Studies in the pig-to-NHPs islet xenoTx model are needed to provide evidence of safety and efficacy prior to clinical phase 1 and 2 trials. To meet these criteria, preclinical studies should be performed in diabetic NHPs; to obtain these, three different approaches have been used. Spontaneous diabetes, reported in several strains of monkeys including cynomolgus
, is usually associated with age, obesity, β-cell degeneration, and extensive amyloid deposition [33
]; the availability of these monkeys is limited, and their diabetes could not be considered as a faithful T1D model. Surgical pancreatectomy is a second option, but its execution is difficult and it has associated morbidity problems. This technique is probably the most convincing in the generation of irreversible diabetes, but pancreatectomy needs to be total to avoid potential residual function and possibly β-cell regeneration, and it has the disadvantage of eliminating the endogenous exocrine pancreatic function. The third approach currently used is chemical induction of hyperglycemia through streptozotocin (STZ) [34
]. STZ is a compound derived from the bacterium Streptomyces achromogenes
with the ability to selectively destroy β cells by DNA damage and nicotinamide adenine dinucleotide depletion [34
]. Its intravenous administration successfully achieves killing of β cells and subsequent induction of hyperglycemia, but a discussion on the optimal dose remains an open problem.
A single dose of 30 mg/kg of STZ is not sufficient to induce complete diabetes; although larger doses (100–150 mg/kg) are, they are also associated with harmful side effects [11
]. Nephrotoxicity and hepatotoxicity are some of the major disadvantages of high-dose STZ; nonetheless, STZ-induced diabetic monkeys usually remain diabetic and insulin dependent for years, with no significant endogenous C-peptide release even after glucose stimulation and histologic evidence of only a few insulin-staining cells surviving in the pancreas [11
]. There is interspecies variation in the effectiveness of STZ also due to the different expression of low-affinity glucose transporter 2 (GLUT-2), which is the receptor through with STZ accesses the β cells but also, to a lower extent, renal and hepatic cells, where this receptor is also expressed. STZ could thus cause hepato- and nephrotoxicity.
There are other important variables that can influence the effectiveness of STZ after intravenous administration, such as the age of the NHPs and an intrinsic sensitivity of the β cells to the drug [36
]. Post-STZ hyperglycemia is typically the main indication that a diabetic status has been established; however, C-peptide levels are usually measured for a correct diagnosis. Primate C-peptide levels less than 0.9 ng/mL are considered sufficient to indicate a diabetic status; however, our group has established that diabetes is durable when at least a 75% reduction in C-peptide levels post STZ is recorded [35
] and a negative response (lack of C-peptide increase) after a challenge is confirmed [4
]. The intravenous glucose tolerance and arginine stimulation tests are the most used challenge tests. The diabetic status of NHPs can be further evaluated by checking the variations in hemoglobin A1c
) levels [38
]. The effect of STZ on β cells can also be evaluated by histologic examination of pancreatic biopsies, but this requires a surgical procedure and, because some residual insulin immunostaining is always present, it is difficult to quantify the real extent of the damage. The combination of hyperglycemia, insulin dependence, failure to increase C-peptide levels after stimulation, and increase in HbA1c
levels allows for a correct diagnosis of STZ-induced diabetes.