This transcript has been edited for clarity.
Ileana L. Piña, MD, MPH: Hello. I'm Ileana Piña, professor of medicine at Thomas Jefferson University in Philadelphia. I am part of the heart failure transplant team and the chief quality officer. This is a more recent position for me.
I am absolutely thrilled to have with me Dr Bart Griffith, whom I've known for a long time, and his reputation precedes him as a superb cardiac surgeon transplant surgeon. He made the news this year with a xenotransplantation. Since I was in medical school, I've heard about xenotransplantation, but it's never been a reality. We've asked him to come here today. He is chair of cardiac surgery at the University of Maryland, where he has been for how many years now, Bart?
Bartley P. Griffith, MD: Some would say too many. More than 20 years.
Piña: That is where I first met him long ago. Welcome, Bart. Tell me, how long have you been thinking about a xenotransplant into a human?
Xenotransplant Hype and Hurdles
Griffith: I'm probably a good person to ask that question to because like many folks my age or just a little bit younger, I'm someone who looked at xenotransplant as one of those things that very few people have gotten into for many reasons. It seemed that over the decades, somebody would get very interested, go to the meetings wearing a necktie with a pig on it, give a couple of talks, and then they would be lost to follow-up. They'd lose funding or interest or there would be a hurdle.
I never really took it that seriously until about 7 years ago, when my now-partner Muhammad Mohiuddin, MBBS, moved from the National Institutes of Health (NIH) to join us at the University of Maryland in the research laboratory. He did so after having contributed significantly to the xeno field using a gene-edited animal and obtaining in the heterotopic, intra-abdominal, non–life sustaining model of the baboon greater than 3 years of survival without rejection.
He showed that the antirejection medication, which he helped to pioneer, in fact, was critical for that survival because when he stopped it after 3 years, those hearts stopped beating in the abdomen. It appeared that he had — I don't know whether it was luck or just a combination — got the right combination of immune suppression such that the hearts seemed to be regularly surviving well past a year. Again, these were nonstressed hearts; these were in the abdomen.
He was recruited to work with someone like me who knew how to work in a laboratory in a large animal model, and we converted his lessons to the intrathoracic, orthotopic heart transplant model. We struggled a little bit because the hearts kept going into this postimplant dysfunction. Chris McGregor, MB, MD, had termed this xenograft heart dysfunction postimplant, and he thought it was maybe related to calcium metabolism.
Well, it wasn't until our good friends in Munich were able to demonstrate, by way of a wonderful paper, 3-month survival in the nonhuman primate orthotopic position of the gene-edited pig heart that we cracked the secret. The secret was that if we put those pig hearts immediately on a nonischemic perfusion system... And in our case, we used that which the Germans had used, which was ex vivo, and has a lot of Steen solution and some blood and all kinds of stuff in there.
Basically, I don't think there's any magic, but it kept the heart nice and cool. It was a cool preservation and it had a perfusion pressure at the root of the aorta of about 20 mL, so not high pressure, just a nice gentle kind of flow through, and we were able to suddenly get excellent survival. Animals were extubated in a cage within 2 hours of coming off bypass. Then we took off. All of those frustrations in the past and very expensive modeling seemed like a distant past to us. We got routine survivals at 3 months, then 6 months, and now we're beginning to look at each other and say, "Wow, this is going to work."
Our sponsor, which was Revivicor, a subsidiary of United Therapeutics, was in the process of beginning to think about a US Food and Drug Administration (FDA)–sponsored new drug evaluation, a New Drug Application (NDA). We're not used to following the Investigational New Drugs (INDs) process for devices, but the gene-edited pig heart is considered a drug so it goes through that regulatory quagmire, if you wish. We were beginning to meet with the FDA for premeetings, so they had a portfolio of our outcomes. We knew it was going to be maybe 2 years before that multicenter trial might get launched — and more likely 3 years — based upon what we're hearing back about additional studies that needed to be done.
Dr Mohiuddin and I looked at each other and said, "What are we going to learn? We've learned a ton to this point. Once we get into that IND, we can't change anything." I think we're maxed out. Are we going to wait 3 years before we expose a patient to what we already think we know? Is there a group of patients that might be critically ill without option that we just might help? That's how we got to it. Sorry if that was long-winded.
Piña: I think that's perfect. My first question was about the immunosuppression protocols that he used at the NIH on the baboon. How different are they from what we use today with the mycophenolate and the steroids?
