ADCs: Conjugation Methods of Linker and Antibody

  • Nevertheless, scientists have not given up this highly potential technology. After Mylotarg, four ADCs drugs have been approved by the FDA. They are Adcetris (brentuximab vedotin, jointly developed by Takeda/Seattle Genetics, in 2011, for the treatment of Hodgkin’s lymphoma and systemic anaplastic large cell lymphoma.); Roche’s Kadcyla (ado-trastuzumab emtansine, 2013, for the treatment of HER2-positive breast cancer); Wyeth/Pfizer’s Besponsa (inotuzumab ozogamicin, 2017, for the monotherapy of relapsed or refractory CD22+ adult B cells Prodromal acute lymphoblastic leukemia (ALL)). AstraZeneca’s Lumoxiti (moxetumomabpasudotox-tdfk, 2018, for the treatment of adult patients with at least two systemic treatments, relapsed or refractory hairy cell leukemia (HCL).) And in September 2017, Mylotarg was approved by the FDA Remarketed for the treatment of newly diagnosed CD33+ adult acute myeloid leukemia (AML) and refractory CD33+ acute myeloid leukemia (AML) in children over 2 years old who did not respond to initial treatment. From a purely technical analysis, ADCs can be roughly divided into the first, second and third generations.

    Among the first-generation ADC drugs, they are mainly combined with mouse monoclonal antibodies through non-degradable linkers, and their efficacy and activity are low. There are many reasons for the failure of the first generation of ADC drugs. The first is insufficient drug efficacy, the concentration of the drug in the blood is lower than the therapeutically effective concentration, and the low expression of the target antigen results in a small amount of drug delivery, and the intracellular drugs are not enough to kill the cells. Secondly, the first-generation ADC drugs are not strong in targeting tumors, and the localization rate is low, and the linkers used at that time are also unstable, which makes the drugs more toxic. Finally, because the monoclonal antibodies in early ADCs were of murine origin rather than humanized antibodies, it led to immune responses and the production of human anti-mouse antibodies (HAMAs). The above are all factors that caused the failure of the first-generation drugs, but the research is endless, and soon the second-generation ADC drugs have entered people's field of vision.

    In the development of second-generation ADC drugs, mAb technology has been improved, and monoclonal antibodies have been carefully selected to improve tumor cell targeting and reduce cross-reaction with healthy cells. More importantly, the early use of the small molecule drugs used to treat cancer at the time as a toxic load lacked clinical research. Later, more effective small molecule substances were discovered. Compared with the first-generation ADC, the second-generation ADC has better CMC characteristics. From the three second-generation ADC drugs approved by the FDA at that time (vedotin, emtansine, and ozogamicin), the second-generation ADC drugs showed good clinical efficacy and safety. However, due to off-target toxicity, the presence of unbound antibodies, and ADC aggregation or rapid clearance caused by a drug-to-antibody ratio (DAR) of 8, currently most second-generation ADCs show a narrow therapeutic window. In addition, ADCs with DAR>4 have been shown to have low tolerance, low in vivo efficacy but high plasma clearance. So far, the second-generation ADC drugs are also difficult to meet the needs of patients. Optimizing monoclonal antibodies (mAbs), linkers and binding chemicals can improve the efficacy of third-generation ADCs, and site-specific coupling is now considered to be the key to the successful development of ADCs.

    The third-generation ADC drugs integrate the failure factors of the first and second generations, using the site-specific binding of small molecule drugs and monoclonal antibodies to produce ADCs with DARs of 2 or 4. This ADC drug has reduced toxicity and no unbound single Cloned antibodies have greatly improved stability and pharmacokinetics, lower coupling and shedding speed, high drug activity, and high cell activity at low antigen levels. In summary, the third-generation ADC drugs overcome the factors that led to the failure of the first-generation and second-generation drugs, allowing patients to receive better treatment.

