Cytotoxic activity of 6-alkynyl- and 6-alkenylpurines

2-Arylamino-6-ethynylpurines are cysteine-targeting irreversible inhibitors of Nek2 kinase

Renewed interest in covalent inhibitors of enzymes implicated in disease states has afforded several agents targeted at protein kinases of relevance to cancers. We now report the design, synthesis and biological evaluation of 6-ethynylpurines that act as covalent inhibitors of Nek2 by capturing a cysteine residue (Cys22) close to the catalytic domain of this protein kinase. Examination of the crystal structure of the non-covalent inhibitor 3-((6-cyclohexylmethoxy-7H-purin-2-yl)amino)benzamide in complex with Nek2 indicated that replacing the alkoxy with an ethynyl group places the terminus of the alkyne close to Cys22 and in a position compatible with the stereoelectronic requirements of a Michael addition. A series of 6-ethynylpurines was prepared and a structure activity relationship (SAR) established for inhibition of Nek2. 6-Ethynyl-N-phenyl-7H-purin-2-amine [IC50 0.15 μM (Nek2)] and 4-((6-ethynyl-7H-purin-2-yl)amino)benzenesulfonamide (IC50 0.14 μM) were selected for determination of the mode of inhibition of Nek2, which was shown to be time-dependent, not reversed by addition of ATP and negated by site directed mutagenesis of Cys22 to alanine. Replacement of the ethynyl group by ethyl or cyano abrogated activity. Variation of substituents on the N-phenyl moiety for 6-ethynylpurines gave further SAR data for Nek2 inhibition. The data showed little correlation of activity with the nature of the substituent, indicating that after sufficient initial competitive binding to Nek2 subsequent covalent modification of Cys22 occurs in all cases. A typical activity profile was that for 2-(3-((6-ethynyl-9H-purin-2-yl)amino)phenyl)acetamide [IC50 0.06 μM (Nek2); GI50 (SKBR3) 2.2 μM] which exhibited >5-10-fold selectivity for Nek2 over other kinases; it also showed > 50% growth inhibition at 10 μM concentration against selected breast and leukaemia cell lines. X-ray crystallographic analysis confirmed that binding of the compound to the Nek2 ATP-binding site resulted in covalent modification of Cys22. Further studies confirmed that 2-(3-((6-ethynyl-9H-purin-2-yl)amino)phenyl)acetamide has the attributes of a drug-like compound with good aqueous solubility, no inhibition of hERG at 25 μM and a good stability profile in human liver microsomes. It is concluded that 6-ethynylpurines are promising agents for cancer treatment by virtue of their selective inhibition of Nek2.

Synthesis and anticancer activity of multisubstituted purines and xanthines with one or two propynylthio and aminobutynylthio groups

A synthesis of new 2,6-disubstituted and 2,6,8-trisubstituted 7-methylpurines as well as 8-substituted 3,7-dimethylxanthines containing a triple bond chain have been worked out. Purinethiones and xanthinethiones were converted into propynylthio derivatives, which were then further transformed via a Mannich reaction into aminobutynylthio derivatives (amine = pyrrolidine, piperidine, morpholine, and diethylamine). The products thus obtained represent various types of the purine and xanthine structure: 8-mono-, 2,6- and 6,8-dipropynylthio, 6- and 8-monoaminobutynylthio, 2,6- and 6,8-diaminobutynylthio derivatives. All of these compounds were tested for their anticancer activity against human glioblastoma SNB-19, human adenocarcinoma MDA-MB-231, and melanoma C-32 cell lines. The anticancer activity depends on the nature of the substituent and its localization in the purine and xanthine framework. Generally, compounds possessing two alkynylthio groups (propynylthio or aminobutynylthio) were more active than those possessing only one group. Some compounds exhibited stronger or similar anticancer activity to cisplatin. All compounds were also tested for their cytotoxic activity against normal human fibroblasts (HFF-1). The most promising anticancer compounds were found to be 2,6-dipropynylthio-7-methylpurine 4, 2-chloro-6,8-dipropynylthio-7-methylpurine 14, and 2-chloro-6,8-di(N-morpholinylbutynylthio)-7-methylpurine 15c acting selectively on glioblastoma SNB-19, melanoma C-32, and adenocarcinoma MDA-MB-231 with the IC₅₀ = 0.07-4.08 μg/mL.

