Lead Optimization Optimizing Access to the Target (Part 6)

6. Lead Optimization: Optimizing Access to the Target

Aims of Lead Optimization

  • Pharmacokinetic Properties: Aim to enhance the pharmacokinetic properties of lead compounds.

  • Chemical and Metabolic Stability: Focus on improving the compound's stability against chemical reactions and metabolic processes.

  • Hydrophilic/Hydrophobic Balance: Achieve a balance between hydrophilic and hydrophobic properties.

  • Solubility: Ensure adequate solubility in biological environments.

  • Drug Half-Life: Optimize to prolong the duration the drug remains active in the body.

  • Distribution Characteristics: Improve the distribution profile within the organism.

Key Pharmacokinetic Considerations

  • Polarity for Solubility:

    • Drugs must be sufficiently polar to dissolve in aqueous environments.

    • Sufficient polarity is also needed to interact effectively with biological targets.

    • Conversely, drugs need lipophilic characteristics to cross cell membranes and prevent rapid excretion.

  • Compounds' Features:

    • Many therapeutic agents are weak bases with pKa values ranging from 6 to 8.

    • The compound must feature a balance of hydrophilic and lipophilic qualities for desired absorbance and efficacy.

Addressing Barriers to Drug Targeting

  • Beyond Binding Interaction:

    • A drug's binding affinity is crucial, but overcoming physiological barriers is necessary to deliver the drug effectively to its target in the body.

  • Drug Design Strategies:

    • Investigate design modifications necessary to enhance delivery and stability, potentially utilizing carrier systems or antibody linkages.

    • Aim should focus on drugs that are absorbed into the bloodstream, effectively reaching targets, surviving metabolic processes, and eliminating in a controllable timeframe (pharmacokinetics).

Optimizing Hydrophilic/Hydrophobic Properties

Importance in ADME

  • Impact on Pharmacokinetics:

    • Hydrophilic/hydrophobic balance influences key pharmacokinetic properties (Absorption, Distribution, Metabolism, Excretion - ADME).

Issues with Polarity

  • Too Polar:

    1. Ineffective traversal across cell membranes.

    2. Cannot effectively target intracellular sites.

    3. More prone to plasma binding and quicker metabolic turnover.

  • Excessively Hydrophobic:

    1. Poor absorption if administered orally since they bind in fat globules.

    2. Low solubility in blood if injected, leading to accumulation in adipose tissues.

    3. Increased likelihood of producing toxic metabolites.

Quantitative Analysis

  • Partition Coefficient (P): A metric calculated as:

    • P = Concentration of drug in octanol / Concentration of drug in water.

Ionized and Non-Ionized Forms

  • Distribution Measurement:

    • Log P captures un-ionized species between aqueous and lipophilic environments.

    • Log D indicates distribution of both ionized and un-ionized forms and is pH-dependent, particularly noted at blood physiological pH (7.4).

Strategies for Adjusting Properties

Masking and Modification Techniques

  • Masking Functional Groups:

    • Employ alkyl or acyl groups to decrease polarity, converting alcohols and carboxylic acids into less polar formats like ethers or esters.

  • Adding/Removing Groups:

    • Modify polarity by adding polar groups (e.g., converting tioconazole to fluconazole for enhanced solubility).

  • Substituent Variation:

    • Include or exchange hydrophobic groups to adjust polarity; longer alkyl chains increase hydrophobicity, while removing bulky groups can enhance polarity.

    • Utilize halogenation for increasing molecular hydrophobicity.

Methylene Shuffle Technique

  • Selective Group Alteration:

    • Methylene shuffling is a strategy used to improve drug selectivity and bioactivity through structural modifications.

pKa Adjustments

Importance of pKa

  • Ionization Impact:

    • Adjusting pKa is essential to maintain optimal drug absorption, typically within the range of 6-9 to ensure efficient passage through membranes.

Techniques for pKa Variation

  1. N-Alkyl Substituent Modification: Adjusting N-alkyl groups influences ionization and absorption potential.

  2. Aromatic Ring Substituents: Modifying electronegative substituents can help balance pKa and improve biologic compatibility.

Utilization of Bioisosteres

  • Functional Group Alternatives:

    • Bioisosteres serve as substitutes for polar groups, maintaining desirable interactions while enhancing absorption characteristics. For example, replacing carboxylic acids with tetrazole can enhance lipophilicity, improving absorption and reducing metabolic liabilities.

Enhancing Drug Resistance against Degradation

Protective Strategies

  • Steric Shields: Addition of large groups near reactive functionalities to prevent hydrolysis.

  • Bioisostere Utilization: Replace unstable functionalities with less reactive bioisosteres to maintain activity while improving stability.

Practice in Drug Design

  • Example Adjustments: Renowned adjustments in drug structures (e.g., changing esters to amides) have proven effective in increasing stability against metabolic systems and improving medication efficacy.

General Strategies Against Metabolic Degradation

  • Introduce Metabolic Blockers and Substitutions: Implement groups at potential metabolic sites to inhibit metabolism, which can enhance drug efficacy and lifespan in treatment contexts.

  • Restructuring Groups: Shifting metabolically prone groups to hinder enzymatic recognition while ensuring binding efficacy.