Nip it in the bud: potential for the early treatment of osteoarthritis

Abstract

Osteoarthritis (OA) is considered a complex multifactorial disease of weight-bearing joints, with extensive psychosocial impact. The etiology of primary OA remains unknown and the molecular and cellular mechanisms of OA initiation and progression are not fully elucidated. Investigation of post-traumatic OA has shed some light on the early molecular and cellular events that orchestrate changes in the knee because the nature and the timing of the initial injury leading to OA are often known1Rai M.F. Brophy R.H. Sandell L.J. Osteoarthritis following meniscus and ligament injury: insights from translational studies and animal models.Curr Opin Rheumatol. 2019; 31: 70-79PubMed Google Scholar,2Neuman P. Englund M. Kostogiannis I. Friden T. Roos H. Dahlberg L.E. Prevalence of tibiofemoral osteoarthritis 15 years after nonoperative treatment of anterior cruciate ligament injury: a prospective cohort study.Am J Sports Med. 2008; 36: 1717-1725Crossref PubMed Scopus (283) Google Scholar. Studies on meniscus and ligament injuries, which constitute a majority of cases of post-traumatic OA3Englund M. Roos E.M. Lohmander L.S. Impact of type of meniscal tear on radiographic and symptomatic knee osteoarthritis: a sixteen-year followup of meniscectomy with matched controls.Arthritis Rheum. 2003; 48: 2178-2187Crossref PubMed Scopus (427) Google Scholar,4Roos H. Adalberth T. Dahlberg L. Lohmander L.S. Osteoarthritis of the knee after injury to the anterior cruciate ligament or meniscus: the influence of time and age.Osteoarthritis Cartilage. 1995; 3: 261-267Abstract Full Text PDF PubMed Scopus (430) Google Scholar, demonstrate that acute molecular alterations in the joint take place decades before the structural whole-joint changes become detectable by conventional radiographs5Favero M. Ramonda R. Goldring M.B. Goldring S.R. Punzi L. Early knee osteoarthritis.RMD Open. 2015; 1e000062Crossref PubMed Scopus (67) Google Scholar. Molecular and cellular mechanisms including, metabolic processes, inflammatory pathways, and epigenetic modifications6Shen J. Abu-Amer Y. O'Keefe R.J. McAlinden A. Inflammation and epigenetic regulation in osteoarthritis.Connect Tissue Res. 2017; 58: 49-63Crossref PubMed Scopus (82) Google Scholar,7Arra M. Swarnkar G. Ke K. Otero J.E. Ying J. Duan X. et al.LDHA-mediated ROS generation in chondrocytes is a potential therapeutic target for osteoarthritis.Nat Commun. 2020; 11: 3427Crossref PubMed Scopus (29) Google Scholar perturb joint homeostasis ultimately leading to joint failure. At this stage of the disease, little can be done to curb progression as there are currently no disease-modifying OA drugs available. A prerequisite for the development of early, effective, and targeted therapies for OA is the understanding of disease status at the molecular level immediately after injury. While movement of the identification of OA from the terminal point of joint death to early molecular detection has begun, there is an unmet need to identify and treat OA when the disease is still in the early molecular phase and before structural changes have set in. In the absence of treatment modalities targeting OA at the very onset, the prevalence of OA will continue to grow exponentially, with more joint replacements – often in patients at a younger age. Although, many treatment options have shown striking results in preclinical studies, only a few have been selected for clinical trials, and none have made it to the clinic. Take, for instance, IL-1Ra, which is a competitive inhibitor of IL-1 and has long been recognized as a therapeutic candidate in preclinical studies. It has failed to elicit therapeutic effects in patients with post-traumatic OA in a multicenter, randomized, double-blind, placebo-controlled study (NCT00110916). There are a variety of reasons as to why conventional treatments for early OA have fallen short; a major roadblock is that many of the therapeutic agents target a branch of the upstream signaling cascade or one of the many inflammatory mediators. There are clear knowledge gaps in terms of identification of a common upstream pathway upon which many related inflammatory pathways should converge, as well as in the quest for candidates that elicit a quick therapeutic effect in a more effective, specific and synergistic fashion. In this issue of Osteoarthritis and Cartilage, Fukui et al.8Fukui T. Yik J.H.N. Doyran B. Davis J. Haudenschild A.K. Adamopoulos I.E. et al.Bromodomain-containing protein-4 and cyclin-dependent kinase-9 inhibitors interact synergistically in-vitro and combined treatment reduces post-traumatic osteoarthritis severity in mice.Osteoarthritis Cartilage. 