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Introduction
On April 14, 2026, CrossBridge Bio announced a definitive agreement to be acquired by Eli Lilly and Company for up to $300 million in cash, inclusive of an upfront payment and a subsequent payment upon achieving a specified development milestone1. Founded in 2023, the Houston-based startup utilizes innovative linker technology to develop next-generation antibody-drug conjugates (ADCs) that deliver multiple therapeutic payloads. This proprietary platform, originally developed at UTHealth Houston, employs proprietary glutamic acid-based EGCit linkers designed to enhance stability in the bloodstream while facilitating the release of drugs within tumor cells. Their lead candidate, CBB-120, is a dual-payload ADC targeting TROP2 that aims to overcome common cancer resistance mechanisms. Research conducted by the company suggests these advancements improve therapeutic efficacy and safety compared to traditional ADC designs. The acquisition positions Eli Lilly to accelerate these oncology treatments into clinical trials scheduled for 2026.
Here, we expand on what differentiates CrossBridge Bio’s ADC platform.
ADC Capabilities and Limitations
We covered what an antibody-drug conjugates (ADCs) is and how they came about in our discussion of the Gilead-Tubulis acquisition, but to summarize ADCs consist of three component parts:
The Antibody (The GPS): This is a protein designed to find and “lock onto” a specific target that is only (or mostly) found on cancer cells.
The Payload (The Warhead): This is a highly potent chemotherapy drug. It is far too toxic to be injected into the blood on its own, as it would damage everything it touches.
The Linker (The Safety Pin): This is the chemical bond that holds the drug to the antibody. It is designed to stay stable in the bloodstream and only break once it is safely inside the cancer cell.
While the antibodies and the toxic payloads (the medicine) are important, the most significant leaps across generations have been in the linker chemistry and the conjugation site (where the drug attaches to the antibody). Targeted delivery aims to widen the therapeutic window by increasing the concentration of the drug at the tumor site relative to systemic exposure. By designing the chemical bond to tether the ‘poison’ to the antibody until internalization, ADCs aim to minimize damage to healthy tissues while allowing for higher, more effective doses at the tumor site.
Despite this promise, 1st generation ADCs struggled with leaky chemistry. In the early 2000s, the goal was simply to prove that an antibody could carry a toxin. However, the technology was blunt. Researchers used random conjugation. Toxic payloads were mixed together with antibodies and stuck wherever they landed, leading to a mix of antibodies with 0, 2, or 8 drugs attached. The chemical bonds were often acid-labile and unstable in human blood. The toxic payload would often fall off while the drug was still circulating, leading to systemic toxicity, poisoning the patient rather than the tumor. Mylotarg (CD33 ADC for AML), the first FDA approved ADC, was first approved in 2000, voluntarily withdrawn in 2010 due to safety concerns, then re-approved at a lower dose and different dosing schedule in 2017 with an expanded indication in 2020.
By the 2010s, scientists moved toward more stable linkers and “humanized” antibodies, which the body’s immune system was less likely to reject. Because 1st-generation drugs weren’t potent enough, the 2nd generation introduced extremely powerful toxins like DM1 or MMAE. These are thousands of times more toxic than standard chemotherapy. Better maleimide linkers were used, which stayed attached in the bloodstream much longer, aimed to enhance the amount of the drug that actually reached the tumor. While better, they still used random conjugation. If an antibody carried too many drugs (a high DAR or Drug-to-Antibody Ratio), it became “greasy” and was cleared by the liver too quickly, reducing its effectiveness.
We are currently in the era of 3rd generation of ADCs (2020–2026), where the focus has shifted to site-specific conjugation. Scientists can now place the drug at an exact spot on the antibody. This creates a uniform drug (nearly every molecule is identical), which makes the medicine much more predictable and safer. Modern payloads are designed to pass through cell membranes. Once the ADC kills the primary cancer cell, the toxin can drift into neighboring cancer cells that might not even have the target protein. This is crucial because tumors are heterogeneous; messy mixtures where not every cell looks the same. Furthermore, scientists can now load more toxic payloads onto a single antibody without making the ADC toxic to the rest of the body or unstable in the blood.
