Trends in Pharmacological Sciences
OpinionGPCR ligand–dendrimer (GLiDe) conjugates: future smart drugs?
Introduction
The mechanism of action of at least one-quarter of the drugs on the market involves modulation of rhodopsin-like G protein-coupled receptors (GPCRs) on the cell surface [1]. A recurrent issue in the discovery and clinical development of new GPCR ligands, agonists and antagonists is the occurrence of a given receptor in multiple tissues, which leads to side effects. The same receptor might occur in a target tissue (in which a desired pharmacological effect can be demonstrated) and in off-target tissues (in which the effect can be less than beneficial or even harmful). Methods of designing GPCR modulators that function as ‘smart drugs’ are required [2]. This need is especially evident for receptor classes such as purine receptors, which encompasses adenosine receptors (ARs) [3] and purinergic P2Y receptors [4] for extracellular nucleotides that are expressed at varying levels and in diverse combinations in nearly every organ. Even with the promise of structure-based design of GPCR ligands 5, 6 (now with an increasing number of X-ray crystallographic structures available), tissue- or organ-selectivity would provide a great therapeutic advantage.
Pro-drug approaches have sought to achieve this goal of organ-selective delivery of GPCR ligands [7]. However, their success is dependent upon the presence of a specific enzyme for the release of the free drug species in the target tissue, which is often unfeasible. Instead, research has focused on using covalent conjugates of chemically functionalized GPCR ligands tethered from macromolecular carriers, which emerged from the early work of Ringsdorf and others [8]. The polyamidoamine (PAMAM) dendrimers form a particularly versatile and biocompatible class of polymeric drug carriers. First introduced by Tomalia [9], these tree-like polymers have found use in diagnostic imaging and in research for delivering nucleic acids and peptides to cells for internalization 10, 11. Charge and size parameters that affect internalization of PAMAM dendrimers have been studied [12]. PAMAM dendrimers can be tailored to a wide range of molecular weights by adding outer layers that bifurcate in successive ‘generations’ that multiply the number of terminal groups geometrically. Also, the nature of the terminal functionality can be altered for conjugation purposes or for ‘tuning’ pharmacokinetic properties in vivo. Enhanced stability of ligands when conjugated to PAMAM dendrimers has been noted [13]. Most of the examples of use of PAMAM dendrimers as drug carriers involve internalization of the complex followed by dissociation or cleavage of the drug cargo to act within the cell. In one example, arginine–glycine–aspartic acid (RGD) peptides were conjugated to PAMAM dendrimers for binding to cell-surface integrin receptors before internalization [14]. With GPCR ligand–dendrimer (GLiDe) conjugates 15, 16, 17, 18, 19, 20, 21, we have taken a different approach that requires strategic functionalization of the small molecular drug such that it displays the desired biological property outside the cell while still tethered covalently to peripheral groups of the dendrimer. A peptide hormone was also coupled to a PAMAM dendrimer to serve as a targeting moiety through binding to its GPCR [22].
Section snippets
PAMAM conjugates as multivalent ligands of GPCRs
GLiDe conjugates have recently been described as interacting with various adenosine and P2Y receptors (Table 1). These are the first covalent conjugates of PAMAM dendrimers with small molecular GPCR agonists and antagonists that display high potency at a target receptor (nM to μM range). Dissociation of the drug and/or internalization of the complex are, in general, not necessary or desirable using this approach, which maintains or enhances the binding affinity of the functionalized congeners
Polyethylene glycol (PEG)ylated unimolecular micellar conjugates
A difficulty encountered with the first amide- or thiourea-linked A2AAR agonists was reduced aqueous solubility which was probably due to H-bonding-induced aggregation of the dendrimer 15, 16. Aggregation can reduce bioavailability and increase nonspecific membrane binding. Therefore, we tested if appending PEG chains to free terminal groups of the carrier would allow efficient binding of the nucleosides to their binding sites [17]. The effects of increasing the chain length of PEG and the
Multiple pharmacophores on the same carrier
These two examples of agonists of the A3AR and the P2Y14 receptor were used as a test of the ability to combine two different pharmacophores, i.e. selective nucleoside and nucleotide agonist ligan into the same GLiDe conjugate [20]. Thus, the conjugate shown in Figure 3 could bind to and activate either receptor with high potency. We recently reported that the RBL-2H3 mast cell line, known previously to express functional A3ARs, also expresses the P2Y14 receptor, and that both receptors
Conclusions
We have introduced a general methodology for designing, in stages, multivalent GLiDe conjugates that might display qualitatively different pharmacological properties in comparison with the monomeric drugs. The ability of these polymeric derivatives to cross biological membranes is probably reduced and structural heterogeneity is present, but there are advantages over the monomeric drugs. Nevertheless, major problems must be addressed before significant therapeutic advances are achieved. The
Acknowledgement
Support from the NIDDK Intramural Research Program, National institutes of Health (Bethesda, MD, USA) is acknowledged.
Glossary
- AR
- adenosine receptor (family of four GPCRs activated by extracellular adenosine and its nucleoside analogs).
