Structure-activity investigations of polyamine-anthracene conjugates and their uptake via the polyamine transporter

Synthesis and evaluation of a 68Ga-labeled spermine derivative for tumor PET imaging

BACKGROUND

The polyamine transporter system (PTS), which renders it a promising target for tumor therapy and imaging applications, facilitates the transmembrane transport of polyamines. We reported a novel derivative of spermine labeled with gallium-68 ([68Ga]Ga-NOTA-Spermine) for the imaging of the PTS in mouse models of tumor.

RESULTS

The radiochemical yield of [68Ga]Ga-NOTA-Spermine was determined to be 64-69 %, demonstrating exceptional stability and radiochemical purity (>98 %). Cellular uptake experiments revealed that A549 cells exhibited peak uptake of [68Ga]Ga-NOTA-Spermine at 90 min (15.4 % ± 0.68 %). Biodistribution analysis demonstrated significant accumulation of [68Ga]Ga-NOTA-Spermine in kidneys and liver, while exhibiting low uptake levels in muscle, brain, and bones. Furthermore, Micro-PET/CT scans conducted on A549 tumor-bearing mouse models indicated substantial uptake of [68Ga]Ga-NOTA-Spermine, with maximum tumor/muscle (T/M) ratios reaching 3.71.

CONCLUSION

These results suggest that [68Ga]Ga-NOTA-Spermine holds potential as a PET imaging agent for tumors with high levels of PTS.

Synthesis and in vitro cytotoxic activity of dye-linker-macrocycle conjugates with variable linker length and components

The study investigated the structure-activity relationship of newly synthesized dye-linker-macrocycle (DLM) conjugates and the effect of each component on various biological properties, including cytotoxicity, cellular uptake, intracellular localization, interaction with DNA and photodynamic effects. The conjugates were synthesized by combining 1,8-naphthalimide and thioxanthone dyes with 1,4,7,10-tetraazacyclododecane (cyclen) and 1-aza-12-crown-4 (1A12C4) using alkyl linkers of different lengths. The results revealed significant differences in biological activity among the various series of conjugates. Particularly, 1A12C4 conjugates exhibited notably higher cytotoxicity compared to cyclen conjugates. Conjugation with 1A12C4 proved to be an effective strategy for increasing cellular uptake and cytotoxicity of small-molecule conjugates. In addition, the results highlighted the critical role of linker length in modulating the biological activity of DLM conjugates. It became clear that the choice of each component (dye, macrocycle and linker) could significantly alter the biological activity of the conjugates.

Discovery of Anthranilic Acid Derivatives as Difluoromethylornithine Adjunct Agents That Inhibit Far Upstream Element Binding Protein 1 (FUBP1) Function

Polyamine biosynthesis is regulated by ornithine decarboxylase (ODC), which is transcriptionally activated by c-Myc. A large library was screened to find molecules that potentiate the ODC inhibitor, difluoromethylornithine (DFMO). Anthranilic acid derivatives were identified as DFMO adjunct agents. Further studies identified the far upstream binding protein 1 (FUBP1) as the target of lead compound 9. FUBP1 is a single-stranded DNA/RNA binding protein and a master controller of specific genes including c-Myc and p21. We showed that 9 does not inhibit 3H-spermidine uptake yet works synergistically with DFMO to limit cell growth in the presence of exogenous spermidine. Compound 9 was also shown to inhibit the KH4 FUBP1-FUSE interaction in a gel shift assay, bind to FUBP1 in a ChIP assay, reduce both c-Myc mRNA and protein expression, increase p21 mRNA and protein expression, and deplete intracellular polyamines. This promising hit opens the door to new FUBP1 inhibitors with increased potency.

Development of Polyamine Lassos as Polyamine Transport Inhibitors

Nine- and twelve-membered triaza-macrocycles were appended to one end of homospermidine to make polyamine lassos. These compounds were shown to be potent polyamine transport inhibitors (PTIs) using pancreatic ductal adenocarcinoma L3.6pl cells, which have high polyamine transport activity. The smaller triazacyclononane-based lasso significantly reduced the uptake of a fluorescent polyamine probe and inhibited spermidine uptake and reduced intracellular polyamine levels in difluoromethylornithine (DFMO)-treated L3.6pl cells. Both designs were shown to be effective inhibitors of ³H-spermidine uptake, with the smaller lasso outperforming the larger lasso. When the smaller lasso was challenged to inhibit each of the three radiolabeled native polyamines, it had similar K _i values as those of the known PTIs, Trimer44NMe and AMXT1501. Because of these promising properties, these materials may have future anticancer applications in polyamine blocking therapy, an approach that couples a polyamine biosynthesis inhibitor (DFMO) with a PTI to lower intracellular polyamines and suppress cell growth.

