Solution-phase synthesis of 2,6,9-trisubstituted purines

Adamantane-Substituted Purines and Their β-Cyclodextrin Complexes: Synthesis and Biological Activity

Cyclin-dependent kinases (CDKs) play an important role in the cell-division cycle. Synthetic inhibitors of CDKs are based on 2,6,9-trisubstituted purines and are developed as potential anticancer drugs; however, they have low solubility in water. In this study, we proved that the pharmaco-chemical properties of purine-based inhibitors can be improved by appropriate substitution with the adamantane moiety. We prepared ten new purine derivatives with adamantane skeletons that were linked at position 6 using phenylene spacers of variable geometry and polarity. We demonstrated that the adamantane skeleton does not compromise the biological activity, and some of the new purines displayed even higher inhibition activity towards CDK2/cyclin E than the parental compounds. These findings were supported by a docking study, which showed an adamantane scaffold inside the binding pocket participating in the complex stabilisation with non-polar interactions. In addition, we demonstrated that β-cyclodextrin (CD) increases the drug’s solubility in water, although this is at the cost of reducing the biochemical and cellular effect. Most likely, the drug concentration, which is necessary for target engagement, was decreased by competitive drug binding within the complex with β-CD.

Promising 2,6,9-Trisubstituted Purine Derivatives for Anticancer Compounds: Synthesis, 3D-QSAR, and Preliminary Biological Assays

We designed, synthesized, and evaluated novel 2,6,9-trisubstituted purine derivatives for their prospective role as antitumor compounds. Using simple and efficient methodologies, 31 compounds were obtained. We tested these compounds in vitro to draw conclusions about their cell toxicity on seven cancer cells lines and one non-neoplastic cell line. Structural requirements for antitumor activity on two different cancer cell lines were analyzed with SAR and 3D-QSAR. The 3D-QSAR models showed that steric properties could better explain the cytotoxicity of compounds than electronic properties (70% and 30% of contribution, respectively). From this analysis, we concluded that an arylpiperazinyl system connected at position 6 of the purine ring is beneficial for cytotoxic activity, while the use of bulky systems at position C-2 of the purine is not favorable. Compound 7h was found to be an effective potential agent when compared with a currently marketed drug, cisplatin, in four out of the seven cancer cell lines tested. Compound 7h showed the highest potency, unprecedented selectivity, and complied with all the Lipinski rules. Finally, it was demonstrated that 7h induced apoptosis and caused cell cycle arrest at the S-phase on HL-60 cells. Our study suggests that substitution in the purine core by arylpiperidine moiety is essential to obtain derivatives with potential anticancer activity.

New 2,6,9-trisubstituted purine derivatives as Bcr-Abl and Btk inhibitors and as promising agents against leukemia

Bcr-Abl and Btk kinases are among the targets that have been considered for the treatment of leukemia. Therefore, several strategies have focused on the use of inhibitors as chemotherapeutic tools to treat these types of leukemia, such as imatinib (for Bcr-Abl) or ibrutinib (for Btk). However, the efficacy of these drugs has been reduced due to resistance mechanisms, which have motivated the development of new and more effective compounds. In this study, we designed, synthesized and evaluated 2,6,9-trisubstituted purine derivatives as novel Bcr-Abl and Btk inhibitors. We identified 5c and 5d as potent inhibitors of both kinases (IC₅₀ values of 40 nM and 0.58/0.66 μM for Abl and Btk, respectively). From docking and QSAR analyses, we concluded that fluorination of the arylpiperazine system is detrimental to the activity against two kinases, and we also validated our hypothesis that the substitution on the 6-phenylamino ring is important for the inhibition of both kinases. In addition, our studies indicated that most compounds could suppress the proliferation of leukemia and lymphoma cells (HL60, MV4-11, CEM, K562 and Ramos cells) at low micromolar concentrations in vitro. Finally, we preliminarily demonstrated that 5c inhibited the downstream signaling of both kinases in the respective cell models. Therefore, 5c or 5d possessed potency to be further optimized as anti-leukemia drugs by simultaneously inhibiting the Bcr-Abl and Btk kinases.

