Evaluation of WO-2014132220, selective PDGFR inhibitors for the treatment of pulmonary arterial hypertension

Abstract

Introduction: Pulmonary arterial hypertension (PAH) is a rare disease currently treated by a range of vasodilator agents and/or endothelin antagonists. Inhibition of platelet derived growth factor receptor (PDGFR) kinases has been suggested to provide an additional therapeutic modality, and clinical studies with the non-selective PDGFR inhibitor imatinib appear to validate this hypothesis. However, side-effects associated with a lack of selectivity suggest clinical utility requires the identification and development of selective PDGFR inhibitors.

Areas covered: This application claims derivatives and crystalline forms of two previously claimed PDGFR inhibitors and their use for the treatment of PAH. N-(5-(2-(2,2-dimethylpyrrolidin-1-yl)ethylcarbamoyl)-2-methylpyridin-3-yl)-6-(1-methyl-1H-pyrazol-4-yl)pyrazolo[1,5-a]pyridine-3-carboxamide and N-(5-(2-(2,6-cis-dimethylpiperidin-1-yl)ethylcarbamoyl)-2-fluorophenyl)-7-(1-methyl-1H-pyrazol-5-yl)imidazo[1,2-a]pyridine-3-carboxamide have respective IC₅₀ values of 3 and 45 nM in a cellular proliferation assay.

Expert opinion: These two compounds are likely to be selective PDGFR inhibitors. The nature of this filing suggests that Novartis intends to develop at least one of these compounds for the treatment of PAH.

Keywords:

• platelet derived growth factor
• platelet derived growth factor receptor kinase inhibitor
• pulmonary arterial hypertension
• rare disease

1. Introduction

1.1 Pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare disease that is generally idiopathic rather than genetic in origin. In spite of its rarity, its currently reported prevalence is approximately 200,000 patients in Europe and the US [1]. But these prevalence figures are widely regarded as underestimates since diagnosis is generally confined to patients showing signs of more advanced disease [2]. Early diagnosis of PAH is rare due to mild, relatively non-specific symptoms with accurate diagnosis only achievable via invasive procedures. PAH is characterised by the symptoms in shown in Figure 1. Clinical guidelines based on these symptoms have been established both in Europe [3] and in the US [4].

Figure 1. Characteristic symptoms of PAH.

The principal consequence of advanced PAH is right ventricular failure arising from the increase in pulmonary vascular resistance due to pulmonary arterial obstruction. PAH has a 15% annual mortality rate although prognosis varies between different subsets of patients and is influenced by age and comorbid conditions such as systemic scleroderma. The underlying disease pathology is characterised by endothelial dysfunction and vascular remodelling [5]. These changes are accompanied by impaired production of endogenous regulators of vascular tone, for example, nitric oxide and prostacyclin (PGI₂) as well as elevated levels of vasoconstrictors such as endothelin and serotonin, due to prolonged overexpression or enhanced release respectively.

Currently approved treatment options for PAH focus on trying to alleviate these problems. The current mainstays of treatment are inhaled and oral vasodilators (respectively IP receptor agonists and PDE5 inhibitors) and oral endothelin antagonists [6,7]. Exogenous PGI₂ (epoprostenol sodium) is also used but the need to administer this chemically unstable agent via intravenous infusions render it less convenient. The vast majority of the approved treatments for PAH have been developed as orphan drugs, often by repurposing of drugs developed for other indications [8]. To date, only two novel chemical entities, the ET receptor antagonist macitentan and the vasodilator riociguat, have been developed specifically for the treatment of PAH although a number of other novel agents are currently in clinical development for the treatment of this condition [9,10].

1.2 Platelet derived growth factor

Platelet derived growth factor (PDGF) is one of a number of mitogenic growth factors. The effects of PDGF are mediated by its activation of specific receptors: PDGFRα and PDGFRβ. Both PDGF and its receptors have been shown to be upregulated in PAH [11]. Studies in animal models have revealed that PDGF appears to play a significant role in the vascular remodelling that is characteristic of PAH [12,13]. Such evidence suggests that modulating PDGF activity could be beneficial in the treatment of PAH [14]. Furthermore, studies with agents that inhibit PDGFR suggest that such a hypothesis is valid [15].

