D-Valine as a mirror-image antifibrotic agent: Inhibiting fibroblast proliferation and attenuating airway fibrosis

Postintubation tracheal stenosis is a fibroproliferative airway disorder with limited pharmacological treatment options. Excessive fibroblast proliferation and extracellular matrix deposition play a central role in its pathogenesis. D-amino acids such as D-Valine are thought to modulate L-Valine-dependent protein synthesis, thereby suppressing fibroblast activity. This study aimed to evaluate the antifibrotic effects of D-Valine in experimental models.

This study evaluated the antifibrotic effects of D-Valine using both in vitro and in vivo models. Primary human bronchial epithelial cells and fibroblasts were cultured and treated with increasing concentrations of D-Valine (120 mM), and cell viability was assessed with the MTT assay (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide). For the in vivo model, tracheal stenosis was surgically induced in 20 New Zealand rabbits, randomized into four groups: Control, physiological saline (PS), dexamethasone, and D-Valine. Submucosal thickness, fibrosis, and inflammatory infiltration were analyzed histopathologically.

In vitro assays showed a concentration-related decrease in fibroblast viability, as assessed by the MTT assay; however, this trend did not reach statistical significance (mean absorbance values ranged from 0.84±0.31 to 0.97±0.44 compared with 0.95±0.31 in the PS group; p=0.326). Epithelial cell viability also declined slightly at higher doses (mean 0.85±0.36 vs. 0.90±0.72, p=0.214). A moderate positive correlation was observed between epithelial and connective tissue viability at 20 mM D-Valine (r=0.494, p=0.002, n=6). In vivo, submucosal thickness was highest in the saline group (1.08±0.44 mm) and lowest in the control group (0.28±0.08 mm; p=0.011). The dexamethasone and D-Valine groups showed reduced submucosal thickening (0.98±0.13 mm and 0.86±0.24 mm, respectively), though differences versus saline were not statistically significant (p=0.989 and p=0.534). The fibrosis rate was greatest in the saline group (74±15.17%) and lowest in controls (37±12.55%, p=0.001). D-Valine (56±11.4%) and dexamethasone (56.25±4.79%) groups exhibited markedly lower fibrosis than saline (p=0.057 and p=0.056, respectively).

This study provides experimental evidence that D-Valine suppresses fibroblast proliferation and attenuates fibrosis in a New Zealand White rabbit model of tracheal stenosis. The antifibrotic effects observed may be related to interference with valine-dependent protein synthesis. However, considering the concurrent effects on epithelial cells and the inherent limitations of experimental models, further mechanistic and translational studies are required before potential clinical implications can be considered. These findings suggest that D-Valine represents a promising experimental antifibrotic candidate for airway fibrosis research.