The adenosine A(2B) receptor is of considerable interest as a new drug target for the treatment of asthma, inflammatory diseases, pain, and cancer. In the present study we investigated the role of the cysteine residues in the extracellular loop 2 (ECL2) of the receptor, which is particularly cysteine-rich, by a combination of mutagenesis, molecular modeling, chemical and pharmacological experiments. Pretreatment of CHO cells recombinantly expressing the human A(2B) receptor with dithiothreitol led to a 74-fold increase in the EC(50) value of the agonist NECA in cyclic AMP accumulation. In the C78(3.25)S and the C171(45.50)S mutant high-affinity binding of the A(2B) antagonist radioligand [(3)H]PSB-603 was abolished and agonists were virtually inactive in cAMP assays. This indicates that the C3.25-C45.50 disulfide bond, which is highly conserved in GPCRs, is also important for binding and function of A(2B) receptors. In contrast, the C166(45.45)S and the C167(45.46)S mutant as well as the C166(45.45)S-C167(45.46)S double mutant behaved like the wild-type receptor, while in the C154(45.33)S mutant significant, although more subtle effects on cAMP accumulation were observed - decrease (BAY60-6583) or increase (NECA) - depending on the structure of the investigated agonist. In contrast to the X-ray structure of the closely related A(2A) receptor, which showed four disulfide bonds, the present data indicate that in the A(2B) receptor only the C3.25-C45.50 disulfide bond is essential for ligand binding and receptor activation. Thus, the cysteine residues in the ECL2 of the A(2B) receptor not involved in stabilization of the receptor structure may have other functions.

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A three-dimensional model of the human adenosine A2B receptor was generated by means of homology modelling, using the crystal structures of bovine rhodopsin, the beta2-adrenergic receptor, and the human adenosine A2A receptor as templates. In order to compare the three resulting models, the binding modes of the adenosine A2B receptor antagonists theophylline, ZM241385, MRS1706, and PSB601 were investigated. The A2A-based model was much better able to stabilize the ligands in the binding site than the other models reflecting the high degree of similarity between A2A and A2B receptors: while the A2B receptor shares about 21% of the residues with rhodopsin, and 31% with the beta2-adrenergic receptor, it is 56% identical to the adenosine A2A receptor. The A2A-based model was used for further studies. The model included the transmembrane domains, the extracellular and the intracellular hydrophilic loops as well as the terminal domains. In order to validate the usefulness of this model, a docking analysis of several selective and nonselective agonists and antagonists was carried out including a study of binding affinities and selectivities of these ligands with respect to the adenosine A2A and A2B receptors. A common binding site is proposed for antagonists and agonists based on homology modelling combined with site-directed mutagenesis and a comparison between experimental and calculated affinity data. The new, validated A2B receptor model may serve as a basis for developing more potent and selective drugs.

Adenosine A2B receptors, which play a role in inflammation and cancer, are of considerable interest as novel drug targets. To gain deeper insights into ligand binding and receptor activation, we exchanged amino acids predicted to be close to the binding pocket. The alanine mutants were stably expressed in CHO cells and characterized by radioligand binding and cAMP assays using three structural classes of ligands: xanthine (antagonist), adenosine, and aminopyridine derivatives (agonists). Asn2827.45 and His2807.43 were found to stabilize the binding site by intramolecular hydrogen bond formation as in the related A2A receptor subtype. Trp2476.48, Val2506.51, and particularly Ser2797.42 were shown to be important for binding of nucleosidic agonists. Leu813.28, Asn1865.42, and Val2506.51 were discovered to be crucial for binding of the xanthine-derived antagonist PSB-603. Leu813.28, which is not conserved among adenosine receptor subtypes, may be important for the high selectivity of PSB-603. The N1865.42A mutant resulted in an increased potency for agonists. The interactions of the non-nucleosidic agonist BAY60–6583 were different from those of the nucleosides: while BAY60–6583 appeared not to interact with Ser2797.42, its interactions with Trp2476.48 and Val2506.51 were significantly weaker compared to those of NECA. Moreover, our results discount the hypothesis of Trp2476.48 serving as a “toogle switch” because BAY60–6583 was able to activate the corresponding mutant. This study reveals distinct interactions of structurally diverse ligands with the human A2B receptor and differences between closely related receptor subtypes (A2B and A2A). It will contribute to the understanding of G protein-coupled receptor function and advance A2B receptor ligand design.

Targeting the epidermal growth factor receptors (EGFRs) with small inhibitor molecules has been validated as a potential therapeutic strategy in cancer therapy. Pyrazolo[3,4‐d]pyrimidine is a versatile scaffold that has been exploited for developing potential anticancer agents. On the basis of fragment‐based drug discovery, considering the essential pharmacophoric features of potent EGFR tyrosine kinase (TK) inhibitors, herein, we report the design and synthesis of new hybrid molecules of the pyrazolo[3,4‐d]pyrimidine scaffold linked with diverse pharmacophoric fragments with reported anticancer potential. These fragments include hydrazone, indoline‐2‐one, phthalimide, thiourea, oxadiazole, pyrazole, and dihydropyrazole. The synthesized molecules were evaluated for their anticancer activity against the human breast cancer cell line, MCF‐7. The obtained results revealed comparable antitumor activity with that of the reference drugs doxorubicin and toceranib. Docking studies were performed along with EGFR‐TK and ADMET profiling studies. The results of the docking studies showed the ability of the designed compounds to interact with key residues of the EGFR‐TK through a number of covalent and noncovalent interactions. The obtained activity of compound 25 (IC50 = 2.89 µM) suggested that it may serve as a lead for further optimization and drug development.