Researchers at the University of Alabama at Birmingham, and Southern Research, have identified a new drug candidate that they claim could represent a “distinct and innovative” approach to treating type 1 and type 2 diabetes. The small molecule drug, designated SRI-37330, inhibits the expression of a protein known as TXNIP—which the team had previously identified as a top glucose-induced gene—in both mouse and human islets.
Results from the researchers’ preclinical studies suggested that SRI-37330 acts on pancreatic islet cells that produce glucagon and insulin, and also acts on the liver. The findings showed that the drug could have therapeutic effects against diabetes, in both lean and obese individuals. Tests on isolated human and mouse pancreatic islets, on mouse and rat cell cultures, and in animal models of both type 1 and type 2 diabetes, demonstrated that SRI-37330 improved diabetes-related hyperglycemia, and hyperglucagonemia; reduced the excessive production of glucose by the liver; and reduced fatty liver, or hepatic steatosis.
Research head Anath Shalev, MD, director of UAB’s Comprehensive Diabetes Center, commented, “compared to currently available diabetes therapies, the compound may provide a distinct, effective and highly beneficial approach to treat diabetes.” And while Shalev acknowledged that the safety and efficacy of SRI-37330 in humans still remains to be confirmed, she noted that the drug compound “… is highly effective in human islets, is orally bioavailable and is well tolerated in mice … Together with the fact that SRI-37330 was also effective after the onset of overt diabetes, as well as when just dosed twice a day by oral gavage, is particularly promising and raises the possibility that SRI-37330 may ultimately lead to a much-needed oral drug that could also be used for type 1 diabetes.”
Shalev and colleagues report on their developments in Cell Metabolism, in a paper titled, “Identification of an Anti-diabetic, Orally Available Small Molecule that Regulates TXNIP Expression and Glucagon Action.”
Diabetes affects 425 million people worldwide and more than 30 million in the U.S. Diabetes represents a growing epidemic, with 1.5 million Americans newly diagnosed each year. Type 2 diabetes (T2D), particularly, has become a worldwide health issue and poses major therapeutic challenges, the authors noted.
Diabetes is a disease affecting two hormones, insulin and glucagon. In healthy individuals, insulin helps cells take up glucose from the blood when glucose levels are high, and glucagon helps the liver release glucose into the bloodstream when glucose levels are low. In diabetes, insulin release is diminished, cell sensitivity to insulin can decrease, and glucagon release is excessive. This can cause a vicious cycle of escalating blood glucose levels.
Commenting on the global epidemic of type 2 diabetes, the investigators commented, “over the last several decades, it has been recognized as a bihormonal disease distinguished by the combination of hypoinsulinemia due to loss of functional pancreatic beta cell mass and hyperglucagonemia due to persistent alpha-cell-mediated glucagon secretion … “Insufficient production of the glucose-lowering hormone insulin and high levels of the gluconeogenic hormone glucagon combine to exacerbate the hyperglycemia that is the hallmark of T2D, leading to a vicious cycle of glucose toxicity that furthers beta cell dysfunction.” And while management of type 2 diabetes has improved significantly over recent years, they continued “…therapies that target these underlying processes are still lacking.”
The path to discovery of SRI-37330 began 18 years ago when Shalev and colleagues identified TXNIP as the top glucose-induced gene in human islets, which are the cell groups in the pancreas that produce insulin and glucagon This was followed by their work showing that TXNIP negatively affected islet function and survival, suggesting that TXNIP might play an important detrimental role in diabetes. “We previously identified thioredoxin-interacting protein (TXNIP) as the top glucose-induced gene in a human islet gene expression microarray study, and as a critical link between glucose toxicity and beta cell loss, suggesting that this arrestin protein may play an important role in T2D,” the investigators commented.
In previous research, Shalev and colleagues also showed that TXNIP was increased in different mouse models of diabetes and in diabetic human islets, and that deletion of the TXNIP gene protected mice from diabetes and had beneficial effects on pancreatic islet biology. Altogether, these data suggested that a search for a TXNIP inhibitor could provide a novel approach to diabetes treatment.
SRI-37330 was discovered through a high-throughput screen of 300,000 compounds, and then extensive medicinal chemistry optimization at Southern Research. Initial tests showed that SRI-37330 inhibited activity of the TXNIP promoter by 70%, and dose-dependently inhibited TXNIP mRNA and protein. “In order to mimic diabetic conditions, we also conducted experiments in the context of high (25 mM) glucose and found that SRI-37330 was able to significantly inhibit TXNIP expression in rat INS-1 cells, primary mouse islets, and isolated human islets,” the investigators commented.
