Resorcinarene-Based Polymer Conjugated for Pharmaceutical Applications

Abstract

Resorcinarenes are polyhydroxylated platforms consisting of 4, 5, 8, or more units of resorcinol. The numbers refer to the number of resorcinol units, with 4-unit platforms being the most stable. Investigation into their use in pharmaceutical applications has increased due to high versatility and functionalization. They exhibit significant flexibility due to their methylene bridges and to the interactions of hydrogen bridges and van der Waals forces. These platforms can be used in an increasing number of applications, which include the functionalization of nanoparticles and relevant materials, the synthesis of catalysts, the removal of contaminants, and analytical separations in analytes such as benzodiazepines and norepinephrine. For this last application, resorcinarenes are functionalized with specific important functional groups. Polymers were developed in the 20th century for the development of materials with significant improvements in thermal and mechanical properties. They are cross-linked polymeric structures, mainly made up of monomers such as styrene, divinylbenzene acrylate, vinylpyridine, and vinyl acetate, among others. They often have a homogeneous, porous structure, but this structure can vary significantly depending on the type of solvent used. Therefore, they have been applied in the functionalization of the polyhydroxylated platforms. In this review, the structure, properties, and synthesis of resorcinarenes, as well as the use of polymeric matrices, are analyzed, emphasizing the functionalization of organic polymers using resorcinarenes. Furthermore, the respective applications in controlled drug delivery, pharmaceutical transport, and therapeutics, which are diverse and show promising growth, will be explored.

1. Introduction

Resorcinarenes are part of polyhydroxylated platforms made up of units of phenolic derivatives linked by methine groups. These macrocycles have demonstrated unique properties that make them attractive for the design of new materials and the generation of supramolecular receptors. This is due to the stability of their conformations and their versatile scaffolds [1,2]. The initial investigation began with the chemist Baeyer, who synthesized phenol-based dyes, finding that the addition of concentrated sulfuric acid to a mixture of benzaldehyde and phenol derivatives resulted in colored products; however, he was unable to characterize them [3,4]. Afterward, Niederl and Vogel reacted acetaldehyde with resorcinol [5]. They reported a structural approach to the cyclic tetramer type. Högberg demonstrated that investigation of the structure via X-ray diffraction also indicated that the most common resorcinarene macrocycles can be obtained with high yield under key conditions (solvency, temperature, and time, among others) [6].

These macrocycles stand out among the many versatile compounds due to their easy modification, and thus, they are used in various applications, among which are as catalysts in the reduction of nitroaromatic compounds [7,8], the synthesis of Pd nanoparticles applied in the Suzuki–Miyuara cross-coupling reactions [9], and as carrier dendrimers for molecules such as ibuprofen with anticancer activity [10].

Regarding other functions, the application of zirconium–resorcinarene complexes in the goat-skin tanning process has been documented, thus promoting a new strategy for chrome-free tanning [11]. Resorcinarenes can also be applied to the transport of drugs or in the formation of complexes in the synthesis of biomedical materials (biosensors, bioimages) [3,12] because they participate in non-covalent interactions, which result in good adaptability to the response to external stimuli, and therefore they can be used in the manufacture of materials sensitive to such stimuli [3]. Macrocycles have been used as synthetic building blocks and some of them have biological activity and can be used in therapeutic treatments [13]. Furthermore, resorcinarenes are applicable in analytical separations through the development of stationary phases for HPLC [14] due to their ability to modify polymeric surfaces such as organic monoliths based on their high selectivity in carrying out functionalization at both the lower and upper edge of the respective aromatic bonds [15].

These macrocycles can be classified as belonging to the group of polyhydroxylated platforms because they are generally obtained by carrying out a condensation reaction between a phenol, resorcinol, or pyrogallol compound and an aldehyde (aliphatic or aromatic) in an acid medium. One important characteristic is that aliphatic or aromatic portions can be substituted into their lower edge, which gives them greater interaction with other molecules, providing them with a great deal of versatility, which in turn leads to the presence of an increasing number of applications such as those mentioned above. However, the resorcinarenes themselves are hydrophobic, which reduces their bioavailability, but this problem can be overcome by functionalizing them at the top edge via reactions such as sulfomethylation [16,17] or using specifically functionalized aldehydes [18].

This review provides a comprehensive analysis of resorcinarenes, highlighting their growing importance in the pharmaceutical field. Key aspects such as its molecular structure, advances in synthesis, and functionalization strategies are addressed, with a specific focus on drug-delivery systems, pharmaceutical transport, and therapeutic and analytical applications. The integration of resorcinarenes into polymeric matrices has promoted the development of innovative therapeutic platforms, optimizing controlled drug release and improving biocompatibility. In addition, their ability to form highly selective host–guest complexes through non-covalent interactions highlights their potential in biodetection and molecular recognition. The chemical versatility of these macrocycles, combined with functionalized polymer engineering, opens new opportunities for the development of smart materials in precision medicine and advanced pharmaceutical formulations.

