6-Benzylthioinosine analogues: promising anti-toxoplasmic agents as inhibitors of the mammalian nucleoside transporter ENT1 (es)

Targeting MYC Dependence by Metabolic Inhibitors in Cancer

MYC is a critical growth regulatory gene that is commonly overexpressed in a wide range of cancers. Therapeutic targeting of MYC transcriptional activity has long been a goal, but it has been difficult to achieve with drugs that directly block its DNA-binding ability. Additional approaches that exploit oncogene addiction are promising strategies against MYC-driven cancers. Also, drugs that target metabolic regulatory pathways and enzymes have potential for indirectly reducing MYC levels. Glucose metabolism and oxidative phosphorylation, which can be targeted by multiple agents, promote cell growth and MYC expression. Likewise, modulation of the signaling pathways and protein synthesis regulated by adenosine monophosphate-activated protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) can also be an effective route for suppressing MYC translation. Furthermore, recent data suggest that metabolism of nucleotides, fatty acids and glutamine are exploited to alter MYC levels. Combination therapies offer potential new approaches to overcome metabolic plasticity caused by single agents. Although potential toxicities must be carefully controlled, new inhibitors currently being tested in clinical trials offer significant promise. Therefore, as both a downstream target of metabolism and an upstream regulator, MYC is a prominent central regulator of cancer metabolism. Exploiting metabolic vulnerabilities of MYC-driven cancers is an emerging research area with translational potential.

Synergistic cell death in FLT3-ITD positive acute myeloid leukemia by combined treatment with metformin and 6-benzylthioinosine

Current therapy for acute myeloid leukemia (AML) primarily includes high-dose cytotoxic chemotherapy with or without allogeneic stem cell transplantation. Targeting unique cellular metabolism of cancer cells is a potentially less toxic approach. Monotherapy with mitochondrial inhibitors like metformin have met with limited success since escape mechanisms such as increased glycolytic ATP production, especially in hyperglycemia, can overcome the metabolic blockade. As an alternative strategy for metformin therapy, we hypothesized that the combination of 6-benzylthioinosine (6-BT), a broad-spectrum metabolic inhibitor, and metformin could block this drug resistance mechanism. Metformin treatment alone resulted in significant suppression of ROS and mitochondrial respiration with increased glycolysis accompanied by modest cytotoxicity (10-25%). In contrast, 6-BT monotherapy resulted in inhibition of glucose uptake, decreased glycolysis, and decreased ATP with minimal changes in ROS and mitochondrial respiration. The combination of 6-BT with metformin resulted in significant cytotoxicity (60-70%) in monocytic AML cell lines and was associated with inhibition of FLT3-ITD activated STAT5 and reduced c-Myc and GLUT-1 expression. Therefore, although the anti-tumor and metabolic effects of metformin have been limited by the metabolic reprogramming within cells, the novel combination of 6-BT and metformin targets this bypass mechanism resulting in reduced glycolysis, STAT5 inhibition, and increased cell death.

Carbocyclic 6-benzylthioinosine analogues as subversive substrates of Toxoplasma gondii adenosine kinase: biological activities and selective toxicities

