Dr. Tamer Coskun: The Eli Lilly Scientist Behind Tirzepatide (Mounjaro & Zepbound)
Profiles in Metabolic Medicine · A Long-Form Scientific Profile
The Architect of the Dual Agonist
Dr. Tamer Coskun and the making of tirzepatide — how a Turkish-trained physician-scientist, working quietly inside Eli Lilly, turned a two-hormone hypothesis into one of the most consequential medicines of the twenty-first century.
There are moments when the center of gravity of an entire field of medicine shifts, and for a while almost nobody notices. May 2022 was one of them. When the United States Food and Drug Administration approved tirzepatide — sold as Mounjaro — for type 2 diabetes, the decision looked like another entry in a long line of incretin-based therapies. Within eighteen months, the same molecule, rebranded as Zepbound for chronic weight management, had become one of the most discussed, most prescribed, and most politically scrutinized medicines on the planet. Pharmacies reported shortages. Insurers rewrote formularies. Heads of state commented on it. A drug class once dismissed as a niche of diabetology had become a cultural phenomenon.
Yet tirzepatide is not merely “another GLP-1 drug.” It is the first approved medicine in history to fuse two gut hormones — glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) — into a single molecule, and the first to validate the idea that the body’s own post-meal hormone chorus could be conducted, deliberately, by one engineered peptide. That idea was controversial for more than a decade. Many experts believed GIP agonism was a dead end, even counterproductive. The bet that it could work — and the program that proved it — is, in no small measure, the story of one scientist.
His name is Tamer Coskun. A physician trained at Istanbul’s storied Cerrahpaşa School of Medicine, a physiologist with a PhD from Marmara University, a former assistant professor at Indiana University’s medical school, and, since 2003, a career Lilly man who rose to Vice President–Medical, Coskun is the lead author of the foundational 2018 paper that introduced tirzepatide to the world and an inventor on numerous patents covering it. Within Lilly he has occupied, since 2018, a deliberately unusual role: simultaneous responsibility for discovery research and for early-phase clinical studies in diabetes and obesity — the exact seam where tirzepatide was forged.
This article is a profile of that scientist and of his singular contribution to the development of tirzepatide. It argues that the molecule cannot be understood apart from the kind of researcher Coskun is: a physician-physiologist who studied the gut as an endocrine organ for decades, who crossed oceans and disciplines, and who never allowed the chemistry to forget the patient. It is also, inevitably, a story about how modern medicines are actually made — not in a single eureka moment, but in a long, disciplined conversation between bench and bedside, conducted by people fluent in both languages.
The Bosphorus Years
Istanbul, 1983–1999: a physician is formed
The public record of Tamer Coskun’s life begins, as so many Turkish success stories do, with a rigorous education in Istanbul. According to his aggregated professional profiles, he was schooled at Kadıköy Anadolu Lisesi, one of the city’s respected Anatolian high schools on its Asian side — an institution of the kind that sorts Turkey’s most academically promising teenagers through fierce entrance examinations. What followed was a commitment of far greater magnitude: in September 1983 he entered the Cerrahpaşa School of Medicine of Istanbul University, one of the oldest and most demanding medical faculties in the country, from which he graduated with his MD in early 1991 after nearly eight years of training.
Cerrahpaşa in the 1980s was a school of the old tradition: enormous patient volumes, bedside teaching, and a clinical discipline that expected a physician to reason from physiology before reaching for technology. Graduates of that era often describe an education in which the mechanisms of disease — not merely its management — were the daily currency. For a young doctor whose later career would be defined by gut-hormone physiology, that environment mattered. The habits formed there, of treating the patient and the mechanism as a single problem, would become the signature of his science.
Rather than remain purely a clinician, Coskun moved toward research. Between 1993 and 1997 he completed a PhD at Marmara University School of Medicine in Istanbul, with his doctoral work rooted in gastroenterology and physiology — the study of the gut not only as a digestive organ but as a sensing, signaling, hormone-secreting one. It was a prescient choice. In the 1990s the gut’s endocrine role was still, in many clinical circles, a backwater; the therapeutic exploitation of its hormones was in its infancy. By anchoring himself in gut physiology, Coskun placed himself at the headwaters of what would become the incretin revolution.