2. Synthesis of Calix[4]resorcinarenes

2.1. Reflux Synthesis of Calix[4]resorcinarenes

The process of synthesis—the optimum solubilization of resorcinol together with the aldehyde used, whether aromatic or aliphatic—in general must be carried out in an ethanol:water mixture. A condensation reaction is then carried out between the aldehyde used and resorcinol, using an inorganic acid such as HCl or H₂SO₄ as a catalyst for the process, after which the mixture is refluxed [19,20]. Lewis acid, Sc(OTf)₃, can also be used as a process catalyst in the case of a cyclocondensation reaction between 1,3-dialkoxybenzenes, thus forming resorcin[4]arenes (Figure 1) [21]. Solvent-free and microwave irradiation processes can also be used and are explained below.

2.2. Solvent-Free Synthesis of Calix[4]resorcinarenes

Synthesis is based on vigorously grinding equimolar amounts of resorcinol and selected aromatic aldehyde in the presence of a catalytic amount of p-toluensulfonic acid (TsOH), as shown in Figure 1 (1c) [22,23] in order to increase the surface area of the particles and decrease the particle size, thus directly facilitating the reaction of the reactants [22]. The reaction takes place under mild, solvent-free conditions because the reactants dissolve into each other, i.e., they are miscible and, therefore, form a solid solution, even though no solvents are used.

After the grinding process of the reagents, the reaction mixtures appear to be viscous pastes, even though the reagents are solid. This mixture returns to a solid state in a few minutes, after which the product obtained is washed with deionized water to eliminate the catalyst (p-toluensulfonic acid). For example, the synthesis of tetra-(p-hydroxyphenyl)calix[4]resorcinarene can be carried out with high yields by directly reacting solid resorcinol and p-hydroxybenzaldehyde. Other resorcinarenes have also been synthesized with an aromatic substituent on a methine bridge (Figure 2) [22]. This methodology is environmentally friendly because it does not use toxic solvents and thus has a significantly lower requirement for water and thermal energy in the system. This latter requirement is reduced because it is not applied to the high-milling reaction mixture since it is itself a viscous liquid, in contrast with previous procedures, in which large volumes of solvent and slow reactions were used [24]. Therefore, it can be affirmed that this reaction employs the principles of green chemistry since it involves the implementation of sustainable chemical processes.

2.3. Synthesis of Calix[4]resorcinarenes Through Microwave Irradiation

In the specialized literature [25], there are several reports of the use of microwaves to obtain resorcinarenes. An interesting example is the reaction between resorcinol and vanillin (4-hydroxy-3-methoxybenzaldehyde). Once all the synthesis and purification protocols have been carried out, the desired product is obtained with a good yield of above 90%, and the microwave-assisted reaction decreases the reaction time by between 5 and 10 min (Figure 3) [25].

2.4. Conformations of Calix[4]resorcinarenes

Resorcinarenes can be classified as calixarenes. The prefix “calix” derives from the fact that these macrocycles exhibit a structure analogous to the shape of the Greek “calyx” crater vessels, which was proposed by Gutsche [26]. Resorcinarene is a macrocyclic molecule made up of resorcinol subunits with a characteristic calyx-shaped structure generally due to its flexible bonds [20], which in turn are classified into an upper rim and lower rim, and the weak interactions are essential in the macrocycle because they increase its stability [4].

Many conformations can be found in resorcinarenes as a consequence of the flexibility in their methine groups, interactions of the macrocycle with the solvent, and interactions of hydrogen bonds, as well as not being flat. The most common crown conformation of resorcinarenes that have been observed are chair, saddle, boat, and diamond. In the crown conformation, all hydroxyl groups point upwards, forming a ring of intramolecular hydrogen bonds that stabilize the structure [19], while in the chair conformation, two opposite aromatic rings point upwards, and the other two are almost parallel to the plane of the methyne carbon. When studying the kinetic products of resorcinarene condensation, where at least one aliphatic (R) is in the exo orientation, the saddle and diamond conformations have one or two aromatic rings pointing downwards. However, only the all-endo conformations, including the crown, are in a bowl shape, which is more favorable in host–host complexation (Figure 4) [2,20,27].

They occur in two isomeric forms: cis–cis–cis isomers (rccc) and cis–trans–trans isomers (rctt). These distributions are due to the different solubilities of these conformers (Figure 5); and can be distinguished by the comparison of their ¹H NMR spectra. For example, for the rctt isomer, two sets of peaks can be identified from the resonances of the OH, Ha, and Hb groups, which are characteristic of C_2h symmetry [27]. This symmetry can also be verified by crystal diffraction studies, indicating that the rctt isomer of calix[4]resorcinarene crystallizes in the chair isomer with the aforementioned symmetry.

