Regulatory barriers to xenotransplantation

Abstract

Purpose of review

There is a grave discordance between supply and demand for patients with failing organs largely due to an insufficient donor pool for transplantation. Xenotransplantation has been proposed as a solution to bridge this gap.

Recent findings

Recent success over the last decade in nonhuman primate models, due to emerging gene-editing technologies combined with novel immunosuppression regimens, has produced promising results in pancreatic islet cell, heart, lung, kidney and liver xenotransplantations.

Summary

As the prospect of xenotransplantation is realized, safety and ethical considerations have come to the forefront of discussion. The WHO and World Health Assembly have encouraged member states to form regulatory bodies to govern human xenotransplantation studies with the highest standards. Here, we summarize the current regulatory landscape governing preclinical advances toward the first human clinical trials.

INTRODUCTION

Although over 135 000 transplants are conducted worldwide each year, it is estimated to be less than 10% of the actual global need for failing hearts, lungs, kidneys, livers, pancreas and small bowel [1]. This discordance between supply and demand is largely due to an insufficient donor pool. Living kidney transplants have been performed as early as the 1960s (and now account for ~40% of overall kidney transplants) and lung and liver transplants to a much less extent over the last 2 decades. Despite living donor transplants supplementing the overall transplantation volume, the demand for transplant organs remains largely unmet. This has inspired physicians and scientists around the world to develop technologies to ‘bridge the gap’, allowing capabilities to transplant porcine cells, tissues and organs to humans (i.e. xenotransplantation). Moreover, recent success in preclinical nonhuman primate (NHP) models for organ transplantation has further energized this prospect. As the prospect of clinical success with xenotransplantation increases, further consideration for the establishment of international regulations and guidelines of nonhuman products for human use has become necessary.

As the prospect of xenotransplantation is realized, safety and ethical considerations have come to the forefront of discussion. Fundamentally, the theoretical benefits and broad applications to a number of end-organ diseases afflicting the heart, lung, liver, kidney and pancreas must be balanced with the potential negative effects it may have on the transplant patient population and/or the community at large. Most apparent may be the potential risk for the transmission of zoonotic infections through xenotransplantation. The WHO produced a call to action for the international community to address several of these issues in 2001 and again in 2004 with the ‘WHO Guidance on Xenogeneic Infection/Disease Surveillance and Response: A strategy for International Cooperation and Coordination’. These initial discussions subsequently led to the 1st Changsha Communiqué in 2008 [2,3].

First held in Changsha, China, this commission proposed several shared beliefs among international experts that urged adherence by both member states of the WHO and future clinical investigators of xenotransplantation. It included several declarations and principles, namely, animal sources should be from closed herds that are pathogen-free with high standards for animal welfare and human clinical trials should be carefully planned using guidance from rigorous preclinical data. Furthermore, governments of member countries should transparently regulate xenotransplantation clinical trials with the highest scientific and ethical standards. Finally, patients undergoing clinical trials must have transparent and informed consent and be willing to accept compliance to a set of standards to minimize risk to themselves and society. Moreover, a system for surveillance of xenotransplantation-related infection and the ability for medical teams to recognize and respond them in a timely manner should be in place. Now, within a 10-year timeframe, two additional updates of the Changsha Communiqué have been produced. The 2nd Changsha Communiqué in Geneva, Switzerland focused on reviewing the first regulated xenotransplantation clinical trial and demonstrated the need for further regulation of ongoing unregulated clinical trials in some member states. In addition, it called for a further need for surveillance of possible infectious disease scenarios for clinical trials. The 3rd iteration, which consists of 36 experts, is currently underway [4^▪▪].

The WHO and World Health Assembly statements have encouraged several regulatory bodies including the US Food and Drug Administration (FDA), Council of Europe and United Kingdom Department of Health and stances from both the International Xenotransplantation Association (IXA) and TTS [5,6^▪,7,8]. Perhaps the most progressive of regulatory documents is the FDA’s guidance on xenotransplantation, which regulates all aspects of xenotransplantation product development and production in the United States. This includes ‘live organs, tissues or cells from a nonhuman animal source or xenotransplantation product materials used in encapsulated form or in which nonhuman live organs, tissues or cells have ex-vivo contact with human body fluids, cells, tissues or organs that are subsequently given to a human recipient’. The FDA also approves all xenotransplantation products undergoing clinical investigation in the United States. Considerations prior to clinical trials include animal donor safety and welfare, appropriately qualified herd sources, surveillance of donors and recipients, proven efficacy of xenotransplantation products for transplantation to recipient. Outlined is the general regulatory framework governing xenotransplantation today, summarized by the FDA, WHO and IXA, with European Union guidelines being very similar.

