Theranostics — Radioimmunoconjugates that can be used in both diagnosis and therapy — show significant promise for complementing and supplementing immunotherapies as personalized cancer treatments. As theranostics are high-value and complex products comprising vectors, linkers, and payloads, specialized expertise in bioconjugation is essential to the development and manufacture of optimal candidates that can provide highly targeted diagnostic and therapeutic benefits. GBI has extensive experience helping clients rapidly bring safe and effective theranostic products to the market.

The Nature of Theranostics for Cancer

Theranostics is derived from the words therapeutics and diagnostics. A patient receiving theranostic treatment is simultaneously or sequentially diagnosed and treated using one or more drugs and/or techniques in a single “package.” By diagnosing and treating patients in one session, time and money are saved, and, in many cases, undesired side effects that occur when these steps are implemented separately can be avoided.¹

For cancer patients, the combined use of diagnosis and therapy tools also enables a personalized approach to treatment.² Typically, positron emission tomography (PET) imaging is used to determine whether specific tumor receptors are present on a patient’s cancer cells. If detected, a radioactive drug is delivered in a highly targeted manner to the cancer cells to destroy them without harming healthy cells.³

This targeted delivery is achieved by using a radioimmunoconjugate comprising an antibody covalently linked to (or chelating) a radioactive payload. The antibody is the delivery system that brings the radioisotope or radionuclide to the targeted disease tissue. The intensity of the PET scans indicates the level of the disease and the extent of treatment required.

The somatostatin receptor (SSTR2) on the tumor cell membrane is a common target.⁴ The radioactive diagnostic drug Ga-68 DOTATOC binds to this receptor, making any tumors present in the body visible and detectable on a PET scan. If tumors are detected, the patient can then be injected with a different radiopharmaceutical, such as lutetium-177 (Lu-177) or yttrium-90 (Y-90), bound to the same somatostatin analog peptide that binds to the SSTR2 receptors, allowing the drug to target and kill the tumor cells.

Figure 1. Standard theranostic design targeting the somatostatin receptor (SSTR2) on the tumor cell membrane. ^4,12,13

Theranostic treatment regimens are designed specifically for each individual based on information gained using pharmacogenetic, proteomic, and biomarker profiling data, which ensures that the right drug is delivered to each patient at the correct time.⁵ Because they involve an “intimate connection of diagnosis and therapeutics” that draws diagnosis and therapy closer, theranostic protocols have the potential to provide better outcomes than traditional therapy approaches.

Initially, iodine 131 (I131) was widely used as a diagnostic and therapeutic agent. Most theranostic developers today, however, are moving away from radioactive iodine, which is a beta emitter, and pivoting to other options (such as alpha emitter and the use of an appropriate hetero-bifunctional linker that is covalently conjugated to the antibody and chelated to an appropriate radionuclide — see below), owing to the poor signal-to-noise ratio associated with I131. Furthermore, since I131 is directly conjugated to the antibody without using a linker, the drug-design possibilities are relatively limited.

Vast Market Potential

The field of theranostics grew out of the field of diagnostic imaging, according to Steven R. Deitcher, M.D., President, CEO, and Board Member of radiation therapeutic innovator Radimmune. The first step was the development of solutions for the targeted delivery of gamma-emitting radioisotopes for nuclear medicine imaging. This technology was then applied to the targeted delivery of beta-emitting radioisotopes to treat the same diseases. Today, theranostics includes the use of a single antibody to deliver both a diagnostic agent and a therapeutic anticancer agent to specific tumor cells. A typical goal in the field is to develop a single antibody that delivers a single radioisotope that can serve as the diagnostic and therapeutic agent.

The potential for the theranostics market directly reflects the significant unmet needs that remain with respect to cancer treatment. “With over 1.9 million new cases and over 600,000 cancer deaths predicted for 2022 in the United States alone, there is plenty of room for improvement,” Deitcher states. Advances in PET technology, the growing use of alpha emitters, and improved manufacturing processes are also contributing to the growth of the theranostics market.

The possible applications of theranostics are numerous and include visualization of biological processes or tumor biology in vivo, diagnosis and tumor staging, therapy planning, and treatment of specific tumors.⁶ Importantly, theranostics accelerates the translation of potential tumor targets from preclinical research into first-in-human clinical studies.

One challenge of some significance is the supply and sourcing of the raw radioisotopes used in the production of radionuclide payloads, particularly for alpha-therapeutics and theranostics. Fortunately, investments in new production capabilities are being undertaken to address this issue. What is needed, observes Deitcher, is more favorable data from robust clinical trials to drive patient and physician demand that would prompt production innovation and investment geared toward alleviating the current supply issues.

