Natural product drug delivery: A special challenge?

Natural products have a long-standing and critical role in drug development and medical use. The structural and physicochemical properties of natural products, while derived evolutionarily to be effective in living systems, may create challenges in translation to a pharmaceutical product. Molecular complexity, low solubility, functional group reactivity and general instability are among the challenges that typically need to be overcome. This review looks at some of the ways that natural products have been formulated and delivered to enable the successful application of these vitally important medicines to patients.

α-Halo carbonyls enable meta selective primary, secondary and tertiary C-H alkylations by ruthenium catalysis.

A catalytic meta selective C-H alkylation of arenes is described using a wide range of α-halo carbonyls as coupling partners. Previously unreported primary alkylations with high meta selectivity have been enabled by this methodology whereas using straight chain alkyl halides affords ortho substituted products. Mechanistic analysis reveals an activation pathway whereby cyclometalation with a ruthenium(ii) complex activates the substrate molecule and is responsible for the meta selectivity observed. A distinct second activation of the coupling partner allows site selective reaction between both components.

An inhibitor of NADPH oxidase-4 attenuates established pulmonary fibrosis in a rodent disease model.

Idiopathic pulmonary fibrosis is a chronic progressive disease of increasing prevalence for which there is no effective therapy. Increased oxidative stress associated with an oxidant-antioxidant imbalance is thought to contribute to disease progression. NADPH oxidases (Nox) are a primary source of reactive oxygen species within the lung and cardiovascular system. We demonstrate that the Nox4 isoform is up-regulated in the lungs of patients with IPF and in a rodent model of bleomycin-induced pulmonary fibrosis and vascular remodeling. Nox4 is constitutively active, and therefore increased expression levels are likely to contribute to disease pathology. Using a small molecule Nox4/Nox1 inhibitor, we demonstrate that targeting Nox4 results in attenuation of an established fibrotic response, with reductions in gene transcripts for the extracellular matrix components collagen 1α1, collagen 3α1, and fibronectin and in principle pathway components associated with pulmonary fibrosis and hypoxia-mediated vascular remodeling: transforming growth factor (TGF)-ß1, plasminogen activator inhibitor-1, hypoxia-inducible factor, and Nox4. TGF-ß1 is a principle fibrotic mediator responsible for inducing up-regulation of profibrotic pathways associated with disease pathology. Using normal human lung-derived primary fibroblasts, we demonstrate that inhibition of Nox4 activity using a small molecule antagonist attenuates TGF-ß1-mediated up-regulation in expression of profibrotic genes and inhibits the differentiation of fibroblast to myofibroblasts, that is associated with up-regulation in smooth muscle actin and acquisition of a contractile phenotype. These studies support the view that targeting Nox4 may provide a therapeutic approach for attenuating pulmonary fibrosis.

The discovery of potent, orally bioavailable pyrazolo and triazolopyrimidine CXCR2 receptor antagonists.

A hit-to-lead optimisation programme was carried out on the Novartis archive screening hit, pyrazolopyrimidine 2-methyl-5-((phenylthio)methyl)pyrazolo[1,5-a]pyrimidin-7-ol 1, resulting in the discovery of CXCR2 receptor antagonist 2-benzyl-5-(((2,3-difluorophenyl)thio)methyl)-[1,2,4]triazolo[1,5-a]pyrimidin-7-ol 14. The SAR was investigated by systematic variation of the pendant thiol, alkyl and pyrimidinol groups. Replacement of the pyrazolopyrimidine core with a triazolo alternative led to a dual series of antagonists with favourable biological and pharmacokinetic properties.

The discovery of potent, orally bioavailable pyrimidine-5-carbonitrile-6-alkyl CXCR2 receptor antagonists.

A hit-to-lead optimisation programme was carried out on the Novartis archive screening hit, pyrimidine 2-((2,6-dichlorobenzyl)thio)-5-isocyano-6-phenylpyrimidin-4-ol 4, resulting in the discovery of CXCR2 receptor antagonist 2-((2,3-difluorobenzyl)thio)-6-(2-(hydroxymethyl)cyclopropyl)-5-isocyanopyrimidin-4-ol 24. The SAR was investigated by systematic variation of the aromatic group at c-6, the linker between c-2 and the halogenated ring, and the c-5 nitrile moiety.

