A Novel Redox-Sensitive Drug Delivery System Based on Trimethyl-Locked Polycarbonate

Stimuli-responsive polymer nanocarriers, capable of exploiting subtle changes in the tumor microenvironment for controlled drug release, have gained significant attention in cancer therapy. Notably, NAD(P)H: quinone oxidoreductase 1 (NQO1), found to be upregulated in various solid tumors, represents a promising therapeutic target due to its effective capability to enzymatically reduce trimethyl-locked (TML) benzoquinone structures in a physiological condition. In this study, a novel redox-sensitive carbonate monomer, MTC, was synthesized, and its amphiphilic block copolymers were prepared through ring-opening polymerization. By successfully self-assembling poly(ethylene glycol)-b-PMTC micelles, the model drug doxorubicin (DOX) was encapsulated with high efficiency. The micelles exhibited redox-responsive behavior, leading to rapid drug release. In vitro assessments confirmed their excellent biocompatibility and hemocompatibility. Furthermore, the inhibition of the NQO1 enzyme reduced drug release in NQO1-overexpressed cells but not in control cells, resulting in decreased cytotoxicity in the presence of NQO1 enzyme inhibitors. Overall, this study showcases the potential of MTC-based polycarbonate micelles to achieve targeted and specific drug release in the NQO1 enzyme-mediated tumor microenvironment. Therefore, the self-assembly of MTC-based polymers into nanomicelles holds immense promise as intelligent nanocarriers in drug delivery applications.

Enzyme Models-From Catalysis to Prodrugs

Enzymes are highly specific biological catalysts that accelerate the rate of chemical reactions within the cell. Our knowledge of how enzymes work remains incomplete. Computational methodologies such as molecular mechanics (MM) and quantum mechanical (QM) methods play an important role in elucidating the detailed mechanisms of enzymatic reactions where experimental research measurements are not possible. Theories invoked by a variety of scientists indicate that enzymes work as structural scaffolds that serve to bring together and orient the reactants so that the reaction can proceed with minimum energy. Enzyme models can be utilized for mimicking enzyme catalysis and the development of novel prodrugs. Prodrugs are used to enhance the pharmacokinetics of drugs; classical prodrug approaches focus on alternating the physicochemical properties, while chemical modern approaches are based on the knowledge gained from the chemistry of enzyme models and correlations between experimental and calculated rate values of intramolecular processes (enzyme models). A large number of prodrugs have been designed and developed to improve the effectiveness and pharmacokinetics of commonly used drugs, such as anti-Parkinson (dopamine), antiviral (acyclovir), antimalarial (atovaquone), anticancer (azanucleosides), antifibrinolytic (tranexamic acid), antihyperlipidemia (statins), vasoconstrictors (phenylephrine), antihypertension (atenolol), antibacterial agents (amoxicillin, cephalexin, and cefuroxime axetil), paracetamol, and guaifenesin. This article describes the works done on enzyme models and the computational methods used to understand enzyme catalysis and to help in the development of efficient prodrugs.

Trimethyl Lock: A Multifunctional Molecular Tool for Drug Delivery, Cellular Imaging, and Stimuli-Responsive Materials

Trimethyl lock (TML) systems are based on ortho-hydroxydihydrocinnamic acid derivatives displaying increased lactonization reactivity owing to unfavorable steric interactions of three pendant methyl groups, and this leads to the formation of hydrocoumarins. Protection of the phenolic hydroxy function or masking of the reactivity as benzoquinone derivatives prevents lactonization and provides a trigger for controlled release of molecules attached to the carboxylic acid function through amides, esters, or thioesters. Their easy synthesis and possible chemical adaption to several different triggers make TML a highly versatile module for the development of drug-delivery systems, prodrug approaches, cell-imaging tools, molecular tools for supramolecular chemistry, as well as smart stimuliresponsive materials.

Substituent and catalyst effects on GAC lactonization of γ-hydroxy esters

The effect of catalyst, substituent and leaving group on reactivity was probed experimentally and computationally rationalized.

Design, synthesis and in vitro kinetic study of tranexamic acid prodrugs for the treatment of bleeding conditions

Based on density functional theory (DFT) calculations for the acid-catalyzed hydrolysis of several maleamic acid amide derivatives four tranexamic acid prodrugs were designed. The DFT results on the acid catalyzed hydrolysis revealed that the reaction rate-limiting step is determined on the nature of the amine leaving group. When the amine leaving group was a primary amine or tranexamic acid moiety, the tetrahedral intermediate collapse was the rate-limiting step, whereas in the cases by which the amine leaving group was aciclovir or cefuroxime the rate-limiting step was the tetrahedral intermediate formation. The linear correlation between the calculated DFT and experimental rates for N-methylmaleamic acids 1-7 provided a credible basis for designing tranexamic acid prodrugs that have the potential to release the parent drug in a sustained release fashion. For example, based on the calculated B3LYP/6-31G(d,p) rates the predicted t1/2 (a time needed for 50 % of the prodrug to be converted into drug) values for tranexamic acid prodrugs ProD 1-ProD 4 at pH 2 were 556 h [50.5 h as calculated by B3LYP/311+G(d,p)] and 6.2 h as calculated by GGA: MPW1K), 253 h, 70 s and 1.7 h, respectively. Kinetic study on the interconversion of the newly synthesized tranexamic acid prodrug ProD 1 revealed that the t1/2 for its conversion to the parent drug was largely affected by the pH of the medium. The experimental t1/2 values in 1 N HCl, buffer pH 2 and buffer pH 5 were 54 min, 23.9 and 270 h, respectively.

