In January 2012, the Islet Sheet Project will embark on the long-awaited and critical phase of metabolic studies in larger animals. The definitive experimental model for implantation with the Islet Sheet is mammals such as dogs, pigs, or monkeys made diabetic by total removal of the pancreas. In contrast to rats and mice, such animals can easily accommodate the 5-centimeter-square Islet Sheet designed for humans.
The Canine Model
Most of what we know about how islets and insulin work come from studies in a certain large animal, the dog. That the pancreas is the source of the “anti-diabetes hormone” was discovered in 1889, when dogs that had had their pancreases removed were found to be diabetic. Dogs have continued to be essential in experiments to analyze metabolism and diabetes physiology.
In all matters of importance to the Islet Sheet’s function, dogs are similar to humans. Surgical aspects are similar, including the size and location of the pancreas. And unlike smaller mammals, the pancreas of a dog is possible to remove intact. Pancreatectomy is reliable, and islets do not regenerate.
The immune system of the dog is similar to that of humans, especially in the vitally important foreign body reaction. Dog hormones can be measured easily, and the potency of dog insulin is essentially the same as human insulin. Even statistically, dogs are like humans. That is, only a few diabetic dogs—say, 6 to 12—are needed to prove that a therapy is working. This is because, as in human autoimmune diabetes, spontaneous remission of disease is rare.
The team’s animal physiologists—Richard Bergman and Marilyn Ader—are acknowledged experts in studying diabetes with canine models. This model for large-animal studies was selected on their recommendation.
Collaboration by Freeway
The logistics of this research phase are much streamlined because all the work will take place in greater Los Angeles: in the physiology lab of Drs. Bergman and Ader at Cedars-Sinai Diabetes and Obesity Research Center, and at the UC Irvine facility directed by Dr. Lakey. The sequence is:
- Recipient dogs are rendered diabetic by total pancreatectomy at Cedars-Sinai, with Dr. Lakey assisting. Removing 100 percent of the pancreas is extremely challenging surgery. Diabetes is confirmed with metabolic testing.
- Islets are obtained by performing pancreatectomies on other dogs, at Cedars-Sinai or Irvine. The procedure is similar but technically easier and with different goals: this surgery is designed to secure the pancreatic structure so that islets can be successfully isolated and digestion can be supported in the recipient dogs. The pancreases may be transported on ice from one lab to the other.
- Islets are isolated and extracted using the digestive enzyme collagenase, injected into the pancreatic duct, which runs through the head, body, and tail of the pancreas. Delivered this way, the enzyme solution causes distension of the pancreas, which is subsequently cut into small chunks and transferred into a Ricordi’s chamber, where digestion takes place, until the islets are liberated and removed from the solution. Isolated islets are then separated from the exocrine tissue and debris, using density gradients and centrifuges.
- Islet Sheets are fabricated containing these islets at UC Irvine, under the direction of Rick Storrs of Islet Sheet Medical. A 4 x 6 cm sheet can encapsulate about 100,000 islets. The sheets may be cultured for days or even frozen for storage.
- Islet Sheets are transported to Cedars-Sinai and implanted in the subject dogs by Drs. Bergman, Ader, and Lakey. Normally islets from two donors are pooled for implantation into a single recipient. A number of implant sites are under consideration, including the wall of the peritoneal cavity and the subcutaneous space. Sheets are held in place by sutures at the corners.
- Over a period of 6 to 24 months, the Bergman-Ader team monitors and tests the recipient dogs’ metabolism. The crucial test is the IVGTT (intravenous glucose tolerance test), which measures the hormonal secretions of the Islet Sheet as well as the metabolic response under the influence of an intravenous glucose bolus. In addition to metabolic testing, the reaction of the implant site is assessed through tissue analysis and histology.
- Data from these experiments are compiled and analyzed for eventual publication, and to support regulatory applications for clinical studies.
- For the upcoming large-animal studies, the team’s physiologists recommended a canine model.
- Dogs are similar to humans in ways important to how the Islet Sheet functions: for example, immune response and potency of insulin they produce.
- The process of removing the canine pancreases, extracting islets, fabricating and implanting Islet Sheets, and monitoring the study animals is highly complex and challenging, but our team members are the best in their fields and proximity will streamline the effort.