Inhibitors of NTPDase: key players in the metabolism of extracellular purines

Identification of Novel CD39 Inhibitors Based on Virtual Screening and Enzymatic Assays

The accumulation of adenosine in the tumor microenvironment mediates immunosuppression and promotes tumor growth and proliferation. Intervention of the adenosine pathway is an important direction of antitumor immunity research. CD39 is an important ecto-nucleotidases for adenosine generation, therefore targeting the CD39-adenosine pathway is an emerging immune checkpoint for anticancer treatment. However, currently no CD39 inhibitor has been approved by the U.S. Food and Drug Administration. The development of CD39 drugs is urgent for clinical application. In this study, we combined homology modeling, virtual screening, and in vitro enzymatic activity to characterize the structural features of the CD39 protein and identify a triazinoindole-based compound as a CD39 inhibitor. The identified inhibitor and one of its analogues could effectively prevent the enzymatic activity of CD39 with IC₅₀ values of 27.42 ± 5.52 and 79.24 ± 12.21 μM, respectively. At the same time, the inhibitor significantly inhibited the adenosine monophosphate production in colorectal cancer cell lines (HT29 and MC38) and thereafter prevented cell proliferation. Molecular docking studies, mutagenesis, and microscale thermophoresis indicated that residues such as R85 could be the main contributor in binding triazinoindole compounds. The binding mode can potentially be utilized for hit-to-lead optimization, and the identified inhibitor can be further tested for its anticancer activity in vivo or may serve as a chemical agent to study CD39-related functions.

Characterization of a monoclonal antibody as the first specific inhibitor of human NTP diphosphohydrolase-3 : partial characterization of the inhibitory epitope and potential applications

The study and therapeutic modulation of purinergic signaling is hindered by a lack of specific inhibitors for NTP diphosphohydrolases (NTPDases),which are the terminating enzymes for these processes. In addition, little is known of the NTPDase protein structural elements that affect enzymatic activity and which could be used as targets for inhibitor design. In the present study, we report the first inhibitory monoclonal antibodies specific for an NTPDase, namely human NTPDase3 (EC 3.6.1.5), as assessed by ELISA, western blotting, flow cytometry, immunohistochemistry and inhibition assays. Antibody recognition of NTPDase3 is greatly attenuated by denaturation with SDS, and abolished by reducing agents, indicating the significance of the native conformation and the disulfide bonds for epitope recognition. Using site-directed chemical cleavage, the SDS-resistant parts of the epitope were located in two fragments of the C-terminal lobe ofNTPDase3 (i.e. Leu220-Cys347 and Cys347-Pro485), which are both required for antibody binding. Additional site-directed mutagenesis revealed the importance of Ser297 and the fifth disulfide bond (Cys399-Cys422) for antibody binding, indicating that the discontinuous inhibitory epitope is located on the extracellular C-terminal lobe of NTPDase3. These antibodies inhibit recombinant NTPDase3 by 60-90%, depending on the conditions. More importantly, they also efficiently inhibit the NTPDase3expressed in insulin secreting human pancreatic islet cells in situ. Because insulin secretion is modulated by extracellular ATP and purinergic receptors, this finding suggests the potential application of these inhibitory antibodies for the study and control of insulin secretion.

Selective nucleoside triphosphate diphosphohydrolase-2 (NTPDase2) inhibitors: nucleotide mimetics derived from uridine-5'-carboxamide

Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases, subtypes 1, 2, 3, 8 of NTPDases) dephosphorylate nucleoside tri- and diphosphates to the corresponding di- and monophosphates. In the present study we synthesized adenine and uracil nucleotide mimetics, in which the phosphate residues were replaced by phosphonic acid esters attached to the nucleoside at the 5'-position by amide linkers. Among the synthesized uridine derivatives, we identified the first potent and selective inhibitors of human NTPDase2. The most potent compound was 19a (PSB-6426), which was a competitive inhibitor of NTPDase2 exhibiting a K i value of 8.2 microM and selectivity versus other NTPDases. It was inactive toward uracil nucleotide-activated P2Y 2, P2Y 4, and P2Y 6 receptor subtypes. Compound 19a was chemically and metabolically highly stable. In contrast to the few known (unselective) NTPDase inhibitors, 19a is an uncharged molecule and may be perorally bioavailable. NTPDase2 inhibitors have potential as novel cardioprotective drugs for the treatment of stroke and for cancer therapy.

Polyoxometalates--a new class of potent ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) inhibitors

Polyoxotungstates were identified as potent inhibitors of NTPDases1, 2, and 3. The most potent compound was K(6)H(2)[TiW(11)CoO(40)], exhibiting K(i) values of 0.140 microM (NTPDase1), 0.910 microM (NTPDase2), and 0.563 microM (NTPDase3). One of the compounds, (NH(4))(18)[NaSb(9)W(21)O(86)], was selective for NTPDases2 and 3 versus NTPDase1. NTPDase inhibition might contribute to the described biological effects of polyoxometalates, including their anti-cancer activity.

Differences in nucleotide-binding site of isoapyrases deduced from tryptophan fluorescence

Comparative studies of intrinsic and extrinsic fluorescence of apyrases purified from two potato tuber varieties (Pimpernel and Desirée) were performed to determine differences in the microenvironment of the nucleotide binding site. The dissociation constants (K(d)) of Pimpernel apyrase for the binding of different fluorescent substrate analogs: methylanthranoyl (MANT-), trinitrophenyl (TNP-), and epsilon -derivatives of ATP and ADP were determined from the quenching of Trp fluorescence, and compared with K(d) values previously reported for Desirée enzyme. Binding of non-fluorescent substrate analogues decreased the Trp emission of both isoapyrases, indicating conformational changes in the vicinity of these residues. Similar effect was observed with fluorescent derivatives where, in the quenching effect, the transfer of energy from tryptophan residues to the fluorophore moiety could be additionally involved. The existence of energy transfer between Trp residues in the Pimpernel enzyme was demonstrated with epsilon -analogues, similar to our previous observations with the Desirée. From these results we deduced that tryptophan residues are close to or in the nucleotide binding site in both enzymes. Experiments with quenchers like acrylamide, Cs(+) and I(-), both in the presence and absence of nucleotide analogues, suggest the existence of differences in the nucleotide binding site of the two enzymes. From the results obtained in this work, we can conclude that the differences found in the microenvironment of the nucleotide binding site can explain, at least in part, the kinetic behaviour of both isoenzymes.