1.3 PDGFR inhibitors

Both PDGFRα and PDGFRβ receptors are associated with specific receptor tyrosine kinases. Thus, PDGFR inhibitors are seen as having the potential to be useful in the treatment of PAH [16,17]. Inhibition of PDGFR by the (non-specific) PDGFR inhibitor imatinib has been shown to inhibit PDGF-induced proliferation and migration of cultured pulmonary artery smooth muscle cells from patients with idiopathic PAH [18], and to reverse experimentally induced PAH in rats [15]. Such results, and individual case reports, prompted Novartis to initiate clinical studies in PAH with both imatinib and nilotinib. Although Novartis completed Phase III studies with imatinib and filed for additional approval for its use in PAH, it subsequently withdrew both its US and European marketing applications after the US FDA had requested additional data. The FDA’s request was assumed to relate to concerns about adverse events with imatinib, while Phase II studies with nilotinib were terminated after significant cardiovascular adverse events were observed [19].

Such adverse effects have raised concerns about the utility of kinase inhibitors in the treatment of PAH [20]. The patent application [21], from Novartis, that is the subject of this evaluation specifically claims crystalline forms of the PDGFR inhibitors (1) and (2) for the treatment of PAH (Figure 2), and implies that Novartis has a different view on the viability of such an approach to the treatment of PAH.

2. Chemistry

The application claims two derivatives of each of two compounds. The tartrate salt and a saccharin complex of N-(5-(2-(2,2-dimethylpyrrolidin-1-yl)ethylcarbamoyl)-2-methylpyridin-3-yl)-6-(1-methyl-1H-pyrazol-4-yl)pyrazolo[1,5-a]pyridine-3-carboxamide (1), and the maleate and saccharinate salts of N-(5-(2-(2,6-cis-dimethylpiperidin-1-yl)ethylcarbamoyl)-2-fluorophenyl)-7-(1-methyl-1H-pyrazol-5-yl)imidazo[1,2-a]pyridine-3-carboxamide (2) (Figure 2). Crystalline forms of all four derivatives are also claimed as are their use for the treatment of PDGFR-mediated disorders, respiratory, inflammatory and fibrotic diseases including the specific example of PAH.

The preparation of both of these compounds, a number of intermediates and the claimed salts and crystalline forms is exemplified. Their preparation is illustrated in the two schemes.

7-Bromoimidazo[1,2-a]pyridine-3-carboxylic acid (3) was prepared in three steps from ethyl 2-chloroacetate and potassium 2-methylpropan-2-olate (Figure 3). Methyl 5-amino-6-methylnicotinate was prepared in two steps from 6-methyl-5-nitro-2-oxo-1,2-dihydropyridine-3-carboxylic acid. Compound (1) was prepared as shown in scheme 1. 7-Bromoimidazo[1,2-a]pyridine-3-carboxylic acid was converted to its acid chloride (4) and then condensed with the nicotinate. Suzuki coupling with the 1-methylpyrazole boronate (5) was followed by coupling with 2-(2,2-dimethylpyrrolidin-1-yl)ethanamine to give (1) (Figure 3).

Figure 3. The preparation of compound (1). (a) SOCl₂, toluene, 6 h, 110°C. (b) pyridine, 18 h, 20°C. (c) Cs₂CO_3, PdCl₂(dppf).CH₂Cl₂ dimethoxyethane, H₂O, 7 h, 100°C. (d) iPr₂EtN, HATU, DMF, 90 min, 20°C.

Compound (2) was prepared in a similar manner as shown in (Figure 4). Methyl 3-amino-4-fluorobenzoate and 2-(2,6-cis-dimethylpiperidin-1-yl)ethanaminium chloride were condensed in the presence of base and followed by acylation with 7-bromoimidazo[1,2-a]pyridine-3-carboxyl chloride to give 7-bromo-N-(5-(2-(2,6-cis-dimethylpiperidin-1-yl)ethylcarbamoyl)-2-fluorophenyl)imidazo[1,2-a]pyridine-3-carboxamide (6). Suzuki coupling as before gave compound (2). The preparation and characterisation of 1:1 saccharin and tartrate adducts of (1), and 1:1 saccharin and maleate adducts of (2) is also exemplified.

Figure 4. The preparation of compound (2). (a) 2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine, THF, 40 h, 80°C. (b) (4), pyridine, 18 h, 20°C. (c) Cs₂CO_3, PdCl₂(dppf).CH₂Cl₂ dimethoxyethane, H₂O, hν, 1 h, 100°C.

3. Biology

The current application contains no biological data. However, data on these compounds was presented in the earlier application [22]. Their potency as PDGFR inhibitors was evaluated by measuring inhibition of the proliferation of rat A10 cells. Rat A10 cells (ATCC) were resuspended in DMEM supplemented with 1% FBS and 10 ng/ml recombinant rat PDGF-BB at 20,000 cells/ml. The cells were aliquoted into 384 well plates at 50 μl/well and incubated for 4 h at 37°C then for 68 h after addition of varying concentrations of test compound solution (0.5 μl/well). Luminescence was then read using a CLIPR CCD camera after addition of CellTiter-Glo and 15 min incubation. IC₅₀ values were calculated by non-linear regression fitting to sigmoidal dose response curves.