Investigating how amine structure influences drug-amine ion-pair formation and uptake via the polyamine transporter in A549 lung cells

Transiently associating amines with therapeutic agents through the formation of ion-pairs has been established both in vitro and in vivo as an effective means to systemically direct drug delivery to the lung via the polyamine transport system (PTS). However, there remains a need to better understand the structural traits required for effective PTS uptake of drug ion-pairs. This study aimed to use a structurally related series of amine counterions to investigate how they influenced the stability of theophylline ion-pairs and their active uptake in A549 cells. Using ethylamine (mono-amine), ethylenediamine (di-amine), spermidine (tri-amine) and spermine (tetra-amine) as counterions the ion-pair affinity was shown to increase as the number of protonated amine groups in the counterion structure increased. The mono and diamines generated a single hydrogen bond and the weakest ion-pair affinities (pK_FTIR: 1.32 ± 0.04 and 1.43 ± 0.02) whereas the polyamines produced two hydrogen bonds and thus the strongest ion-pair affinities (pK_FTIR: 1.93 ± 0.05 and 1.96 ± 0.04). In A549 cells depleted of endogenous polyamines using α-difluoromethylornithine (DFMO), the spermine-theophylline uptake was significantly increased (p < 0.05) compared to non-amine depleted cells and this evidenced the active PTS sequestering of the ion-pair. The mono-amine and di-amine failed to enhance theophylline uptake in these A549 cells, but the tri-amine and tetra-amine both almost doubled the theophylline uptake into the cells when compared to the uptake of free drug. As the data indicated that polyamines with at least 3 amines were required to form ion-pairs that could enhance A549 cell uptake, it suggested that at least two amines were required to physically stabilise the ion-pair and one to interact with the PTS.

The discovery of indolone GW5074 during a comprehensive search for non-polyamine-based polyamine transport inhibitors

The native polyamines putrescine, spermidine, and spermine are essential for cell development and proliferation. Polyamine levels are often increased in cancer tissues and polyamine depletion is a validated anticancer strategy. Cancer cell growth can be inhibited by the polyamine biosynthesis inhibitor difluoromethylornithine (DFMO), which inhibits ornithine decarboxylase (ODC), the rate-limiting enzyme in the polyamine biosynthesis pathway. Unfortunately, cells treated with DFMO often replenish their polyamine pools by importing polyamines from their environment. Several polyamine-based molecules have been developed to work as polyamine transport inhibitors (PTIs) and have been successfully used in combination with DFMO in several cancer models. Here, we present the first comprehensive search for potential non-polyamine based PTIs that work in human pancreatic cancer cells in vitro. After identifying and testing five different categories of compounds, we have identified the c-RAF inhibitor, GW5074, as a novel non-polyamine based PTI. GW5074 inhibited the uptake of all three native polyamines and a fluorescent-polyamine probe into human pancreatic cancer cells. GW5074 significantly reduced pancreatic cancer cell growth in vitro when treated in combination with DFMO and a rescuing dose of spermidine. Moreover, GW5074 alone reduced tumor growth when tested in a murine pancreatic cancer mouse model in vivo. In summary, GW5074 is a novel non-polyamine-based PTI that potentiates the anticancer activity of DFMO in pancreatic cancers.

Syntheses and Evaluation of New Bisacridine Derivatives for Dual Binding of G-Quadruplex and i-Motif in Regulating Oncogene c-myc Expression

The c-myc oncogene is an important regulator for cell growth and differentiation, and its aberrant overexpression is closely related to the occurrence and development of various cancers. Thus, the suppression of c-myc transcription and expression has been investigated for cancer treatment. In this study, various new bisacridine derivatives were synthesized and evaluated for their binding with c-myc promoter G-quadruplex and i-motif. We found that a9 could bind to and stabilize both G-quadruplex and i-motif, resulting in the downregulation of c-myc gene transcription. a9 could inhibit cancer cell proliferation and induce SiHa cell apoptosis and cycle arrest. a9 exhibited tumor growth inhibition activity in a SiHa xenograft tumor model, which might be related to its binding with c-myc promoter G-quadruplex and i-motif. Our results suggested that a9 as a dual G-quadruplex/i-motif binder could be effective in both oncogene replication and transcription and become a promising lead compound for further development with improved potency and selectivity.