Despite the potential headwinds, the value of the global theranostics market is expected to maintain a compound annual growth rate (CAGR) of ~9.5–12.2% in the coming years, reaching $124–$153 billion by 2027/2028. ^7,8

Moving from Diagnostics to Therapeutics

Many companies developing theranostic solutions start with a purely diagnostic product, typically a radioisotope conjugated to a specific antibody that can be used to determine whether a patient has cancer and the severity of the disease. Nearly in parallel, they develop a complementary therapeutic using a different antibody–chelated payload combination.

This approach enables the “assess it and treat it” concept while also allowing these companies to demonstrate the effectiveness of the antibody for targeting the tumor and time to raise money for the development of companion therapeutics.

Alpha, Beta, and Gamma Emitters

Radionuclides used as payloads in theranostic applications are either alpha-, beta-, or gamma-emitters. Alpha particles are essentially the nuclei of helium atoms (two protons and two neutrons), while beta particles are electrons, and gamma particles are photons.

A notable difference between beta and alpha particles, beyond size and charge, lies in their respective linear energy transfer (LET) properties, according to Deitcher. LET describes the rate at which energy is transferred per unit length of track. Beta particles are lower energy and travel farther than alpha particles; thus, alpha particles can have LETs that are hundreds of times higher than those of beta particles. That makes them attractive for therapeutic applications, because if delivered very close to the cancer cell nuclei, they can kill tumors while avoiding or minimizing collateral damage to healthy cells.

It is also worth noting, however, that the use of radioactive — and nonradioactive — payloads that have already been approved in other applications confers an advantage with respect to providing a level of comfort for patients, physicians, and regulatory authorities that the agent is known to work in a clinical setting and has well-characterized safety risks.

Theranostic Delivery Systems

Antibodies currently serve as the most widely used delivery systems for theranostic applications, and they are often biosimilars with demonstrated binding behaviors. Therefore, these radioconjugates represent a subtype of antibody–drug conjugates (ADCs). The specific nature of the disease will dictate the optimum delivery vehicle, linker, and payload.

While antibodies effectively target specific cell receptors, their use has some limitations. For instance, different cancers at different stages in different groups of patients may require different delivery vectors, according to Deitcher. Tumor affinity, tumor selectivity, the pharmacokinetics of the vector and the chosen radioisotope, clinical activity, and safety must all be considered.

Non-antibody delivery vectors can be attractive, because it is much easier to quickly screen large numbers of them. Examples include peptides, small molecules, and nanoscale systems. Peptides and small molecules,⁹ in theory, are easier to make and penetrate tumors more quickly than full-sized antibodies, but they are cleared from the body more quickly.

Nanoscale formulations can be derived from liposomes, dendrimers, polymeric nanoparticles, metallic nanoparticles, silica nanoparticles, magnetic nanoparticles, quantum dots, carbon nanotubes, polymeric micelles, scFV fragments and nanobodies (from camelids), among others.^1,4,10 More recently, bacteriophages have attracted attention as possible drug-delivery systems. Because they are so small, nanoscale theranostics have the advantage of enabling the detection of tumors less than 0.5 cm in size.¹¹ They also make it possible to prepare biomimetic materials.¹⁰ Challenges to their use include difficulty in producing particles that are uniform in size and highly reproducible, particularly when scaling up processes.¹¹

Choosing the Right Chelator

It is not only important to choose the right vector and payload, but also the right linker or chelator. In addition to the necessary linkage chemistry for given vectors and payloads, it is also essential to consider whether the product will be used for diagnostic or therapeutic purposes. For diagnostic applications, the radionuclide must remain bound to the chelator, while for therapeutics, the chelator must release the payload at the target site.

To date, there have only been a limited number of commercially available chelators for use in theranostic applications, including cyclic 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), [(R)-2-Amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (acyclic “CHX-A” linkers), TCMC (also known as DOTAM), and deferoxamine (DFO). DOTA is commonly used to produce diagnostic products, while CHX-A is used for therapeutics.

It is crucial for theranostic developers not to consider the linker to be less important than the delivery vehicle and the payload. While it may be the least expensive of the three components, it is equally important. It provides the means by which the payload is attached to the vector, without which there is no product. It also controls the release of the payload, which directly impacts therapeutic performance.

