Elsevier

Drug Discovery Today

Volume 23, Issue 8, August 2018, Pages 1556-1563
Drug Discovery Today

Review
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Supermolecular drug challenge to overcome drug resistance in cancer cells

https://doi.org/10.1016/j.drudis.2018.05.037Get rights and content

Highlights

  • Supermolecular drugs will cancel drug resistance from cancer cells.

  • Anti-MDR facility of supermolecular drugs depends on allosteric regulation by induced-fit model.

  • Supermolecular drugs react catalytically with the variety substrates of target.

  • Supermolecular drugs will form a robust signal feedback control system loop.

  • Supermolecular drugs will be evaluated for robustness from Hill Eq. by BIBO Stability.

Overcoming multidrug resistance (MDR) of cancer cells can be accomplished using drug delivery systems in large-molecular-weight ATP-binding cassette transporters before entry into phagolysosomes and by particle–cell-surface interactions. However, these hypotheses do not address the intratumoral heterogeneity in cancer. Anti-MDR must be related to alterations of drug targets, expression of detoxification, as well as altered proliferation. In this study, it is shown that the excellent efficacy and sustainability of anti-MDR is due to a stable ES complex because of the allosteric facilities of artificial enzymes when they are used as supermolecular complexes. The allosteric effect of supermolecular drugs can be explained by the induced-fit model and can provide stable feedback control systems through the loop transfer function of the Hill equation.

Introduction

Cancer cells can acquire resistance to anticancer drugs by mechanisms that include alterations of drug targets, expression of detoxification, ATP-binding cassette transporters and altered proliferation, which are all found in the tumor microenvironment. Specifically, ATP-binding cassette transporters do not function when the other anticancer drug mechanisms do not act. In the case of normal enzyme reactions in cells, ATP-binding cassette transporters will not act as enzymes. These enzyme reactions are controlled by their allosteric interactions. Allosteric regulation of enzymes can be easily explained by the ‘induced-fit’ model [1] in comparison with the ‘lock and key’ hypothesis [2]. Allosteric ligands are often involved in the drug discovery process.

Until now, polymer drug delivery systems (DDSs) have been regarded as supermolecular nanodevices that enable enhanced permeability and retention (EPR) effects [3] and RES evasion 4, 5, and the curative effect of a polymer DDS was considered to be caused by superior delivery of drugs to their targets. However, it was found that DDSs developed efficacy without new knowledge regarding the intracellular distribution of the endosomal lysosome or the Golgi body and mitochondria. In addition, a recent study showed that DDSs act without the polymer micelle releasing the carried drug into the cell after endocytosis [6]. It is thought that the internalization of the drug by the polymer DDS and the change of the intracellular distribution contribute to its efficacy. Nishiyama and colleagues showed that gene onset in a cisplatin-polymer micelle is partially different from gene onset of cisplatin in its free form and that ability of a gene cluster to participate in vascularization and metastasis is restrained by the drug in the micellar body [7]. Therefore, the unified polymer DDS and carried drug might be able to act as a supermolecular assembly.

Recent studies have shown that the polymer DDS of doxorubicin (DXR) or paclitxel (PTX) prevents anti-multidrug resistance (MDR) of cancer cells 8, 9, 10. Table 1 lists the supermolecular complexes that have anti-MDR effects in cancer cells. Previous studies have reported that, through an endocytic process in the target cell, this type of carrier system could inhibit ABCB5 anticancer-agent discharge transporters, which are ABC transporters, and evade MDR 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. However, in previous experiments using polyisohexyl cyanoacrylate (PIHCA) nanoparticles, particle accumulation was not observed in a MDR cell line under a fluorescent microscope, but accumulation was observed in the phagocytes, such as macrophages [21]. Furthermore, this hypothesis of nanoparticle and cell interactions was supported by an experiment that indicated that doxorubicin-loaded polyisobutyl cyanoacrylate (PIBCA) nanoparticles had a highly cytotoxic effect on resistant P388/Adr cells. The active doxorubicin-related drug concentration rose at a five-times higher rate than that of the target cell. In contrast to the uptake of nanoparticles via endocytosis, it was shown that the nanoparticle–cell interaction was the base mechanism of this phenomenon 22, 23. However, the nanoparticle–cell interaction is not clear when considering the mechanism by which doxorubicin is inserted between chromosome base pairs in the nucleus or how it controls the biosynthesis of strands of DNA and RNA.

