A USF Health-led team tests 40 million compounds and finds lead candidate that selectively relaxes airway smooth muscle cells with no detectable drug desensitization

TAMPA, Fla. (Dec. 2, 2021) – Beta-agonists (β-agonists) are the only drugs that directly open narrowed airways and make it easier to breathe for millions of people with asthma, a chronic respiratory disease. These inhaled medications activate the β₂-adrenergic receptors (β₂AR ) on airway smooth muscle cells and relax them, dilating airways and increasing air flow.

However, for a significant proportion of asthmatics, the effectiveness of existing β-agonists is insufficient to open tightly constricted airways and the clinical benefits realized appear to wane over time, leaving them constantly struggling with the disease.

“A lack of more effective therapies to treat or prevent shortness of breath is a major issue for patients with severe-to-moderate asthma,” said Stephen Liggett, MD, vice dean for research and a professor of medicine, molecular pharmacology and physiology, and biomedical engineering at the University of South Florida Health (USF Health) Morsani College of Medicine. “As regular use of β-agonists increases, the body becomes less sensitive to these bronchodilators.”

This process, known as tachyphylaxis or drug desensitization, contributes to insufficient asthma control, which leads to increased emergency department visits and hospitalizations — impacting the quality of life and extracting an economic toll in increased medical costs and missed days of work and school. Dr. Liggett’s laboratory works with collaborators across the country to understand the mechanisms of tachyphylaxis, with the aim of improving β-agonists.

Over the last three years, a multi-institutional research team led by USF Health studied 40 million compounds to identify those that activated β₂AR (β-agonists) without causing tachyphylaxis. The investigators found one such agonist, which was structurally distinct from all known traditional β-agonists. Their preclinical research suggests that a different class of β-agonists, known as biased agonists, offers promise for selectively treating asthma and other obstructive lung diseases. Such biased agonists offer a therapeutic option without causing the rapid turndown of these receptors (β₂AR) when the drug is used on an as-needed basis, or the even greater loss of effectiveness observed with chronic use.

The drug discovery study, published online recently in the Proceedings of the National Academy of Sciences of the United States (PNAS), was conducted by scientists with expertise in biochemistry, physiology, and computational biology. The team used molecular modeling driven by high speed, high-capacity supercomputers to define how this atypical agonist, named C1-S, works at the molecular level.

“This is the first β-agonist ever known to relax airway smooth muscle and treat asthma without any detectable tachyphylaxis and represents a significant breakthrough in asthma therapy,” said principal investigator Dr. Liggett, the PNAS paper’s senior author.

USF Health’s Stephen Liggett, MD, led a multi-institutional research team that studied 40 million compounds to identify those that activated β₂AR (beta-agonists) without causing tachyphylaxis (drug desensitization). — Photo by Allison Long, USF Health Communications and Marketing

β2-adrenergic receptors are G protein-coupled receptors (GPCR), present in airway smooth muscle cells to mediate various functions. The existing β-agonists used to treat asthma are all unbiased. That means the drug equally favors activating a G-protein signaling pathway that promotes airway smooth muscle cell relaxation (thus easier breathing) as well as engaging a beta arrestin (β-arrestin) signaling pathway that leads to the unwanted outcome of tachyphylaxis.

“Beta-arrestin is a protein that upon interaction with the G protein-coupled receptor begins to uncouple (inhibit) the receptor from stimulating the clinically important signaling pathway we want to preserve,” Dr. Liggett explained. “With unbiased beta agonists you have these dueling signaling processes essentially competing with each other.”

Research is underway to design biased agonists to help alleviate pain without addiction and to better treat certain cardiovascular conditions with minimal side effects; however, no GPCR-biased agonists are yet being developed for asthma.