The IC₅₀ values of the two compounds claimed in the current application were 3.0 nM (1) and 45 nM (2). No further biological data, on these compounds, were presented in the earlier application.

4. Expert opinion

This application is interesting both because of its focus on two specific compounds, and because of the claim of the use of PDGFR inhibitors for the treatment of PAH. While these examples clearly represent the compounds of greater interest from an earlier filing, which also focused on the PAH indication, more detailed examination of previous filings from the same group indicates a substantial level of activity around these bicyclic scaffolds.

A set of five filings claiming N-phenyl-imidazo[1,2-a]pyridine-3-carboxamide derivatives as inhibitors of the tyrosine kinases PDGFR and/or c-kit were published on 7 March 2013. The key selectivity difference, judged by the claims in these applications, is attributable to the cyclic substituent attached to the N-phenyl group. Where an attached oxadiazole ring was explicitly claimed, the compounds were claimed as inhibitors of both c-kit and PDGFR [23-25]. In contrast, where a second aryl group is attached via an amide linker, the compounds are claimed solely as inhibitors of PDGFR and for use in the treatment of PAH [26]. The genesis for this scaffold appears to stem from a filing claiming pyrimidinyloxy substituted heterocyclic carboxamides as kinase inhibitors, particularly of VEGFR [27]. That application included a number of examples of pyrazolo[1,5-a]pyridine derivatives. However, Novartis’s use of the pyrazolo[1,5-a]pyridine-3-carboxamide motif appears confined to the filing under evaluation and the earlier filing from which the two specific examples were selected.

Assuming that compounds (1) and (2) are selective PDGFR inhibitors, how do they compare with well-documented PDGFR inhibitors? Crenolanib (Figure 5) was originally developed as a PDGFR inhibitor by Pfizer, with low nM potency against both PDGFRα and PDGFRβ [28], but has since been shown to also be a potent and relatively selective inhibitor of the tyrosine kinase Flt3 [29,30]. In contrast, CP-673451 was reported to be a potent and highly selective PDGFR inhibitor [31], inhibiting PDGFRα and PDGFRβ with respective IC₅₀ values of 10 and 1 nM and showing > 250-fold selectivity over other kinases. AZD-2932 also has nanomolar potency as a PDGFR inhibitor but is equipotent against both VEGFR2 and Flt3 [32] and never progressed beyond preclinical development. None of these three PDGFR inhibitors have been evaluated in models of PAH.

Figure 2. The two compounds that are the subject of the application under discussion.

Figure 5. Known inhibitors of PDGFR kinase.

Pulmokine has recently reported the activity of PK-10453, described as a non-selective PDGFR inhibitor, in such a model [33-35]. PK-10453 inhibits PDGFRα and PDGFRβ with respective IC₅₀ values of 10.1 and 35 nM. Pulmokine is aiming to develop an inhaled formulation of PK-10453 for the treatment of PAH, citing the efficacy of imatinib in the IMPRES study [36]. In this study, 24 weeks of treatment with imatinib, as an add on therapy, improved exercise capacity and haemodynamics in patients with advanced PAH, but its use was accompanied by a higher rate of serious adverse events and there was a higher drop out rate in the imatinib treatment arm. However, imatinib is not a selective inhibitor of PDGFR, nor a particularly potent inhibitor of PDGFR [37]. Thus, the use of more selective (and potent) PDGFR inhibitors delivered orally or via inhalation is anticipated to provide better tolerated and more effective treatments for PAH.

With regard to the two compounds claimed in the application under discussion, it appears that both are more potent PDGFR inhibitors than either imatinib or PK-10453. And the changing claim structure within this patent family implies that the two claimed compounds are also more selective PDGFR inhibitors. The discrepancy in potency between the two claimed compounds suggests either that the less potent inhibitor (2) is much better absorbed if the intention is to develop the compounds for oral administration, or that both are being considered for delivery via inhalation. Since Novartis has already evaluated both imatinib and nilotinib for the treatment of PAH and has developed a variety of inhaled formulations for the treatment of asthma and COPD, it may be intending to explore both routes of administration with these PDGFR inhibitors. There is currently no evidence to indicate that either compound has progressed beyond exploratory development.

Declaration of interest

The author has received an honorarium from Informa for the preparation of this manuscript. The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.