Pentraxin-2 is a member of the pentraxin family of proteins, which includes C-reactive protein (CRP), pentraxin-2 and pentraxin-3.^1–3 When initially discovered and characterized, pentraxin-2 was known as serum amyloid P (SAP) due to its isolation from amyloid deposits in humans; subsequently, SAP was found to be in the sera from circulating blood and, hence, its designation as SAP.^4–6 Subsequent research revealed multiple biological properties and functions of SAP (Table 1). SAP is produced in the liver by hepatocytes and secreted into the circulating blood.⁶ As SAP is an acute-phase protein in mice, its serum levels can increase by up to 20-fold in response to an inflammatory stimulus; however, SAP is not an acute-phase protein in humans.⁷ SAP and CRP have similar structures, having circular pentamers shaped like a flat disc with a gap in the middle.^1–3,8 As each SAP molecule binds to two Ca²⁺ atoms, the pentamer displays a total of 10 Ca²⁺ atoms on one side of its disc-like structure.^1–3 Ca²⁺ cations facilitate the binding to various moieties, including amyloid deposits, toxins and polysaccharides from bacteria and debris from apoptotic cells.^9–11

Patients with idiopathic pulmonary fibrosis (IPF) progressively and irreversibly lose lung function over time, although there is considerable variability among patients in the pace of lung function decline, with a 5-year survival rate estimated to range from 20 to 40%.¹² The elderly population (especially males) is predominantly affected by IPF, with an incidence ranging from three to nine cases per 100,000 person-years.¹³ The anti-fibrotic drugs, pirfenidone and nintedanib, have been shown to significantly attenuate the rate of decline in lung function over time for patients with IPF, and more recent clinical trial results show the efficacy of these agents for other forms of progressive fibrosing interstitial lung disease, such as that associated with scleroderma, rheumatoid arthritis and other autoimmune disorders.^14–18 However, while decline in lung function may be slowed by anti-fibrotic therapy, not all patients benefit from such anti-fibrotic pharmacotherapies, resulting in declining lung function in these patients. These drugs can have significant side effects, leading some patients to be unable to tolerate them. There remains an urgent need for anti-fibrotic drug therapies that can be both effective and well tolerated by patients.

Numerous early investigations indicated that SAP can modulate wound healing and can have an inhibitory effect on various forms of fibrosis that lead to organ dysfunction. Such findings fostered numerous clinical studies to assess the ability of SAP to inhibit scar tissue formation in animal models and humans. Forms of fibrosis that can cause severe morbidity and lead to death range from cardiac fibrosis, cirrhosis of the liver and end-stage diabetic kidney disease to various forms of pulmonary fibrosis (PF).¹⁹

Preclinical investigations

Studies on the role that SAP may play in wound healing and tissue fibrosis have led to important insights supporting its efficacy as an anti-fibrotic moiety.^20–23 Fibrocytes, which promote scar formation and stimulate resident fibroblasts to produce excessive collagen, are found in high numbers in healing wounds, fibrotic lesions in patients with PF and animal models of pulmonary and other forms of fibrosis.^2,24,25 SAP has been shown to be an endogenous inhibitor of fibrocyte differentiation.²⁶ Additionally, SAP has an inhibitory effect on pro-fibrotic macrophages and has been shown to promote the formation of immune-regulatory macrophages.²⁷ Bleomycin has been found to induce a heightened and persistent inflammatory response accompanied by increased fibrosis in an SAP-knockout mouse model, and exogenous SAP blunted the accumulation of pro-inflammatory macrophages and attenuated PF.^20,22,28

