Stimuli responsive systems constructed using cucurbit[n]uril-type molecular containers.

Conspectus This Account focuses on stimuli responsive systems that function in aqueous solution using examples drawn from the work of the Isaacs group using cucurbit[n]uril (CB[n]) molecular containers as key recognition elements. Our entry into the area of stimuli responsive systems began with the preparation of glycoluril derived molecular clips that efficiently distinguish between self and nonself by H-bonds and π-π interactions even within complex mixtures and therefore undergo self-sorting. We concluded that the selectivity of a wide variety of H-bonded supramolecular assemblies was higher than previously appreciated and that self-sorting is not exceptional behavior. This lead us to examine self-sorting within the context of CB[n] host-guest chemistry in water. We discovered that CB[n] homologues (CB[7] and CB[8]) display remarkably high binding affinity (Ka up to 10(17) M(-1)) and selectivity (ΔΔG) toward their guests, which renders CB[n]s prime components for the construction of stimuli responsive host-guest systems. The CB[7]·adamantaneammonium ion complex, which is particularly privileged (Ka = 4.2 × 10(12) M(-1)), was introduced by us as a stimulus to trigger constitutional changes in multicomponent self-sorting systems. For example, we describe how the free energy associated with the formation of host-guest complexes of CB[n]-type receptors can drive conformational changes of included guests like triazene-arylene foldamers and cationic calix[4]arenes, as well as induced conformational changes (e.g., ammonium guest size dependent homotropic allostery, metal ion triggered folding, and heterochiral dimerization) of the hosts themselves. Many guests display large pKa shifts within their CB[n]-guest complexes, which we used to promote pH controlled guest swapping and thermal trans-to-cis isomerization of azobenzene derivatives. We also used the high affinity and selectivity of CB[7] toward its guests to outcompete an enzyme (bovine carbonic anhydrase) for a two-faced inhibitor, which allowed stimuli responsive regulation of enzymatic activity. These results prompted us to examine the use of CB[n]-type receptors in both in vitro and in vivo biological systems. We demonstrated that adamantaneammonium ion can be used to intracellularly sequester CB[7] from gold nanoparticles passivated with hexanediammonium ion·CB[7] complexes and thereby trigger cytotoxicity. CB[7] derivatives bearing a biotin targeting group enhance the cytotoxicity of encapsulated oxaliplatin toward L1210FR cells. Finally, acyclic CB[n]-type receptors function as solubilizing excipients for insoluble drugs for drug delivery purposes and as a broad spectrum reversal agent for the neuromuscular blocking agents rocuronium, vecuronium, and cis-atracurium in rats. The work highlights the great potential for integration of CB[n]-type receptors with biological systems.

Introduction

Herein, I present an Account of research performed in my group directed toward the development of stimuli responsive systems using cucurbit[n]uril (CB[n]) molecular containers and other glycoluril derived systems as key recognition components. In the 1990s, the supramolecular chemistry field began to use its fundamental knowledge of noncovalent interactions (e.g., H-bonds, metal–ligand interactions, electrostatic interactions, and the solvophobic effect) to enable the preparation of more complex multicomponent aggregates formed in self-assembly processes. For the purpose of this Account, we define self-assembly as those processes that result in the transformation of a set of building blocks into one well-defined aggregate under a specific set of conditions (Scheme 1a). Self-sorting can then be defined as the concurrent self-assembly of multiple sets of building blocks into multiple well-defined aggregates under a specific set of conditions (Scheme 1b). The focus of this Account, stimuli responsive systems, is those self-assembling or self-sorting systems whose constituents can be reconfigured in response to suitable stimuli (e.g., pH change, photochemistry, electrochemistry, addition of further constituents). In stimulus responsive systems (Scheme 1c), the most potent stimuli are those that result in large favorable changes in the overall free energy (ΔΔG) after application of the stimulus that can drive substantial changes in constitution.