Identification of Trypanosoma cruzi Polyamine Transport Inhibitors by Computational Drug Repurposing

Trypanosoma cruzi is the causative agent of Chagas disease, a parasitic infection endemic in Latin America. In T. cruzi the transport of polyamines is essential because this organism is unable to synthesize these compounds de novo. Therefore, the uptake of polyamines from the extracellular medium is critical for survival of the parasite. The anthracene-putrescine conjugate Ant4 was first designed as a polyamine transport probe in cancer cells. Ant4 was also found to inhibit the polyamine transport system and produced a strong trypanocidal effect in T. cruzi. Considering that Ant4 is not currently approved by the FDA, in this work we performed computer simulations to find trypanocidal drugs approved for use in humans that have structures and activities similar to Ant4. Through a similarity ligand-based virtual screening using Ant4 as reference molecule, four possible inhibitors of polyamine transport were found. Three of them, promazine, chlorpromazine, and clomipramine, showed to be effective inhibitors of putrescine uptake, and also revealed a high trypanocidal activity against T. cruzi amastigotes (IC₅₀ values of 3.8, 1.9, and 2.9 μM, respectively) and trypomastigotes (IC₅₀ values of 3.4, 2.7, and 1.3 μM, respectively) while in epimastigotes the IC₅₀ were significantly higher (34.7, 41.4, and 39.7 μM, respectively). Finally, molecular docking simulations suggest that the interactions between the T. cruzi polyamine transporter TcPAT12 and all the identified inhibitors occur in the same region of the protein. However, this location is different from the site occupied by the natural substrates. The value of this effort is that repurposing known drugs in the treatment of other pathologies, especially neglected diseases such as Chagas disease, significantly decreases the time and economic cost of implementation.

Synthesis and antiproliferative evaluation of some iron chelators as polyamine transporter targeting agents

A series of kojic acid derivatives based on monoamines, diamines, and triamines have been synthesized in good yield and purity. A spermidine (spd) rescue experiment was applied against MCF-7 cells to evaluate the polyamine transporter selectivity, and in vitro antiproliferative effects were determined against Hela and DU-145 cell lines. Overall, 5b showed the best selectivity for the polyamine transporter and antiproliferative effects. Therefore, the in-silico metabolism profile and ADMET properties of the title compounds were calculated by the PreADMET server. Additionally, physicochemical properties of ligands were predicted by using the Molinspiration online property calculation server.

Conjugation with polyamines enhances the antitumor activity of naphthoquinones against human glioblastoma cells

Glioblastoma multiform (GBM) is the most common and devastating type of primary brain tumor, being considered the deadliest of human cancers. In this context, extensive efforts have been undertaken to develop new drugs that exhibit both antiproliferation and antimetastasis effects on GBM. 1,4-Naphthoquinone (1,4-NQ) scaffold has been found in compounds able to inhibit important biological targets associated with cancer, which includes DNA topoisomerase, Hsp90 and monoamine oxidase. Among potential antineoplastic 1,4-NQs is the plant-derived lapachol (2-hydroxy-3-prenyl-1,4-naphthoquinone) that was found to be active against the Walker-256 carcinoma and Yoshida sarcoma. In the present study, we examined the effect of polyamine (PA)-conjugated derivatives of lapachol, nor-lapachol and lawsone on the growth and invasion of the human GBM cells. The conjugation with PA (a spermidine analog) resulted in dose-dependent and time-dependent increase of cytotoxicity of the 1,4-NQs. In addition, in-vitro inhibition of GBM cell invasion by lapachol was increased upon PA conjugation. Previous biochemical experiments indicated that these PA-1,4-NQs are capable of inhibiting DNA human topoisomerase II-α (topo2α), a major enzyme involved in maintaining DNA topology. Herein, we applied molecular docking to investigate the binding of PA-1,4-NQs to the ATPase site of topo2α. The most active molecules preferentially bind at the ATP-binding site of topo2α, which is energetically favored by the conjugation with PA. Taken together, these findings suggested that the PA-1,4-NQ conjugates might represent potential molecules in the development of new drugs in chemotherapy for malignant brain tumors.