N-Benzyl-9-isopropyl-9H-purin-6-amine

The asymmetric unit of the title compound, C₁₅H₁₇N₅, consists of two mol­ecules in which the dihedral angles between the best planes of the purine ring system (r.m.s. deviations = 0.0060 and 0.0190 Å) and the benzene ring are 89.21 (3) and 82.14 (4)°. The mol­ecules within the asymmetric unit are linked into dimers by pairs of N—H⋯N hydrogen bonds. Weak C—H⋯π contacts and π–π inter­actions [centroid–centroid = 3.3071 (1) Å] further connect the mol­ecules into a three-dimensional network.

2-Chloro-N-ethyl-9-isopropyl-9H-purin-6-amine

In the title compound, C(10)H(14)ClN(5), the purine ring system is essentially planar, with an r.m.s. deviation from the least-squares plane defined by the nine constituent atoms of 0.0063 (11) Å. In the crystal, mol-ecules are linked by weak N-H⋯N and C-H⋯π inter-actions.

Design, synthesis and structure-activity relationship (SAR) studies of 2,4-disubstituted pyrimidine derivatives: dual activity as cholinesterase and Aβ-aggregation inhibitors

A novel class of 2,4-disubstituted pyrimidines (7a-u, 8a-f, 9a-e) that possess substituents with varying steric and electronic properties at the C-2 and C-4 positions, were designed, synthesized and evaluated as dual cholinesterase and amyloid-β (Aβ)-aggregation inhibitors. In vitro screening identified N-(naphth-1-ylmethyl)-2-(pyrrolidin-1-yl)pyrimidin-4-amine (9a) as the most potent AChE inhibitor (IC(50)=5.5 μM). Among this class of compounds, 2-(4-methylpiperidin-1-yl)-N-(naphth-1-ylmethyl)pyrimidin-4-amine (9e) was identified as the most potent and selective BuChE inhibitor (IC(50)=2.2 μM, selectivity index=11.7) and was about 5.7-fold more potent compared to the commercial, approved reference drug galanthamine (BuChE IC(50)=12.6 μM). In addition, the selective AChE inhibitor N-benzyl-2-(4-methylpiperazin-1-yl)pyrimidin-4-amine (7d), exhibited good inhibition of hAChE-induced aggregation of Aβ(1-40) fibrils (59% inhibition). Furthermore, molecular modeling studies indicate that a central pyrimidine ring serves as a suitable template to develop dual inhibitors of cholinesterase and AChE-induced Aβ aggregation thereby targeting multiple pathological routes in AD.

Synthesis of purin-2-yl and purin-6-yl-aminoglucitols as C-nucleosidic ATP mimics and biological evaluation as FGFR3 inhibitors

Two new series of C-nucleosidic ATP mimics have been synthesized using an efficient and versatile synthetic pathway. These compounds were designed as FGFR3 inhibitors using purine as a central scaffold. The two substituents, a polyhydroxylated ribose mimic and a lipophilic moiety, were linked either in position 2 or 6 of the purine ring in order to explore any possible binding mode. All the compounds were able to inhibit FGFR3 kinase activity at a concentration of 50 μM. Unexpectedly, the best inhibitor was found to be one of the synthetic intermediates 13 bearing an iodine atom in position 2. Docking studies have confirmed its location in the ATP binding site and revealed halogen bonding among key interactions.

2-Chloro-9-isopropyl-N,N-dimethyl-9H-purin-6-amine

In the title compound, C(10)H(14)ClN(5), the imidazole and pyrimidine rings are essentially planar [maximum deviation = 0.0013 (14) and 0.0207 (13) Å, respectively]. In the crystal, the mol-ecules are linked by weak C-H⋯N inter-actions into chains parallel to the c axis and the crystal packing is stabilized by additional weak C-H⋯N and C-H⋯Cl inter-actions.

(1-Adamantyl){4-[(2-chloro-9-isopropyl-9H-purin-6-yl)aminomethyl]phenyl}methanone trichloromethane solvate

In the title compound, C26H30ClN5O·CHCl3, the purine molecule consists of essentially planar benzene and purine ring systems [maximum deviation 0.010 (4) Å for both ring systems] forming a dihedral angle of 85.52 (9)°. Intermolecular N—H...N hydrogen bonds link adjacent molecules into centrosymmetric dimers. The structure also contains intermolecular C—H...O and C—H...N interactions. The benzene rings form offset face-to-face π–π stacking interactions with an interplanar distance of 3.541 (4) Å and a centroid-to-centroid distance of 4.022 (4) Å.

