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Optimising ADCs

through three critical components

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Antibody drug conjugates (ADCs) are a promising form of cancer treatment that directly target cancerous cells leaving healthy cells unharmed. Comprising of an antibody and a cytotoxic payload joined through a linker, ADCs combine the high selectivity of the antibody with the potency of the toxin.1

ADC technology has progressed rapidly in recent years, with 11 ADCs receiving commercial approval since 2011. As a result, bioconjugation research and development has expanded to explore different target molecules, linkers and toxins, which can be applied to treat indications beyond oncology, including a variety of infectious, inflammatory and autoimmune diseases. Regardless of indication, each and every bioconjugation project requires careful selection of these three critical components and a partner that has extensive experience working with the full diversity of each.

Greater than the sum of their parts

The true promise of bioconjugation lies in its ability to synergise the respective strengths of each individual component. On their own, toxins are too highly potent to use safely in treatment, while target molecules are not potent enough to kill cancerous cells. In an ADC, however, the potency of the toxin combines with the selectivity of the antibody via a linker to enable effective treatment that is both highly targeted and highly potent.1

Click on the three critical components of the ADC below to learn about each.


Target Molecule

Target molecules are responsible for the selectivity of a bioconjugate. While antibody drug conjugates currently dominate, research on target molecules has expanded to include antibody fragments, peptides, proteins, and small molecules.


Linkers include elements to confer stability in circulation, to enhance solubility and include cancer specific trigger mechanism for release of the toxin. ADC elements can be combined using chemical and enzymatic processes to generate stochastic and site specific ADCs.


Historically a toxic payload which directly killed cancer cells was incorporated into ADCs. In recent years different modes of action, both direct and indirect, have been exploited in oncology and payloads with different modes of action exploited in other indications, such as infection and inflammation.

Delivering tailored, effective ADCs

At Sterling, we harness our experience working with a variety of target molecules, linkers, conjugation chemistries, and toxins to support a range of bioconjugation projects. Select a piece to learn more.

adc interactive diagram

Antibodies (IgG1, 2, 4)

As the traditional and most commercially successful target molecule utilised in targeted cancer therapies to date, monoclonal antibodies have been widely explored in bioconjugation. The antibody is responsible for recognizing cancerous cells and binding on the surface and promoting internalisation of the ADC into them.

At Sterling, we have worked with human, humanized, chimeric, murine and rat antibodies of various isotypes.

Antibody Fragments

Antibody fragment drug conjugates (FDCs) utilise smaller antibody fragments than ADCs. Some studies suggest they are less toxic, result in fewer side effects, and are more tolerable than ADCs with equivalent cytotoxic payloads. While none are currently on the market, FDCs have shown promise in preclinical development.2

At Sterling, we have worked with minibodies, FAB / FAB 2, and scFV fragments.


With the development of better strategies to overcome poor pharmacokinetic properties associated with peptides, peptide drug conjugates (PDCs) have started rising to prominence. Key benefits of peptides include low immunogenicity, inexpensive syntheses, and greater tissue penetration. One PDC, Lu-dotatate, has received commercial approval to date, but many are in various stages of clinical development.3

At Sterling, we have worked with linear and cyclic peptides.


As with peptide drug conjugates, advances in synthesis and customisation capabilities have enabled the development of effective protein drug conjugates. Several protein drug conjugates have reached clinical trials, and ongoing research is anticipated to enhance their clinical and commercial success.4

At Sterling, our experience and skill with antibody bioconjugation is directly transferrable to protein bioconjugates.

Small Molecules

Small molecule drug conjugates (SMDCs) are an area of increased interest because compared to antibodies, small molecules are non-immunogenic, offer more manageable syntheses, and have lower molecular weights that enable better cell penetration.5


As a well-known anti-inflammatory, dexamethasone has been explored in ADC development for non-cancer indications. One ADC in the preclinical stage, CYMAC-001, pairs CD-163 with dexamethasone.9 This product has shown promise in treating chronic inflammatory diseases while mitigating unpleasant side effects of dexamethasone like weight loss and suppressed cortisol levels.10


Exatecan is a pyranoindolizinoquinoline which acts as a topoisomerase I inhibitor. Used in clinical trials for various cancers, it is a completely synthetic analogue of camptothecin that displays increased aqueous solubility and greater tumour efficacy, with manageable toxicity. One ADC using Deruxtecan, a derivative of exatecan, has been approved commercially – ENHERTU.