Animal donor safety and welfare

Just as the welfare of animals is regulated by the Institutional Animal Care and Use Committee and housing facilities are accredited by the Association for Assessment and Accreditation of Laboratory Animal Care, animals used in preclinical and clinical trials must also hold these standards for husbandry, harvesting and euthanasia [9,10]. In addition, source animals should only be used from closed herds that are closely monitored, but should also be quarantined and regularly screened prior to use for infectious diseases dangerous to both the donor and recipient, including transmissible spongiform encephalopathies, in which applicable. Procedures should be developed to identify and prevent harm to source animals [6^▪,11].

Appropriately qualified herd sources

Full consideration of zoonotic-related infectious diseases is paramount to reducing the risk to benefit burden to patients and society as we progress to conducting clinical trials. However, this risk may be less than previously thought. While typical infections in allotransplantation such as HIV, hepatitis C virus and hepatitis B virus have strong implications in recipients, they can generally only infect human cells [12^▪]. As such, screening and routine monitoring should be conducted to ensure harmful infectious agents are not present or introduced into a closed herd that are xenotransplant-specific. Herds absent of predetermined agents may cause harm to recipients of a potential xenotransplant and can be ‘designated pathogen free’ (DPF). Thus, the control of transplant-related infections may be even superior to that of allotransplantation due to strict control of source organs. Regardless, potential designated pathogens have been compiled and described by Fishman et al. [11,12^▪] as well as through WHO guidelines. Periodic screening by necropsy is recommended to monitor for subclinical infections [6^▪].

Surveillance of donors and recipients

As it pertains to recipient surveillance of xenotransplantation and the public health at large, archiving samples of the source animals for each individual recipient via necropsy or at the time of donor death is essential. This would facilitate investigation of an inciting event after an unanticipated complication from xenotransplantation. However, the storage time-period remains a point of contention. The FDA recommends 50 years from initial sample requisition, compared with the United Kingdom’s Department of Health’s recommendation of 30 years [6^▪,13]. Both of recommendations represent challenges from a logistical storage standpoint. Furthermore, the requirement may somewhat be arbitrary as it is unknown that the lag-time truly exists between transplantation and potential immunogenic complications of the recipient or close contacts susceptible to infectious complications [14].

Moreover, assays must be validated that have acceptable specificities, sensitivities and reproducibility for infectious agents. Common human-relevant bacteria, fungi and micafungin already have clinical assays that meet this standard in allotransplantation as they are used routinely in standard clinical settings. However, porcine endogenous retroviruses (PERV) present another unique set of circumstances, as standard clinical assays for these viral infections do not exist and it is not known the clinical or societal impact of these infections. Recent advances in knowledge of PERV have addressed some of these concerns; there are some assays available to detect PERV in serum and there are some active antiretroviral drugs that have activity against PERV but indeed are limited [15]. Also, all known 62 copies of PERV have been shown to be inactivated in immortal porcine cell lines through multiplexed genome engineering, preventing transmission to human cells [16]. In addition, first human clinical trials in xenotransplantation have failed to show actual transmission of PERV [17]. Regardless, PERV surveillance programs should still be prioritized.

The WHO has also published recent consensus guidelines as how to conduct surveillance activities in human xenotransplantation [11]. They center on routine screening of source animals, recipients, casual and sexual contacts of recipients and immediate cessation of clinical trials until evaluation of infectious syndromes as they develop or are detected until further evaluation. Evaluation includes typical allotransplantation screening (serologic testing and cultures of blood, sputum, cerebrospinal fluid and urine as appropriate for suspected bacterial, viral, fungal and parasitic pathogens) and screening for typical porcine-derived pathogens, such as PERV, swine influenza, porcine lymphotropic herpesvirus and porcine cytomegalovirus. Testing of the source animal would depend on archived samples as previously mentioned.

Disposal genetically modified animals and animal products for xenotransplantation also presents a unique set of circumstances, especially in species (like swine), which have traditionally been used for breeding or as a food source. The FDA’s chief concern is to prevent edible xenotransplantation products from entering the food chain. Although the FDA does not entirely rule out the possibility of using these products as potential human food sources, it does require consultation with FDA’s Center for Veterinary Medicine [7]. It does, however, prohibit the use of these products for use as food for other animals. In addition, the FDA suggests disposing these animals and byproducts as medical waste in compliance with federal, state and local law by incineration, composting or burial [6^▪].

Proven efficacy of xenotransplantation products

Several preclinical considerations must also be met prior to bringing xenotransplantation to clinical trials, which are equally important to infection prevention and containment considerations. First and foremost, xenotransplantation should only be offered to patients with serious or life-threatening diseases for whom proven standard-of-care therapies are not available. In addition, preclinical studies should prove physiologic similarities to its human analog. For example, in cells, tissues or organs producing a gene product (e.g. protein), it should be of similar function of the replacement human protein or therapy. Insulin, in a pancreas islet cell xenotransplant product, should be of similar pharmacodynamic properties of human insulin and be able to sustain normoglycemia in diabetic models. As another example, cardiac solid organ xenotransplantation, should be able to produce enough cardiac output to perfuse organs and sustain life in the human recipient for enough time as either destination therapy to improve longevity or as a bridge to transplantation at similar rates of current therapies. In addition, the survival of xenogeneic cells, tissues or organs in animal models should be demonstrated along with the characterization of its immunologic tolerance or rejection. This includes graft versus host disease, fibrotic encapsulation’s effect on xenogeneic transplant function, any predisposition of rejection of subsequent xenotransplantation or allotransplantation products as a result of prior rejection.