Preclinical studies summarized in Table 2 support the notion that SAP/pentraxin-2 has the ability to suppress fibrotic responses that can be triggered by injury and inflammation.^{20–22,28–33} In a model of bleomycin-induced PF, Pilling et al. found that intraperitoneal injections of SAP reduced leucocyte and fibrocyte accumulation and attenuated PF in both rats and mice.²¹ This line of research was extended by Murray et al. by assessing the effect of SAP on bleomycin-induced PF in mice and measuring SAP levels in human subjects with IPF versus controls.²⁸ They found that SAP significantly diminished tissue fibrosis and collagen deposition in their mouse model, which was associated with a reduction in pro-inflammatory M2 macrophages and a probable increase in M1 regulatory macrophages. Additionally, SAP levels in circulating venous blood from patients with usual interstitial pneumonia (UIP)/IPF were lower than those in controls, and levels of specific plasma proteins were consistent with a skewing of macrophages from M1 to M2 phenotypes. Subsequently, Murray et al. examined SAP levels in human subjects with UIP/IPF and the effect of SAP treatment on fibrocyte accumulation in their bleomycin/PF mouse model in which transforming growth factor (TGF)-β1 could be overexpressed.²² As reported previously, plasma levels of SAP were significantly lower in patients with UIP/IPF versus control subjects and correlated inversely with the degree of forced vital capacity (FVC) impairment. When human monocytes were cultured in vitro, SAP exposure reduced the secretion of M2 macrophage-associated proteins and promoted the emergence of M1 macrophage responses. In the mouse model, SAP attenuated collagen accumulation in a dose-dependent fashion and was associated with decreased fibrocytes in both bronchoalveolar lavage and lung tissue. Further experiments by Pilling et al. using a bleomycin/PF mouse model, in which the SAP gene was knocked out, showed that bleomycin induced a persistent inflammatory response and increased PF in the knockout mice.²⁰ Treatment with exogenous SAP reduced the accumulation of inflammatory macrophages and prevented the fibrotic change in both knockout and wild-type mice exposed to bleomycin.

In addition to preclinical studies of SAP in models of PF, numerous studies have examined the effects of SAP on experimental models of non-pulmonary tissues/organs. Murray et al. examined experimental radiation-induced damage in a hamster cheek pouch model and reported that intraperitoneally administered SAP delayed the onset of oral mucositis, diminished myofibroblast infiltration into tissues, reduced collagen deposition and gene expression and promoted injury resolution.²⁹ Moreira et al. examined the effects of SAP on airway inflammation and remodelling in a murine model that used Aspergillus fumigatus conidia to induce airway sensitivity, performing both in vivo and in vitro experiments to determine the effects of SAP on monocyte/macrophage in Fcγ chain receptor-deficient mice versus wild-type mice.³⁰ They reported that SAP suppressed M2 macrophage activation via an FcγR-dependent mechanism, inhibited an increase in airway resistance induced by methacholine challenge and reduced airway inflammation and remodelling but did not impair the clearance of fungi. Haudek et al. examined the effects of SAP on cardiac fibrosis in a mouse model of ischaemia/reperfusion cardiomyopathy induced by intermittent coronary artery occlusion.³¹ They reported that SAP significantly reduced fibroblast flux into tissue and completely prevented ischaemia/reperfusion-induced fibrosis and ventricular dysfunction. Additional experiments reported by Haudek et al. using the murine cardiomyopathy model in FcγR^–/–-knockout mice showed that SAP provided protection against ischaemia/reperfusion fibrotic injury and cardiac dysfunction in wild-type mice but not in the knockout mice, but SAP had to be present prior to monocyte trans-endothelial migration when applied to an in vitro membrane model.³² Finally, Castaño et al. examined the effect of SAP administration in a mouse model of kidney fibrosis caused by unilateral ureteric obstruction while also measuring SAP serum concentrations in patients with chronic kidney disease.³³ Human SAP (hSAP) given intraperitoneally every 48 h suppressed kidney fibrosis at days 7 and 14, and myofibroblast activation was markedly inhibited. hSAP induced an anti-inflammatory cellular signature in macrophages that infiltrated inflamed tissue, concentrations of hSAP were markedly increased in injured kidneys and predominantly associated with apoptotic/necrotic cells and hSAP was shown to bind to Fcγ receptors. It was also found that patients with more severe kidney diseases had lower SAP levels in circulating blood.

