PDGF receptor blocker for pulmonary hypertension: a new agent in therapeutic arsenal

Keywords:

• imatinib mesylate
• microRNA
• PDGF
• pulmonary hypertension

Pulmonary hypertension (PH) is a serious disorder with poor life expectancy. A variety of systemic diseases and drug toxicity can lead to PH. Irrespective of the underlying cause, PH is characterized by the loss of vascular relaxation response, activation of proliferative and anti-apoptotic pathways leading to vascular remodeling, narrowing of the pulmonary arteries, elevated pressure and right ventricular hypertrophy. As the disease progresses, the appearance of right ventricular failure eventually leads to premature death. The diagnosis of PH is often made late, because the symptoms tend to be vague; by the time the diagnosis is made, significant pathological changes have already taken place in the pulmonary vasculature. Furthermore, multiple signaling pathways have been implicated in the pathogenesis of PH. Therefore, the PH therapy poses a serious challenge. Currently available therapy includes i) prostacyclin and its analogs to increase cAMP generation, ii) endothelin receptor blockers to inhibit cell contraction and proliferation and iii) phosphodiesterase 5 inhibitors to prevent degradation of cGMP. These drugs are used as monotherapy or in combination depending on the disease state and the patients' response. The advent of these drugs has improved hemodynamic parameters, exercise tolerance and the time leading to deterioration. These drugs, however, do not cure the disease but delay the progression and the eventual outcome. Therefore, it is essential to find new targets to develop innovative therapy to reverse or at least to halt the progression of the disease.

Studies with experimental and human PH suggest that the endothelial injury may play a key role in initiating the activation of multiple signaling pathways leading to PH. Initial endothelial injury in the monocrotaline (MCT) model of PH is accompanied by progressive loss of endothelial caveolin-1, and reciprocal activation of proliferative and anti-apoptotic pathways (PY-STAT3 and Bcl-xL) occur before the onset of PH. As the disease progresses, endothelial cells begin to lose cytosolic proteins, indicating progression of the endothelial damage [1]. Increased expression of other mitogens such as PDGF and EGF has also been reported in PH [2]. Caveolin-1 interacts with multiple signaling molecules within the caveolae and negatively regulates proliferative pathways. Rescue of endothelial caveolin-1 inhibits PY-STAT3 activation and prevents MCT-induced PH, but once the PH is established, most therapeutic measures fail to rescue endothelial caveolin-1 or to attenuate PH [3]. Thus, the loss of inhibitory function of endothelial caveolin-1 leading to deregulated proliferative pathways may be a pivotal event in the pathogenesis of PH.

Endothelial damage in PH is progressive leading to extensive damage/loss of endothelial cells accompanied by enhanced expression of caveolin-1 in smooth muscle cells (SMC) with subsequent neointima formation as has recently been reported in immunosuppressant-induced pulmonary arterial hypertension (PAH) [4]. Enhanced expression of caveolin-1 in SMC probably is a result of its translocation from caveolae, thereby losing its inhibitory capacity, and actively participating in cell proliferation and cell migration, thus facilitating the disease progression. This dual role of caveolin-1 is a well-described phenomenon in several types of cancer [3]. Deregulated proliferation of pulmonary vascular cells, angiogenic proliferation of endothelial cells and associated inflammation observed in PH have led some investigators to liken PH to cancer [5]. With the exception of cell transformation, PH shares several aspects of pathological changes with cancer such as dual role of caveolin-1 and the activation of proliferative and anti-apoptotic pathways including PY-STAT3, PDGF, survivin, Bcl₂ and Bcl-xL. Therefore, adaptation from cancer therapy to improve PH management is a valuable strategy.

Imatinib mesylate (STI 571 or Gleevec), approved for the treatment of chronic myeloid leukemia and gastrointestinal stromal tumors, belongs to a group of tyrosine kinase inhibitors, which targets tyrosine kinase activity of BCR-Abl, c-kit and PDGF-α and -β receptors. Several studies have shown increased expression of PDGF receptor β (PDGFR β) in patients with idiopathic PAH and also in MCT and hypoxia models of PH. Furthermore, inhibition of PDGF receptor with imatinib reverses MCT and hypoxia-induced PH [2,6]. Phase II clinical trial with imatinib in PAH has revealed improvement in hemodynamic parameters, although improvement in 6-min walk was not significant. The interesting aspect of this study was that the patients with more severe PH and higher pulmonary vascular resistance responded with better hemodynamic parameters and improved 6-min walk compared with patients with less severe PH [7]. Although imatinib was considered relatively safe in this group, 39% of patients exhibited serious side effects. However, cardiotoxicity was not observed in this group [7]. In last month's issue ten Freyhaus et al. [8] have provided a comprehensive review of the usage of imatinib mesylate in the treatment of PAH. In addition to the case reports cited in the ten Freyhaus paper imatinib has been found to be extremely useful in managing perioperative intractable PH in congenital diaphragmatic hernia and portopulmonary hypertension, and also in scleroderma-associated PAH not responding well to conventional therapy [9-11]. However, imatinib has not proved effective in other cases [12]. In a small group of patients, it was recently shown that the patients with high serum PDGF-BB levels responded to imatinib initially, but the longer therapy did not have any effect. Thus, once the PDGF-BB levels are decreased, the treatment with imatinib may not be effective, because imatinib induces SMC apoptosis in the presence of PDGF-BB. Interestingly, imatinib increases apoptosis in SMC from PAH patients exposed to PDGF, but has no effect on control SMC [13].