Scheme 1

Concepts of (a) Self-Assembly, (b) Self-Sorting, and (c) Chemical Stimulus Responsive Systems

Lehn conceptualized self-recognition processes in the context of the self-assembly of a series oligo(bipyridine) compounds (1) in the presence of Cu+ ions (Scheme 2a).[1] Individually, compounds 1a–1d self-assemble with Cu+ to give the double helicates [1a2·Cu2]2+–[1d5·Cu5]5+. Remarkably, the mixture of the four structurally similar strands (1a–d) and Cu+ gives [1a2·Cu2]2+–[1d5·Cu5]5+ in a high fidelity self-recognition process. The Sanders group first used the term self-sorting to describe the behavior of a mixture of building blocks (2 and 3) under thermodynamically controlled transesterification conditions (Scheme 2b).[2] Interestingly, a mixture of the two homomeric macrocycles 4 and 5 is formed in a high fidelity self-sorting process. In both of these pioneering works, the focus was on self-processes where each compound recognizes other identical building blocks within a mixture.

Scheme 2

Self-Sorting Processes

In this Account, we use glycoluril derived systems whose development can be traced to the early work of Mock, Nolte, and Rebek.[3−5] In 1981, Mock reported that the reaction of glycoluril and formaldehyde under strongly acidic conditions yields cucurbit[6]uril (CB[6]); subsequent work by Kim, Day, and Isaacs using milder reaction conditions produced CB[n] homologues CB[5], CB[7], CB[8], and CB[10] (Figure 1).[6−8] CB[n] chemistry is the subject of comprehensive reviews.[9] Mock showed that CB[6] shows high affinity toward alkanediammoniums (+H3N(CH2)NH3+, 6) in HCO2H–water mixtures in a process driven by the hydrophobic effect and ion–dipole interactions between the ammoniums and the two symmetry equivalent C=O portals of CB[n]. CB[6] also displays high selectivity based on length with 6 being preferred.

Figure 1

Structures of CB[n] and CB[6]·6 complex.

Self-Sorting Systems Based on Self-Recognition

As part of our synthetic and mechanistic studies toward CB[n]-type receptors[10,11] we prepared molecular clips 7–9, which feature a central glycoluril dimer framed by aromatic walls that display two H-bonding arms in different three-dimensional arrangements (Figure 2a).[12,13] We found that 7–9 individually undergo self-assembly in CDCl3 to yield homodimeric assemblies 82 and 92 and heterochiral homodimer (+)-7·(−)-7 (Figure 2b) driven by π–π interactions and H-bonds. We found that the three-component mixture 7–9 undergoes high fidelity self-sorting because the hypothetical heterodimers cannot satisfy the geometrical constraints of π–π interactions and simultaneously maximize the number of H-bonds formed.

Figure 2

Chemical and X-ray structures of self-sorting molecular clips.

The results with 7–9 lead us to question whether self-sorting was exceptional behavior or whether the wide variety of H-bonded aggregates known in the literature could simply be mixed together to generate a multicomponent self-sorting system. For this purpose, we selected ten components (7, 8, 10–16, and barium picrate), which are the components of Davis’ self-assembled ionophores, Reinhoudt’s rosettes, Rebek’s tennis ball and calixarene tetraurea capsule, Meijer’s ureidopyrimidinone, and our molecular clips.[14] We found that the 1H NMR spectrum of the 10-component mixture (Figure 3I) was simply equal to the sum of the NMR spectra of its individual self-assembled species (Figure 3A–H), which is the spectroscopic earmark of a self-sorting system. These studies suggested that simply selecting components with different H-bond donor/acceptor patterns and spatial orientations whose aggregates feature closed networks of H-bonds would enable the preparation of complex self-sorting systems. This work lead supramolecular chemists to appreciate that the subset of molecular aggregates that are capable of self-sorting was much larger than previously appreciated, which inspired numerous research groups to design and study self-sorting systems.

Figure 3

Ten-component self-sorting system.

Self-Sorting and Stimuli Responsive Systems Based on CB[n]-type Receptors

This section details our use of CB[n]-type molecular containers as prime components for the construction of stimuli responsive (self-sorting) systems including those that display allostery, pH and conformational responsiveness, regulation of enzymatic activity, and regulation of in vitro and in vivo biological activity. As discussed, CB[n] compounds are prime components for self-sorting systems because of their high affinity and high selectivity interactions, which commonly display slow kinetics of exchange on the chemical shift time scale, which allows for convenient monitoring of the constitution of the mixture by 1H NMR spectroscopy.

