Biotech company Shasqi and Johnson & Johnson have entered into a research collaboration to use Click Activated Protodrugs Against Cancer (CAPAC) platform to develop new cancer therapies.
Previously, Shasqi's click-activated drug delivery system passed its first human safety trial and is currently in Phase II clinical trials. Based on this success, other research teams are preparing to test click chemistry in clinical settings, hoping that this technology can overcome some limitations of existing targeted methods, such as antibody-drug conjugates (ADCs).
Figure 1. CAPAC platform, source: Shasqi official website
What's Click Chemistry?
On October 5, 2022, the Royal Swedish Academy of Sciences announced that the 2022 Nobel Prize in Chemistry would be awarded to Carolyn R. Bertozzi, Morten Meldal, and K. Barry Sharpless in recognition of their contributions to "click chemistry and bioorthogonal chemistry."
Twenty years ago, K. Barry Sharpless from the Scripps Research Institute in La Jolla, California, first proposed the concept of click chemistry. Shortly thereafter, he and Morten Meldal from the University of Copenhagen independently developed the first classic click chemistry reaction: the copper-catalyzed azide-alkyne cycloaddition (CuAAC) (Figure 2).
Figure 2. CuAAC
"Click chemistry" is a molecular reaction method similar to Lego bricks, allowing molecules to be "clicked" together quickly and selectively to construct complex drug molecules. It is fast, reliable, and does not produce any byproducts, but it also relies on copper catalysts, which are toxic to cells.
Subsequently, in 2004, Carolyn R. Bertozzi from the University of California, Berkeley, developed a copper-free click reaction, the strain-promoted azide-alkyne cycloaddition (SPAAC), and demonstrated that it could be used to track glycans in cells. This reaction can occur in biological environments without disrupting the surrounding biochemistry, which is referred to as "bioorthogonal" chemistry.
How does click chemistry translate into therapeutic or diagnostic agents?
Researchers can label a patient's cells or tissues with click chemistry molecules, then inject drugs or imaging agents linked to complementary click partners. The payload is delivered specifically to the target site when the two click molecules bind together.
In the human body, effective targeted drug delivery requires fast click reactions to deliver the payload within a useful therapeutic timeframe. Therefore, in 2008, two research groups from the University of Delaware and Massachusetts General Hospital/Harvard Medical School independently invented the tetrazine ligation, a fast reaction that combines nitrogen-rich tetrazines with partners containing strained carbon-carbon double bonds. Shasqi, Tagworks, and MSKCC are all utilizing this reaction, employing tetrazine and trans-cyclooctene (TCO) as click partners.
Once the drug reaches its target, it must be released from its click partner to be effective. To achieve this, Tagworks developed a variant of tetrazine ligation called "click-to-release." In this system, the click partners are designed so that when tetrazine and TCO click together, it triggers a subsequent reaction called carbamate cleavage, which releases the drug (Figure 3).
Figure 3. Tagworks' MMAE delivery in mice. Adapted from Rossin, R. et al. Nat. Commun. 9, 1484 (2018).
What did Shasqi's research achieve?
Shasqi conducted experiments utilizing a hyaluronic acid biopolymer with tetrazine moieties and a prodrug of the anticancer agent doxorubicin linked with TCO. Clinicians injected the tetrazine-containing polymer into the tumor site, followed by intravenous infusion of the prodrug five times daily. Upon interaction and click formation, the carbamate cleavage reaction released doxorubicin, effectively delivering the anticancer drug to the tumor.
This method offers several advantages:
First, it overcomes the toxicity associated with doxorubicin. Despite being used for over 40 years in treating solid tumors, the toxicity of doxorubicin has limited both the dosage per administration and the lifetime dose. However, Shasqi's prodrug has approximately 80 times lower toxicity than doxorubicin, thereby avoiding systemic side effects.
Second, click release of the prodrug leads to high concentrations of doxorubicin at the tumor site, which cannot be achieved using traditional treatment methods.