It is thought that the PIHCA and PIBCA nanoparticles used here constitute a supermolecular body with a DXR-containing anti-DNA topoisomerase. These DXR supermolecular bodies can become anti-MDR because of the allosteric functions described by the induced-fit model. In the tumor microenvironment, these supermolecular bodies must develop an anti-MDR ability as a result of alterations of drug targets, expression of detoxification, reduced susceptibility to apoptosis, increased ability to repair DNA damage and/or altered proliferation in relation to their allosteric functions according to the induced-fit model.

The term supermolecular was proposed by Jean-Marie Lehn and others [24], and supermolecular describes ‘host guest’ compounds that contain both molecules and an ion ‘guest’ in the host molecules that make molecular interactions. Proteins, LB films, and liquid crystals have been studied as supermolecular assemblies. In addition, new supermolecular biomimetic macromolecules, such as artificial enzymes, are expected to have structure-specific interactions and to be highly molecularly selective. However, it is thought that a supermolecular assembly that has flexible characteristics, such as self-structural changes, resembles an enzyme consisting of a high-molecular-weight carrier and low-molecular-weight active groups, leading to structure-dependent binding to substrates and intermediate flexibility that is controlled by the conformation. In other words, many reactions in living bodies are controlled by allosteric adjustments that change their activity by binding ligands distant from the active site. We can replicate the elaborate system of a living body if we can imitate this type of binding artificially. Using DEAE-dextran-MMA graft copolymer (DDMC) as the host and PTX as the guest, a DDMC–PTX supermolecular complex was developed, and its anticancer properties were investigated.

Cancer cells change anticancer drug genes (ADGs) to neutralize anticancer drugs, leading to MDR, even if another anticancer drug is administered [25]. This effect was described by Vincent Theodore DeVita, Jr, director of the NCI, during Congressional Hearings in 1985. A large number of subclones can be found in one tumor. Considering the variety of anticancer agent substrates, intratumoral inhomogeneity of cancer could be the basic cause of MDR in cancer. The newly developed supermolecular DDMC–PTX complex will have conformational flexibility and it will be matched one-to-one with the enzyme’s active site in a substrate reaction model in the case of multiple substrate reactions, but its allosteric activity according to the induced-fit model will occur without exposure to MDR because it is believed to act as a substrate subtype that has wide substrate selectivity and could have remarkable efficacy even at a low concentration.

The relationship between supermolecular and molecular recognition is very important. Cyclodextrin (CD) is a representative host that can interact with neutral molecules. Guest inclusion of CD occurs via hydrophobic interactions in water, which could be a model of molecular recognition in the living body.

Ueno prepared a γ-CD with one naphthalene ring and examined its inclusion behavior [26]. The naphthalene ring of the side-chain can move freely in the γ-CD entrance because size of cavity of the γ-CD (0.9–1.0 nm). The naphthalene ring functions like a spacer when it enters the cavity with a guest (Fig. 1ai). An AE double-substituted γ-CD was prepared, with the glucose residue of the γ-CD being A, B, C, D or E; the γ-CD showed a coupling pattern that reflected asymmetric torsion by the second naphthalene ring in its cavity. Nevertheless, addition of a guest decreased the strength of binding without changing the remarkable excimer fluorescence. These results show that the naphthalene ring moves outside of the cavity (Fig. 1aii).

With the AD two-substituted β-CD in the small cavity (0.7–0.8 nm), one naphthalene ring moves outside of the cavity and one other naphthalene ring remains in the cavity. The naphthalene ring in the cavity moves out of the cavity owing to the addition of a guest, and an interaction between naphthalene rings take place (Fig. 1aiii). In this way, the interaction between rings resembles an enzyme–substrate reaction because the selectivity changed from an induced-fit model as the conformation changed with the inclusion of a guest. However, the interaction between rings becomes an obstacle to binding because the size of the cavity of γ-CD is fixed. It is necessary to have intelligent conformation flexibility of the cavity to imitate an enzyme–substrate reaction.

