The Safety Potential Of Conditionally Active Biologics

By Tyler Menichiello, Chief Editor, Bioprocess Online

Off-target toxicity has historically been a challenge for biopharmaceuticals, even with highly targeted molecules like antibody-drug conjugates (ADCs); though advances in linker technology have markedly reduced this toxicity. Despite an increase in validated targets, this safety issue still hinders meaningful progress in the clinic. This is especially true of molecules like cytokines, which can be highly effective in treating cancer for some patients, but can present serious systemic toxicities for most.

In 2025, good targets are no longer enough to succeed in the clinic. As such, developers are increasingly exploring innovative ways to reduce toxicity, which has resulted in an emerging class of molecules known as conditionally active biologics (CABs). As the name suggests, these biologics are designed to provide a therapeutic effect only once they’ve been activated by another molecule or factor like specific physiological conditions (e.g., pH, temperature, oxidation state).

At this year’s BIO International Convention, I attended a session titled “Beyond Bispecifics and ADCs: Conditionally Active Biologics,” moderated by Michael Rice, an advisor and SVP of CGT at Lumanity. The panel featured Alexandra Cantley, Ph.D., a partner at Polaris Partners; Stephen Demarest, Ph.D., CSO at Tentarix Biotherapeutics; Randi Isaacs, MD, CMO at Werewolf Therapeutics; John Lin, MD, Ph.D., SVP of immune oncology and head of bispecific R&D at Regeneron Pharmaceuticals; and Neela Patel, Ph.D., chief business officer at Bonum Therapeutics. The panelists talked about the current CABs landscape, discussing the most promising technologies, as well as investor interest.

Designing Molecular On/Off Switches

“The problems that we’re trying to solve with conditional activation have been problems that have been out there for a long time,” said Isaacs. “And now, we’ve got some really interesting tools.”

Finding ways to design biologics with conditional “on/off” switches can improve their therapeutic index, leading to better efficacy and reduced adverse effects, Rice explained. Protease-activated biologics may be the most familiar example. “That is probably one of the most active areas,” Rice said, “and it’s been pursued for quite a long time.”

Rice also alluded to some other common examples, like avidity-based CABs, which only activate when there is high target density, and biologics that are only activated when co-administered with a small molecule. Some CABs can even be turned on physically using sonication or lasers.

The therapeutics companies represented on the panel are all developing different kinds of CABs:

• Bonum Therapeutics uses dual-binding antibodies with complementarity-determining regions (CDRs) that bind two different antigens. This allows for a conditional switch where the therapy is only active in the presence of a specific antigen.
• For oncology, Regeneron is developing conditionally activated CD3 bispecifics and costimulatory bispecifics for more precise T-cell activation. The platform involves using independent tumor antigens, with one serving as the anchor while the other activates a co-receptor.
• Tentarix Therapeutics is developing acidity-driven CABs, which are designed to activate in disease-specific microenvironments.
• Werewolf Therapeutics’ platform utilizes a masking domain to essentially hide therapeutic cytokines from the immune system until they’re in the tumor microenvironment (TME). These conditionally active cytokines are also equipped with half-life extension domains and protease-cleavable linkers, which allow for increased exposure in circulation and localized activation in the TME. This masking approach allows for tumor-specific activation without the systemic toxicity commonly seen in cytokine therapies.

I was most intrigued by Werewolf’s masking approach. The combination of masking domains, protease-cleavable linkers, and half-life extending domains represent the kind of patchwork of technologies that allow for next-generation CABs. While the masking is critical for safety, the platform’s flexibility comes from the protease-cleavable linkers.

Proteases are dysregulated in cancer, not only between indications, but also in different lesions in the same patient, according to Isaacs. That’s why, “It was very important to choose linker sequences that were not specific for any one protease class,” she explained. “And that’s the secret sauce of Werewolf, is you get into the microenvironment, and it doesn’t really matter what the protease is.”

Finding New Approaches To Old Targets

In the early 2000s, biopharma saw no shortage of clinical and commercial success with molecules like mAbs. A big part of this success was the abundance of biological targets. This “low-hanging fruit,” Demarest said, drove a lot of early innovation that carried the industry to its present day. However, in 2025, “a lot of the low-hanging fruit is gone,” he said.

Today, protein engineering is going through a renaissance thanks to computational design and AI, said Demarest, built on learnings from the clinic around understanding mechanisms. As a result, companies are exploring new ways to go after targets that have historically failed — not because the biology wasn’t sound, but because the off-target toxicity wasn’t manageable.

“There are targets that have been mechanistically and biologically de-risked that we’re trying to open up in a new way by looking at therapeutic index,” Cantley said. Familiar targets can make investors more comfortable, according to Cantley. “We know this target works; we know the issue is toxicity. If there’s therapeutic index, we’re taking a risk on the masking approach or whatever sort of your technology is, but there’s less of the fear of a novel target.”

Conditional activation offers an opportunity to address the safety issues surrounding even the most validated targets (e.g., HER2). The panel agreed that a target’s popularity shouldn’t dissuade developers from going after it. “If you have a better technology, you should always apply it to the most validated target,” said Lin.

Off-target toxicity can sink even the most promising programs. If the CABs in development today get to the clinic and can demonstrate radical improvements in safety — regardless of target or modality — it will mark an important shift in the industry. One where conditionality isn’t merely a concept, but a strategy that gives promising targets a better chance of turning into cures for patients. It’s inevitable that the science and technology driving these therapies will continue to advance, but as Demarest cautioned, “With powerful biology, you want to be careful and make it as conditional as possible.”