Research & Development
Acute Respiratory Distress Syndrome (ARDS)
Acute respiratory distress syndrome (ARDS), also known as acute lung injury, is a massive reaction of the lung in response to damaging factors associated with a rapid onset of widespread inflammation. Causes can be respiratory viral infections (e.g. SARS-CoV2-induced COVID-19), respiratory bacterial infections, trauma, and aspiration. The underlying mechanism includes an activation of the immune system and a diffuse damage to cells that form the blood-air barrier – the barrier between the pulmonary microvasculature and the microscopic air sacs of the lungs.
In fact, ARDS affects the ability of the lungs to exchange oxygen and carbon dioxide. ARDS affects more than 3 million people worldwide every year. The syndrome is associated with a mortality rate between 35 and 50%.
Pathophysiology of ARDS
Impairment of O2/CO2 gas exchange
The blood–air barrier (or alveolar–capillary barrier) exists at the very end of the respiratory tree – the gas exchanging region of the lungs. The barrier prevents the formation of air bubbles in the blood, and from blood entering the alveoli. It is formed by the type-1 alveolar epithelial cells of the alveolar wall, the endothelial cells of the capillaries, the basement membrane between the two cell types and the space between the two membranes called perimicrovascular interstitium. The barrier is permeable to molecular oxygen, carbon dioxide, carbon monoxide and many other gases (Figure 2, left panel).
Increased capillary permeability is one hallmark of acute respiratory distress syndrome (ARDS)
The pathophysiology of ARDS involves fluid accumulation in the alveoli of the lungs caused by vascular inflammation (Figure 2, right panel). In case of ARDS this edema formation is typically provoked by inflammation of the lung tissue which is often caused by an excessive response to pathogens or trauma. The insult to the tissue usually results in an initial release of biochemical signals and other inflammatory mediators which are secreted by local epithelial and endothelial cells. These inflammatory mediators are recruiting immune cells but are also leading to a disintegration of the endothelial barrier by disruption of endothelial cell-cell contacts. Subsequently, this allows immune cell infiltration through the perivascular interstitium to infected alveolar cells. As a secondary event, referred to as vascular leakage or permeability, protein-rich fluids from vascular capillaries enter the perimicrovascular interstitium and the alveolar space in the alveolus. The flooding of the lungs is impairing the exchange of gases such as oxygen and carbon dioxide with capillaries in the lung.
Biological regulation of endothelial barrier stabilization
Role of Angiopoietin-Tie2 signaling pathway
The endothelial Tie2 growth factor receptor pathway regulates vascular permeability and pathological vascular remodeling. Angiopoietin 1 (Ang1) and Angiopoietin 2 (Ang2) are both multimeric ligands for the receptor tyrosine kinase Tie2.
In healthy conditions, Ang1 is constantly secreted by microvascular pericytes and causes an inactive, dormant endothelium through strong Tie2 activation in microvascular endothelial cells (Figure 3, left panel). Ang2 in contrast acts as a competitive antagonist for Tie2 and is usually stored in endothelial cell vesicles called Weibel-Palade bodies. During vascular inflammation, endothelial cells release Ang2 upon stimulation by inflammatory signals resulting in a strong increase of Ang-2 concentration in the blood. Inflammation induced Ang2 binding to Tie2 inhibits Tie2 signalling causing an increase in vascular permeability and edema formation (Figure 3, right panel).
Due to the predisposition of Ang1 to undergo strong multimerization, recombinant Ang1 is very difficult to produce in mammalian cells due to low solubility and aggregation.
Activation of Tie2 signaling by PAN004
PAN004, Panthernas first development program in ARDS, is focusing on the mRNA directed expression of a hyperactive angiopoetin 1 derivative (Ang1*) to prevent vascular leakage and to restore endothelial barrier function in the inflamed pulmonary vasculature.
The proprietary therapeutic mRNA (PTXmRNA) of PAN004 is packaged in an optimized cationic lipid nanoparticle (cLNP) formulation. Due to the positive charge of the PTXcLNPs, PAN004 specifically targets pulmonary microvascular endothelial cells potentially by interaction with negatively charged glycocalyx on endothelial cell surface. After endocytosis of PAN004, Ang1* PTXmRNA is released from endosomes and translated into Ang1* protein by cellular ribosomes. Hyperactive Ang1* protein is secreted as a multimer to bind and activate Tie2 receptors. This synthetic PTXmRNA-directed Ang1* expression leads to Tie2 activation of PAN004-transfected endothelial cells (autocrine Tie2 activation) and to Tie2 activation in neighboring, non-transfected endothelial cells (paracrine Tie2 activation).
Ultimately, this highly localized Tie2 pathway activation in lung regions with elevated immune cell infiltration and tissue damage will restore vascular barrier function. This Tie2 dependent improvement of pulmonary tissue integrity should accelerate recovery from inflammation-induced tissue damage in ARDS patients.