Modification of Biodistribution and Brain Uptake of Copper Bis(thiosemicarbazonato) Complexes by the Incorporation of Amine and Polyamine Functional Groups

The synthesis of new bis(thiosemicarbazonato)copper(II) complexes featuring polyamine substituents via selective transamination reactions is presented. Polyamines of different lengths, with different ionizable substituent groups, were used to modify and adjust the hydrophilic/lipophilic balance of the copper complexes. The new analogues were radiolabeled with copper-64 and their lipophilicities estimated using distribution coefficients. The cell uptake of the new polyamine complexes was investigated with preliminary in vitro biological studies using a neuroblastoma cancer cell line. The in vivo biodistribution of three of the new analogues was investigated in vivo in mice using positron-emission tomography imaging, and one of the new complexes was compared to [⁶⁴Cu]Cu(atsm) in an A431 squamous cell carcinoma xenograft model. Modification of the copper complexes with various amine-containing functional groups alters the biodistribution of the complexes in mice. One complex, with a pendent ( N, N-dimethylamino)ethane functional group, displayed tumor uptake similar to that of [⁶⁴Cu]Cu(atsm) but higher brain uptake, suggesting that this compound has the potential to be of use in the diagnostic brain imaging of tumors and neurodegenerative diseases.

Polyamine transport system-targeted nanometric micelles assembled from epipodophyllotoxin-amphiphiles

Micelle-forming amphiphilic drug conjugates were synthesized starting from a biologically active epipodophyllotoxin derivative which was covalently inserted in between a hydrophilic targeting spermine unit, and a hydrophobic stearyl chain.

A Novel Polyamine-Targeted Therapy for BRAF Mutant Melanoma Tumors

Mutant serine/threonine protein kinase B-Raf (BRAF) protein is expressed in over half of all melanoma tumors. Although BRAF inhibitors (BRAFi) elicit rapid anti-tumor responses in the majority of patients with mutant BRAF melanoma, the tumors inevitably relapse after a short time. We hypothesized that polyamines are essential for tumor survival in mutant BRAF melanomas. These tumors rely on both polyamine biosynthesis and an upregulated polyamine transport system (PTS) to maintain their high intracellular polyamine levels. We evaluated the effect of a novel arylpolyamine (AP) compound that is cytotoxic upon cellular entry via the increased PTS activity of melanoma cells with different BRAF mutational status. Mutant BRAF melanoma cells demonstrated greater PTS activity and increased sensitivity to AP compared to wild type BRAF (BRAF^WT) melanoma cells. Treatment with an inhibitor of polyamine biosynthesis, α-difluoromethylornithine (DFMO), further upregulated PTS activity in mutant BRAF cells and increased their sensitivity to AP. Furthermore, viability assays of 3D spheroid cultures of mutant BRAF melanoma cells demonstrated greater resistance to the BRAFi, PLX4720, compared to 2D monolayer cultures. However, co-treatment with AP restored the sensitivity of melanoma spheroids to PLX4720. These data indicate that mutant BRAF melanoma cells are more dependent on the PTS compared to BRAF^WT melanoma cells, resulting in greater sensitivity to the PTS-targeted cytotoxic AP compound.

Polyamines and Cancer

This chapter provides an overview of how the polyamine pathway has been exploited as a target for the treatment and prevention of multiple forms of cancer, since this pathway is disrupted in all cancers. It is divided into three main sections. The first explores how the polyamine pathway has been targeted for chemotherapy, starting from the first drug to target it, difluoromethylornithine (DFMO) to the large variety of polyamine analogues that have been synthesised and tested throughout the years with all their potentials and pitfalls. The second section focuses on the use of polyamines as vectors for drug delivery. Knowing that the polyamine transport system is upregulated in cancers and that polyamines naturally bind to DNA, a range of polyamine analogues and polyamine-like structures have been synthesised to target epigenetic regulators, with encouraging results. Furthermore, the use of polyamines as transport vectors to introduce toxic/bioactive/fluorescent agents more selectively to the intended target in cancer cells is discussed. The last section concentrates on chemoprevention, where the different strategies that have been undertaken to interfere with polyamine metabolism and function for antiproliferative intervention are outlined and discussed.

