Identification of FTY720 and COH29 as novel topoisomerase I catalytic inhibitors by experimental and computational studies.

The development of novel topoisomerase I (TOP1) inhibitors is crucial for overcoming the drawbacks and limitations of current TOP1 poisons. Here, we identified two potential TOP1 inhibitors, namely, FTY720 (a sphingosine 1-phosphate antagonist) and COH29 (a ribonucleotide reductase inhibitor), through experimental screening of known active compounds. Biological experiments verified that FTY720 and COH29 were nonintercalative TOP1 catalytic inhibitors that did not induce the formation of DNA-TOP1 covalent complexes. Molecular docking revealed that FTY720 and COH29 interacted favorably with TOP1. Molecular dynamics simulations revealed that FTY720 and COH29 could affect the catalytic domain of TOP1, thus resulting in altered DNA-binding cavity size. The alanine scanning and interaction entropy identified Arg536 as a hotspot residue. In addition, the bioinformatics analysis predicted that FTY720 and COH29 could be effective in treating malignant breast tumors. Biological experiments verified their antitumor activities using MCF-7 breast cancer cells. Their combinatory effects with TOP1 poisons were also investigated. Further, FTY720 and COH29 were found to cause less DNA damage compared with TOP1 poisons. The findings provide reliable lead compounds for the development of novel TOP1 catalytic inhibitors and offer new insights into the potential clinical applications of FTY720 and COH29 in targeting TOP1.

Design, Synthesis, and Investigation of the Pharmacokinetics and Anticancer Activities of Indenoisoquinoline Derivatives That Stabilize the G-Quadruplex in the MYC Promoter and Inhibit Topoisomerase I.

G-quadruplexes are noncanonical four-stranded DNA secondary structures. MYC is a master oncogene and the G-quadruplex formed in the MYC promoter functions as a transcriptional silencer and can be stabilized by small molecules. We have previously revealed a novel mechanism of action for indenoisoquinoline anticancer drugs, dual-downregulation of MYC and inhibition of topoisomerase I. Herein, we report the design and synthesis of novel 7-aza-8,9-methylenedioxyindenoisoquinolines based on desirable substituents and π-π stacking interactions. These compounds stabilize the MYC promoter G-quadruplex, significantly lower MYC levels in cancer cells, and inhibit topoisomerase I. MYC targeting was demonstrated by differential activities in Raji vs CA-46 cells and cytotoxicity in MYC-dependent cell lines. Cytotoxicities in the NCI-60 panel of human cancer cell lines were investigated. Favorable pharmacokinetics were established, and in vivo anticancer activities were demonstrated in xenograft mouse models. Furthermore, favorable brain penetration, brain pharmacokinetics, and anticancer activity in an orthotopic glioblastoma mouse model were demonstrated.

Minocycline and photodynamic priming significantly improve chemotherapy efficacy in heterotypic spheroids of pancreatic ductal adenocarcinoma.

The prognosis for patients with advanced-stage pancreatic ductal adenocarcinoma (PDAC) remains dismal. It is generally accepted that combination cancer therapies offer the most promise, such as Folforinox, despite their associated high toxicity. This study addresses the issue of chemoresistance by introducing a complementary dual priming approach to attenuate the DNA repair mechanism and to improve the efficacy of a type 1 topoisomerase (Top1) inhibitor. The result is a regimen that integrates drug-repurposing and nanotechnology using 3 clinically relevant FDA-approved agents (1) Top1 inhibitor (irinotecan) at subcytotoxic doses (2) benzoporphyrin derivative (BPD) as a photoactive molecule for photodynamic priming (PDP) to improve the delivery of irinotecan within the cancer cell and (3) minocycline priming (MNP) to modulate DNA repair enzyme Tdp1 (tyrosyl-DNA phosphodiesterase) activity. We demonstrate in heterotypic 3D cancer models that incorporate cancer cells and pancreatic cancer-associated fibroblasts that simultaneous targeting of Tdp1 and Top1 were significantly more effective by employing MNP and photoactivatable multi-inhibitor liposomes encapsulating BPD and irinotecan compared to monotherapies or a cocktail of dual or triple-agents. These data are encouraging and warrant further work in appropriate animal models to evolve improved therapeutic regimens.