Trimethyl lock: A trigger for molecular release in chemistry, biology, and pharmacology

The trimethyl lock is an o-hydroxydihydrocinnamic acid derivative in which unfavorable steric interactions between three pendant methyl groups encourage lactonization to form a hydrocoumarin. This reaction is extremely rapid, even when the electrophile is an amide and the leaving group is an amino group of a small-molecule drug, fluorophore, peptide, or nucleic acid. O-Acylation of the phenolic hydroxyl group prevents reaction, providing a trigger for the reaction. Thus, the release of an amino group from an amide can be coupled to the hydrolysis of a designated ester (or to another chemical reaction that regenerates the hydroxyl group). Trimethyl lock conjugates are easy to synthesize, making the trimethyl lock a highly versatile module for chemical biology and related fields.

Computer-assisted design for atenolol prodrugs for the use in aqueous formulations

Based on stability studies on the drugs atenolol and propranolol and some of their derivatives it is believed that increasing the lipophilicity of the drug will lead to an increase in the stability of its aqueous solutions and will provide a prodrug system with the potential for releasing atenolol in a controlled manner. Using DFT theoretical calculations we have calculated an intramolecular acid catalyzed hydrolysis in nine maleamic (4-amino-4-oxo-2butenoic) acids (Kirby's N-alkylmaleamic acids), 1-9. The DFT calculations confirmed that the acid-catalyzed hydrolysis mechanism in these systems involves: (1) a proton transfer from the hydroxyl of the carboxyl group to the adjacent amide carbonyl carbon, (2) an approach of the carboxylate anion toward the protonated amide carbonyl carbon to form a tetrahedral intermediate; and (3) a collapse of the tetrahedral intermediate into products. Furthermore, DFT calculations in different media revealed that the reaction rate-limiting step depends on the reaction medium. In aqueous medium the rate-limiting step is the collapse of the tetrahedral intermediate whereas in the gas phase the formation of the tetrahedral intermediate is the rate-limiting step. Furthermore, the calculations establish that the acid-catalyzed hydrolysis efficiency is largely sensitive to the pattern of substitution on the carbon-carbon double bond. Based on the experimental t(1/2) (the time needed for the conversion of 50% of the reactants to products) and EM (effective molarity) values for processes 1-9 we have calculated the t(1/2) values for the conversion of the two prodrugs to the parental drug, atenolol. The calculated t(1/2) values for ProD 1-2 are predicted to be 65.3 hours and 11.8 minutes, respectively. Thus, the rate by which atenolol prodrug undergoes cleavage to release atenolol can be determined according to the nature of the linker of the prodrug (Kirby's N-alkylmaleamic acids 1-9).

Prodrugs of aza nucleosides based on proton transfer reaction

DFT calculation results for intramolecular proton transfer reactions in Kirby's enzyme models 1-7 reveal that the reaction rate is quite responsive to geometric disposition, especially to distance between the two reactive centers, r (GM), and the angle of attack, α (the hydrogen bonding angle). Hence, the study on the systems reported herein could provide a good basis for designing aza nucleoside prodrug systems that are less hydrophilic than their parental drugs and can be used, in different dosage forms, to release the parent drug in a controlled manner. For example, based on the calculated log EM, the cleavage process for prodrug 1ProD is predicted to be about 10¹⁰ times faster than that for prodrug 7ProD and about 10⁴ times faster than prodrug 3ProD: rate( 1ProD ) > rate( 3ProD ) > rate( 7ProD ). Hence, the rate by which the prodrug releases the aza nucleoside drug can be determined according to the structural features of the linker (Kirby's enzyme model).

Computer-assisted design of pro-drugs for antimalarial atovaquone

Density Functional Theory (DFT) and ab initio calculation results for the proton transfer reaction in Kirby's enzyme models 1-6 reveal that the reaction rate is largely dependent on the existence of a hydrogen bonding net in the reactants and the corresponding transition states. Further, the distance between the two reacting centers and the angle of the hydrogen bonding formed along the reaction path has profound effects on the rate. Hence, the study on the systems reported herein could provide a good basis for designing antimalarial (atovaquone) pro-drug systems that can be used to release the parent drug in a controlled manner. For example, based on the calculated log EM, the cleavage process for pro-drug 1Pro may be predicted to be about 10¹¹ times faster than that for a pro-drug 4Pro and about 10⁴ times faster than pro-drug 2Pro: rate (1Pro) > rate (2Pro > rate (4Pro). Thus, the rate by which the pro-drug releases the antimalarial drug can be determined according to the nature of the linker (Kirby's enzyme model 1-6).

The effective molarity (EM)--a computational approach

The effective molarity (EM) for 12 intramolecular S(N)2 processes involving the formation of substituted aziridines and substituted epoxides were computed using ab initio and DFT calculation methods. Strong correlation was found between the calculated effective molarity and the experimentally determined values. This result could open a door for obtaining EM values for intramolecular processes that are difficult to be experimentally provided. Furthermore, the calculation results reveal that the driving forces for ring-closing reactions in the two different systems are proximity orientation of the nucleophile to the electrophile and the ground strain energies of the products and the reactants.

The effective molarity (EM) puzzle in proton transfer reactions

The DFT and HF calculation results for the proton transfer reactions of three different systems reveal that the reaction mechanism (transfer of a proton to a nucleophile) is largely determined by the distance between the two reactive centers (r). Systems with relatively large r values tend to abstract a proton from a molecule of water, whereas, these with a relatively small r values prefer to be engaged intramolecularly and their interaction with water is only via hydrogen bonding. Further, the results indicate that the effective molarity (logEM) for an intramolecular process is strongly correlated with the distance between the two reacting centers (r) in accordance with Menger's "spatiotemporal hypothesis".