Synthesis and Bioevaluation of Macrocycle-Polyamine Conjugates as Cell Migration Inhibitors

The motuporamines are natural products isolated from the New Guinea sea sponge Xestospongia exigua. Dihydromotuporamine C contains a large macrocycle and an appended polyamine component and was shown to be both antimetastatic and cytotoxic to human L3.6pl pancreatic cancer cells. A series of macrocycle-polyamine conjugates were prepared, and the sequence of the polyamine component was varied to optimize the antimigration properties (as measured in L3.6pl cells) of this molecular class. A one-carbon spacer between the 15-membered carbocycle and the appended polyamine showed improved antimigration properties. A survey of different polyamine sequences containing two, three, or four carbon spacers revealed that the natural polyamine sequence (norspermidine, a 3,3-triamine) was superior in terms of inhibiting the migration of L3.6pl cells in vitro. An investigation of the respective ceramide and sphingomyelin populations in L3.6pl cells revealed that these molecules can modulate both ceramide and sphingomyelin pools in cells and inhibit cell migration.

A novel polyamine blockade therapy activates an anti-tumor immune response

Most tumors maintain elevated levels of polyamines to support their growth and survival. This study explores the anti-tumor effect of polyamine starvation via both inhibiting polyamine biosynthesis and blocking the upregulated import of polyamines into the tumor. We demonstrate that polyamine blockade therapy (PBT) co-treatment with both DFMO and a novel polyamine transport inhibitor, Trimer PTI, significantly inhibits tumor growth more than treatment with DFMO or the Trimer PTI alone. The anti-tumor effect of PBT was lost in mice where CD4⁺ and CD8⁺ T cells were antibody depleted, implying that PBT stimulates an anti-tumor immune effect that is T-cell dependent. The PBT anti-tumor effect was accompanied by an increase in granzyme B⁺, IFN-γ⁺ CD8⁺ T-cells and a decrease in immunosuppressive tumor infiltrating cells including Gr-1⁺CD11b⁺ myeloid derived suppressor cells (MDSCs), CD4⁺CD25⁺ Tregs, and CD206⁺F4/80⁺ M2 macrophages. Stimulation with tumor-specific peptides elicited elevated antigen-specific IFN-γ secretion in splenocytes from PBT-treated mice, indicating that PBT treatment stimulates the activation of T-cells in a tumor-specific manner. These data show that combined treatment with both DFMO and the Trimer PTI not only deprives polyamine-addicted tumor cells of polyamines, but also relieves polyamine-mediated immunosuppression in the tumor microenvironment, thus allowing the activation of tumoricidal T-cells.

Targeting polyamine metabolism for cancer therapy and prevention

The chemically simple, biologically complex eukaryotic polyamines, spermidine and spermine, are positively charged alkylamines involved in many crucial cellular processes. Along with their diamine precursor putrescine, their normally high intracellular concentrations require fine attenuation by multiple regulatory mechanisms to keep these essential molecules within strict physiologic ranges. Since the metabolism of and requirement for polyamines are frequently dysregulated in neoplastic disease, the metabolic pathway and functions of polyamines provide rational drug targets; however, these targets have been difficult to exploit for chemotherapy. It is the goal of this article to review the latest findings in the field that demonstrate the potential utility of targeting the metabolism and function of polyamines as strategies for both chemotherapy and, possibly more importantly, chemoprevention.

Motuporamine Derivatives as Antimicrobial Agents and Antibiotic Enhancers against Resistant Gram-Negative Bacteria

Dihydromotuporamine C and its derivatives were evaluated for their in vitro antimicrobial activities and antibiotic enhancement properties against Gram‐negative bacteria and clinical isolates. The mechanism of action of one of these derivatives, MOTU‐N44, was investigated against Enterobacter aerogenes by using fluorescent dyes to evaluate outer‐membrane depolarization and permeabilization. Its efficiency correlated with inhibition of dye transport, thus suggesting that these molecules inhibit drug transporters by de‐energization of the efflux pump rather than by direct interaction of the molecule with the pump. This suggests that depowering the efflux pump provides another strategy to address antibiotic resistance.

