In 1997, the US Food and Drug Administration (FDA) approved the first antibody drug—rituximab—for the treatment of non-Hodgkin’s lymphoma, the most common form of blood cancer in adults. Rituximab didn’t just improve patient outcomes with few side effects; it represented a novel approach to developing cancer therapies: find a molecular target displayed on the surface of a cancer cell, then engineer an antibody against it.
Since then, many antibody-based cancer therapies have been developed against external-facing or extracellular molecules on cancer cells. But what about intracellular targets—surely cancer cells are as different from normal cells on the inside as they are on the outside? Qi Zeng, a Research Director at A*STAR’s Institute of Molecular Cell Biology (IMCB), and an Adjunct Professor at the National University of Singapore’s Yong Loo Lin School of Medicine, has answered that question with a definitive yes.
In fact, her research has shown that an intracellular protein, PRL3, can be targeted by an antibody for cancer therapy. She has since founded Intra-ImmuSG, an A*STAR spin-off company, to test the efficacy of PRL3-zumab, a first-in-class humanized monoclonal antibody, in patients suffering from a range of solid tumors. The drug has been approved by the US FDA’s Investigational New Drug program for Phase II clinical trials on patients in the US with any solid cancers.
Describing the process of bringing a scientific discovery from bench to bedside as “a duty and a calling,” Zeng is optimistic about the trial results. She tells A*STAR Research how the careful use of animal models, self-belief and perseverance have brought her to the brink of pioneering a new class of anticancer therapies.
1. Why did you choose to focus on PRL3 as a target for cancer treatment?
My research on PRL3 began more than two decades ago, when I first identified and characterized the PRL3 gene in 1998. The gene is also known as PTP4A3 and encodes a phosphatase—an enzyme that removes a chemical tag (a phosphate group) from other proteins inside cells.
Back then, I had overexpressed the gene in mammalian cells, which caused those cells to undergo very drastic changes in morphology and behavior—some cells became five to ten times larger than usual and developed multiple nuclei (normal cells only have one nucleus containing the cell’s genome). The membranes of those cells also became very ‘spiky’ with protrusions known as pseudopodia. I called those cells ‘monster cells.’
Later, in 2001, Professor Bert Vogelstein’s group from Johns Hopkins University in the US demonstrated a tight association between PRL3 and cancer—specifically, they observed high expression of PRL3 in metastatic colon cancers, but not in benign or normal colon tissues. Professor Vogelstein is a renowned cancer researcher and I learned a lot from him.
Since 2001, my research lab and many others worldwide have thoroughly characterized PRL3 as a cancer protein (oncoprotein) that is overexpressed and associated with multiple human cancer types.
2. Most antibody-based therapies target extracellular proteins. Why did you choose to develop an antibody targeting PRL3 which is found inside cells?
The development of antibody therapies has focused mainly on targeting a few extracellular proteins—those secreted by cells or found on the surface of cells. This is because antibodies are generally believed to be too large to enter cells. What this means is there remains a largely untapped pool of intracellular therapeutic targets, such as phosphatases, kinases and transcription factors.
Since 2008, my lab has presented evidence suggesting that intracellular proteins highly expressed in cancer cells can be targeted by monoclonal antibody-based or vaccination immunotherapies. Although our results repeatedly proved the concept, people were still skeptical. But that did not deter me from further testing my approach.
After developing an antibody called PRL3-zumab, we demonstrated its mechanism of action against an intracellular target. When cancer cells are stressed or dying, PRL3 gets turned ‘inside-out’—it becomes exposed on the surface of cancer cells, or released into the tumor microenvironment, making it available for antibody binding.
We tested PRL3-zumab in multiple mouse models of cancer and showed that PRL3-zumab specifically bound to PRL3 to trigger an immune response against tumors. This, in turn, results in a ‘kill-and-leak’ domino effect that causes tumors to shrink by as much as 90 percent. These results have been published in Nature Communications.
We have used more than seven different mouse models for more than ten years to test this unconventional cancer immunotherapy, and the anticancer effects of PRL3-zumab have been consistent against tumors that express PRL3. Here, I want to emphasize the importance of using the right mouse models for the testing of anticancer compounds—an animal model, if used correctly, gives very clinical-like insights on the efficacy of treatment.
3. What are the advantages of using PRL3-zumab instead of a small molecule drug that targets PRL3?
The main advantage of PRL3-zumab is that it binds very specifically to PRL3, which is specifically expressed in cancer cells but not in normal cells. Researchers usually use small molecule drugs to target intracellular PRL3, but by virtue of their small size, they travel everywhere in the body and get taken up by many different organs, which can lead to undesirable side effects.
In 2016, the Health Sciences Authority (HSA) of Singapore recognized the novelty of our treatment approach and the potential of our preclinical results. This paved the way for us to conduct first-in-man trials in Singapore using PRL3-zumab for targeted antibody therapy of cancer. I am very grateful to HSA and many local clinicians who continue to support this home-grown project.
Working with a team of oncologists in the department of hematology-oncology at the National University Cancer Institute, Singapore, we completed a Phase I clinical trial in 2018 to evaluate the safety of PRL3-zumab in 23 cancer patients. We concluded that the drug is very safe and showed some early efficacies—there is no dose-limiting toxicity.
4. What were some of the challenges you faced in starting Intra-ImmuSG?
The foremost difficulty was getting people to trust and have confidence in PRL3-zumab, to believe in the therapeutic potential of an antibody that targets an intracellular protein. But I think the years of research and the promising findings surrounding the use of PRL3-zumab in various cancer models speak for themselves.
Another major challenge was producing fresh PRL3-zumab for clinical trials. The production of the drug is very expensive, yet we provide the drug for free and cover most of the medical costs to patients enrolled in clinical trials.
In 2015, A*ccelerate Technologies Pte Ltd, the commercialization arm of A*STAR, encouraged me to spin off Intra-ImmuSG to further develop PRL3-zumab for the clinic. When that happened, I was very lucky to have received the backing of an angel investor to finance my R&D and clinical trials. A*STAR still supports Intra-ImmuSG in terms of maintaining the patent portfolio family for PRL3-zumab, with worldwide coverage.
Currently, we face challenges in recruiting patients for clinical trials. Patients eligible for PRL3-zumab treatment must have run out of all standard-of-care treatments. By then, the patients’ immune system is very weak and their response to treatment may not be as ideal as we hope for. Nevertheless, I am very excited to see that the drug has been found to be safe and has shown early signs of efficacy.
5. Going forward, what are your plans for Intra-ImmuSG?
We have an ongoing HSA-approved Phase II clinical trial at the National Cancer Center Singapore where we’re testing the efficacy of PRL3-zumab in late-stage cancer patients with solid tumors. So far, we have enrolled one liver cancer patient, and our treatment has stabilized the patient’s acute condition, so we’re optimistic. We are also carrying out a Phase Ib extension clinical trial with the National University Hospital, Singapore to further validate the safety of PRL3-zumab in patients suffering from leukemia.
Having said that, there is a sense of urgency to recruit more patients because our patent on PRL3-zumab will eventually expire. Hence, I am very delighted that we have just received approval under the US FDA’s Investigational New Drug program to carry out Phase II clinical trials in the US on patients with any type of solid tumors. We hope to initiate the trial in the US as soon as possible.
We are also applying to the National Medical Products Administration, China, and the drug administrations of other countries to run clinical trials. This way, we may be able to speed up the patient recruitment process.
Finally, in addition to PRL3-zumab, we have developed a pipeline of drug/vaccine candidates for further testing and validation. We are awaiting a new era of cancer immunotherapy in the near future.