Dr.Ira Pastan（Co-Chief，National Cancer Institute，Member of National Academy of Sciences

The Chinese Antibody Society (“the Society”) gratefully thanks Dr. Ira Pastan for participating in an interview with the Society’s volunteers Peng Lin, Ph.D., J.D., and Cong Yao, Ph.D., J.D. on December 20, 2018. During the interview, Dr. Pastan discussed his research experience, including the research which eventually led to an FDA approved drug moxetumomab pasudotox, shared his insights on the challenges and opportunities for immunotoxin-based immunotherapy, and provided helpful suggestions to the Society, especially on the Society’s peer-reviewed journal Antibody Therapeutics. Dr. Pastan also kindly agreed to speak at the Society’s third annual conference, which was held on April 7th, 2019, in Cambridge, MA. Below is the transcript for the interview. This transcript has been edited for publication purpose

Interviewers: This is Peng Lin and Cong Yao from the Chinese Antibody Society. Thank you so much for agreeing to do an interview with us. Cong and I are both volunteers for the Chinese Antibody Society. Before the interview, we would like to briefly introduce our society. Our society is a non-profit organization, established in 2017, and serves as a platform fostering communication and collaboration among its members worldwide. The mission of Chinese Antibody Society is to bring together industry leaders, healthcare providers, researchers, academics, and investors to connect and collaborate on the discovery, development, manufacturing, and marketing of therapeutic antibodies.

Interviewers: We had many successful events in the past, including our annual conference, Webinar series, and PharmaConnect. This interview is part of our new program Experts’ Perspective. As part of this program, we plan to interview renowned experts in the therapeutic antibody field, and we will publish the interview transcripts on the Society’s website and social media channels. We feel much honored to have you as our first expert in this program.

Interviewers: Today’s interview will include three parts. The first part centers on your background, research, and your current role in the NIH. For the second part, we hope to have your insights on the challenges and opportunities of immunotoxin based immunotherapy. For the third part, we will seek your suggestions on our society, especially on our society’s peer-reviewed journal Antibody Therapeutics.

Interviewers: We understand that you are the founder of the Laboratory of Molecular Biology (LMB) in the National Cancer Institute, and you are also the expert of immunotoxin therapies. Could you please briefly talk about your past experience, what made you decide to become a physician and then a scientist, and what your experience was like at the LMB of the NIH?

Dr. Pastan: I was born in Winthrop Massachusetts. I grew up in a working-class community.

When I was a kid, I was very good in science. At that time, being a doctor was a good career track for kids who were good at science. So I thought I probably would become a physician. I went to the Tufts College and majored in biology, and then to Tufts Medical School. In my first year of medical school, I talked to my advisor and told him that I wanted to do something related to medicine as a summer job, and asked him if he could give me some advice. My advisor was a clinician but also had some space in a lab that was headed by Dr. Ted Astwood. Ted was a professor of medicine and very famous endocrinologist. He had discovered anti-thyroid drugs that were used to treat hyperthyroidism. And he was also an editor of Endocrinology, the major endocrine journal at that time. So the summer after my first year in medical school I worked in an endocrine lab for my advisor Bill Vanderlaan and he later moved to California. Thus, I was able to work with Dr. Astwood. I worked in his lab all summers and was able to do research during the school year. That was when I fell in love with research and actually published my first paper in Endocrinology, when I was a medical student. Remarkably, I was the sole author. Imagine what a generous person my boss was.

Then I went to Yale in New Haven to do my residency training in medicine. At the end of my two-year residency training, I was drafted for military service and needed to go into the Army or the Navy, or do some other form of military service. At that time, if you were a doctor and did well in the medical school, you could apply to the NIH and become an officer in the Commissioned Corps of the Public Health Service, which would fulfill the military obligations. So many of us applied to the NIH and became officers in the US Public Health Service. Many young physicians participated in this program, which led to the expansion of NIH.

I joined the NIH in 1959. In the first two years, I worked in the clinical endocrinology branch of what was then called the National Institute of Arthritis and Metabolic Diseases. It is now called National Institute of Diabetes and Kidney Disease. I spent about 20% of my time seeing patients with endocrine disorders and thyroid disorders, and 80% of my time working in the lab. I discovered I liked working in a lab more than seeing the patients. I began to work on polypeptide hormones, e.g., thyroid stimulating hormone, and tried to figure out how it worked to stimulate thyroid growth and function. After doing that for two years, I realized that I did not have enough knowledge of biochemistry to do the research. Remember, molecular biology did not yet exist at that time. So I went to work with a well-known biochemist Earl Stadtman, who is known for his pioneering work on glutamine synthetase. He was also the president of the American Society of Biological Chemistry. Earl was a great biochemist and also a great mentor. I worked with him for two years. At the end of the two years (around 1963), I was offered a full-time job back in the endocrinology branch doing research and seeing patients.