Social Self-Sorting

The self-sorting systems discussed above are based on H-bond driven self-recognition processes in CDCl3 where two or more identical molecules self-assemble. We envisioned that self-sorting processes in water based on host–guest complexation events, social self-sorting, would be both more biologically relevant and stimuli responsive since addition of exogenous guest could trigger transformation from one self-sorted state to another. For our initial demonstration of social self-sorting, we selected several host–guest pairs (CB[6]·6, CB[8]·17·18, β-cyclodextrin·19, and 20·K+) and other aggregates (212 and 222·232) and asked whether their components would faithfully reform these aggregates upon mixing or whether alternate host–guest processes would impinge on the self-sorting process (Figure 4).[15] We found that the 1H NMR spectrum of the 12-component mixture is a superposition of the spectra of the six complexes listed above, which means that this mixture is self-sorted. This work showed that self-sorting proceeds efficiently even in biologically relevant H2O as a medium using inherently less directional electrostatic interactions and the hydrophobic effect. Furthermore, the high fidelity self-sorting suggested to us that the high affinity and selectivity observed for CB[6][16] would also be observed for CB[n] homologues.

Figure 4

Social self-sorting in water.

CB[n] Exhibit High Affinity and High Selectivity Host–Guest Interactions

The social self-sorting process described above prompted us to determine the binding constants of CB[7] and CB[8] toward numerous guests by 1H NMR competition experiments referenced to absolute Ka values determined by direct UV/vis titrations.[17] Figure 5a shows a selection of the Ka values determined. For example, we discovered that CB[7] exhibits picomolar binding affinity toward adamantaneammonium and ferrocene ammonium ions, 24 and 25, in 50 mM NaOAc buffered D2O;[17] the high binding affinity of CB[7]·25 was subsequently confirmed by ITC competition experiments performed by Inoue, Kim, and Kaifer.[7]uril Host: A Comparative Study of the Cucurbituril and Cyclodextrin Host Families. J. Am. Chem. Soc.. 2005 ">18] Equally interesting was the fact that the CB[8]·24 complex is more than 5000-fold weaker than CB[7]·24. Conversely, the CB[8]·26 complex is more than 107-fold tighter than CB[7]·26. Similar to many protein–ligand complexes, the addition of Me-groups greatly destabilizes the CB[7]·26 complex due to unfavorable steric interactions between the convex surface of the guest and the concave surface of the host. Recent work by Scherman, DeSimone, and Nau suggests that an important factor in the high affinity binding of CB[n] is the release of high energy waters from the CB[n] cavity upon complexation.[19] We also measured the Ka values between guest 27 (available from a concurrent study)[8] and CB[7] and CB[8]; the encapsulation of both ammonium arms within CB[8]·27 leads to 3200-fold higher affinity and conformational control. Intrigued by the high affinity binding of CB[7]·24, several groups sought to reach even higher levels of affinity. In 2007, a collaboration between the Inoue, Kim, Kaifer, Gilson, and Isaacs groups studied the CB[7]·28 complex in pure water and measured Ka = 3 × 1015 M–1, which is comparable to that of avidin–biotin;[20] the ability of 28 to engage in ion–dipole interactions with both C=O portals was deemed of critical importance. Additional ferrocene and [2.2.2]bicyclooctane diammonium ions (29) have been explored in this context.[21,22] Recently, the Glaser–Majerski–Isaacs team reported that diamantanediammonium ion 30 forms CB[7]·30 with Kd = 1.4 aM![7]uril·Guest Pair with an Attomolar Dissociation Constant. Angew. Chem., Int. Ed.. 2014 ">23] This remarkable result was attributed to the presence of 14 optimal ion–dipole interactions, the colinearity of the symmetry axes of CB[7] and 30 within CB[7]·30, and the greater hydrophobicity of diamantane relative to adamantane or ferrocene skeletons based on X-ray crystallographic results (Figure 5b).

Figure 5

(a) Ka values for selected CB[7] and CB[8] complexes determined in 50 mM NaOAc buffer and (b) X-ray structure of CB[7]·30.