Design, synthesis, and biological evaluation of novel benzo[6,7]indolo[3,4-c]isoquinolines as anticancer agents with topoisomerase I inhibition.

A novel series of benzo[6,7]indolo[3,4-c]isoquinolines 3a-3f was designed by scaffold hopping of topoisomerase I inhibitor benzo[g][1]benzopyrano[4,3-b]indol-6(13H)-ones (BBPIs), which were developed by structural modification of the natural marine product lamellarin. The unconventional pentacycle was constructed by Bischler-Napieralski-type condensation of amide 11 and subsequent intramolecular Heck reaction. In vitro anticancer activity of the synthesized benzo[6,7]indolo[3,4-c]isoquinolines was evaluated on a panel of 39 human cancer cell lines (JFCR39). Among the compounds tested, N-(3-morpholinopropyl) derivative 3e showed the most potent antiproliferative activity, with a mean GI50 value of 39 nM. This compound inhibited topoisomerase I activity by stabilizing the enzyme-DNA complex.

Identification and characterization of topoisomerase III beta poisons.

We designed and carried out a high-throughput screen for compounds that trap topoisomerase III beta (TOP3B poisons) by developing a Comparative Cellular Cytotoxicity Screen. We found a bisacridine compound NSC690634 and a thiacyanine compound NSC96932 that preferentially sensitize cell lines expressing TOP3B, indicating that they target TOP3B. These compounds trap TOP3B cleavage complex (TOP3Bcc) in cells and in vitro and predominately act on RNA, leading to high levels of RNA-TOP3Bccs. NSC690634 also leads to enhanced R-loops in a TOP3B-dependent manner. Preliminary structural activity studies show that the lengths of linkers between the two aromatic moieties in each compound are critical; altering the linker length completely abolishes the trapping of TOP3Bccs. Both of our lead compounds share a similar structural motif, which can serve as a base for further modification. They may also serve in anticancer, antiviral, and/or basic research applications.

Identification of N-(3-(methyl(3-(orotic amido)propyl)amino)propyl) oleanolamide as a novel topoisomerase I catalytic inhibitor by rational design, molecular dynamics simulation, and biological evaluation.

DNA topoisomerase I (TOP1) catalytic inhibitors are a promising class of antitumor agents. Oleanolic acid derivatives are potential TOP1 catalytic inhibitors. However, their inhibitory activity still needs to be enhanced, and the stability and hotspot residue sites of their interaction with TOP1 remain to be elucidated. Herein, a novel oleanolic acid derivative, OA4 (N-(3-(methyl(3-(orotic amido)propyl)amino)propyl)oleanolamide), was identified by rational design. Subsequently, molecular dynamics simulations were performed to explore the stability and conformational dynamics of the TOP1-OA4 complex. The molecular mechanics/generalized Born surface area method calculated the binding free energy and predicted Arg488, Ile535, and His632 to be hotspot residues. Biological experiments verified that OA4 is a nonintercalative TOP1 catalytic inhibitor. OA4 exhibits better proliferation inhibitory activity against tumor cells than normal cells. Furthermore, OA4 can induce apoptosis and effectively suppress the proliferation and migration of cancer cells. This work provides new insights for the development of novel TOP1 catalytic inhibitors.

Molecular Docking and Dynamics Simulation Revealed the Potential Inhibitory Activity of New Drugs against Human Topoisomerase I Receptor.

