Hormones in Concert

In many respects, the work was very successful, at least in the short term. Young patients with Type 1 diabetes, many with early signs of microvascular complications, achieved their target glucose control levels for the first time since diagnosis. Obese patients, after losing 10% or more of their body weight, saw marked improvements in their liver function tests, cardiovascular risk factors, and/or sleep apnea.

But the limitations of our treatment approaches became apparent over the long term. Upon intensifying insulin treatment, most diabetes patients gained weight and many grew frustrated with the unpredictable glucose swings and constant insulin dose adjustments. Likewise, the vast majority of patients in our obesity clinic saw a relentless regain of their body weight.

In the clinical setting, it was not uncommon for doctors to advise their patients to try harder and be more disciplined. After all, with adequate willpower and meticulous tracking of blood sugars and ingested calories, there had to be a way to do better. From a scientific perspective, however, it was quite evident that the root of the problem was far more complex.

I was drawn to endocrinology because I was intrigued by the complexity and elegance with which hormonal signaling systems govern whole-body metabolism and many other vital functions. Most hormones have multiple actions that are well coordinated, and naturally integrated with other hormonal systems. It is, in many respects, the equivalent of individual musicians playing together in a philharmonic orchestra producing the most melodic, beautiful symphonies. Some hormones, such as insulin, thyroid hormone, or cortisol, are “major players,” and their deficiency or excess can result in life-threatening metabolic derangements. Others, such as calcitonin, pancreatic polypeptide, or amylin can be viewed as complementary signals that enhance, or “fine-tune,” a tightly regulated metabolic process. In many cases, the central nervous system (CNS) orchestrates and balances these hormonal interactions, serving as the role of conductor.

Working model of pramlintide/metreleptin mechanism of action, inferred from preclinical and clinical studies with monotherapy and combination therapy.

A number of important discoveries and developments occurred in the mid-1990s that ushered in a new era of endocrinology-based research and treatment approaches for diabetes and obesity. In diabetes, I had followed with great interest the emerging recognition within the medical scientific community that glucose control was governed by a number of glucoregulatory hormones other than insulin. In obesity, the whole field was electrified by a seminal discovery from Rockefeller University. Using positional cloning, Jeff Friedman and colleagues had identified a fat-derived cytokine hormone, called leptin (from the Greek word leptos, for thin). Mice lacking the leptin gene (ob/ob) displayed a slowed metabolism, a marked increase in food intake, and a profoundly obese phenotype. These abnormalities were corrected by replacing the missing hormone. This discovery not only provided scientific evidence that body weight was regulated by a complex biological feedback system, it also pointed to a pivotal role of hormonal signals in body weight regulation. Clearly, in the expanding panoply of existing and newly discovered hormones, leptin looked like a major player.

While the exact roles of leptin and some of the newer, glucoregulatory hormones—amylin and glucagon-like peptide 1 (GLP-1)—were still to be defined, I wondered whether they might be some of the “missing links” that could explain my patients’ struggles with glucose and weight control. The more I immersed myself into the evolving science, the more I felt compelled toward clinical research rather than clinical practice.

In late 1997, I moved to the United States to work as a visiting researcher with the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), in Phoenix, Arizona. For more than 3 decades, the Phoenix branch of the NIDDK had been conducting pioneering studies on the epidemiology and pathophysiology of obesity and diabetes in Pima Indians, a population with the highest reported prevalence rates of both disorders in the world.

With respect to obesity research, our group was world-renowned for its work on energy expenditure (metabolic rate). We conducted a series of short- and long-term studies showing that weight loss was accompanied by a prompt and sustained drop in metabolic rate and leptin levels. Seeing these results firsthand, I was convinced that the weight regain I had seen in many of my patients was not merely due to a lack of willpower, but rather the manifestation of a powerful, neuroendocrine-mediated metabolic compensation—what we call weight loss counterregulation. In other words, as people lose weight, their bodies mount a response to conserve energy and boost appetite to resist further weight loss, and eventually promote weight regain.

While the line of research I pursued at NIDDK contributed important insights into the pathophysiology of obesity and diabetes, there was little opportunity to translate scientific findings into novel treatment approaches.