He stayed on at Marmara as an assistant professor from 1997, teaching and researching in the same institution that had granted him his doctorate. Colleagues from those years, and the publication record that survives them, show a young academic interested in the borderlands — between physiology and pharmacology, between animal models and human disease. He was, by training and temperament, a translational scientist before the word had become a buzzword. And he was, crucially, a physician who happened to do research, rather than a researcher who occasionally remembered patients. That ordering of identities would shape everything that followed.
It should be noted that Coskun guards his private life carefully; no public source documents his childhood, family, or personal affairs, and this profile respects that boundary. What the record does show is a pattern: every formative choice — the Anatolian high school, Cerrahpaşa, Marmara, gastroenterology — pointed toward the interface of the gut, its hormones, and human metabolic disease. Istanbul gave him his medicine. America would give him his molecule.
The American Apprenticeship
UCLA, IUPUI, and the decision to join Lilly, 1995–2003
In the mid-1990s Coskun crossed the Atlantic for postdoctoral fellowship work at the University of California, Los Angeles, where in 1995–1996 he studied brain–gut interactions. The topic seems, in retrospect, almost prophetic. Two decades later, the decisive clinical advantage of incretin therapies — profound appetite suppression and weight loss — would be traced to precisely those brain–gut circuits. The young fellow measuring how gut signals reach the brain was, unknowingly or not, laying the conceptual groundwork for understanding why his future drug would shrink waistlines as well as glucose curves.
By 1999 he had settled in Indianapolis, the company town of Eli Lilly, where he served as an assistant researcher and assistant professor at the Indiana University School of Medicine — the successor institution to IUPUI — in the Department of Cellular and Integrative Physiology between 1999 and 2003. Indianapolis is not a neutral location in the history of diabetes; it is the city where insulin was first commercialized in the United States, and where Lilly built, over the twentieth century, the world’s most storied diabetes franchise. A gut physiologist teaching and researching in Indianapolis was, whether by design or by destiny, standing in the right place at the right time.
In November 2003 he made the move that defined his career: he left academia and joined Eli Lilly and Company as a Principal Research Scientist. The decision carried risk. Academic physiologists who cross into industry are sometimes viewed as having abandoned the purity of scholarship. But Coskun’s choice reflected a clear-eyed judgment about where his kind of science could matter most: the questions he cared about — can gut hormones be turned into medicines? — required chemistry teams, clinical units, and capital that only a pharmaceutical company could marshal.
His ascent inside Lilly was steady and, eventually, steep: Principal Research Scientist through 2013, then Research Advisor, Senior Research Advisor, Senior Medical Advisor, and Medical Fellow, before his elevation to Associate Vice President–Medical in 2022 and Vice President–Medical in 2024. The inflection point came in 2018, when Lilly gave him a deliberately hybrid mandate — continuing responsibility for discovery efforts while also leading early-phase clinical research within its Diabetes, Obesity and Complications Therapeutic Area. It was an organizational acknowledgment of what his colleagues already knew: Coskun was the rare executive who could read a receptor-signaling curve in the morning and a first-in-human safety dossier in the afternoon.
The Two-Hormone Hypothesis
Why GIP + GLP-1, when everyone said GIP was dead
To appreciate Coskun’s contribution, one must appreciate the heresy he helped vindicate. The incretin effect — the observation that oral glucose provokes a greater insulin response than injected glucose — had been described decades earlier, and its two principal hormonal mediators, GIP and GLP-1, were identified in the 1970s and 1980s. When drug hunters industrialized the pathway, it was GLP-1 that won: exenatide, liraglutide, semaglutide. GLP-1 agonism lowered glucose, trimmed weight, and eventually protected hearts. GIP, its sibling hormone, was left behind.
The case against GIP seemed strong. In type 2 diabetes, the insulinotropic effect of GIP is famously blunted — the hormone appears to fail precisely where it is needed. Genetic experiments complicated the picture further: mice lacking the GIP receptor resist diet-induced obesity, which some read as evidence that GIP signaling promotes fatness, and that blocking GIP, not activating it, might be the sensible strategy. For years, the field’s informal consensus held that a GIP agonist was, at best, redundant and, at worst, fattening.
