Enhanced potency of nucleotide-dendrimer conjugates as agonists of the P2Y14 receptor: multivalent effect in G protein-coupled receptor recognition

Application of Nanoparticles for Targeting G Protein-Coupled Receptors

Nanoparticles (NPs) have attracted unequivocal attention in recent years due to their potential applications in therapeutics, bio-imaging and material sciences. For drug delivery, NP-based carrier systems offer several advantages over conventional methods. When conjugated with ligands and drugs (or other therapeutic molecules), administrated NPs are able to deliver cargo to targeted sites through ligand-receptor recognition. Such targeted delivery is especially important in cancer therapy. Through this targeted cancer nanotherapy, cancer cells are killed with higher specificity, while the healthy cells are spared. Furthermore, NP drug delivery leads to improved drug load, enhanced drug solubility and stability, and controlled drug release. G protein-coupled receptors (GPCRs) are a superfamily of cell transmembrane receptors. They regulate a plethora of physiological processes through ligand-receptor-binding-induced signaling transduction. With recent evidence unveiling their roles in cancer, GPCR agonists and antagonists have quickly become new targets in cancer therapy. This review focuses on the application of some notable nanomaterials, such as dendrimers, quantum dots, gold nanoparticles, and magnetic nanoparticles, in GPCR-related cancers.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Design, synthesis, pharmacological characterization of a fluorescent agonist of the P2Y₁₄ receptor

The P2Y14R is a G(i/o)-coupled receptor of the P2Y family of purinergic receptors that is activated by extracellular UDP and UDP-glucose (UDPG). In an earlier report we described a P2Y14R fluorescent probe, MRS4174, based on the potent and selective antagonist PPTN, a naphthoic acid derivative. Here, we report the design, preparation, and activity of an agonist-based fluorescent probe MRS4183 (11) and a shorter P2Y14R agonist congener, which contain a UDP-glucuronic acid pharmacophore and BODIPY fluorophores conjugated through diaminoalkyl linkers. The design relied on both docking in a P2Y14R homology model and established structure activity relationship (SAR) of nucleotide analogs. 11 retained P2Y14R potency with EC50 value of 0.96 nM (inhibition of adenylyl cyclase), compared to parent UDPG (EC50 47 nM) and served as a tracer for microscopy and flow cytometry, displaying minimal nonspecific binding. Binding saturation analysis gave an apparent binding constant for 11 in whole cells of 21.4±1.1 nM, with a t1/2 of association at 50 nM 11 of 23.9 min. Known P2Y14R agonists and PPTN inhibited cell binding of 11 with the expected rank order of potency. The success in the identification of a new P2Y14R fluorescent agonist with low nonspecific binding illustrates the advantages of rational design based on recently determined GPCR X-ray structures. Such conjugates will be useful tools in expanding the SAR of this receptor, which still lacks chemical diversity in its collective ligands.

Exploring a 2-naphthoic acid template for the structure-based design of P2Y14 receptor antagonist molecular probes

The P2Y₁₄ receptor (P2Y₁₄R), one of eight P2Y G protein-coupled receptors (GPCR), is involved in inflammatory, endocrine, and hypoxic processes and is an attractive pharmaceutical target. The goal of this research is to develop high-affinity P2Y₁₄R fluorescent probes based on the potent and highly selective antagonist 4-(4-(piperidin-4-yl)-phenyl)-7-(4-(trifluoromethyl)-phenyl)-2-naphthoic acid (6, PPTN). A model of hP2Y₁₄R based on recent hP2Y₁₂R X-ray structures together with simulated antagonist docking suggested that the piperidine ring is suitable for fluorophore conjugation while preserving affinity. Chain-elongated alkynyl or amino derivatives of 6 for click or amide coupling were synthesized, and their antagonist activities were measured in hP2Y₁₄R-expressing CHO cells. Moreover, a new Alexa Fluor 488 (AF488) containing derivative 30 (MRS4174, K_i = 80 pM) exhibited exceptionally high affinity, as compared to 13 nM for the alkyne precursor 22. A flow cytometry assay employing 30 as a fluorescent probe was used to quantify specific binding to P2Y₁₄R. Known P2Y receptor ligands inhibited binding of 30 with properties consistent with their previously established receptor selectivities and affinities. These results illustrate that potency in this series of 2-naphthoic acid derivatives can be preserved by chain functionalization, leading to highly potent fluorescent molecular probes for P2Y₁₄R. Such conjugates will be useful tools in expanding the SAR of this receptor, which still lacks chemical diversity in its collective ligands. This approach demonstrates the predictive power of GPCR homology modeling and the relevance of newly determined X-ray structures to GPCR medicinal chemistry.

