Pyrazolo[3,4-d]pyrimidines Endowed with Antiproliferative Activity on Ductal Infiltrating Carcinoma Cells

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.

Three-Dimensional Interactions Analysis of the Anticancer Target c-Src Kinase with Its Inhibitors

Src family kinases (SFKs) constitute the biggest family of non-receptor tyrosine kinases considered as therapeutic targets for cancer therapy. An aberrant expression and/or activation of the proto-oncogene c-Src kinase, which is the oldest and most studied member of the family, has long been demonstrated to play a major role in the development, growth, progression and metastasis of numerous human cancers, including colon, breast, gastric, pancreatic, lung and brain carcinomas. For these reasons, the pharmacological inhibition of c-Src activity represents an effective anticancer strategy and a few compounds targeting c-Src, together with other kinases, have been approved as drugs for cancer therapy, while others are currently undergoing preclinical studies. Nevertheless, the development of potent and selective inhibitors of c-Src aimed at properly exploiting this biological target for the treatment of cancer still represents a growing field of study. In this review, the co-crystal structures of c-Src kinase in complex with inhibitors discovered in the past two decades have been described, highlighting the key ligand-protein interactions necessary to obtain high potency and the features to be exploited for addressing selectivity and drug resistance issues, thus providing useful information for the design of new and potent c-Src kinase inhibitors.

Development of a 4-aminopyrazolo[3,4-d]pyrimidine-based dual IGF1R/Src inhibitor as a novel anticancer agent with minimal toxicity

BACKGROUND

Both the type I insulin-like growth factor receptor (IGF1R) and Src pathways are associated with the development and progression of numerous types of human cancer, and Src activation confers resistance to anti-IGF1R therapies. Hence, targeting both IGF1R and Src concurrently is one of the main challenges in combating resistance to the currently available anti-IGF1R-based anticancer therapies. However, the enhanced toxicity from this combinatorial treatment could be one of the main hurdles for this strategy, suggesting the necessity of developing a novel strategy for co-targeting IGF1R and Src to meet an urgent clinical need.

METHODS

We synthesized a series of 4-aminopyrazolo[3,4-d]pyrimidine-based dual IGF1R/Src inhibitors, selected LL28 as an active compound and evaluated its potential antitumor effects in vitro and in vivo using the MTT assay, colony formation assays, flow cytometric analysis, a tumor xenograft model, and the Kras ^G12D/+ -driven spontaneous lung tumorigenesis model.

RESULTS

LL28 markedly suppressed the activation of IGF1R and Src and significantly inhibited the viability of several NSCLC cell lines in vitro by inducing apoptosis. Administration of mice with LL28 significantly suppressed the growth of H1299 NSCLC xenograft tumors without overt toxicity and substantially reduced the multiplicity, volume, and load of lung tumors in the Kras ^G12D/+ -driven lung tumorigenesis model.

CONCLUSIONS

The present results suggest the potential of LL28 as a novel anticancer drug candidate targeting both IGF1R and Src, providing a new avenue to efficient anticancer therapies. Further investigation is warranted in advanced preclinical and clinical settings.

Opening the door to the development of novel Abl kinase inhibitors

The discovery of the importance of kinase activity and its relationship to the emergence and proliferation of cancer cells, due to changes in normal physiology, opened a remarkable pathway for the treatment of chronic myelogenous leukemia through intense search of drug candidates. Six Abl kinase inhibitors have received the US FDA approval as chronic myelogenous leukemia treatment, and continuous efforts in obtaining new, more effective and selective molecules are being carried out. Herein we discuss the mechanisms of Abl inhibition, structural features and ligand/protein interactions that are important for the design of new Abl kinase inhibitors. This review provides a broad overview of binding mode predictions, through molecular docking, which can be an approach to discover novel Abl kinase inhibitors.

