Pyrazolo[4,3-d]pyrimidines as new generation of cyclin-dependent kinase inhibitors

Lead Optimization of the 5-Phenylpyrazolopyrimidinone NPD-2975 toward Compounds with Improved Antitrypanosomal Efficacy

Human African trypanosomiasis (HAT) still faces few therapeutic options and emerging drug resistance, stressing an urgency for novel antitrypanosomal drug discovery. Here, we describe lead optimization efforts aiming at improving antitrypanosomal efficacy and better physicochemical properties based on our previously reported optimized hit NPD-2975 (pIC50 7.2). Systematic modification of the 5-phenylpyrazolopyrimidinone NPD-2975 led to the discovery of a R4-substituted analogue 31c (NPD-3519), showing higher in vitro potency (pIC50 7.8) against Trypanosoma brucei and significantly better metabolic stability. Further, in vivo pharmacokinetic evaluation of 31c and experiments in an acute T. brucei mouse model confirmed improved oral bioavailability and antitrypanosomal efficacy at 50 mg/kg with no apparent toxicity. With good physicochemical properties, low toxicity, improved pharmacokinetic features, and in vivo efficacy, 31c may serve as a promising candidate for future drug development for HAT.

Structural Optimization of BIPPO Analogs as Potent Antimalarials

Malaria continues to pose a significant health threat, causing thousands of deaths each year. The limited availability of vaccines and medications, combined with the emergence of drug resistance, further complicates the fight against this disease. In this study, we aimed to enhance the antimalarial potency of the previously reported hit compound BIPPO (pIC₅₀ 5.9). Through systematic modification of pyrazolopyrimidinone analogs, we discovered the promising analog 30 (NPD-3547), which exhibited approximately one log unit higher in vitro potency (pIC₅₀ 6.8) against Plasmodium falciparum. Furthermore, we identified several other BIPPO analogs (23, 28, 29 and 47a) with potent antimalarial activity (pIC₅₀ > 6.0) and favorable metabolic stability in mouse liver microsomes. These compounds can serve as new tools for further optimization towards the development of potential candidates for antimalarial studies.

Pyrimidine-fused Dinitrogenous Penta-heterocycles as a Privileged Scaffold for Anti-Cancer Drug Discovery

Pyrimidine-fused derivatives that are the inextricable part of DNA and RNA play a key role in the normal life cycle of cells. Pyrimidine-fused dinitrogenous penta-heterocycles including pyrazolopyrimidines and imidazopyrimidines is a special class of pyrimidine-fused compounds contributing to an important portion in anti-cancer drug discovery, which have been discovered as core structure for promising anti-cancer agents used in clinic or clinical evaluations. Pyrimidine-fused dinitrogenous penta-heterocycles have become one privileged scaffold for anti-cancer drug discovery. This review consists of the recent progress of pyrimidine-fused dinitrogenous penta-heterocycles as anti-cancer agents and their synthetic strategies. In addition, this review also summarizes some key structure-activity relationships (SARs) of pyrimidine-fused dinitrogenous penta-heterocycle derivatives as anti-cancer agents.

Pyrazolopyrimidines as anticancer agents: A review on structural and target-based approaches

Pyrazolopyrimidine scaffold is one of the privileged heterocycles in drug discovery. This scaffold produced numerous biological activities in which anticancer is important one. Previous studies showed its importance in interactions with various receptors such as growth factor receptor, TGFBR2 gene, CDK2/cyclin E and Abl kinase, adenosine receptor, calcium-dependent Protein Kinase, Pim-1 kinase, Potent Janus kinase 2, BTK kinase, P21-activated kinase 1, extracellular signal-regulated kinase 2, histone lysine demethylase and Human Kinesin-5. However, there is a need of numerous studies for the discovery of target based potential compounds. The structure activity relationship studies may help to explore the generation of potential compounds in short time period. Therefore, in the present review we tried to explore the structural aspects of Pyrazolopyrimidine with their structure activity relationship against various targets for the development of potential compounds. The current review is the compilation of significant advances made on Pyrazolopyrimidines reported between 2015 and 2020.

Recent Progress on Anticancer Agents Incorporating Pyrazole Scaffold

The search of new anticancer agents is considered as a dynamic field of medicinal chemistry. In recent years, the synthesis of compounds with anticancer potential has increased and a large number of structurally varied compounds displaying potent anticancer activities have been published. Pyrazole is an important biologically active scaffold that possessed nearly all types of biological activities. The aim of this review is to collate literature work reported by researchers to provide an overview on in vivo and in vitro anticancer activities of pyrazole based derivatives among the diverse biological activities displayed by them and also presents recent efforts made on this heterocyclic moiety regarding anticancer activities. This review has been driven from the increasing number of publications, on this issue, which have been reported in the literature since the ending of the 20th century (from 1995-to date).

