Synthesis, biological evaluation and SAR of 3-benzoates of ingenol for treatment of actinic keratosis and non-melanoma skin cancer

The Role of Ingenane Diterpenes in Cancer Therapy: From Bioactive Secondary Compounds to Small Molecules

Diterpenes are a class of critical taxonomic markers of the Euphorbiaceae family, representing small compounds (eg, molecules) with a wide range of biological activities and multi-target therapeutic potential. Diterpenes can exert different activities, including antitumor and multi-drug resistance-reversing activities, and antiviral, immunomodulatory, and anti-inflammatory effects, mainly due to their great structural diversity. In particular, one polycyclic skeleton has been highlighted: ingenane. Besides this natural diterpene, promising polycyclic skeletons may be submitted to chemical modification—by in silico approaches, chemical reactions, or biotransformation—putatively providing more active analogs (eg, ingenol derivatives), which are currently under pre-clinical investigation. This review outlines the current mechanisms of action and potential therapeutic implications of ingenol diterpenes as small cancer molecules.

Ingenane and jatrophane-type diterpenoids from Euphorbia kansui with multidrug resistance reversal activity

Bioassay-guided purification on the ethanolic extract of the roots of Euphorbia kansui Liou ex S.B.Ho (Euphorbiaceae) led to the isolation of one unreported ingenane-type (euphorksol A) and six unreported jatrophane-type (euphorksjats A-F) diterpenoids, together with twenty-five known diterpenoids. Their structures were elucidated based on extensive NMR analysis and high-resolution mass spectrometry. Euphorksol A is a rare example of an ingenane-type diterpenoid with a 6,7-expoxy fragment. All compounds were examined for cytotoxicity against adriamycin (Adr)-sensitive HepG-2 and Adr-resistant HepG-2/Adr cell lines, but none showed significant activity. Then, all isolates were evaluated for their ability to reverse multidrug resistance (MDR). 6β,7β-Epoxy-3β,4β,5β-trihydroxyl-20- deoxyingenol and 3,5,7,15-tetraacetoxy-9-nicotinoyloxy-14-oxojatropha-6(17),11-diene showed significant MDR reversal activity in HepG-2/Adr cells (reversal fold: RF = 186.4 and 143.8, respectively) versus the positive control verapamil (Ver, RF = 93.7). Euphorksol A and kansuinin B exhibited moderate MDR reversal activity (RF = 57.4 and 68.9, respectively). These compounds are the first ingenane-type diterpenoids reported to show MDR reversal activity, which will provide new insights toward the development of MDR regulatory agents.

Ingol and Ingenol-Type Diterpenes from Euphorbia trigona Miller with Keratinocyte Inhibitory Activity

Ingenol mebutate, isolated from Euphorbia peplus, is an ingenane-type diterpenoid, primarily used for the topical treatment of actinic keratosis, a premalignant skin condition. The aim of our work was to investigate other Euphorbia species to find structurally similar diterpenes that can be used as alternatives to ingenol mebutate. Pharmacological investigation of Euphorbia candelabrum, Euphorbia cotinifolia, Euphorbia ramipressa, and Euphorbia trigona revealed the potent keratinocyte (HPV-Ker cell line) inhibitory activity of these spurge species. From the methanolic extract of the aerial parts of Euphorbia trigona Miller, the most active species, five ingol (1–5) and four ingenane-type diterpenoids (6–9) were isolated by various chromatographic separation techniques, including open column chromatography, vacuum liquid chromatography, thin-layer chromatography, and high-performance liquid chromatography. The structures of the compounds were determined by NMR spectroscopic analysis and by comparison of the assignations with the literature data. The cytotoxic activity of the compounds against keratinocytes was tested in vitro by using ingenol mebutate as a positive control. Among the isolated compounds, two ingenane derivatives (6 and 7) exhibited remarkably stronger cytotoxic activity (IC₅₀ values 0.39 μM and 0.32 μM, respectively) on keratinocytes than ingenol mebutate (IC₅₀ value 0.84 μM). These compounds could serve as starting materials for further investigations to find alternatives to Picato^® (with active substance ingenol mebutate), which was withdrawn from marketing authorization in the European Union.

Prodrugs of PKC modulators show enhanced HIV latency reversal and an expanded therapeutic window

AIDS is a pandemic disease caused by HIV that affects 37 million people worldwide. Current antiretroviral therapy slows disease progression but does not eliminate latently infected cells, which resupply active virus, thus necessitating lifelong treatment with associated compliance, cost, and chemoexposure issues. Latency-reversing agents (LRAs) activate these cells, allowing for their potential clearance, thus presenting a strategy to eradicate the infection. Protein kinase C (PKC) modulators-including prostratin, ingenol esters, bryostatin, and their analogs-are potent LRAs in various stages of development for several clinical indications. While LRAs are promising, a major challenge associated with their clinical use is sustaining therapeutically meaningful levels of the active agent while minimizing side effects. Here we describe a strategy to address this problem based on LRA prodrugs, designed for controllable release of the active LRA after a single injection. As intended, these prodrugs exhibit comparable or superior in vitro activity relative to the parent compounds. Selected compounds induced higher in vivo expression of CD69, an activation biomarker, and, by releasing free agent over time, significantly improved tolerability when compared to the parent LRAs. More generally, selected prodrugs of PKC modulators avoid the bolus toxicities of the parent drug and exhibit greater efficacy and expanded tolerability, thereby addressing a longstanding objective for many clinical applications.