Therapeutic effect of quantum dots for cancer treatment

The therapeutic effect of Qdots for cancer treatment arises from ROS-induced cell apoptosis and inhibited tumor growth in vivo.

Recent advances in cancer treatment by iron chelators

The development of new therapeutic alternatives for cancers is a major public health priority. Among the more promising approaches, the iron depletion strategy based on metal chelation in the tumoral environment has been particularly studied in recent decades. After a short description of the importance of iron for cancer cell proliferation, we will review the different iron chelators developed as potential chemotherapeutics. Finally, the recent efforts to vectorize the chelating agents specifically in the microtumoral environment will be discussed in detail.

Conjugation with polyamines enhances the antibacterial and anticancer activity of chloramphenicol

Chloramphenicol (CAM) is a broad-spectrum antibiotic, limited to occasional only use in developed countries because of its potential toxicity. To explore the influence of polyamines on the uptake and activity of CAM into cells, a series of polyamine–CAM conjugates were synthesized. Both polyamine architecture and the position of CAM-scaffold substitution were crucial in augmenting the antibacterial and anticancer potency of the synthesized conjugates. Compounds 4 and 5, prepared by replacement of dichloro-acetyl group of CAM with succinic acid attached to N4 and N1 positions of N⁸,N⁸-dibenzylspermidine, respectively, exhibited higher activity than CAM in inhibiting the puromycin reaction in a bacterial cell-free system. Kinetic and footprinting analysis revealed that whereas the CAM-scaffold preserved its role in competing with the binding of aminoacyl-tRNA 3′-terminus to ribosomal A-site, the polyamine-tail could interfere with the rotatory motion of aminoacyl-tRNA 3′-terminus toward the P-site. Compared to CAM, compounds 4 and 5 exhibited comparable or improved antibacterial activity, particularly against CAM-resistant strains. Compound 4 also possessed enhanced toxicity against human cancer cells, and lower toxicity against healthy human cells. Thus, the designed conjugates proved to be suitable tools in investigating the ribosomal catalytic center plasticity and some of them exhibited greater efficacy than CAM itself.

Polyamine transport inhibitors: design, synthesis, and combination therapies with difluoromethylornithine

The development of polyamine transport inhibitors (PTIs), in combination with the polyamine biosynthesis inhibitor difluoromethylornithine (DFMO), provides a method to target cancers with high polyamine requirements. The DFMO+PTI combination therapy results in sustained intracellular polyamine depletion and cell death. A series of substituted benzene derivatives were evaluated for their ability to inhibit the import of spermidine in DFMO-treated Chinese hamster ovary (CHO) and L3.6pl human pancreatic cancer cells. Several design features were discovered which strongly influenced PTI potency, sensitivity to amine oxidases, and cytotoxicity. These included changes in (a) the number of polyamine chains appended to the ring system, (b) the polyamine sequence, (c) the attachment linkage of the polyamine to the aryl core, and (d) the presence of a terminal N-methyl group. Of the series tested, the optimal design was N(1),N(1'),N(1″)-(benzene-1,3,5-triyltris(methylene))tris(N(4)-(4-(methylamino)butyl)butane-1,4-diamine, 6b, which contained three N-methylhomospermidine motifs. This PTI exhibited decreased sensitivity to amine oxidases and low toxicity as well as high potency (EC50 = 1.4 μM) in inhibiting the uptake of spermidine (1 μM) in DFMO-treated L3.6pl human pancreatic cancer cells.

Polyamines and membrane transporters

In recent years, our understanding of the importance of membrane transporters (MTs) in the disposition of and response to drugs has increased significantly. MTs are proteins that regulate the transport of endogenous molecules and xenobiotics across the cell membrane. In mammals, two super-families have been identified: ATP-binding cassette (ABC) and solute carrier (SLC) transporters. There is evidence that MTs might mediate polyamines (PA) transport. PA are ubiquitous polycations which are found in all living cells. In mammalian cells, three major PA are synthesised: putrescine, spermidine and spermine; whilst the decarboxylated arginine (agmatine) is not produced by mammals but is synthesised by plants and bacteria. In addition, research in the PA field suggests that PA are transported into cells via a specific transporter, the polyamine transport system(s) (PTS). Although the PTS has not been fully defined, there is evidence that some of the known MTs might be involved in PA transport. In this mini review, eight SLC transporters will be reviewed and their potential to mediate PA transport in human cells discussed. These transporters are SLC22A1, SLC22A2, SLC22A3, SLC47A1, SLC7A1, SLC3A2, SLC12A8A, and SLC22A16. Preliminary data from our laboratory have revealed that SLC22A1 might be involved in the PA uptake; in addition to one member of ABC superfamily (MDR1 protein) might also mediate the efflux of polyamine like molecules.