3. Functionalization and Host–Guest Interactions of Calix[4]resorcinarenes

3.1. Functionalization Calix[4]resorcinarenes

Functionalization at the top edge can go two ways: in the hydroxyl oxygens or at the ortho position. Along the same lines, electrophilic attacks, specifically electrophilic aromatic substitution reactions (EAS), occur in the aromatic ring, which has a high electronic density in p orbitals, forming a stable aromatic system. These π electrons can be arranged to generate a new bond with strong electrophile properties, thus forming a carbocation [28]. Therefore, hydroxyl groups of resorcinarene stabilize ortho position ring, inducing SEAr reactions to bond to electrophiles.This is how this reaction can be carried out in the ortho carbon to both hydroxyl groups. In that position, reactions such as acetylation [29], alkylation [20], sulfonation [30], and diazotization [31] can take place. Formulation of the Duff reaction can take place in formylated calix[4]resorcinarenes, which, in turn, can be reacted with aliphatic or aromatic amines (Figure 5) [32]. Also, aminomethylation [33] and resorcinarene can undergo the Mannich reaction, which can be carried out in order to significantly improve the solubility of the macrocycle in water, which is an important property in medical applications [34]. Therefore, these molecules are reacted with amino acids such as glycine, alanine, etc., allowing possible applications as cavitands. N-oxides, acetates, pyridine, nitriles, or alkyl chlorides can also be used in the synthesis of functionalized resorcin[4]arene cavitands, which can be synthetic receptors that generate a deep, vase-like space in which small molecules can reside. These modifications generally result in a marked improvement in the interaction of resorcinarenes with amino acids. Furthermore, click-chemistry strategies have also been applied in the modification of these macrocycles [35].

The specific reaction of the bottom-edge functionalization is carried out on the methylene bridge in bifunctional aldehydes such as 11-undecylenicaldehyde. The functionalization thus provides double bonds and hydroxyl or sulfonate groups, among others [13,36]. A very important detail is the protection of the hydroxyl groups at the upper edge in synthetic transformations with acetyl or silyl groups. These hydroxyl groups can also be protected with methylene ether, giving rise to resorcinarenes with rigid bonding cavities, commonly called resorcinarene cavitands. The lower rim of resorcinarenes can also be functionalized with alkenes, the next step being to transform them into thiols with the aim of carrying out applications in the production of self-assembled monolayers through covalent bonds of resorcinol molecules on gold sheets [37].

Another example is the functionalization of calix[4]resorcinarenes at methylene bridges to generate resorcinarene tetra-hydrazide (RTH), which can be prepared through two synthetic steps (Figure 6):

• Starting from resorcinol and 4-formylbenzoic acid, two of the carboxyl groups are esterified and prevent the esterification of the other two carboxyl groups.
• Subsequently, RTH is generated via nucleophilic substitution in the carboxyl groups of the substituents of the methylene bridge. It is used to stabilize gold nanoparticles and in the detection of phenylalanine in human serum [38].

3.2. Host–Guest Interactions of Calix[4]resorcinarenes with Molecules of Biological Interest

Non-covalent interactions such as cation–π, π–π stacking, anion–π, van der Waals interactions, and H-bonding drive host–guest phenomena [39]. Through these interactions, calix[4]resorcinarenes form inclusion complexes with organic molecules [40], trimethylammonium cations [41], and quaternary ammonium halides [42]. The size and shape complementarity of calix[4]resorcinarene leads to high selectivity and affinity to biomolecules such as carnitine and choline. Receptors based on calix[4]resorcinarenes, with a concave binding site as well as a suitable size, have been developed for these molecules [43,44]. A first example of host–guest interactions with molecules of biological interest is found in the study of Ballester et al. [43]. They succeeded in synthesizing cavitands based on calix[4]resorcinarenes, with the octa-amine receptor being the most promising (Figure 7) [2,45] since it showed the best interaction with these molecules due to the cation–π attractions between the aromatic portions of this and the positive charge of the guest.

Stability constants were determined for octa-amine receptors using ¹H-NMR titrations. The free energies of formation (

Table 1. 1:1 complexes between guests with octa-amine receptor: Stability constants (K_S) and free energies of formation [43].

Guest	KS (M−1)	ΔGo (kcal/mol)
Choline Chloride	12,000 ± 2400	5.5 ± 0.1
Acetylcholine Chloride	4000 ± 800	4.9 ± 0.1
L-carnitine	15,000 ± 3000	5.6 ± 0.1
Acetylcarnitine·HCl	30 ± 6	2.0 ± 0.1

) were also calculated. The complexation of ChCl is 0.6 kcal/mol higher than acetylcholine because ChCl is a good hydrogen bond donor. It can be seen that the complex formation with greater stability is with carnitine due to its structure (carboxyl and hydroxyl groups) forming stabilizing interactions of hydrogen bonds with the amino groups of the receptor.

Another work to highlight is that of Velásquez et al., where ¹H-NMR titrations for resorcinarenes with an alkyl chain demonstrated the formation of complexes with choline, which is a molecule with an important versatility in the synthesis of phospholipids of biological membranes, transmembrane signaling, and lipid–cholesterol synthesis, among others [46]. These titrations reveal important changes in the chemical shifts for both the CH bridge of the resorcinarenes as well as in the signal of the methyl groups of the choline molecule. Therefore, changes may be due to a strong interaction between the meta-protons of the resorcinarene and the methylene protons of the choline molecule. In this study, the interaction of some resorcinarenes with the aliphatic lower rim, especially that of C-tetra(methyl)calix[4]resorcinarene with the choline molecule, is clarified since the association constant log β 2.96 was calculated, which shows the high degree of host–host affinity due to the thermodynamic stability generated by the secondary hydrogen bond-type interactions and π [47]. This resorcinarene has also been used in the interaction in ruthenocene host molecules, finding a new 2D triangular brick-wall framework [48]. This work is promising in the rational design of new supramolecular solids and the understanding of ruthenium-based anticancer agents [49]. On the other hand, in the work of Pinalli R. et al., in which complexes with amino acids are generated from tetraphosphonate cavitands based on calix[4]resorcinarene, the interactions responsible for molecular recognition in amino acids are determined, considering enthalpic and entropic effects. From this study, it is possible to predict, to a large extent, the binding behavior of phosphate cavitands to proteins [50].