    ADC drugs and indications that have been approved by the FDA

    Brentuximab vedotin: three of them are mainly antibodies (human CD30 specific antibody cAC10), cytotoxic drugs (microtubule disrupting agent MMAE, which inhibits tubulin polymerization, leading to G2M cell cycle arrest and apoptosis) and junction bridge (to MMAE Protease cleavable link bridge covalently linked to cAC10). After Brentuximad vedotin was approved by the FDA for its first indication in 2011, with the continuous disclosure of clinical research data, as of November 2018, the FDA has approved a total of 6 indications. In order to facilitate understanding, it is summarized into 3 diseases and 6 indications.

    The first type of lymphoma: Classical Hodgkin Lymphoma (cHL).

    The indication is combined with doxorubicin, vincristine and dacarbazine chemotherapy regimen for the treatment of stage III or IV classic Hodgkin’s lymphoma without pre-treatment (meaning combined with AVD regimen for first-line treatment)

    For the treatment of classic Hodgkin's lymphoma with high risk of recurrence or progression of autologous stem cell bone marrow transplantation (auto-HSCT)

    For the treatment of classic Hodgkin's lymphoma after failure of autologous bone marrow transplantation or not suitable for autologous bone marrow transplantation after at least 2 failed previous chemotherapy

    The second type of lymphoma: CD-30 positive non-Hodgkin lymphoma (NHL)

    Combined with cyclophosphamide, doxorubicin, and prednisone for the treatment of systemic anaplastic large cell lymphoma or other CD-30-positive peripheral T-cell lymphomas (PTCL) without pre-treatment, including vascular immunity Blastic T-cell lymphoma and unclassified PTCL (meaning first-line treatment of CD-30-positive peripheral T-cell lymphoma in combination with chemotherapy)

    For the treatment of systemic anaplastic large cell lymphoma after at least one chemotherapy regimen has failed

    The third type of lymphoma: primary cutaneous anaplastic large cell lymphoma or CD-30 positive mycosis fungoides (MF)

    For the treatment of primary cutaneous anaplastic large cell lymphoma or CD-30 positive mycosis fungoides (MF) after pre-treatment

    Trastuzumab emtansine: It is an ADC linked by trastuzumab targeting HER2 and microtubule inhibitor DM1, which can be used for HER2-positive metastatic breast cancer. The indication approved by the FDA in 2013 is single-drug treatment for Her-2 positive metastatic breast cancer that has previously received trastuzumab or paclitaxel therapy (single or combined). In May 2019, the indications for adjuvant treatment of early breast cancer were obtained, specifically for the adjuvant (postoperative) treatment for patients with HER2-positive early breast cancer who had previously received taxane-based and Herceptin (trastuzumab). There are still residual lesions after neoadjuvant (preoperative) treatment.

    Mylotarg (gemtuzumab ozogamicin; GO): Approved by the FDA in 2000, it contains gemtuzumab targeting CD33 and the cytotoxin ozogamicin (a calicheamicin) linked to it for the treatment of newly diagnosed adult patients with CD33-positive acute myeloid leukemia (AML) and adults with CD33-positive relapse or refractory and acute myeloid leukemia (AML) patients over 2 years old.

    Inotuzumab ozogamicin: is an antibody-conjugated drug (ADC) that contains a monoclonal antibody targeting CD22. CD22 is a cell surface antigen expressed on approximately 90% of B-cell malignancies) coupled with cytotoxic agents. For the treatment of adult patients with relapsed or refractory precursor B-cell acute lymphoblastic leukemia (ALL).

    Polatuzumab vedotin: anti-CD79bADC, conjugated by anti-CD79b antibody and anti-mitotic agent MMAE (monomethyl auristatin E), approved by the U.S. Food and Drug Administration (FDA) on June 10, 2019. The indication is a combination Bendamustine is used for patients with relapsed or refractory diffuse large B-cell lymphoma (R/R DLBCL).