PKR inhibitor imoxin fails to ease diaphragm disease in male mdx mice

Duchenne muscular dystrophy (DMD) remains one of the most devastating genetic diagnoses in medicine, a fatal X-linked disorder caused by the absence of functional dystrophin that progressively strips skeletal muscle of its strength and, ultimately, its ability to sustain life. For decades, researchers have chased the molecular cascades that fan the flames of degeneration in dystrophin-deficient muscle, and one candidate has recently drawn intense scrutiny: protein kinase R, or PKR, a central player in the cell’s integrated stress response. A new study published in Physiological Reports tested whether long-term pharmacological inhibition of PKR could slow disease in the diaphragm of mdx mice, the standard animal model of DMD, and the results deliver a sobering but instructive answer. Twenty-four weeks of treatment with the small-molecule PKR inhibitor imoxin failed to reduce PKR phosphorylation, failed to preserve muscle function, and failed to blunt the inflammatory and endoplasmic reticulum stress signaling that characterizes dystrophic respiratory muscle in male mice.

The rationale for targeting PKR was compelling. PKR, encoded by the EIF2AK2 gene, is canonically activated by double-stranded RNA during viral infection, but it can also be switched on by inflammatory signaling, oxidative stress, and ER stress, all of which are hallmarks of dystrophic muscle. Once active, PKR phosphorylates the alpha subunit of eukaryotic initiation factor 2 (eIF2α) at serine 51, shutting down global protein synthesis, and it simultaneously amplifies inflammatory pathways through NFκB, MAPK, and the NLRP3 inflammasome. Prior work from the same research group had shown that PKR expression and activation are elevated in dystrophic skeletal muscle, and separate studies demonstrated that imoxin could alleviate ER stress in cultured muscle cells and suppress lipopolysaccharide-induced inflammation and atrophy gene expression in rodent muscle. The hypothesis was straightforward: if PKR is a driver of pathology, blocking it chronically should preserve muscle function and dampen disease progression.

To test this, researchers at Iowa State University and the University of Florida treated male C57BL/10 control mice and mdx mice beginning at eight weeks of age, injecting imoxin subcutaneously at 0.5 mg/kg five days per week for approximately 24 weeks, with vehicle-treated animals serving as controls. The dose and delivery route mirrored protocols that had successfully suppressed PKR activity in models of obesity, neuroinflammation, hypertension, and acute lung injury. At roughly 28 weeks of age, mice underwent in vitro functional testing of the costal diaphragm, the respiratory muscle that most faithfully reproduces the degenerative course of human DMD. Investigators measured isometric tetanic specific force, half-relaxation time, time to peak tension, and both dynamic and elastic passive forces across a range of stretches, providing a comprehensive mechanical portrait of each muscle before tissue was apportioned for histology and biochemical analysis.

The first surprise came at the molecular level. Western blot analysis revealed that dystrophin deficiency drove dramatic increases in total PKR and phosphorylated PKR at Thr446, along with a constellation of PKR regulatory proteins including PACT, phosphorylated PACT, PP1α, TRBP, and both total and phosphorylated eIF2α. Compared with vehicle-treated controls, mdx mice showed 180 to 300 percent higher levels of PKR and pPKR and 112 to 409 percent higher levels of eIF2α and peIF2α. Yet imoxin treatment changed none of these measures. Phosphorylated PKR remained just as elevated in drug-treated mdx mice as in their vehicle-treated counterparts, meaning the dosing strategy failed to achieve its primary biochemical objective of inhibiting PKR activation in the dystrophic diaphragm.

Functional testing told a similarly disappointing story. Specific force production was reduced by roughly 44 to 46 percent in mdx diaphragms regardless of treatment, half-relaxation time was prolonged by disease, and both dynamic and elastic passive forces, indicators of muscle stiffness, were significantly elevated in dystrophic muscle. Imoxin did not improve specific tension, relaxation kinetics, or time to peak tension in either healthy or dystrophic animals. The only flicker of benefit appeared in the histological data: trichrome staining showed that fibrosis occupied a dramatically larger fraction of mdx diaphragm than control tissue, a 477 to 564 percent increase, but imoxin reduced that fibrotic area by 18 percent in mdx mice. Intriguingly, this histological improvement was not accompanied by any reduction in the protein abundance of fibrosis drivers such as fibronectin, TGF-β1, or collagens I and III, all of which remained strongly elevated in drug-treated dystrophic muscle.

