Xenotransplantation

The usefulness and limitations of the diabetic macaque model in evaluating long-term porcine islet xenograft survival

BACKGROUND

  Various groups have reported prolonged diabetes reversal and graft function after porcine islet transplantation into diabetic macaques using different experimental designs (macaque source, islet source, type of immunosuppression): subsequently, the International Xenotransplantation Association has published recommendations for entering a clinical trial. Our experiments showed limitations that affected consistent achievement of long-term survival. We aimed to identify these limitations and underlying causes to emphasize the translational value of this highly relevant type 1 diabetic macaque model.

METHODS

  We reviewed data from our institution and literature data on long-term porcine islet xenograft survival in the diabetic macaque model, especially focusing on aspects of incomplete diabetes reversal relative to macaque normal values. This phenomenon was compared with diabetes reversal in an allo-islet transplant model in macaques and with chronic insulin treatment of diabetic macaques, all with 180-day follow-up. This comparison enabled to identify potential model limitations and underlying causative factors.

RESULTS

  Especially in the xenograft model, the achievement of long-term graft survival revealed limitations including chronic, mild hyperglycemia and absence of body weight (BW) gain or even progressive BW loss. Metabolic incompatibilities in glycemic control (i.e., insulin kinetics) between the pig and macaque species underlie chronic, mild hyperglycemia. This phenomenon might not bear relevance for the pig-to-human species combination because the glycemic control in pigs and humans is similar and differs from that in nonhuman primates (NHP). Weight loss could be related to changes in the gastrointestinal tract related with local high exposure to orally administered immunosuppressants; these must be given at higher dose levels because of low bioavailability in macaques to achieve systemic exposure at therapeutic levels. This is aggravated by insufficient graft insulin production in proportion to the needs of macaques: this model limitation has no translational value to the pig-to-human setting. Nutritional deficits can result in incorrect interpretation of blood glucose levels and C-peptide levels regarding graft function. Likewise, nutritional status alters physiologic responses, influencing susceptibility to infectious and noninfectious complications.

CONCLUSION

  THE model-induced confounding described interferes with accurate interpretation of safety and efficacy studies, which affects the translational value of pig-to-NHP islet cell transplant studies to the pig-to-human transplant condition.

Management of adverse side-effects after chemotherapy in macaques as exemplified by streptozotocin: case studies and recommendations

The chemotherapeutic streptozotocin is used for induction of diabetes in animal models including non-human primates. Being a cytotoxic nitrosourea compound, it can be associated with adverse events (AEs), mainly nausea and emesis, nephrotoxicity, elevated liver transaminase levels, pulmonary oedema and, most prominently, metabolic acidosis: these can be severe in some cases. The incidence and gravity are to some extent related to the characteristics of the individual animal, diagnostic tools, prompt recognition of symptoms and supportive measures. Careful animal selection, dose adaptation and supportive actions such as renal protective hydration are the main tools in managing AEs, but do not fully eliminate unavoidable and sometimes life-threatening conditions. In our centre we have built experience in a cohort of 78 cynomolgus and rhesus macaques in which six cases manifested severe AEs (8%). This experience has prompted implementation of strategies for early detection and management of adverse effects, together with an animal refinement programme. We present here specific pretreatment regimens, post-infusion laboratory evaluations, and flow charts to assess/treat metabolic acidosis and precipitating factors. Case reports of the six animals with severe AEs are presented to illustrate management of AEs, especially metabolic acidosis, and criteria for early euthanasia where appropriate. We conclude that improved monitoring and validated tools allow for optimal management of adverse effects in an early stage of their manifestation. Reduced morbidity and mortality not only improve individual animal wellbeing but also avoid model-induced confounding that diminishes the translational value of the experimental protocol.