Rapid metabolic control after primary islet transplantation: relevance for stem cell-derived islet product development

oxygenation before imposing a high secretory demand on the engrafted β-cells (2). Since primary islets isolated from deceased donors-derived islets (also referred to as primary islets) are developmentally and functionally mature, they continue to sense glucose and secrete insulin immediately post-transplant, despite the stress of isolation and the relatively hypoxic transplant environment before arterialization. For this reason, administering exogenous insulin to reduce the need for β-cell insulin secretion, along with avoiding hyperglycemia, is a crucial strategy to reduce metabolic demand on transplanted islets during the engraftment period. The inevitable glucose-dependent nutrient-triggered insulin secretion from transplanted islets nonetheless contributes to achieving glycemic control targetstarget glucose control, with less glucose variability and less exogenous insulin requirements. To illustrate this, we present metabolic control outcomes during the first 75 days following an initial primary islet infusion for our previously described cohort (3) (Fig. 1).The eleven individuals (four men, seven women) were median (range) age 49 years (range 28-63), with median type 1 diabetes duration 34 years (range 12-50), BMI 24 kg/m 2 (19-26), and insulin requirements 0.47 units/kg/day (0.24-0.78) at the time of receiving a median of 7,879 islet equivalents per kg body weight (range 5,277-12,073). The islets for each infusion were isolated from a single donor pancreas (4). Induction immunosuppression was with antithymocyte globulin and etanercept, and maintenance immunosuppression with low-dose tacrolimus and sirolimus as previously described (1; 2). Blood glucose was monitored >4 times daily, averaged daily and every 7 days to obtain a weekly average blood glucose. Weekly glycemic lability indexes were calculated from all blood glucose measurements recorded at least an hour apart, as previously described (5), to measure temporal glucose variability. Insulin requirements were totaled each day and averaged every 7 days to calculate a weekly insulin requirement. Endogenous insulin secretion was estimated from serum C-peptide levels measured fasting and following meal stimulation, as follows: pre-transplant and on Dday 75 post-transplant, C-peptide was measured fasting and at 60-and 90-minutes following ingestion of 6 mL/kg (up to 360 mL) Boost® High Protein. On Ddays 3 and 7 post-transplant, C-peptide was measured fasting and at 60 and 120 minutes following a routine breakfast, with bolus insulin administered. On Ddays 28 and 56 post-transplant, C-peptide was measured fasting only. The C-peptide lower limit of detection was 0.05 ng/mL. Glucose control improved immediately after transplant, as indicated by substantial pre-to posttransplant reductions in blood glucose (median 148 to 104 mg/dL; P<0.01; Fig. 1A) and glycemic lability (median 14.5 to 1.0 [mmol/L] 2 /hour; P<0.01; Fig. 1B) starting within the first week following islet infusion. This restoration ofimproved glucose homeostasis was accompanied by a reductionn initial modest decrease in insulin requirement (median 27 to 12 units/day; P<0.001; Fig. 1C), which further declined weekly until achieving a plateau between weeks 4 and 8 post-transplant at <50% of the pre-transplant insulin requirement. Between From Ddays 56 andto 75 post-transplant, all islet recipients tapered off exogenous insulin. While blood glucose increased modestly during this interval, all recipients met glucose targets with minimal glucose variability, completing a Day 75 mixed-meal tolerance test (MMTT) off exogenous insulin. Fasting and stimulated C-peptide was were undetectable (<0.05 ng/mL) in all recipients prior to islet transplantation but was were detectable at clinically meaningful concentrations at Day 3, and progressively increased increasing through to the Day 75 MMTT (Fig. 1D). At Day 75, all recipients had a fasting glucose <120 mg/dL, and; 7 of the 11 had a 90minute MMTT glucose <180 mg/dL, while 4 had a 90-minute MMTT glucose ≥180 mg/dL that qualified them to receive a second islet infusion (data not shown). Of these 4 recipients, two had peak MMTT-stimulated C-peptide below, and two above, the 2.93 ng/mL threshold reported by the Collaborative Islet Transplant Registry for predicting insulin-independence (6). All 7 recipients with 90-minute MMTT glucose <180 mg/dL had MMTT-stimulated C-peptide above this threshold.These data demonstrate the functional consequences of primary human islet transplants on metabolic control for primary human islet transplants. Insulin therapy to provide "islet rest" avoids glucose toxicity while islet oxygenation and functionality improve, as evidenced by gradual increases in fasting and stimulated C-peptide levels. Despite the continued intensive insulin therapy, primary human islets function immediately after transplantation and contribute to the achievement of on-target glucose control for the recipient within the first days posttransplant. By Day 75, full Full function of each primary human islet graft was previously demonstrated for this cohort by Day 75, with comparable β-cell secretory capacity by glucosepotentiated arginine testing between Day 75 and Day 365 (2). Moreover, the MMTT-stimulated C-peptide levels at Day 75 are consistentis evident by the consistency of C-peptide levels with those identified through long-term follow-up in the Collaborative Islet Transplant Registry as predicting optimal outcomes for islet transplant alone recipients. Unlike primary islets, embryonic or pluripotent stem cell-derived islet cell products (SC-islets) are not molecularly and functionally immature (7; 8), and lack the precision, kinetics, and extent of insulin release seen in adult islets (9). This delays the therapeutic benefit of SC-islet transplants, with >120 days needed on average for intraportal SC-islet transplant recipients to lower exogenous insulin needs by 50% (10), in contrast to ~21 days for primary islet recipients (Fig. 1C). The ~>5-fold time delay reflects the fact that SC-islets require a "maturation" period to gain increasing glucose-dependent insulin secretion beyond the engraftment period, as reflected by MMTT results (10)to become fully functional. The SC-islet transplantation study estimated cell dosing based on primary islet experience, following the same protocol for intraportal delivery and immunosuppression as in CIT07. Ten of 12 recipients of SC-islets eventually became insulin-independent with a single infusion, similar to the 7 of 11 in our CIT07 cohort. As achieving insulin independence supports the establishment of a reserve capacity for insulin secretion, these results suggest that delayed functional maturation, rather than insufficient engrafted SC-islet mass, explains the delay in glucose improvement.Future developments in SC-islet engineering may wish to bridge this critical gap in functionality that delays assessment of adequacy of the delivered islet dose, engraftment, and immune evasion for each recipient. Primary graft non-function, IBMIR, and acute rejection can all be detected through metabolic functional testing over the first days to weeks of primary islet transplantation. However, these mechanisms for graft failure may not be identified for SC-islets while awaiting in vivo functional maturation. Very eEarly metabolic control outcomes for primary islets may serve as useful functional benchmarks for a fully mature stem cell-derived SC-islet product when assessing longer-term functional outcomes. Fwith future advancements in SCislet development that enable early functional assessment may also help align expectations with evolving regulatory standards (11).