Macrocycle Pharmaceuticals

Macrocyclic medicine

Macrocyclic drugs are generallyA group of molecules with a macrocyclic skeleton composed of 12 or more atomsrefers to. While they fall into the category of "bRo5 (Beyond Rule of 5)" molecules—which exceed Lipinski's Rule of 5, traditionally emphasized in small-molecule drug discovery—they are characterized by their ability to bind with high affinity and selectivity to flat protein-protein interaction (PPI) surfaces and shallow binding pockets, leveraging their steric molecular conformation derived from a cyclic structure. Therefore, they are attracting attention as a new drug discovery modality that can act on targets that were difficult to drug using conventional small molecules.

Representative examples of macrocyclic drugs

Tacrolimus (FK506)

Tacrolimus (FK506) is a natural product-derived 23-membered macrolide immunosuppressant. It binds to FKBP12 to form a complex and suppresses immune responses by inhibiting calcineurin.A pioneering example of a “molecular glue” mechanism that acts through a complex of a target protein and a binding proteinand it is also important for its high target selectivity due to the macrocyclic structure and as a prototype for ternary complex formation.

Lorlatinib

Lorlatinib is a third-generation ALK tyrosine kinase inhibitor targeting ALK/ROS1. By macrocyclizing an acyclic small molecule to increase molecular rigidity,Combines high blood-brain barrier (BBB) permeability with activity against resistance mutations such as G1202RThis is a prime example of a “conformational restriction” design.

Glecaprevir

Glecaprevir is an inhibitor targeting the NS3/4A protease of the hepatitis C virus (HCV). By fixing the P2-P4 positions with a macrocyclic ring, it achieves high affinity for the enzyme active site,High genetic barrier to drug-resistant mutant strainsIt is also linked to this. It is a successful example that lies at the boundary between low-molecular-weight and medium-molecular-weight compounds.

Key challenges in macrocyclic drug development

The Difficulty of Predicting "Membrane Permeability, Water Solubility, and Absorbency" Associated with Chameleon-Like Conformation Changes

Macrocyclic molecules exhibit “chameleon-like behavior,” forming and breaking intramolecular hydrogen bonds in response to their environment, thereby altering their polar surface area. Therefore,in vitro physical property values such as LogP and LogD tend to easily lose correlation with actual membrane permeability, gastrointestinal absorption, and blood solubility...which makes it difficult to predict PK translation.

Complexity in Profiles Due to Molecular Flexibility and Off-Target Binding

Although the molecular conformation is fixed by macrocyclization, some flexibility remains depending on the ring size and substituents. As a result,Unintended binding to other kinases, receptors, ion channels, and the like occurs, and the high affinity observed in cell-free systems is not reproduced in vivo.As a result, the activity profile—including off-target effects—becomes more complex.

Complex Interactions with CYP Transporters and Tissue Accumulation

Due to their molecular weight and hydrophobic surface area, macromolecules interact in complex ways with metabolic enzymes such as CYP3A4 and transporters such as P-gp and OATP. As a result, the risk of drug-drug interactions (DDIs) increases, andCaution is also required regarding delayed-onset toxicity caused by excessive accumulation of the substance in tissues such as the liver, kidneys, and small intestine.Pharmacodynamics(PD) Studies

Nonclinical Evaluations Required for Macrocycle Drugs

Evaluation 1: “Conformation Analysis × High-Sensitivity Membrane Permeability Assay” to Elucidate Chameleon-Like Behavior

In macrocycle studies, relying solely on Caco-2 or standard PAMPA may not fully capture the permeability mechanisms associated with structural changes, which could lead to an inaccurate evaluation of candidate compounds. By analyzing conformational shifts using solvent-dependent NMR and MD simulations,Evaluate true membrane permeability and absorbability in combination with Chiral-PAMPA and lipid bilayer permeabilityWe will do so. Furthermore, by correlating these two properties, we will derive design guidelines for intramolecular hydrogen-bonding networks that achieve both transparency and water solubility.

Evaluation 2: “Residence Time & RO Analysis in Live Cells” to Capture Binding Dynamics of PPIs and Hard-to-Targeted Molecules

In macrocycle drug design, it is important to consider not only binding affinity but also the dissociation rate from the target (Koff).Tracking Target Residence Time and Target Occupancy (RO) in Living Cells Using SPR, NanoBRET, and CETSAFurthermore, by integrating this with in vivo PK/PD data, we can accurately predict the duration of action and dosing intervals.

Evaluation 3: “Multimodal PK/PD and MSI Analysis” for Visualizing Complex Pharmacokinetics and Tissue Localization

For macrocycle molecules, blood concentrations alone may not fully explain their therapeutic effects or toxicity.Visualize the tissue distribution of parent compounds and metabolites using MSI (MALDI-MSI/DESI-MSI) and integrate the results with PK data obtained via LC-MS/MS and microdialysisThis allows us to evaluate the relationship between local drug exposure and drug efficacy and tissue toxicity.

Evaluation 4: “Integrated Safety Profiling” to Identify Off-Target Transporter Risks for bRo5 Molecules

For bRo5 molecules, it is important to evaluate at an early stage the risk of drug-drug interactions (DDIs) resulting from nonspecific interactions due to molecular size and structural characteristics, as well as CYP and transporter inhibition.Combines evaluation of the Kinome GPCR Panel, CYP/P-gp, BCRP, OATP, BSEP, and others, and integrates them with a PBPK modelThis ensures a safety margin and improves the accuracy of dose setting based on the MABEL and NOAEL values from the FIH study.

Key points when choosing a contract research organization (CRO) for non-clinical studies

For macrocycle drugs, comprehensive preclinical evaluation is required that takes into account complex conformational changes, tissue distribution, and off-target effects. A CRO that specializes solely in PK evaluation is insufficient; selecting a CRO capable of providing end-to-end support—from efficacy evaluation to pathological and resistance models—is the key to successful development.