A Computational Investigation of the Uncatalysed and Water-Catalysed Acyl Rearrangements in Ingenol Esters

Ingenol esters have been identified as potent anticancer and HIV latency reversing agents. Ingenol-3-angelate was recently approved as a topical treatment for precancerous actinic keratosis skin lesions. It was found, however, that ingenol esters can undergo a series of acyl rearrangements, which may affect their biological potency and the shelf-life of drug formulations. We use double-hybrid density functional theory to explore the mechanisms for the uncatalysed and water-catalysed acyl migrations in a model ingenol ester. The uncatalysed reaction may proceed either via a concerted mechanism or via a stepwise mechanism that involves a chiral orthoester intermediate. We find that the stepwise pathway is kinetically preferred by a significant amount of ΔΔH‡298 = 44.5 kJ mol−1. The uncatalysed 3-O-acyl to 5-O-acyl and 5-O-acyl to 20-O-acyl stepwise rearrangements involve cyclisation and ring-opening steps, both concomitant with a proton transfer. We find that the ring-opening step is the rate-determining step for both rearrangements, with reaction barrier heights of ΔH‡298 = 251.6 and 177.1 kJ mol−1 respectively. The proton transfers in the cyclisation and ring-opening steps may be catalysed by a water molecule. The water catalyst reduces the reaction barrier heights of these steps by over 90 kJ mol−1.

Chemical Proteomics Identifies SLC25A20 as a Functional Target of the Ingenol Class of Actinic Keratosis Drugs

The diterpenoid ester ingenol mebutate (IngMeb) is the active ingredient in the topical drug Picato, a first-in-class treatment for the precancerous skin condition actinic keratosis. IngMeb is proposed to exert its therapeutic effects through a dual mode of action involving (i) induction of cell death that is associated with mitochondrial dysfunction followed by (ii) stimulation of a local inflammatory response, at least partially driven by protein kinase C (PKC) activation. Although this therapeutic model has been well characterized, the complete set of molecular targets responsible for mediating IngMeb activity remains ill-defined. Here, we have synthesized a photoreactive, clickable analogue of IngMeb and used this probe in quantitative proteomic experiments to map several protein targets of IngMeb in human cancer cell lines and primary human keratinocytes. Prominent among these targets was the mitochondrial carnitine-acylcarnitine translocase SLC25A20, which we show is inhibited in cells by IngMeb and the more stable analogue ingenol disoxate (IngDsx), but not by the canonical PKC agonist 12-O-tetradecanoylphorbol-13-acetate (TPA). SLC25A20 blockade by IngMeb and IngDsx leads to a buildup of cellular acylcarnitines and blockade of fatty acid oxidation (FAO), pointing to a possible mechanism for IngMeb-mediated perturbations in mitochondrial function.

Ingenol Disoxate: A Novel 4-Isoxazolecarboxylate Ester of Ingenol with Improved Properties for Treatment of Actinic Keratosis and Other Non-Melanoma Skin Cancers

Introduction

Ingenol mebutate gel (Picato^®, LEO Pharma A/S) is approved for the field treatment of actinic keratosis and is characterized by high sustained clearance of actinic lesions. The inherent propensity of ingenol mebutate towards chemical rearrangement necessitates refrigeration of the final product. We sought to identify novel ingenol derivatives with enhanced chemical stability and similar or improved in vitro potency and in vivo efficacy.

Methods

A number of ingenol esters were synthesized with full regiocontrol from ingenol. Chemical stability was determined in aqueous buffer at physiological pH and hydroalcoholic gel at lower pH. Acute cytotoxicity was determined in HeLa or HSC-5 cells. Keratinocyte proliferation, viability and caspase 3/7 activation was measured in primary epidermal keratinocytes. Relative gene expression levels were determined by real-time quantitative PCR. Evaluation of in vivo tumor ablating potential was performed in the murine B16 melanoma mouse model and in the UV-induced skin carcinogenesis model in hairless SKH-1 mice following topical treatment for two consecutive days with test compounds formulated at 0.1% in a hydroalcoholic gel.