An appraisal on synthetic and pharmaceutical perspectives of pyrazolo[4,3-d]pyrimidine scaffold

Pyrazolo[4,3-d]pyrimidine, a fused heterocycle bearing pyrazole and pyrimidine portions has gained a significant attention in the field of bioorganic and medicinal chemistry. Pyrazolo[4,3-d]pyrimidine derivatives have demonstrated numerous pharmacological activities particularly, anti-cancer, anti-infectious, phosphodiesterase inhibitors, adenosine antagonists and cytokinin antagonists etc. This review extensively unveils the synthetic and pharmacological diversity with special emphasis on structural variations around pyrazolo[4,3-d]pyrimidine scaffold. This endeavour has thus uncovered the medicinal worthiness of pyrazolo[4,3-d]pyrimidine framework. To the best of our knowledge this review is the first compilation on synthetic, medicinal and structure activity relationship (SAR) aspects of pyrazolo[4,3-d]pyrimidines since 1956.

Virtual screening to successfully identify novel janus kinase 3 inhibitors: a sequential focused screening approach

In an effort to identify novel Janus kinase 3 inhibitors, a sequential focused screening approach was adopted to search our in-house chemical database. By biologically testing only 79 selected compounds, we successfully identified 19 compounds showing IC 50 < 20 microM, with four of them in the nanomolar range. Particularly, a 3,5-disubstituted pyrazolo[4,3- d]pyrimidine scaffold emerged as a promising candidate for further lead optimization. With the advantages of efficiency and flexibility, this approach may be utilized to identify leads for other therapeutic targets.

Structural analysis identifies imidazo[1,2-b]pyridazines as PIM kinase inhibitors with in vitro antileukemic activity

Much attention has recently been focused on PIM kinases as potential targets for the treatment of hematopoietic malignancies and some solid cancers. Using protein stability shift assays, we identified a family of imidazo[1,2-b]pyridazines to specifically interact with and inhibit PIM kinases with low nanomolar potency. The high-resolution crystal structure of a PIM1 inhibitor complex revealed that imidazo[1,2-b]pyridazines surprisingly interact with the NH(2)-terminal lobe helix alphaC rather than with the kinase hinge region. Thus, the identified inhibitors are ATP competitive but not ATP mimetic compounds, explaining their enhanced selectivity with respect to conventional type I kinase inhibitors. One of the identified imidazo[1,2-b]pyridazines (K00135) was further tested in several hematopoietic cellular systems. First, K00135 dose-dependently impaired survival of murine Ba/F3 cells that have been rendered cytokine independent by overexpression of human PIMs. Second, K00135 impaired survival and clonogenic growth of a panel of human acute leukemia cells. Third, exposure of K00135 significantly suppressed in vitro growth of leukemic blasts from five acute myelogenous leukemia patients but not of normal umbilical cord blood mononuclear cells. In vitro kinase assays and immunoblotting using lysates from human MV4;11 leukemic cells showed inhibition of phosphorylation of known PIM downstream targets, such as BAD and eukaryotic translation initiation factor 4E-binding protein 1, by K00135. Taken together, we report a family of small molecules that selectively interact and block PIM kinases and could serve as a lead to develop new targeted antileukemic therapeutics.

Classical Anticytokinins Do Not Interact with Cytokinin Receptors but Inhibit Cyclin-dependent Kinases

Cytokinins are a class of plant hormones that regulate the cell cycle and diverse developmental and physiological processes. Several compounds have been identified that antagonize the effects of cytokinins. Based on structural similarities and competitive inhibition, it has been assumed that these anticytokinins act through a common cellular target, namely the cytokinin receptor. Here, we examined directly the possibility that various representative classical anticytokinins inhibit the Arabidopsis cytokinin receptors CRE1/AHK4 (cytokinin response 1/Arabidopsis histidine kinase 4) and AHK3 (Arabidopsis histidine kinase 3). We show that pyrrolo[2,3-d]pyrimidine and pyrazolo[4,3-d]pyrimidine anticytokinins do not act as competitors of cytokinins at the receptor level. Flow cytometry and microscopic analyses revealed that anticytokinins inhibit the cell cycle and cause disorganization of the microtubular cytoskeleton and apoptosis. This is consistent with the hypothesis that they inhibit regulatory cyclin-dependent kinase (CDK) enzymes. Biochemical studies demonstrated inhibition by selected anti-cytokinins of both Arabidopsis and human CDKs. X-ray determination of the crystal structure of a human CDK2-anticytokinin complex demonstrated that the antagonist occupies the ATP-binding site of CDK2. Finally, treatment of human cancer cell lines with anticytokinins demonstrated their ability to kill human cells with similar effectiveness as known CDK inhibitors.