CrossBridge Bio and Dual Payloads
CrossBridge Bio and their proprietary EGCit Linker platform advances the cutting edge of ADCs in five distinct ways:
Enables dual payload ADCs: Traditional ADCs often rely on single-mechanism payloads, leaving them vulnerable to tumor heterogeneity and the rapid onset of multidrug resistance (MDR). When a therapeutic agent hits only one molecular target, the emergence of antigen-negative clones or the upregulation of efflux pumps frequently results in clinical relapse. For patients with aggressive solid tumors, these limitations translate into a narrow therapeutic index where the dose required for efficacy is inextricably linked to dose-limiting systemic toxicities. CrossBridge Bio’s platform represents a strategic response to these constraints. Overcoming intratumor heterogeneity and multidrug resistance (MDR) requires moving beyond the “one-antibody, one-drug” model. CrossBridge utilizes click-chemistry-based branched linkers (di-arm and tri-arm) to install two distinct payloads with absolute precision. The CrossBridge Bio’s candidate, CBB-120, is a TROP2-targeting ADC utilizing a novel dual-payload architecture that combines a Topoisomerase 1 inhibitor (TOP1i) with an ATR inhibitor (ATRi). The combination of TOP1i and ATRi in CBB-120 creates a state of synthetic lethality. While TOP1 inhibitors create DNA single-strand breaks during replication, ATR inhibitors block the cell’s ability to repair those breaks, hypothesized to trap replication forks and induce apoptosis. Preclinical models suggest this mechanism demonstrates greater potency in vitro than the sum of its parts, and could potentially provide enough differentiation to make CBB-120 the best-in-class TROP2 ADC.
Potentially better brain exposure: CrossBridge Bio has identified ADC homogeneity as the key to Blood-Brain Barrier (BBB) penetration. Heterogeneous ADCs often contain “high-DAR” species (DAR 6–8) that are excessively hydrophobic. These species form multimolecular complexes with albumin, significantly increasing the apparent hydrodynamic radius of the conjugate and preventing it from crossing the BBB. In preclinical models, the platform was observed to reduce hydrophobicity and albumin binding, thereby lowering the hydrodynamic radius of the ADC. This is designed to facilitate better passive diffusion across the blood-brain barrier, a feat that other high-DAR ADCs have struggled to achieve consistently. This research is intended to explore the potential for better treatment and control of tumors that have metastasized to the brain, or could unlock efficacy in glioblastoma (GBM), which prior TROP2 ADCs have struggled with.
Site-specific conjugation: Unlike stochastic conjugation, which produces a heterogeneous mixture of species with varying Drug-to-Antibody Ratios (DAR), CrossBridge’s platform ensures site-specific conjugation. This precision is essential for maintaining a predictable safety profile and ensuring the therapeutic index remains broad enough for clinical success. The technical centerpiece of this acquisition is the EGCit (Glutamic acid-glycine-citrulline) linker.
Potentially better mouse-to-human translation: Standard VCit linkers, the backbone of 3rd gen ADCs, are rapidly degraded by the extracellular carboxylesterase Ces1c in mice, leading to premature toxicity and false negatives in efficacy trials. CrossBridge’s EGCit linker features a negatively charged glutamic acid (red bubble in figure below) that effectively is designed to repel the Ces1c enzyme, demonstrating improved stability in mouse circulation. While resistant to mouse enzymes, the EGCit linker remains highly sensitive to human intracellular cathepsins, ensuring a traceless and rapid release of the payload once internalized. By eliminating species-specific degradation in mice, EGCit allows scientists to generate more translatable preclinical data and prevent the premature abandonment of candidates due to species-specific instability.
Potentially better linker stability in human plasma: While the addition of glutamic acid at the P3 position solved the issue of mouse plasma instability, researchers discovered that human neutrophil elastase preferentially cleaves the amide bond between citrulline (P1) and valine (P2). Replacing valine with glycine (blue bubble in figure above) is designed to shield the linker from degradation by neutrophil-secreted proteases and enhances linker stability in human plasma.
Conclusion
The acquisition of CrossBridge Bio by Eli Lilly marks a sophisticated evolution in oncology, signaling a shift from simply hitting a target to ensuring the target cannot escape via resistance. By integrating dual-payload synthetic lethality with a branched EGCit linker platform that is designed to address the long-standing mouse-to-human translation gap and improve the potential for brain penetration, Eli Lilly is positioning itself to explore whether therapeutic windows can be expanded by the application of this specific linker technology. These capabilities could expand Eli Lilly’s growing ADC pipeline, which currently includes FR-alpha ADC sofetabart mipitecan (Phase 1 ongoing in Platinum-Resistant Ovarian Cancer) and three unnamed ADCs in Phase 1 trials (two NECTIN-4 ADCs and a PTK7 ADC). It also complements Eli Lilly’s acquisition of Mablink Bioscience in 2023, for their stable linker technology.
As CBB-120 moves toward its IND filing in late 2026, the success of this deal will likely depend on whether the candidate can successfully dismantle the resistance mechanisms and reduce the side effects that have historically limited current TROP2-targeted therapies. If CBB-120 delivers on its preclinical promise, the industry may look back on this deal as the moment that dual-payload ADCs emerged as a new gold-standard for the modality.
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