James R.
Woods
a,
Huaping
Mo
ab,
Andrew A.
Bieberich
a,
Tanja
Alavanja
c and
David A.
Colby
*ac
aDepartment of Medicinal Chemistry and Molecular Pharmacology, Purdue University, West Lafayette, IN 47907, USA
bInterdepartmental NMR Facility, Purdue University, West Lafayette, IN 47907, USA
cDepartment of Chemistry, Purdue University, West Lafayette, IN 47907, USA. E-mail: dcolby@purdue.edu; Fax: +1 765-494-1414; Tel: +1 765-496-3962
First published on 16th August 2012
The sesquiterpene lactone class of natural products displays a diverse array of biological activities due to the presence of the α-methylene-γ-lactone motif. However, clinical translation of this class has been hampered by poor aqueous solubility and non-selective binding as a Michael acceptor at undesired targets. A prodrug approach has been developed to overcome these problems in which an amine is added into the α-methylene-γ-lactone to mask this group from nucleophiles and increase solubility. The medicinal chemistry of amino-derivatives of the sesquiterpene lactones is described, beginning with synthetic development, moving into pharmacological applications, and finishing with clinical translation.
Fig. 1 Conversion of α-methylene-γ-lactones into amino-derivatives. |
Originally, the conversion of an α-methylene-γ-lactone into its amino-derivative was used as a method to protect the α,β-unsaturation from a subsequent hydrogenation. In 1968, the sesquiterpene lactone dehydrocostus lactone (1) was transformed into its dimethylamino-adduct to enable selective reduction of two other alkenes (Fig. 2).4N-Alkylation and subsequent pyrolysis reformed the conjugated α-methylene-γ-lactone. Although biological evaluation of the novel derivatives was not conducted, these studies were the first successful application of this strategy to the sesquiterpene lactones.
Fig. 2 Synthesis of amino-derivative of dehydrocostus lactone (1) as a protection strategy for the α,β-unsaturated enone. |
Fig. 3 Synthesis and antiproliferative activities of amino-derivatives of COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundhelenalin (2). |
The mechanism of the sesquiterpene lactones, such as COMPOUND LINKS
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Download mol file of compoundhelenalin, is typically due to addition of biological thiols to the α-methylene-γ-lactone motif (Fig. 4).8 Accordingly, complete reduction of both of the α,β-unsaturated groups of COMPOUND LINKS
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Download mol file of compoundhelenalin provides an inactive analogue (the COMPOUND LINKS
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Download mol file of compoundcyclopentenone can also serve as a reactive enone).5 The mechanism of action of COMPOUND LINKS
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Download mol file of compoundhelenalin (2) is the inhibition of the NF-κB signalling pathway and the induction of reactive oxygen species (ROS),9 likely by COMPOUND LINKS
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Download mol file of compoundglutathione binding.8
Fig. 4 Mechanism of action of α-methylene-γ-lactones by thiol trapping. |
Fig. 5 Structures of ambrosin (6) and its amino-derivatives 7–8 along with antiproliferative assay data. |
Cushman posited that conversion of the amino-derivatives back into the parent compound could be accomplished through metabolic N-oxidation of the tertiary amine followed by a Cope elimination of the corresponding N-oxide to give ambrosin 6 (Fig. 6).10 This mechanistic hypothesis, coupled with the parity of the biological data, indicated that the amino-derivatives served as water-soluble prodrugs of ambrosin 6. Indeed, a prodrug strategy involving the one-step preparation of amino-adducts with enhanced COMPOUND LINKS
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Download mol file of compoundwater solubility, yet without altered biological activity, was a powerful notion. Similar to COMPOUND LINKS
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Download mol file of compoundhelenalin2, ambrosin 6 inhibits NF-κB signalling,11 and the loss of both α,β-unsaturated groups of ambrosin 6 also provides inactive derivatives.10 Helenalin 2 and ambrosin 6 inhibit inducible nitric oxide synthase (iNOS)-dependent nitric oxide (NO) synthesis by decreasing nitrite accumulation in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages.11
Fig. 6 Conversion of the ambrosin prodrug8 into ambrosin 6. |
Fig. 7 Structures of COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundcostunolide (9) and saussureamines A–E (10–14). |
Compd | Gastric lesions | Inflammation | |
---|---|---|---|
Inhibition (%)a | Inhibition (%)b | IC50 (μg mL−1)c | |
a Inhibition of water-immersion induced lesions.