Overcoming the blood-brain barrier for delivering drugs into the brain by using adenosine receptor nanoagonist

The extremely low permeability of the blood-brain barrier (BBB) poses the greatest impediment in the treatment of central nervous system (CNS) diseases. Recent work indicated that BBB permeability can be up-regulated by activating A2A adenosine receptor (AR), which temporarily increases intercellular spaces between the brain capillary endothelial cells. However, due to transient circulation lifetime of adenosine-based agonists, their capability to enhance brain delivery of drugs, especially macromolecular drugs, is limited. In this work, a series of nanoagonists (NAs) were developed by labeling different copies of A2A AR activating ligands on dendrimers. In vitro transendothelial electrical resistance measurements demonstrated that the NAs increased permeability of the endothelial cell monolayer by compromising the tightness of tight junctions, the key structure that restricts the entry of blood-borne molecules into the brain. In vivo imaging studies indicated the remarkably up-regulated brain uptake of a macromolecular model drug (45 kDa) after intravenous injection of NAs. Autoradiographic imaging showed that the BBB opening time-window can be tuned in a range of 0.5-2.0 h by the NAs labeled with different numbers of AR-activating ligands. By choosing a suitable NA, it is possible to maximize brain drug delivery and minimize the uncontrollable BBB leakage by matching the BBB opening time-window with the pharmacokinetics of a therapeutic agent. The NA-mediated brain drug delivery strategy holds promise for the treatment of CNS diseases with improved therapeutic efficiency and reduced side-effects.

Pharmacoinformatic approaches to understand complexation of dendrimeric nanoparticles with drugs

Nanoparticle based drug delivery systems are gaining popularity due to their wide spectrum advantages over traditional drug delivery systems; among them, dendrimeric nano-vectors are the most widely explored carriers for pharmaceutical and biomedical applications. The precise mechanism of encapsulation of drug molecules inside the dendritic matrix, delivery of drugs into specific cells, interactions of nano-formulation with biological targets and proteins, etc. present a substantial challenge to the scientific understanding of the subject. Computational methods complement experimental techniques in the design and optimization of drug delivery systems, thus minimizing the investment in drug design and development. Significant progress in computer simulations could facilitate an understanding of the precise mechanism of encapsulation of bioactive molecules and their delivery. This review summarizes the pharmacoinformatic studies spanning from quantum chemical calculations to coarse-grained simulations, aimed at providing better insight into dendrimer-drug interactions and the physicochemical parameters influencing the binding and release mechanism of drugs.

Design of biocompatible dendrimers for cancer diagnosis and therapy: current status and future perspectives

In the past decade, nanomedicine with its promise of improved therapy and diagnostics has revolutionized conventional health care and medical technology. Dendrimers and dendrimer-based therapeutics are outstanding candidates in this exciting field as more and more biological systems have benefited from these starburst molecules. Anticancer agents can be either encapsulated in or conjugated to dendrimer and be delivered to the tumour via enhanced permeability and retention (EPR) effect of the nanoparticle and/or with the help of a targeting moiety such as antibody, peptides, vitamins, and hormones. Imaging agents including MRI contrast agents, radionuclide probes, computed tomography contrast agents, and fluorescent dyes are combined with the multifunctional nanomedicine for targeted therapy with simultaneous cancer diagnosis. However, an important question reported with dendrimer-based therapeutics as well as other nanomedicines to date is the long-term viability and biocompatibility of the nanotherapeutics. This critical review focuses on the design of biocompatible dendrimers for cancer diagnosis and therapy. The biocompatibility aspects of dendrimers such as nanotoxicity, long-term circulation, and degradation are discussed. The construction of novel dendrimers with biocompatible components, and the surface modification of commercially available dendrimers by PEGylation, acetylation, glycosylation, and amino acid functionalization have been proposed as available strategies to solve the safety problem of dendrimer-based nanotherapeutics. Also, exciting opportunities and challenges on the development of dendrimer-based nanoplatforms for targeted cancer diagnosis and therapy are reviewed (404 references).