4. Calix[4]resorcinarene Application Overview

Considering the host–guest interaction capacity of calix[4]resorcinarenes with ions and molecules [13,51], its applications have increased in areas such as chromatography [52], environmental remediation processes [53], ion-selective electrodes [54], solid phase extraction [55], and many others. This is because of their structure, which has a hollow cup-shaped cavity surrounded by hydrophilic groups, so they are used as base materials for medical applications [13]. They are also important building blocks in supramolecular architecture because in these macrocycles, the bottom-up principle can be applied, and through this principle, they generate an alternative way to control its physicochemical properties [56]

Regarding their function as reducing and stabilizing agents, the compound tetra-methoxy-resorcin[4]arene tetra-hydrazide (TMRTH) should be mentioned. This has been used as a reducing and coating agent in the synthesis of stable palladium nanoparticles (TMRTH-PdNPs) (Figure 8). This activity, particularly against Gram-positive bacterial strains, demonstrates a dual functional profile: it exhibits noteworthy catalytic performance in the Suzuki–Miyaura cross-coupling reaction and displays marked antibacterial efficacy, positioning it as a promising candidate for innovative applications at the interface of synthetic chemistry and antimicrobial research [9].

With NaBH₄ and employing AuNPs as a catalyst, another application of resorcinarenes is their use in the synthesis of PAMAM ibuprofen dendrimer conjugates with eight or sixteen ibuprofen molecules. Ibuprofen is released from the dendrimers in a dependent manner; thus, Hernández et al. found that nanoresorcinarene dendrimers that are conjugated with the drug exhibit greater cellular uptake than free ibuprofen [9]. Another application of resorcinarenes is the synthesis of a zirconium–resorcinarene complex in the goat-skin tanning process. Using this complex, the tear resistance increased by 77%, thus offering a new strategy for chrome-free tanning. Here, resorcinarene can act as a ligand for zirconium, promoting zirconium permeability as well as decreasing the amount of zirconium usage. It also increases the biodegradability of wastewater after the tanning process. This promotes the development of clean technology in leather manufacturing [11].

The easy derivatization of resorcinarenes is one of the key features that makes them unique among other macrocycles. The hydroxyl groups at the lower rim provide excellent handholds for the incorporation of other elements via reactions with electrophiles, as exemplified by O-acylation and O-alkylation. Calix[4]resorcinarenes have found application in a number of areas, as mentioned above, along with liquid crystals [57], HPLC stationary phases [58], heavy metal ion-extraction agents [54], and important applications in the modification of polymers as explained below.

5. Resorcinarenes in the Modification of Polymers

The chemical or physical modification of polymer surfaces is an effective strategy for optimizing polymer performance. By modifying the polymer matrix with functional molecules such as resorcinarenes, specific interactions with various types of substances are enhanced, improving both the adsorption capacity and the selectivity. These modifications allow the formation of stable complexes between modified polymers and various types of analytes, especially in the case of compounds that have pharmacological activity.

5.1. Surface Interaction Overview

Molecular interactions and surface forces are critical factors governing the behavior and properties of materials, particularly soft material systems such as polymeric nanomaterials. These forces, acting at both the molecular and surface levels, play a pivotal role in precisely controlling the physicochemical properties of polymers. A clear example of their influence is the way the architecture and arrangement of self-assembled structures of these materials are dictated by a delicate balance between attractive and repulsive interactions affecting molecular segments. Fundamental properties, including melting and boiling points, thermal stability, and the formation of crystalline structures, are profoundly influenced by the intermolecular forces interacting with polymer molecules. Among the key forces involved are van der Waals interactions, electrostatic forces, and depletion forces (Figure 9), which regulate molecular organization in the presence of solvents, and steric forces, which impose spatial constraints on molecular segments in high-density environments [59,60].

There are three basic mechanisms of adhesion between a cylindrical surface and a solid substrate (Figure 10). In the first diagram (a), van der Waals forces create a weak and reversible interaction based on transient charge fluctuations. In the second case (b), electrostatic forces induce attraction between opposite charges on the contacting surfaces, promoting a more stable adhesion. Finally, in the third scenario (c), molecular coupling is observed, resulting in specific chemical bonds that ensure a robust and permanent bond. These processes are of paramount importance in nanotechnology applications and the optimization of polymeric materials.