Clinical development and regulatory trials of PRM-151

As pentraxin-2 had been shown to potently inhibit the differentiation of monocytes into pro-inflammatory macrophages and profibrotic fibrocytes and suppress the production of TGF-β1, a key mediator of tissue fibrosis, IPF, was considered as one form of fibrotic disease that could be targeted in a clinical trial. Furthermore, preclinical studies suggested that patients with IPF as well as other fibrotic disorders appeared to have deficient circulating levels of SAP. As SAP was often confused with another protein, serum amyloid A, the nomenclature of SAP was changed to pentraxin-2, and the recombinant SAP used for clinical trials was named PRM-151.²

The results of the first study in normal volunteers and patients with IPF (Table 3) were published in 2013.^34–37 This blinded, placebo-controlled, randomized clinical trial (RCT) was performed to evaluate the safety, tolerability and pharmacokinetics of PRM-151 using single ascending doses (range 0.1–20 mg/kg) of PRM-151 administered via continuous intravenous (IV) infusion over 30 min while fasting. While mostly healthy subjects were evaluated, a modest number of patients with IPF were also studied. Plasma concentrations of SAP were elevated with the 5, 10 and 20 mg/kg protocols and persisted for up to 72 h, and pharmacokinetic profiles for 10 mg/kg dosing were similar for both healthy volunteers and patients with IPF. Fibrocyte percentages (CD45+/procollagen-1+ cells in whole blood samples) declined by 30–50% at 24 h following PRM-151 administration.

The results of the subsequent phase Ib RCT were published in 2016.³⁵ Three successive cohorts of patients with IPF received multiple ascending doses of PRM-151 (1, 5 or 10 mg/kg versus placebo) on days 1, 3, 5, 8 and 15, and recipients were followed up to day 57. No serious adverse reactions to the study drug were observed, and SAP levels in plasma increased in a dose-dependent fashion up to eightfold, although considerable interindividual variability in drug half-life was observed. Although subject numbers were limited, a trend towards an improvement in 6MWT distance and stabilization of interstitial changes on high-resolution computed tomography (HRCT) occurred in some participants.

The results of the phase I studies set the stage for a phase II efficacy and safety trial, the results of which were reported in 2018.³⁶ The phase II RCT enrolled eligible patients aged 40–80 years, with 111 of the 116 randomized patients (96%) completing the study at 28 weeks with 77 randomized to PRM-151 and 39 receiving placebo. Drug (10 mg/kg IV infusion over 60 min on days 1, 3 and 5 and then one infusion every 4 weeks) or placebo infusions were administered every 4 weeks for 24 weeks, and the majority of study participants were also receiving nintedanib or pirfenidone. The FVC% predicted showed a significantly less decline at week 28 for the PRM-151 recipients versus those given placebo (-2.5 versus -4.8 for treated versus placebo groups, p=0.001), and this trend was also noted for the subset of patients not receiving pirfenidone or nintedanib. The change in 6MWT distance was also attenuated (-0.5 m versus -31.8 m for treated versus placebo groups, p<0.001), but there was no significant difference in diffusion capacity of the lung for carbon monoxide or changes in HRCT appearance for drug versus placebo.

Nearly all study subjects (111 of 116) opted to enter the long-term extension study; the 37 subjects who had been on placebo began taking PRM-151, and the 74 subjects who had been randomized to PRM-151 continued to receive the drug.³⁸ The open-label extension participants were followed up to week 128. Although treatment-emergent adverse events (TEAEs) led to permanent discontinuation of the study drug in 28 (25%) participants prior to week 128, PRM-151 was relatively well tolerated, and its safety profile was acceptable; no serious TEAEs were found to be related to the drug.^38,39 While the trajectory of decline in FVC and 6MWT distance suggested a sustained effect of PRM-151 over time as noted in the initial RCT, participant numbers were too limited to allow conclusive evidence of a sustained drug effect.³⁹

The positive results (attenuation of FVC decline and stabilization of 6MWT distance as compared with patients who received placebo) in the phase II study compelled the US Food and Drug Administration to fast-track PRM-151 for a phase III trial (A Study to Evaluate the Efficacy and Safety of Recombinant Human Pentraxin-2 (rhPTX-2; PRM-151) in Participants With Idiopathic Pulmonary Fibrosis [STARSCAPE]; ClinicalTrials.gov identifier: NCT04552899) with recombinant human pentraxin-2 (PRM-151 relabelled as zinpentraxin alfa).³⁷ Change in FVC from baseline was chosen as the primary endpoint, and enrolment was initiated in 2021 following the dosing regimen used in the phase II RCT. An open-label extension study (A Study to Evaluate Long Term Safety and Efficacy of Recombinant Human Pentraxin-2 [rhPTX-2; PRM-151] in Participants With Idiopathic Pulmonary Fibrosis [STARSCAPE-OLE]; ClinicalTrials.gov identifier: NCT04594707) enrolling both phase II and phase III participants was also planned in anticipation of positive results from the phase III RCT.⁴⁰ Unfortunately, interim data analyses of phase III RCT results showed a lack of efficacy, and the trial was terminated by the sponsor in the fourth quarter of 2022.