Development of flavonoid-based inverse agonists of the key signaling receptor US28 of human cytomegalovirus

A series of 31 chalcone- and flavonoid-based derivatives were synthesized in good overall yields and screened for their inverse agonist activity on the US28 receptor of human cytomegalovirus (HCMV). With one exception (e.g., 2-(5-bromo-2-methoxyphenyl)-3-hydroxy-4H-chromen-4-one), halogen-substituted flavonoids were typically more potent inverse agonists than their related hydro derivatives. While toxicity could be used to partially explain the inverse agonist activity of some members of the series, 5-(benzyloxy)-2-(5-bromo-2-methoxyphenyl)-4H-chromen-4-one (11b) acted on the US28 receptor as a nontoxic, inverse agonist. The full inverse agonism (efficacy, -89%) and potency (EC50 = 3.5 μM) observed with flavonoid 11b is especially important as it provides both a new tool to study US28 signaling and a potential platform for the future development of HCMV-targeting drugs.

Anthracene-polyamine conjugates inhibit in vitro proliferation of intraerythrocytic Plasmodium falciparum parasites

Anthracene-polyamine conjugates inhibit the in vitro proliferation of the intraerythrocytic human malaria parasite Plasmodium falciparum, with 50% inhibitory concentrations (IC50s) in the nM to μM range. The compounds are taken up into the intraerythrocytic parasite, where they arrest the parasite cell cycle. Both the anthracene and polyamine components of the conjugates play a role in their antiplasmodial effect.

Recent advances in the molecular biology of metazoan polyamine transport

Very limited molecular knowledge exists about the identity and protein components of the ubiquitous polyamine transporters found in animal cells. However, a number of reports have been published over the last 5 years on potential candidates for metazoan polyamine permeases. We review the available evidence on these putative polyamine permeases, as well as establish a useful "identikit picture" of the general polyamine transport system, based on its properties as found in a wide spectrum of mammalian cells. Any molecular candidate encoding a putative "general" polyamine permease should fit that provided portrait. The current models proposed for the mechanism of polyamine internalization in mammalian cells are also briefly reviewed.

NPC-16, a novel naphthalimide-polyamine conjugate, induced apoptosis and autophagy in human hepatoma HepG2 cells and Bel-7402 cells

The antitumor effects and molecular mechanism of NPC-16, a novel naphthalimide-polyamine conjugate, were evaluated in HepG2 cells and Bel-7402 cells. Apoptosis and necrosis were evaluated by Annexin V-FITC detection kit, and autophagy by acridine orange and Lyso-Tracker Red staining. The change of mitochondrial transmembrane potential was measured using rhodamine 123 staining. The protein expression of Beclin 1, LC3 II and mTOR, p70S6 K, 14-3-3, caspase, and Bcl-2 family members was detected by immunofluorescence assays and Western Blot. Here, we elucidated the nature of cellular response of HepG2 cells and Bel-7402 cells to NPC-16 at IC(50). NPC-16 induced caspase-dependent apoptosis via the mitochondrial pathway and death receptor pathway in Bel-7402 cells. Differently, NPC-16 triggered HepG2 cells both apoptosis and autophagy, further autophagy facilitated cellular apoptosis. Furthermore, mTOR signal pathway was involved in NPC-16-mediated autophagy in HepG2 cells. Thus, NPC-16 may be useful as a potential template for investigation the molecular mechanism of naphthalimide-polyamine conjugate against hepatocellular carcinoma.