My early research focused on how thyroid stimulating hormone controls thyroid function and my research interest later expanded into insulin and cyclic AMP (cAMP). One big question at that time was how peptide hormones outside the cells can regulate something inside the cells. We provided some of the first evidences that the receptors for polypeptide hormones were located on the outside of the cells. Polypeptide hormones do not enter cells to carry out their function, but they bind to the receptors on the outside of cells and somehow activate signals inside the cells. I also became interested in the function of cyclic AMP in cells and I began to collaborate with Bob Perlman. Together, we showed cyclic AMP plays a very important role in regulating gene expression in E. coli. Receptors and cyclic AMP were my two early research areas of interest.

Within a few years, I was asked to move to the National Cancer Institute and received additional resources to do my research. In the National Cancer Institute, I began to think about cancer. I realized that receptors probably have an important role in controlling cancer cell growth and that overexpression of some receptors could actually cause cancer. In one of my early projects, we showed that EGF receptors were overexpressed in many cancer cell lines. We further showed introducing an EGF receptor gene into the cells can transform the normal cells to cancer cells.

Later on Dave Fitzgerald joined my lab as a postdoctoral fellow. He came to my lab because he was interested in how toxins enter cells and we had developed methods to study this. David was interested in Pseudomonas exotoxin A and after a few months’ work, we showed that it enter the cell by a common endocytic pathway. While doing these studies we were impressed with how effective Pseudomonas toxin was in killing cells. So we thought that it might be a good idea to attach it to a monoclonal antibody to target and kill the cancer cells. Around that time, there was a lot of interest in using monoclonal antibodies to deliver radioisotopes, cytotoxic drugs and toxins to kill cancer cells.

Our initial efforts focused on figuring out the functions of different domains of Pseudomonas exotoxin A. We determined that one region of the toxin binds to the cell, and another region can kill the cell. Thus, we could get rid of the cell binding region, and replace it with an antibody or a piece of an antibody. Originally, we just coupled the modified toxin to an antibody. But then we became more sophisticated and one of my fellows, Vijay Chaudhary, constructed a fusion protein composed of the Fv portion of an antibody directly fused to the toxin. We called these fusion proteins recombinant immunotoxins (RITs). That was really a great breakthrough. We did this for several different immunotoxin targets. We made an immunotoxin targeting CD25 named LMB-2. It has been in clinical trials and has produced responses in patients with T cell leukemia. But the number of leukemias expressing CD25 is relatively low so then we decided to focus on CD22, because CD22 is expressed on many lymphomas and other B cell malignancies. We identified a monoclonal antibody that bound to CD22, cloned the Fv portion, and attached it to PE38 to make a RIT named BL22. We carried out clinical trials with BL22 and showed it produced complete remissions in some patients with drug-resistant Hairy Cell Leukemia, but it also produced hemolytic uremic syndrome (HUS) in a few patients.

To overcome this side effect, they altered the binding region of BL22 to increase its affinity, making it about 10-fold more active. This change reduced the frequency and severity of HUS. This new protein was named HA22 and was eventually licensed to Medimmune for clinical development. Medimmune named the agent Moxetumomab pasudotox (Moxe) and sponsored several clinical trials in which Moxe was shown to be safe and effective. These findings led to a world-wide multi-center clinical trial that was completed in 2018 in which 80% of patients achieved hematologic remission and 41%, achieved complete durable remissions. On the basis of these results FDA approval was granted in September 2018 and the agent is now marketed as Lumoxiti. Although approved for Refractory Hairy Cell Leukemia, Lumoxiti has shown anti-cancer activity in other B cell malignancies and clinical trials in other diseases are underway.

Interviewers: This is really a great story. Thank you for sharing the story with us. What is your current research interest? Are you still working in this particular area?

Dr. Pastan: Yes. Because targeting CD22 for B cell malignancies works really well, I decided to work on multiple myeloma. Multiple myeloma is another B cell malignancy, but the cells are more mature. One of my new projects is to develop an immunotoxin targeting BCMA. We have published a few articles, and we have shown that anti-BCMA immunotoxins are really effective in many models. A company is now working on it for clinical trials. I think one of the articles on targeting BCMA was published in your Society’s new journal, Antibody Therapeutics.

We also have some evidence showing that there is a synergy between immunomodulator therapies (e.g., checkpoint inhibitors) and immunotoxins. We have evidence that immunotoxins produce immunogenic cell death. We are now doing clinical trials combining an immunotoxin targeting mesothelioma with a checkpoint inhibitor.