Whether pentraxin-2 may yet be found to have efficacy in treating fibrotic disorders other than IPF remains to be seen. Although PRM-151 (zinpentraxin alfa) had also advanced to phase II status for the treatment of anaemia caused by bone marrow fibrosis, these myelofibrosis trials in Canada, France and the USA have been discontinued.⁴¹

Serum amyloid P/pentraxin-2 as a treatment for fibrotic disorders in perspective

Despite the potential utility of SAP as a treatment for fibrotic diseases as suggested by preclinical studies and the potential efficacy and an acceptable safety profile of recombinant human pentraxin-2 for patients with IPF as suggested by the early RCT results, interim data analyses did not support efficacy. Considering the apparent lack of benefit for zinpentraxin alfa, it is unlikely that the drug will be further studied in any RCT for fibrosing lung disease or other fibrotic disorders. However, this is not to say that pentraxin-2 lacks anti-fibrotic properties. Even well-powered phase III RCTs can go off the rails for a multitude of reasons, including skewed participant randomization or the impact of confounding variables.

Many potential therapies for the treatment of idiopathic pulmonary fibrosis (other than pentraxin 2) that have made it to phase III status when the phase II RCT outcomes suggested possible efficacy and acceptable safety profiles have been abandoned due to interim data analyses showing a lack of efficacy. One potential pitfall of using pentraxin-2 to treat IPF is that virtually all patients enrolled in an RCT have well-established and fairly advanced diseases. The preclinical study using the cardiomyopathy model suggested that pentraxin-2 had to be administered prior to the trans-endothelial migration of pro-inflammatory monocyte/macrophages to prevent tissue invasion and transformation of these cells to pro-fibrotic, tissue-resident fibrocytes to attenuate fibrosis.³² This observation may, in part, explain why pentraxin-2 infusions in patients with IPF with established, advanced disease did not appear to have a significant effect on the primary endpoint of FVC in the STARSCAPE RCT. Additionally, the burden of monthly IV infusions with associated costs and inconvenience would need to be outweighed by a definitive, robust therapeutic response that prevents the loss of lung function over time to justify the use of PRM-151/zinpentraxin alfa as a stand-alone agent or an adjunctive therapy combined with pirfenidone or nintedanib use.

Although PRM-151 (aka SAP, pentraxin-2 or zinpentraxin alfa) had a good safety profile in the phase II RCT (A Phase 2 Trial to Evaluate the Efficacy of PRM-151 in Subjects With Idiopathic Pulmonary Fibrosis [IPF]; ClinicalTrials.gov identifier: NCT02550873) and in the long-term extension follow-up studies, as with many phase III RCTs of promising drug therapies for IPF, efficacy was not supported by the interim data analysis of the phase III STARSCAPE trial.^36,38,39 Nonetheless, the basic and clinical research into this endogenous modulator of fibrotic pathways as described in this review provides valuable information concerning the difficulties involved in developing effective pharmacotherapeutic agents for IPF.

Key points

• Preclinical investigations in various models of fibrosis, including the bleomycin mouse model, supported pentraxin-2 as a relatively potent inhibitor of tissue fibrosis.

• The phase II trial of PRM-151 in patients with idiopathic pulmonary fibrosis showed a significant impact on both forced vital capacity decline (primary endpoint) and 6MWT distance (secondary endpoint); these outcomes were observed even when patients were receiving the US Food and Drug Administration-approved anti-fibrotic agents nintedanib or pirfenidone.

• Significant changes in DLCO, total lung volume or HRCT imaging were not observed over a 28-week treatment course in the phase II RCT.

• Long-term follow-up of patients enrolled in the phase II trial showed a good safety profile for PRM-151.

• An interim analysis of the data from the phase III (STARSCAPE) trial revealed a lack of efficacy, which led to its early termination.

• Zinpentraxin alfa is no longer a candidate drug for the treatment of pulmonary fibrosis and is unlikely to enter clinical trials targeting other forms of tissue/organ fibrosis in humans.