- CGS21680
- 2-[p-(2-carboxyethyl)phenyl-ethylamino]-5′-N-ethylcarboxamidoadenosine
- GLiDe
- covalently conjugated GPCR ligand–dendrimer.
- GPCR
- G protein-coupled receptor.
- Ligand docking
- process of computational identification of an energetically favorable binding mode of a small molecule in its receptor site.
- MRS2500
References (33)
Dendrimer-based drug and imaging conjugates: design considerations for nanomedical applications
Drug Disc. Today
(2010)Multivalent dendrimeric and monomeric adenosine agonists attenuate cell death in HL-1 mouse cardiomyocytes expressing the A3 adenosine receptor
Biochem. Pharmacol.
(2010)The targeted delivery of cancer drugs across the blood-brain barrier: chemical modifications of drugs or drug-nanoparticles?
Drug Disc. Today
(2008)- et al.
The growing impact of click chemistry on drug discovery
Drug Discov. Today
(2003) UDP-glucose acting at P2Y14 receptors is a mediator of mast cell degranulation
Biochem. Pharmacol.
(2010)- et al.
Molecular modeling of a PAMAM-CGS21680 dendrimer bound to an A2A adenosine receptor homodimer
Bioorg. Med. Chem. Lett.
(2008) How many drug targets are there?
Nat. Rev. Drug Disc.
(2006)Intelligent drug delivery systems
Bioconjugate Chem.
(2009)- et al.
Adenosine receptors as therapeutic targets
Nat. Rev. Drug Disc.
(2006) International Union of Pharmacology LVIII. Update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy
Pharmacol. Rev.
(2006)
Structure-based discovery of novel chemotypes for adenosine A2A receptor antagonists
J. Med. Chem.
Structure-based discovery of adenosine A2A receptor ligands
J. Med. Chem.
Prodrug approaches for enhancing the bioavailability of drugs with low solubility
Chem. Biodivers.
Polymer–drug conjugates: towards a novel approach for the treatment of endrocine-related cancer
Endocrine-Related Cancer
Macromolecules, dendrimers, and nanomaterials in magnetic resonance imaging: the interplay between size, function, and pharmacokinetics
Chem. Rev.
Self-assembled ternary complex of cationic dendrimer, cucurbituril, and DNA: noncovalent strategy in developing a gene delivery carrier
Bioconjug. Chem.
Cited by (18)
Dendrimers as nanocarriers for nucleoside analogues
2017, European Journal of Pharmaceutics and BiopharmaceuticsCitation Excerpt :Such constructs were elaborated to improve the activity and stability of ligand in biological systems, and to enhance its pharmacokinetic and pharmacodynamic properties compared to the monomeric ligands. The research involved both adenosine receptors (ARs) and purinergic P2Y receptors that are expressed at various levels and in diverse combinations in nearly every tissue [97–100]. In order to synthesize polymer-bound adenine nucleotides with high coenzymatic activities, adenine nucleotides (ATP and ADP) were covalently bound with PAMAM dendrimers.
Anti-ischemic effects of multivalent dendrimeric A <inf>3</inf> adenosine receptor agonists in cultured cardiomyocytes and in the isolated rat heart
2012, Pharmacological ResearchCitation Excerpt :Assuming proper functionalization of a ligand for covalent conjugation, the resulting GPCR ligand–dendrimer (GLiDe) conjugates have displayed dramatically increased potency or selectivity in comparison to the monomeric, small molecular ligands [19–22]. The use of nanocarriers for stably conjugated drugs that act at the cell surface may provide pharmacokinetic and pharmacodynamic advantages, such as impeded metabolic degradation or the possibility of tissue selectivity [18,22–24]. The aim of our study was to investigate the cardioprotective effects in neonatal rat primary cardiac cell cultures and in isolated heart models (12-week-old rats) of two recently reported multivalent A3AR agonists [25], MRS5216 1 and MRS5246 2, and a newly synthesized conjugate MRS5539 6 (Fig. 1), which allows for the incorporation of a fluorescent reporter group.
Molecular probes for the A<inf>2A</inf> adenosine receptor based on a pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine scaffold
2011, Bioorganic and Medicinal Chemistry LettersMolecular Pharmacology, Physiology, and Structure of the P2Y Receptors
2011, Advances in PharmacologyCitation Excerpt :32P]-labeled (Houston et al., 2006) and [125I]-labeled (Ohlmann et al., 2010) MRS2500 have been developed as selective and high-affinity radioligands for recombinant and natively expressed P2Y1 receptors. An interesting and novel approach for development of targeted pharmacological tools is the synthesis of multivalent congeners by attaching, for example, multiple bisphosphate P2Y1 receptor antagonists to a polyamidoamine dendrimer carrier (de Castro et al., 2010; Jacobson, 2010). The P2Y1 receptor antagonists described thus far are polar structures that are expected to exhibit poor absorption when given orally.
Structural analysis, molecular docking and molecular dynamics simulations of G-protein-coupled receptor (kisspeptin) in fish
2020, Journal of Biomolecular Structure and DynamicsCHAPTER 4: Poly(amidoamine) (PAMAM) dendrimers: Synthesis and biological applications
2020, Monographs in Supramolecular Chemistry