Full circle
The porcine heart was protected along two fronts against assault from Mr. Bennett’s immune system and other inhospitable aspects of his physiology, either of which could also have been obstacles to success: Genetic modification (Revivicor) of the pig that provided the heart, and a singularly aggressive antirejection drug regimen for the patient.
The knockout of three genes targeting specific porcine cell-surface carbohydrates that provoke a strong human antibody response reportedly averted a hyperacute rejection response that would have caused the graft to fail almost immediately.
Other genetic manipulations, some using CRISPR technology, silenced genes encoded for porcine endogenous retroviruses. Others were aimed at controlling myocardial growth and stemming graft microangiopathy.
Mr. Bennett himself was treated with powerful immunosuppressants, including an investigational anti-CD40 monoclonal antibody (KPL-404, Kiniksa Pharmaceuticals) that, according to UMSOM, inhibits a well-recognized pathway critical to B-cell proliferation, T-cell activation, and antibody production.
“I suspect the patient may not have had rejection, but unfortunately, that intense immunosuppression really set him up – even if he had been half that age – for a very difficult time,” David A. Baran, MD, a cardiologist from Sentara Advanced Heart Failure Center, Norfolk, Va., who studies transplant immunology, said in an interview.
“This is in some ways like the original heart transplant in 1967, when the ability to do the surgery evolved before understanding of the immunosuppression needed. Four or 5 years later, heart transplantation almost died out, before the development of better immunosuppressants like cyclosporine and later tacrolimus,” Dr. Baran said.
“The current age, when we use less immunosuppression than ever, is based on 30 years of progressive success,” he noted. This landmark xenotransplantation “basically turns back the clock to a time when the intensity of immunosuppression by definition had to be extremely high, because we really didn’t know what to expect.”
Emerging role of xeno-organs
Xenotransplantation has been touted as potential strategy for expanding the pool of organs available for transplantation. Mr. Bennett’s “breakthrough surgery” takes the world “one step closer to solving the organ shortage crisis,” his surgeon, Dr. Griffith, announced soon after the procedure. “There are simply not enough donor human hearts available to meet the long list of potential recipients.”
But it’s not the only proposed approach. Measures could be taken, for example, to make more efficient use of the human organs that become available, partly by opening the field to additional less-than-ideal hearts and loosening regulatory mandates for projected graft survival.
“Every year, more than two-thirds of donor organs in the United States are discarded. So it’s not actually that we don’t have enough organs, it’s that we don’t have enough organs that people are willing to take,” Dr. Baran said. Still, it’s important to pursue all promising avenues, and “the genetic manipulation pathway is remarkable.”
But “honestly, organs such as kidneys probably make the most sense” for early study of xenotransplantation from pigs, he said. “The waiting list for kidneys is also very long, but if the kidney graft were to fail, the patient wouldn’t die. It would allow us to work out the immunosuppression without putting patients’ lives at risk.”
Often overlooked in assessments of organ demand, Dr. West said, is that “a lot of patients who could benefit from a transplant will never even be listed for a transplant.” It’s not clear why; perhaps they have multiple comorbidities, live too far from a transplant center, “or they’re too big or too small. Even if there were unlimited organs, you could never meet the needs of people who could benefit from transplantation.”
So even if more available donor organs were used, she said, there would still be a gap that xenotransplantation could help fill. “I’m very much in favor of research that allows us to continue to try to find a pathway to xenotransplantation. I think it’s critically important.”
Unquestionably, “we now need to have a dialogue to entertain how a technology like this, using modern medicine with gene editing, is really going to be utilized,” Dr. Mehra said. The Bennett case “does open up the field, but it also raises caution.” There should be broad participation to move the field forward, “coordinated through either societies or nationally allocated advisory committees that oversee the movement of this technology, to the next step.”
Ideally, that next step “would be to do a safety clinical trial in the right patient,” he said. “And the right patient, by definition, would be one who does not have a life-prolonging option, either mechanical circulatory support or allograft transplantation. That would be the goal.”
Dr. Mehra has reported receiving payments to his institution from Abbott for consulting; consulting fees from Janssen, Mesoblast, Broadview Ventures, Natera, Paragonix, Moderna, and the Baim Institute for Clinical Research; and serving on a scientific advisory board NuPulseCV, Leviticus, and FineHeart. Dr. Baran disclosed consulting for Getinge and LivaNova; speaking for Pfizer; and serving on trial steering committees for CareDx and Procyrion, all unrelated to xenotransplantation. Dr. West has declared no relevant conflicts.
A version of this article first appeared on Medscape.com.