Toxoplasma gondii adenosine kinase (EC 2.7.1.20) is the major route of adenosine metabolism in this parasite. The enzyme is significantly more active than any other enzyme of the purine salvage in T. gondii and has been established as a potential chemotherapeutic target for the treatment of toxoplasmosis. Several 6-benzylthioinosines have already been identified as subversive substrates of the T. gondii but not human adenosine kinase. Therefore, these compounds are preferentially metabolized to their respective nucleotides and become selectively toxic against the parasites but not its host. In the present study, we report the testing of the metabolism of several carbocyclic 6-benzylthioinosines, as well as their efficacy as anti-toxoplasmic agents in cell culture. All the carbocyclic 6-benzylthioinosine analogues were metabolized to their 5'-monophosphate derivatives, albeit to different degrees. These results indicate that these compounds are not only ligands but also substrates of T. gondii adenosine kinase. All the carbocyclic 6-benzylthioinosine analogues showed a selective anti-toxoplasmic effect against wild type parasites, but not mutants lacking adenosine kinase. These results indicate that the oxygen atom of the sugar is not critical for substrate binding. The efficacy of these compounds varied with the position and nature of the substitution on their phenyl ring. Moreover, none of these analogues exhibited host toxicity. The best compounds were carbocyclic 6-(p-methylbenzylthio)inosine (IC(50)=11.9 microM), carbocyclic 6-(p-methoxybenzylthio)inosine (IC(50)=12.1 microM), and carbocyclic 6-(p-methoxycarbonylbenzylthio)inosine (IC(50)=12.8 microM). These compounds have about a 1.5-fold better efficacy relative to their corresponding 6-benzylthioinosine analogues (Rais et al., Biochem Pharmacol 2005;69:1409-19 [29]). The results further confirm that T. gondii adenosine kinase is an excellent target for chemotherapy and that carbocyclic 6-benzylthioinosines are potential anti-toxoplasmic agents.

Synthesis and biological evaluation of phloridzin analogs as human concentrative nucleoside transporter 3 (hCNT3) inhibitors

Nucleoside transporter inhibitors have potential therapeutic applications as anticancer, antiviral, cardioprotective and neuroprotective agents. Although quite a few potent inhibitors of the equilibrative nucleoside transporters are known, largely missing are the concentrative nucleoside transporter inhibitors. Phloridzin (3, K(i)=16.00 microM) is a known moderate inhibitor of the concentrative nucleoside transporters. We have synthesized and evaluated analogs of phloridzin at the hCNT3 nucleoside transporter. Within the series of synthesized analogs compound 16 (K(i)=2.88 microM), possessing a ribofuranose sugar unit instead of a glucopyranose as present in phloridzin, exhibited the highest binding affinity at the hCNT3 transporter. Phloridzin and compound 16 have also been shown to be selective for the hCNT3 transporter as compared with the hENT1 transporter. Compound 16 can serve as a new lead which after further modifications could yield selective and potent hCNT3 inhibitors.

Identification of 6-benzylthioinosine as a myeloid leukemia differentiation-inducing compound

As the pathophysiology of acute myelogenous leukemia (AML) involves a block of myeloid maturation, a desirable therapeutic strategy is to induce leukemic cell maturation to increase the efficacy and to avoid the side effects of traditional chemotherapeutics. Through a compound library screen, 6-benzylthioinosine (6BT) was identified as a promising differentiation-inducing agent. 6BT induces monocytic differentiation of myeloid leukemia cell lines such as HL-60 and OCI-AML3, as well as primary patient samples as evidenced by morphology, immunophenotyping, and nitroblue tetrazolium reduction. Not only can 6BT induce differentiation but a subset of AML cell lines such as MV4-11 and HNT34 instead undergo 6BT-mediated cell death. Despite inducing cell death in some leukemic cells, 6BT exhibits extremely low toxicity on several nonmalignant cells such as fibroblasts, normal bone marrow, and endothelial cells. This toxicity profile may relate to the function of 6BT as an inhibitor of the nucleoside transporter, ent1, which is thought to prevent it from entering many cell types. In contrast, 6BT likely enters at least some leukemic cell lines as shown by its requirement for phosphorylation for its differentiation activity. 6BT is also able to synergize with currently used myeloid differentiation agents such as ATRA and decitabine. Early studies indicate that the mechanism of action of this compound may involve ATP depletion that leads to growth inhibition and subsequent differentiation. Besides in vitro activity, 6BT also shows the ability to impair HL-60 and MV4-11 tumor growth in nude mice. 6BT is a promising new monocytic differentiation agent with apparent leukemic cell-specific activity.