Novel potent inhibitors of A. thaliana cytokinin oxidase/dehydrogenase

The synthesis of a new group of 2-X-6-anilinopurines, including compounds with potential cytokinin-like activities, with various substitutions (X=H, halogen, amino, methylthio or nitro) on the phenyl ring is described. The prepared compounds have been characterized using standard physico-chemical methods, and the influence of individual substituents on biological activity has been compared in three different bioassays, based on the stimulation of tobacco callus growth, retention of chlorophyll in excised wheat leaves and the dark induction of betacyanin synthesis in Amaranthus cotyledons. The biological activity of the prepared compounds was also assessed in receptor assays, in which the ability of the compounds to activate the cytokinin receptors AHK3 and AHK4/CRE1 was studied. Finally, the interactions of the compounds with the Arabidopsis cytokinin oxidase/dehydrogenase AtCKX2 (heterologously expressed) were investigated. Systematic testing led to the identification of two very potent inhibitors of AtCKX2: 2-chloro-6-(3-methoxyphenyl)aminopurine and 2-fluoro-6-(3-methoxyphenyl)aminopurine.

Selective Amidation of 2,6-Dihalogenopurines: Application to the Synthesis of New 2,6,9-Trisubstituted Purines

We report herein the palladium(0)/Xantphos-catalyzed cross-coupling of various amides with 2,6-dihalogenopurines, with substituent-dependent regioselectivity. Furthermore, subjecting the same 2,6-dihalogenopurines to SNAr conditions with amide/NaH in DMF leads to inverted regioselectivity albeit in lower yield. These methodologies allow the two-step synthesis of new 2,6,9-trisubstituted purines from readily available 2,6-dihalogenopurines.

Cassette dosing pharmacokinetics of a library of 2,6,9-trisubstituted purine cyclin-dependent kinase 2 inhibitors prepared by parallel synthesis

Determination of pharmacokinetic properties in the intact animal remains a major bottleneck in drug discovery. Cassette dosing involves administration of a cocktail of drugs to individual animals. Here we describe the cassette dosing properties of a 107-membered library of 2,6,9-trisubstituted purine cyclin-dependent kinase 2 (CDK2) inhibitors. A three-step parallel synthesis approach produced compounds with purity ranging from 63% to 100%. Cassette dosing was validated by comparing the pharmacokinetic parameters obtained following i.v. administration of a mixture of olomoucine, R-roscovitine (CYC202), and bohemine, each at 16.6 mg/kg, with results for administration of single agents at 50 mg/kg. No significant difference was observed between the pharmacokinetic parameters of agents when dosed in combination compared with those of individual compounds. CYC202 showed the highest area under the curve (AUC) and the longest elimination half-life (t1/2). Further cassettes evaluated the library of trisubstituted purines with CYC202 and purvalanol A included as pharmacokinetic standards in a validated limited sampling strategy. The ratios of pharmacokinetic parameters to that of CYC202 [AUC, maximum concentration (Cmax), and t1/2] remained similar when compounds were tested in two different cassettes or as individual compounds. Following dosing of the same cassette on three different days, there was less than 20% variation in pharmacokinetic parameters between days. The structure-pharmacokinetics relationship showed that the favored purine substituents are benzylamine and veratrylamine at position 6, amino-2 propanol at position 2, and methylpropyl or hydroxyethyl at position 9. Without cassette dosing, this study would have used 3 times as many animals and would have taken 4 times longer, illustrating the power of this method in lead optimization.

Recycling and reuse of a polymer-supported scavenger for amine sequestration

The regeneration and reuse of a supported scavenger 1 for amine sequestration has been achieved up to three times without significant loss of activity. The scavenging process between the aldehyde resin 1 and a range of amines has been investigated in detail to determine the scope of this scavenger. Its application for the rapid purification of a small library of secondary amines 7a-j has also been demonstrated, and it has been shown that the large excess of scavenger resin used can be recovered and recycled, making this a more cost-effective process.