Human Topoisomerase I (hTop1p) is a ubiquitous enzyme that relaxes supercoiled DNA through a conserved mechanism involving transient breakage, rotation, and binding. Htop1p is the molecular target of the chemotherapeutic drug camptothecin (CPT). It causes the hTop1p-DNA complex to slow down the binding process and clash with the replicative machinery during the S phase of the cell cycle, forcing cells to activate the apoptotic response. This gives hTop1p a central role in cancer therapy. Recently, two artesunic acid derivatives (compounds c6 and c7) have been proposed as promising inhibitors of hTop1p with possible antitumor activity. We used several computational approaches to obtain in silico confirmations of the experimental data and to form a comprehensive dynamic description of the ligand-receptor system. We performed molecular docking analyses to verify the ability of the two new derivatives to access the enzyme-DNA interface, and a classical molecular dynamics simulation was performed to assess the capacity of the two compounds to maintain a stable binding pose over time. Finally, we calculated the noncovalent interactions between the two new derivatives and the hTop1p receptor in order to propose a possible inhibitory mechanism like that adopted by CPT.

Insight on the Interaction between the Camptothecin Derivative and DNA Oligomer Mimicking the Target of Topo I Inhibitors.

The understanding of the mechanism of Topo I inhibition by organic ligands is a crucial source of information that has led to the design of more effective and safe pharmaceuticals in oncological chemotherapy. The vast number of inhibitors that have been studied in this respect over the last decades have enabled the creation of a concept of an 'interfacial inhibitor', thereby describing the machinery of Topo I inhibition. The central module of action of this machinery is the interface of a Topo I/DNA/inhibitor ternary complex. Most of the 'interfacial inhibitors' are primarily kinetic inhibitors that form molecular complexes with an "on-off" rate timing; therefore, all of the contacts between the inhibitor and both the enzyme and the DNA are essential to keep the complex stable and reduce the "off rate". To test this hypothesis, we designed the compound using a C-9-(N-(2'-hydroxyethyl)amino)methyl substituent in an SN38 core, with a view that a flexible substituent may bind inside the nick of a model of the DNA and stabilize the complex, leading to a reduction in the "off rate" of a ligand in a potential ternary complex in vivo. Using docking analysis and molecular dynamics, free energy calculations on the level of the MM-PBSA and MM-GBSA model, here we presented the in silico-calculated structure of a ternary complex involving the studied compound 1. This confirmed our suggestion that compound 1 is situated in a groove of the nicked DNA model in a few conformations. The number of hydrogen bonds between the components of a ternary complex was established, which strengthens the complex and supports our view. The docking analysis and free energy calculations for the receptor structures which were obtained in the MD simulations of the ternary complex 1/DNA/Topo I show that the binding constant is stronger than it was for similar complexes with TPT, CPT, and SN38, which are commonly considered as strong Topo I inhibitors. The binary complex structure 1/DNA was calculated and compared with the experimental results of a complex that was in a solution. The analysis of the cross-peaks in NOESY spectra allowed us to assign the dipolar interactions between the given protons in the calculated structures. A DOSY experiment in the solution confirmed the strong binding of a ligand in a binary complex, having a Ka of 746 mM-1, which was compared with a Ka of 3.78 mM-1 for TPT. The MALDI-ToF MS showed the presence of the biohybrid, thus evidencing the occurrence of DNA alkylation by compound 1. Because of it having a strong molecular complex, alkylation is the most efficient way to reduce the "on-off" timing as it acts as a tool that causes the cog to brake in a working gear, and this is this activity we want to highlight in our contribution. Finally, the Topo I inhibition test showed a lower IC50 of the studied compound than it did for CPT and SN38.

Topoisomerase Enzyme Inhibitors as Potential Drugs Against Cancer: What Makes Them Selective or Dual? - A Review.