Functionalized congeners of P2Y1 receptor antagonists: 2-alkynyl (N)-methanocarba 2'-deoxyadenosine 3',5'-bisphosphate analogues and conjugation to a polyamidoamine (PAMAM) dendrimer carrier

The P2Y(1) receptor is a prothrombotic G protein-coupled receptor (GPCR) activated by ADP. Preference for the North (N) ring conformation of the ribose moiety of adenine nucleotide 3',5'-bisphosphate antagonists of the P2Y(1) receptor was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute. A series of covalently linkable N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphates containing extended 2-alkynyl chains was designed, and binding affinity at the human (h) P2Y(1) receptor determined. The chain of these functionalized congeners contained hydrophilic moieties, a reactive substituent, or biotin, linked via an amide. Variation of the chain length and position of an intermediate amide group revealed high affinity of carboxylic congener 8 (K(i) 23 nM) and extended amine congener 15 (K(i) 132 nM), both having a 2-(1-pentynoyl) group. A biotin conjugate 18 containing an extended epsilon-aminocaproyl spacer chain exhibited higher affinity than a shorter biotinylated analogue. Alternatively, click coupling of terminal alkynes of homologous 2-dialkynyl nucleotide derivatives to alkyl azido groups produced triazole derivatives that bound to the P2Y(1) receptor following deprotection of the bisphosphate groups. The preservation of receptor affinity of the functionalized congeners was consistent with new P2Y(1) receptor modeling and ligand docking. Attempted P2Y(1) antagonist conjugation to PAMAM dendrimer carriers by amide formation or palladium-catalyzed reaction between an alkyne on the dendrimer and a 2-iodopurine-derivatized nucleotide was unsuccessful. A dialkynyl intermediate containing the chain length favored in receptor binding was conjugated to an azide-derivatized dendrimer, and the conjugate inhibited ADP-promoted human platelet aggregation. This is the first example of attaching a strategically functionalized P2Y receptor antagonist to a PAMAM dendrimer to produce a multivalent conjugate exhibiting a desired biological effect, i.e., antithrombotic action.

GPCR ligand-dendrimer (GLiDe) conjugates: future smart drugs?

Unlike nanocarriers that are intended to release their drug cargo at the site of action, biocompatibile polyamidoamine (PAMAM) conjugates are designed to act at cell surface G protein-coupled receptors (GPCRs) without drug release. These multivalent GPCR ligand-dendrimer (GLiDe) conjugates display qualitatively different pharmacological properties in comparison with monomeric drugs. They might be useful as novel tools to study GPCR homodimers and heterodimers as well as higher aggregates. The structure of the conjugate determines the profile of biological activity, receptor selectivity, and physical properties such as water solubility. Prosthetic groups for characterization and imaging of receptors can be introduced without loss of affinity. The feasibility of targeting multiple adenosine and P2Y receptors for synergistic effects has been shown. Testing in vivo will be needed to explore the effects on pharmacokinetics and tissue targeting.

Multivalent dendrimeric and monomeric adenosine agonists attenuate cell death in HL-1 mouse cardiomyocytes expressing the A(3) receptor

Multivalent dendrimeric conjugates of GPCR ligands may have increased potency or selectivity in comparison to monomeric ligands, a phenomenon that was tested in a model of cytoprotection in mouse HL-1 cardiomyocytes. Quantitative RT-PCR indicated high expression levels of endogenous A(1) and A(2A) adenosine receptors (ARs), but not of A(2B) and A(3)ARs. Activation of the heterologously expressed human A(3)AR in HL-1 cells by AR agonists significantly attenuated cell damage following 4h exposure to H(2)O(2) (750 microM) but not in untransfected cells. The A(3) agonist IB-MECA (EC(50) 3.8 microM) and the non-selective agonist NECA (EC(50) 3.9 microM) protected A(3) AR-transfected cells against H(2)O(2) in a concentration-dependent manner, as determined by lactate dehydrogenase release. A generation 5.5 PAMAM (polyamidoamine) dendrimeric conjugate of a N(6)-chain-functionalized adenosine agonist was synthesized and its mass indicated an average of 60 amide-linked nucleoside moieties out of 256 theoretical attachment sites. It non-selectively activated the A(3)AR to inhibit forskolin-stimulated cAMP formation (IC(50) 66nM) and, similarly, protected A(3)-transfected HL-1 cells from apoptosis-inducing H(2)O(2) with greater potency (IC(50) 35nM) than monomeric nucleosides. Thus, a PAMAM conjugate retained AR binding affinity and displayed greatly enhanced cardioprotective potency.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.

Nucleoside conjugates of quantum dots for characterization of G protein-coupled receptors: strategies for immobilizing A2A adenosine receptor agonists

Background

Quantum dots (QDs) are crystalline nanoparticles that are compatible with biological systems to provide a chemically and photochemically stable fluorescent label. New ligand probes with fluorescent reporter groups are needed for detection and characterization of G protein-coupled receptors (GPCRs).