Some of the problems associated with imatinib are that it is poorly tolerated by some patients. Imatinib has been shown to induce ultrastructural abnormalities of mitochondria and myocyte loss. Cardiotoxicity and LV dysfunction are thought to be due to the inhibition of c-Abl by imatinib [8]; however, reported incidence of cardiotoxicity is relatively low. Hepatotoxicity and fatal liver failure have been reported in patients who took acetaminophen while on imatinib therapy. Tyrosine kinase inhibitors decrease acetaminophen glucuronidation resulting in increased accumulation of acetaminophen, thus leading to hepatotoxicity [14]. Caregivers and patients need to be aware of the effect of coadministration of imatinib and acetaminophen.

PDGF was first identified in 1970s as a serum growth factor for cells including fibroblasts and SMC. In the embryonic stage, PDGF-B and PDGFR-β are essential for normal cardiovascular development. PDGF-B expression is concentrated in endothelial cells and PDGF-β receptors are predominantly found in SMC and pericytes. PDGF and VEGF belong to a superfamily of signaling molecules containing cystine-knot structure. VEGF-A and PDGF-BB both are critical factors promoting recruitment and proliferation of vascular cells. VEGF-A stimulates both PDGF-α and -β receptors. Inhibition of either receptor significantly attenuates VEGF-A-induced call migration. Both receptors mediate VEGF-A and PDGF signaling, and VEGF-A is a crucial factor in promoting the recruitment and proliferation of vascular SMC both under physiological and pathological conditions [15,16]. Both STAT3 and PDGF-B are essential for normal embryonic development. In addition to cytokines, growth factors such as PDGF can activate STAT3. Furthermore, in several cell systems, activation of STAT3 is required for PDGF-induced cell proliferation; and the inhibition of PDGF receptor results in the suppression of cell proliferation via inactivation of PY-STAT3 signaling [17]. PDGF receptors colocalize with caveolin-1 in caveolae, and the scaffolding domain of caveolin-1 interacts with PDGF receptor and inhibits receptor kinase activity. Overexpression of caveolin-1 induces vascular SMC apoptosis after PDGF stimulation, and PDGF stimulation upregulates caveolin-1 mRNA but facilitates caveolin-1 protein degradation via lysosomal pathway [18,19].

Increased expression of PDGF ligand and its receptor occurs after vascular injury. PDGF downregulates SMC genes, thus altering SMC phenotype from contractile to undifferentiated synthetic type, which is required for vascular repair. After repair, SMC revert to contractile phenotype, but deregulated synthetic phenotype leads to vascular disease. Micro (mi) RNA-221 is considered essential for PDGF-induced cell migration. miRNA-221 is highly expressed in various cancerous cells and promotes proliferation through binding to the 3'-untranslated region of cell cycle inhibitor and inhibiting p27/kip1 expression; miRNA-221-induced inhibition of p27/kip1 is thought to facilitate cell proliferation. Furthermore, miRNA-221 reduces c-Kit mRNA. C-kit changes vascular SMC phenotype from synthetic to contractile type. Thus, the inhibition of c-Kit leads to PDGF-mediated downregulation of SMC gene expression resulting in undifferentiated synthetic phenotype [20]. It is tempting to consider that the inhibition of miRNA-221 may abrogate PDGF-induced SMC proliferation and phenotype change without the side effects of imatinib. Interestingly, as opposed to SMC, in endothelial cells, miRNA-221 blocks cell proliferation and migration via inhibiting c-kit [18]. Montani et al. have recently shown increased expression of c-kit⁺ cells in the experimental and human PAH [21]. These authors have further shown that the inhibition of c-kit attenuates hypoxia-induced PH. The role of c-kit in vascular remodeling is not entirely clear. It is likely that c-kit function depends on the cell types involved. Figure 1 depicts the possible PDGF-BB-mediated pathways in PAH and the sites of imatinib effect.

Figure 1. Initial injury leads to the loss or dysfunction of endothelial caveolin-1 and increased expression of PDGF-BB. This figure depicts the possible PDGF-mediated pathways and the sites of inhibitory activity of imatinib.

Favorable results obtained with imatinib therapy in severe and intractable PH are very encouraging. Some of the side effects are of concern, although most patients seemed to have tolerated the drug with minor side effects. Ongoing Phase III clinical trial will clarify some of the questions such as i) the incidence of side effects, ii) type of PH patients responsive to therapy and iii) whether the long-term treatment is beneficial and sustained.

Expert opinion

Currently available therapy has not proved efficient in reversing the disease or halting its progression. Imatinib is a welcome addition to the PH therapeutic arsenal. It has proved very effective in cases with severe and intractable PH. Imatinib therapy, however, has not been uniformly successful. It is likely that PDGF may not have a significant role in all forms of PH or perhaps at different stages of the disease. Furthermore, once the PDGF levels are lowered with imatinib, the drug may no longer be effective. Although the incidence of reported cardiotoxicity has been low in patients treated with imatinib for other diseases, it is of concern in PH patients. Interaction of imatinib with other drugs could also pose a problem. Occurrence of PH has recently been reported in several patients treated with an imatinib-related compound for chronic myeloid leukemia. In view of these potential problems, a careful PH patient selection for imatinib therapy may be crucial for successful results.

Deregulation of vascular smooth muscle cell phenotype plays a critical role in the progression of PH. Importance of miRNAs in vascular diseases including PH is emerging, and several miRNAs have been discovered that regulate cell proliferation and SMC phenotype change. PDGF-mediated increase in miRNA-221 promotes SMC phenotype change, cell proliferation, cell migration and possibly facilitating neointima formation. Identification and modulation of miRNA involved in vascular disease is an attractive therapeutic modality.

Declaration of interest

The author states no conflict of interest and has received no payment in preparation of this manuscript.

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