2020; 29: 68-77Google Scholar have reported the therapeutic effects of two compounds, namely Flavopiridol and JQ1, which respectively target the cyclin-dependent kinase 9 (CDK-9) and bromodomain-containing protein 4 (Brd-4) pathways of early inflammatory response gene transactivation. This concept stems from the finding that numerous inflammatory signaling pathways in OA converge on a common rate-limiting step, during the transactivation of primary response genes, which is controlled by the transcriptional regulator CDK-9 and its recruitment factor Brd-4. Under physiological conditions, transcription of primary response genes remains inactive due to the paused RNA polymerase II. However, upon inflammatory stimulation, Brd-4 recruits CDK-9 to the transcriptional complex and phosphorylates the C-terminus of polymerase II, allowing transcriptional activation. With this key role in the transactivation of primary response genes, both Brd-4 and CDK-9 are attractive therapeutic targets. In pioneering studies, Haudenschild's laboratory has shown that Flavopiridol is a potent inhibitor of CDK-9 kinase activity. The authors found that inhibition of CDK-9 in primary chondrocytes not only effectively suppresses the expression of the catabolic genes induced by different inflammatory stimuli, but also reduces the release of the matrix degradation product glycosaminoglycan and cleaved collagen type IIA peptides9Yik J.H. Hu Z. Kumari R. Christiansen B.A. Haudenschild D.R. Cyclin-dependent kinase 9 inhibition protects cartilage from the catabolic effects of proinflammatory cytokines.Arthritis Rheum. 2014; 66: 1537-1546Crossref PubMed Scopus (25) Google Scholar. In a follow-up study, the authors demonstrated that pharmacological inhibition of CDK-9 by Flavopiridol markedly reduces the expression of injury-induced catabolic genes, attenuates cellular apoptosis, and reduces cartilage matrix degradation10Hu Z. Yik J.H. Cissell D.D. Michelier P.V. Athanasiou K.A. Haudenschild D.R. Inhibition of CDK9 prevents mechanical injury-induced inflammation, apoptosis and matrix degradation in cartilage explants.Eur Cell Mater. 2016; 30: 200-209Crossref PubMed Scopus (12) Google Scholar. Hitherto, studies on the inhibition of Brd-4 in chondrocytes are lacking, but it is known that a small molecule – JQ1 – inhibits Brd-4, impairs inflammatory responses in macrophages, and diminishes the ‘cytokine storm’ in endotoxemic mice by reducing the levels of inflammatory cytokines11Belkina A.C. Nikolajczyk B.S. Denis G.V. BET protein function is required for inflammation: Brd2 genetic disruption and BET inhibitor JQ1 impair mouse macrophage inflammatory responses.J Immunol. 2013; 190: 3670-3678Crossref PubMed Scopus (267) Google Scholar. Furthermore, JQ1-mediated Brd-4 inhibition has been shown to attenuate inflammatory responses in microglia and to promote functional recovery subsequent to spinal cord injury in rodents12Wang J. Chen J. Jin H. Lin D. Chen Y. Chen X. et al.BRD4 inhibition attenuates inflammatory response in microglia and facilitates recovery after spinal cord injury in rats.J Cell Mol Med. 2019; 23: 3214-3223Crossref PubMed Scopus (27) Google Scholar. As Brd-4 and CDK-9 work in concert, and since their potent inhibitors are in clinical trials for cancer therapy13Pervaiz M. Mishra P. Gunther S. Bromodomain drug discovery – the past, the present, and the future.Chem Rec. 2018; 18: 1808-1817Crossref PubMed Scopus (54) Google Scholar,14Wiernik P.H. Alvocidib (Flavopiridol) for the treatment of chronic lymphocytic leukemia.Expet Opin Invest Drugs. 