In a mouse model of colorectal cancer, this therapy improved mouse survival rates by 63% compared to using doxorubicin alone. Phase I clinical trials involved 40 patients with late-stage or metastatic solid tumors. Results from the first 22 patients indicated that each cycle of prodrug injection could deliver doxorubicin to the tumor at 12 times the conventional tolerable dose, with manageable toxicity and side effects. Data presented at the American Association for Cancer Research (AACR) conference held in Orlando, Florida in April revealed increased cytotoxic T-cell activity in patients' bodies, indicating doxorubicin's expected efficacy. Shasqi has since dosed nearly 12 patients in Phase II follow-up trials and anticipates completing the first stage of this trial by the end of 2024 (Figure 4).
Figure 4. SQ3370 combines the targeting agents SQL70 and SQP33
CAPAC presents several advantages over ADCs.
ADCs, a major focus in targeted cancer therapy, combine specific monoclonal antibodies with cytotoxic agents capable of killing tumor cells.
However, ADCs have limitations:
ADCs typically depend on intracellular biological processes within tumor cells (such as proteases) to cleave the conjugate and release the drug payload, which is constrained by the types of targetable receptors available.
Click Chemistry for Improving Radiographic Imaging
While radiolabeled antibodies are commonly used for diagnostic imaging, they have their limitations. It can take several days for enough antibodies to accumulate at the target site, and even longer for unbound conjugates to clear from the body, which can impact the clarity of the resulting image.
In a Phase I trial at MSKCC, researchers are exploring how click chemistry can enhance tumor PET imaging with radiolabeled isotopes. Their approach involves first positioning the targeted antibody and then introducing the PET radiolabeled isotope. During the trial, patients receive an injection of a monoclonal antibody called hu5B1, modified with a click chemistry TCO moiety. This antibody targets the CA 19-9 antigen on pancreatic cancer cells. When injected into the bloodstream along with a tetrazine molecule carrying 64 copper, it binds to the TCO on tumor cells within minutes. Once the isotope is securely locked in place, PET scans can precisely display the location of the cells in the body. In practice, patients could receive the antibody during a routine visit and return the following week for the radiolabeled portion, generating images within an hour.
Likewise, this method seeks to eradicate tumors post-imaging. "It's essentially the same chemically—just transitioning from using diagnostic isotopes to therapeutic isotopes."
The Phase I trial at MSKCC aims to assess the system's safety, determine optimal component ratios, and identify the best imaging timeframe. If successful, Lewis aims to integrate a second therapeutic isotope phase into subsequent Phase II trials. Additionally, Tagworks is developing click-assisted radiographic imaging agents with a primary focus on targeted therapy. In contrast to Shasqi's current trials, which employ physical injection for targeting specific tumor sites, Tagworks employs antibodies to position the click molecules. This approach enables them to target any cancer cells with suitable target receptors throughout the body.
Conclusion
Beyond offering targeted therapies and imaging agents, some researchers see click chemistry as versatile, potentially allowing for the assembly of large drug molecules within the patient's body. This method is especially valuable for delivering high-volume drugs to tissues that are otherwise hard to access.
To sum up, many companies and research centers are dedicated to leveraging click chemistry in cancer-targeted therapies, opening doors for broader clinical applications.
Biopharma PEG proudly nurtures this energy by being a leading provider of click chemistry reagents worldwide. We supply PEG products & reagents functionalized with azide, alkyne, DBCO and other cyclooctyne. Contact us at sales@biochempeg.com now.
References:
1. Peplow M. 'Clicked' drugs: researchers prove the remarkable chemistry in humans. Nat Biotechnol. 2023 Jul;41(7):883-885. doi: 10.1038/s41587-023-01860-2. PMID: 37407704.
2. Wu K, Yee NA, Srinivasan S, et al. Click-activated protodrugs against cancer increase the therapeutic potential of chemotherapy through local capture and activation [published correction appears in Chem Sci. 2021 May 21;12(21):7583]. Chem Sci. 2021;12(4):1259-1271. Published 2021 Jan 5. doi:10.1039/d0sc06099b.
3. Srinivasan S, Yee NA, Wu K, et al. SQ3370 Activates Cytotoxic Drug via Click Chemistry at Tumor and Elicits Sustained Responses in Injected & Non-injected Lesions. Adv Ther (Weinh). 2021;4(3):2000243. doi:10.1002/adtp.202000243.