The Belgrade Biochemist Who Built the Drug Sanofi Could Not Build Alone

On March 20, 2025, Sanofi published a press release announcing that it would acquire DR-0201, a bispecific antibody developed by a small private California company called Dren Bio. The upfront payment was $600 million. Total potential milestone payments reached $1.9 billion. Dren Bio had been operating for less than six years. Its CEO and co-founder, Nenad Tomašević, had spent the previous two decades working in corners of immunology that the pharmaceutical industry had consistently ranked below higher-profile targets. He received his doctorate from the University of Belgrade, and named his company after the Serbian word for dogwood: a tree long associated in the Balkans with resilience and medicinal use. What he had been building, across three companies and a postdoctoral posting at the National Institutes of Health, was a thesis about where durable biological insight actually hides.

The Biochemist Who Looked Where the Consensus Had Stopped

Tomašević earned his undergraduate degree in Chemistry and his doctorate in Biochemistry from the University of Belgrade, completing one of the most rigorous scientific training paths available in Serbia. From Belgrade he crossed the Atlantic for a postdoctoral position at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health in Bethesda, Maryland. Working in the Genetics and Biochemistry Branch, he characterized two proteins that had not previously been formally described. The work was precise, foundational, and unspectacular by the standards of a field always hunting the next clinical blockbuster. It was also exactly the kind of research that rewards sustained attention over short-term positioning.

He moved from NIH to Cytokinetics, a South San Francisco biotechnology company, where he led research into small-molecule inhibitors of proteins involved in actin dynamics and cell motility. The habit that formed was more consequential than the publications: Tomašević was building a career around targets the field had visited briefly and left behind.

The pattern crystallized at Allakos, a company he co-founded around a therapeutic opportunity he had identified: antibodies targeting Siglec-8, a receptor expressed on specific myeloid-lineage immune cells involved in allergic and inflammatory diseases. Siglec-8 was not among the decade's priority targets when he built around it. Allakos went on to develop a clinical antibody program around the receptor. As Vice President of Research, Tomašević was responsible for all research and nonclinical development functions through the company's early clinical years.

When he left to co-found Dren Bio in May 2019 with Ivan Trifunović, another Serbian scientist, he was not searching for a new thesis. He had been working one for years. The company they built is American: incorporated and headquartered in the Bay Area, financed by US investors. What it carries from Serbia is its founders and its name. That name was the founders' quiet declaration of origin: dren is the Serbian word for dogwood, a plant common in Serbian forests and long valued in traditional Balkan medicine.

The Myeloid Cell Hypothesis That Pharma Had Filed Away

The bet Dren Bio made in 2019 was biologically specific. Through the 2010s, the center of gravity in cancer immunotherapy and autoimmune research had settled on T cells and natural killer cells. These were the effectors that checkpoint inhibitors had made famous, the mechanisms behind clinical results that had reshaped oncology. The field had organized its capital and attention accordingly.

Myeloid cells occupied a different position in this landscape. As tissue-resident macrophages and related cell types, myeloid cells are responsible for clearing cellular debris and pathogens through phagocytosis: the process by which a cell engulfs and digests its targets. The mechanism was well understood at the level of basic biology. What it had not become, by 2019, was a tractable focus for targeted therapeutic development. The consensus had examined it and moved elsewhere.

Tomašević and Trifunović founded Dren Bio on the premise that this departure was premature. Their platform, which they called the Targeted Myeloid Engager and Phagocytosis Platform, was built to create bispecific antibodies that would activate a phagocytic receptor selectively expressed on myeloid cells, but only in the presence of the relevant disease target. The design aimed for potent, precise depletion of disease-causing cells without the toxicity patterns associated with older immunotherapies.

Dren Bio raised $6 million in a seed financing in 2019, led by 8VC with participation from Mission BioCapital and individual industry investors. The lead oncology asset, DR-01, targeted autoreactive CD8 T cells implicated in cytotoxic lymphomas and specific autoimmune diseases. A second program, DR-0201, applied the phagocytosis platform to B-cell biology: a CD20-directed bispecific antibody designed to deplete B cells more completely than existing anti-CD20 therapies by enlisting myeloid cells as the effector mechanism, bypassing the limitations of T-cell or natural killer cell-based depletion.