Interactions between the etoposide derivative F14512 and human type II topoisomerases: implications for the C4 spermine moiety in promoting enzyme-mediated DNA cleavage

F14512 is a novel etoposide derivative that contains a spermine in place of the C4 glycosidic moiety. The drug was designed to exploit the polyamine transport system that is upregulated in some cancers. However, a preliminary study suggests that it is also a more efficacious topoisomerase II poison than etoposide [Barret et al. (2008) Cancer Res. 68, 9845-9853]. Therefore, we undertook a more complete study of the actions of F14512 against human type II topoisomerases. As determined by saturation transfer difference (1)H NMR spectroscopy, contacts between F14512 and human topoisomerase IIα in the binary enzyme-drug complex are similar to those of etoposide. Although the spermine of F14512 does not interact with the enzyme, it converts the drug to a DNA binder [Barret et al. (2008)]. Consequently, the influence of the C4 spermine on drug activity was assessed. F14512 is a highly active topoisomerase II poison and stimulates DNA cleavage mediated by human topoisomerase IIα or topoisomerase IIβ. The drug is more potent and efficacious than etoposide or TOP-53, an etoposide derivative that contains a C4 aminoalkyl group that strengthens drug-enzyme binding. Unlike the other drugs, F14512 maintains robust activity in the absence of ATP. The enhanced activity of F14512 correlates with a tighter binding and an increased stability of the ternary topoisomerase II-drug-DNA complex. The spermine-drug core linkage is critical for these attributes. These findings demonstrate the utility of a C4 DNA binding group and provide a rational basis for the development of novel and more active etoposide-based topoisomerase II poisons.

Fluorescent substrates for polyamine catabolic enzymes and transport

The most widely used methods for measuring polyamine enzyme activities are radioisotope methods that measure the radioactivity of compounds produced from radiolabeled substrate by the enzyme reaction. Several fluorescent polyamines have been developed for the measurement of the polyamine transport system (PTS) or transglutaminase. Although fluorophores in the fluorescent polyamines may affect the affinity of the polyamine moiety to the enzyme protein, the assays that use fluorescent substrate are sensitive and simple for common laboratory usage.In this chapter, the uses of dansyl polyamines with a simple high-performance liquid chromatography system for the measurement of the PTS and polyamine catabolic enzymes including spermidine/spermine N¹-acetyltransferase and N¹-acetylpolyamine oxidase are described.

A fluorescent biomarker of the polyamine transport system to select patients with AML for F14512 treatment

The polyamine transport system (PTS), hyperactive in cancer cells, can constitute a gate to deliver F14512, a novel spermine epipodophyllotoxin conjugate recently selected for clinical development in AML phase I. We investigated in vitro the high antiproliferative effect of F14512 against 13 leukemia cell lines, and demonstrated a statistically significant correlation with the level of PTS activity, using a novel fluorescent marker F96982. This labelling protocol was then adapted for clinical applications for blood, bone marrow and AML samples with CD45 gating. Within the patient samples, the PTS activity varied significantly in AML cells, as compared to normal lymphocytes. In conclusion, the identification of PTS-positive AML with F98982 probe offers new perspectives to select patients prone to respond to F14512.

A putrescine-anthracene conjugate: a paradigm for selective drug delivery

Increased polyamine concentrations play an important role in the development of cancer at all stages, from initiation through to maintenance of the transformed phenotype. One way cancer cells accumulate increased concentrations of polyamines is by increased uptake of preformed polyamines via their PTS (polyamine transport system). The PTS is promiscuous and will transport a range of polyamine-based molecules. Therefore it may be that cytotoxic drugs could be attached to polyamine vectors and targeted selectively to cancer cells by utilizing the PTS. The aim of the present study was to investigate the potential of Ant 4, a putrescine-anthracene conjugate, to target cytotoxic agents to human cancer cells as a paradigm for a novel method of selective drug delivery. Ant 4 induced cytotoxicity after only 24 h exposure. Apoptosis was the predominant type of cell death, with mechanistic studies revealing that oxidative stress and DNA damage may have a part to play. For the first time, uptake of Ant 4 via the PTS was demonstrated both directly and indirectly in human cell lines. In addition, Ant 4 significantly reduced putrescine uptake, demonstrating that this conjugate not only used the PTS, but also could successfully compete with its native polyamine for uptake. However, the most interesting finding was the intracellular depletion of the polyamine pools, providing an additional mode of toxicity for Ant 4 and the possibility that this molecule may act as a 'double-edged sword': preventing cell growth by delivery of the toxic moiety and by depletion of intracellular polyamine content.