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18 BioPharm International ® Emerging Therapies eBook September 2024 www.biopharminternational.com Antibody-Drug Conjugates to release the payload (2). This choice has a direct im- pact on the efficacy and toxicity of an ADC, but many other decisions are vital to shaping the final product's characteristics. The choice of conjugation chemistry, linker backbone, number of payloads per linker, and linker architecture also impact ADC properties such as drug-to-antibody ratio (DAR), ADC aggregation, yield, blood clearance, and the ability of the antibody to bind to its target (3). All of these linker elements can be modified to fine-tune the characteristics of the ultimate ADC product, improving safety, efficacy, stability, manufacturability, and more. Additionally, linker design can be leveraged to over- come chemical and pharmacological limitations posed by some drug payloads. For example, many payloads are hydrophobic, potentially resulting in poor conju- gation yield, aggregation, and increased clearance. Because altering the payload itself to improve hydro- philicity can compromise its activity, linker design can instead be leveraged to modulate ADC solubility and reduce these effects (4). Including a linker with a hy- drophilic backbone such as polyethylene glycol (PEG) or modifying the linker architecture to include one or more hydrophilic arms without changing the proxim- ity of the payload to the antibody are options that can effectively shield the hydrophobic payload from the environment, improving conjugation yield and pre- venting aggregation. Importantly, the variable param- eters of ADC components are not independent of one another and must be carefully modified and assessed to maximize desirable features without compromising other functional traits. As researchers and drug developers have gained a deeper appreciation of the impact of linker design, increasingly complex and sophisticated architectures have been developed to better modulate ADC proper- ties. Advances in linker technology and conjugation chemistry, including site-specific conjugation methods, hydrophilic backbones, and specialized architectures, have driven a new wave of development and enabled greater control and consistency of DAR, homogeneity, and other key traits (5). However, this growing appreci- ation for the complexity of linker design has elucidated the need for specialized knowledge and experience in chemical biology and linker manufacturing to harness the full potential of these tunable parameters. Challenges in linker design and manufacturing While the role of linker design in ADC performance is now bet ter understood, many researchers and early stage drug developers still face considerable challenges in designing and manufacturing linkers to screen for their impact on ADC performance. The stakes are high, as selecting a suboptimal linker for the antibody-drug combination of interest can sig- nificantly derail development timelines and budgets and hold back an otherwise promising drug candi- date. For example, ADC product heterogeneit y is highly dependent on the conjugation method used and has significant implications for the drug's ther- apeutic potential. Depending on the reactive group present on a linker, as well as the conjugation site and processes used to join the linker to the antibody and payload, the resulting drug substance can contain a mixture of ADC molecules with a range of DARs or even some portion of unconjugated antibody. These heterogeneous A DC molecules can var y in phar- macological properties and compromise the safety, efficacy, and performance of the ADC product (6). However, heterogeneity does not completely preclude safety and efficacy, as many approved ADCs do pos- sess some degree of heterogeneity, but increased ho- mogeneity simplifies the thorough characterization of ADCs (7). Linker design not only impacts the het- erogeneity of ADCs but also the efficiency of conjuga- tion reactions. As discussed previously, the inclusion of hydrophilic elements in linker design can increase both performance and yield, while traditional hydro- phobic linkers may require special solvents during conjugation and still result in low yields. Adding in a short PEG spacer to a linker can improve conjugation yield and reduce aggregation of the ADC (8). Because antibody-based approaches to site-spe- cific conjugation typically require recombinant en- gineering to introduce novel functional groups to the antibody, linker-based approaches to optimizing the homogeneity and performance of ADCs present the advantage of being compatible with proven, "off- the-shelf " antibodies (9). As the ADC landscape has grown and evolved, some drug developers have estab- lished the knowledge and infrastructure necessary to reliably synthesize and purif y their own, some- times proprietary, linkers. However, the specialized chemical biology expertise required to put advanced novel linker design strategies to work is beyond the scope of many drug developers' in-house capabilities. For example, many manufacturers can now produce short monodisperse PEG-based linkers with linear structures but lack the capacity to successfully syn- thesize longer or more complex linker architectures with the necessary purity and scale. Deep, specialized expertise and infrastructure are necessary to realize the full potential of linker design and synthesis for generating safe and effective ADCs, leading many companies to turn to expert providers of advanced ADC linkers to supplement in-house capabilities. Continued progress in ADC development To accelerate the path of promising ADC candidates to market, biopharma companies must leverage the field's growing understanding of how linker design can optimize ADC performance and invest in these