Old Drugs, New Tricks: How Repurposing Could Crack Cancer’s Resistance Problem

Cancer’s most stubborn enemy is not the tumor that first appears on a scan, but the one that comes back after treatment has seemingly worked. Drug resistance remains the central obstacle in oncology, blunting the activity of otherwise powerful therapies, narrowing the therapeutic window until patients suffer toxicity without commensurate benefit, and ultimately driving progression and relapse. A new review published in the journal Molecular Cancer argues that the field has been attacking this problem with too blunt an instrument, and that a disciplined, mechanism-first approach to drug repurposing could turn already-approved medicines into precision weapons against the specific biological programs that make tumors resistant.

The review, led by Haowen Zhang and Xinyuan Zhu of West China Hospital, Sichuan University, together with colleagues, begins from a deceptively simple observation: resistance is rarely the product of a single lesion. Instead, it emerges from interacting biological programs that conspire to keep chemotherapy and targeted agents from doing their jobs. The authors organize these programs into four therapeutically actionable domains, and the elegance of the framework lies in how it maps each domain onto vulnerabilities that drugs developed outside oncology were, in many cases, already designed to hit.

The first domain is the most classically understood: the failure of drugs to accumulate inside tumor cells. ATP-binding cassette transporters, a family of molecular pumps that includes P-glycoprotein, multidrug resistance-associated protein 1, and the breast cancer resistance protein, actively expel a wide range of chemotherapeutics from the cell before they can reach lethal concentrations. This efflux machinery is a textbook cause of multidrug resistance, and it has long tempted researchers to find inhibitors. The problem, historically, has been that transporter inhibitors tested in the clinic produced disappointing results and unacceptable interactions with co-administered drugs. The review’s mechanism-first filter asks a harder question of any candidate: not merely whether it blocks a pump in a dish, but whether pump blockade can be confirmed as the operative mechanism in resistant tumors in patients.

The second and third domains move the focus outside and inside the tumor cell itself. Within the tumor microenvironment, niches built from cancer-associated fibroblasts, hypoxic regions, and stromal signaling provide extrinsic protection that shields malignant cells from therapy. Meanwhile, cancer stem cells, the slow-dividing, plastic subpopulation capable of regenerating an entire tumor, survive standard regimens through stress-adaptive metabolic programs, including shifts toward oxidative phosphorylation, altered lipid metabolism, and developmental signaling pathways such as Hedgehog and YAP/TAZ. The fourth domain concerns dormant, quiescent cells, the disseminated tumor cells that sit quietly in distant tissues and seed late recurrences years after the primary tumor has been eradicated. Each of these programs depends on transport, metabolic, stromal, inflammatory, and stress-adaptive pathways, and that dependency is precisely what makes repurposing attractive.

The logic of repurposing is straightforward and, in an era of billion-dollar drug development budgets, financially compelling. Many agents approved for entirely unrelated indications, metabolic inhibitors, anti-inflammatory drugs, antiparasitic and antiviral medications, happen to act on the very pathways that resistant tumors co-opt. Because these compounds already possess human pharmacokinetic data, established formulations, and documented safety profiles, their development timelines can in principle be shortened and early attrition reduced relative to de novo discovery. A molecule that has already been dosed in thousands of patients does not need to start from first-in-human trials. The authors are careful, however, to temper this optimism: mechanism confirmation and prospective clinical validation remain obligatory, and the shortcut applies only to the safety and exposure side of the equation, not to proving that the drug works against resistance in the intended context.

What distinguishes this review from many previous calls for repurposing is its insistence on separating preclinical signals from clinically validated outcomes. The authors apply a single mechanism-first filter to representative candidates in each of the four resistance domains, and they state explicitly where a conclusion rests on one primary study. This is a rare degree of candor in a literature crowded with promising cell-culture results that never survive contact with patients. Candidate selection and mechanistic attribution in resistant tumors, the authors note, remain genuinely difficult: a drug that appears to reverse resistance in a cell line may be acting through an entirely different pathway in a patient, or may fail to reach sufficient concentrations in the relevant tissue compartment at all.

The review also emphasizes that resistance biology is only half of the translational equation. The other half is biomarker-defined patient selection. If ABC transporter expression, metabolic rewiring, or a specific stem-cell signature defines the population most likely to benefit from a repurposed agent, then trials must enroll patients on the basis of those markers rather than tumor type alone. Circulating tumor DNA and minimal residual disease assays, both increasingly routine in modern oncology, offer potential tools for identifying patients harboring dormant cells or emerging resistant clones before clinical relapse, creating a window in which an anti-resistance agent could be deployed preemptively rather than reactively.

To carry these context-restricted mechanistic signals into the clinic, the authors turn to trial architecture. Traditional randomized trials, powered to detect benefit across a broad, unselected population, would dilute precisely the signal that makes a repurposed agent interesting. Instead, the review advocates basket, umbrella, and platform designs. Basket trials test one agent across multiple tumor types united by a shared biomarker or resistance mechanism; umbrella trials assign different agents to subgroups of a single cancer defined by molecular features; platform trials allow multiple interventions to enter and exit a standing protocol as evidence accumulates. These designs, already proven in precision oncology, are well suited to repurposing because the hypothesis being tested is mechanism-specific rather than disease-specific.

The practical implications are considerable. Repurposed agents arrive with known dosing, known drug-interaction profiles, and, in many cases, established routes of administration, which lowers the barrier to combination regimens with existing anticancer therapies. The authors also point toward physiologically based pharmacokinetic modeling as a tool for predicting whether an approved drug can achieve the exposures needed to engage its resistance target in a tumor, before expensive clinical resources are committed. Cellular thermal shift assays and related target-engagement methods offer ways to confirm, in patient-derived systems, that a repurposed compound is actually binding the intended resistance machinery rather than acting through an accidental pathway.

None of this guarantees success, and the review does not pretend otherwise. The history of resistance-modifying agents is littered with compounds that looked transformative in the laboratory and failed in the clinic, often because the resistance mechanism being targeted was not the one driving relapse in the enrolled patients. But by insisting on explicit mechanistic attribution, honest separation of evidence tiers, biomarker-driven enrollment, and trial designs built for context-restricted signals, the framework offers something the field has often lacked: a disciplined path from resistance biology to clinical translation. If the approach holds up under prospective testing, the next breakthrough against treatment-resistant cancer may not come from a new molecule at all, but from an old one, deployed against the right target in the right patient at the right time.

Subject of Research: Mechanism-guided drug repurposing to overcome therapeutic resistance in cancer

Article Title: Mechanism-guided drug repurposing against therapeutically actionable resistance domains in cancer: from resistance biology to clinical translation

Article References: Mechanism-guided drug repurposing against therapeutically actionable resistance domains in cancer: from resistance biology to clinical translation. (n.d.). https://doi.org/10.1186/s12943-026-02815-2

Image Credits: AI Generated

DOI: 10.1186/s12943-026-02815-2

Keywords: drug repurposing, cancer drug resistance, ABC transporters, tumor microenvironment, cancer stem cells, tumor dormancy, biomarker-guided therapy, clinical trial design, precision oncology, translational oncology, mechanism of action, Molecular Cancer