In late 2000, I decided to join San Diego–based Amylin Pharmaceuticals, Inc., a company I had followed with interest for several years. Amylin was the only company I was aware of that was exclusively focused on developing peptide hormone therapeutics for metabolic disorders. The company had just submitted a New Drug Application (NDA) for its lead drug candidate pramlintide. It is an analog of amylin, a hormone co-secreted with insulin by β-cells in the pancreas. Amylin had emerged as a possible “partner hormone” to insulin, complementing insulin’s action in postprandial metabolism. It acts as a neurohormone that binds to receptors in the area postrema of the hindbrain. That binding slows gastric emptying via signals from the vagus nerve and suppresses excess glucagon, a hormone that normally raises blood sugar by triggering the release of glucose stores from the liver.¹ The net result is a moderation of glucose influx after a meal, to better match the rate of insulin-mediated glucose disposal.

In parallel, the company was conducting early clinical studies with a second peptide hormone candidate. This was exendin-4 (exenatide), a GLP-1 agonist that held great promise in the management of Type 2 diabetes. GLP-1, a hormone secreted by enteroendocrine cells in the gut, had the intriguing and unique ability to stimulate insulin and amylin secretion in a glucose-dependent manner—that is, only when blood glucose levels are above normal levels. Interestingly, it was noticed that both amylin and GLP-1 appeared to act as satiety signals, curbing food intake by sending a neurohormonal signal to the brain to stop eating.

In early 2005, the hard work of many came to fruition, when both first-in-class drug candidates were approved by the FDA. Pramlintide, the amylin analog, became the first novel, noninsulin peptide therapeutic for Type 1 and Type 2 diabetes in over 80 years. It helps address some of the well-known limitations of insulin therapy by reducing the excessive rise in blood sugar after a meal, erratic glucose swings, and insulin-induced weight gain.² Exenatide became an attractive alternative to injectable insulin for patients with Type 2 diabetes who were no longer able to control their diabetes with oral agents alone. It offered similar glycemic benefits to basal insulin, but with weight loss rather than weight gain, and without the inherent risk of hypoglycemia or the need for meticulous dose adjustments.³ Virtually overnight, diabetes became a disease that was not only recognized as a multihormonal disorder, but could also be treated as such.

With both of our approved diabetes drugs having the unique attribute of reducing food intake and body weight, we ventured to tackle obesity as our next therapeutic focus. The scientific community was at that point gaining an increased appreciation that food intake and body weight were governed by a complex interplay between various hormonal signals and the brain. However, in stark contrast to the potent glucose-lowering effect of insulin and GLP-1 agonists in diabetes, no single hormonal pathway caused sufficient weight loss to be considered a therapeutic breakthrough. The once-hailed “magic bullet” for obesity, metreleptin, Amgen’s analog of human leptin, had failed to induce meaningful weight loss in people with general obesity, and was suddenly discarded by many pharmaceutical companies as a failed target.

While metreleptin treatment was not effective in inducing weight loss in patients with general obesity, several studies of rare diseases had emerged hinting at leptin’s therapeutic potential in several “low-leptin” states. Steven O’Rahilly and Sadaf Farooqi had identified a handful of children with congenital leptin deficiency caused by a frame-shift mutation in the leptin gene, named “ob.” As with the ob/ob knockout mice, these leptin-deficient (ob/ob) children displayed a voracious appetite and marked, early-onset obesity. Leptin replacement with metreleptin led to a profound reduction in food intake and body weight. Meanwhile, teams led by Phil Gorden from the NIDDK and Abhimanyu Garg from the University of Texas Southwestern, had administered metreleptin to patients with severe lipodystrophy. This rare syndrome, characterized by a severe loss of adipose tissue and a leptin deficiency, commonly manifests with extremely high circulating lipids, marked insulin resistance, and diabetes that is difficult to control even with very high doses of insulin. Metreleptin dramatically improved these metabolic problems in most patients.⁴ In a third example of patients with a “low-leptin state,” Christos Mantzoros and colleagues from Harvard demonstrated that metreleptin therapy was able to restore normal ovulatory menstrual cycles in female athletes whose menstrual cycle had stopped because of very low body fat (a syndrome called hypothalamic amenorrhea). Perhaps most relevant to the problem of weight regain in obesity, Rudy Leibel and Michael Rosenbaum from Columbia University showed that using metreleptin to restore normal leptin levels in people who had lost 10% of their body weight almost completely mitigated the counterregulatory responses that drive weight regain.⁵

