AACR 2017: PSMA Update

Jaspreet Batra_AACR 2017_PSMA
Jaspreet Batra presents this research at AACR 2017

At the 2017 Annual Meeting for the Annual Association for Cancer Research (AACR), we presented research highlighting how we’re targeting PSMA – a marker on the surface of most prostate cancer cells – with radioimmunotherapy to kill cancer cells.

Radioimmunotherapy involves attaching radioactive particles to targeted immunotherapies that go directly to the cancer cells. The monoclonal antibody we use is called J591 and will bind only to PSMA. In this research, we attached the radioactive isotope actinium-225 (225Ac) to J591. We have used J591 linked with lutetium 177 (177Lu) and yttrium 90 (90Y) to treat patients in many clinical trials in the past. We believe that since 225Ac is an alpha particle emitter with much greater energy released that each individual antibody will lead to more tumor cell death. In our experimental models presented at the 2017 AACR meeting, 225Ac-J591 was not significantly more toxic than control (similar to placebo) in mice without tumors. When we treated mice with prostate cancer tumors, there was significant tumor killing following a single injection of 225Ac-J591.

Given our long history of administering radiolabeled J591 to hundreds of men in different clinical trials, we have plans to launch a phase I dose-escalation clinical trial (to determine safe doses and later look at tumor response) later this year. We and others are quite enthusiastic about this approach.

Using Alpha and Beta Radioisotopes to Kill Cancer Cells

Radionuclides, also known as radioisotopes, are particles that emit energy. The different particles they emit vary and some types emit damaging radiation (also called ionizing particles). This is a good thing when we’re using radiation as a way to kill cancer cells. The two main categories of radiation particles used to kill cancer cells are alpha and beta particles.

Several radioisotopes – using both alpha and beta particles — have been approved by the Food and Drug Administration (FDA) for clinical use in cancer treatment. Historically, bone-seeking radioisotopes were used for patients with painful tumors in the bone. For example, Strontium-89 (Metastron) and samarium-153 (Quadramet) are beta-emitters that are taken up like calcium into bone and were approved to decrease pain. More recently, the alpha-emitting agent radium-223 (Xofigo) was approved for men with metastatic castration-resistant disease that has spread to the bone. However, unlike the previous beta-emitting agents, radium-223 was FDA-approved because it leads to longer overall survival rather than just symptom relief. Radium-223 is an alpha particle that mimics calcium and is delivered and taken up by the bone cells. This generally occurs near tumor cells, and while we don’t know the exact mechanism of action, we suspect that in addition to being in close proximity to some tumor cells, this creates a less hospitable environment for the tumor cells that have spread to the bone.

Additionally, we can now utilize different targeting agents to take radionuclides directly to the tumor cells. Radioimmunotherapy or radioligand therapy involves the practice of attaching a radioactive isotope to a cancer-targeting antibody or small molecule that binds only to a specific cancer-related molecule on a tumor cell. This is similar to a “lock and key” scenario, where the antibody or molecule resembles the key that will only recognize a very specific lock (the cancer-related molecule).

As it turns out, essentially all prostate cancer cells have a specific “lock” called prostate-specific membrane antigen (PSMA). This lock sits on the surface of each prostate cancer cell. We have engineered very specific monoclonal antibodies and molecules that will bind only to PSMA, leading to the opportunity for “molecularly targeted” (radio-)therapy.

In terms of attaching the radioactive isotopes, we can use both alpha and beta particles depending on the location and size of the tumor.Alpha vs beta radiationAlpha particles have the advantage of a very high amount of energy and a short path length. The amount of energy is high enough so that only a small number (1-10) of alpha particles lead to lethal damage to cells. An advantage of the short path length is that only the cells in close proximity to the alpha particle are destroyed, sparing other healthy and normal tissues. However, because of the short path length travelled, the alpha particle needs to be delivered into or right next to the tumor cell. In fact, even a piece of paper (or skin) is enough to block an alpha particle. Other alpha particles are being developed to be delivered as lethal payloads when attached to carrier molecules. One of these, actinium-225 (225Ac) is an alpha-emitting radionuclide that emits 4 alpha particles. In humans the 225Ac particle has been used as part of a compound linked to an antibody to treat leukemia and it also has been linked to a PSMA-recognizing peptide to treat men with late-stage prostate cancer with initial examples published last year.

