The History of Three Generations of Antibody-drug Conjugates

What is antibody-drug conjugate? It is a novel immunotherapy that has aroused wide attention these years owing to its excellent performance on the treatment of breast cancer.  Antibody-drug conjugate typically consists of a fully humanized monoclonal antibody (mAb), a cytotoxic drug, a suitable linker, which is capable of maintaining the drug cytotoxicity, targeting ability, and ADC stability in the systemic circulation.

After the antibody binds to the targeted antigen on the surface of tumor cells, the tumor cells will endocytose ADC. ADC drugs then break down in lysosomes, releasing active chemical poisons that damage DNA or prevent tumor cells from dividing, killing cells. The ideal linker should be stable so as not to cause off-target toxicity and efficient release of the toxicant inside the cell. Therefore, a correct combination of these components is the key to the successful ADC development.

The first-generation ADC, such as mitomycin C, idarubicin, anthracyclines, N-acetyl melamine, adriamycin, venka alkaloids, and methotrexate, were conjugated to mouse mAbs primarily via noncleavable linkers using amide or succinimide spacer. KS1/4-methotrexate and BR 96-doxorubicin are two typical 1st generation ADCs. The former binds methotrexate via an amide bond to the mouse mAb KS1/4 to treat non-small cell lung cancer. The latter consists of the doxorubicin-conjugated chimeric mAb BR 96, which binds to doxorubicin via a hydrazone bond of an unstable acid to treat metastatic breast cancer. However, the in vitro and in vivo assessments indicate that these ADCs are only moderately potent and generally less active than the parent drug.

Over the past 10 + years, mAb technology has developed rapidly, and more powerful anticancer small molecules have been identified. Second-generation ADCs have better CMC characteristics than first-generation ADCs. And three FDA-approved ADCs, which are brentuximab vedotin, ado-trastuzumab emtansine, and inotuzumab ozogamine, reflect the progress of the second-generation ADCs.

However, most second-generation ADCs currently have a narrow therapeutic window due to off-target toxicity, competition with unbound antibodies, and aggregation or rapid clearance induced by drug-to-antibody ratio (DAR) of 8. Second-generation ADCs are different drug loads (DAR from 0 to 8), with a typical mean DAR of 3.5 (e.g., trastuzumab emtansine) or 4 (e.g., brentuximab vedotin). Approximately 5% of naked antibodies compete with ADCs for antigen binding. Molecules with DAR over 4 were shown to have lower tolerance, higher plasma clearance and reduced in vivo efficacy.

Last decade, the third generation of ADCs are developed based on lessons learned from first- and second-generation development, in which therapeutic indicators are greatly enhanced by optimization of mAbs, linkers, and chemical conjugation. Site-specific binding plays a critical role in the successful development of ADCs, which guarantee a uniform ADC for DARs. Third-generation ADCs utilize site-specific binding of small molecule drugs to engineered mAbs, equipped with DARs of 2 or 4, without increasing toxicity and unbound mAbs, which improve stability and pharmacokinetics, slow disaggregation rate with high potency and high activity against cells with lower antigen levels.

Through years of persistent efforts from researchers and scientists, a series of progresses have been made in the antibody-drug conjugates field. More achievements are expected for the treatment of more cancers.