Novel strategies for solid-phase construction of small-molecule combinatorial libraries

During the past decade we witnessed a rapid advance in the new field of chemical science, combinatorial chemistry. The pharmaceutical industries invested heavily in accelerating the development of this new technology. As a result, it has become an extremely important tool in lead identification and optimization in current pharmaceutical research. It also quickly crossed the boundaries of the original chemical discipline and demonstrated great potential in many other important areas, such as searching for novel and highly efficient catalysts and superconductive material. Researchers from both academic and industrial laboratories have directed great effort towards the development of novel strategies for combinatorial synthesis.

Synthesis of bicyclic pyrimidine derivatives as ATP analogues

A highly efficient and general solid-phase synthesis of bicyclic pyrimidine derivatives that target purine dependent proteins is reported. The synthesis of the key intermediate, 4,6-disubstituted-5-amino-pyrimidine, involved reduction of the corresponding nitro derivatives using 1,1'-dioctyl-viologen in a triphasic milieu. The mild reduction conditions enable the use of any acid labile solid support as well as a wide range of combinatorial substituents, thus enabling the synthesis of large libraries of highly diverse bicyclic pyrimidines. Alternative reduction conditions with tin(II) chloride and structure-reactivity studies are discussed as well.

Solid-phase synthesis of 2,4-diaminoquinazolines

A highly efficient and versatile solid-phase synthesis of 2,4-diaminoquinazoline library from 2,4-dichloroquinazolines and amines using 3,5-dimethoxy 4-formylphenoxy-polystyrene resin is described.

Royal Society of Chemistry and The Biochemical Society's international symposium on functional genomics: from gene to commercial reality? September 19-22, 1999, Ambleside, UK

All aspects of genomics were covered during the meeting from gene identification and characterization to exploitation of genomic targets. Speakers highlighted the strategies used together with recent examples, indicating the advantages together with the disadvantages of the methodologies used.

ATP site-directed competitive and irreversible inhibitors of protein kinases

Several tyrosine and serine/threonine protein kinases have emerged in the last few years as attractive targets in the search for new therapeutic agents being applicable in many different disease indications. Initially, inhibition of these protein kinases by ATP site-directed inhibitors was considered less prone to success, but medicinal chemists from both academia and industry have been able to impart potency and selectivity to a limited number of scaffolds by modulating and fine-tuning the interactions of the modified template with the ATP binding site of the selected kinase. The chemical templates that have been used in the synthesis of ATP site-directed protein kinase inhibitors are reviewed with emphasis on the kinase inhibitors that have entered or are about to enter clinical trials. Examples have been selected to illustrate how structure-based design approaches and new methods to increase compound diversity have had an impact on this area of research.

Synthesis and application of functionally diverse 2,6,9-trisubstituted purine libraries as CDK inhibitors

BACKGROUND

Purines constitute a structural class of protein ligands involved in mediating an astonishing array of metabolic processes and signal pathways in all living organisms. Synthesis of purine derivatives targeting specific purine-binding proteins in vivo could lead to versatile lead compounds for use as biological probes or drug candidates.

RESULTS

We synthesized several libraries of 2,6, 9-trisubstituted purines using both solution- and solid-phase chemistry, and screened the compounds for inhibition of cyclin-dependent kinase (CDK) activity and human leukemic cell growth. Lead compounds were optimized by iterative synthesis based on structure-activity relationships (SARs), as well as analysis of several CDK-inhibitor cocrystal structures, to afford several interesting compounds including one of the most potent CDK inhibitors known to date. Unexpectedly, some compounds with similar CDK inhibitory activity arrested cellular proliferation at distinctly different phases of the cell cycle and another inhibitor directly induced apoptosis, bypassing cell-cycle arrest. Some of these compounds selectively inhibited growth of cells derived from specific tumors.

CONCLUSIONS

2,6,9-Trisubstituted purines have various and potent biological activities, despite high concentrations of competing endogenous purine ligands in living cells. Purine libraries constitute a versatile source of small molecules that affect distinct biochemical pathways mediating different cellular functions.

2,6,9-trisubstituted purines: optimization towards highly potent and selective CDK1 inhibitors

Novel 2,6,9-substituted purine derivatives represent a class of potent and selective inhibitors of CDK1/cyclinB. The synthesis, SAR and biological profile of selected compounds are described.