In silico identification and biological evaluation of novel selective serum/glucocorticoid-inducible kinase 1 inhibitors based on the pyrazolo-pyrimidine scaffold

The serum/glucocorticoid-inducible kinase 1 (Sgk1) has demonstrated antiapoptotic function and the capability to regulate cell survival, proliferation, and differentiation. A pivotal role of Sgk1 in carcinogenesis and in resistance to anticancer therapy has been suggested. With the aim of identifying new Sgk1 modulators, 322 pyrazolo-pyrimidine derivatives have been virtually screened with respect to a crystallographic model of Sgk1. The top five ranked compounds have been evaluated demonstrating Sgk1 inhibition in vitro and selectivity compared to RAC-alpha serine/threonine-protein kinase (Akt1).

Pyrazolo[3,4-d]pyrimidine Prodrugs: Strategic Optimization of the Aqueous Solubility of Dual Src/Abl Inhibitors

Design and synthesis of prodrugs of promising drug candidates represents a valid strategy to overcome the lack of favorable ADME properties, in particular aqueous solubility and bioavailability. We report herein the successful application of this strategy with two representative pyrazolo[3,4-d]pyrimidine derivatives (1 and 2), which led to the development of the corresponding and highly water-soluble antitumor prodrugs (7 and 8). In vitro studies confirmed a significant improvement of aqueous solubility and, for compound 8, good plasma stability, suggesting superior in vivo bioavailability. As expected, the uncleaved water-soluble prodrugs 7 and 8 showed no activity toward the enzymatic targets (c-Src and c-Abl) but revealed promising antiproliferative activity in myeloid cell lines, as a consequence of the in vitro hydrolysis of the selected solubilizing moiety, followed by the release of the active compounds (1 and 2).

Synthesis, characterization and RHF/ab initio simulations of 2-amino-1,3,4-thiadiazole and its annulated ring junction pyrimidine derivatives

Michael addition reaction of the 2-amino-1,3,4-thiadiazole to chalcone as biselectrophile afforded 5,7-diphenyl-6-[1,3-diphenylpropan-1-on-3-yl][1,3,4]thiadiazolo[3,2-a]pyrimidine (3) instead of 5,7-diphenyl-5H-[1,3,4]thiadiazolo[3,2-a]pyrimidine (5) via further Michael addition at C₅ in pyrimidine moiety. The structure 3 was established through the aspect of ab initio calculations, elemental analysis and spectral data.

A combined targeted/phenotypic approach for the identification of new antiangiogenics agents active on a zebrafish model: from in silico screening to cyclodextrin formulation

A combined targeted/phenotypic approach for the rapid identification of novel antiangiogenics with in vivo efficacy is herein reported. Considering the important role played by the tyrosine kinase c-Src in the regulation of tumour angiogenesis, we submitted our in-house library of c-Src inhibitors to a sequential screening approach: in silico screening on VEGFR2, in vitro screening on HUVEC cells, ADME profiling, formulation and in vivo testing on a zebrafish model. A promising antiangiogenic candidate able to interfere with the vascular growth of a zebrafish model at low micromolar concentration was thus identified.

Molecular characterization of c-Abl/c-Src kinase inhibitors targeted against murine tumour progenitor cells that express stem cell markers

Background

The non-receptor tyrosine kinases c-Abl and c-Src are overexpressed in various solid human tumours. Inhibition of their hyperactivity represents a molecular rationale in the combat of cancerous diseases. Here we examined the effects of a new family of pyrazolo [3,4-d] pyrimidines on a panel of 11 different murine lung tumour progenitor cell lines, that express stem cell markers, as well as on the human lung adenocarcinoma cell line A549, the human hepatoma cell line HepG2 and the human colon cancer cell line CaCo2 to obtain insight into the mode of action of these experimental drugs.

Methodology/Principal Findings

Treatment with the dual kinase inhibitors blocked c-Abl and c-Src kinase activity efficiently in the nanomolar range, induced apoptosis, reduced cell viability and caused cell cycle arrest predominantly at G0/G1 phase while western blot analysis confirmed repressed protein expression of c-Abl and c-Src as well as the interacting partners p38 mitogen activated protein kinase, heterogenous ribonucleoprotein K, cyclin dependent kinase 1 and further proteins that are crucial for tumour progression. Importantly, a significant repression of the epidermal growth factor receptor was observed while whole genome gene expression analysis evidenced regulation of many cell cycle regulated genes as well integrin and focal adhesion kinase (FAK) signalling to impact cytoskeleton dynamics, migration, invasion and metastasis.

