It Takes Two to Tango, Part 2: Applying Bioconjugation

The enhanced functionality unlocked by bioconjugation is an asset to targeted cancer therapies. Traditional chemotherapeutics lack the selectivity to distinguish between healthy and diseased cells, hence very high doses are administered to achieve potency, with severe negative ramifications on healthy cells. To overcome this, drug innovators began to use bioconjugation to attach the therapeutic to an antibody that can specifically target tumor-associated antigens on cancer cells. Thus, the bioconjugate is selectively taken up by only tumor cells, where the cytotoxin can exert its effects, and the healthy cells are exempted from cytotoxicity. Glycans are ideal targets for ADCs, as cancer cells display different glycan epitopes from their healthy counterparts. For example, mucin – an important surface peptide that is aberrantly O-glycosylated in various cancers (e.g., through extensive sialylation and fucosylation), is often a viable target for ADCs (monoclonal antibody (mAb) conjugated drugs) [6].  

The successful targeting of ADCs has been complemented with the radiolabeling of antibodies to detect tumor cells, ensure targeted delivery, and monitor drug mechanisms of action. Today, various radiolabeled antibodies are undergoing clinical trials for diagnostic and therapeutic purposes against breast cancer, lymphoma, lung cancer, prostate cancer, and gastric tumors [7].

Identifying and detecting disease-associated proteins requires cutting-edge diagnostics assays, where bioconjugation finds a central role. One example is enzyme-linked immunosorbent assays (ELISA) that investigate antigen-antibody interactions often using a secondary antibody for detection and quantification. This is achieved by conjugating the secondary antibody to an enzyme, such as horseradish peroxidase (HRP) or a fluorophore to generate detectable signals. The same protocol is applied to flow cytometry and imaging methods by conjugating antibodies or probes with fluorophores. This allows researchers to quantify and visualize differences between healthy and diseased cells.

Besides therapeutic and diagnostic applications, we also encounter bioconjugation in biochemistry research in general. Biotin is perhaps the most widely used detection label for protein detection and isolation studies. The conjugation of biotin to antibodies and cell surface receptors can be used to highly enhance the detection signal since streptavidin has a high affinity towards biotin and can be used as a replacement of secondary detection antibody or as a tertiary detection reagent. In addition, bioconjugation plays a central role in creating fusion proteins that allow us to study protein-protein interactions. Several strategies, such as genetic fusion, enzyme-mediated conjugation, and chemical ligation e.g. click chemistry, tetrazine ligation, oxime/hydrazone ligation, etc., are often employed to covalently link protein domains that enable two proteins to function as a single unit.

Nanotechnology is an emerging field in biomedical research aiming to harness the unique properties of nanoparticles to improve the selectivity of therapeutics and imaging agents. For example, superparamagnetic iron oxide nanoparticles (SPION) are frequently studied as magnetic resonance imaging (MRI) agents of brain tumors due to their small diameter and ability to cross the blood-brain barrier. Shevtsov et al. used SPION-EGF nanoconjugates to selectively target EGFR-overexpressing C6 glioma cells, achieving a robust MRI contrast of tumor cells in mouse models [8]. Another study by Huang et al. developed a multifunctional folic acid-modified SPION conjugate that not only enabled selective MRI imaging of tumors but also mediated the targeted delivery of the anticancer drug Doxorubicin to the mouse tumor cells with high potency [9]. 

Another noteworthy example is a nanoparticle-based biosensor used to quantify a biomolecular interaction or a cellular process. This method is a crucial part of medical diagnostics, such as glucose monitoring for diabetic patients [10] and biomarker detection as well as drug screening methods to evaluate potency, dosage, and pharmacokinetics. Unsurprisingly, bioconjugation is involved in the development of biosensors when attaching the biomolecule used in recognition to the nanosurface. 

In summary, bioconjugation gives biochemical modalities that can serve as promising tools in targeted therapies and diagnostics and can be integrated with nanotechnology. Detailed explanations of our bioconjugation products can be found in the Vector Laboratories Bioconjugation Resource Guide and Bioconjugation Application Page. In Part 3, we’’ll be diving into the numerous advantages and drawbacks to bioconjugation.