It should also be noted that not all isotopes require a linker; some can be directly bound to the vector.^17,18, In addition, some linkers can only bind to a limited number of isotopes, while others work with a wide range of different types of isotopes, such that the same vector with one particular linker could be used to bind a diagnostic isotope or a therapeutic isotope. In this case, the same antibody–linker product could be used to support the diagnostic imaging and the therapeutics, with only the payload needing to be changed.

Fighting Melanoma with Theranostics

Radimmune was originally formed in 2016 with a primary focus of developing a universal targeted alpha-therapeutic for all subtypes of melanoma. The technology involves engineering an antibody that binds to extracellular accumulations of melanin (an intracellular molecule), unlike other companies in the field that are targeting cancer cell surface antigens or receptors. In addition, the company is looking to add to their pipeline and is considering some cell surface targets.

Melanin is a common feature of all types and cases of melanoma, whether cutaneous, uveal, mucosal, or acral melanoma. Melanin is not a protein but a pigment that is very difficult to bind. Radimmune’s anti-melanin antibodies bring isotopes into the tumor, and it is believed, according to Deitcher, that the antibodies bind to melanin that has leaked out of a few dead melanoma cells. The goal is to bind to that melanin and kill more melanoma cells, which further enriches the target tool.

Radimmune’s engineered humanized antibodies against melanin linked with several different alpha-emitters have been tested in animals using different tumor models. The company has generated some promising data and continue to assess opportunities for further optimization.

According to Deitcher, the company will continue to concentrate on some of the hardest-to-treat cancers and look for additional targets and opportunities that will hopefully make a difference in the lives of people with cancer.

Development and Manufacturing Challenges for Theranostics

The potential combinations of vectors, linkers, and payloads afford theranostic developers significant flexibility and numerous options for finding optimum solutions but also create challenges for development. There are many different permutations and combinations that must be evaluated. It can also be challenging, because access to proper imaging devices during preclinical and clinical development is essential.

With respect to manufacturing, the chemistry is fairly straightforward in most instances. The chelator is a heterobifunctional crosslinking reagent with reactive and metal-chelating groups at each end that selectively react with the antibody (or another vector) and chelate to the payload. The challenges derive more from the fact that the payload is a radioactive metal compound. It is critical to ensure that no metal contaminants, particularly heavy and transition metals, are present that could potentially bind to the chelator. Buffers and other reagents must be de-metalized before use, typically using a complexing agent, such as ethylene-diamine-tetra-acetic acid (EDTA) or metal-chelating resin column. Those complexing reagents and any complexed metals are then removed.

The process involves the conjugation of the linker, such as an amine-reactive (or thiol-reactive) chelator, to the antibody. This reaction is optimized with respect to the molar ratio of the chelator to the antibody (usually exhibits a Gaussian distribution of about 1.0 to 3.0 molar ratio of the chelator to antibody). The resulting mixture is subject to tangential flow filtration (TFF) to remove excess chelator and achieve buffer exchange into the final buffer at the desired concentration. TFF is ideal for this step, because it is widely used in pharma and is readily scalable. Other factors taken into consideration for developing and optimizing such conjugates include conjugation time and temperature, antibody concentration, appropriate conjugation buffer, use of organic solvents (to solubilize the chelator if required), pH, and mixing, among others.

A challenge at this point is the comprehensive characterization of the conjugate. Characterizing antibodies on their own can be difficult, because they are such large molecules; it is even more challenging when the antibody is conjugated to a linker. It is necessary to characterize the whole molecule, as well as its individual components. Using liquid chromatography (LC)-mass spectrometry and other advanced analytical techniques may be necessary to precisely determine the molar chelator–antibody ratio.

The final step is the addition of the radioisotope to the linker. While this chemistry is not necessarily difficult, characterization can be complicated, including varying half-lives of the radioisotope used, generally from about an hour to a few days. In addition to physicochemical characterization, it is necessary to use radio detection equipment to determine the level of radioactivity.

In general, because the manufacturing process is multi-step and the product comprises three distinct components, — viz: the antibody, chelator, and radionuclide constituting the radioimmune conjugate—  there are some manufacturing and regulatory complexities. The regulatory complexities are borne from the fact that one has to follow the regulatory guidelines for not only the GMP manufacturing of the antibody and antibody–chelator conjugate (drug intermediate) but also radionuclide manufacturing and regulations to afford the radioimmune conjugate (drug product) and handling facility.¹⁹ Other issues include sourcing and handling of the radioactive materials, obtaining the appropriate licenses, accessing skilled operators and specialized training, and managing waste.