b Inhibition of HCl/COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundEtOH-induced lesions. c Inhibition of nitric oxide production in LPS-activated mouse macrophages. |
|||
Dehydrocostus lactone (1) | 19.0 | 96.2 | 1.2 |
COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundCostunolide (9) |
14.3 | 93.2 | 1.2 |
COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundSaussureamine A (10) |
57.0 | 59.8 | 2.8 |
COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundSaussureamine B (11) |
19.0 | 87.0 | 2.8 |
COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundSaussureamine C (12) |
4.7 | 68.1 | 16 |
Sixteen amino-adducts of costunolide 9 were synthesized and tested in eight different cancer cell lines.16 Typically, the amine-analogues derived from COMPOUND LINKS
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Download mol file of compounddimethylamine, pyrrolidines, or piperidines displayed significant antiproliferative activity similar to the parent 9; however, all of the compounds prepared from piperazines were inactive or weakly active. A subset of data from the active derivatives 15–19 is illustrated in Fig. 8. The COMPOUND LINKS
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Download mol file of compounddimethylamine15, the 3-hydroxypyrrolidine 17, and the 4-hydroxypiperidine 19 exhibited enhanced antiproliferative activities in MiaPaca2 (pancreatic cancer), K562 (leukemia), and PA1 (ovarian cancer) cells compared to 9. In contrast, piperidine18 displays a dramatic loss of activity compared to 9 in SW-620 (colon cancer) cells. These data demonstrated that understanding structure–activity relationships across amine-derivatives of a sesquiterpene lactone is fundamental to identifying candidate compounds for different cancers. All of the analogues were additionally screened against normal mouse fibroblast (NIH3T3) cells to establish a safety profile, and typically, the amine-adducts demonstrated less cytotoxicity than 9. Indeed, it is known from studies by Blagg and co-workers that adding a basic nitrogen atom into the structure of a cytotoxic agent can enhance selectivity for malignant cell lines.17
Fig. 8
Amine-adducts of COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundcostunolide (9) and antiproliferative activities. |
Fig. 9 Structures and antiproliferative activities of diethylamino-adducts of COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundalantolactone (20) and isoalantolactone (21). |
Fig. 10
COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundParthenolide and its amino-prodrugs 25–29 with anti-HCV effects in cellular assays. |
Following a report that COMPOUND LINKS
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Download mol file of compoundparthenolide selectively targets and initiates apoptosis in leukemia stem cells,19 Crooks and co-workers synthesized a series of amino-derivatives of COMPOUND LINKS
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Download mol file of compoundparthenolide from primary and secondary amines.21,22 The dimethylamino parthenolide 25 (also referred to as DMAPT or LC-1) was shown to have excellent oral bioavailability, superior aqueous solubility, and also the superb anti-leukemic activity of COMPOUND LINKS
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Download mol file of compoundparthenolide24.23,24 Structure–activity investigations in 60 human cancer cell lines (NCI-60) of analogues of 24 from the fourteen primary amines and three secondary amines identified compound 30 with antiproliferative activity that is superior to 24 (Fig. 11).21 Specifically, analogue 30 manifests GI50 < 0.01 μM in CCRF-CEM (lymphoblastic leukemia) cells compared to GI50 = 8.38 μM for COMPOUND LINKS
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Download mol file of compoundparthenolide24. Most other analogues displayed similar potencies to 24. Subsequent studies by Crooks et al. detailed anti-leukemic activity in AML (acute myelogenous leukemia) cells for thirty-two aminoparthenolides.22 A portion of these data are listed for analogues 25–29 for comparisons (Table 2). A larger range of cell killing was demonstrated across all of these analogues, likely due to the broad range of structures of the amines examined. DMAPT 25 was determined to be the candidate compound for subsequent advancement to pre-clinical studies and clinical studies.23–25 The anti-leukemic activity of parthenolide 24 manifests from its ability to inhibit NF-κB as well as initiate oxidative stress by increasing ROS.19 DMAPT 25 also inhibits NF-κB and causes oxidative stress.23,26 Analogous to prior structure–activity studies for other sesquiterpene lactones, reduction of the α,β-unsaturation of the α-methylene-γ-lactone of parthenolide 24 provides an inactive derivative.27