From Seed Round to Sanofi in Six Years

In June 2022, Dren Bio closed a $65 million Series B, co-led by Aisling Capital and HBM Healthcare Investments. Pfizer joined as a new investor, alongside ArrowMark Partners, Revelation Partners, SR One, 8VC, Taiho Ventures, BVF Partners, Mission BioCapital, and Alexandria Venture Investments. Total capital raised to that point exceeded $156 million. The financing funded DR-01's entry into clinical evaluation in patients with Large Granular Lymphocytic leukemia and cytotoxic lymphomas, and accelerated development of additional programs from the phagocytosis platform.

The first major external validation did not arrive as a clinical result. It arrived as a partnership announcement from Novartis.

In July 2024, Dren Bio announced a strategic collaboration with Novartis Pharma AG for bispecific antibodies targeting cancer using the myeloid engager platform. Novartis paid $150 million upfront, including a $25 million equity investment, with up to $2.85 billion available in milestone payments. It was the first institutional signal from global pharma that the myeloid phagocytosis approach had crossed from scientific premise to priced asset.

Nine months later, Sanofi moved on the B-cell program.

On March 20, 2025, Sanofi announced it would acquire DR-0201 through the acquisition of a Dren Bio corporate entity, Dren 0201, Inc. The upfront payment was $600 million, with up to $1.3 billion in further milestone payments, for total potential value of $1.9 billion. Sanofi's immunology leadership described DR-0201 as capable of inducing deep B-cell depletion through a mechanism distinct from standard anti-CD20 approaches, with clinical data suggesting the potential for sustained treatment-free remission in autoimmune patients. The asset was renamed SAR448501 and entered two Phase 1 clinical studies. The acquisition closed on May 27, 2025. Dren Bio retained its remaining pipeline and continued to operate as an independent company.

Sanofi had seen enough of the biology on the first program to want to extend the relationship before the second one was clinically proven.

In December 2025, Sanofi returned. On December 15, the two companies announced an expanded strategic collaboration for the discovery and development of a second B-cell depleting program for autoimmune diseases. The upfront payment was $100 million, with eligibility for up to $1.7 billion in development, regulatory, and commercial milestone payments. Dren Bio retained the option to co-fund future development in exchange for US co-promotion rights and a 50/50 share of US profits and losses on the American market.

The two Sanofi transactions, nine months apart, carried a combined $700 million in upfront payments. Added to the Novartis collaboration, Dren Bio's partnerships had generated more than $850 million in upfront capital before either the oncology or the immunology programs had reached Phase 3 evaluation. Each deal had built directly from the one before it.

The Biology Hidden One Step Behind the Consensus

The pattern across Tomašević's career is consistent. At the NIH, he characterized proteins the field had passed over. At Allakos, he co-founded a company around a receptor the literature had catalogued without building a therapy around. At Dren Bio, he built a platform around a cellular mechanism the mainstream had classified as background biology rather than a therapeutic target. In each case, the insight was not hidden from the field. Siglec-8 was in the published literature. Myeloid phagocytosis was well understood at the basic science level. The question was not whether the biology existed, but whether anyone would stay long enough to build a company around it.

Large pharmaceutical companies optimize for targets with established clinical precedent. The structural gap they create is not in capital or scientific competence but in biological attention. The founder who builds in that gap, and stays long enough to develop a platform rather than a single molecule, creates something that a large organization cannot quickly replicate.

Tomašević named his company after a tree. The dogwood survives across Serbian hillsides and has been used in traditional Balkan medicine for generations, valued by those who looked carefully enough to notice it before it was commercially recognized. The science behind Dren Bio operated the same way: present in the literature for anyone reading with the right question, available to the researcher willing to stay close enough to build around it.

The deal Sanofi came back to negotiate in December 2025, nine months after spending $600 million on the first program, was the answer to the question Dren Bio had been asking since 2019. The founder who stayed close to the biology, rather than migrating toward whatever the consensus had most recently certified, had built the platform that pharma needed but had not built for itself.