  As Patrick Walsh and I have said for years in our books, Knowledge is power: Saving your life may start with you going to the doctor, and knowing the right questions to ask. I hope all men will put prostate cancer on their radar. Get a baseline PSA blood test in your early 40s, and if you are of African descent, or if cancer and/or prostate cancer runs in your family, you need to be screened regularly for the disease. Many doctors don’t do this, so it’s up to you to ask for it.

 ©Janet Farrar Worthington

News flash:  All metastasis is not alike, and the basic category of “metastatic prostate cancer” is being redefined by doctors and scientists even as we speak.  It’s not just either-or anymore – either cancer is confined to the prostate area, or it has escaped.  It’s actually more of a spectrum, and it is very likely that there’s wiggle room – and still the potential for cure – between cancer escaping the local area around the prostate and full-blown, widespread, metastatic cancer, if we can catch it in time.

I’ve written previously about the work of Johns Hopkins radiation oncologist Phuoc Tran, because I really like what he’s trying to do:  widen the window of curability of prostate cancer.  Great news:  he’s not alone!  Doctors all over the world are rethinking metastasis in prostate cancer and other cancers, as well.  Recently, Tran was one of several experts to take part in a seminar the Prostate Cancer Foundation (PCF) put together.  I was lucky enough to be able to cover it for the PCF, and now I want to make sure you know about the kind of go-getters there are out there who don’t just accept that, if cancer leaves its primary organ, it can’t still be treated and maybe even cured with local treatment.  Better imaging is making it possible to see these cancers sooner than we ever could before.  The reason I want you to know this: if your doctor says you have a couple bits of cancer outside the prostate, so it’s time to start your lifetime of ADT – I want to encourage you to ask around and see if there’s another possibility.

Rethinking Metastasis

For a very long time, many doctors believed, and many still believe, that if we don’t cure cancer while it’s confined to the prostate, then that’s it.  Game over, it’s not curable.  Note:  That doesn’t mean it can’t be treated, sometimes for many years!   But in terms of treatment, traditionally, metastasis has meant bye-bye, local therapy, and hello, systemic therapy – androgen deprivation therapy (ADT), androgen receptor-blocking drugs such as apalutamide or enzalutamide, and chemotherapy.  For patients with metastatic prostate cancer who see their doctors every three months for just a few minutes at a time, that can feel, as one patient’s son put it, like “Lupron and a handshake.”

But a lot of things have come together recently to make doctors and scientists say, “Not so fast!  Maybe there’s a window, and maybe the window is wider than we thought.”  One of these things is the recent ORIOLE study, led by Johns Hopkins radiation oncologist and PCF-funded investigator Phuoc Tran, M.D., Ph.D.  Another is the development over the last decade of better imaging, such as PSMA-PET, which allows tiny bits of cancer to be seen months before they could be seen on conventional imaging, such as a CT scan or bone scan.   Better imaging has sparked an idea:  “If we can see it, we can treat it.”

Is it true?  Can treating little spots of cancer, before full-blown metastasis develops, prolong life?  Recently, the PCF brought together some of the country’s best and brightest – experts in radiation oncology, oncology, urology, and basic science – for a worldwide exchange of knowledge, a webinar attended by more than 300 scientists around the world.  The topic was oligometastasis.  Oligometastasis is just a little bit of metastasis; definitions vary, but generally, scientists who use this word are generally talking about fewer than 3 or 5 spots of cancer that have escaped from the main tumor.  It’s not widespread; it’s limited.  That doesn’t mean it can’t go on to cause trouble later.  If your kids or grandkids are into Pokémon, it’s like catching a little monster before it evolves into something more powerful.

Is oligometastasis treatable?  It is in some other cancers.   In colon cancer, for example, oligometastasis is treated with surgery or spot radiation in addition to removing the primary tumor, and sometimes it’s cured!   Phuoc Tran’s ORIOLE study, and now promising early results from other studies, including ORIOLE’s successor, the RAVENS study, suggest that treating oligometastasis – in Tran’s case, with SABR (stereotactic ablative body radiation, also called SRBT, a highly focused, intense dose of radiation therapy) – in addition to treating the primary prostate tumor can change the course of metastasis in some patients.