Intramolecular nitrone dipolar cycloadditions: control of regioselectivity and synthesis of naturally-occurring spirocyclic alkaloids.

The intramolecular nitrone dipolar cycloaddition of in situ-generated nitrones such as compound 26 has been used for the synthesis of cyclic isoxazolidines 27 and 29. The regioselectivity of the intramolecular cycloaddition depends on the nature of the terminal substituent on the dipolarophile. The influence of the substituent on the regioselectivity of the cycloaddition has been examined using several model systems and two methods of nitrone formation. These studies demonstrated that the cyano-substituent plays a special role in favouring the formation of the 6,6,5-ring fused adduct 27 under thermodynamically controlled conditions. The utility of the cyclo-adduct 57 (see Scheme 12) as a precursor for the naturally occurring histrionicotoxins is illustrated by the synthesis of three "unsymmetrical" (i.e. with each side chain bearing different functional groups) members of the histrionicotoxin family HTX-259A, HTX-285C and HTX-285E (2, 3 and 4 respectively).

A synthetic, catalytic and theoretical investigation of an unsymmetrical SCN pincer palladacycle.

The SCN ligand 2-{3-[(methylsulfanyl)methyl]phenyl}pyridine, 1, has been synthesized starting from an initial Suzuki-Miyaura (SM) coupling between 3-((hydroxymethyl)phenyl)boronic acid and 2-bromopyridine. The C-H activation of 1 with in situ formed Pd(MeCN)4(BF4)2 has been studied and leads to a mixture of palladacycles, which were characterized by X-ray crystallography. The monomeric palladacycle LPdCl 6, where L-H = 1, has been synthesized, and tested in SM couplings of aryl bromides, where it showed moderate activity. Density functional theory and the atoms in molecules (AIM) method have been used to investigate the formation and bonding of 6, revealing a difference in the nature of the Pd-S and Pd-N bonds. It was found that S-coordination to the metal in the rate determining C-H bond activation step leads to better stabilization of the Pd(II) centre (by 13-28 kJ mol(-1)) than with N-coordination. This is attributed to the electron donating ability of the donor atoms determined by Bader charges. The AIM analysis also revealed that the Pd-N bonds are stronger than the Pd-S bonds influencing the stability of key intermediates in the palladacycle formation reaction pathway.

CGH2466, a combined adenosine receptor antagonist, p38 mitogen-activated protein kinase and phosphodiesterase type 4 inhibitor with potent in vitro and in vivo anti-inflammatory activities.

Theophylline, a phosphodiesterase inhibitor and adenosine receptor antagonist, is used in asthma and chronic obstructive pulmonary disease (COPD) treatment. However, the relatively low effectiveness of theophylline have recently led to reduced usage. The goal of the present study was to identify a theophylline-like compound with improved effectiveness. We discovered CGH2466, which not only antagonised the adenosine A1, A2b and A3 receptors with IC50 values of 19 +/- 4, 21 +/- 3 and 80 +/- 14 nM, respectively, but also inhibited the p38 mitogen-activated protein (MAP) kinases alpha and beta and the phosphodiesterase 4D (PDE4D) isoenzyme with IC50 values of 187 +/- 18, 400 +/- 38 and 22 +/- 5 nM, respectively. Despite similar potencies on individual targets, CGH2466 inhibited the production of cytokines and oxygen radicals by human peripheral blood leucocytes in vitro, more potently (IC50 values between 30 and 50 nM) than the standard p38 MAP kinase inhibitor SB203580 (30 nM to >1 microM), the PDE4 inhibitor cilomilast (120-400 nM) and the broad spectrum adenosine receptor antagonist CGS15943 (>10 microM). When given either orally or locally into the lungs, CGH2466 (3 to 10 mg kg(-1)) inhibited the ovalbumin- or lipopolysaccharide-induced airway inflammation in mice more potently than the single receptor antagonists or enzyme inhibitors used alone. In conclusion, CGH2466 through its combined activities at multiple targets exerted a powerful anti-inflammatory effect and therefore may have beneficial therapeutic value in diseases such as asthma and COPD.

Catalytic meta-selective C-H functionalization to construct quaternary carbon centres.