Auristatins, which are synthetic, water soluble analogues of Dolastatin 10, interfere with microtubule dynamics to cause cell apoptosis. Because of high potency, water solubility, stability, and suitability for linker attachment, they have been popular in ADC development, particularly the analogues MMAE and MMAF. Three ADCs utilising MMAE—ADCETRIS, POLIVY, and PADCEV, and one ADC utilising MMAF—BLENREP, have been approved commercially.


Maytansine, first isolated in 1972, works by interfering with microtubule assembly to prevent cell division. On their own, maytansines display low therapeutic index and tumour specificity. With high potency, stability, and solubility, however, they have been more successful when utilised in ADCs—KADCYLA, which uses the DM1 maytansinoid, received commercial approval in 2013.


First isolated from Streptomyces bacteria in 1988, Duocarmycins bind to the minor groove of DNA to cause damage. While no commercially-approved ADCs harness this toxin, their extreme toxicity makes them appealing in killing cancerous cells.11

PBD Dimers

Anthramycin, a pyrrolobenzodiazepine antibiotic, was first isolated in the 1950s. PBDs block DNA replication by binding to the minor groove and forming a DNA adduct. When two PBDs are combined into a dimer, they display significantly higher potency than the monomer. This makes them an appealing payload in ADCs, with several in clinical development.12 Recently approved ZYNLONTA, which Sterling’s Deeside facility worked on developing, is the first commercialised ADC to harness a PBD dimer.


Doxorubicin, an antineoplastic antibiotic, comes from the bacterium Streptomyces peucetius var. caesius. It interacts with DNA, and is used as a standalone chemotherapy treatment for a variety of cancers.13 An early ADC that reached clinical trials, BR-96 doxorubicin, paired doxorubicin with an IgG1 antibody. Despite showing antitumour activity in breast cancer patients, it was deemed no more effective than doxorubicin on its own.14 Today, doxorubicin continues to be trialed in other ADCs.


Nemorubicin is an analogue of doxorubicin that works as a DNA-intercalator and inhibits topoisomerase and RNA synthesis. On its own, it reached phase II clinical trials for treatment of solid tumours.15 After it failed to progress to commercial approval as a standalone treatment, it has been actively explored in the development of ADCs.


Paclitaxel was initially isolated from a Pacific yew tree, and it works by binding to tubulin to interfere with cell division and cause cell death.16 It is an FDA-approved chemotherapeutic for treatment in Kaposi sarcoma, breast cancer, non-small cell lung cancer, and ovarian cancer.17 Because it displays poor solubility on its own and has not seen therapeutic success in treating carcinomas, it has been tested in ADCs to expand its application.18


Alpha-Amantin, possibly the most deadly of amatoxins which is present in several poisonous mushroom species, inhibits mRNA transcription and offers high selectivity. While no amatoxin-based ADCs have received commercial approval to date, several have shown promise in clinical development due to their strong tolerability, significant activity against different tumours, and solubility in water.


Camptothecin was initially isolated in China and Tibet and used in traditional Chinese medicine. It interacts with Topoisomerases I to inhibit DNA replication. Several camptothecins have been widely utilised in traditional chemotherapy, and their ability to be synthesised and modified to produce desired analogues make them appealing in ADC development. Two camptothecin-based ADCs are available on the market today, TRODELVY and ENHERTU.

Linker Customisation

Linkers influence ADC stability and support the antibody in directing ADCs to cancerous cells.6 Linkers can be cleavable or non-cleavable—cleavable linkers incorporate chemical motifs such as dipeptides, disulfides, sugars, carbonates, and a variety of other compounds, whose cleavage triggers toxin release, while non-cleavable linkers have no designed cleavage element. To date, most commercially approved ADCs harness cleavable linkers.7

At Sterling, we have worked with a variety of different tethers, polarity, triggers, and immolation methods to optimise ADC linkers for our customers.

Conjugation Chemistry

Conjugation can be site-specific or stochastic. Stochastic conjugation has been the traditional method, utilising lysine and cysteine residues, and to date all approved ADCs use these conjugation methodologies. Site-specific conjugation has gained popularity because of its ability to deliver greater homogeneity and reproducibility. Last year, all ADCs that entered clinical trials utilised site-specific conjugation approaches, with both chemical and enzymatic methods represented.8

At Sterling, we have experience working with a range of different conjugation chemistries, including cysteine, lysine, enzymatic, and protein ligation, as well as both site-specific and stochastic conjugation methods.