Patients reach oligometastasis in different ways.  Some reach it by biochemical recurrence – the dreaded rise of PSA after treatment of the primary tumor in the prostate with surgery or radiation.  Others are diagnosed from the get-go with cancer that has already spread outside the prostate.  The standard of care for most of these latter patients is not only not to treat the main tumor, but not to zap or surgically remove the few sites of metastasis. 

Why not?  Why the heck not?  Or, as Tran says, “It makes so much sense, so why don’t we do it?  Because we have tried periodically over the past five decades to treat metastatic disease aggressively with local therapies, and because of lack of imaging, treatment technology and just general lack of our ability to take care of patients, this approach did not work.”  In fact, he continues, “it was actually a resounding failure, and made many who lived through these periods very scared of doing much more harm than good.  One of the first texts on this concept, called ‘Solitary Metastases,’ actually started out with a chapter called “Illusion or Reality.’”

But that was then.  Even now, there’s not yet definitive proof that it works.  But take heart:  the winds of change are blowing! 

This brings us to the PCF 2020 Global Knowledge Exchange on Oligometastatic Prostate Cancer.  Eric Klein, M.D., Chairman of the Glickman Urological & Kidney Institute at the Cleveland Clinic, who moderated the discussion, set the stage with a story about a patient, seen by him and medical oncologist Howard Scher, M.D., of Memorial Sloan Kettering Cancer Center (MSKCC).  The patient was in his 50s, diagnosed with Gleason 9 cancer that extended slightly past the prostate, into the seminal vesicles.  He also had cancer in a lymph node.  The man received ADT for six months, had a radical prostatectomy, then was on abiraterone plus prednisone for a year afterward.  A bone scan showed one spot of cancer; it was treated with radiation at MSKCC.  “He’s about eight or nine years out now,” says Klein.  “He has an undetectable PSA and a normal testosterone.”

As the PCF’s CEO, Jonathan Simons, M.D., says, “One clinical case well studied can change the course of medical history.”  This patient’s exceptional clinical course has led Klein ask to the big question:  “If we can seemingly cure one man with metastatic prostate cancer, can we cure others?  And are we at a place now in the field to be asking the right questions, with the right trial behind them?”

Ralph Weichselbaum, M.D., Chair of the Department of Radiation and Cellular Oncology at the University of Chicago, is the radiation oncologist who coined the term, “oligometastasis.”  He specializes in treating it in various forms of cancer.  Not only does metastasis represent a spectrum of disease, he says, “depending on the number of metastases, the organs involved, and the pace of progression,” but patients represent a spectrum, too.  “There are subsets of patients who are potentially curable with metastasis-directed therapies” (treating breakout tumors directly, and not relying on systemic therapy alone).  What accounts for these subsets?  Genetic factors, and also the robustness of the patient’s immune system.  Weichselbaum’s research suggests that patients with a well-functioning immune system are better able to hold metastasis in check than others.  In other words, whether oligometastasis responds to treatment depends on “the complex relationship between tumor and host.”

It May Require the Proverbial Kitchen Sink

Scher and Mary-Ellen Taplin, M.D., medical oncologist and Director of Clinical Research at the Dana-Farber Cancer Institute’s Lank Center for Genitourinary Oncology, collaborated on the design of a multi-arm, multi-modality therapy clinical trial with funding from a PCF Challenge Award.  “Our focus is the patient with high-risk localized disease, or low-volume or recurrent metastatic disease,” said Taplin. The trial will be looking at many things, including potential biomarkers for sensitivity and resistance to treatment.  But one of the objectives is of particular interest:  “to eliminate all disease in patients largely incurable with any single treatment.” 

In other words: to kill prostate cancer that has escaped the prostate, these doctors and others believe, in addition to targeting the primary tumor with prostatectomy or radiation, it may well take a short course of ADT, perhaps also chemotherapy, maybe further external-beam radiation to the area around the prostate, and then radiation or radiofrequency ablation to the metastatic sites themselves.   But then, the hope is that these patients will have an undetectable PSA and that they will get their testosterone back.