Conclusions/Significance

Our experiments and recently published in vivo engraftment studies with various tumour cell lines revealed the dual kinase inhibitors to be efficient in their antitumour activity.

Antiproliferative and pro-apoptotic effects afforded by novel Src-kinase inhibitors in human neuroblastoma cells

Background

Neuroblastoma (NB) is the second most common solid malignancy of childhood that usually undergoes rapid progression with a poor prognosis upon metastasis. The Src-family tyrosine kinases (SFKs) are a group of proteins involved in cancer development and invasiveness that seem to play an important role in the NB carcinogenesis.

Methods

To determine cell proliferation, the growth rate was evaluated by both MTT test and cells counted. Analysis of DNA content was performed for the evaluation of the cell cycle and apoptosis. To characterize the mechanisms underlying the antiproliferative effects induced by SI 34, a novel pyrazolo-pyrimidine derivative provided with Src inhibitory activity, the involvement of some cellular pathways that are important for cell proliferation and survival was investigated by western blot assays. In particular, the contribution of cyclins, Src and ERK were examined. Finally, experiments of cell adhesion and invasiveness were performed.

Results

Treatment of SH-SY5Y human NB cells and CHP100 human neuroepithelioma (NE) cultures with three novel pyrazolo[3,4-d]pyrimidine derivatives, namely SI 34, SI 35 and SI 83, inhibits the cell proliferation in a time and concentration-dependent manner. The maximal effect was obtained after 72 hours incubation with SI 34 10 μM. Fluorescence microscopy experiments, flow cytometry analysis and determination of caspase-3 activity by fluorimetric assays showed that SI 34 induced SH-SY5Y apoptosis. Moreover, SI 34 determined cell cycle arrest at the G0/G1 phase, paralleled by a decreased expression of cyclin D1. Furthermore, our data indicate that SI 34 reduces the SH-SY5Y cells adhesion and invasiveness. Evidence that SI 34 inhibits the Src and the ERK-phosphorylation, suggests the mechanism through which it exerts its effects in SH-SY5Y cells.

Conclusions

Our study shows the ability of this pyrazolo-pyrimidine Src inhibitor in reducing the growth and the invasiveness of human NB cells, suggesting a promising role as novel drug in the treatment of neuroblastoma.

Novel dual Src/Abl inhibitors for hematologic and solid malignancies

IMPORTANCE OF THE FIELD

c-Src and Bcr-Abl are two non-receptor or cytoplasmic tyrosine kinases (TKs) that play important roles in the development of solid and hematological malignancies. Indeed, Src is overexpressed or hyperactivated in a variety of solid tumors, while Bcr-Abl is the causative agent of chronic myeloid leukemia (CML), where Src is also involved. The two enzymes share significant sequence homology and remarkable structural resemblance.

AREAS COVERED IN THIS REVIEW

ATP-competitive compounds originally developed as Src inhibitors, showed to be also potent Abl inhibitors. Dasatinib, the first dual Src/Abl inhibitor approved by the US FDA in 2006 for the treatment of imatinib-resistant CML, is currently being tested in several clinical trials for the treatment of different solid tumors. SKI-606 and AZD0530 are two other important dual Src/Abl inhibitors extensively tested in animal models and in clinical trials, but not entered into therapy yet.

WHAT THE READER WILL GAIN

In this review we will report the latest results regarding dasatinib, SKI-606 and AZD0530, but also the knowledge on new compounds that have appeared in the literature in the last few years, including AP24163, AP24534, XL228, DC2036. We will focus on the most recent clinical trials or on preclinical studies that are in progress on these small-molecule TK inhibitors that represent a targeted therapy with high potential against cancer.

TAKE HOME MESSAGE

Molecularly targeted therapies, including the inhibition of specific TKs hyperactivated or overexpressed in many human cancers, could be less toxic than the classical non-specific cytotoxic chemotherapeutic agents; they could offer important therapeutic effects, especially if used in association with other agents such as monoclonal antibodies.