Moreover, polymer systems involve not only molecular interactions such as van der Waals or electrostatic interactions but also more complex phenomena such as entropic coupling, which affects the conformation and mobility of the chains depending on their topology. In addition, the nonlinear interactions between chains in three-dimensional polymer networks can induce unprecedented mechanical and thermal behaviors, depending on the structure and entanglement of the chains. Molecular friction and localized energy dispersion play a critical role in the wear resistance of polymers under dynamic conditions [61]. In turn, forces between polymers and their environment, such as on surfaces exposed to liquids or gases, generate adsorption or surface-swelling phenomena that alter their physical properties. Finally, in multifunctional materials, dynamic molecular interactions can induce adaptive responses to external stimuli, allowing the design of polymers with controlled response characteristics to environmental changes [14]. These advanced mechanisms, although still poorly understood, offer promising prospects for the development of new polymeric materials with exceptional properties.

When polymer molecules in solution do not adsorb on the surfaces, an attractive surface force is produced due to an entropic origin, which is called the depletion force. Consider two solid surfaces in a solution of polymer molecules whose average radius is Rg (Figure 11). When the distance between the two surfaces decreases to about 2Rg, the molecules are pushed out of the gap. Then, the polymer concentration in the gap is lower than the bulk polymer concentration, and an attractive depletion force occurs due to osmotic pressure [61].

On the other hand, polymers can self-assemble and respond dynamically to their environment, allowing the creation of functional structures. The interaction between a dispersed phase and specifically organized polymer chains is observed (Figure 11), suggesting an intelligent design of the material. These interactions can be modulated by factors such as chemical composition and the distribution of segments in the chain, allowing controlled responses. Unlike conventional materials, polymers can adapt in real time, modifying their structure and functionality according to external stimuli. This structural flexibility enables innovative applications in multiple fields, from biomedicine to advanced technologies.

5.2. Surface-Modified Polymeric Materials in the Adsorption Process

The adsorption of molecules onto polymer surfaces is a highly dynamic process, and outside of thermodynamic equilibrium, the interaction between adsorbates and polymer chains is a crucial phenomenon that influences properties such as adhesion, permeability, and mechanical stability. The formation of chemical bonds between the adsorbate and the polymer matrix is determined by the energy band between the electronic states of the adsorbate and the functional groups of the polymer [62]. However, it should be noted that the adsorption of molecules in polymeric systems is not only governed by the conventional mechanisms of chemical adsorption and physical adsorption (Figure 12) but also by molecular confinement effects, dynamic surface restructuring, and nanoscale heterogeneities.

On the other hand, in the field of polymeric systems, the adsorption of molecules on the surface of polymers is a critical process that is highly dependent on the microstructure of the material, particularly its porosity and surface area. The porosity of a polymer not only facilitates the diffusion of adsorbates into the matrix but also provides additional active sites for the formation of chemical bonds [63]. These bonds can be covalent, ionic, or even hydrogen bonds, depending on the reactivity of the functional groups present on the surface of the polymer.

Surface morphology, including pore size and pore distribution, also influences adsorption kinetics, as it affects the accessibility of active sites and the diffusion of adsorbed molecules. In addition, surface area is a determining factor in the adsorption capacity of a polymeric material. A larger surface area increases the number of sites available for interaction with adsorbates, which translates into greater efficiency in processes such as pollutant capture, controlled drug release, or heterogeneous catalysis.

5.3. Selectivity and Specificity in Polymeric Materials: A Molecular Perspective for Pharmaceutical Applications

The ability of polymeric materials to exhibit high selectivity and specificity in molecular recognition has revolutionized their application in the pharmaceutical, therapeutic, and medical fields. Unlike conventional materials, polymers can be designed at the molecular level to interact with target biomolecules through a combination of chemical functionalization, conformational adaptability, and controlled microenvironments. This precise control of interactions enables drug-delivery systems, biosensors, and implantable devices to achieve unprecedented efficacy and biocompatibility.

Nevertheless, the chemical architecture of the polymers allows the incorporation of recognition elements that improve specificity with biological targets. Functional groups such as carboxyls, hydroxyls, amines, and thiols can be introduced to establish hydrogen bridges, electrostatic interactions, and covalent immobilization with biological molecules. In therapeutic applications, stimuli-responsive polymers exhibit dynamic behavior, modifying their structure in response to pH, temperature, or enzymatic activity, ensuring controlled and specific drug release at the site of action.

In the pharmaceutical field, polymer specificity is used to design controlled drug-release systems. These systems can release the active ingredient in response to specific conditions, such as changes in pH or the presence of enzymes, allowing more precise dosing and reducing side effects. In addition, polymers can be modified to improve the solubility and stability of drugs, optimizing their therapeutic efficacy.

In the medical field, polymer selectivity is crucial in the development of biomaterials for implants and medical devices. Biocompatible polymers can be designed to specifically interact with biological tissues, promoting integration and reducing the risk of rejection. Furthermore, in diagnostic applications, functionalized polymers can be used to detect specific biomarkers in body fluids, facilitating early diagnosis of diseases.

Lastly, the ability to design polymeric materials with high selectivity and specificity offers new opportunities in chemistry, pharmaceuticals, therapeutics, and medicine. These advances enable the development of more efficient and personalized technologies that improve human health and quality of life [64,65].