Control over Allosteric Processes with CB[n]

In 2005, we reported that addition of guest 27 to CB[10]·CB[5] gave CB[10]·272, which upon reaction with Ac2O followed by washing delivered free CB[10].[8] In searching for guests to fill the voluminous (870 Å3) cavity of CB[10], we found that tetracationic calixarene 32 forms the CB[10]·1,3-alt-32 complex in which 32 preferentially assumes the less stable 1,3-alternate conformation (Scheme 3). Quite interestingly, we found that addition of adamantane derivative 19 resulted in the formation of the CB[10]·cone-32·19, which amounts to a stimuli responsive conformational control of 32 within the macromolecular complex. Cationic adamantane 24 was much less efficient a stimulus than 19 presumably due to unfavorable electrostatic interactions in the CB[10]·cone-32·14 ternary complex. To reverse the conformational change, we added CB[7] as a stimulus, which sequesters 19 as its CB[7]·19 complex. The formation of CB[7]·19 (Ka = 3.2 × 108 M–1) provides the free energy needed to drive the conformational change. In this manner, alternate addition of 19 and CB[7] can be used as chemical stimuli to exert allosteric control over the conformation of calixarene 32.

Scheme 3

Conformational Responsiveness of 32 within CB[10]

Size Dependent Homotropic Allostery with Bis-nor-seco-CB[10]

During our studies of the mechanism of CB[n] formation, we isolated bis-ns-CB[10], which is formed by step-growth polymerization.[24] Bis-ns-CB[10] features two cavities, each shaped by five glycolurils and connected by one CH2–bridge (Figure 6). Bis-ns-CB[10] forms ternary complexes with a variety of guest molecules including 24, 31, 33, and 34. Molecular modeling shows that the cavity volume (450 to 740 Å3) and H2C···CH2 nonbonded distance (5.5 to 9.3 Å) increase systematically as the size of the included guest increases. Therefore, we hypothesized that the binding of the first guest to give bis-ns-CB[10]·guest complexes would change the size of the second cavity (Figure 6c). Further, we suspected that the second cavity would display a preference to bind a guest of similar size by homotropic allostery. To test this hypothesis, we prepared mixtures comprising bis-ns-CB[10] and pairs of guests (e.g., 31 + 33; 24 + 34) and monitored the process by 1H NMR spectroscopy. We observed that mixtures of small and large guests (e.g., 31 + 33) do indeed undergo size dependent homotropic allostery to yield bis-ns-CB[10]·312 and bis-ns-CB[10]·332; whereas guests of comparable size (24 + 34) form mixtures of the two homomeric and the heteromeric ternary complex bis-ns-CB[10]·24·34. The binding of the first guest acts as a stimulus, which instructs the second binding site to assume a specific size.

Figure 6

Size dependent homotropic allostery with bis-ns-CB[10].

pH Controlled Guest Swapping

CB[n] compounds bind substantially tighter to cationic guests (GH+) than to the corresponding neutral form (G), which is reflected in a pKa shift within the CB[n]·GH+ complex. We designed a four component system comprising two guests and two hosts to undergo controlled guest swapping in response to a change in pH (Scheme 4).[25] As the hosts, we selected β-cyclodextrin (β-CD), which is not selective based on guest charge, and CB[6], which is highly selective for cationic over neutral guests. We selected guest 35H+, which is pH sensitive, and guest 36, which remains cationic over the full pH range. Guests 35H+ and 36 contain adamantaneammonium binding epitopes to promote binding toward β-CD and hexylammonium tails to promote CB[6] binding. At pH 7, an equimolar mixture of all four components mainly forms CB[6]·35H+ and β-CD·36 (84%). At pH 13, 35H+ becomes deprotonated to 35, whereas 36 remains cationic. Accordingly, the dominant species (89%) at pH 13 are CB[6]·36 and β-CD·35 as monitored by 1H NMR spectroscopy. Thermodynamic calculations using Gepasi show that the magnitude of the pKa shift (e.g., ΔΔG) between CB[6]·35H+ and 35H+ is the critical parameter governing the guest swapping process.