New pyrazolo-[3,4-d]-pyrimidine derivative Src kinase inhibitors lead to cell cycle arrest and tumor growth reduction of human medulloblastoma cells

Medulloblastoma is the most common malignant brain tumor in children, and despite improvements in the overall survival rate, it still lacks an effective treatment. Src plays an important role in cancer, and recently high Src activity was documented in medulloblastoma. In this report, we examined the effects of novel pyrazolo-[3,4-d]-pyrimidine derivative Src inhibitors in medulloblastoma. By MTS assay, we showed that the pyrimidine derivatives indicated as S7, S29, and SI163 greatly reduce the growth rate of medulloblastoma cells by inhibiting Src phosphorylation, compared with HT22 non-neoplastic nerve cells. These compounds also halt cells in the G(2)/M phase, and this effect likely occurs through the regulation of cdc2 and CDC25C phosphorylation, as shown by Western blot. Moreover, the exposure to pyrimidine derivatives induces apoptosis, assayed by the supravital propidium iodide assay, through modulation of the apoptotic proteins Bax and Bcl2, and inhibits tumor growth in vivo in a mouse model. Notably, S7, S29, and SI163 show major inhibitory effects on medulloblastoma cell growth compared with the chemotherapeutic agents cisplatin and etoposide. In conclusion, our results suggest that S7, S29, and SI163 could be novel attractive candidates for the treatment of medulloblastoma or tumors characterized by high Src activity.

Synthesis, characterization, crystal structure and ab initio studies on 5-ethoxycarbonly-6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine

The title compound, 5-ethoxycarbonly-6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine, has been synthesized and characterized by elemental analysis, IR, UV-vis and X-ray single crystal diffraction. Ab initio calculations have been carried out for the title compound by using B3LYP and HF methods at 6-31G* basis set. The calculated results show that the predicted geometries can reproduce the structural parameters, although the intermolecular interactions have some influences on the molecular geometry in the solid state. Predicted vibrational frequencies by two methods are consistent with each other. They have been assigned and compared with experimental IR spectra. The theoretical electronic absorption spectra have been calculated by both TD-DFT and HF-CIS methods, and the results indicate that only the former can approximately predict the electronic spectra for the title compound. Molecular orbital coefficients analyses suggest that the electronic transitions are mainly assigned to n-->pi* and pi-->pi* electronic transitions. On the basis of vibrational analyses, the thermodynamic properties of the title compound at different temperatures have been calculated, revealing the correlations between C(p,m)(0), S(m)(0), H(m)(0) and temperatures.

Antiproliferative and proapoptotic activities of new pyrazolo[3,4-d]pyrimidine derivative Src kinase inhibitors in human osteosarcoma cells

Osteosarcoma is the most frequent primitive malignant tumor of the skeletal system, characterized by an extremely aggressive clinical course that still lacks an effective treatment. Src kinase seems to be involved in the osteosarcoma malignant phenotype. We show that the treatment of human osteosarcoma cell lines with a new pyrazolo[3,4-d]pyrimidine derivative Src inhibitor, namely SI-83, impaired cell viability, with a half-maximal inhibitory concentration of 12 microM in nonstarved cells and a kinetic different from that known for the Src inhibitor PP2. Analysis by terminal deoxynucleotidyl transferase-mediated nick end labeling, Hoechst, and flow cytometric assay showed that SI-83 induced apoptosis in SaOS-2 cells. Moreover, SI-83, by inhibiting Src phosphorylation, decreased in vivo osteosarcoma tumor mass in a mouse model. Finally, SI-83 showed selectivity for osteosarcoma, since it had a far lower effect in primary human osteoblasts. These results show that human osteosarcoma had Src-dependent proliferation and that modulation of Src activity may be a therapeutic target of this new compound with low toxicity for nonneoplastic cells.