Topoisomerase inhibitors are extensively used in cancer chemotherapy. In the process of identifying novel anticancer compounds, biological evaluations are crucial and include, among others, the use of in silico and in vitro approaches. This work aimed to present recent research involving the obtainment and in silico and in vitro evaluation of topoisomerase I, II, and double inhibitors, of synthetic and natural origin, as potential compounds against tumor cells, in addition to proposing the construction of a desirable enzyme catalytic site. Therefore, it was observed that most Topoisomerase I inhibitors presented medium to large structures, with a rigid portion and a flexible region. In contrast, Topoisomerase IIα inhibitors showed medium and large structural characteristics, in addition to the planarity of the aromatic rings, which are mitigated due to flexible rings but may also present elements that restrict conformation. Most compounds that exhibit dual inhibitory activity had relatively long chains, in addition to a flat and rigid portion suggestive of affinity for Topo I and a flexible region characteristic of selective drugs for Topo II. Besides, it is noticed that most compounds that exhibit dual inhibitory showed similarities in the types of interactions and amino acids when compared to the selective compounds of Topo I and II. For instance, selective Topoisomerase I inhibitors interact with Arginine364 residues, and selective Topoisomerase II inhibitors interact with Arginine487 residues, as both residues are targets for dual compounds.

Discovery of dual inhibitors of topoisomerase I and Cyclooxygenase-2 for colon cancer therapy.

Novel tolfenamic acid derivatives based on the structure of I-1 were designed and synthesized to improve its poor target inhibition and solubility. Among them, W10 was identified as a potent dual-target inhibitor of Topo I (IC50 = 0.90 ± 0.17 µM) and COX-2 (IC50 = 2.31 ± 0.07 µM) with improved water solubility (32.33 µg/mL). Moreover, W10 also exhibited fairly potent anti-proliferative and pro-apoptosis activity via the mitochondrial pathway, as well as suppressed aberrant NF-κB/IκB activation in colon cancer cells in vitro. Additionally, W10 possessed favorable pharmacokinetic properties and excellent antitumor effects in vivo. In general, our study has demonstrated the potency of a novel Topo I/COX-2 dual inhibitor, which can potentially be developed into a chemotherapeutic candidate for colon cancer.

Discovery of novel benzofuro[3,2-b]quinoline derivatives as dual CDK2/Topo I inhibitors.

Uncontrolled cell proliferation is a hallmark of cancer. The major regulator of the cell cycle, cyclin dependent kinase 2 (CDK2), has become a mature target for cancer treatment. Herein, we describe our efforts toward the discovery of a series of benzofuro[3,2-b]quinoline alkaloid derivatives as CDK2 inhibitors through a scaffold hopping strategy. Compound ZLHQ-5f has topoisomerase I (Topo I) inhibitory activity due to the unique structure of benzofurano[3,2-b]quinoline. Resultantly, ZLHQ-5f exhibited promising anti-proliferative and CDK2 inhibitory activities. It also arrests the cell cycle in S-phase, triggers apoptosis in HCT116 cells, and has a good safety profile in vivo. There has yet to be a report on dual CDK2/Topo I inhibitor, thus this will be a novel attempt.

Total Synthesis, Mechanism of Action, and Antitumor Efficacy of Camptothecin and Some of its Analogues.

Over the past 55 years of research, various experimental methods have been developed for the total synthesis of the anticancer camptothecin, a potent antitumor antibiotic, and its numerous active derivatives. The discoveries made in synthetic pathways of the camptothecin heterocyclic core have contributed significantly to the theory and strategy of directed organic synthesis aimed at finding effective anticancer drugs. The synthetic, medicinal chemistry of camptothecin, the development of structures of anticancer camptothecin analogues, and the mechanism of their activity in inhibiting the growth of different types of cancers, such as lung, ovarian, breast, pancreas, and stomach cancers are analyzed. Various structural modifications in the A, B, C, D, and E-rings of the camptothecin molecule have been thoroughly studied to improve bioavailability and diminish toxicity. Modern synthetic approaches to the camptothecin analogues and several semi-synthetic methods are reviewed.

Topoisomerase I inhibitors: Challenges, progress and the road ahead.