6. Use of Resorcinarene-Based Polymers Conjugated for Pharmaceutical Applications

Given the great versatility of polyhydroxylated platforms in the physical or chemical modification of polymeric structures, resorcinarenes have presented broad pharmaceutical applicability in four well-defined lines, including drug release and controlled release (Figure 13). In the literature, those macrocycles are conjugated to polymers. Thus, from methylsulfonated resorcinarenes in the upper part of the ring, polystyrene-based nanoparticles have been observed to be stable at a wide range of pH (6–12). This characteristic is important for the controlled release of substrates [66]. PLA polymers with resorcinarene cores should also be highlighted. They employ poly(ε-caprolactone) with promising applications in drug-delivery systems depending on molecular weight, thermal and mechanical properties, and their biodegradability [67,68]. Likewise, other cases are highlighted below.

6.1. Resorcinarene-Based Polymers: Analytic Applications

Polymers modified with guest molecules such as resorcinarenes are highly efficient in processes of analysis of molecules of pharmacological interest since they confer selectivity towards various types of substances. In this way, they can be used in selective extraction processes, the separation of analytes, and molecular identification processes. A first example is demonstrated in the micro-extraction of noradrenaline, an analyte found in urine, using the rotating-disk sorption extraction (RDSE) technique, which is an environmentally friendly technique. This technique uses a polymer modified with a chiral derivative of aromatic resorcinarene, which works as a new sorbent phase for the quantitative micro-extraction of noradrenaline [69]. This was carried out by fixing resorcinarene, using a direct reaction with the glycidyl group, reactive structural portion of a copolymer consisting of glycidyl methacrylate (GMA) and ethyl dimethacrylate (EDMA), in basic media with the hydroxyl group on the lower edge of the chiral resorcinarene [69,70].

In this line of applications, there is also the use of polymeric surfaces derived from methacrylates for the preconcentration of molecules of pharmacological interest such as carnitine [71,72]. This study shows the feasibility of efficiently modifying copolymers with resorcinarenes of different chain lengths. The results are promising because these composite materials facilitate the extraction of carnitine for analysis in aqueous solutions. Another interesting work is the one presented by Ruderish et al. [73]. The authors present the production of a composite material consisting of resorcinarene and poly(hydromethyl)dimethylsiloxane, which they used as a chiral stationary phase for chromatographic separation processes. The solid phase obtained in this way confers a special selectivity towards various types of amino acids, allowing their efficient separation.

In processes of the molecular identification of compounds of pharmacological interest, a first reference is found in the work of Umar et al. [74]. This work presents the synthesis of a carboxylic derivative of calix[4]resorcinarene, which was used to obtain nanoparticles. The development of this type of material was successfully used as an ultra-trace-level voltammetric sensor for Methylene Blue in human plasma. Other authors [75] have focused on obtaining a new polymer-membrane scaffold consisting of polysulfone and resorcinarene. Polymer modification was achieved using the polymer-membrane scaffold method containing resorcinarenes and showing multiple non-covalent interactions with the template. This molecularly imprinted polymer targeted at α-tocopherol presented promising results in selective retention under different conditions.

6.2. Resorcinarene-Based Polymers: Delivery or Transport Systems for Drugs

Another important area of application is in the obtaining of conjugates of resorcinarenes and polymers for the transportation and delivery of drugs. In this context, for example, conjugates of resorcinarene and methoxy-polyethylene glycol (mPEG) and their improved use of pH-sensitive low-toxicity supramolecular drug-delivery systems such as Methylene Blue [76] was presented by Shumatbaeva et al., who showed the obtaining of two calix[4]resorcinarenes in chair and boat conformations for the modification of polyethylene glycol (PEG). The authors highlight the potential use of calix[4]resorcinarene–mPEG conjugates bearing acylhydrazone bonds as supramolecular drug-delivery systems such as Methylene Blue.

Sergeeva et al. [77] also investigated the use of sulfonated resorcinarenes in obtaining nanocarriers for glucose-controlled insulin administration. The methodology consists of linking the resorcinarenes to a polymeric shell linked by means of phenylborate bonds. The glucose-sensitive polymeric nanocarrier based on sulfonated resorcinarene, at normal glucose concentrations, is stable in blood plasma and water.

Another example of using resorcinarenes as polymer modifiers for the transport of drugs was reported by Ermakova et al. [78]. In this work, the authors reported the obtaining of tetra(undecyl)calix resorcinarene functionalized by methoxy-poly(ethylene glycol)chains. In an aqueous solution, it forms nano-associates that can encapsulate drugs such as doxorubicin, naproxen, ibuprofen, or quercetin. From these results, the authors suggest that the properties of the obtained calix resorcinarene–mPEG conjugate show promising potential for use as a supramolecular drug-delivery system due to the combination of the low toxicity and encapsulation properties. In this same context, Shumatbaeva et al. [79] reported the synthesis of the conjugate formed between tetra(phenyleneoxypentyl)calix[4]resorcinarene and methoxy-poly(ethylene-glycole) (C₅OPh-mPEG). The novel calix[4]resorcinarene-PEG conjugate presents good stability and encapsulation efficiency, which is a result of the affinity of the conjugate low-rim substituents. The encapsulation process was efficient with substrates such as naproxen, ibuprofen, and doxorubicin drugs [80].