There are several other important trials underway to treat oligometastasis in prostate cancer.  Of all the things scientists hope to learn from these trials, perhaps most important, says medical oncologist Ana Aparicio, M.D., of MD Anderson Cancer Center, is “how do these site-directed therapies work?”  Will the success come from messing up the circulating tumor microenvironment?  One idea is that, as cancer spreads, it sends messengers back for supplies to the other sites where cancer is already established, using the bloodstream as a liquid version of Fed Ex.  “Or, are we modulating the immune response?  Does the primary tumor have an immunosuppressive effect that limits the ability of the patient’s immune system to control the disease?  Or, are we having an immune-stimulatory effect with treatments?  We may need to build on that, and combine radiation with some novel immunotherapies.  Or, are we decreasing the tumor burden,” by zapping sites of oligometastasis?

Two Icebergs

Aparicio draws a picture for her patients to help explain:  There are two icebergs, one blue, one yellow.  “The blue one, most of it is above the water,” she notes.  “If you get rid of what you see, it is likely that the iceberg is going to take a long time to grow again and become a problem.  So, if what we see on the scans is most of the disease that’s present, then yes, addressing all the sites we can see can be beneficial.  But if it’s just the tip of the iceberg (like the yellow picture), and there’s a large burden of tumor we are not able to detect with our imaging tools, we’ll find that the disease grows very quickly.”

Better imaging, such as PSMA-PET, will undoubtedly help determine the true state of tumor burden, “particularly when the PSA is rising, but it’s less than 10; conventional imaging really is not useful when the PSA is 5 or 10,” says Phuoc Tran.  He believes the number of patients with oligometastasis in the U.S. is huge, “much higher than the number of men diagnosed each year.”  Right now, “systemic therapy is the standard of care for patients with metastatic disease,” says Tran.  “But in that gray area of biochemical recurrence (PSA creeping back up after prostatectomy or radiation of the primary tumor), as men are approaching low-volume metastasis, there’s a perfectly reasonable period in which you can ask the question, does local therapy change the metastatic process?” That was the question behind the ORIOLE trial.

“If the oligometastatic state didn’t exist, if this were not a spectrum, and if local therapy could not alter that natural history of metastasis, then we shouldn’t be able to affect progression at all with local therapy alone.  Patients should progress no matter what.  We did not see that.  Obviously, stronger evidence is needed,” but the results of the ORIOLE trial and early results of the RAVENS trial have been very encouraging.

It may be, says Weichselbaum, that we are dealing with multiple, different disease states, “requiring entirely different kinds of treatments.  We need to define really what metastasis is, and how the systemic treatments and ablative treatments fit together for optimal therapeutic outcome.”

And maybe one day, says Tran, “we can alter the natural history of metastasis, and cure these patients with formerly incurable disease.”

In addition to the book, I have written about this story and much more about prostate cancer on the Prostate Cancer Foundation’s website, pcf.org. The stories I’ve written are under the categories, “Understanding Prostate Cancer,” and “For Patients.”  As Patrick Walsh and I have said for years in our books, Knowledge is power: Saving your life may start with you going to the doctor, and knowing the right questions to ask. I hope all men will put prostate cancer on their radar. Get a baseline PSA blood test in your early 40s, and if you are of African descent, or if cancer and/or prostate cancer runs in your family, you need to be screened regularly for the disease. Many doctors don’t do this, so it’s up to you to ask for it.

 ©Janet Farrar Worthington

 

Oligometastasis:  Good News from the ORIOLE Study

To the growing and hopeful list of strategies for attacking prostate cancer, let us add this approach:  Whack-a-Mole.

That’s how Johns Hopkins radiation oncologist Phuoc Tran, M.D., Ph.D., describes it to his patients.  The actual scientific name for this highly sophisticated strategy is stereotactic ablative radiotherapy (SABR, highly focused, intense doses of radiation), for men with oligometastasis – up to three small bits of cancer that have broken away from the main prostate tumor and started to grow elsewhere.

Previously, I wrote on Vital Jake and also on the Prostate Cancer Foundation’s website, about the Baltimore ORIOLE study, led by Tran, who also contributed greatly to our book.  Tran was enrolling patients in a small study to see if men with oligometastasis would benefit from SABR in addition to treatment of their primary tumor.

His strategy was a new one – part of a general rethinking of what represents curable prostate cancer.  The boundary used to be very clear:  prostate cancer was either confined to the prostate or prostate bed, or it wasn’t.   Like a light switch with no dimmer, there was no in-between:  a man with only one metastasis was believed to face the same fate, eventually, as a man with widespread metastases.  It was just a matter of time.