Topoisomerase IB (Top1), a subcategory of DNA topoisomerase enzymes is expressed much higher in several tumor cells. Therefore, modulating the activity of Top1 in tumor cells to prevent DNA replication and subsequent cell division made it an important drug target for anticancer therapy. FDA-approved camptothecin (CPT) derivatives topotecan and irinotecan exert anticancer activity through stabilization of enzyme-mediated DNA cleavage complex forming a ternary complex between DNA-Top1-drug. However, CPT derivatives suffer from several limitations which prompted interest in the development of 'non-camptothecin' Top1 poisons as anticancer agents. This review aims to provide chronological development of different classes of Top1 poisons from both natural and synthetic sources through strategic structure-activity relationship (SAR) analysis with insight into the important structural features in different chemotypes that imparted Top1 inhibition along with the understanding of the structural basis of inhibition. This review also provides a snapshot of the application of Top1 poisons in various combination therapies in recent times. We believe such a comprehensive review is going to be beneficial for the medicinal chemistry community to design efficient drug development strategies using existing knowledge.

Pyridoxal hydrochloride thiosemicarbazones with copper ions inhibit cell division via Topo-I and Topo-IIɑ.

Topoisomerase (Topo) accelerates cell growth and division, and has been a theoretical target for anti-cancer drugs for decades. A series of pyridoxal thiosemicarbazone (PLT) ligands were designed and synthesized, and the dependence of their antiproliferative activity on copper was investigated. The insertion of N-cyclohexyl-2-((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methylene)-N-methylhydrazinecarbothioamide hydrochloride (compound 9) and Chlorido(N-cyclohexyl-2-((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methylene)-N-methylhydrazinecarbothioamide hydrochloride-O,N,S)­copper(II) nitrate (9-Cu complex) into Topo-I and Topo-II prevented uncoiling of DNA through hydrogen bonds and intermolecular forces. The combination of PLT derivatives and copper gluconate (CuGlu) improved their anti-tumour activity against a cell line with high expression of topoisomerase (SK-BR-3). The non-linear regression equations of the inhibitory activity and anti-tumour activity of Topo-I and Topo-IIɑ in SK-BR-3 cells had R2 values of 0.93 and 0.94, respectively. In addition to lipophilicity, inhibition of topoisomerase also affected the activity of PLT ligands by coordinating with copper ions. At the cellular level, PLTs and CuGlu penetrate the cell membrane to form metabolites in the cell, thus selectively inhibiting the activity of Topo-I and Topo-IIɑ, and ultimately inhibiting cell division. These findings will inform the design of future anti-cancer thiosemicarbazone drugs.

Poloxamer-linked prodrug of a topoisomerase I inhibitor SN22 shows efficacy in models of high-risk neuroblastoma with primary and acquired chemoresistance.

High-risk solid tumors continue to pose a tremendous therapeutic challenge due to multidrug resistance. Biological mechanisms driving chemoresistance in high-risk primary and recurrent disease are distinct: in newly diagnosed patients, non-response to therapy is often associated with a higher level of tumor "stemness" paralleled by overexpression of the ABCG2 drug efflux pump, whereas in tumors relapsing after non-curative therapy, poor drug sensitivity is most commonly linked to the dysfunction of the tumor suppressor protein, p53. In this study, we used preclinical models of aggressive neuroblastoma featuring these characteristic mechanisms of primary and acquired drug resistance to experimentally evaluate a macromolecular prodrug of a structurally enhanced camptothecin analog, SN22, resisting ABCG2-mediated export, and glucuronidation. Together with extended tumor exposure to therapeutically effective drug levels via reversible conjugation to Pluronic F-108 (PF108), these features translated into rapid tumor regression and long-term survival in models of both ABCG2-overexpressing and p53-mutant high-risk neuroblastomas, in contrast to a marginal effect of the clinically used camptothecin derivative, irinotecan. Our results demonstrate that pharmacophore enhancement, increased tumor uptake, and optimally stable carrier-drug association integrated into the design of the hydrolytically activatable PF108-[SN22]2  have the potential to effectively combat multiple mechanisms governing chemoresistance in newly diagnosed (chemo-naïve) and recurrent forms of aggressive malignancies. As a macromolecular carrier-based delivery system exhibiting remarkable efficacy against two particularly challenging forms of high-risk neuroblastoma, PF108-[SN22]2 can pave the way to a robust and clinically viable therapeutic strategy urgently needed for patients with multidrug-resistant disease presently lacking effective treatment options.