Regioselectivity of the C-metalation of 6-furylpurine: importance of directing effects

We report the C-metalation of 6-furylpurine with Pt(2+), Pd(2+), and Hg(2+). The ligand binds the metal ions in a bidentate fashion, involving the N7 purine atom and one of the furyl carbon atoms. The regioselectivity is determined by the metal ion. Pt(2+) and Pd(2+) coordinate the furyl moiety in its β position and Hg(2+) in its electronically preferred α position.

Synthesis and anticancer activity of thiosubstituted purines

New thiopurines with the propargylthio, pyrrolidinobutynylthio, sulfenamide, and sulfonamide groups in the pyrimidine ring were synthesized. The anticancer activity of these compounds and previously obtained 2- or 6-substituted azathioprine analogs and dialkylaminoalkylthiopurines were tested in vitro against three cell lines: glioblastoma SNB-19, melanoma C-32, and human ductal breast epithelial tumor T47D. 2-Chloro-7-methyl-6-pyrrolidinobutynylthiopurine (5b) was the most potent compound against SBN-19 and C-32 cell lines with the activity similar to cisplatin (EC₅₀ = 5.00 and 7.58 μg/ml, respectively). The dialkylaminoalkylthio derivatives (4b, 4c, 4e, and 4f) showed good activity against SBN-19 cell line (EC₅₀ < 10 μg/ml). The azathioprine analogs (2a, 2b, and 3a) were more active than azathioprine against SBN-19 and C-32 cell lines. The sulfenamide and sulfonamide derivatives of purine were very weak active against tested cell lines. All studied thiopurines were less toxic than cisplatin.

Efficient Pd-catalyzed coupling of tautomerizable heterocycles with terminal alkynes via C-OH bond activation using PyBrOP

The direct alkynylation of tautomerizable heterocylcles is described via a two-step process involving in situ C-OH activation with bromotripyrrolidinophosphonium hexafluorophosphate (PyBrOP) followed by Sonogashira coupling with a wide range of alkyl or aryl terminal alkynes using a copper-free system employing PdCl(2)(CH(3)CN)(2) and 2-(dicyclohexylphosphino)biphenyl.

S(N)Ar displacements with 6-(fluoro, chloro, bromo, iodo, and alkylsulfonyl)purine nucleosides: synthesis, kinetics, and mechanism1

SNAr reactions with 6-(fluoro, chloro, bromo, iodo, and alkylsulfonyl)purine nucleosides and nitrogen, oxygen, and sulfur nucleophiles were studied. Pseudo-first-order kinetics were measured with 6-halopurine compounds, and comparative reactivities were determined versus a 6-(alkylsulfonyl)purine nucleoside. The displacement reactivity order was: F > Br > Cl > I (with BuNH2/MeCN), F > Cl approximately Br > I (with MeOH/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/MeCN), and F > Br > I > Cl [with K+ -SCOCH3/dimethyl sulfoxide (DMSO)]. The order of reactivity with a weakly basic arylamine (aniline) was: I > Br > Cl > F (with 5 equiv of aniline in MeCN at 70 degrees C). However, those reactions with aniline were autocatalytic and had significant induction periods ( approximately 50 min for the iodo compound and approximately 6 h for the fluoro analogue). Addition of trifluoroacetic acid (TFA) eliminated the induction period, and the order then was F > I > Br > Cl (with 5 equiv of aniline and 2 equiv of TFA in MeCN at 50 degrees C). The 6-(alkylsulfonyl)purine nucleoside analogue was more reactive than the 6-fluoropurine compound with both MeOH/DBU/MeCN and iPentSH/DBU/MeCN and was more reactive than the Cl, Br, and I compounds with BuNH2 and aniline/TFA. Titration of the 6-halopurine nucleosides in CDCl3 with TFA showed progressive downfield 1H NMR chemical shifts for H8 (larger) and H2 (smaller). The major site of protonation as N7 for both the 6-fluoro and 6-bromo analogues was confirmed by large upfield shifts ( approximately 16 ppm) of the 15N NMR signal for N7 upon addition of TFA (1.6 equiv). Mechanistic considerations and resolution of prior conflicting results are presented.

Reductive Dimerization of 2- and 6-Iodopurines: Side Reaction in Pd-Catalyzed Cross-Coupling of Iodopurines

In the presence of a Pd catalyst and a base, 6- and 2-iodopurine derivatives undergo reductive C-C dimerization with the formation of the corresponding 6,6'- or 2,2'-dimers. The best results of the dimerization were obtained in the presence of i-Pr2NEt as a base in DMF. Phosphine-free catalysts as well as catalysts containing phosphines can be used. In the presence of catalytic systems containing PPh3 the dimerization does not proceed. This dimerization may become an important side reaction in the Stille or the Suzuki-Miyaura reactions of iodopurines.