A catalytic meta-selective C-H functionalization of 2-phenylpyridines using a range of tertiary halides is described. The protocol is simple to perform and uses commercially available reagents to construct challenging quaternary carbon centres in a regioselective manner. Preliminary studies suggest the C-H functionalization proceeds through a radical process directed via a remote σ-activation.

Efficient and facile synthesis of acrylamide libraries for protein-guided tethering.

A kinetic template-guided tethering (KTGT) strategy has been developed for the site-directed discovery of fragments that bind to defined protein surfaces, where acrylamide-modified fragments can be irreversibly captured in a protein-templated conjugate addition reaction. Herein, an efficient and facile method is reported for the preparation of acrylamide libraries from a diverse range of amine fragments using a solid-supported quaternary amine base.

Discovery and Optimization of 4-(8-(3-Fluorophenyl)-1,7-naphthyridin-6-yl)transcyclohexanecarboxylic Acid, an Improved PDE4 Inhibitor for the Treatment of Chronic Obstructive Pulmonary Disease (COPD).

Herein we describe the optimization of a series of PDE4 inhibitors, with special focus on solubility and pharamcokinetics, to clinical compound 2, 4-(8-(3-fluorophenyl)-1,7-naphthyridin-6-yl)transcyclohexanecarboxylic acid. Although compound 2 produces emesis in humans when given as a single dose, its exemplary pharmacokinetic properties enabled a novel dosing regime comprising multiple escalating doses and the resultant achievement of high plasma drug levels without associated nausea or emesis.

New highly potent and selective adenosine A(3) receptor antagonists.

A class of potent, selective adenosine A(3) receptor antagonists was obtained via optimisation of the screening hit N-[4-(4-methoxyphenyl)-thiazol-2-yl]-acetamide. Structural modifications of this hit revealed very quickly that a 5-(pyridin-4-yl) substituent on the 2-aminothiazole ring was optimal for high potency at the adenosine A(3) receptor. Structure activity relationship studies led to both potent and selective A(3) receptor antagonists, including N-[5-pyridin-4-yl-4-(3,4,5-trimethoxyphenyl)-thiazol-2-yl]-acetamide, a highly potent aden-osine A(3) receptor antagonist with greater than 100- fold selectivity against the related adenosine receptors. As well as demonstrating selective in vitro binding on the human A(3) adenosine receptor, this compound was also shown to selectively block the rat A(3) receptor in vivo. This important new compound can be readily synthesised in four steps from commercially available starting materials.

The total synthesis of alkaloids (-)-histrionicotoxin 259A, 285C and 285E.

The first total syntheses of three "unsymmetrical" (i.e. different terminal groups in the side chains) members of the histrionicotoxin family of alkaloids have been accomplished via stepwise introduction of the two side chain moieties onto a common tricyclic core.

Targeting the serotonin pathway for the treatment of pulmonary arterial hypertension.

As we uncover the complex pathophysiology underlying idiopathic and familial pulmonary arterial hypertension, multiple disease associated pathways, cell types and processes reveal links to elements of the serotonin system. Beyond the original 'serotonin hypothesis' observed with anorexigens, and the latterly demonstrated association with vascular tone and pulmonary artery smooth muscle cell proliferation, recent studies suggest links to BMPR2, PDGF and RhoK pathways, as well as an impact upon more complex lesion formation and pathologic bone marrow progenitor mobilization. Clinical experience with antagonists targeting the various elements of the serotonin pathway has been unsatisfactory, yet perhaps this is less than surprising given our expanding knowledge around serotonin production and signaling biology, which indicate opportunities for novel therapeutic options.

Solubility-driven optimization of phosphodiesterase-4 inhibitors leading to a clinical candidate.

The solubility-driven optimization of a series of 1,7-napthyridine phosphodiesterase-4 inhibitors is described. Directed structural changes resulted in increased aqueous solubility, enabling superior pharmacokinetic properties with retention of PDE4 inhibition. A range of potent and orally bioavailable compounds with good in vivo efficacy in animal models of inflammation and reduced emetic potential compared to previously described drugs were synthesized. Compound 2d was taken forward as a clinical candidate for the treatment of COPD.

Microwave-mediated synthesis of an arylboronate library.