But Tran believed that the lines of prostate cancer were not so clear-cut as scientists had assumed; that instead of two circles – localized and metastatic cancer – that didn’t connect, we might be dealing with a Venn diagram, with oligometastasis as the critical area where the two circles overlap.  “It may be that the window of curability is wider than we thought,” he said last year, and we all hoped that he was right.

Tran and colleagues at Johns Hopkins, Stanford, and Thomas Jefferson University recently published results of the ORIOLE Phase 2 clinical trial in JAMA Oncology.  The results are promising:  54 men with oligometastasis were randomly assigned either to treatment with SABR or to observation.  To detect and keep track of the oligometastases, the study used PSMA-PET scanning, which uses a small molecule linked to PSMA (prostate-specific membrane antigen, found on the surface of prostate cancer cells) as a radioactive tracer.  This PSMA-targeting tracer can highlight areas of cancer as small as a BB – much smaller than can be seen on regular PET or CT imaging.  “PSMA-PET allows us to treat lesions we otherwise couldn’t see,” Tran explains.  “A CT or bone scan would miss those lesions, and patients would presumably not do as well.”

At six months, 61 percent of the men in the observation group progressed – compared to only 19 percent of the men who received SABR.  “We also saw a significantly decreased risk of new metastatic lesions using PSMA PET-CT” says Tran.  “The men in the SABR group did considerably better.  This is a definite signal that we can perhaps modify metastatic disease.”

This was a Phase 2 study, and “we need larger Phase 3 trials,” he says.  “But this is very positive, and we hope that in the future, we will be able to change the course of metastatic disease in some men.” 

Some interesting points here:  First, Tran and colleagues hope that “complete metastatic ablation of oligometastatic prostate cancer may provide an alternative to early initiation of androgen deprivation therapy (ADT).”  The key question, Tran says, is, “can we alter the natural history of metastatic prostate cancer with metastasis-directed therapy (MDT)?”  They don’t know the answer yet.  Most men with oligometastatic disease who get these spots of cancer zapped don’t experience a complete drop in PSA.  This, Tran says, suggests that “residual micrometastases are present but undetectable.” Does SABR simply reset the clock – does it keep pushing the snooze button?  Or, as the scientists hope, does it make the cancer less likely to form new metastases?

Bad Pioneers on a Bad Journey:  Tran and other scientists theorize that the spread of cancer is a story of colonization.  A few pioneers set forth on a journey to a new land.  At first, it’s touch-and-go; their survival is tenuous.  Just think of the early colonists in the U.S., from England, France, or Spain.  Until they took root in the new land, these nascent colonies were frail:  they needed reinforcements from their mother countries – medicine, weapons, tools, food – and “eventually they did survive.”  So it may be with the seeds of cancer; either the cancer cells themselves, or their messages (in the form of genetic and chemical telegrams) are dispatched to the primary tumor, the mother country.  If the mother country is no more – if it has been eradicated by surgery or radiation – then small cancer outposts might get similar support from visiting each other.  But if those outposts are destroyed by SABR, even if there are a few cancer cells remaining in the tissue or bloodstream, it doesn’t matter:  the environment is too hostile, and the numbers are too few for new colonies to survive – “or, if they did, it would take much longer.”

“It’s like Whack-a-Mole”:  In the ORIOLE trial, Tran and colleagues looked for circulating tumor DNA (ctDNA), and identified certain gene signatures that can tell if a man is more likely to respond to SABR.  “Patients who don’t have these mutations responded very well,” he says.  They also have learned from this and other research that men with oligometastasis fall into three basic groups.  “Some men do really well after one course of SABR,” with no recurrence of cancer.  A second group of men have a small recurrence.  “Another site pops up; a microscopic metastasis that we couldn’t see before establishes itself into a macroscopic metastasis.  It’s a limited return of cancer and it responds to another round of SABR.”  Then some men, after a few months, have multiple new areas of cancer.  “For these men, the SABR doesn’t control the disease at all.”

Imagine a green lawn, with one or two dandelions, Tran tells his patients:  “You can pluck those two or three weeds, and wait and see.  Sometimes you get lucky; sometimes another weed or two pops up, and you pluck them.  It’s like Whack-a-Mole.  You can do that for a while,” with repeated SABR treatments.  As the scientists reported:  “If a single round of MDT arrests the progression of some, but not all, lesions, subsequent rounds of MDT might salvage the remaining disease, until what remains is inadequate to support a metastatic phenotype.” Basically, for some men, a treatment strategy might be to keep knocking the cancer down until, like a prizefighter who’s taken one too many blows to the head, it can’t get back up.  Imagine:  punch-drunk prostate cancer that may still be staggering around, but can’t raise a fist.  Wouldn’t that be nice!