Preparation of Highly Substituted 6-Arylpurine Ribonucleosides by Ni-Catalyzed Cyclotrimerization. Scope of the Reaction

Transition metal complex catalyzed cocyclotrimerization of protected alkynylpurine ribonucleosides 1 with various diynes 2 gave rise to a series of 6-arylpurine nucleosides 3 that were further deprotected to free nucleosides 4. Generally, the best yields of cyclotrimerizations were obtained with a catalytic system Ni(cod)2/2PPh(3). On the other hand, CoBr(PPh(3))3 proved to be a superior catalyst for cyclotrimerization of 1 with dipropargyl ether 2g. In addition, Ni catalysis is also suitable for direct cyclotrimerization of unprotected alkynylpurine ribonucleosides 5 to the corresponding 6-arylpurinylribosides 4.

Oncology exploration: charting cancer medicinal chemistry space

Approaches for the experimental determination of protein-ligand molecular interactions are reliant on the quality of the compounds being tested. The application of large, randomly designed combinatorial libraries has given way to the creation of more-focused 'drug-like' libraries. Prior to synthesis, we wish to screen the potential compounds to remove undesired chemical moieties and to be within a required range of physiochemical properties. We have used a principal-component analysis (PCA) computational approach to analyze the 3D descriptor space of active and non-active (hit-like) cancer medicinal chemistry compounds. We define hit-like those molecules passing the unmodified OpenEye FILTER program. Our analysis indicates that these compounds occupy quite different regions in space. Cancer-active compounds exist in a much greater volume of space than generic hit-like space and most of them fail the commonly applied filters for orally bioavailable drugs. This is of great significance when designing orally bioavailable cancer target drugs.

Selective anti-tubercular purines: Synthesis and chemotherapeutic properties of 6-aryl- and 6-heteroaryl-9-benzylpurines

6-Aryl- and 6-heteroaryl-9-benzylpurines have been synthesized employing palladium-catalyzed coupling reactions in the step forming the C-C or C-N bond between the aryl- or heteroaryl and the purine. The compounds were screened for activity against Mycobacterium tuberculosis as well as representative Gram+ and Gram- bacteria, and for cytotoxic effects on mammalian cells. Several potent antimycobacterials were identified. These compounds probably act by a novel and selective mechanism; they exhibit low toxicity toward other bacteria as well as mammalian cells.

6-Substituted Purines as Inhibitors of 15-Lipoxygenase; a Structure-Activity Study

15-lipoxygenase (15-LO) has been implicated in oxidation of low-density lipoproteins (LDL), a process believed to be important for the development of atherosclerosis, as well as other pathogenic conditions. Potent and selective inhibitors of 15-LO may have a drug potential. In this study, purines with a variety of substituents have been examined as inhibitors of 15-lipoxygenase (15-LO) from soybeans. Several 6-substitued purines where the purine ring and a phenyl ring in the substituent were separated by a "spacer" were synthesized and their ability to inhibit the enzyme was explored. Sepa ration of the purine and the phenyl rings with none, one or two sp3-carbons resulted in essentially inactive compounds, trans-styrylpurines and phenylethynylpurines, on the other hand, they exhibited activity close to the well-known 15-LO inhibitor quercetin. High activity was also found when the "spacer" was a trans-cyclopropyl ring. The shape of the spacer was important; a corresponding cis-cyclopropylpurine exhibited much less affinity for the enzyme. Only minor differences in inhibitory activity against 15-LO were found regardless of whether an N-substituent was situated on N-9 or N-7, even when the N-substituent was relatively large. Also, a variety of substituents in the purine 2- and 8-position were well tolerated.

SNAr Iodination of 6-Chloropurine Nucleosides: Aromatic Finkelstein Reactions at Temperatures Below −40 °C1

Mesitoyl or toluoyl esters of inosine and 2'-deoxyinosine were deoxychlorinated at C6 to give the crystalline 6-chloropurine nucleoside derivatives, which underwent quantitative conversion to the 6-iodo analogues with NaI/TFA/butanone at -50 to -40 degrees C. The 6-iodo compounds were efficient substrates for SNAr, Sonogashira, and Suzuki-Miyaura reactions, in contrast with the 6-chloro analogues, and gave good to high yields of C-N and C-C coupled products.