Griffith: It was surprisingly very similar. From my previous understandings of this work, it required radiation therapy and all kinds of intense immune suppression. Well, other than some early induction therapy with antithymocyte globulin (ATG) and rituximab to knock B cells down and T cells early, which is unusual for us in clinical work unless there's a high panel reactive antibody (PRA) or some kind of antibody-mediated issue or renal failure, and you want to be tacrolimus-avoiding.
Piña: We don't usually do that.
Griffith: Basically, other than that introduction, it looked like a pretty standard three-drug regimen. The odd part was that we avoided the use of tac, and the backbone of our therapy was an anti-CD40 antibody. Anti-CD40 attacks the receptor site of the B cell and it doesn't enable the B cell to activate when trying to be engaged by the T-cell population. It is very true that xenograft rejection likely is B-cell oriented, much more so than allo–T-cell business.
Piña: I would think so.
Griffith: The T cells are involved but much less so than they would be. The regimen was anti-CD40, a standard dose of mycophenolate mofetil (MMF), and then really quick-wean steroids.
Piña: You even weaned them quickly. That's interesting. The patient expired eventually. I know that you guys have looked at that graft very carefully. What did it look like?
Griffith: Well, it's remarkable, actually. I don't think you would believe it if you looked at it. I guess at 32 days, we did our first endomyocardial biopsy. He was bone marrow deficient and had a very low platelet count. We were very concerned to do biopsies.
Good Biopsy Then A Decline
Griffith: We were nervous that we might get into trouble because of the very low platelet count. We did an elective biopsy at 32 weeks because we couldn't stand waiting much longer. The scientific need to know...
Griffith: We looked at it; it was perfectly normal. I ran around the hospital showing it. I took the slide, and I just showed it to everybody, not that there were microscopes around. I was so excited because it looked like an absolutely normal biopsy. There wasn't one inflammatory cell that we could find in the interstitium.
At that time, the heart was squeezing like a rock star. We had beautiful strain echoes, beautiful diastolic relaxation, good systolic function. Really, almost unbelievable, right? If I were to show you this, you would just shake your head in disbelief. That's a pig heart that's been in there a month. Are you kidding?
It was very unnerving, but exciting, too. We did get into more bone-marrow white-cell trouble. We had to drop the MMF out. We reduced the steroids very low. We were basically on monotherapy with anti-CD40 for a long period of time and we gave that regularly in order to keep the level at a point that we thought might be adequate — that drug had never been in a human. We were guessing from what our laboratory had told us. It was all guesstimating.
What happened, we believe, is one of three things. He got sick at about 45 days. He looked septic. His circulating immunoglobulin G (IgG) was very low, at around 170 mg/dL. I think it's supposed to be 1000 mg/dL, so it was supposed to be much higher than it was. I remember we were all sitting around on the weekend and saying, "Well, my goodness, that's not good. He's getting septic and he may not be here tomorrow." We gave him antibiotics, antifungals, antivirals, and then intravenous immunoglobulin (IVIG).
Well, IVIG is a pooled sample. We were very careful about not knowingly giving him antibodies against his heart, but he did have anti-porcine antibodies in that pooled sample of IVIG we gave him. Unfortunately, those antibodies were shown to coat the pig heart but they were noncytolytic. They didn't seem to cause any injury to the clone cells that we could test them against. Still, could it have triggered something? Maybe. He got through that episode. Again, treated with all kinds of stuff.
Piña: COVID negative?
Griffith: Yes, we tested over and over. We had the world's best people looking at him, in my opinion. Everything came out pretty negative, but he got better. He got better for another 6 or 7 days, was perfect and sitting in a chair for the first time in 3 months by himself. We were talking about sending him home to a rehab center.
Then, bang, overnight, his heart just showed a devastating change on echo, which I had never seen before. It just began to swell. The walls of the heart doubled in thickness, literally overnight. It was almost as if you turned your ankle and your ankle were to swell up. Biopsy showed there was tremendous edema in the heart. Again, no inflammatory cells.
Piña: That's so unusual, but his IgG was so low of his own. Maybe he just couldn't mount it.
Griffith: We didn't know what was going on, frankly. We hadn't seen this in the animals. We've seen swelling of the heart, and we thought that could be a manifestation of rejection, but never so fast. Not overnight.
He got into trouble. His last 10 days, he was on extracorporeal membrane oxygenation (ECMO) to support him with the hopes that we could reverse it. We never could. We plexed him, we gave him all the kinds of stuff you might imagine. The heart just kept getting worse and thicker. We had systolic preservation initially and no diastolic function, but eventually we lost everything. The end result of the tissue shows, again, no inflammatory infiltrate but an odd loss of various capillary networks where the endothelium had just blown away, just lysed away.
Piña: Almost ischemic in origin, do you think?
