Click Chemistry is a term coined by Barry Sharpless to describe chemical reactions that are modular, wide in scope, give very high yields, and generate only inoffensive byproducts. It has been found wide application in bioconjugation, enabling control over the site of modification in biomolecules. Over the past two decades, this type of chemistry has been increasingly used by bio-inorganic and medicinal chemists, most notably in the development of targeted medicines, such as antibody-drug conjugates (ADCs).
ADCs combine the specificity of antibodies for tumor targeting with highly cytotoxic small molecules through chemical linkers. To date, the US Food and Drug Administration (FDA) has approved eight ADCs, all cancer treatments, including the Genentech's Polivy (polatuzumab vedotin-piiq), a humanized anti-CD79b IgG1 mAb conjugated to the antimitotic agent monomethyl auristatin E for the treatment of diffuse large B-cell lymphoma. However, Polivy and other first-generation ADCs are mixtures of many different biologic conjugates due to the inability of traditional coupling reactions to control how many drug molecules conjugate to the monoclonal antibody (mAb) or to which position within the mAb. Consequently, the mixture of ADCs in a manufacturing batch can reduce the efficiency and increase toxicity of the product.
A click chemistry reaction developed by Carolyn Bertozzi takes place between azide and alkyne (here a cyclooctyne) components, and because these groups are not present in biomolecules, it leaves other chemical groups untouched and can be used in living cells. Image source: https://www.nature.com/
Introducing bioorthogonal click chemistry, a bioorthogonal handle on the mAb, offers a way to control the number and position of conjugated drug molecules, and to identify the most effective ADC formulation. This method is being used by companies to create a new generation of ADCs and some of these ADC candidates are already in Phase I clinical trials as shown in Table 1.
|Table 1 | Selected Biotech Companies Working with Click Chemistry for ADC Preparation
||Leading candidates (partner)
||Progress (date initiated)
||HER2 targeting antibody + Amberstatin269
||Advanced cancers with HER2 expression
||Phase 1 (March 2016)
|Catalent/ Redwood Bioscience
||Anti-CD22 antibody + maytansine
||TRPH-222 (Triphase Accelerator Corp.)
||Phase 1 (April 2019)
||Anti-CD74 IgG1 antibody (SP7219) + maytansine
||Multiple myeloma and non-Hodgkin lymphoma
||Phase 1 (April 2018)
||Anti-FolRa human IgG1 antibody (SP8166) + 3-aminophenyl hemiasterlin
||Ovarian and endometrial cancers
||Phase 1 (March 2019)
||Anti-AXL IgG1 antibody + pyrrolobenzodiazepine dimer SG3199
||ADCT-601 (ADC Therapeutics)
||Solid tumors with AXL expression
||Phase 1 (January 2019)
||Breast cancer, liposarcoma
||Preclinical: human trials expected 2021
||CC49 fragment + monomethyl auristatin E
||Tumor-associated glycoprotein 72 (TAG72) in various cancers
Clinical Drug Candidates
ARX788 is an ADC that targets HER2 over-expressing tumors including breast, gastric, colon, pancreatic and ovarian cancers. ARX788 is based on our next-generation ADC technology platform whereby a HER2 specific monoclonal antibody is site-specifically conjugated with Amberstatin269, a potent cytotoxic tubulin inhibitor. In pre-clinical models, ARX788 has demonstrated an improved efficacy and safety profile relative to currently available HER2 targeting ADCs. ARX788 has the potential to benefit a broader spectrum of HER2+ cancer patients than those that can be treated with the currently available HER2 targeted therapies and to improve outcomes for those patients that are eligible for current HER2-targeted therapies. Preclinical studies demonstrated that ARX788 is effective in controlling xenograft tumors, and safety studies in rodents and non-human primates demonstrated that ARX788 is well tolerated at doses significantly higher than the doses required to demonstrate efficacy.
TRPH-222 is a novel, site-specific ADC targeting CD22, a B-cell-restricted sialo-glycoprotein that is an important modulator of B-cell signaling and survival, which is expressed on nearly all B-cell malignancies. CD22 is a clinically validated ADC target with potential in Non-Hodgkin lymphoma (NHL) and acute lymphoblastic leukemia (ALL). The compound itself combines a site-specific modified humanized antibody conjugated to a cytotoxic payload using SMARTag® conjugation technology with Hydrazino-Pictet-Spengler (HIPS) chemistry and a proprietary 4AP linker. Pre-clinical data have shown that this optimization of payload placement and linker composition, combined with the stability afforded by HIPS chemistry, leads to better tolerability and expanded therapeutic index. TRPH-222 is currently in a Phase 1 clinical study for patients with relapsed and/or refractory B-cell lymphoma.
Sutro Biopharma is developing STRO-001 for the treatment of B-cell malignancies such as multiple myeloma and diffuse large B-cell lymphoma. The drug candidate is a monoclonal antibody conjugate that targets cells expressing CD74. Sutro's cell-free protein synthesis and site-specific conjugation platform technology was used to discover and develop STRO-001. In 2019, Sutro announced positive interim Phase I safety data from a Phase I dose escalation trial of STRO-001.
Sutro Biopharma is also developing STRO-002, a monoclonal antibody conjugate that acts by targeting folate receptor alpha (FolRα) (3). STRO-002 is a drug candidate for the treatment of relapsed/refractory cancers like epithelial ovarian cancer, endometrial carcinoma, fallopian tube cancer and peritoneal cancer. In 2019, Sutro Biopharma initiated a Phase I clinical trial of STRO-002 in patients with ovarian and endometrial cancers (2).
ADCT-601 is an ADC composed of a humanized monoclonal antibody that binds to human AXL (licensed from BerGenBio), conjugated using Glycoconnect™ technology (licensed from Synaffix BV) to a linker with a pyrrolobenzodiazepine (PBD)-dimer toxin. Once bound to an AXL-expressing cell, ADCT-601 is internalized into the cell where enzymes release the PBD-based warhead. AXL is an ideal target for an ADC approach, as it is highly overexpressed in many solid tumors (e.g., lung, breast, prostate, pancreas, glioma and esophageal) and hematological malignancies (e.g., acute and chronic myeloid leukemia). ADCT-601 is being evaluated in a Phase 1 clinical trial in patients with selected advanced solid tumors.
ADCs have emerged as a promising drug class in the field of oncology medicine. Click chemistry is playing an important role in constructing ADCs with defined and uniform sites of drug conjugation.
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 M. Peplow, Click chemistry targets antibody-drug conjugates for the clinic. Nature Biotechnology 2019, 37, 835-837.
 GlobalData. 2019
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