To date, the FDA has approved 15 Antibody-Drug Conjugates (ADCs), with over a thousand more currently in various stages of clinical and preclinical development. However, a significant bottleneck persists: most potent cytotoxic payloads (such as MMAE, SN-38, or PBD dimers) are inherently hydrophobic, which often induce ADC aggregation, rapid in vivo clearance and compromised therapeutic effects.
This article provides an independent summary of a recent review published in the Journal of Controlled Release. It systematically analyzed how hydrophilic linkers serve as the definitive solution to these challenges by masking payload lipophilicity and enhancing the overall druggability of ADCs.
The Hydrophobicity Hurdle in ADC Development
ADCs consist of three main parts: a recombinant humanized antibody, a cytotoxic small molecule, and a chemical linker to bridge the former two components. While the linker must remain stable in circulation to prevent systemic toxicity, it must also facilitate efficient payload release within the tumor cell.
The use of hydrophobic payloads (like MMAE or MMAF) inevitably leads to a cascade of developmental failures:
- ● Physical Aggregation: High DAR values usually cause ADC aggregation and precipitation, thereby impairing ADC efficacy to a large extent.
- ● Metabolic Clearance: The immune system (specifically the mononuclear phagocyte system) identifies these aggregates as impurities, swiftly clearing them from the blood and drastically reducing the drug’s half-life.
- ● Off-Target Toxicity: Excessive hydrophobicity promotes non-specific uptake by healthy tissues. This causes the drug to release its toxic cargo before reaching the tumor, significantly narrowing the therapeutic window.
To circumvent these problems, hydrophilic linkers are designed to mitigate the hydrophobicity of payloads as well as the entire ADCs to achieve maximized therapeutic effects.
Common Linkers in ADC Development
Classic Dipeptides/Tetrapeptides: While Valine-Citrulline (VC) remains the most widely utilized linker, its hydrophilicity falls short in the context of high-DAR applications. In contrast, the Gly-Gly-Phe-Gly (GGFG) tetrapeptide—the cornerstone of Enhertu (DS-8201)—leverages superior hydrophilicity (ClogP ≈ -3.40) to successfully support potent DAR 8 configurations.
Frontier Scaffolds: Emerging tripeptides, such as Ala-Ala-Ala (AAA) and Ala-Ala-Asn (AAN), demonstrate exceptional enzymatic cleavage specificity and enhanced hydrophilicity (ClogP reaching -3.91). Furthermore, the TMALIN platform exhibits remarkable hydrophilicity under physiological conditions (ClogD -4.74) and features a sophisticated dual-release mechanism responsive to both the tumor microenvironment (TME) and the lysosome.
Hydrophilic Linkers in ADC Development
Hydrophilic modification of existing linkers is the primary strategy, with current approaches categorized into four main types: polyethylene glycol (PEG), Polysarcosine (PSAR), Saccharides, and ionizable functional groups.
PEG in ADC Linkers: Balancing Hydrophilicity and Pharmacokinetics
PEG is the most widely utilized hydrophilic modifier in drug delivery and has become the gold standard for the hydrophilic engineering of ADC linkers. Two approved ADCs use PEGylated linkers, such as Sacituzumab govitecan (CL2A DAR=7.6) and Loncastuximab tesirine (PEG8-modified VA, DAR=2.3). Its primary function is to enhance aqueous solubility, mitigate molecular aggregation, extend plasma half-life, and minimize non-specific uptake by healthy tissues.
However, the efficacy of PEG modification does not scale linearly with its chain length. Research has identified that in vivo clearance of ADCs is slowed down as PEG unit length increases, up to a threshold of PEG8. Beyond this point, longer PEGs have a negligible impact on pharmacokinetics.
And, the current optimization trend has shifted toward the use of Branched PEG (Multi-arm PEG) architectures. ADCs with branched PEG formats have shown improved pharmacokinetic behavior and in vivo efficacy compared to those with a linear PEG configuration.
Despite its dominance, an increasing number of studies revealed that PEG is immunogenic in humans, with accelerated plasma clearance being widely observed upon repeated administration.
Polysarcosine (PSAR) Modification: The Emerging Power Beyond PEG
To address the immunogenicity concerns associated with PEG, Polysarcosine (PSAR) has emerged as one of the most promising alternative hydrophilic polymers.