The researchers approached this massive study with “no preconceived notions” about what compounds might work best, Dr. Liggett said. Among their key findings:

• Of the 40 million compounds screened, 12 agonists activated the target receptor (β₂AR), stimulating cyclic AMP production that causes airway smooth muscle relaxation. But only one of these 12 (C1-S) appeared to be strongly biased away from the b-arrestin signaling that limits airway smooth muscle response (and thus drug effectiveness) due to receptor desensitization.
• Through a series of biochemical experiments, the researchers verified for the first time that it was possible for an agonist to “split the signal” mediated by a G coupled-protein receptor (β₂AR). This split preferentially activates, or switches on, a signaling pathway beneficial for treating obstructive lung disease rather than a pathway believed to be physiologically harmful, Dr. Liggett said.
• In addition to measuring signaling at the cellular level, the researchers employed the magnetic twisting cytometry, a method pioneered by co-author Steven An, PhD, at Rutgers University that measures changes in human airway smooth muscle cell relaxation and contraction. All the biochemistry results correlated with the physiological response the researchers expected — relaxation of airway smooth muscle without desensitization.
• Computer modeling and docking was performed by investigators at Caltech (William Goddard III, PhD, and now graduate student Alina Tokmakova). These studies helped identify molecular contact points between the receptor and biased agonist C1-S; some of these binding sites were not seen with any other agonist before and thus point to the basis of the properties of this unique drug. The collection of 40 million compounds was assembled and maintained by Marc Giulianotti, PhD, of Florida International University.

As regular use of β-agonists increases, the body becomes less sensitive to these inhaled bronchodilators, a process known as as tachyphylaxis (drug desensitization) that contributes to insufficient asthma control.

The researchers plan to evaluate the safety and efficacy of their lead drug candidate C1-S for potential use in humans, Dr. Liggett said.

“Every day we see breakthrough asthma symptoms in patients using albuterol, a beta-2 receptor agonist that is the cornerstone of treatment. When exacerbated, these symptoms sometimes require hospitalization, use of a ventilator, and occasionally even result in death,” said Kathryn S. Robinett, MD, assistant professor of medicine at the University of Maryland School of Medicine’s Division of Pulmonary and Critical Care Medicine, who was not involved in the research. “A new class of beta-agonists that do not cause tachyphylaxis, like the one characterized in this study, could provide rapid relief and add a powerful tool to our belt in the treatment of asthma.”

The study’s co-lead authors were Donghwa Kim, PhD, of the USF Health Morsani College of Medicine, and Alina Tokmakova, currently a graduate student at University of California San Francisco.

The work was supported by grants from the National Heart, Lung, and Blood Institute, part of the National Institutes of Health.

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Studies led by USF’s Dr. Stephen Liggett shed light on genetic variability of adrenergic receptors and how they might best be used to treat disease

Dr. Stephen Liggett, who leads the research enterprise for the Morasani College of Medicine and for USF Health, also oversees a genomics laboratory working on NIH-funded studies. Behind him is a radioligand binding machine used to determine the number of receptors in each cell.

While significant progress has been made managing asthma over the last two decades, about half of all asthmatics achieve optimal control of this chronic inflammatory disease using currently available medications.  Similarly, only about 50 percent of patients with congestive heart failure, which occurs when the heart is too weak to pump enough blood to meet the body’s needs, have an average life expectancy of more than five years.

More still needs to be  known at the molecular level about these common diseases to identify potential new targets for drug therapies, said Stephen B. Liggett, MD, associate vice president for research at USF Health, vice dean for research at the Morsani College of Medicine, and professor of internal medicine and molecular pharmacology and physiology.

What ties these two diseases together are the receptors on cardiac muscle and on smooth muscle of the airways. Dr. Liggett’s laboratory helps shed light on the genetic variability of adrenergic receptors and on how these receptors can best be used for treatment. The genetic studies have been particularly useful in developing the concept of pharmacogenetics, a tailoring of therapy based on an individual’s genetic makeup, for heart failure and asthma.

“Twenty years ago we had a handful of medicines for high blood pressure, and today we don’t use any of them. Now, we have a whole new group of more effective (antihypertensive) drugs with much fewer side effects,” he said.  “And, I’m sure that one day, we’ll have more tools in our toolbox to better treat heart failure and asthma – drugs that work better for subgroups of people as defined by their genetic makeup and environmental exposures.”