The broader signaling landscape was equally resistant to the drug. Endoplasmic reticulum stress markers, including BiP, the phosphorylated-to-total PERK ratio, ATF4, and spliced XBP1, were elevated 79 to 438 percent in mdx muscle, and imoxin left them untouched. Inflammatory signaling proteins told the same story: TLR4, MyD88, IKKα, total and phosphorylated NFκB, TNFα, pro- and cleaved IL-1β, and NLRP3 were all substantially increased by dystrophin deficiency, with TNFα rising nearly fourfold, and none of these measures differed between vehicle- and imoxin-treated mdx groups. The drug that had quieted inflammatory cascades in cell culture and in other disease models appeared powerless against the chronic, self-sustaining inflammatory milieu of the dystrophin-deficient diaphragm.

Why did imoxin fail where it had previously succeeded? The authors offer several technical and biological explanations. Pharmacologically, imoxin blocks the autophosphorylation of inactive PKR but may not strip phosphate groups from already-activated enzyme, and prior successes often involved acute inflammatory insults rather than lifelong, genetically driven stress. In cancer cachexia models, a fivefold higher dose of 5 mg/kg prevented muscle loss while 1 mg/kg did not, suggesting a therapeutic threshold that the 0.5 mg/kg regimen used here may not have reached. More fundamentally, the dystrophic intracellular environment appears poised to sustain PKR activation through multiple redundant inputs: chronic inflammation activates PACT, an upstream PKR kinase, while ER stress and oxidative stress provide additional activation routes that persist even without double-stranded RNA. The researchers observed that even endogenous inhibitors of PKR phosphorylation, TRBP and PP1α, were elevated alongside the activating signals, indicating that dystrophic muscle is differentially regulating the kinase but that the net balance favors activation, a situation the authors liken to the difficulty of pharmacologically inhibiting calpain in dystrophic muscle.

The study’s implications cut both ways. On one hand, it is a negative result: a widely used PKR inhibitor, delivered chronically at a dose validated in other contexts, does not attenuate disease severity in the dystrophic diaphragm of male mice. On the other hand, the consistent, coordinated upregulation of PKR alongside ER stress and inflammatory markers reinforces the kinase’s status as a potential central regulatory node in DMD pathology. The authors suggest that future strategies might bypass autophosphorylation altogether, instead preventing activators from engaging PKR or enhancing its endogenous inhibitors, approaches that could prove more effective against a kinase embedded in such a densely interconnected stress-signaling web. Whether higher imoxin doses, alternative inhibitors, or combination therapies could succeed remains an open question, as does whether female mdx mice, which experience a milder phenotype, would respond differently.

For the DMD research community, the work is a reminder that respiratory muscle, the ultimate arbiter of patient survival, presents a uniquely hostile pharmacological environment. The diaphragm of the mdx mouse endures decades of contraction against a fibrotic, inflamed, calcium-dysregulated substrate, and interventions that succeed in limb muscle or in acute injury models may simply be overwhelmed there. As clinical trials of gene therapies, exon skipping, and anti-inflammatory agents continue to reshape the DMD treatment landscape, this study tempers expectations for stress-kinase inhibitors while sharpening the field’s understanding of why dystrophic muscle is so difficult to quiet. PKR remains a suspect worth pursuing, but catching it, the findings suggest, will require more than blocking a single phosphorylation site.

Subject of Research: PKR inhibition with imoxin and disease progression in the dystrophic diaphragm of mdx mice, a model of Duchenne muscular dystrophy

Article Title: Long‐term imoxin treatment does not attenuate disease severity in dystrophic diaphragm of male mice

Article References: Lee, J. H., Vorwald, M. E., Roths, M., Selsby, J. T., & Valentine, R. J. (2026). Long‐term imoxin treatment does not attenuate disease severity in dystrophic diaphragm of male mice. Physiological Reports, 14(18), Article e71115. https://doi.org/10.14814/phy2.71115

Image Credits: AI Generated

DOI: 10.14814/phy2.71115

Keywords: Duchenne muscular dystrophy, mdx mice, protein kinase R, imoxin, diaphragm, ER stress, inflammation, fibrosis, muscle function, eIF2alpha, NFkB, integrated stress response