A paper in Nature Cancer  details exciting research about a new drug that could be the first to prevent and treat fibrosis. The drug, called PXS-5505, inhibits crosslinking and stabilisation of scar tissue (the collagen matrix) produced by myofibroblasts, reducing tissue stiffness. Myofibroblasts are key players in fibrosis, and these cells also occur around most tumours, so this drug could treat both conditions. In fibrosis pathology, myofibroblasts produce the excessive amounts of collagen that contracts and deforms tissues. In arthrofibrosis, this results in painful joints that no longer have a normal range of motion, preventing normal functioning of the limb. By inhibiting collagen crosslinking, PXS-5505 inhibits the powerful mechanical feedback effects that maintain myofibroblast gene activity . More clinical trials are needed before PXS-5505 becomes widely available, however, this if new approach proves to be an effective fibrosis therapy it will be the first non-invasive method to treat both established and recently diagnosed fibrosis, including arthrofibrosis.
Arthrofibrosis is a relatively common side effect of injury and surgery. Arthrofibrosis means fibrosis (the pathological production of scar tissue) in a joint, and the biology of arthrofibrosis is common to all fibrotic diseases . Fibrosis pathology begins when cells called fibroblasts are transformed by an inflammatory stimulus, such as a wound, and become activated myofibroblasts (see below). Myofibroblasts are highly active and produce large amounts of scar tissue. There are currently no drugs that will stop or reverse arthrofibrosis, and surgery is the only approach that sometimes helps. However, surgery can also worsen symptoms by re-stimulating the wound healing response. A medication to stop the pathological processes of arthrofibrosis is badly needed.
To understand why PXS-5505 could be an exciting development for people with arthrofibrosis, we need to understand some cell biology, because the success of any therapy ultimately depends on how it impacts our biology. In healthy tissues, cells called fibroblasts are surrounded by, and attached to, a soft collagen matrix that provides the mechanical support, strength and structure of tissues . The fibroblasts produce and maintain the collagen matrix they’re embedded in . Fibroblasts to adhere to their collagen scaffold using specialised protein complexes on the cell surface called Focal Adhesion Complexes.
The physical forces created by fibroblasts pulling on collagen they are adhered to regulates the amounts collagen and collagen degrading enzymes made by fibroblasts. In this way the cells sense and react to their environment in a process called mechanoregulation.
The physical forces created by fibroblasts pulling on collagen they are adhered to regulates the amounts collagen and collagen degrading enzymes made by fibroblasts. In this way the cells sense and react to their environment in a process called mechanoregulation . The physical properties of the collagen, such as stiffness, regulate what fibroblasts do [2,4], creating a complex, self-regulated sensory network of cells and collagen.
Under normal conditions fibroblasts maintain a healthy balance between the amounts collagen and collagen degrading enzymes they produce. However, when there is an insult like surgery, acute injury signals are generated  and the picture changes dramatically. Almost instantly, the immune system leaps into action, producing signalling molecules like TGF-β and TNF-α that transform fibroblasts into myofibroblasts . Myofibroblasts are responsible for the task of pulling the edges of the wound together and repairing it. To perform this amazing feat, myofibroblasts form internal stress fibres containing the contractile protein, α smooth muscle actin. These fibres terminate at the surface Focal Adhesion Complexes (see figure 1) where the collagen matrix adheres. As the stress fibres inside the cells contract, considerable tensile forces are generated on the collagen matrix and surrounding tissues, creating tension that contracts tissues and seals the wound.
These physical stress forces play pivotal roles in tissue injury and repair . With healthy healing collagen scar tissue is produced to heal the wound and then largely broken down over time. This reduces the tension on the myofibroblasts and causes them to revert back into fibroblasts (normal tissue) or disappear.
Things start to go wrong when the feedback effects from contracting myofibroblasts pulling on the collagen matrix causes them to make more collagen  (see Figure 1). However, it’s not only the excessive amounts of collagen that creates fibrosis pathology, the quality of it is central to disease progression . As the collagen matrix matures, crosslinking between and within the collagen fibres stiffens the matrix and stabilises it, making it almost impossible for the body’s enzymes to degrade . The increased stiffness generates more mechanical tension that maintains myofibroblast activity. At this point, the myofibroblasts become effectively immortal and resist apoptosis (programmed cell death).
A family of enzymes called lysyl oxidases (LOX) are essential for crosslinking collagen fibrils  and stiffening the matrix (see Figure 2 below). LOX activity is increased temporarily after an injury in response to the growth hormone TGF-β, hypoxia and inflammation, and is necessary for normal healing. However, in fibrosis, LOX activity remains high  resulting in stiffened and stabilised scar tissue.
Since crosslinking is fundamental to fibrosis pathology, the LOX enzymes that create collagen crosslinks are potentially a powerful treatment target.
Since crosslinking is fundamental to fibrosis pathology, the LOX enzymes that create collagen crosslinks are potentially a powerful treatment target . PXS-5505 is the first drug to irreversibly inhibit all members of the LOX family, and collagen crosslinking . To date it has demonstrated excellent safety outcomes and target specificity, however, more clinical trials are needed. The IAA is excited to see what the future holds for arthrofibrosis treatment with this drug.
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