Experimental Drug CS18 Restores Sensitivity to Existing Therapies in Drug-Resistant Cancer Models

Clinical oncologists generally operate on a grim assumption: once a tumor evolves to survive a targeted therapy, the drug is permanently exhausted, and the cancer's new defenses must be attacked with entirely different, often more toxic, alternatives. The prevailing consensus is that cancer cells cannot be made to drop their acquired shields. But an experimental compound developed by researchers at Baylor College of Medicine directly contradicts that model. The drug, called CS18, has demonstrated in animal models that treatment-resistant cancer cells can be forced to dismantle their defenses and regain sensitivity to the exact therapies they had learned to evade.[1][3]

The findings, published in the journal Science Advances in August 2026, outline a fundamentally different approach to dismantling therapeutic resistance. Rather than attempting to block a single survival pathway or a specific genetic mutation, the Baylor team targeted a central molecular switchboard that coordinates multiple cancer-promoting systems simultaneously. By disabling this structural hub, the experimental drug weakened the tumor's ability to repair its own DNA, continue dividing, and suppress cell death, leaving the malignant cells highly vulnerable to secondary attacks.[3][5]

"Therapeutic resistance is a main obstacle to achieve effective and durable cancer treatments," said Dr. Weei-Chin Lin, a professor of medicine, hematology, and oncology at Baylor College of Medicine and the corresponding author of the study. "While some therapies are effective at the beginning, many patients eventually relapse because cancer cells can activate compensatory and convergent biological pathways that allow them to overcome the toxic effects of therapy, promoting survival." This biological adaptability has long frustrated efforts to cure aggressive disease.[5]

To bypass these redundant networks, Lin and his colleagues focused on a multifunctional scaffold protein known as topoisomerase IIβ-binding protein 1, or TopBP1. In aggressive cancers, TopBP1 is frequently overexpressed and acts as a biological control center, integrating signals that manage replication stress with broader oncogenic networks. The protein contains exactly nine distinct BRCT domains, which function as physical docking interfaces for other regulatory molecules, allowing the tumor to coordinate its survival strategies across multiple fronts simultaneously.[4][5]

The researchers honed in on one specific interface within the protein: the BRCT7/8 domain. This localized region interacts with several key regulators of cancer growth, including MIZ1, a suppressor of the cancer-driving protein MYC; mutant forms of the p53 protein, which can acquire functions that actively promote cancer; and PLK1 and CIP2A, proteins that help malignant cells survive and divide under stress. By targeting this single domain, the team hypothesized they could sever multiple survival lifelines at once.[5]

"All together, these diverse roles position TopBP1-BRCT7/8 as a promising target for intervention," Lin noted. To exploit this vulnerability, the research team screened more than 1,000 candidate molecules to find one that could bind exclusively to this specific domain. An initial hit, a compound designated 3B6, provided a chemical starting point. Through multiple rounds of structure-activity relationship-driven optimization, the team chemically modified 3B6 to produce CS18, a highly potent and selective inhibitor capable of dismantling the protein complex.[5]

When CS18 binds to the BRCT7/8 domain, it physically disrupts the oncogenic TopBP1 complexes without interfering with normal, healthy DNA replication. The downstream effects are severe for the tumor architecture. The cancer-promoting activities of MYC and mutant p53 decrease, proteins involved in DNA repair become markedly less active, and genes that restrict uncontrolled cell growth are switched back on. The ultimate result is a state of mitotic catastrophe, where the cancer cells are driven irreversibly toward programmed cell death.[3][5]

The researchers tested CS18 across five distinct types of malignant cells: triple-negative breast cancer, ovarian cancer, lung adenocarcinoma, lung squamous cell carcinoma, and acute myeloid leukemia. In laboratory tests, the compound exhibited broad-spectrum anticancer activity across all five models, suggesting that TopBP1 inhibition is relevant across tumors driven by entirely different genetic abnormalities, rather than being limited to a single cancer subtype. Crucially, the experimental drug proved significantly less toxic to non-cancerous cells during these initial in vitro evaluations.[1][5]

The most striking results emerged when CS18 was deployed not as a standalone treatment, but in direct combination with two existing classes of cancer medications. The compound significantly enhanced the activity of PARP inhibitors, a class of drugs that prevent cells from repairing certain forms of DNA damage. Because tumor cells often operate under heavy replication stress, they rely heavily on backup repair pathways. When CS18 disrupts TopBP1 signaling, those remaining repair mechanisms fail, leaving the cancer cells highly vulnerable to the PARP blockade.[4][5]

CS18 also strengthened the effect of osimertinib, a targeted therapy used against non-small cell lung cancers driven by mutations in the epidermal growth factor receptor (EGFR). In patient-derived xenograft models—specialized mice carrying human breast and lung tumors—the combination of CS18 and osimertinib was particularly effective in cells that had already developed complete, documented resistance to the EGFR inhibitor, a scenario that typically leaves human patients with very few viable treatment options and a rapidly worsening prognosis.[3][5]

"In the case of lung cancer cells that were already resistant to osimertinib, adding CS18 restored the cells' sensitivity to osimertinib, increasing cancer cell death," Lin explained. The addition of the experimental compound essentially dismantled the acquired resistance mechanisms from the inside out, rather than simply adding an independent source of toxicity to the regimen. The animal models showed a significant reduction in overall tumor growth, with no major weight loss or other obvious signs of systemic toxicity observed during the experiments.[5]

Despite the highly promising preclinical data, CS18 remains strictly an experimental candidate for further pharmacological development, not an available treatment for current patients. The Baylor research team, operating within the Dan L Duncan Comprehensive Cancer Center, must now establish the compound's precise pharmacokinetics, optimal dosing parameters, and long-term safety profile in much more complex biological models before any human testing can be authorized by regulatory agencies. The path from laboratory efficacy to an approved oncological drug requires years of rigorous validation.[4][5]

The transition from laboratory success to human clinical trials is notoriously difficult, particularly for drugs that disrupt foundational cellular processes like DNA repair and replication stress. Researchers must ensure that the targeted blockade of TopBP1 does not inadvertently trigger catastrophic damage in rapidly dividing healthy tissues, such as the bone marrow or the gastrointestinal lining, which rely on similar baseline mechanisms to maintain normal physiological function and daily cellular turnover. Balancing this therapeutic window is the primary hurdle for the next phase of research.[2][4]

If future clinical studies ultimately confirm its safety and efficacy, CS18 could represent a structural shift in the practice of precision oncology. Rather than constantly chasing individual mutations as tumors evolve and adapt, oncologists could deploy combination therapies that preemptively disable the biological switchboards tumors use to survive. This approach has the potential to extend the lifespan of existing drugs indefinitely, offering durable, long-term responses for patients facing refractory disease who currently have no other options. The next verifiable checkpoint will be the initiation of Phase I safety trials, which will determine if this mechanism can be safely tolerated by the human body.[1][6]