As a polymer composed of endogenous amino acids, PSAR offers inherent advantages, including zero immunogenicity and full biodegradability. Compared to PEG, PSAR exhibits superior masking capabilities for hydrophobic payloads in high-DAR constructs. Its moderate steric hindrance profile ensures that the ADC’s ability to penetrate tumor tissues remains uncompromised. Preclinical data indicate that PSAR-modified ADCs achieve deeper penetration into solid tumors than those modified with long-chain PEG, while simultaneously and significantly extending plasma half-life.
Despite its potential, PSAR faces several hurdles that must be addressed before widespread clinical adoption:
- ● Manufacturing Scalability: The large-scale synthesis of PSAR is technically demanding, resulting in higher production costs compared to traditional PEG.
- ● Limited Long-term Safety Data: Comprehensive longitudinal safety profiles are currently lacking; the long-term physiological impact of its metabolic by-products requires further clinical validation.
- ● Weak Stealth Properties: PSAR exhibited weak stealth properties compared to PEG.
Saccharides-Based Modifications: The Pinnacle of Biocompatibility
Saccharides and glyco-mimetics possess exceptional biocompatibility and biodegradability. Their multi-hydroxyl structure makes it highly soluble in water and resistant to aggregation, and certain glycosides can be selectively cleaved by tumor-specific glycosidases. This dual functionality allows them to serve as linkers that provide both hydrophilic masking and site-specific release. These are generally categorized into amino sugar-modified linkers and glycosidase-cleavable linkers.
Amino Sugar-Modified Linkers (non-cleavable): These linkers are covalently modified with amino sugars such as galactose, glucamine, or cyclodextrin. By introducing these polyhydroxy hydrophilic groups, researchers can achieve low aggregation even at high DAR.
Glycosidase-Cleavable Linkers: These designs exploit glycosidases (such as β-glucuronidase, α-L-iduronidase, and β-galactosidase) that are highly expressed in tumors or lysosomes but are expressed at low levels in normal tissues. By integrating these sugar moieties, the linker simultaneously improves hydrophilicity and ensures the tumor-specific release of the cytotoxic payload.
However, the performance of glycosylation is highly case-dependent. While some carbohydrate-modified ADCs exhibit superior pharmacokinetics compared to their PEGylated counterparts, others do not. Additionally, the tissue-specific expression of certain glycosidases requires further clinical validation to ensure precise targeting.
Ionizable Functional Groups: Charge-Driven Enhancement of ADC Performance
Incorporating ionizable functional groups—such as carboxylates, phosphates, and sulfates—into the linker architecture stands as one of the most direct and effective strategies for hydrophilic modification. By leveraging charge-induced hydration under physiological conditions, these modifications do more than just improve aqueous solubility; they also enhance the plasma stability of ADCs through strategic electrostatic interactions.
For example, the introduction of specific residues, such as Glutamic acid (Glu), has been shown to significantly reduce non-specific uptake, leading to a substantial 60% decrease in off-target toxicity. This allows for a much broader therapeutic window.
Phosphate-based modifications are particularly revolutionary for exceptionally hydrophobic cargoes, such as corticosteroids (steroids). By utilizing these ionizable groups, researchers can transform potent but previously "undruggable" hydrophobic molecules into high-performance ADC candidates with excellent pharmacological profiles.
Future Outlook
The technological evolution of hydrophilic linkers is propelling ADCs into a new era characterized by the "Three Highs": High DAR, High Stability, and High Specificity.
Future R&D will shift away from isolated modifications toward a more sophisticated, multi-dimensional equilibrium. Key challenges include maintaining exceptional hydrophilicity without introducing excessive steric hindrance that might impair antibody-antigen binding affinity, as well as achieving the robust, large-scale synthesis of novel materials like Polysarcosine (PSAR).
In essence, hydrophilic linkers have transcended their role as mere "solubility patches." They have become the core engine for broadening the ADC therapeutic spectrum and fundamentally enhancing clinical safety.
References:
Fu Q, Kong X, Liu Y, Liu M, Huang K, Wang PG, Wu K. The role of hydrophilic linkers in next-generation antibody-drug conjugates. J Control Release. 2026 Jan 6;391:114612. doi: 10.1016/j.jconrel.2026.114612. Epub ahead of print. PMID: 41506379.