That probably won’t work in every man, Tran says.  “Unfortunately, sometimes there will be a whole bunch of seeds all at once, and at that point, you need weed killer all over the lawn,” or systemic therapy.  However, SABR plus ADT, androgen-blocking drugs, or chemo might one day provide “the multipronged attack required to cure this disease.”

Looking ahead:  In a follow-up trial, called RAVENS, men with oligomestatic prostate cancer are randomly given either SABR alone, or SABR plus radium-223 (Xofigo).  “What we have seen in the men in that second group – the ones who have more isolated spots of cancer popping up – is, they’re not recurring where they received the SABR, but in areas that were microscopic, and commonly in the bone.”  Radium-223 targets cancer in bone.  “It releases a radioactive particle that is very toxic but is so focused that it only kills in a radius of two-three cell depths.  It’s ideal for microscopic disease.”

More and larger studies are needed, but in the future, Tran envisions, men with oligometastasis will require more vigilant monitoring, and ideally, regular follow-up PSMA-PET scanning.  “This has the potential to be practice-changing.  We are very excited by our results, and by the potential to modulate the course of metastatic prostate cancer.”

In addition to the book, I have written about this story and much more about prostate cancer on the Prostate Cancer Foundation’s website, pcf.org. The stories I’ve written are under the categories, “Understanding Prostate Cancer,” and “For Patients.”  As Patrick Walsh and I have said for years in our books, Knowledge is power: Saving your life may start with you going to the doctor, and knowing the right questions to ask. I hope all men will put prostate cancer on their radar. Get a baseline PSA blood test in your early 40s, and if you are of African descent, or if cancer and/or prostate cancer runs in your family, you need to be screened regularly for the disease. Many doctors don’t do this, so it’s up to you to ask for it.

 ©Janet Farrar Worthington

 

Remember these letters:  PSMA.  If you haven’t heard of PSMA-targeted agents yet, you probably will soon.

Imagine a heat-seeking missile – except the tiny target locked onto by this particular missile is PSMA (prostate-specific membrane antigen), a protein that sits on the surface of prostate cancer cells.  The weapon itself is a small molecule, originally designed as an imaging agent by a team led by Johns Hopkins investigator Martin Pomper, M.D., Ph.D., and scientists are still discovering what it can do.

How specific is it? Imagine a bit of tissue the size of a teardrop.  Two of the cells inside it are prostate cancer cells; the rest are not.  With a PSMA-targeting tracer, like Pomper’s small molecule or any of its next-gen relatives, only those two cells would light up.

We’re talking molecular LEGOs here: With Pomper’s small molecule, PSMA can be linked to different chemical bricks.  One kind of brick is a radioactive tracer that can show on a PET scan exactly where small bits of cancer are hiding.  But wait!  There’s more:  PSMA can also be hooked up to a radiopharmaceutical agent, called a radionuclide, that can seek out and kill those tiny pockets of cancer and potentially even stop metastatic disease.

It’s like the old commercial for the miracle product called Shimmer on “Saturday Night Live.” It’s a floor wax! It’s a dessert topping! No, it’s both!

“It truly has excellent potential and we are just scratching the surface here of what PSMA-targeting can do,” says medical oncologist Jonathan Simons, M.D., CEO of the Prostate Cancer Foundation, “in metastatic disease and also in localized disease.”   I recently interviewed Simons and Pomper for the PCF’s website, and Pomper for the newly released Fourth Edition of my book with Patrick Walsh.

This momentum has been building for two decades. “We started working on PSMA-based imaging agents back in the late 1990s,” says Pomper, Director of Nuclear Medicine and Molecular Imaging at Johns Hopkins. Pomper’s team was not the first to try to harness PSMA as a way to get to prostate cancer; in 1996, scientist Neil Bander created an antibody that can target PSMA and used SPECT imaging to see hidden prostate cancer cells.  But antibodies are cumbersome; it takes several days from the time they are administered until they clear the bloodstream and reach the target cells. They are also very large. Continuing the building-block theme here, it’s like trying to attach toddler-friendly DUPLO blocks to the much more svelte LEGOs. “We want to be able to scan within an hour or so after injection,” Pomper explains. “We prefer the small molecules for therapy, too.”

