Dogs May Not Respond to Thalidomide, Genetic Study Warns

Thalidomide is one of the most infamous drugs in modern medicine, a sedative withdrawn from the market in 1961 after causing severe birth defects in thousands of children. Yet in the decades since, the drug has been carefully repurposed for human cancers such as multiple myeloma and for inflammatory skin conditions, and veterinary oncologists have increasingly explored its use in dogs with cancer. A new genetic analysis now throws that practice into serious doubt. By examining the genomes of nearly 2,000 canids together with tumor sequencing data from multiple canine cancers, researchers found that dogs carry the same molecular signature that renders mice largely insensitive to thalidomide, raising the possibility that the drug may be ineffective, or at least behave very differently, in canine patients.

The study, led by Maja Louise Arendt of the University of Copenhagen and Jennifer R. S. Meadows of Uppsala University and SciLifeLab, focused on cereblon, the protein that thalidomide binds. Cereblon was only identified as the drug’s primary target in 2010, when researchers showed that thalidomide attaches to this substrate receptor of the cullin-4 E3 ubiquitin ligase complex, known as CUL4-RBX1-DDB1. When thalidomide occupies cereblon, the degradation machinery’s specificity changes: it begins recruiting, ubiquitinating and destroying an unnatural set of proteins called neo-substrates, including transcription factors such as SALL4, p63, Ikaros and Aiolos. These proteins govern critical developmental and immunological processes, which helps explain both the drug’s teratogenic tragedy and its therapeutic benefits. At the same time, native substrates such as MEIS2 are no longer degraded, adding further biological consequences. To date, cereblon remains the only described mammalian target of thalidomide.

The crux of the new findings lies in a single amino acid position within the cereblon protein. The C-terminal region of human cereblon, spanning roughly amino acids 318 to 426, forms the thalidomide-binding CULT domain, and variation within this 109-amino-acid stretch is known to determine how different species respond to the drug. In humans, the amino acid at position 388 is a valine, abbreviated V388. In mice, the equivalent position holds an isoleucine instead. This seemingly minor substitution does not prevent thalidomide from binding to cereblon, but it creates steric hindrance that blocks neo-substrates from entering the binding pocket, preventing their ubiquitination and degradation. The result is that thalidomide and its derivatives are pharmacologically ineffective in mice, a fact that contributed to the drug’s original licensing as a supposedly safe sedative, since rodent tests failed to predict its devastating effects in humans.

When the researchers mined the Dog10K consortium datasets, which include single nucleotide and structural variant calls from 1,929 and 1,879 canids respectively, they found that every single dog examined encodes an isoleucine at the position equivalent to human V388. Across the roughly 30-kilobase CRBN gene region, 384 variable sites were detected, but only three had any potential to alter the protein’s coding sequence, and each of these was extraordinarily rare, with allele frequencies below 0.2 percent. Two structural deletions overlapping the gene were also identified, both at frequencies below 0.5 percent. In other words, the canine cereblon protein is essentially uniform across the species, and it uniformly carries the mouse-like isoleucine that has been shown to abolish thalidomide’s neo-substrate degradation activity.

To place this finding in evolutionary context, the team turned to two massive comparative genomics resources: the Zoonomia alignment of 240 mammalian species and the TOGA alignment covering 344 eutherian mammals. At the nucleotide level, the codon encoding position 388 showed little evolutionary constraint, with phyloP conservation scores of 0.40, 4.66 and -1.74 across its three bases; only the second position showed purifying selection. This wobbling tolerance is mirrored by amino acid variability across the mammalian tree. Among 344 species, 72 percent encode an isoleucine at the equivalent position, including all 55 available species of the order Carnivora, to which dogs belong. Only bats, rodents and rabbits, and the order Primates, which includes humans, were found to encode both valine and isoleucine at this site. Notably, rabbits and guinea pigs carry a valine, consistent with decades of toxicology data showing that rabbits, unlike mice, do develop thalidomide-induced birth defects similar to those seen in humans.

The researchers also considered whether cancer itself might change the equation. Tumors accumulate mutations that can alter drug sensitivity, so the team searched for non-silent, protein-altering mutations in the CRBN gene within two published canine tumor-normal sequencing datasets, comprising 55 mammary tumors and 43 diffuse large B-cell lymphoma cases, as well as 723 samples from the cBioportal Canine Cancer Genome Atlas. The search yielded almost nothing: a single non-synonymous mutation, p.CRBN A10T, was found in one lymphoma sample, affecting the N-terminal region far from the thalidomide-binding CULT domain. No mutations were identified that would plausibly make canine cancer cells more susceptible to thalidomide than normal cells. In human multiple myeloma, somatic CRBN mutations are likewise reported to be neutral or to confer resistance, never enhanced sensitivity.

These genetic findings cast a revealing light on the veterinary literature. Thalidomide has been tested in dogs with splenic hemangiosarcoma, mammary carcinoma, lung carcinoma, lymphoma and multiple myeloma, usually in combination with other therapies, making efficacy difficult to assess. One recent multicentre retrospective study reported that 71 percent of seven dogs with relapsed multiple myeloma maintained or achieved complete remission on thalidomide as a single-agent rescue therapy, with moderate lethargy in two dogs as the only recorded side effect. Strikingly, a 53-week toxicology study in beagles using doses up to 1,000 milligrams per kilogram daily, roughly fifty times the human clinical dose for a ten-kilogram dog, found no clinically significant adverse effects, no hematological or biochemical changes and no evidence of the peripheral neuropathy that plagues human patients. The authors argue that this remarkable tolerance is itself a warning sign: a drug with no side effects at massive doses may simply have no pharmacological activity.

There are caveats. The sedative effect of thalidomide appears to be independent of cereblon, and one canine study reported reduced vascular endothelial growth factor staining in metastatic tissue from treated dogs, hinting that anti-angiogenic mechanisms independent of the CRBN-CUL4-RBX1-DDB1 complex might operate in dogs. A pharmacokinetic study in tumor-bearing dogs also reported a range of clinical signs, though the authors of the new analysis note these could reflect advanced cancer rather than drug toxicity. The researchers are careful not to claim that thalidomide is definitively useless in dogs; rather, they conclude that the molecular function of the drug in this species must be directly investigated before its therapeutic role in canine cancer can be understood or justified.

Beyond its immediate veterinary implications, the study is a compelling demonstration of how publicly available genomic data can inform clinical pharmacology. Because DNA-damaging chemotherapy agents tend to work comparably across humans and dogs, veterinary oncologists have reasonably borrowed human anticancer drugs. But for targeted agents such as thalidomide, whose action depends on the precise structure of a single protein, species differences in that target can silently nullify the drug’s effect. Given thalidomide’s tragic history and its strict regulation in human medicine, the authors advocate that molecular evidence of function should be established before the drug is used therapeutically in dogs, a lesson that applies equally to the growing arsenal of cereblon-targeting protein degraders now entering human clinical development.

Subject of Research: Genetic variation in the canine cereblon protein and its implications for thalidomide therapy in dogs with cancer

Article Title: Genetic evidence supports that thalidomide should not be used therapeutically in dogs

Article References: Arendt, M. L., & Meadows, J. R. S. (2026). Genetic evidence supports that thalidomide should not be used therapeutically in dogs. Veterinary Oncology, 3(1), Article 14. https://doi.org/10.1186/s44356-026-00067-2

Image Credits: AI Generated

DOI: 10.1186/s44356-026-00067-2

Keywords: thalidomide, cereblon, CRBN, dogs, canine cancer, veterinary oncology, comparative genomics, neo-substrates, ubiquitin ligase, drug repurposing, teratogenicity, Dog10K