6.3. Resorcinarene-Based Polymers: Therapeutic Applications

Another interesting example was reported by Shumatbaeva et al. [81]. In this work, using nanoparticles capped by a calix[4]resorcinarene–mPEG conjugate, the authors examined the system formed by a novel isatin derivative that presented antimicrobial activity. The results showed that the solubility of that system is increased by molecular association with carboxy resorcinarenes [82].

Some articles in the literature appear to make use of resorcinarenes binding to biomolecules or biopolymers with biological activity, especially towards bacteria. This is of utmost relevance since, according to the World Health Organization, antimicrobial resistance is among the top 10 threats to public health. In turn, an alternative to mitigate this problem is the generation of new treatment agents; thus, calix[4]resorcinarenes assume relevance in this field [83] because they form relevant conjugates, as indicated below.

Also relevant is the work of Cristófalo et al. [84], in which they synthesize an octavalent glycocluster featuring a thiodisaccharide mimetic of the hyaluronic acid repeating unit, built on a calix[4]resorcinarene scaffold. Through DOSY-NMR and DLS experiments, they demonstrated the formation of spherical micelles. Furthermore, STD-NMR experiments provided evidence for the interaction between the synthetic glycocluster and langerin, a C-type lectin, which is a protein that acts as an endocytic receptor inducing the formation of Birbeck granules, as well as a pathogen-binding receptor capable of regulating both innate and adaptive immune responses [84,85,86].

Calix[4]resorcinarenes have been used in the synthesis of peptide conjugates derived from LfcinB, evaluating their antibacterial activity against E. Coli and E. faecalis strains. In this study, one resorcinarene was functionalized with maleimide groups using aminomethylation reaction with N-(1,5-diamino-1-oxopentan-2-yl)-6-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)hexanamide, which is the ornithine–maleimide motif. The obtained mono, di, and tri-functionalized compounds with ornithine–maleimide motifs showed activity against Gram-positive sepsis with MICs of 247 µM, 101 µM, and 75 µM. Subsequently, the (maleimide)n-calix[4]resorcinarenes were conjugated by Michael, using the thiol–maleimide addition reaction with peptides between 6 and 13 residues using solid phase peptide synthesis, obtaining antibacterial activity with the peptide RRWQWRC and the monosubstituted resorcinarene MIC 53 µM against the E. Faecalis (ATCC 29212) strain [87].

In the same line, other studies have been developed using resorcinarenes conjugated to peptides derived from LfcinB (20–25): RRWQWR and BF (32–34). In this study, the biological activity of the conjugates, the antimicrobial activity against reference strains and clinical isolates of bacteria and fungi, and the cytotoxic activity against bacteria and fungi were evaluated, as were the cytotoxic activity on erythrocytes, fibroblasts, MCF-7, and HeLa cell lines. This led to the identification of promising molecules that may lead to advances in the development of new therapeutic agents [88].

Finally, this shows a summary of the pharmaceutical applications of resorcinarene-based polymer conjugates (

Table 2. Summary of the pharmaceutical applications of resorcinarene-based polymer conjugates.