Target Molecule

The target molecules of ADCs deliver high selectivity and demonstrate anti-tumour activity, but are not sufficiently cytotoxic on their own to kill cancerous cells. Monoclonal antibodies (mAb) have been widely utilised as target molecules in bioconjugation to deliver targeted cancer treatment, with 11 ADCs receiving commercial approval to date. With the rising clinical success of ADCs in recent years, several other target molecules have been explored in developing alternative forms of targeted treatment, including peptides, proteins, and small molecules.


Often, cytotoxic payloads in ADCs are too toxic and not selective enough on their own to serve as an independent treatment, posing risk to human health. When paired with a target molecule, however, cytotoxins are deactivated until reaching tumour cells. Upon reaching a cancerous cell, the cytotoxin regains activity and causes cell death.1 Other payloads used in bioconjugation may be existing standalone cancer treatments, or treatments for other indications.

Extensive expertise for efficient and quality-centred ADC development

At Sterling, our dedicated bioconjugation team has more than 35 years of combined expertise in ADC discovery and development. Our experience working with a wide range of target molecules, linkers, conjugation chemistries, and toxins enables us to develop customised ADCs that are tailored to our customers’ unique requirements. As a PDMO, or partnership development and manufacturing organisation, we place our customers’ products at the centre of everything we do, and we deliver the close scientific collaboration required to navigate complex challenges and support a robust bioconjugation programme.

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  1. What are Antibody-drug Conjugates? ADC Review [Online] 2019. (accessed May 27, 2021).
  2. Hofland, P. Strategic Alliance Helps the Development of Antibody Fragment-Drug Conjugates. ADC Review [Online] 2019.,Antibody%20fragment%2Ddrug%20conjugates,antibody%20against%20tumor%2Dassociated%20targets (accessed May 26, 2021).
  3. Cooper, B.; Iegre, J,; O’Donovan, D.; Halvarsson, M.; Spring, D. Peptides as a platform for targeted therapeutics for cancer: peptide-drug conjugates (PDCs). Chem. Soc. Rev [Online]. December 21, 2020, 1480-1494. Royal Society of Chemistry. (accessed May 26, 2021).
  4. Vhora, I.; Sushikumar, P.; Bhatt, P.; Misra, A. Chapter One – Protein- and Peptide-Drug Conjugates: An Emerging Drug Delivery Technology. Advances in Protein Chemistry and Structural Biology [Online], 2015, 1-55. ScienceDirect. (accessed May 26, 2021).
  5. Zhuang, C.; Guan, X.; Ma, H.; Cong, H.; Zhang, W.; Miao, Z. Small molecule-drug conjugates: A novel strategy for cancer-targeted treatment. Eur J Med Chem. [Online], February 1, 2019. (accessed May 26, 2021).
  6. Lu, J.; Jiang, F.; Lu, A.; Zhang, G. Linkers Having a Crucial Role in Antibody-Drug Conjugates. Int J Mol Sci [Online], April 14, 2016, 561. National Center for Biotechnology Information. (accessed May 26, 2021).
  7. Su, Z.; Xiao, D.; et. al. Antibody-drug conjugates: Recent advances in linker chemistry. Acta Pharmaceutica Sinica B [Online], April 6, 2021. ScienceDirect. (accessed May 27, 2021).
  8. Sadiki, A.; Vaidya, S.; et. al. Site-specific conjugation of native antibody. Antibody Therapeutics [Online], December 18, 2020, 271-284. Oxford Academic. (accessed May 27, 2021).
  9. CYMAC-001. ADC Review. Retrieved from
  10. Liu, R.; Wang, R.; Wang, F. Antibody-drug conjugates for non-oncological indications. Expert Opinion on Biological Therapy [Online], March 21, 2016, 591-593. Taylor & Francis Online. (accessed May 27, 2021).
  11. What are Duocarmycin analogues? ADC Review. Retrieved from
  12. Pyrrolobenzodiazepine (PBD). ADC Review. Retrieved from
  13. Doxorubicin. National Library of Medicine. Retrieved from
  14. Tolcher, A.W. Antibody drug conjugates: lessons from 20 years of clinical experience. Annals of Oncology [Online] 2016, 12, 2168-2172. (accessed May 26, 2021).
  15. Nemorubicin. National Center for Advancing Translational Sciences. Retrieved from
  16. Paclitaxel. National Cancer Institute, Retrieved from
  17. Paclitaxel. National Cancer Institute. Retrieved from
  18. Shao, T.; Chen, T.; et. al. Construction of paclitaxel-based antibody-drug conjugates with a PEGylated linker to achieve superior therapeutic index. Signal Transduction and Targeted Therapy [Online] 2020, 5. (accessed May 27, 2021).