 Dr. Liggett comments on some of his laboratory’s contributions to the field over his career.

The research team led by Dr. Liggett, center, includes Ashley Goss, Hiwot Zewdie, Donghwa Kim, PhD, and Maria Castano. Not pictured: Alexa Woo, PhD.

Mining a “superfamily” of receptors for better drug targets

Dr. Liggett leads a USF team that studies the genetic, molecular biology, structure and function of G-coupled protein receptors, or GPCRs, the largest family of human proteins.  More than 800 GPCRs have been discovered within cell membranes in the human body, Dr. Liggett said, and one or more of these receptors plays a role in virtually everything the body does, including controlling thoughts in the brain, sight and smell, uterine contraction and relaxation, blood pressure, cardiac, lung and kidney function, to name just a few.

Consequently, malfunctions of GPCR signaling pathways are implicated in many chronic diseases including asthma and cardiovascular diseases.  Already this “superfamily” of receptors accounts for nearly half the targets of all prescribed drugs. But, a deeper understanding of the dynamics of the GPCR signaling network and how it maintains a healthy cell or responds to pathogens could lead to the design of drugs that more precisely target diseases with greater effectiveness and fewer side effects.

Dr. Liggett began his work with GPCRs in 1988 as a Howard Hughes Institute postdoctoral research fellow in the Duke University Medical Center laboratory of mentor Robert Lefkowitz, MD. Dr. Lefkowitz was awarded the 2012 Nobel Prize in Chemistry with Brian Kobilka, MD, for groundbreaking discoveries revealing the inner workings of GPCRs.

Building upon his interest and advanced training in pulmonary and critical care medicine, Dr. Liggett began early in his career to concentrate on one of the classes of GPCRs known as adrenergic receptors, which are stimulated by the hormone epinephrine and the neurotransmitter norepinephrine. They are involved in increasing the rate and force of contraction of the heart, as well as constriction and dilation of blood vessels throughout the body and of airways in the lung. For the last 28 years, he has been continuously funded by the National Institutes of Health (NIH) to study the molecular basis of beta-adrenergic receptors in asthma.

Biological scientist Ashley Goss

Dr. Liggett is the principal investigator of a four-year, $1.12-million R01 grant from the NIH’s National Heart, Blood and Lung Institute (NHBLI) that seeks to understand how beta-adrenergic signaling is regulated to influence the development and treatment of asthma. Over his career, he has also been awarded millions of dollars in NIH funding to explore the role of genetic variations of GPCRs in heart failure, including whether those variations may alter how effectively drugs work in individual patients.

Bitter taste receptors in a new place

Dr. Liggett is also currently a project principal investigator for a five-year, $2-million NHBLI P01 grant examining how airway smooth muscle bitter taste receptors might be applied as new treatments for asthma and chronic obstructive pulmonary disease.

Using a genomics-based method that Dr. Liggett pioneered, his team had previously identified bitter taste receptors, initially thought only to exist on the tongue, deep inside the lung at the airway smooth muscle and demonstrated they act to open the airway. “When activated, they appear far superior to the beta-agonists commonly prescribed to patients to open their airways during an asthma attack,” said Dr. Liggett, who published the discovery and the need for alternatives to current bronchodilators in Nature Medicine and other journals.

Overall, discoveries emerging from Dr. Liggett’s research have yielded more than 250 peer-reviewed papers, many highly cited and appearing in top journals such as Nature Medicine, Science, Proceedings of the National Academy of Sciences, and the New England Journal of Medicine. His work has been cited by other papers more than 26,000 times. He also holds 18 patents detailing potential new targets for drug therapy or genetic variations of known drug targets and how they might be used to predict response to medications and customize treatment.