When this condition is satisfied, the host compounds can include various substrates with additional guests. According to the induced-fit model of conformational change, these enzyme changes are different for each substrate and can lead to catalysis. It is easy to use supermolecular polymers, and there is a large amount of the self-organizing supermolecular aggregate; the polymers are carefully connected by spontaneous interactions with each other via intermolecular interactions, with the exception of covalent bonding. As for core-shell type supermolecules, which aggregate as an amphipathic block copolymer in water, a guest is clathrated as a hydrophobic drug in the inner core.

Nishiya et al. applied micellar DDS to this polymer, but the DDS’s ability to release drugs into the cell was inhibited owing to hydrophobicity [27]. Enzymes composed of a high-molecular-weight carrier and low-molecular-weight active-sight react one-to-one with substrates, unlike with a general catalyst. If the association with a polymer supermolecule is stable, an integral reaction occurs, making this polymer an ideal biocatalyst that is similar to an artificial enzyme, which produces conformation flexibility [28]. Ishihara et al. [29] from CREST assumed that the conformation flexibility caused by the induced-fit model was necessary for a supermolecular catalyst to act as an artificial enzyme (Fig. 1b). The late Professor Tabuse of Kyoto University established the concept of molecular recognition according to multicomponent and multiphase systems [29].

Section snippets

2-Diethyl aminoethyl-dextran-methyl methacrylate graft copolymer: DDMC

DDMC was prepared by using a graft-polymerized methyl methacrylate ester (MMA) on DEAE-dextran with a tetravalent cerium salt that was composed of the branch polymer Poly(methyl methacrylate) (PMMA) and backbone polymer DEAE-dextran. DDMC formed a hydrophilic and hydrophobic microphase that was separated by a hydrophilic domain in DEAE-dextran and a hydrophobic domain of the branch polymer PMMA.

DDMC–PTX supermolecular complexes

The DDMC–PTX supermolecular complex is shown in Fig. 2a. It is believed that the incorporation of PTX

In vitro anticancer activity of the supermolecular DDMC–PTX complex using melanoma B16F10 cells

The in vitro anticancer activity of the supermolecular DDMC–PTX complex is shown using melanoma B16F10 cells. Figure 3a shows the cell survival rate measured by the WST8 test using PTX-resistant melanoma B16F10 cells. The tolerance of melanoma cells to PTX clearly becomes a convex curve of the survival rate (Fig. 3a). It is believed that a convex curve is generated because the positive factor of phenomenon A and negative factor of phenomenon B depend on the PTX concentration, so the survival

Artificial enzymatic behaviors

Artificial enzymatic behaviors are observed in the presence of DDMC–PTX. The relationship is expressed by the Heel-type model [37] of the allosteric environment and according to the Michaelis–Menten model [38], which assumes a one-on-one enzymatic reaction at the activity point between a substrate and enzyme, as given by Eq. (1).v=kcat[E]0[S]n/([S]n+Km)Where the Michaelis constant (Km) contributes to the stability of the Michaelis complex of the enzyme–substrate from the lock and key

Concluding remarks

The Hill equation (Eq. (1)) shows an alternative dependent enzyme–substrate reaction according the induced-fit model (Fig. 2c), and the allosteric environment consists of supermolecular complexes and cancer cells.

The general form of the Hill equation is:F(X)=βXnKnXn,where, Kn < Xn, and β is the active allosteric factor.

If X is an inlet signal and Y is an outlet signal in a tumor microenvironment, then, F(X) = β, F(X) = αY and ΔF(X) = dY/dt at α > 0, β > 0 in allosteric regulation in this tumor

Conflicts of interest

The authors have no conflicts of interest to declare.

Grant sponsor

MHLW; grant number: H23-Shinko-Ippan-010.

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