Fig. 11 Antiproliferative activity of parthenolide 24 and aminoparthenolide 30. |
Compd | % Cell deatha | LD50 (μM)b |
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a Anti-leukemic activity at 10 μM dose. b Concentration causing death of 50% of the population of primary AML cells. | ||
COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundParthenolide (24) |
84 | 1.4 |
25 | 93 | 1.7 |
26 | 60 | nd |
27 | 82 | 2.7 |
28 | 76 | 2.4 |
29 | 20 | 14 |
Preliminary stability studies determined that less than 3% of DMAPT 25 degraded to parthenolide 24 over 24 h.22 Only a small amount of COMPOUND LINKS
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Download mol file of compoundparthenolide was found in blood plasma of rats eight hours after dosing with DMAPT 25. Oral bioavailability for DMAPT 25 was 70% in rats,22 mice, and canine models.23 On the other hand, parthenolide 24 has a poor pharmacokinetic profile, and phase I clinical studies determined that 24 was undetectable in patients' plasma samples after oral dosing.28 Concentration maximum (Cmax) and half-life (T1/2) for oral dosing of DMAPT 25 (100 mg kg−1) was 25 μM and 0.63 hours in mice and 61 μM and 1.9 hours in canines.23 Parthenolide 24 displays Cmax = 200 nM in mice after a dose of 40 mg kg−1, which is the highest dose allowed due to its poor solubility. Also, parthenolide 24 is highly protein-bound, through its action as a Michael acceptor, after incubation in human plasma and human serum albumin.29 DMAPT 25 has an excellent solubility profile and is over 1000-fold more soluble in COMPOUND LINKS
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Download mol file of compoundwater relative to COMPOUND LINKS
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Download mol file of compoundparthenolide24.23,26
Fluorinated aminoparthenolides, derived from pyrrolidines and piperidines, were synthesized by Colby and co-workers for mechanistic analysis by 19F NMR.30 Six fluorinated piperidine analogues and three fluorinated pyrrolidine derivatives were made and compared to 24, 27, and 28 in an antiproliferative assay in HL-60 cells; 4-trifluoromethylpiperidine 31 was determined to have biological activity most similar to 24 (Fig. 12). Using 19F NMR in a deuterium-free environment, it was found that the fluorinated amine was released from the aminoparthenolide 31 in the presence of COMPOUND LINKS
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Download mol file of compoundglutathione, yet this effect was minimal in its absence (Fig. 13). These data support that the conversion of an aminoparthenolide to COMPOUND LINKS
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Download mol file of compoundparthenolide is promoted in the presence of COMPOUND LINKS
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Download mol file of compoundglutathione, which is normally more prevalent in malignant cells, and also that an aminoparthenolide can serve as a prodrug of COMPOUND LINKS
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Download mol file of compoundparthenolide. 19F NMR observes interesting fluorinated drug candidates and their fates under physiological conditions with almost zero background interference and minimal efforts in sample preparation.30 Using deuterium-free environments during concentration analysis by NMR is preferable when replicating processes that occur in aqueous systems.30,31 Moreover, it is particularly advantageous to apply this method when complex systems are present or when multiple metabolic events/biochemical pathways can take place. In this example, 19F NMR clearly showed the dissociation of the fluorinated amine from prodrug31 (as expected) with minimal interference.30
Fig. 12 Fluorinated aminoparthenolide 31. |
Fig. 13
19F NMR observation of the release of fluorinated amine from the fluorinated prodrug31 in the absence (left) and presence (right) of COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundglutathione. |
Fig. 14 Proposed activation of COMPOUND LINKS Read more about this on ChemSpider Download mol file of compoundβ-amino-ethyl ketone for irreversible binding at active site. |
Fig. 15
Thiol trapping by COMPOUND LINKS Read more about this on ChemSpider Download mol file of compound3-aminopropanamideinhibitor of EGFR. |
Footnote |
† This article is part of a MedChemComm ‘New Talents’ issue highlighting the work of outstanding rising scientists in medicinal chemistry research. |
This journal is © The Royal Society of Chemistry 2013 |