4-arylazo-3,5-diamino-1H-pyrazole CDK inhibitors: SAR study, crystal structure in complex with CDK2, selectivity, and cellular effects

In a routine screening of our small-molecule compound collection we recently identified 4-arylazo-3,5-diamino-1H-pyrazoles as a novel group of ATP antagonists with moderate potency against CDK2-cyclin E. A preliminary SAR study based on 35 analogues suggests ways in which the pharmacophore could be further optimized, for example, via substitutions in the 4-aryl ring. Enzyme kinetics studies with the lead compound and X-ray crystallography of an inhibitor-CDK2 complex demonstrated that its mode of inhibition is competitive. Functional kinase assays confirmed the selectivity toward CDKs, with a preference for CDK9-cyclin T1. The most potent inhibitor, 4-[(3,5-diamino-1H-pyrazol-4-yl)diazenyl]phenol 31b (CAN508), reduced the frequency of S-phase cells of the cancer cell line HT-29 in antiproliferation assays. Further observed cellular effects included decreased phosphorylation of the retinoblastoma protein and the C-terminal domain of RNA polymerase II, inhibition of mRNA synthesis, and induction of the tumor suppressor protein p53, all of which are consistent with inhibition of CDK9.

Synthesis and biological activity of 8-azapurine and pyrazolo[4,3-d]pyrimidine analogues of myoseverin

The trisubstituted purine myoseverin has been recently identified as a novel inhibitor of microtubule assembly. To analyze the effects of modifying its heterocyclic skeleton, we prepared 8-azapurine and pyrazolo[4,3-d]pyrimidine analogues of myoseverin and compared their biological activities. Rearrangement of nitrogen atoms in the heterocycle changes the affinity of the compounds to purified tubulin, as demonstrated by the results of polymerization assays, and affects the proliferation of cancer cell lines. Surprisingly, compound E2GG, a pyrazolo[4,3-d]pyrimidine analogue of myoseverin, displayed inhibitory activity towards both tubulin polymerization and the activity of cyclin-dependent kinases 1, 2 and 7. Such a dual specificity-inhibitor offers a starting point for developing a novel class of antiproliferative agents.

The purines: potent and versatile small molecule inhibitors and modulators of key biological targets

The goal of this review is to highlight the wide range of biological activities displayed by purines, with particular emphasis on new purine-based agents which find potential application as chemical-biology tools and/or therapeutic agents. The expanding interest in the biological properties of polyfunctionalized purine derivatives issues, in large part, from the development of rapid high-throughput screening essays for new protein targets, and the corresponding development of efficient synthetic methodology adapted to the construction of highly diverse purine libraries. Purine-based compounds have found new applications as inducers of interferon and lineage-committed cell dedifferentiation, agonists and antagonists of adenosine receptors, ligands of corticotropin-releasing hormone receptors, and as inhibitors of HSP90, Src kinase, p38alpha MAP kinase, sulfotransferases, phosphodiesterases, and Cdks. The scope of application of purines in biology is most certainly far from being exhausted. Testing purine derivatives against the multitude of biological targets for which small molecule probes have not yet been found should thus be a natural reflex.

Structural basis of inhibitor specificity of the human protooncogene proviral insertion site in moloney murine leukemia virus (PIM-1) kinase

The kinase PIM-1 plays a pivotal role in cytokine signaling and is implicated in the development of a number of tumors. The three-dimensional structure of PIM-1 is characterized by an unique hinge region which lacks a second hydrogen bond donor and makes it particularly important to determine how inhibitors bind to this kinase. We determined the structures of PIM-1 in complex with bisindolylmaleimide (BIM-1) and established the structure-activity relationship (SAR) for this inhibitor class. In addition, we screened a kinase targeted library and identified a number of high affinity inhibitors of PIM-1 such as imidazo[1,2-b]pyridazines, pyrazolo[1,5-a]pyrimidines, and members of the flavonoid family. In this paper we present an initial SAR of the identified scaffolds determined on the basis of a thermostability shift assay, calorimetric binding data, and biochemical assays which may find applications for the treatment of PIM-1 dependent cancer types.

8-Azapurines as new inhibitors of cyclin-dependent kinases

Purine inhibitors of cyclin-dependent kinases (CDK) seem to be a potential anticancer drug candidate as one of the first representatives, roscovitine, is passing Phase II clinical trials for cancer and glomerulonephritis. In this article, we describe a novel modification of the purine scaffold influencing CDK2 inhibitory activities as well as anticancer properties in cell lines of different histopathological origin. The introduced N at position 8 of the purine ring generally lowered CDK2 inhibitory activity of new 8-azapurines (1,2,3-triazolo[4,5-d]pyrimidines) in comparison to the model trisubstituted purines, whereas the antiproliferative potential of some derivatives remained very high, reflecting their ability to activate p53 tumor suppressor.

2,6,8,9-tetrasubstituted purines as new CDK1 inhibitors

Purine inhibitors of cyclin-dependent kinases attract attention as potential anticancer drugs because their first representative roscovitine recently entered clinical trials. Although well described in terms of structure-activity relationships, we still present here a novel modification of the purine scaffold influencing their inhibitory properties. The introduced C-8 substituents, however, lowered the CDK inhibitory activity of roscovitine, whereas the antiproliferative potential of several derivatives remained high.