On-Demand Activation of a Bioorthogonal Prodrug of SN-38 with Fast Reaction Kinetics and High Releasing Efficiency In Vivo.

Although a myriad of bioorthogonal prodrugs have been developed, very few of them present both fast reaction kinetics and complete cleavage. Herein, we report a new bioorthogonal prodrug strategy with both fast reaction kinetics (k2: ∼103 M-1 s-1) and complete cleavage (>90% within minutes) using the bioorthogonal reaction pair of N-oxide and boron reagent. Distinctively, an innovative 1,6-elimination-based self-immolative linker is masked by N-oxide, which can be bioorthogonally demasked by a boron reagent for the release of both amino and hydroxy-containing payload in live cells. Such a strategy was applied to prepare a bioorthogonal prodrug for a camptothecin derivative, SN-38, resulting in 10-fold weakened cytotoxicity against A549 cells, 300-fold enhanced water solubility, and "on-demand" activation upon a click reaction both in vitro and in vivo. This novel bioorthogonal prodrug strategy presents significant advances over the existing ones and may find wide applications in drug delivery in the future.

pH-responsive and folate-coated liposomes encapsulating irinotecan as an alternative to improve efficacy of colorectal cancer treatment.

Irinotecan (IRN) is a semisynthetic derivative of camptothecin that acts as a topoisomerase I inhibitor. IRN is used worldwide for the treatment of several types of cancer, including colorectal cancer, however its use can lead to serious adverse effects, as diarrhea and myelosuppression. Liposomes are widely used as drug delivery systems that can improve chemotherapeutic activity and decrease side effects. Liposomes can also be pH-sensitive to release its content preferentially in acidic environments, like tumors, and be surface-functionalized for targeting purposes. Herein, we developed a folate-coated pH-sensitive liposome as a drug delivery system for IRN to reach improved tumor therapy without potential adverse events. Liposomes were prepared containing IRN and characterized for particle size, polydispersity index, zeta potential, concentration, encapsulation, cellular uptake, and release profile. Antitumor activity was investigated in a murine model of colorectal cancer, and its toxicity was evaluated by hematological/biochemical tests and histological analysis of main organs. The results showed vesicles smaller than 200 nm with little dispersion, a surface charge close to neutral, and high encapsulation rate of over 90%. The system demonstrated prolonged and sustained release in pH-dependent manner with high intracellular drug delivery capacity. Importantly, the folate-coated pH-sensitive formulation had significantly better antitumor activity than the pH-dependent system only or the free drug. Tumor tissue of IRN-containing groups presented large areas of necrosis. Furthermore, no evidence of systemic toxicity was found for the groups investigated. Thus, our developed nanodrug IRN delivery system can potentially be an alternative to conventional colorectal cancer treatment.

Ruthenium(II) Diphosphine Complexes with Mercapto Ligands That Inhibit Topoisomerase IB and Suppress Tumor Growth In Vivo.