2016; 25: 729-734Crossref PubMed Scopus (42) Google Scholar, Fukui et al. have taken a smart approach in systematically examining the synergistic effects of Flavopiridol and JQ1 treatment in inflammatory and post-traumatic OA. Prior to undertaking therapeutic studies, the authors determined optimal drug dosages using statistical ‘design of experiment’ and ‘lack of fit test’ models, and established that, when combined, both drugs confer a synergistic effect at lower doses. The authors then set up an in vitro model of inflammatory arthritis by treating human primary chondrocytes with various proinflammatory stimuli (IL-1β, TNFα, and IL-6 and its receptor) and showed that the synergistic effect of Flavopiridol and JQ1 led to suppression of transactivation of numerous inflammatory response genes during the inflammatory response elicited by the exogenous inflammatory stimuli. Moreover, these drugs protected against catabolic degradation of cartilage explants induced by inflammatory conditions. Next, the authors examined the synergistic treatment effects of the aforementioned inhibitors in a murine model of post-traumatic OA induced by the mechanical rupture of the anterior cruciate ligament (ACL). This model results in cartilage damage, synovitis, and OA, and enables the study of the natural progression of the early events following injury15Rai M.F. Duan X. Quirk J.D. Holguin N. Schmidt E.J. Chinzei N. et al.Post-traumatic osteoarthritis in mice following mechanical injury to the synovial joint.Sci Rep. 2017; 7: 45223Crossref PubMed Scopus (24) Google Scholar,16Christiansen B.A. Anderson M.J. Lee C.A. Williams J.C. Yik J.H. Haudenschild D.R. Musculoskeletal changes following non-invasive knee injury using a novel mouse model of post-traumatic osteoarthritis.Osteoarthritis Cartilage. 2012; 20: 773-782Abstract Full Text Full Text PDF PubMed Scopus (121) Google Scholar. Multiple systemic injections of Brd-4 and CDK-9 inhibitors in mice following ACL rupture suppressed the expression of proinflammatory cytokines, inhibited MMP-13 activity in the joint, mitigated synovitis, reduced mechanical deterioration of cartilage, and ameliorated post-traumatic OA. This is the first study to show synergistic interaction of Brd4 and CDK-9 inhibitors in the suppression of inflammation and post-traumatic OA. In contrast to those on conventional treatment options, this study targets a common upstream signaling pathway but lacks specificity when it comes to the readouts. For instance, without additional mechanistic studies it is not possible to discern whether Flavopiridol and JQ1 inhibit MMP-13 activity exclusively via the CDK-9 pathway or if they also target other pathways that govern the transcription of MMP-13, such as the NFκB pathway. Thus, this study provides several opportunities for further investigations. First, it would be desirable to test whether the combined treatment option applies to other rodent models of OA, such as destabilization of the medial meniscus and meniscal–ligamentous injury models. Second, it remains to be determined whether therapeutic effects are maintained in the long term and hold true in a large animal model of knee injury and OA. Third, it would be highly desirable to ascertain whether the combined therapy exerts beneficial effects on functional outcomes, such as gait, activity, pain, and behavior. Fourth, efforts need to be directed at a more robust approach where frequency of injections could be reduced to increase feasibility prior to undertaking clinical trials. Finally, a ‘window of opportunity’ needs to be defined across a range of injury models and species in order to maximize therapeutic benefits. In summary, this study suggests that targeting mRNA transcription of primary response genes after knee trauma via a ‘nip it in the bud’ approach is a viable therapeutic strategy to mitigate post-traumatic OA. While there is still a long way to go, I expect that with such an appealing approach, we may eventually find the holy grail of treatment for (post-traumatic) OA. I am the sole contributor. None declared.