Application	Examples	Advantages	Disadvantages
Controlled delivery drug	Resorcinarene centered on (SPCL-b-PEG) used in drug release [80].	This conjugate delivery indomethacin stands out for its stability in aqueous solutions.	The synthesis of the conjugate is developed using a very unaffordable catalyst.
		There is a strong interaction of the conjugate with the drug, so its delivery took 40 h.	This work can be complemented using hydrophobic drugs.
		This study demonstrates their low hemolytic activity as cytotoxics against human hepatocyte cells.
	Resorcinarene centered on methoxy-PEG conjugates [76].	Doxorubicin was used as the base drug, whose release increases at acidic pH, which is relevant in tumor environments.	The conjugate structure contains acylhydrazone bonds, susceptible to unexpected hydrolysis.
		The associations of the drug with the conjugate showed an increase in toxicity against tumor cells in contrast to the free drug.	This work can be complemented with in vivo investigations.
	Nanocarrier for glucose-regulated insulin for controlled insulin delivery [77].	Conjugates exhibit low hemotoxicity and cytotoxicity in changing liver cells.	This study could evaluate the use of resorcinarene with aromatic substituents.
		The capacity of the nanocarrier for insulin encapsulation was 76% using a sulfonated resorcinarene.	This study can be complemented by assessing the interference of biomolecules.
		This material is stable in blood plasma and water.
Pharmaceutical transport	Tetraundecylcalixresorcin-arene–mPEG encapsulates organic compounds and some drugs [78].	The macrocycle obtained can form nano-associates in aqueous solution for drug encapsulation and has low hemotoxicity.	Macrocycle does not address long-term toxicity tests.
		The critical association concentration of this conjugate is 0.01 mg/mL, which is the minimum value of the conjugate in micelle formation.	This study lacks biodegradability studies of the macrocycle.
		This macrocycle, being amphiphilic, makes it possible to encapsulate both hydrophilic and hydrophobic drugs.
	Dendrimer resorcinarene for gene-delivery systems [89].	PAMAM-calix-dendrimers (PCD) can bind siRNA more efficiently and interact with cancer cells better than conventional PAMAM dendrimers.	There is an inefficient delivery of siRNA for the first generation (G1-alt).
		PCD shows high internalization capacity in HeLa cells.	This study could further investigate the effect of the interaction of the conjugate with some biological enzymes.
		The larger the PCD size, the less toxic effect on blood cells has been found.
Therapeutic applications	Peptide resorcinarene with antibacterial activity potential [87,88].	Broad therapeutic versatility and potential for controlled drug- and gene-delivery applications. Significant activity against specific bacterial strains is reported, supporting the therapeutic potential of the compound. Basis for the development of new antimicrobial agents.	The antibacterial activity of the conjugates studied against E. coli was low. The study is promising for further in vivo experiments. Limited toxicity assessment.
	Chlorambucil resorcinarene utilized for therapy in human myelogenous leukemia cells [90].	Basis for the development of new antimicrobial agents.	Limited toxicity assessment.
		Resorcinarene facilitates the internalization of chlorambucil in tumor cells.	Comparison with various chemotherapeutic agents is limited to free chlorambucil and cisplatin.
		This work shows an effective way of retaining or improving the alkylating activity of chlorambucil.	Limited to in vitro results.
Therapeutic applications	Glycocluster-resorcinarene and thiodisaccharide: Langerin recognition studies [84].	Creation of a glycocluster octavalent based on resorcinarene with hyaluronic acid mimetic.	Absence of advanced biological studies.
		Specific interaction with Langerin demonstrated by STD-NMR, showing a multivalent effect.	The activity was verified only against Langerin; affinity against other relevant receptors was not studied.
	Study the conjugates between resorcinarene dendrimers and ibuprofen [10].	Biological activity tests showed that the synthesized compounds have high potential activity against cancer.	The biocompatibility of dendrimers is not comprehensively evaluated.
		They showed relevant cytotoxic effects against human glioblastoma and mammary adenocarcinoma cell lines.	Dendrimeric conjugates can be costly and difficult to scale up.
	Calix[4]resorcinarene–mPEG and its antimicrobial activity [81].	The macrocycle obtained has low hemotoxicity. High drug encapsulation capacity. Controlled drug release by self-assembly in nanostructures.	Dependence on the type of drug because not all compounds encapsulate with equal efficiency. Results are limited to in vitro tests.
Analytic applications	Methacrylate resorcinarenes: Sorbent in noradrenalide extraction [69].	Methacrylate-based polymers modified with resorcinarenes show significant potential for the micro-extraction of biomolecules.	Lack of studies with other analytes to validate the versatility of the material.
		The chiral cavities introduced by the resorcinarenes allow enantioselective interactions.	Possible interference of other similar biomolecules in real biological matrices.
	Resorcinarene-based sorbent butylmethacrylate and ethylenedimethacrylate in the enrichment of 3-hydroxy-4-trimethylaminobutyrate [71].	The material demonstrates promising capability enrichment of L-carnitine from aqueous solutions.	Techniques based on the proposed copolymer can be improved, including the fabrication of sorbents for the preconcentration of other neurotransmitters.
		Conjugate can be used in several extraction cycles without significant loss of efficiency.	Specific conditions for improved performance.
	New material, calix[4]resorcinarenes, as modifier of glycidyl methacrylate (GMA), promissory in extraction of amino acid [72].	Enhanced surface polarity and the introduction of functionality present groups capable of interacting with peptides, which are about 60% at pH = 7.0.	The modifications could degrade under severe conditions (extreme pH or high temperatures).
		This is a promising stationary phase material for the separation of biomolecules by HPLC.	Specific reaction conditions (N2 atmosphere, DMF, basic agents) could restrict industrial scalability.

).

7. Conclusions

Resorcinarene-based polymers are promising systems that enable pharmaceutical processes such as drug release (doxorubicin, naproxen, and ibuprofen, among others) thoroughly, as well as the separation and more efficient analysis of analytes of biological interest, such as norepinephrine, carnitine, and choline, among others. Some of them also show anticancer or antimicrobial activity. The characteristics of these systems are largely a function of the versatility in the functionalization of calix[4]resorcinarene. That system is growing in potential with the generation of relevant scientific studies presented in this review. In the first part, calix[4]resorcinarene macrocycles are described in terms of their synthesis, functionalization, host–guest interactions, and main applications. Subsequently, the physicochemical properties of the polymers, as well as their surface interactions, are presented. Finally, relevant scientific works on pharmaceutical applications using resorcinarene-based polymer conjugates, such as controlled drug delivery, pharmaceutical transport, therapeutic applications, and analytic applications, are analyzed. This article enables the exploration of the development of calix[4]resorcinarene with more efficient polymer interactions, with a focus on the development of possible solutions to address current challenges in pharmacology and medicine.