Scheme 4

Four Component Guest Swapping Ensemble

Azobenzene Cis–Trans Isomerization

A more complex pH responsive system comprises CB[7] and azobenzene derivative 37 (Scheme 5a).[26] Azobenzene derivatives typically prefer the trans-form over the cis-form by approximately 10 kcal mol–1. However, when we mixed CB[7] with 37 (D2O, pH 4.74), we observed the purple color that is typical of cis-azobenzenes. We performed a pH titration (pH 0.5 to 12.5) of 37 alone and did not observe any purple color; the three pKa values of 37·H+ (3.54), 37·2H2+ (2.11), and 37·3H3+ (−0.5) agreed with previous literature reports. Subsequently, we performed a UV/vis titration for the CB[7]·37 complex from pH 0.17 to 13.35. Plots of absorbance (320, 554, and 391 nm) as a function of pH allowed us to extract three pKa values for the CB[7]·37·H+ (5.55), CB[7]·37·2H2+ (5.09), and CB[7]·37·3H3+ (3.38) complexes (Scheme 5b). The magnitude of the pKa shifts induced upon CB[7] binding for 37·3H3+ and 37·2H2+ are quite substantial and approach the magnitude of shifts observed by Nau[27] and Raymond[28] for other container complexes. Quite interestingly, decreasing the pH (7 to 5) results in the formation of CB[7]·trans-37·H+ and then CB[7]·trans-37·2H2+, which undergoes thermal isomerization to the CB[7]·cis-37·2H2+ complex (Scheme 5a). We suggest that the very high Ka values typically observed for CB[7] complexes and the better match of cis-37·2H2+ than trans-37·2H2+ toward the hydrophobic cavity and electrostatically negative C=O portals of CB[7] provide the potent thermodynamic driving force to overcome the 10 kcal mol–1 preference for trans-azobenzenes. Decreasing pH below 3.4 results in cis-to-trans isomerization to yield CB[7]·trans-37·3H3+. We used this highly pH responsive UV/vis active system to construct indicator displacement assays for acetylcholine, caffeine, nicotine, dopamine, l-adrenaline, ephedrine, and pseudoephedrine and were able to quantitate pseudoephedrine concentration in the over-the-counter nasal decongestant Sudafed in the 5–350 μM range.

Scheme 5

Equilibria in the System Comprising CB[7] and 37

Controlled Folding Processes

Our earlier experience with the binding of 27 with CB[7], CB[8], and CB[10] prompted us to use CB[n] complexation to control the conformation of longer triazene–arylene oligomer 38 and related structures.[29] By virtue of its four exocyclic triazene–NHAr bonds (green), oligomer 38 populates 24 conformations of which 10 are unique (Scheme 6). We studied the interaction of 38 with CB[7], CB[8], and CB[10] by a combination of 1H NMR spectroscopy and X-ray crystallography. Based on symmetry arguments and the observed direction and magnitude of changes in chemical shift upon complexation, we established that two CB[7] bind to the terminal benzylammonium rings of 38 to give CB[7]·a,s,s,a-38·CB[7], whereas CB[8] forms the CB[8]·a,a,a,s-38, and CB[10] forms the CB[10]·a,a,a,a-38 complex. We envisioned that we could fold, unfold, and refold a single oligomer strand in response to chemical stimuli. Initially, 38 populates the full 10-component conformational ensemble. However, upon addition of CB[8], the CB[8]·a,a,a,s-38 conformation is selectively stabilized. Addition of 2 equiv of CB[7] and 26 results in ejection and unfolding of 38 followed by the refolding into the CB[7]·a,s,s,a-38·CB[7] conformation. The formation of the CB[8]·26 complex provides the thermodynamic driving force to promote this process. Interestingly, we observed that the addition of 1 equiv of CB[7] enhances the rate of dissociation of the CB[8]·a,a,a,a-38 by formation of ternary complex CB[8]·a,a,s,s-38·CB[7]. Finally, the addition of 2 equiv of 24 and CB[10]·CB[5] results in the expulsion and forced unfolding of 38 into free solution where it displaces CB[5] and folds to form CB[10]·a,a,a,a-38 driven by the free energy of formation of the CB[7]·24 complex. The work is reminiscent of chaperone proteins, which forcibly unfold misfolded proteins and provide an environment for the refolding.