A minority disagreed. Building on the emerging science of unimolecular co-agonists — single engineered peptides that activate multiple hormone receptors, an approach pioneered by peptide chemists such as Richard DiMarchi and metabolic biologists including Brian Finan and their collaborators — this camp proposed that GIP and GLP-1 might be complementary rather than competitive. GIP, they argued, contributes meaningfully to post-meal insulin secretion in the presence of GLP-1 activity, may improve adipose-tissue insulin sensitivity, and might even blunt the nausea that limits GLP-1 drugs. The hypothesis was elegant. What it lacked was decisive biological proof — in disease models, and then in people.
That proof was a biological problem, not a chemical one, and biology was Coskun’s territory. Within Lilly’s diabetes and obesity research organization, his teams sat exactly where the hypothesis met the test: whole-animal models of diabetes and obesity, gut-hormone physiology, and, increasingly, the early human studies that would confirm or bury the idea. The chemistry platform could propose dozens of dual-agonist peptides; only rigorous physiological characterization could identify the one worth a billion-dollar clinical program.
This is the context in which LY3298176 — the molecule later named tirzepatide — was born. It was not an accident of screening. It was the product of a deliberate, contrarian strategy, executed by a team in which Coskun’s translational judgment was central: choosing the right balance of GIP and GLP-1 activity, the right dosing interval, the right safety profile, and having the conviction to carry that choice into humans despite a decade of skepticism.
Engineering LY3298176
A GIP backbone, a GLP-1 soul, and the pharmacology of imbalance
Tirzepatide’s design is a lesson in purposeful molecular architecture. The peptide is built on the sequence of GIP, engineered to carry meaningful GLP-1 receptor activity as well — one molecule, two hormonal identities. Strategic substitutions with non-proteinogenic amino acids shield it from the enzyme DPP-4 and from other proteases that destroy native incretins within minutes. A long-chain fatty diacid side chain allows the peptide to dock onto albumin in the bloodstream, slowing its clearance and stretching its half-life to roughly five days — the pharmacokinetic foundation of convenient once-weekly dosing.
But the molecule’s most intellectually interesting feature is not its durability; it is its asymmetry. Careful pharmacological work by the Lilly team, published in 2020, showed that tirzepatide is an imbalanced dual agonist — considerably more potent at the GIP receptor than at the GLP-1 receptor — and a biased one, recruiting less β-arrestin and provoking less GLP-1 receptor internalization than native GLP-1, a signature associated with sustained signaling. Far from a flaw, this imbalance was the hypothesis made flesh: enough GLP-1 activity to deliver glucose-lowering and appetite suppression, embedded in a dominant GIP signal that might amplify insulin secretion, improve energy disposal, and soften the gastrointestinal side effects that plague pure GLP-1 agonists.
The foundational public statement of this science arrived in 2018, when Coskun, as lead author, introduced LY3298176 to the world. The paper paired extensive preclinical characterization — receptor pharmacology, animal models of diabetes and obesity — with the first signals of human efficacy, and it did something rare in drug discovery: it made the case for an entire new therapeutic paradigm, not just a compound. The work has since been cited more than a thousand times, making it one of the most influential industry-authored papers in modern metabolism.
Tirzepatide at a glance
- Class: first-in-class dual GIP/GLP-1 receptor agonist (“twincretin”).
- Design: GIP-based peptide engineered for GLP-1 activity; protease-resistant substitutions; albumin-binding fatty diacid for once-weekly dosing.
- Pharmacology: imbalanced (GIP > GLP-1 potency) and biased (reduced β-arrestin recruitment at GLP-1R).
- Approvals: Mounjaro (type 2 diabetes, US, May 2022); Zepbound (chronic weight management, US, late 2023).
- Legacy: validated the co-agonist paradigm now pursued across the entire industry.
Equally important was the process by which the molecule matured, and here Coskun’s 2018 dual mandate proved its worth. In traditional pharma, discovery hands a molecule “over the wall” to clinical development, and knowledge is lost in transit. Coskun’s integrated role collapsed that wall. Observations from first-in-human studies fed back into biological understanding within the same leadership circle; pharmacology informed dose selection; dose selection informed the design of the next analogue. Tirzepatide was thus not merely discovered and then tested — it was iterated, in a continuous bench–bedside loop of the kind that translational medicine preaches but rarely achieves.