It should be noted though, that robust preclinical data requirements should not be so prohibitive as to prevent initial clinical trials in patients with end-stage organ failure when all other current medical therapies have been exhausted. There is a clear, direct benefit to the introduction of xenotransplantation products toward novel clinical applications that have not been met by more traditional approaches [18]. Thus, while no regulations are in place, consensus documents from several international organizations have been developed for heart, lung, kidney and pancreas islet cell xenotransplantations. For thoracic organs, the advisory committee of the International Society for Heart and Lung Transplantation suggests that preclinical efficacy is supported when a majority (>60%) of transplants of porcine to NHP models’ life sustaining survival is greater than 3 months, with some evidence that these organs can sustain life for greater than 6 months [19]. The IXA published consensus guidelines for clinical efficacy of pancreatic islet cell transplantation prior to human clinical trials. The consensus guidelines state that the majority (four of six, or five of eight) NHP models should have reversal of diabetes or greatly reduced insulin burden for normoglycemia in a 6-month follow-up period (but ideally 12-month follow-up) [20]. Consensus guidelines for kidney have yet to be developed, but some have suggested a 90% 1-year survival rate off of hemodialysis in highly sensitized individuals [21]. Lastly, the success of solid organ transplant to date has largely been as a result of novel immunosuppression regimens, namely, costimulation blockade that has not been approved by the FDA. However, the FDA has made clear that proven efficacy and safety of these drugs, as in most drugs formally approved by the FDA, will not preclude them from being used for a xenotransplantation clinical trial [22].

Although there are some subtle differences between the regulatory frameworks of cell-based therapies compared with organ-based therapies those are not differentiated here. In addition, these guidelines are not specific regulatory law, as those are based on local governing authorities. However, the FDA, as an example, does have regulatory authority as granted by the US Congress. The aforementioned guidelines solely serve as a regulatory framework for the basis of safe xenotransplantation, an emerging technology surpassing current specific law in most countries.

Initial regulated clinical trials with xenografts have been performed successfully; New Zealand was the first country to approve a clinical trial under these regulatory frameworks. It was approved by the Minister of Health of New Zealand and also approved by a national ethics committee. It was also registered with the US National Institute of Health. Preclinical efficacy data and need were demonstrated and source DPF herds were developed. The study enrolled 14 patients between July 2009 and March 2011 with unstable type I diabetes despite maximum medical therapy. These patients underwent transplantation of porcine pancreas beta islet cells that produce porcine insulin encapsulated in an alginate, which allowed isolation of these cells from the immune system. Transplanted into the peritoneal cavity with these alginate microcapsules, patients did not require immunosuppression. While 60% of patients showed some circulating porcine insulin 1 year after xenotransplantation, there were no significant changes in baseline exogenous insulin requirements or hemoglobin A1c levels. However, most importantly, there were no instances of PERV or other xenotransplantation-related infections [17,23]. The first xenotransplantation trial in the United States was just approved by the FDA and began enrolling patients in March 2019. It is a Phase I, open-label, nonrandomized trial, to assess the safety and tolerability of porcine skin xenografts for patients with severe burns [24].

CONCLUSION

Just as the advent of novel immunosuppression agents in the 20th century paved the way for allotransplantation, exciting advances transgenic engineering in the last decade has allowed for substantial progress in the field of xenotransplantation and provided renewed optimism for the prospect of clinical applications. Novel gene-editing technologies such as CRISPR/Cas-9 allow scientists to engineer pigs with genetic knock-outs to highly immunogenic epitopes such as α1–3 galactosidase transferase or engineer expression of human transgenes that regulate the coagulation cascade and compliment regulatory proteins with unprecedented immunocompatibility for cross-species transplant. While only a few regulated human xenotransplantation trials have been approved to date, preclinical results in many other xenotransplantation domains have produced encouraging results, most notably in islet-cell, heart and kidney transplants that would suggest that the first clinical trials in solid organ xenotransplantation may also be on the horizon [25–28].

KEY POINTS.

• There is a clear, direct benefit to the introduction of xenotransplantation products toward novel clinical applications when all other traditional medical options have been exhausted.

• Consensus documents from several international organizations have been developed for heart, lung and pancreas islet cell xenotransplantations for suggested preclinical efficacy requirements acceptable for clinical trials xenotransplantation.

• Zoonotic infection screening, prevention and mitigation are paramount to the safety and welfare of the recipient and society at large.

• Surveillance of donors and recipients is required for infection mitigation and immunologic characterization of xenograft/recipient response. This requires some level of biologic sample archiving of donor and recipient tissue and plasma.