Griffith: I would agree to that. It's not uncommon in the xeno world to have platelet plugs when you have uncontrolled rejection. That's really how they die. They die an ugly-looking death where they turn purple. That wasn't this case. It looked like there were no platelets. It almost looked as if you'd gone in with a micro rake and you just pulled out all the capillaries in certain areas. Downstream, where those capillaries were flowing, myocytes just didn't get adequate perfusion and those myocytes had died.
Piña: They weren't perfused.
Griffith: We're left with still an unknown, and it could be that the IVIG stimulated something because the timing was kind of right. Then again, remember, he got sick, and we treated him. Whether the treatment was only evanescent, we're not sure. We did learn and had been following, actually, from day 20, using DNA analysis of his serum, the recipient serum. We found porcine cytomegalovirus (CMV).
Piña: I was going to ask you about viruses jumping species.
Griffith: Yes. Well, of course we're all worried about that. Porcine CMV is a little misnamed. It's a herpes virus and it can be very sleepy. If it's not activated, it can be hard to pick up in the blood. What we were picking up in the blood was DNA from shed cells. As any organ will shed its cells and then lyse the cells, you might expect to find some DNA if you had a virus that was within that heart. Even though we tested the pig four times, and it was negative for this virus...
Piña: It could be dormant.
Griffith: We just didn't pick it up. Although they had a decade of experience using different things, they were quite comfortable that it wasn't there, but it was obvious that it was.
Piña: That's fascinating.
Griffith: It didn't really infect the patient. It infected the heart that we put into the patient.
Piña: It was targeted to the heart. It had a myocardial target.
Xenotransplantation Next Steps
Piña: We heard about the xenotransplantation into cadavers at NYU. What are you going to do next?
Griffith: We've done a lot of work with the NIH and our own pathologists of international experts looking at this tissue. We think we have it figured out enough. We think we should take a less sick patient. We have a better polymerase chain reaction (PCR) now to screen for that inactive virus so I think we can exclude it. We can even biopsy spleen at the same time. I think we're going to go a little gentle into the next immune suppression since we didn't really see obvious rejection. Although it could have been, in the end, rejection. I don't think so.
Piña: It doesn't sound like it without inflammatory cells around.
Griffith: It would be quite odd. Too odd for my liking.
Piña: Would you then prep the patient with an antiviral specifically for this porcine CMV?
Griffith: Yes, we would. We did. We treated it, but it might have been too little too late. I think we have to avoid that virus for the time being.
Piña: Pretreatment might exclude it.
Griffith: Well, on the animal, yeah.
Piña: Yeah. On the animal.
Griffith: We'll take a less sick patient with a better bone marrow reserve; we'll be able to stick to our gun; we'll go less T-cell suppression. I think we'll avoid ATG altogether as part of induction. We do believe the B-cell rituximab is important and we probably shouldn't give that up yet. I think we'll go for it and we're going to put that in a compendium. I'm certain a patient is going to pop up. When we find a patient, we will readdress the FDA for permission to try a second patient on a kind of expedited, extended trial.
Piña: To us who do this every day, you get a really frail, sick patient with all the comorbidities, it's much harder to show anything. I mean, this gentleman's course obviously was going in a very wrong direction without the xenotransplantation.
Incredible work, Bart. You'll go down into textbooks over this work. Unfortunately, the gentleman didn't make it, but I think you have learned tremendous amounts.
Griffith: We're going to keep plugging. There's much more to be learned. I'd like to be sure that if we do get permission to go forward with an IND, we get a little bit more patient experience so that IND will be a much better protocol. It will be safer and more likely to represent it because, frankly, if we hadn't done the experimental part of this with a patient, we might not have learned about that viral likelihood.
Piña: I want to thank you for the time that you've spent with us. It's been absolutely fascinating, and I know how busy you are. We appreciate your time with me.
Griffith: It's been an honor, and I hope this is useful.
Piña: It certainly will be. I hope the audience enjoys listening to this. We've learned so much. All the pieces that Bart has talked about are really important to this whole immuno cascade, which can be so incredibly challenging for those of us who often take care of the transplant patients.
I'm signing off from my blog. Thank you for joining me today. I'm Ileana Piña, signing off. Have a great day.
Ileana L. Piña, MD, MPH, is a heart failure and cardiac transplantation expert. She serves as an advisor/consultant to the FDA's Center for Devices and Radiological Health and has been a volunteer for the American Heart Association since 1982. Originally from Havana, Cuba, she is passionate about enrolling more women and minorities in clinical trials. She also enjoys cooking and taking spin classes.
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