An even bigger question that must ultimately be addressed by theranostics makers, according to Deitcher, is whether the drug should have the radioactive payload added by the pharmacist in the hospital or whether manufacturing should be done centrally, with the radioactive product then delivered to the hospital or clinic ready to administer. It is a complex question, and the answer often depends on the isotope and its half-life. Most companies are currently attempting to use isotopes that allow for central manufacturing.

There is also the question about how theranostics are developed: linking a radionuclide to an antibody that is known to bind to a particular tumor cell or looking at which cancers are not yet benefitting from recent innovations and seeking to find antibodies that can deliver alpha-emitting radioisotopes.

Streamlining Theranostics Manufacturing at GBI^12–18

GBI has been developing and manufacturing a wide range of bioconjugates for more than two decades. Many projects have involved radioisotopes, fluorescent dyes, and other payloads that are suitable for theranostic applications (Table 1). We have experience in working with both biomolecules and small molecules and have a deep understanding of the interface between organic chemistry and protein chemistry, including what is required to characterize bioconjugates. Furthermore, we have the expertise and knowhow to perform conjugation process optimization and scale-up and GMP manufacturing with process economics and compliance considerations to generate clinical, therapeutic-grade bioconjugates from lab to commercial scale. This includes the utilization of systems such as TFF, Mettler-Toledo Easy Max advanced synthesis workstation to precisely control conjugation and chemical processes, and size-exclusion chromatography, among others.

Table 1. Selected Radioimmune Conjugates Manufactured by GBI.

*Denotes projects including process development (PD), scale-up, and cGMP manufacturing

For theranostics, a key aspect of process development is the optimization of the molar ratio of payload, linker, and vector. Knowledge of which chelators are suitable for diagnostic and therapeutic applications is also essential. For poorly water-soluble linkers and payloads, the ability to balance the use and amount of organic solvents and reagents to ensure that the antibody vector remains intact is crucial, as proteins may aggregate under such conditions. In addition, some antibodies are more fragile than others, and the propensity to aggregate or degrade may increase once they are covalently linked to small molecules.

For each new project, GBI applies the learnings from hundreds of previous and ongoing projects (including complex biologics such as bioconjugates, monoclonal and polyclonal antibodies, Fc-fusion proteins, conjugate vaccines, bispecific antibodies and proteins, cytokines, enzymes, and other recombinant proteins) to the design of streamlined optimization studies. In addition, we are a single-source provider, and therefore manage the entire supply chain for our customers, viz: GMP cell banking, process development, scale-up and GMP manufacturing of the “naked” antibody (or recombinant protein), conjugation of the antibody onto the payload, and aseptic fill and finish.

Furthermore, GBI also manages other activities, such as IND-enabling activities, including characterization of the antibody–chelator conjugate (drug intermediates), stability studies with a range of fit-for-purpose buffers, and creation of reference standards, among others. Since GBI’s production site is not an organic synthetic chemistry facility, we work closely with partners that have the necessary expertise and licenses to produce chelators and small molecule payloads, including radionuclides.

It is also worth noting that, for antibody–chelator conjugates leveraging CHX-A” DTPA as the linker, GBI has developed a proprietary analytical method for quantification of this chelator, which can be quite challenging due to its complex structure. We have shown that the results obtained with this fluorescent spectroscopy method correlate well with the results obtained using LC-MS, while results are available immediately rather than taking weeks to receive data from the contract analytical lab. This proprietary rapid analytical technique contributes to our ability to accelerate optimization development to achieve the appropriate molar ratio of the antibody–chelator conjugate, which is particularly important for theranostic candidates that have received Fast Track, Breakthrough Therapy, or Accelerated Approval designations.

Ultimately, GBI’s experience and awareness of the unique development and manufacturing requirements associated with bioconjugates, combined with our ability to analyze antibodies and linkers to determine the best approach for delivering payloads, including radioisotopes, is significantly valuable to customers.

Conclusion

Theranostics show significant promise for complementing and supplementing immunotherapies as personalized cancer treatments. As high-value and complex products comprising vectors, linkers, and payloads, specialized expertise in bioconjugation is essential to developing and manufacturing optimal candidates that can provide highly targeted diagnostic and therapeutic benefits. Working with a contract development and manufacturing organization such as GBI with demonstrated experience in bioconjugation from the lab to the clinic and commercialization ensures that new safe, and effective theranostic products reach the market in an accelerated and cost-effective manner.

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