Pomper’s versatile small molecule and derivatives of it have galvanized the field of nuclear medicine. PSMA-targeted imaging and therapy has generated huge interest worldwide – especially in Europe, where scientists have linked the small molecule to radionuclides (both alpha- and beta-emitting particles) and are reporting long-term remissions in some men with metastatic prostate cancer. “You just switch what’s attached to the small molecule, and you can go from imaging to irradiating the cancer – cancer you can’t even see, potentially. This would be impossible using external-beam radiation.”

German doctors – who, thanks to a regulatory loophole were able to move right into using PSMA-targeted radiotherapy without having to conduct the clinical trials required in the U.S. – have even reported cures in a few men – but also some side effects, including the loss of the salivary gland, where some PSMA-bearing cells also live. That’s because, although scientists called it “prostate-specific,” PSMA is not solely confined to prostate cancer.

Scientists worldwide are trying to figure out how to tackle the “collateral damage” problem of PSMA. Is there some way to protect the salivary gland, like using potassium iodide to protect the thyroid in the event of a nuclear attack? Some of the salivary-protecting options being explored include botox and anticholinergic drugs.

The Great Promise of PSMA-Targeting Agents

“PSMA is present in the normal prostate, present in the brain, the kidney and the intestines,” Pomper notes, “but it’s really expressed much higher in malignant prostate tissue. It’s also expressed in the neovasculature – the vessels tumors need in order to grow in place or metastasize.”

PSMA is present in many different cancers, too. “Renal cell carcinoma, glioblastoma, pancreas cancer and other cancers have PSMA in the blood vessels around them – not in the tumor itself,” and this is an exciting potential avenue for future research: finding a way to target and kill PSMA-bearing areas around some terrible cancers that desperately need effective treatment.

Pomper keeps tinkering with the molecule and agents that link to it.. Recent work with Hopkins colleagues in Radiology and Radiation Oncology has led to the first published small-molecule alpha-particle emitting agent to treat prostate cancer. A team led by radiation oncologist Ana Kiess, M.D., Ph.D., linked an alpha-particle emitter to Pomper’s small molecule. “Alpha particles are emitted from certain molecules as a consequence of radioactive decay,” explains Pomper. “They are useful for treating cancer because they provide a lethal punch to the DNA of malignant cells – more so than other forms of radiation. The key is to enable the alpha emitter to reach the cancer cells selectively, leaving normal tissues unharmed.”

In the lab, “using this agent, we were able to prolong the lives of immunocompromised mice bearing human prostate tumors,” says Pomper. This study lays the groundwork for future clinical trials in men with prostate cancer, and for the design of even safer, next-generation alpha particle agents. Also, it “represents a pivot by our group from developing imaging agents to finding better agents for therapy.” The group is now leading a phase I clinical trial for beta particle-emitting agents it has developed.

The very good news for men with advanced prostate cancer is that numerous clinical trials are opening in the U.S. and Australia to test similar PSMA-targeted radiopharmaceutical agents. In fact, the PCF is funding three research projects – in Australia, at UCLA, and Weill Cornell – and all of these have clinical trials.

PSMA-PET Can Help Clarify Localized Prostate Cancer, Too         

So far, efforts with PSMA-targeting molecules have mostly been focused on what ancient Romans called the disjecta membra, the scattered bits and pieces of cancer that started out in the prostate and moved to the lungs, bone, liver, or someplace else.

But what about the cancer that’s right there in the prostate – cancer that hasn’t spread yet?  That’s what investigator Steve Cho, M.D., has been working to find out.  Cho, on the faculty at Johns Hopkins before joining the faculty at the University of Wisconsin, led the first human imaging study of Pomper’s PSMA-targeting agent.   He showed how well PSMA-PET could pick up metastatic prostate cancer – better than a bone scan and CT combined. Then he thought: “There’s a low level of PSMA in the prostate itself. How well does this agent pick up primary prostate cancer?”   With Movember funding through the Prostate Cancer Foundation, Cho led another study for prostatectomy patients – men with localized prostate cancer who have it taken out surgically. The benefit here is that Cho and colleagues could compare what they saw on the PSMA-PET images with what the pathologists found in the needle biopsy tissue and in the actual removed prostate specimens. They learned a couple of very important things:

One, in localized disease “this specific agent doesn’t show up in all prostate cancer patients.” (Note: other PSMA-targeting molecules might be found to work better in this situation.) But “it does show up in men with higher-grade cancers,” Gleason grade 8 or 9 tumors.

