The Integrin-TGFbeta Axis: Inhibition of Integrin Alpha v Beta6 Prevents Radiation-Induced Lung Fibrosis.
Reviewer: Christine Hill, MD
Abramson Cancer Center of the University of Pennsylvania
Last Modified: October 30, 2007
Presenter: S. Cheng Presenter's Affiliation: New York University Medical Center, New York, NY Type of Session: Scientific
Over 175,000 new cases of thoracic malignancies are diagnosed in the United States each year, and over half of them require treatment with radiation at some point.
Treatment of thoracic malignancies is limited by the risk of radiation-induced lung injury, including pneumonitis and fibrosis, which can be fatal in some cases. Radiation-induced lung fibrosis occurs in between 30 and 50% of patients who undergo lung irradiation for conditions such as non-small cell lung cancer, small cell lung cancer, malignant mesothelioma, and lung metastases from other primary cancers.
Understanding of molecular targets and potential therapeutics has potential to reduce the risk of radiation-induced lung injury.
TGFβ is a potent cytokine that acts as a key mediator in radiation-induced lung injury.
TGFβ is upregulated in the period following radiation, in the phases that precede both radiation pneumonitis and radiation fibrosis. It has been demonstrated to trigger inflammatory and pro-fibrotic responses that contribute to the symptoms and development of radiation-induced lung injury.
TGFβ ligand is secreted in an inactive form, mediated by its association with latency-associated protein (LAP). TGFβ is activated through its release from LAP, which may be accomplished through several mechanisms; among these are interactions with other proteins, and exposure to radiation. Once TGFβ is released, it binds to its receptor and activates downstream mediators of inflammation and fibrosis.
The integrin αvβ6 (Itgb6) is a major activator of TGFβ in the lung. It has previously been demonstrated to play a key role in radiation-induced lung fibrosis (Munger J et al, ASTRO, 2005).
Materials and Methods
Itgb6 knockout mice as well as control (C57BL/6) wildtype mice were anesthetized and positioned in a plexiglass tray. Selective blocks were used to shield the remainder of the body outside of the thoracic cavity.
The bilateral lungs were then treated with 14 Gray (Gy) in a single fraction, delivered from a 60Co source.
The mice were subsequently sacrificed 26 weeks after radiation, and the percent lung fibrosis was calculated through observation of the percent fibrotic area of paraffin-embedded, formalin-fixed tissue sections that were stained with Masson trichrome, as well as measurement of hydroxycholine levels signifying the presence of collagen.
In order to assess the effects of inhibition of Itgb6, mice were injected with a monoclonal antibody directed against αvβ6. Injections were initiated at week 15 after radiation and were given weekly at several concentrations (1, 3, 6, 10 mg/ kg/ week).
Additionally, a separate population of mice were injected on a weekly basis with a soluble TGFβ receptor at a single concentration (5 mg/ kg).
IgG and PBS, respectively, were used as control solutions for these injectable proteins.
In the absence of monoclonal antibody/ solubilized receptor, at 27 weeks post-radiation, wildtype control mice developed significant lung fibrosis (17% +/- 3% mean area), while none of the Itgb6 knockout mice developed any evidence of fibrosis.
In wildtype mice injected with monoclonal antibody against αvβ6 in escalating doses, significant reduction in radiation fibrosis was achieved. Among control mice (injected with IgG), 35/42 developed histologially evident fibrosis. In contrast, fibrosis developed in 25/42, 10/42, 8/42, and 9/42 mice injected weekly with 1 mg/ kg antibody, 3 mg/ kg antibody, 6 mg/ kg antibody, and 10 mg/ kg antibody, respectively.
Levels of fibrosis-associated proteins in brochoalveolar lavage were reduced in mice treated with the monoclonal antibody, even at the lowest concentration.
At high antibody doses (10 mg/ kg), increased lymphocytic inflammation was noted to be present; however, this was not true at lower levels.
Mice injected with solubilized TGFβ receptor also had decreased areas of fibrosis, and no increase in lymphocytic inflammation was observed in these mice.
Survival was not impacted by use of either the monoclonal antibody or the solubilized TGFβ receptor.
The authors conclude that Itgb6 knockout mice, who lack αvβ6, are protected against radiation-induced lung fibrosis.
Additionally, inhibition of integrin αvβ6 with a monoclonal antibody beginning 15 weeks after radiation prevents radiation-induced lung fibrosis.
Although prevention of radiation-induced lung fibrosis did not impact survival in the rodent population investigated in this study, the authors point out that the animals were treated with radiation to their entire, bilateral lung fields. They note that future investigations utilizing partial lung radiation would more accurately approximate actual clinical situations.
Lung cancer and other thoracic malignancies are associated with poor prognosis. Thoracic radiation is essential to the treatment of many of these diseases, and radiation dosing is limited by the risk of potentially fatal radiation-induced lung injury.
The development of protective agents to prevent development of radiation-induced lung injury could potentially allow radiation dose escalation that could improve survival rates in patients with lung cancer.
This study demonstrates that radiation-induced lung injury may be preventable in rodents with use of a monoclonal antibody directed against integrin αvβ6.
The translation of this data and agent into the clinical setting could provide exciting addition to clinical care, and further investigation regarding development and use of a humanized antibody is certainly indicated.
Increase in lymphocytic inflammation was observed with use of high antibody concentration, and trials investigating antibody dose and safety would be essential to implementing use of such an antibody in the clinical setting.
Although the development of an agent to be used for patients will involve many steps, including further animal work, translation of the basic science data presented here to development of a humanized antibody, and implementation of phase I, II, and III prospective clinical trials, this data is an exciting addition to the current armory of treatments for lung cancer and other thoracic malignancies. Clinical use of a protective agent such as the one presented here could allow increased radiation dosing for patients with lung malignancies, potentially improving survival while reducing toxicity.
Partially funded by an unrestricted educational grant from Bristol-Myers Squibb.