Scheme 6

Folding Oligomer 38 with CB[n] Containers

In 2009, we isolated acyclic glycoluril decamer (±)-39 in racemic form.[30] When dissolved in water, (±)-39 exhibits a poorly resolved 1H NMR spectrum indicative of a range of conformations around the central N–CH2–N bridge. Quite interestingly, addition of metal sulfate salts (100 mM Li+, Na+, K+, or Ca2+) triggers folding and heterochiral dimerization processes to yield (+)-39·(−)-39·6Na+ driven by strong ion–dipole interactions between three or four C=O groups and each of the Na+ ions. The X-ray crystal structure of (+)-39·(−)-39·6Na+ is shown in Scheme 7. It is possible to reverse this folding and heterochiral dimerization process by the addition of 31, which gives the (±)-39·312 complex; subsequent addition of CB[7] sequesters 31 as the CB[7]·31 complex and results in the reformation of the (+)-39·(−)-39·6Na+ complex. These processes amount to a metal ion responsive folding and assembly process that is reminiscent of RNA folding processes. CB[n] containers are privileged in that they display protein mimetic (e.g., tight and selective binding) and nucleic acid mimetic (e.g., metal ion responsive) processes.

Scheme 7

Folding and Assembly of (±)-39

Recently, we reported the preparation of CB[6] dimer 40 by the reaction of glycoluril hexamer with 1,2,4,5-tetraformylbenzene.[6]uril. Angew. Chem., Int. Ed.. 2013 ">31] Compound 40 (3 equiv) undergoes self-assembly with oligomer 41 (2 equiv) to yield supramolecular ladder 403·412 as shown in Scheme 8 as established by 1H NMR symmetry considerations and diffusion coefficient measurements (DOSY). Complex 403·412 is comparable in weight (7389 amu) and dimension (40 × 29 × 11 Å3) to small proteins. Interestingly, an equimolar mixture of 40 and 41 yields the folded 40·41 assembly wherein the two terminal hexylene binding sites of 41 are complexed to fully satisfy both cavities of 40. The folded structure of 40·41 was assigned on the basis of complexation induced chemical shifts, symmetry considerations, DOSY, and electrospray ionization mass spectrometry measurements. When more 40 (0.5 equiv) is added, the assembly is transformed into 403·412 in a process that is responsive to host–guest stoichiometry.

Scheme 8

Formation of Supramolecular Ladder 403·412

Control over Enzymatic Catalysis

We envisioned that the extremely high binding constants commonly exhibited by CB[7]–guest complexes could be used to outcompete biological macromolecules for their targets and thereby exert stimuli responsive control over the cognate biological functions. First, we decided to exert control over the catalytic activity of bovine carbonic anhydrase (BCA).[32] We prepared a series of compounds (42a–c) that we refer to as two-faced inhibitors that contain a benzenesulfonamide binding epitope, which is well-known to inhibit BCA via binding to the Zn-cofactor at the active site (Ka ≈ 107–108 M–1), and an ammonium ion binding face, which binds to CB[7] (Scheme 9). Compounds 42a–c contain adamantaneammonium, trimethylsilylmethylammonium, and hexylammonium binding epitopes, which possess vastly different Ka values (∼1012, 109, and 106 M–1, respectively) toward CB[7]. Experimentally, we found that addition of 42a–c to BCA turns off the catalytic ability of BCA as monitored by a standard UV/vis p-nitrophenylacetate hydrolysis assay. Addition of CB[7] sequesters 42a or 42b, but not 42c, as their CB[7] complexes and turns on the catalytic activity of BCA. The lower binding affinity of CB[7]·42c is insufficient to drive the reactivation of BCA. Similarly, we were unable to reactivate the BCA·42a or BCA·42b systems by addition of β-cyclodextrin because of the low Ka values of β-CD–guest complexes. Subsequent addition of 43 to the BCA + CB[7]·42b system competitively displaces two faced inhibitor 42b from its CB[7]·42b complex, which then turns off BCA by formation of BCA·42b. We could not regenerate activity of the BCA + CB[7]·42a system in this manner because the CB[7]·42a complex (Ka ≈ 1012 M–1) exhibits slow dissociation kinetics (koff ≈ 2 × 10–5 s–1; half-life = 9 h) on the time scale of the experiment. Alternate addition of CB[7] and 43 was able to control enzymatic catalysis of BCA over four on–off cycles. Very interestingly, we found that CB[7] enhances the rate of dissociation of BCA·42a (11-fold) and BCA·42b (19-fold) due to the formation of a transient ternary complex (BCA·42·CB[7]). We independently confirmed that the CB[7]·42a and CB[7]·42b complexes are ∼100-fold weaker inhibitors (Ka ≈ 105–106 M–1) of BCA than 42a or 42b presumably due to steric interactions between the convex surface of CB[7] and the active site of BCA. The work suggests a path toward stimuli responsive control of protein function in biological systems.