 The serendipity of finding bitter taste receptors on smooth airway muscle in the lungs

Laboratory assistant Hiwot Zewdie

Among some of his laboratory’s major findings:

– While at the University of Maryland, Dr. Liggett’s team worked with colleagues at the University of Wisconsin-Madison to sequence for the first time the entire genomes (more than 100 different strains) of all known rhinoviruses, a frequent cause of respiratory infections including the common cold. The groundbreaking work, published on the cover of Science, provided a powerful framework for large-scale, genome-based epidemiological studies and the design of antiviral agents or vaccines to combat rhinoviruses. “I originally suggested sequencing 10 strains, and then my collaborator asked why not do them all,” he said. “This made the difference between a mediocre proof-of-concept paper and a full article in Science. I learned that it is important to think big if you want to make a real difference”

–  Discovered and characterized genetic variations that may predict which patients with congestive heart failure respond best to a life-saving beta-blocker drug.  These landmark studies occurred over several years and were published in Nature Medicine twice, and the Proceedings of the National Academy of Sciences three times. “This is a good example of the progression of an idea over time, where every year or so an unexpected turn of events occurred, and new insight was gained,” he said.

– While at the University of Cincinnati, Dr. Liggett, working with colleagues at Washington University and Thomas Jefferson University, found that a genetic variation of an enzyme, which inhibits beta-adrenergic receptor signaling, confers “genetic beta-blockade” in cardiac muscle and protects against early death in African Americans with heart failure.  The findings, published in Nature Medicine, provided insight into individual variations in disease outcomes. Another key study from Cincinnati revealed that a certain combination of genetic variants within a single gene conferred low vs. excellent responses to inhaled beta-agonists in treating asthma. These combinations, called haplotypes, had never been identified in GPCRs. The work was published in Proceedings of the National Academy of Sciences.

Dr. Liggett’s groundbreaking research sequencing all known human rhinoviruses, a frequent cause of respiratory infections, was featured on the April 3, 2009 cover of the journal Science.

Advancing outside his field of study

Dr. Liggett joined USF Health in 2012 from the University of Maryland School of Medicine in Baltimore, where he was associate dean for interdisciplinary research and professor of medicine and physiology. He received his MD degree at the University of Miami and completed both a residency in internal medicine and fellowship in pulmonary diseases and critical care medicine at Washington University School of Medicine and Barnes Hospital in St. Louis, MO.

Within two years, he advanced from a postdoctoral research fellowship in Dr. Lefkowitz’s laboratory at Duke to tenured associate professor and director of pulmonary and critical care medicine at the University of Cincinnati College of Medicine.  By the time he left Cincinnati for the University of Maryland in 2005, he held an endowed chair in medicine and directed the university’s Cardiopulmonary Research Center.

Though he had no significant wet-lab experience, Dr. Liggett was fascinated by the emerging science called “molecular biology” and was undeterred from branching into a field of study in which he had no formal training.

He secured a position as assistant professor at Duke following his fellowship there, and figured out how to sequence adrenergic receptor genes from a patient’s blood. While routine now, such genetic testing had not been done previously.  He unexpectedly kept finding multiple variations (called polymorphisms or mutations) in genes coding for the same receptors, so he sought out the advice of some classic geneticists.  At the time, Dr. Liggett said, their traditional thought was modeled after diseases like cystic fibrosis — if a person had the genetic mutation they developed the disease, if the mutation was absent they did not.

“There was no consideration for common genetic variants and how they might affect disease risk, progression, or response to treatment. It simply was not in their thought process,” Dr. Liggett said. He was told “it’s probably nothing and don’t quit your day job.” He did not take their advice.

 Some advice Dr. Liggett would give to emerging young scientists

Assistant professor Donghwa Kim, PhD

Instead, he returned to the laboratory to sequence and clone receptors from many different populations with asthma and heart failure, showing that the receptor genes did indeed differ from one individual to another, generally with several common “versions.” His team also created “humanized” mice expressing the human genes for asthma and heart failure so they could begin to understand the physiology of the receptors. They began to find that some genetic alterations increased receptor function, some decreased the drug’s affinity to bind (responsiveness) to a receptor, and still others altered how the receptor was regulated.  And, through NIH-supported clinical trials, the researchers correlated outcomes observed in patients undergoing drug therapies with the genetic variations uncovered in the laboratory.