As these academic clinical studies with leptin were progressing, we had established our own internal obesity research program. Rather than focusing on single-hormone pathways, we embarked on a comprehensive preclinical program in diet-induced obese rats that tested various dual- and triple-hormone combinations. Our overarching notion was that in normal physiology, food intake and body weight were not regulated by a single, predominant hormonal signal, but rather by a sophisticated interplay of hormonal signals emanating from fat cells (e.g. leptin), β-cells of the pancreas (e.g., amylin) and from cells in the gut (e.g., PYY3–36). With our team of biologists and peptide chemists, we began to systematically modify the native hormones to identify proprietary analogs that were more potent and efficacious than their naturally occurring counterparts.

The initial results from these preclinical studies were stunning to us. While most of the individual hormones induced modest weight loss, we discovered profound, additive, and even synergistic reductions in body weight when testing various dual- and triple-hormone combinations. Moreover, we identified various hormone analogs that achieved marked weight loss even when given alone. With several regimens, we could essentially normalize body weight in obese rodents. While we realized the complexities of translating these results into pharmaceutically tractable interventions, we sensed that we were onto something big. We coined the acronym INTO, for Integrated Neurohormonal Therapy for Obesity, to describe our new obesity research strategy.⁶

Among the most striking discoveries was a synergistic interaction between amylin and leptin. When given alone, leptin did not reduce body weight in obese rats, but after combining it with amylin, we saw a synergistic effect that led to marked, sustained, fat-specific weight loss (see graphic).^7,8 We reasoned that if these results would translate to human obesity, combined treatment with amylin and leptin receptor agonists (i.e., pramlintide + metreleptin) could have tremendous potential.

In early 2006, Amylin licensed the exclusive rights to leptin from Amgen. Later that year, we initiated our first clinical proof-of-concept study testing the pramlintide/metreleptin combination in approximately 180 overweight and obese volunteers. All subjects followed a strict low-calorie diet for 4 weeks, then were randomized to treatment with pramlintide alone, metreleptin alone, or the pramlintide/metreleptin combination. The stakes were high. Would leptin finally show a clear weight-loss effect, after a dozen or so failed clinical trials?

In late 2007, we unblinded our study and were thrilled to find that our hypothesis on the benefits of combination therapy with pramlintide/metreleptin had borne out in humans. Overweight and obese study participants who completed 20 weeks of combination treatment (after 4 weeks on a low-calorie diet) with pramlintide/metreleptin had lost an average of 13% (>25 lbs) of weight, far more than those receiving pramlintide or metreleptin alone.⁹ Earlier this year, we confirmed and expanded our findings in a second, larger Phase 2b clinical trial (without an initial period of time on a low-calorie diet) in approximately 600 subjects, showing that pramlintide/metreleptin worked best in obese individuals with mild-to-moderate obesity. In those patients, weight loss with pramlintide/metreleptin again exceeded 10% over 28 weeks, with the vast majority of weight loss being attributable to a reduction in fat mass (approximately 20 lbs).¹⁰

Additional studies in larger patient populations are required before the therapeutic potential of the pramlintide/metreleptin combination can be fully assessed. For now, our findings provide solid clinical proof-of-concept for our strategy of pairing hormones that are naturally involved in the regulation of food intake and body weight, and which may have complementary and/or synergistic interactions.

When reflecting on the aforementioned development programs in diabetes and obesity, a number of commonalities emerge. It has become abundantly clear that body weight and glucose metabolism are both regulated by a complex interplay of multiple hormones. Understanding the interaction among these hormones, and combining hormonal signals that have additive or synergistic interactions, can lead to innovative therapeutic approaches. This concept is exemplified by amylin agonism (with pramlintide), which has been successfully developed as an adjunctive therapy to insulin for the treatment of diabetes, and has now also shown promise as an adjunctive therapy to leptin for the treatment of obesity.

While it will take years or decades before the therapeutic utility of peptide hormones in obesity are fully realized, I firmly believe that endocrine solutions will become an integral part of future treatment paradigms.

Christian Weyer, MD is the vice president of Medical Development at Amylin Pharmaceuticals, Inc.