RNA sequencing of isolated human pancreatic islets treated with SRI-37330 showed that the drug inhibited TXNIP signaling, demonstrated by a number of upregulated and downregulated genes. “ … a similar number of genes were up- as well as downregulated, which is consistent with the notion that SRI-37330 does not inhibit general transcription,” the scientists further noted. The studies in addition confirmed that SRI-37330 specifically inhibited TXNIP, and did not inhibit other members of the arrestin family.
Test results indicated that SRI-37330 effectively reduced TXNIP at the nanomolar range and demonstrated 95% oral bioavailability. The drug also showed no in vitro cytotoxicity and no evidence of toxicity in mice, even at doses about 10-fold above its therapeutic dose. The drug in addition tested negative in Ames mutagenicity assays, CYP450 inhibition, hERG inhibition and Eurofins SafetyScreen for off-target liabilities. Of note, SRI-37330 was found not to inhibit calcium channels. This was interesting because the Shalev lab had previously demonstrated that non-specific inhibition of TXNIP signaling by the calcium channel blocker verapamil had beneficial effects in human subjects with recent onset Type 1 diabetes. However, the team pointed out, verapamil is a potent blood pressure drug and its TXNIP-lowering capacity is intricately linked to its function as an L-type calcium channel blocker. This would limit the drug’s use in certain patient populations. “ … in contrast to verapamil, SRI-37330 is not a calcium channel blocker and is therefore not expected to have these limitations.”
Strikingly, addition of SRI-37330 to the drinking water of obese diabetic db/db mice – a genetic model of severe type 2 diabetes—led within days to normalization of the animals’ blood glucose. Similarly, SRI-37330 protected mice from streptozotocin-induced diabetes, a different, non-genetic model of type 1 diabetes. Encouragingly, SRI-37330 achieved even better blood glucose control than two of the leading oral anti-diabetic drugs, metformin and empagliflozin.
“Together with the fact that SRI-37330 was also effective after the onset of overt diabetes, as well as when just dosed twice a day by oral gavage, is particularly promising and raises the possibility that SRI-37330 may ultimately lead to a much-needed oral drug that could also be used for type 1 diabetes,” Shalev said.
Surprisingly, SRI-37330 decreased blood glucose levels primarily via lowering of serum glucagon levels and inhibition of basal glucose production from the liver. This mode of action is very different from that of currently used anti-diabetic drugs. And despite SRI-37330’s reduction of glucagon release from pancreatic islets and reduction of glucose production by the liver, the inhibitor did not cause any low blood glucose events or create a hypoglycemic liability in mice, even in the context of insulin-induced hypoglycemia. “… these results suggest that SRI-37330 inhibits alpha cell glucagon secretion via TXNIP inhibition and that this effect does not occur under low glucose conditions or in the context of glucagon secretion stimulated by counter-regulatory hormones such as norepinephrine,” they wrote. “These features may help limit the hypoglycemic liability of SRI-37330 and explain why we did not notice any problems with blood glucose levels dropping too low under any circumstances. In fact, even in the context of insulin-induced hypoglycemia, SRI-37330-treated mice were able to maintain their blood glucose levels equally well to untreated controls.”
In another surprising result—and in contrast with previous attempts to inhibit glucagon function for the treatment of diabetes—the new drug dramatically improved the severe fatty liver observed in obese diabetic db/db mice. The effectiveness of SRI-37330 in reducing fatty liver in mice suggested it might have potential to treat non-alcoholic fatty liver disease, which affects about 100 million people in the US and 1 billion worldwide.
“This now raises the intriguing possibility,” Shalev said, “that SRI-37330 might also be beneficial in the context of non-alcoholic fatty liver disease, a complication frequently associated with diabetes and/or obesity.”
The authors acknowledged that the safety and efficacy of SRI-37330 in humans still needs to be confirmed. Nevertheless, Shalev summarized, “our studies have identified a novel substituted quinazoline sulfonamide, SRI-37330, that is orally bioavailable, has a favorable safety profile and inhibits TXNIP expression and signaling in mouse and human islets, inhibits glucagon secretion and function, lowers hepatic glucose production and hepatic steatosis, and exhibits strong anti-diabetic effects in mouse models of type 1 and type 2 diabetes.”