7-Deaza-6-benzylthioinosine analogues as subversive substrate of Toxoplasma gondii adenosine kinase: activities and selective toxicities

Toxoplasma gondii adenosine kinase (EC.2.7.1.20) is the major route of adenosine metabolism in this parasite. The enzyme is significantly more active than any other enzyme of the purine salvage in T. gondii and has been established as a potential chemotherapeutic target for the treatment of toxoplasmosis. Certain 6-benzylthioinosines act as subversive substrates of T. gondii, but not human, adenosine kinase. Therefore, these compounds are preferentially metabolized to their respective nucleotides and become selectively toxic against the parasites but not their host. Moreover, 7-deazaadenosine (tubercidin) was shown to be an excellent ligand of T. gondii adenosine kinase. Therefore, we synthesized 7-deaza-6-benzylthioinosine, and analogues with various substitutions at their phenyl ring, to increase the binding affinity of the 6-benzylthioinosines to T. gondii adenosine kinase. Indeed, the 7-deaza-6-benzylthioinosine analogues were better ligands of T. gondii adenosine kinase than the parent compounds, 6-benzylthioinosine and 7-deazainosine. Herein, we report the testing of the metabolism of these newly synthesized 7-deaza-6-benzylthioinosines, as well as their efficacy as anti-toxoplasmic agents in cell culture. All the 7-deaza-6-benzylthioinosine analogues were metabolized to their 5'-monophosphate derivatives, albeit to different degrees. These results indicate that these compounds are not only ligands but also substrates of T. gondii adenosine kinase. All the 7-deaza-6-benzylthioinosine analogues showed a selective antitoxoplasmic effect against wild type parasites, but not mutants lacking adenosine kinase. The efficacy of these compounds varied with the position and nature of the substitution on their phenyl ring. Moreover, none of these analogues exhibited host toxicity. The best compounds were 7-deaza-6-(p-methoxybenzylthio)inosine (IC(50)=4.6 microM), 7-deaza-6-(p-methoxycarbonylbenzylthio)inosine (IC(50)=5.0 microM), and 7-deaza-6-(p-cyanobenzylthio)inosine (IC(50)=5.3 microM). These results further confirm that T. gondii adenosine kinase is an excellent target for chemotherapy and that 7-deaza-6-benzylthioinosines are potential antitoxoplasmic agents.

Constrained NBMPR analogue synthesis, pharmacophore mapping and 3D-QSAR modeling of equilibrative nucleoside transporter 1 (ENT1) inhibitory activity

Conformationally constrained analogue synthesis was undertaken to aid in pharmacophore mapping and 3D-QSAR analysis of nitrobenzylmercaptopurine riboside (NBMPR) congeners as equilibriative nucleoside transporter 1 (ENT1) inhibitors. In our previous study [J. Med. Chem. 2003, 46, 831-837], novel regioisomeric nitro-1,2,3,4-tetrahydroisoquinoline conformationally constrained analogues of NBMPR were synthesized and evaluated as ENT1 ligands. 7-NO(2)-1,2,3,4-Tetrahydroisoquino-2-yl purine riboside was identified as the analogue with the nitro group in the best orientation at the NBMPR binding site of ENT1. In the present study, further conformational constraining was introduced by synthesizing 5'-O,8-cyclo derivatives. The flow cytometrically determined binding affinities indicated that the additional 5'-O,8-cyclo constraining was unfavorable for binding to the ENT1 transporter. The structure-activity relationship (SAR) acquired was applied to pharmacophore mapping using the PHASE program. The best pharmacophore hypothesis obtained embodied an anti-conformation with three hydrogen-bond acceptors, one hydrophobic center, and two aromatic rings involving the 3'-OH, 4'-oxygen, the NO(2) group, the benzyl phenyl and the imidazole and pyrimidine portions of the purine ring, respectively. A PHASE 3D-QSAR model derived with this pharmacophore yielded an r(2) of 0.916 for four (4) PLS components, and an excellent external test set predictive r(2) of 0.78 for 39 compounds. This pharmacophore was used for molecular alignment in a comparative molecular field analysis (CoMFA) 3D-QSAR study that also afforded a predictive model with external test set validation predictive r(2) of 0.73. Thus, although limited, this study suggests that the bioactive conformation for NBMPR at the ENT1 transporter could be anti. The study has also suggested an ENT1 inhibitory pharmacophore, and established a predictive CoMFA 3D-QSAR model that might be useful for novel ENT1 inhibitor discovery and optimization.

