CHK yourself, before you wreck yourself: targeting the DNA damage response in secondary pulmonary hypertension

Abstract

Despite the debilitating and lethal nature of pulmonary hypertension (PH) secondary to interstitial lung disease (group 3 PH), research into the treatment of this unique category of vasculopathy remains in a relatively nascent stage compared with that of pulmonary arterial hypertension (PAH, group 1). A clue to uncovering the underlying pathophysiology of this particular subtype of group 3 PH, however, may be found in overlapping interest in the DNA damage response (DDR) shared by both pulmonary fibrosis and PH researchers. Primarily a pathway dedicated to preventing DNA damage to cellular progeny, the DDR functions via a complex network of cell cycle checkpoint signalling and DNA repair mechanisms. In the context of malignancy, these nonredundant pathways can be exploited by dependent cancerous cells to thwart apoptosis, thus promoting survival and eventual metastasis. Therapeutically, a key target of the DDR is the family of checkpoint kinases: CHK1 primarily affecting the prevention of DNA damage being replicated (resulting in G1/Sphase cell cycle arrest), and CHK2 mediating transmission to the next generation of cells (a G2/Mphase checkpoint). While there are examples of CHK regulation related to endorgan fibrosis, to date, there is a dearth of literature describing the role of CHK in lung fibrosis. The same was true applying this particular segment of the DDR to PH study until recently, when inhibition of CHK1 was demonstrated to be a druggable target in models of PAH. Armed with this knowledge, Wu et al have posed a novel and interesting followup question to their previous prior work: is there a role for CHK inhibition in pulmonary hypertension secondary to fibrotic lung disease? In an elegant set of experiments, the group initially sought to define the relevance of CHK1/2 expression in idiopathic pulmonary fibrosis (IPF) lung samples, with and without PH, noting a significant increase in the enzyme signalling pathway irrespective of underlying pulmonary vascular disease. On demonstrating similar changes in the lungs of rodents administered intratracheal chemotherapeutic agent bleomycin—to induce inflammatory fibrosis—they could also phenocopy relevant changes in the DDR via a novel rat ‘twohit’ model. Herein, they again administered bleomycin, now followed by monocrotaline injection (a plantbased alkaloid capable of inducing PH) at day 14, with a resulting increase in both fibrotic changes—assessed by the semiquantitative modifiedAshcroft score—as well as an increase in pulmonary vessel medial wall thickness, a sine qua non finding associated with elevated pulmonary pressures. To probe the underlying mechanism in more detail, the investigators next treated with either of two CHK1/2 inhibitors (MK-98776 favouring CHK1 inhibition over CHK2, or LY2606368/prexasertib; both undergoing investigation in ongoing clinical oncology trials) in cultured lung fibroblasts or pulmonary artery smooth muscle cells. They showed that both drugs could attenuate apoptotic resistance of IPFderived cells, with inhibition of the promyofibroblast phenotype, including downregulation of fibronectin, connective tissue growth factor and matrix metallopeptidase 2. Intriguingly, they went on to study combined prexasertib with the approved antifibrotic agent nintedanib in vitro, showing a potentially synergistic effect through an incremental decrease in the described myofibroblast response compared with either agent alone. Finally, they went on to treat their rodent models with this CHK1/2 dualinhibitor, demonstrating a small— yet significant—decrease in right ventricular systolic pressure, with a corresponding improvement in cardiac function in those animals administered drug. Thus, these data support the promising role of selective checkpoint kinase inhibition in the treatment of group 3 PH. Given the group’s exciting report, many questions can now be raised, including which combination of drugs should be used in patients with PH associated with interstitial lung disease and whether they are compatible with currently prescribed antifibrotic agents? Additionally, how long should patients be treated with checkpoint kinase inhibitors, and when to initiate therapy? Is upregulation of the DDRpathway in circulating cells enough to serve as a surrogate biomarker for screening IPF patients with a predisposition to PH, similar to BRCA1/2 testing— for example—in highrisk oncology patients? However, the excitement must be tempered as another consideration because, unfortunately, unmitigated DNA damage and activation of the cytosolic DNA signalling pathway have been linked to worse outcomes in diseases that predispose to both pulmonary fibrosis and PH systemic sclerosis. However, evidence would suggest at least a potentially beneficial effect to CHK2inhibition on DNA sensing linked to upregulation of interferonsignalling and improvement in preclinical oncological outcomes. Thus, in our opinion, equipoise is allotted for future study in patients. Enthusiasm should therefore not be completely tempered, as the pathways in the DDR open up a large number of drug classes, with a variety of distinct mechanisms of action, to the treatment of PH, including CHK1, CHK2 and even immune checkpoint inhibition. However, in light of potential offtarget effects, a final question is which specific cell type contributes to protection in the setting of systemic CHK1/2 inhibitor treatment, as described in the accompanying manuscript? For example, antagonism of another related checkpoint kinase WEE1—known to affect G2/Mphase cell cycle arrest— has been hypothesised to have antiangiogenic properties in endothelial cells, particularly. To this end, it is unsurprising that synergistic effects have been described using a CHK inhibitor in the setting of acquired poly (ADPribose) polymeraseinhibitor resistance, as it targets a different and complementary portion of the DDRreliant cell’s Medicine, University of Florida College of Medicine, Gainesville, Florida, USA Medicine, Stanford University, Stanford, California, USA