Scheme 9

Control of Enzymatic Activity of BCA

Control over in Vitro Biological Systems

In collaboration with the Rotello group, we sought to use the high affinity and highly selective binding interactions of CB[7] to activate nanoparticle therapeutics intracellularly in a stimuli responsive manner.[33] For this purpose, gold nanoparticles (2.5 ± 0.4 nm diameter) were decorated with hexanediammonium ion terminated alkanethiolates. The terminal hexanediammonium ions renders nanoparticles 44 cytotoxic and also serves as a recognition site for CB[7] (Figure 7). We envisioned that the CB[7] complexed nanoparticle 44·CB[7] would cloak the cytotoxic hexanediammonium ion units and reduce cytotoxicity toward MCF-7 cells. This prediction was borne out experimentally, and one reason for the reduced cytotoxicity was the fact that the 44·CB[7] nanoparticles become entrapped in endosomes. Furthermore, we found that the addition of 24 to MCF-7 cells that had already been treated with 44·CB[7] sequesters CB[7] from the nanoparticle surface driven by the free energy of formation of CB[7]·24. This process triggers the release of the naked nanoparticle 44 from the endosome and results in higher levels of cytotoxicity. The ability to intracellularly activate nanoparticle therapeutics triggered by 24 offers potential routes to dual (e.g., host and guest) targeted therapeutics with higher levels of site-specific activity.

Figure 7

Intracellular activation of nanoparticle therapeutics.

Recently, we have been studying the preparation and molecular recognition properties of acyclic CB[n]-type molecular containers 45 and 46 (Figure 8a).[34] Compounds 45 and 46 feature a central glycoluril tetramer, two terminal aromatic rings, and four SO3– groups, which dramatically enhance their solubility in water. Compounds 45 and 46 are preorganized into a C-shaped conformation (Figure 8b) and are not significantly self-associated in water. Significantly, the acyclic nature of 45 and 46 and the flexibility of the glycoluril tetramer backbone allow these compounds to flex like a hand to accommodate a wide range of guest sizes. We found that 45 and 46 greatly enhance the water solubility (from 23-fold to 2750-fold) of a number of insoluble drugs (e.g., paclitaxel, melphalan, clopidogrel, amiodarone, and camptothecin, Figure 8c). Acyclic CB[n]-type container 45 does not display significant in vitro toxicity toward kidney (HepG2), liver (HEK 293), and human monocyte (THP-1) cells according to metabolic cell viability and adenylate kinase release cell death assays. The in vivo (Swiss Webster mice) maximum tolerated dose (MTD) studies with 45 indicated an MTD greater than 1.23 g kg–1. Treatment of HeLa cells with paclitaxel solubilized with 45 results in enhanced cell killing relative to the paclitaxel alone. This study establishes proof-of-principle for use of acyclic CB[n]-type containers for drug solubilization and delivery. These in vitro studies rely on drug displacement by competitive binding of 45 toward endogenous compounds whereas the corresponding in vivo application of cyclodextrin·drug complexes also benefits from the dilution of the container·drug complex from above to below Kd.

Figure 8

Acyclic CB[n] solubilizing containers.

Recently, the Isaacs group discovered the templated synthesis of methylene bridged glycoluril hexamer, which can be transformed into monofunctionalized CB[7] derivatives by reaction with glycoluril bis(cyclic ethers).[35] We adapted this chemistry to prepare CB[7] derivative 47, which bears a covalently attached biotin ligand on its convex surface.[7]uril Containers for Targeted Delivery of Oxaliplatin to Cancer Cells. Angew. Chem., Int. Ed.. 2013 ">36] Container 47 is capable of forming complexes with chemotherapeutic agents (oxaliplatin, camtothecin, tamoxifen, temezolomide, albendazole, or irinotecan). We monitored the internalization of the fluorescent complexes 47·48 and CB[7]·48 into murine lymphocytic leukemia cells (L1210) and their derived cell line L1210FR, which overexpresses biotin receptors, by confocal microscopy and flow cytometry (Figure 9). These experiments indicated enhanced uptake of 47·48 by a receptor mediated endocytosis pathway. Subsequently, we monitored the cell viability of L1210 and L1210FR cells after treatment with targeted 47·oxaliplatin versus untargeted CB[7]·oxaliplatin. We observed an approximate 10-fold decrease in EC50 for 47·oxaliplatin relative to CB[7]·oxaliplatin. This work opens the door toward the use of CB[n] derivatives for the targeted delivery of drugs to specific locations in vivo.