A series of arylboronates has been synthesized from the reaction of 2-(2-, (3-, or (4-(bromomethyl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 1{1-3} respectively with a range of N-, S-, and O-nucleophiles, using microwave-mediated chemistry. For the synthesis of N- and S-substituted boronates, a supported base, PS-NMM, was employed, and many reactions were complete within 15 min. With O-nucleophiles, a mixture of tetrabutylammonium bromide, potassium carbonate, and sodium hydroxide was employed. The resulting aminomethyl, mercaptomethyl, or alkoxy-/phenoxymethyl-arylboronates were subjected to microwave-mediated Suzuki Miyaura coupling reactions to afford a range of biaryls in moderate to good yields. The X-ray structures of five boronates were determined.

Progress towards novel adenosine receptor therapeutics gleaned from the recent patent literature.

IMPORTANCE OF THE FIELD: The principle of treating disease with selective adenosine receptor ligands has been demonstrated with drugs on the market, while the lesser understood receptor subtypes are still being probed with new and drug-like pharmaceutical tools. The field of adenosine receptor research is, therefore, highly important as an emerging and proven point of intervention in disease. AREAS COVERED IN THIS REVIEW: From 2008 to 2009, > 120 primary patent applications have claimed adenosine receptor ligands, which we analyze by applicant and target. Particularly significant disclosures are described in detail, paying particular attention to the biological data marshalled to support the case. WHAT THE READER WILL GAIN: The first published disclosure of new compounds, compound uses or drug targets is often in the patent literature, which can be difficult to trawl, interpret and verify as it is not subject to peer review. We have critically reviewed this area and share our conclusions regarding progress, trends and identification of early tool compounds or compounds of potential clinical significance ahead of peer-reviewed publication. TAKE HOME MESSAGE: Adenosine receptor research is a thriving field with continuing claims of exciting new compounds with high specificity and intriguing examples of new uses for such ligands.

Dual PDE3/4 inhibitors as therapeutic agents for chronic obstructive pulmonary disease.

Phosphodiesterase (PDE)4, and to a lesser extent, PDE3/4 inhibitors have attracted considerable interest as potential therapeutic agents for diseases including chronic obstructive pulmonary disease. Indeed, ibudilast and theophylline are utilized clinically, and roflumilast is in late-stage clinical development. Unfortunately, however many PDE4 and dual PDE3/4 inhibitors have failed in early development due to low therapeutic ratios. The majority of these compounds are however orally administered and non-selective for either PDE3(A, B) or PDE4(A, B, C, D) subtypes. Developing an inhaled dual PDE3/4 inhibitor with subtype specificity may represent one strategy to improve the therapeutic index. Indeed combined inhibition of PDE3 and PDE4 inhibitor has additive and synergistic anti-inflammatory and bronchodilatory effects versus inhibition of either PDE3 or PDE4 alone. Given that synergy has been seen in terms of efficacy end points, an obvious concern is that synergy may also be observed in side effects. Interestingly, however, no synergy or additive effects with a combination of a PDE3 and PDE4 inhibitor in a cardiomyocyte assay were observed. This review will summarize the rationale for developing an inhaled dual PDE3/4 inhibitor, as a treatment for chronic obstructive pulmonary disease together with recent advances in trying to understand the pathogenesis of PDE inhibitor-induced mesenteric vasculitis (a key potential dose-limiting side effect of these agents), highlighting potential early and sensitive predictive biomarkers.

Therapeutic potential of adenosine receptor antagonists and agonists.

The adenosine receptors (A(1), A(2A), A(2B) and A(3)) are important and ubiquitous mediators of cellular signalling, which play vital roles in protecting tissues and organs from damage. Launched drugs include the adenosine receptor antagonists theophylline and doxofylline (both used as bronchodilators in respiratory disorders such as asthma), while several compounds are presently in clinical trials for a range of indications, including heart failure, Parkinson's disease, rheumatoid arthritis, cancer, pain and chronic obstructive pulmonary disease. A host of companies and institutions are addressing the huge potential for the development of selective adenosine receptor agonists and antagonists, so that it appears we are on the verge of a new wave of compounds approaching the market for many unmet medical needs. This review presents an analysis of the patenting activity in the area for 2006 and an interpretation and reflection on the developments that we can expect in the future.