Beta particles emit a lower energy, but can travel further distances. Because of their lower energy levels, more particles are required to cause lethal damage to cells.

This video provides a great overview of the process:

Additional research is needed to decipher the best radionuclides to use for which diseases in which clinical situations. We at Weill Cornell Medicine and NewYork-Presbyterian Hospital will have both alpha and beta radionuclides linked to PSMA compounds available in the clinic this year, initially with a clinical trial using 177Lu-PSMA-617, to be followed by 225Ac-J591, then the combination of 177Lu-J591 and 177Lu-PSMA-617.

Promising New Radioligand Treatment for Men with Metastatic Prostate Cancer Using Lutetium 177 (177Lu)

Using small molecules, we are able to target not only the known tumors, but can also treat the unknown tumors.

Physicians and researchers at Weill Cornell Medicine have been utilizing prostate specific membrane antigen (PSMA)-directed radioisotope therapy for more than a decade. Over the years, we have shown that we could use this approach to target the vast majority of prostate cancer tumors (“hitting” essentially all known tumors and avoiding normal organs), demonstrated anti-tumor responses when the J591 antibody is linked to a radioactive particle with a large (single) treatment, and then further improved upon this treatment (while simultaneously reducing the side effects) by dose-fractionation (splitting the dose into two).

Following our lead and with the discovery of new small molecules which also specifically bind to PSMA, European physicians have begun using these compounds tagged with the same radioactive particle. The most common molecule has been termed PSMA-617. They have shown some very nice anti-tumor responses with limited side effects. However, because European laws differ from the U.S., many men are able to pay for treatment outside of the setting of rigorous, organized clinical research studies that clearly define appropriate dosing, efficacy and toxicity.

In January 2017, research was published in the Journal of Nuclear Medicine demonstrating that Lutetium 177 combined with PSMA-617 can reduce the amount of tumors in the body and lead to remission of the cancer as measured by PSA level. Twelve German hospitals reviewed their data and compiled a publication of patients with metastatic prostate cancer who received Lutetium-177 linked to PSMA-617 (177Lu-PSMA-617). Over 18 months, 145 men whose cancer grew despite standard treatments (including abiraterone and/or enzalutamide and chemotherapy) and whose tumors “lit up” on PSMA imaging were treated. While not a proper prospective research study, they were able to determine information about both anti-tumor activity and safety. Most patients who had PSA measured before and after treatment had some decline, with 40% having PSA cut at least in half following a single treatment. Blood counts dropped in less than half (usually to moderate degrees) and some developed dry mouth and/or taste changes. Severe toxicity was rare.

It is encouraging to see that there is a treatment that might lead to reduction in cancer without severe side effects, even in men who previously have received many other lines of treatment. However, both rigorous research as well as access for our patients are current issues. Therefore, we are excited to offer a clinical trial that builds upon our prior experience of anti-PSMA radioimmunotherapy while taking into account the available European data.

This study utilizes the most commonly used molecule, 177Lu-PSMA-617, in a prospective manner. Our prior research has shown that higher doses result in significantly better anti-tumor responses, so one purpose of this study is to perform dose-escalation to determine the safest and most-effective dose without increased side effects. In addition, our research demonstrated that dose-fractionation allowed higher doses with less toxicity, so our treatment schedule will deliver the total dose in 2 fractions.

We look forward to advancing science and also making these treatments available to men in the tri-state area and across the U.S., not just those who can afford to fly to Germany for treatment. At Weill Cornell Medicine and NewYork-Presbyterian, we have an excellent, multidisciplinary team that has led the world in PSMA-targeted radionuclide therapy. We will leverage our combined expertise and experience to translate the exciting knowledge base into true clinical gains for prostate cancer patients.

To learn more about the clinical trial or enroll, click here. Call us at 646-962-2072 to make an appointment or schedule a consultation.