Figure 9

In vitro targeting of 47·oxaliplatin.

Control over in Vivo Biological Systems

Similar to the cyclodextrins, the in vitro applications of acyclic CB[n]-type containers rely on their comparably weak binding toward many drugs, which allows for release upon competitive displacement or dilution. Sugammadex is a γ-cyclodextrin derivative that binds tightly to the neuromuscular blocking agents rocuronium and vecuronium (Ka ≈ 107 M–1).[37] Sugammadex is marketed by Merck under the trade name Bridion in Europe for the clinical reversal of neuromuscular block induced by rocuronium and vecuronium but has not yet been approved by the US FDA.[37] We envisioned that acyclic CB[n]-type containers 45 and 46 would bind strongly to rocuronium, vecuronium, and cis-atracurium because they are hydrophobic steroidal dications (Figure 10a).[38] Using a combination of direct UV/vis titrations and competition assays allowed us to measure the Ka values for 45 and 46 toward neuromuscular blocking agents 49–51 and acetylcholine 52. The affinity of 46 toward rocuronium is 300-fold higher than that of sugammadex[37] and is 19000-fold selective for rocuronium over acetylcholine 52, which prompted us to perform in vivo reversal experiments (Figure 10c). Experimentally, rats were anesthetized with isofluorane, and their neuromuscular function was blocked by administration of rocuronium (3.5 mg kg–1). Subsequent treatment with 46 (30 mg kg–1) resulted in a rapid reversal of neuromuscular block as evidenced by the dramatic decrease in time required for spontaneous breathing and a return to a train-of-four (TOF) ratio of 0.9. Related experiments conducted using 45 demonstrate the reversal of neuromuscular block induced by rocuronium and also cis-atracurium, which has not been demonstrated for sugammadex.[39] Treatment with 45 (≤150 mg kg–1) does not result in changes in heart rate, mean arterial pressure, or blood chemistry (pH, pO2, or pCO2). Further development of 45 and particularly 46 as broad spectrum reversal agents for neuromuscular block is in progress.

Figure 10

Binding of NMBAs by 45 and 46in vitro and reversal of neuromuscular block in vivo in rats.

Concluding Remarks

CB[n] and CB[n]-type receptors display remarkably high affinity toward their target guests in water as exemplified by the CB[7]·30 complex with Kd = 1.4 aM. Furthermore, CB[n]-type receptors are quite selective in that different hosts display quite different affinity toward a common guest and conversely that a given host is quite selective among a pool of guests. Accordingly, CB[n]–guest derived systems are inherently stimuli responsive (e.g., pH, photochemical, electrochemical, chemical). In this Account, we presented the progression of our work in this area beginning with self-sorted H-bonded molecular clips and collections of H-bonded aggregates by self-association processes and continuing to the development of stimuli responsive systems based on the extremely high affinity (Ka up to 1017 M–1) and selectivity displayed by CB[n]-type containers toward their guests, which provides the free energy driving force to reconfigure multicomponent systems. For example, we constructed systems that display size-dependent homotropic allostery, controlled folding–unfolding–refolding transitions of a triazene–arylene oligomer, pH triggered guest swapping, pH promoted azobenzene trans–cis isomerization, and control over enzymatic activity of BCA. In the final sections, we showed that CB[n]-type receptors could be used to express control over biological systems both in vitro and in vivo. For example, CB[7] can be used to cloak the toxicity of gold nanoparticles 44, which can be decloaked by addition of 24 inside MCF-7 cells, biotin-targeted CB[7] 47 enhances the bioactivity of oxaliplatin toward L1210FR cells 10-fold, and acyclic CB[n]-type receptors 45 and 46 can enhance the solubility and bioactivity of drugs like paclitaxel. Finally, compound 46 binds to rocuronium in vitro and reverses neuromuscular block in vivo in rats. Accordingly, the future prospects for the use of CB[n]-type receptors as stimuli responsive components to trigger reconfiguration of a system between two or more distinct states for both technological and biological applications are extremely bright.