The methyl group (-CH3) plays a versatile and crucial role in drug design and optimization, impacting both pharmacodynamic and pharmacokinetic properties. Its small size, monovalency, and lipophilic nature allow it to participate in various molecular interactions, including hydrophobic interactions (displacing water molecules during molecular recognition), van der Waals interactions, and modulation of physicochemical properties such as LogP and aqueous solubility. Critically, methylation can control the conformational properties of a scaffold, a strategy known as conformational restriction. This allows for the stabilization of bioactive conformations, enhancing potency, and can even improve aqueous solubility while increasing lipophilicity by disrupting planarity and symmetry. Methylation can also influence metabolic reactions, preventing unwanted metabolism, avoiding toxic metabolites, or generally making molecules 'softer' targets for metabolic processes. The diverse effects of the methyl group are collectively referred to as the "methyl effect," "methylation effect," or "magic methyl" effect. This review builds upon previous work, focusing on recent examples (last 10 years) illustrating how rational use of the methyl effect has progressed in drug discovery and development.

The paper extensively cites previous reviews on the methylation effect in medicinal chemistry, acknowledging the existing body of knowledge. It references key studies demonstrating the impact of methyl groups on protein-ligand binding, and the challenges associated with predicting the magnitude of the effect. The review also draws upon earlier work from the authors' group, establishing a baseline for the comparison of recent advancements in the field.

This is a review article, not an experimental study. The methodology employed is a systematic literature review focusing on publications within the last 10 years. The authors selected key examples from the literature to illustrate the diverse applications of the methyl effect in drug discovery. The selection criteria were not explicitly stated but implied a focus on examples demonstrating significant potency changes or improvements in pharmacokinetic/physicochemical properties due to the addition or modification of methyl groups. Each example is presented as a case study, detailing the chemical structures, biological targets, observed effects of methylation, and the underlying mechanistic rationales where available. The review organizes these examples by target class (e.g., EZH2 inhibitors, PI3K/mTOR inhibitors, cannabinoid receptor modulators) facilitating a comparative analysis of the methyl effect across diverse therapeutic areas. The authors also discuss the effect of methylation on physicochemical properties such as aqueous solubility and plasma stability, and on pharmacokinetic properties such as metabolic stability and hERG inhibition. The selection of examples and their presentation aim to provide a comprehensive overview of current knowledge on the 'magic methyl' effect and its applications in medicinal chemistry.

The review presents numerous examples showcasing the significant impact of methyl group modification on drug efficacy and properties. Key findings include: 1. **Pharmacodynamic effects:** Methylation significantly increased potency in several examples. For instance, in the discovery of the FDA-approved anticancer drug tazemetostat, strategic methylation led to a >100,000-fold increase in activity. Similar significant potency increases were observed in the development of EZH2-selective inhibitors, PI3K/mTOR inhibitors, selective κ-opioid receptor antagonists, and cannabinoid receptor modulators (CB2R agonists and antagonists). The addition of a single methyl group sometimes produced a 50-fold potency increase, far exceeding the typical success rate reported in literature for such modifications. 2. **Pharmacokinetic/physicochemical effects:** Methylation improved various pharmacokinetic and physicochemical properties of drug candidates. Examples include: * **Increased aqueous solubility:** Methylation of N-acylhydrazone derivatives improved aqueous solubility without significantly altering potency. * **Increased plasma stability:** Methylation of morpholin-2-one derivatives increased their stability in plasma, likely by preventing lactone hydrolysis. * **Reduced hERG liability:** Introduction of methyl groups reduced hERG potassium channel inhibition in CHK1 and mu opioid receptor ligands. * **Improved metabolic stability:** Methyl group addition improved metabolic stability in several examples, such as PI3Kδ inhibitors and NS3/4A protease inhibitors, by blocking oxidation of specific linkers. 3. **Mechanistic insights:** While the exact mechanisms underlying the 'magic methyl' effect are often complex and context-dependent, the review highlights recurring themes: * Improved hydrophobic interactions with the target binding site. * Stabilization of bioactive conformations, often due to conformational restriction. * Blocking of metabolic soft spots, preventing oxidative metabolism. * Altering electronic properties influencing binding and metabolic stability. 4. **Applications across diverse targets:** The 'magic methyl' effect proved beneficial in the development of drugs targeting diverse therapeutic areas, including cancer, pain management, neurodegenerative diseases, and infectious diseases (antibacterial and antifungal agents).

This review effectively demonstrates the widespread and often unpredictable impact of methyl group modifications on drug activity and properties. The significant potency enhancements observed in several case studies, particularly the >100,000-fold improvement in tazemetostat, underline the potential of rational methyl group manipulation. The diverse examples provided showcase the application of this strategy to a broad spectrum of therapeutic targets and drug classes. The successful modification of pharmacokinetic and physicochemical properties through methylation highlights the value of this simple structural change in addressing common drug development challenges, such as poor solubility, metabolic instability, and hERG liability. Although the review doesn't offer a unified mechanistic explanation, the recurring themes suggest the interplay between hydrophobic interactions, conformational effects, and modulation of electronic properties are often crucial. Future research should focus on developing more predictive models to guide the rational design and implementation of methyl group modifications, and ultimately, to increase the success rate of such strategies in drug discovery.

The 'magic methyl' effect remains a powerful tool in drug discovery and development, profoundly impacting both potency and ADMET properties. This review illustrates the versatility and importance of the methyl group in lead optimization, highlighting successful strategies across a variety of therapeutic targets. Future research should concentrate on developing predictive tools and further elucidating the diverse mechanistic contributions of this simple modification.

While this review provides a comprehensive overview of recent examples, the inherent limitations of review articles must be acknowledged. The selection of case studies, although extensive, might not be fully representative of all published findings on the methyl effect. Additionally, the review primarily focuses on examples where methylation led to improvements. Cases where methylation had detrimental effects were not comprehensively examined. Furthermore, the mechanistic explanations offered are often based on correlational evidence, and additional studies might be needed to fully unravel the complex interplay of factors responsible for the 'magic methyl' effect in specific contexts.