Interviewers: That is really impressive. So in your opinion, what will be the future of immunotoxin-based immunotherapies? What will be the biggest challenge in immunotoxin in the next 5-10 years?

Dr. Pastan: I will give you a big picture. There is a lot of progress in this field, but if a patient has a metastatic cancer, except for melanoma, it is very likely that the patient is still going to die because of the metastatic cancer. So I believe we need different kinds of agents that can kill the cancer by different mechanisms, because the cancer cell evolves very quickly and becomes drug-resistant, e.g., resistant to taxane, taxane analoges, and doxorubicin. Anyway, cancer cells become resistant to chemotherapies very quickly. Inhibiting protein synthesis is a novel mechanism to kill the cancer cells. I think we need to purse using that in addition to everything else. One of the main barriers to this therapy is that in patients with normal immune system, anti-drug antibodies appear after one or two cycles, or three to six doses. So I have been working in various ways to remove B and T cell epitopes on the protein so that the patient will have less or delayed formation of ADAs. In summary, the most important advantage of immunotoxins is that they can kill cancer cells by a novel mechanism. The biggest barrier is the development of anti-drug antibodies.

Interviewers: For those immunotoxins, are you also looking for novel targets or novel cancer antigens?

Dr. Pastan: So you are asking me what would be a good target for immunotoxins? I spent a lot of time looking for novel targets for immunotoxins. For these antibodies with toxins that are attached to them, if the normal tissues express an antigen that the antibody is targeting, it will be a problem. Because normal tissues have good blood flow, as soon as the antibody drug conjugate is injected into the patient, the immunotoxin goes to those normal tissues. Some people may think that if the target is not expressed at high level in normal tissues, it might be okay. But I do not think so. I think that we need something that is not expressed in normal cells, e.g., the lineage restricted differentiation antigens, or something that was made in embryos but not an adult. So I think GPC3, CD19, CD20, CD22, CD25 and mesothelin are good targets.

Interviewers: This is very interesting. For the third part of our interview, do you have any suggestions for our society, in particular our society’s official journal, Antibody Therapeutics?

Dr. Pastan: So clearly the field of antibody-based therapies is still rapidly expanding. There is a need for good journals to publish good solid research papers in this field. The journal should not try to compete with the Cell or Nature, as I do not think that should be the function of the Society’s journal. I think the journal should ensure that the Society’s members would have a place to publish quickly and obtain wide publicity. Being able to publish quickly would be very helpful.

Interviewers: So your point is that one primary function of this journal is to provide a platform so that the members of the Society can publish their work very quickly.

Dr. Pastan: Yes, that is important. However, you should also be careful. All articles should be given a careful peer review.

Interviewers: As you may be aware of, Mitchell Ho is the Editor-In-Chief of the journal.

Dr. Pastan: Yes. Mitchell used to work very closely with me. He is a very remarkable man. I think he will do a good job.

Interviewers: We also notice that you have agreed to speak at our annual conference, which will be held on April 7, 2019 in Cambridge, MA. We are very curious what you will talk about at our annual conference.

Dr. Pastan: It is likely that my topic would be Immunotoxins: From Conception to FDA Approval.

Interviewers: We are also curious about your experience as a physician scientist. When we were doing the research, we had the impression that a lot of research are relatively basic and may not be perceived as closely related to immediate clinic use. Do you think that as researchers, we should focus more on something that is more clinically relevant?

Dr. Pastan: I think we have to be very careful about that. I spent most of my life doing basic research on receptors, gene regulation, and endocytosis. And then I did some work in multidrug-resistant cancer and immunotoxins, which are more clinically relevant. But as you know, the most important advancement in cancer therapy right now by is checkpoint inhibitors. These agents came out of basic research on immune responses and regulations. Therefore, if we did not have that kind of research trying to understand what causes inflammation, we would not have checkpoint inhibitors. Both the basic research and translational research are very important. I think we must support both.

Interviewers: After the basic research, how do you think we can facilitate the transition from the basic research to translational medicine?

Dr. Pastan: We need many different players. We need people doing basic research. We also need companies who have people with the right skills to get the drug to the market. This is a very complicated process.

Interviewers: One mission of the Chinese Antibody Society is to build up a platform that can bring everybody together.

Dr. Pastan: Yes, during my career making immunotoxins, I found speaking at meetings where companies were present was very important, because that provided me some context that helped me with the drug development. I do think it is very important to create an environment where everyone can get together and talk to each other.

Interviewers: Great! Hopefully, we can talk to you again at our annual meeting.

Dr. Pastan: OK. See you guys during the meeting!

Interviewers: We look forward to meeting you this April in Cambridge, MA! Thank you for your time.

Dr. Pastan: You are very welcome!