“If there’s a lesson to be learned here by young investigators, I’d say it’s that you can collect information from experts in the field, but you need to use your gut to ultimately decide on whether to pursue a line of research or not,” Dr. Liggett said.

Personalized medicine challenge: Common diseases, multiple genetic variations

Realizing personalized medicine’s full potential will require a better understanding of how environmental variables – including diet, exercise, the gastrointestinal microbiome (gut bacteria) and toxin exposure – combine with genetic variations to affect disease and its treatment, he said. “Personalized medicine faces its greatest challenges in the common diseases like asthma, atherosclerotic heart disease and heart failure, because they involve multiple variations in multiple genes that interact with the environment to give you a disease – and also provide a set-up for unique ways to treat the disease.”

Biological scientist Maria Castano

Dr. Liggett was one of the first physicians recruited for what would become the USF Health Heart Institute.  He recalls that he still had the letter of offer in his pocket when he stood before the Hillsborough County Commission in 2012 to help USF Health leadership pitch the need for a cardiovascular institute to include a focus on genomics-based personalized medicine.  The county joined the state in funding the project, and Dr. Liggett was instrumental in the early planning stages of the Heart Institute before the arrival of its founding director Dr. Samuel Wickline.  The institute is now under construction in downtown Tampa as part of the new Morsani College of Medicine facility, a key anchor of Water Street Tampa. Already, 21 of the 31 institute’s biomedical scientists who will investigate the root causes of heart and vascular diseases with the aim of finding new ways to detect, treat and prevent them, have been recruited.

“There’s an excitement here and philosophy of excellence that’s rewarding to see,” Dr. Liggett said. “We have a strategic plan in place, including moving ahead to expand research in cardiovascular disease, infectious disease and the microbiome, and the neurosciences. Our departments are recruiting at a good pace, and the faculty we’re bringing in all have NIH funding and are highly collaborative.”

Dr. Liggett is an elected fellow of the American Association for the Advancement of Science – one of only five Morsani College of Medicine faculty members to receive that prestigious honor.  He is also an elected Fellow of the National Academy of Inventors and the American College of Chest Physicians. Last year, he was one of 30 scientists nationwide selected to join The Research Exemplar Project – recognition of his outstanding reputation as a leader whose high-impact, federally-funded research yields novel and reproducible results.

Over his career, he has served on several NIH study sections and on the editorial board of high-impact journals relevant to fundamental biochemistry as well as heart and lung diseases.  He is currently editor-in-chief of the Journal of Personalized Medicine.

 The potential of new treatments for asthma and heart failure

Dr. Liggett holds 18 patents detailing potential new targets for drug therapy or genetic variations of known drug targets, which might be used to predict response to medications and customize treatment.

Some things you may not know about Dr. Liggett:

• He has asthma, which helps motivate his research toward finding better treatments for this common lung disease affecting one in 12 people in the United States.
• Restores vintage cars, primarily DeLoreans. Although he recently finished bringing a funky lime green 1974 Volkswagen Thing back to life, and over the holidays restored a 1973 VW camper. 

• Lives with wife Julie on the beach in Treasure Island, where they enjoy surfing, paddle boarding, and photography.
• Has three children – Elliott, an engineer at NASA’s Jet Propulsion Laboratory at Cal Tech in Pasadena, CA; Grace, who recently completed her master’s degree in public health at USF; and Mara, an undergraduate student studying social work at Florida Atlantic University, and two step-children — Madison, an undergraduate at the University of Florida, and Tripp, a senior at St. Petersburg Catholic High School. He also has three grandchildren, ages 2 to 9.

Photos by Sandra C. Roa, and audio clips by Eric Younghans, University Communications and Marketing