Linker Type

Release Mechanism

Evolutionary Status

Key Advantage

Key Limitation (Driver of Evolution)

Cleavable (1st Generation)

Hydrazone

Acid-dependent hydrolysis (lysosomes, pH 4.5-5.0)

First generation (obsolete)

Simple synthesis

Low plasma stability, systemic toxicity

Disulfide

Reduction by glutathione (in tumor cell cytosol)

First generation

Selectivity under oxidative stress conditions

Risk of premature cleavage in plasma

Cleavable (2nd Generation)

Peptide (Cathepsin-B)

Enzymatic cleavage by lysosomal proteases

Industry standard

High plasma stability, selectivity

Dependent on protease expression levels in tumor

β-Glucuronide

Enzymatic cleavage by glucuronidase

Industry standard

High stability, low immunogenicity

Limited activity in tumors with low enzyme expression

Non-cleavable

Thioether

Complete proteolytic degradation of antibody

Industry standard

Maximum stability in systemic circulation

No "bystander effect", requires high and homogeneous antigen expression

Advanced Developments

Multifunctional

Combination of mechanisms (pH + enzyme)

Research

Ultra-high stability, minimization of toxicity

Complexity of synthesis and scaling up

Externally Activated

UV light, ultrasound

Research

Spatiotemporal control

Limited UV penetration depth, technical complexity