Novel C2-purine position analogs of nitrobenzylmercaptopurine riboside as human equilibrative nucleoside transporter 1 inhibitors

Nucleoside transporter inhibitors have potential therapeutic applications as anticancer, antiviral, cardioprotective, and neuroprotective agents. S(6)-(4-nitrobenzyl)mercaptopurine riboside (NBMPR) is a prototype inhibitor of the human equilibrative nucleoside transporter (hENT1), and is a high affinity ligand with a K(d) of 0.1-1.0 nM. We have synthesized and flow cytometrically evaluated the binding affinity of a series of novel C(2)-purine position substituted analogs of NBMPR at the hENT1. The aim of this research was to understand the substituent requirements at the C(2)-purine position of NBMPR. Structure-activity relationships (SAR) indicate that increasing the steric bulk at the C(2)-purine position of NBMPR led to a decrease in binding affinity of these ligands at the hENT1. New high affinity inhibitors were identified, with the best compound, 2-fluoro-4-nitrobenzyl mercaptopurine riboside (7), exhibiting a K(i) of 2.1 nM. This information, when coupled with the information obtained from other structure-activity relationship studies should prove useful in efforts aimed at modeling the NMBPR and analogs pharmacophore of hENT1 inhibitors.

Molecules targeting the purine salvage pathway in Apicomplexan parasites

The need of intracellular parasites to retrieve nutrients and fulfill their energy requirements is achieved by manipulating the host's metabolism. With the spread of AIDS, research on purine metabolism has gained in importance with the aim to develop drugs against opportunistic infections. Many studies over the past ten years have yielded contradictory results, but this review tries to clarify these findings by exposing the latest data concerning purine transport and the specific activities of the major enzymes of the purine salvage pathway of Toxoplasma gondii, Plasmodium falciparum and Cryptosporidium parvum.

Synthesis, biological evaluation and molecular modeling studies of N6-benzyladenosine analogues as potential anti-toxoplasma agents

Toxoplasma gondii is an opportunistic pathogen responsible for toxoplasmosis. T. gondii is a purine auxotroph incapable of de novo purine biosynthesis and depends on salvage pathways for its purine requirements. Adenosine kinase (EC.2.7.1.20) is the major enzyme in the salvage of purines in these parasites. 6-Benzylthioinosine and analogues were established as "subversive substrates" for the T. gondii, but not for the human adenosine kinase. Therefore, these compounds act as selective anti-toxoplasma agents. In the present study, a series of N(6)-benzyladenosine analogues were synthesized from 6-chloropurine riboside with substituted benzylamines via solution phase parallel synthesis. These N(6)-benzyladenosine analogues were evaluated for their binding affinity to purified T. gondii adenosine kinase. Furthermore, the anti-toxoplasma efficacy and host toxicity of these compounds were tested in cell culture. Certain substituents on the aromatic ring improved binding affinity to T. gondii adenosine kinase when compared to the unsubstituted N(6)-benzyladenosine. Similarly, varying the type and position of the substituents on the aromatic ring led to different degrees of potency and selectivity as anti-toxoplasma agents. Among the synthesized analogues, N(6)-(2,4-dimethoxybenzyl)adenosine exhibited the most favorable anti-toxoplasma activity without host toxicity. The binding mode of the synthesized N(6)-benzyladenosine analogues were characterized to illustrate the role of additional hydrophobic effect and van der Waals interaction within an active site of T. gondii adenosine kinase by induced fit molecular modeling.