There are seven human sirtuins—all with a different biology. One challenge is trying to understand which sirtuin to target for a specific disease. Sirtris Pharmaceuticals, a GlaxoSmithKline company, is focusing on identifying small molecule activators that are structurally different from resveratrol to activate SIRT1.
“What’s unique about SIRT1, versus other potential targets, is that it is really a stress-related sensor and only active in the case where the cell is under some type of stress—oxidative stress, DNA damage, or stress related to increased glucose levels,” stated George Vlasuk, Ph.D., president. He added that the company has focused most of its efforts on finding modulators of SIRT1 for metabolic disease.
Finding small molecule activators is difficult because there are few small molecules that can activate enzymes. The observation that reservatrol could enzymatically activate SIRT1, lead the company to develop assays to screen chemical libraries to, not only find molecules to act like resveratrol, but to specifically activate SIRT1. (There is data on resveratrol inhibiting other enzymes and binding to certain receptors). Some of these molecules have been studied in preclinical disease models.
Dr. Vlasuk said that additional challenges are validating molecules that are acting solely through the activation of SIRT1 in cells and tissues. “In some cases, we’ve demonstrated that clearly, and in other cases, we haven’t been able to due to technical challenges.
“We’re trying to understand what the relationship in regulation of SIRT1 is in various cell types. The question is how is this actually regulated in a cell and how does that impact how the small molecule can regulate or activate SIRT1 in a cell or tissue? That’s the fuzzy area now—where we’re trying to develop the biology to the point where we understand how these molecules are impacting SIRT1 in a cell, tissue, and, ultimately, in an animal.” The company’s lead molecule is currently in a Phase IIa trial in type 2 diabetes.
Inhibiting Triglyceride Synthesis
DGAT1 (diacylgylcerol acyltranferase) is the enzyme that catalyzes the final step in triglyceride synthesis. The advantage of DGAT1 inhibitors, said Robert Dow, Ph.D., associate research fellow, CVMD chemistry at Pfizer, is the potential for positive effects on triglyceride levels, especially in the liver, with “a profound effect on normal functioning of insulin action and glucose utilization.”
Animal studies have shown a substantial reduction in triglyceride content in rodent livers, as well as good plasma clearance and a good projected half-life of eight hours. A subsequent single-dose study in healthy male volunteers showed these predictions were good, and based on this initial data, the company plans to go into multidose and proof-of-mechanism studies.
Previous animal studies with DGAT knock-out mice showed highly reduced levels of triglyceride content in the liver, adipose, and skeletal muscle. These animals lose weight and become resistant to further obesity due to a high fat diet. “That was part of the rationale that got us excited about this approach, knowing we would have an impact on diabetes end points, as well as, potentially, obesity endpoints,” stated Dr. Dow.
In order to avoid losing compounds in later stages of development, his group applied its de-risking strategy in early lead optimization. “The next frontier of drug discovery is to drive safety assays early into the discovery process, so we can head off the sorts of problems where we are losing compounds in later stages of development.”
His group used NMR technologies to understand whether they were forming glucuronides, which can often rearrange and interact with proteins in the body to create toxicities. The results showed low amounts of rearrangements and, therefore, low risk. “Rather than waiting for something to happen in a half-percent incident in humans in a Phase IV study, where you’d need thousands of patients to pick up such a small percent, we move it to the discovery phase and derisk it.”
There were also concerns regarding whether the lead compound had photo instability. Dr. Dow said one of their assays looks at cytotoxicity of the compound in the presence and absence of light. The lead compound did show cytotoxicity in the presence of light, but a modified molecule showed no toxicity. Overall, these steps are part of the company’s approach to move initial in vivo tolerability studies early into their lead-development program.
Developing a new therapeutic for diabetes is challenging due to the progressive nature of the disease. “The bottom-line challenge for all of us is to identify compounds that have not only the initial effect of blood glucose lowering, but that will have positive, long-term effects for diabetics.”