As it turns out, PSMA-targeting molecules have discernment.

This is really important, because many men need some extra help. “One of the problems with MRI,” Cho explains, “is that it can pick up a lot of lesions – but sometimes they are benign.” Calculi, stones in the prostate (like kidney stones, but tiny), and enlargement of the prostate (BPH) can show up on an MRI, too, and it’s not always apparent what needs to be treated and what doesn’t.

MRI is sensitive, but not always very specific; it’s “user-dependent, in terms of interpretation and experience.” Understandably, a radiologist who looks at nothing but prostate images all day probably has more expertise at spotting prostate cancer than does a radiologist who looks at images of all sorts of body parts. “PSMA-PET was specific in our study,” says Cho. “If you see a signal by PSMA-PET in the prostate, it typically ends up being a site of prostate cancer, and ends up being clinically significant.

This could be particularly helpful for men with an elevated PSA but a negative biopsy (or biopsies), or men considering Active Surveillance for prostate cancer. Men who are told they have low-grade disease – because the biopsy needle hasn’t picked up anything different – could have extra peace of mind if a PSMA-PET comes up negative for high-grade disease. Or, men who have had one or more inconclusive biopsies may decide to undergo surgery or radiation therapy if PSMA-PET shows high-grade cancer that the needles missed.

Even if a biopsy shows cancer, “the biopsy needle is not always accurate,” Cho notes. “It might show Gleason 6 disease, but maybe there’s Gleason 8 cancer somewhere hidden. ” Similarly, during a rectal exam, “the urologist’s finger can’t always feel cancer in the apex or anterior of the prostate. That’s where I think this technology can really help: it can provide a better way of targeting a specific region of the prostate so the needle has a higher probability of a true hit.”

Combining PSMA-PET with MRI may result in even more accurate and predictive scans, as well.

But wait again! There’s even more! Cho is exploring PSMA-PET in several different studies, aimed at helping men with different stages of prostate cancer.

One of these is for men with high-risk prostate cancer, “we currently have a clinical trial here at the University of Wisconsin, a Department of Defense-supported grant, with medical oncologist Joshua Lang, M.D., urologist David Jarrard, and biomedical engineer David Beebe, Ph.D., who studies the microenvironment of tumor cells. “In these high-risk patients, at the time you take the prostate out, they already have a high probability of having cancer outside the prostate.” But if the disease could be attacked systemically, with three months of hormonal therapy (Degarelix) and chemotherapy (Docetaxel) before prostatectomy, would that help – and could PSMA-PET images show that tiny bits of cancer have disappeared?

In future studies of men with advanced prostate cancer, Cho envisions using PSMA-PET to monitor treatment – any kind of treatment – to make sure it’s working. “Can we tell early on whether a patient is responding or not responding well, so we don’t have to continue to give treatment that’s not working, and we can quickly change course?” Molecular imaging can help doctors “be more nimble” and respond more quickly – either to intensify treatment or, if it’s working, perhaps to dial it back and spare the patient multiple cycles of hormonal therapy or chemotherapy. This is already happening in other cancers, such as lymphoma.

“This whole area is evolving,” says Cho. “We have barely scratched the surface.”

We’ll be talking more about PSMA in future posts.

In addition to the book, I have written about this story and much more about prostate cancer on the Prostate Cancer Foundation’s website, pcf.org. The stories I’ve written are under the categories, “Understanding Prostate Cancer,” and “For Patients.”  As Patrick Walsh and I have said for years in our books, Knowledge is power: Saving your life may start with you going to the doctor, and knowing the right questions to ask. I hope all men will put prostate cancer on their radar. Get a baseline PSA blood test in your early 40s, and if you are of African descent, or if cancer and/or prostate cancer runs in your family, you need to be screened regularly for the disease. Many doctors don’t do this, so it’s up to you to ask for it.

 ©Janet Farrar Worthington