Ruthenium(II) complexes (Ru1-Ru5), with the general formula [Ru(N-S)(dppe)2]PF6, bearing two 1,2-bis(diphenylphosphino)ethane (dppe) ligands and a series of mercapto ligands (N-S), have been developed. The combination of these ligands in the complexes endowed hydrophobic species with high cytotoxic activity against five cancer cell lines. For the A549 (lung) and MDA-MB-231 (breast) cancer cell lines, the IC50 values of the complexes were 288- to 14-fold lower when compared to cisplatin. Furthermore, the complexes were selective for the A549 and MDA-MB-231 cancer cell lines compared to the MRC-5 nontumor cell line. The multitarget character of the complexes was investigated by using calf thymus DNA (CT DNA), human serum albumin, and human topoisomerase IB (hTopIB). The complexes potently inhibited hTopIB. In particular, complex [Ru(dmp)(dppe)2]PF6 (Ru3), bearing the 4,6-diamino-2-mercaptopyrimidine (dmp) ligand, effectively inhibited hTopIB by acting on both the cleavage and religation steps of the catalytic cycle of this enzyme. Molecular docking showed that the Ru1-Ru5 complexes have binding affinity by active sites on the hTopI and hTopI-DNA, mainly via π-alkyl and alkyl hydrophobic interactions, as well as through hydrogen bonds. Complex Ru3 displayed significant antitumor activity against murine melanoma in mouse xenograph models, but this complex did not damage DNA, as revealed by Ames and micronucleus tests.

Novel Nontoxic 5,9-Disubstituted SN38 Derivatives: Characterization of Their Pharmacological Properties and Interactions with DNA Oligomers.

Novel nontoxic derivatives of SN38 with favorable antineoplastic properties were characterized in water solution using NMR. The phenomena observed by NMR were linked to basic pharmacological properties, such as solubility, bioavailability, chemical and stereochemical stability, and binding to natural DNA oligomers through the terminal G-C base pair, which is commonly considered a biological target of Topo I inhibitors. Compound 1, with bulky substituents at both C5(R) and C20(S) on the same side of a camptothecin core, manifests self-association, whereas diastereomers 2, with bulky C5(S) and C20(S) substituents are mostly monomeric in solution. The stereogenic center at C5 is stable in water solution at pH 5-6. The compound with an (N-azetidinyl)methyl substituent at C9 can undergo the retro Mannich reaction after a prolonged time in water solution. Both diastereomers exhibit different abilities in terms of binding to DNA oligomers: compound 1 is strongly bound, whereas the binding of compound 2 is rather weak. Molecular modeling produced results consistent with NMR experiments. These complementary data allow linking of the observed phenomena in NMR experiments to basic preliminary information on the pharmacodynamic character of compounds and are essential for planning further development research.

Discovery of Leishmania donovani topoisomerase IB selective inhibitors by targeting protein-protein interactions between the large and small subunits.

Visceral leishmaniasis (VL) is a fatal infectious disease caused by viscerotropic parasitic species of Leishmania. Current treatment options are often ineffective and toxic, and more importantly, there are no clinically validated drug targets available to develop next generation therapeutics against VL. Topoisomerase IB (TopIB) is an essential enzyme for Leishmania survival. The enzyme is organized as a bi-subunit that is distinct from the monomeric topoisomerase I of human. Based on this unique feature, we synthesized peptides composed of partial amino acid sequences of small subunit of Leishmania donovani (Ld) TopIB to confirm a decrease in catalytic activity by interfering the interaction between the two subunits. One of the synthetic peptides, covering essential amino acids for catalytic activity of LdTopIB, interrupted the enzymatic activity. Next, we examined 151 compounds selected from virtual screening in a functional assay and identified three LRL-TP compounds with a significant decrease in LdTopIB activity (IC50 of LRL-TP-85: 1.3 µM; LRL-TP-94: 2.9 µM; and LRL-TP-101: 35.3 µM) and no effects on Homo sapiens (Hs) TopIB activity. Based on molecular docking, the protonated tertiary amine of inhibitors formed key interactions with S415 of the large subunit. The EC50 values of LRL-TP-85, LRL-TP-94, and LRL-TP-101 were respectively 4.9, 1.4, and 27.8 µM in extracellular promastigote assay and 34.0, 53.7, and 11.4 µM in intracellular amastigote assay. Overall, we validated the protein-protein interaction site of LdTopIB as a potential drug target and identified small molecule inhibitors with anti-leishmanial activity.