Results

This work resulted in the identification of ingenol disoxate (LEO 43204) displaying increased stability in a clinically relevant formulation and in aqueous buffer with minimal pH-dependent acyl migration degradation. Ingenol disoxate exhibited a significantly higher cytotoxic potency relative to ingenol mebutate. Likewise, cell growth arrest in normal human keratinocyte was more potently induced by ingenol disoxate, which was accompanied by protein kinase C dependent transcription of markers of keratinocyte differentiation. Most notably, ingenol disoxate possessed a superior antitumor effect in a B16 mouse melanoma model and significantly increased median survival time relative to ingenol mebutate. A significant effect on tumor ablation was also observed in a murine model of ultraviolet irradiation-induced skin carcinogenesis.

Conclusion

These data illustrate that the favorable in vitro and in vivo pharmacological properties driving ingenol mebutate efficacy are either preserved or improved in ingenol disoxate. In combination with improved chemical stability to potentially facilitate storage of the final product at ambient temperatures, these features support further development of ingenol disoxate as a convenient and efficacious treatment modality of non-melanoma skin cancers.

Funding

LEO Pharma A/S.

Electronic supplementary material

The online version of this article (doi:10.1007/s13555-016-0137-2) contains supplementary material, which is available to authorized users.

Ingenane Diterpenoids

Despite a more recent isolation and chemical characterization when compared to phorbol, along with its chemical instability, limited distribution in Nature, and scarce availability, ingenol is the only Euphorbia diterpenoid that has undergone successful pharmaceutical development, with ingenol 3-angelate (ingenol mebutate, Picato(®)) entering the pharmaceutical market in 2012 for the treatment of actinic keratosis. The phytochemical, chemical, and biological literature on members of the ingenane class of diterpenoids is reviewed from their first isolation in 1968 through 2015, highlighting unresolved issues both common to phorboids (biogenesis, relationship between molecular targets, and in vivo activity) and specific to ingenol derivatives (two-dimensional representation, in-out stereoisomerism, versatility of binding mode to PKC, and inconsistencies in the structural elucidation of some classes of derivatives). The biogenesis of ingenol is discussed in the light of the Jakupovic proposal of a dissection between the formation of the macrocyclic Euphorbia diterpenoids and the phorboids, and the clinical development of ingenol mebutate is chronicled in the light of its "reverse-pharmacology" focus.

C-H Oxidation of Ingenanes Enables Potent and Selective Protein Kinase C Isoform Activation

Ingenol derivatives with varying degrees of oxidation were prepared by two-phase terpene synthesis. This strategy has allowed access to analogues that cannot be prepared by semisynthesis from natural ingenol. Complex ingenanes resulting from divergent C-H oxidation of a common intermediate were found to interact with protein kinase C in a manner that correlates well with the oxidation state of the ingenane core. Even though previous work on ingenanes has suggested a strong correlation between potential to activate PKCδ and induction of neutrophil oxidative burst, the current study shows that the potential to activate PKCβII is of key importance while interaction with PKCδ is dispensable. Thus, key modifications of the ingenane core allowed PKC isoform selectivity wherein PKCδ-driven activation of keratinocytes is strongly reduced or even absent while PKCβII-driven activation of neutrophils is retained.

Biphasic Effects of Ingenol 3,20-Dibenzoate on the Erythropoietin Receptor: Synergism at Low Doses and Antagonism at High Doses

Although ingenol 3,20-dibenzoate (IDB) is known as a selective novel protein kinase C (PKC) agonist, its biologic actions and underlying mechanisms remain incompletely understood. In this study, we identified IDB as a proliferative agent for an erythropoietin (EPO)-dependent cell line, UT-7/EPO, through the screening of a natural compound library. To clarify the underlying mechanism of IDB's EPO-like activities, we thoroughly analyzed the mutual relation between EPO and IDB in terms of in vitro and in vivo activities, signaling molecules, and a cellular receptor. IDB substantially induced the proliferation of UT-7/EPO cells, but not as much as EPO. IDB also lessened the anemia induced by 5-fluorouracil in an in vivo mouse model. Interestingly, IDB showed a synergistic effect on EPO at low concentration, but an antagonistic effect at higher concentration. Physical interaction and activation of PKCs by IDB- and EPO-competitive binding of IDB to EPO receptor (EPOR) explain these synergistic and antagonistic activities, respectively. Importantly, we addressed IDB's mechanism of action by demonstrating the direct binding of IDB to PKCs, and by identifying EPOR as a novel molecular target of IDB. Based on these dual targeting properties, IDB holds promise as a new small molecule modulator of EPO-related pathologic conditions.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial as opposed to marine sources and includes, labdanes, clerodanes, pimaranes, abietanes, kauranes, gibberellins, cembranes and their cyclization products. The literature from January to December, 2014 is reviewed.

A practical deca-gram scale ring expansion of (R)-(−)-carvone to (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1

A route to enantiopure (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1, an intermediate for terpenoids, has been developed and includes a highly chemo- and regioselective Tiffeneau–Demjanov reaction.