The patent record tells the same story of sustained inventorship. Coskun is named on numerous patents covering tirzepatide, its successor retatrutide, and related incretin inventions — the legal sediment of years of hands-on molecular decision-making. And when competitors and academics later dissected tirzepatide’s mechanism, from its biased signaling to its effects on pancreatic islets and brain circuits, they were, whether they cited him or not, arguing on terrain that Coskun’s team had mapped first.
The Clinical Gauntlet
SURPASS, SURMOUNT, and the rewriting of the rulebook
A molecule is only a hypothesis until the clinic votes. The first serious vote came in 2018, when a phase 2b dose-ranging study of LY3298176 reported glucose-lowering and weight reductions that exceeded what the field had come to expect from incretin therapy at that stage. HbA1c fell by up to roughly two percentage points and body weight dropped meaningfully in a matter of months — numbers that made even seasoned diabetologists sit upright. The dual-agonist idea was no longer a provocation; it was a front-runner.
The phase 3 SURPASS program then delivered, trial after trial, some of the most impressive results in the history of type 2 diabetes: HbA1c reductions approaching 2.5 percentage points and weight losses of up to about ten percent or more in people with diabetes — figures that blurred the line between a glucose drug and a weight drug. The symbolic high point was SURPASS-2, the head-to-head trial against semaglutide 1 mg, the reigning champion of the GLP-1 world. Tirzepatide won on both glucose and weight. The student of GIP had outperformed the masterpiece of GLP-1, and the old consensus — that GIP agonism was futile — was formally retired.
Then came obesity itself. The SURMOUNT-1 trial, published in 2022, reported mean weight reductions of up to about 21 percent with the highest dose over 72 weeks in adults with overweight or obesity — results approaching the territory of bariatric surgery and utterly unprecedented for a medicine. When the FDA approved tirzepatide for chronic weight management as Zepbound in late 2023, it ratified what the data already implied: obesity could be treated, chronically and effectively, as the hormonal disease it is. By 2025, the SURPASS-CVOT outcomes trial had further established cardiovascular safety against an active comparator, anchoring the molecule’s place in long-term medicine.
Where is Coskun in this saga? Not on every headline, but woven through the fabric. He co-authored key studies across the program; more importantly, the early-phase trials he personally steered — the first-in-human and proof-of-concept studies that de-risked the molecule — were the gates through which every later triumph had to pass. A phase 3 program enrolling tens of thousands of patients is only launched when early signals are trusted; that trust is manufactured by the credibility of the people who generate them. Coskun’s dual discovery–clinical role meant that the judgment to advance tirzepatide, and the judgment to advance it as a dual agonist rather than a me-too GLP-1, carried his fingerprint.
History will record the approvals under corporate names, as it must. But the scientific record — author lists, patents, and the 2018 paper that started the citation cascade — records the human architecture. Every prescription of Mounjaro and Zepbound is, in a real sense, a downstream event of experiments that Coskun’s teams designed, interpreted, and had the institutional courage to champion.
The Triple-Agonist Horizon
Retatrutide and the paradigm that now runs the industry
Scientists are often prisoners of their first success; Coskun used his as a launchpad. Even as tirzepatide raced through phase 3, his group pushed the co-agonist logic one receptor further. In 2022, with Coskun as lead author, Lilly published the preclinical characterization of LY3437943 — retatrutide — a single molecule activating the GIP, GLP-1, and glucagon receptors simultaneously. The concept sounded like greed: three hormones, one peptide. But the rationale was disciplined — glucagon receptor agonism adds energy expenditure to the GIP/GLP-1 formula — and the early clinical results were staggering, with phase 2 weight losses of up to roughly 24 percent, again approaching surgical territory.
The broader consequence is that the strategy once considered heretical is now the industry’s roadmap. Virtually every major metabolic-medicine program in development today — dual agonists, tri-agonists, amylin–incretin combinations — descends conceptually from the proof that tirzepatide provided and that Coskun helped deliver. When competitors file patents on co-agonists, they are drafting in a space his work opened. When academic centers study brain–gut reward circuits under tirzepatide — a literature Coskun also contributes to — they are extending the very brain–gut thread that began with his UCLA postdoc decades earlier.
As Vice President–Medical, Coskun now oversees the pipeline side of that future: shepherding next-generation molecules from initiation through clinical proof-of-concept within Lilly’s diabetes and obesity franchise. The boy who entered Cerrahpaşa in 1983 now sits at the head of the table where the next decade of metabolic medicine is decided.