Results

Synthetic strategies for coupling the A_2A adenosine receptor (AR) agonist CGS21680 (2-[4-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine) to functionalized QDs were explored. Conjugates tethered through amide-linked chains and poly(ethyleneglycol) (PEG) displayed low solubility and lacked receptor affinity. The anchor to the dendron was either through two thiol groups of (R)-thioctic acid or through amide formation to a commercial carboxy-derivatized QD. The most effective approach was to use polyamidoamine (PAMAM) D5 dendrons as multivalent spacer groups, grafted on the QD surface through a thioctic acid moiety. In radioligand binding assays, dendron nucleoside conjugate 11 displayed a moderate affinity at the human A_2AAR (K_iapp 1.02 ± 0.15 μM). The QD conjugate of increased water solubility 13, resulting from the anchoring of this dendron derivative, interacted with the receptor with K_iapp of 118 ± 54 nM. The fluorescence emission of 13 occurred at 565 nm, and the presence of the pendant nucleoside did not appreciably quench the fluorescence.

Conclusions

This is a feasibility study to demonstrate a means of conjugating to a QD a small molecular pharmacophore of a GPCR that is relatively hydrophobic. Further enhancement of affinity by altering the pharmacophore or the linking structures will be needed to make useful affinity probes.

Polyamidoamine (PAMAM) dendrimer conjugates of "clickable" agonists of the A3 adenosine receptor and coactivation of the P2Y14 receptor by a tethered nucleotide

We previously synthesized a series of potent and selective A(3) adenosine receptor (AR) agonists (North-methanocarba nucleoside 5'-uronamides) containing dialkyne groups on extended adenine C2 substituents. We coupled the distal alkyne of a 2-octadiynyl nucleoside by Cu(I)-catalyzed "click" chemistry to azide-derivatized G4 (fourth-generation) PAMAM dendrimers to form triazoles. A(3)AR activation was preserved in these multivalent conjugates, which bound with apparent K(i) of 0.1-0.3 nM. They were substituted with nucleoside moieties, solely or in combination with water-solubilizing carboxylic acid groups derived from hexynoic acid. A comparison with various amide-linked dendrimers showed that triazole-linked conjugates displayed selectivity and enhanced A(3)AR affinity. We prepared a PAMAM dendrimer containing equiproportioned peripheral azido and amino groups for conjugation of multiple ligands. A bifunctional conjugate activated both A(3) and P2Y(14) receptors (via amide-linked uridine-5'-diphosphoglucuronic acid), with selectivity in comparison to other ARs and P2Y receptors. This is the first example of targeting two different GPCRs with the same dendrimer conjugate, which is intended for activation of heteromeric GPCR aggregates. Synergistic effects of activating multiple GPCRs with a single dendrimer conjugate might be useful in disease treatment.

Functionalized congener approach to the design of ligands for G protein-coupled receptors (GPCRs)

Functionalized congeners, in which a chemically functionalized chain is incorporated at an insensitive site on a pharmacophore, have been designed from the agonist and antagonist ligands of various G protein-coupled receptors (GPCRs). These chain extensions enable a conjugation strategy for detecting and characterizing GPCR structure and function and pharmacological modulation. The focus in many studies of functionalized congeners has been on two families of GPCRs: those responding to extracellular purines and pyrimidines-i.e., adenosine receptors (ARs) and P2Y nucleotide receptors. Functionalized congeners of small molecule as ligands for other GPCRs and non-G protein coupled receptors have also been designed. For example, among biogenic amine neurotransmitter receptors, muscarinic acetylcholine receptor antagonists and adrenergic receptor ligands have been studied with a functionalized congener approach. Adenosine A(1), A(2A), and A(3) receptor functionalized congeners have yielded macromolecular conjugates, irreversibly binding AR ligands for receptor inactivation and cross-linking, radioactive probes that use prosthetic groups, immobilized ligands for affinity chromatography, and dual-acting ligands that function as binary drugs. Poly(amidoamine) dendrimers have served as nanocarriers for covalently conjugated AR functionalized congeners. Rational methods of ligand design derived from molecular modeling and templates have been included in these studies. Thus, the design of novel ligands, both small molecules and macromolecular conjugates, for studying the chemical and biological properties of GPCRs have been developed with this approach, has provided researchers with a strategy that is more versatile than the classical medicinal chemical approaches.