Growth inhibition of medullary thyroid carcinoma cells by pyrazolo-pyrimidine derivates

There is no effective treatment for recurrent or metastatic medullary thyroid carcinoma (MTC), a tumor arising from thyroid C-cells commonly presenting an inherited or acquired RET mutation. In this study we examined the sensitivity of two human MTC cell lines to novel pyrazolopyrimidine derivates, able to inhibit src-family tyrosine kinase activity. In TT cells [carrying the multiple endocrine neoplasia (MEN)2A Ret mutation Cys 634Trp] and MZ-CRC-1 cells (carrying the MEN2B RET mutation Met891Thr), one of these compounds, namely Si 34, determined a significant growth inhibitory effect (approximately 90% vs control for TT, 80% vs control for MZ-CRC-1) mainly due to enhanced cell mortality after a 6-day incubation. At concentrations that increased cell mortality, neither biochemical or morphological characteristics of apoptosis were detected in TT and MZCRC- 1 cells treated with Si 34. These results, when confirmed in other in vivo preclinical models, suggest that this novel tyrosine kinase inhibitor may be useful for the treatment of MTC.

Pyrazolo[3,4-d]pyrimidines c-Src inhibitors reduce epidermal growth factor-induced migration in prostate cancer cells

During its biological progression, prostate cancer frequently develops dependence on growth factor receptors and their downstream signalling messengers, including c-Src. Evidence for this supports the choice of c-Src as a therapeutic target in the prevention of tumour spreading. Two new pyrazolo[3,4-d]pyrimidines c-Src inhibitors, SI35 and SI40, were used to investigate the role of c-Src in the control of the aggressive phenotype of prostate carcinoma cell line, PC3. SI molecules reduced the proliferation of PC3 cells in a time- and dose-dependent manner, with an IC50 of approximately 50 microM. PC3 cells responded to the presence of epidermal growth factor (EGF) by increasing their migratory ability, and this effect was strongly reduced by the addition of SI at concentrations less than IC50. Further observations demonstrated that SI molecules modulated cell morphology and their adhesive capacity on different physiological substrates. The action of SI molecules appeared to involve, in parallel with c-Src inhibition, the down-modulation of the active forms of paxillin and extracellular signal-regulated kinase (ERK). Our data suggest a promising role for pyrazolo[3,4-d]pyrimidines c-Src inhibitors in the control of a highly invasive tumour phenotype.

A Combination of Docking/Dynamics Simulations and Pharmacophoric Modeling To Discover New Dual c-Src/Abl Kinase Inhibitors

A computational protocol was applied to identify molecular scaffolds untested toward the c-Src tyrosine kinase. A combination of docking and dynamics calculations allowed us to build three-dimensional models of the complexes between Src and several of its known inhibitors. Interactions most contributing to activity of the inhibitors, in terms of hydrogen bonds and hydrophobic contacts, were codified into pharmacophoric models that were in turn applied to perform a search of commercially available compounds within the Asinex database. As a result, we identified 1,3,4-thiadiazoles and pyrazolydine-3,5-diones showing inhibitory activity in the submicromolar range in a cell-free assay toward Src. Moreover, since several of the compounds used to generate pharmacophores were also known as Abl inhibitors, we tested the identified hits toward Abl tyrosine kinase, finding activity in the submicromolar range. Such biological data suggested that the computational protocol is an efficient tool for identifying new hits toward both Src and Abl.

Pyrazolo[3,4-d]pyrimidines as Potent Antiproliferative and Proapoptotic Agents toward A431 and 8701-BC Cells in Culture via Inhibition of c-Src Phosphorylation

We report here the synthesis of new pyrazolo[3,4-d]pyrimidine derivatives along with their biological properties as inhibitors of isolated Src and cell line proliferation (A431 and 8701-BC cells). Such compounds block the growth of cancer cells by interfering with the phosphorylation of Src, and they act as proapoptotic agents through the inhibition of the anti apoptotic gene BCL2. Several of them were found to be more active than the reference compound (1-(tert-butyl)-3-(4-chlorophenyl)-4-aminopyrazolo[3,4-d]pyrimidine, PP2) in inhibiting cell proliferation and in inducing apoptosis, and as active as PP2 in the inhibition of the phosphorylation of isolated Src. Moreover, molecular modeling simulations have been performed to hypothesize the way, at the molecular level, by which the inhibitors were able to act as antiproliferative agents.