Background/Purpose: Bosutinib (SKI-606), a second-generation tyrosine kinase inhibitor that blocks the activity of the BCR-ABL kinase responsible for Philadelphia chromosome positive chronic myeloid leukemias, also inhibits the activity of the Src family of nine non-receptor tyrosine kinases. Activation of Src kinases via phosphorylation of their catalytic regions can be stimulated by PDGF and TGF-ß resulting in strong profibrotic cascade signaling. Furthermore, Src kinases are major regulators of the profibrotic activity of focal adhesion kinase (FAK), which promotes the differentiation of quiescent fibroblasts to activated myofibroblasts. Thus, the purpose of this study was to test the antifibrotic activity of Bosutinib in vivo on TGF-ß induced skin and pulmonary fibrosis employing the well-characterized TBRI^caCol1Cre transgenic mouse model of tissue fibrosis.

Methods: TBRI^caCol1Cre transgenic mice with a tamoxifen-inducible constitutively active TGFß receptor 1 allele in cells expressing Cre recombinase under the control of the Col1a promoter were implanted with subdermal osmotic pumps dispensing either 2.5, 5.0 or 10.0 mg/kd/day of Bosutinib or with pumps dispensing saline as a control for 8 weeks following tamoxifen activation of constitutive TBRI expression. Skin sections and lungs isolated from control and Bosutinib-treated animals (n=6 per group) were analyzed by histopathology following hematoxylin/eosin and Masson’s trichrome staining, measurement of tissue hydroxyproline content, and by evaluation of the expression of genes associated with tissue fibrosis, and myofibroblast differentiation and activation.

Results: Constitutive TGFβ-1 signaling in Col1a-expressing cells induced severe cutaneous and pulmonary tissue fibrosis in untreated mice. In contrast to mice receiving saline-containing pumps, a strong decrease in TGF-ß-induced collagen deposition in the dermis and lungs was observed in response to Bosutinib treatment by histopathological analysis of these tissues. Quantitative assays for collagen content showed that control untreated mice displayed a 1.5 fold increase in hydroxyproline levels in skin and a 1.9 fold increase in the lung following tamoxifen administration. These increases were abrograted in a dose-dependent manner in mice receiving Bosutinib. The marked increase in expression of genes associated with tissue fibrosis and with transdifferentiation and activation of myofibroblasts observed following tamoxifen administration was also abrogated in a dose-dependent fashion in response to Bosutinib.

Conclusion: Cutaneous and pulmonary fibrosis induced in mice carrying an inducible constitutively active TBRI receptor transgene was abrogated in a dose-related manner following administration of the tyrosine kinase inhibitor Bosutinib as assessed by histopathology, measurements of tissue hydroxyproline content, and analysis of expression levels of profibrotic and myofibroblast activation-associated genes. These results demonstrate that the BCR/ABL and Src kinase inhibitor Bosutinib may be a potential therapeutic agent for tissue fibrosis in SSc and other fibroproliferative disorders.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/tgf-s-induced-tissue-fibrosis-in-tbricacol1cre-transgenic-mice-is-abrogated-by-the-second-generation-tyrosine-kinase-inhibitor-ski-606-bosutinib/