The Translator’s Philosophy
What kind of scientist is Tamer Coskun?
Institutions reveal their values through their org charts, and Lilly’s creation of a hybrid discovery–clinical role for Coskun in 2018 was a statement about what it believed translational science requires: not committees, but translators — individuals fluent in both dialects of medicine. His official descriptions speak of “key translational expertise between the preclinical and clinical research space,” driving projects “from initiation through clinical proof-of-concept.” Dry words, but they describe a rare human function: the ability to keep a molecule honest by constantly confronting it with patients.
His recognition has followed. Invited as a keynote speaker at the 2025 annual meeting of the American Society for Pharmacology and Experimental Therapeutics, and a plenary presence at international obesity and diabetes congresses through 2026, Coskun is now publicly acknowledged as one of the field’s architects — the quiet author behind more than a hundred peer-reviewed manuscripts and a patent portfolio that underpins tens of billions of dollars in medicine and market value.
And yet the man himself remains deliberately opaque. He keeps no public social media; he grants no interviews about his private life; no record documents his family, his hobbies, his inner world. In an era when scientists are encouraged to build personal brands, Coskun has built something older-fashioned: a body of work. The only autobiography he has published is an author list — Cerrahpaşa to Marmara to UCLA to IUPUI to Lilly, written one landmark paper at a time.
Perhaps that is the deepest lesson of his career. Tirzepatide was not discovered by a committee, nor by a lone genius, but by a sustained argument between chemistry and physiology, arbitrated by people who understood both. Coskun’s genius, if the word must be used, was dispositional: he remained a physician while doing chemistry, a physiologist while doing business, and a student of the gut while doing everything else.
A Life in Milestones
The essential chronology, from public records
- 1983 Enters Cerrahpaşa School of Medicine, Istanbul University, after schooling at Kadıköy Anadolu Lisesi.
- 1991 Graduates with the degree of Doctor of Medicine (MD).
- 1993–97 PhD in gastroenterology/physiology, Marmara University School of Medicine, Istanbul.
- 1995–96 Postdoctoral fellowship at UCLA, studying brain–gut interactions.
- 1997–99 Assistant Professor, Marmara University School of Medicine.
- 1999–2003 Assistant researcher and assistant professor, Indiana University (IUPUI) School of Medicine, Indianapolis.
- 2003 Joins Eli Lilly and Company as Principal Research Scientist.
- 2018 Lead author of the foundational paper introducing LY3298176 (tirzepatide); given Lilly’s hybrid discovery–clinical mandate in diabetes and obesity.
- 2020 Lilly team publishes the defining pharmacology: tirzepatide as an imbalanced, biased dual GIP/GLP-1 agonist.
- 2022 FDA approves tirzepatide as Mounjaro; Coskun leads the preclinical paper on triple-agonist retatrutide.
- 2023 Tirzepatide approved for weight management as Zepbound; retatrutide phase 2 results stun the field.
- 2024–26 Serves as Vice President–Medical, Eli Lilly; keynote and plenary speaker at ASPET 2025 and international congresses.
Conclusion: The Molecule as Monument
Statues are rarely erected for pharmacologists, so their monuments take other forms: a name in a patent column, a molecule in a formulary, a glucose curve bent back toward normal in a body that had stopped obeying. By those measures, Tamer Coskun’s monument is already vast. Every week, millions of syringes of tirzepatide are injected into millions of arms, and each one delivers, in molecular form, an argument he spent his career winning — that the gut’s two hormones, joined in one engineered peptide, could do together what neither could do alone.
It is a satisfying arc: the student who learned medicine on the Bosphorus, learned the gut in Marmara, learned the brain–gut axis in Los Angeles, and learned drug-making in Indianapolis, ending as the executive-physician who stood exactly at the junction where all four educations were needed at once. Tirzepatide required precisely that junction. Perhaps that is why the molecule worked when skeptics said it couldn’t — because the person holding it understood every language it spoke.
The history of medicine is usually written as a history of discoveries. Occasionally, it is better read as a history of discoverers. In the incretin era, one of those discoverers — a private man from Istanbul who lets his author list speak for him — deserves his chapter.
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