TLR8

Pictured: A white blood cell charging an ADC/Taylor Tieden for BioSpace The antibody drug conjugate market is one of the fastest growing in the pharmaceutical industry, expanding from $1 billion to $10 billion in yearly sales between 2015 to 2023—and expected to rise to $28 billion by 2028, according to Evaluate. But despite their high level of efficacy in delivering cytotoxic payloads directly to tumor cells, antibody-drug conjugates (ADCs) are “unfortunately not curing people,” Tim Lowinger, senior vice president and chief science and technology officer at Mersana Therapeutics, told BioSpace. There are real benefits, he said, including prolonged life and extended progression-free survival, yet reemergence occurs. With 15 ADCs approved as of February 2024 and many more in company pipelines, drug developers are expanding their focus to ask how they can build on the success of ADCs. Enter immunostimulatory ADCs (iADCs), sometimes called immune-stimulating antibody conjugates (iSACs). Immunotherapies are already proven cancer drugs, and delivering these treatments via ADC offers the potential to precisely target tumor cells while protecting normal cells, said Jane Chung, president and chief operating officer at Sutro Biopharma. “Engineering very targeted therapies so that you can dial up the potency of whatever payload you’re using, that’s going to . . . really move the needle on advancing cancer care,” Chung told BioSpace. Hitting the Gas on Immune Response The excitement around iADCs stems from the success of checkpoint inhibitors, said Nathan Tumey, an associate professor at Binghamton University’s School of Pharmacy and Pharmaceutical Sciences. Tumey told BioSpace that checkpoint inhibitors essentially take the brakes off of the immune system, enabling it to attack cancer cells. iADCs offer the potential to take this further and “step on the gas,” ramping up the immune response. Some of the cytotoxic payloads carried by current ADCs are also known to cause immunostimulatory cell death by creating more neoantigens in the tumor, Lowinger explained, driving companies like Pfizer to combine its auristatin-based ADC with a checkpoint inhibitor for a synergistic effect. For Lowinger, this aspect raises the question: “If a secondary mechanism of immunostimulatory cell death can have a benefit, what if you made this your primary mechanism of action?” That idea is driving interest now in iADCs. Biopharma companies are targeting a number of pathways to activate the innate immune system. Mersana, for example, is developing a stimulator of interferon genes (STING) agonist. While STING activation has known anti-cancer effects, it’s been a historically difficult target because activating it throughout the body can be toxic. The drug needs to be activated only once bound to tumor cells—which is precisely what ADCs were made to do. Using its immunosynthen platform, Mersana designed its STING agonist to stay put once delivered inside the cell with a stable, cleavable linker scaffold. Its lead asset is currently in Phase I trials against multiple HER2-positive solid tumor types. Instead of using the monoclonal antibody trastuzumab often paired with current HER2-targeting ADCs, Mersana developed a novel antibody. Therefore, it could potentially be used in combination with established, cytotoxic ADCs, as the two would not be competing for the same binding site. Meanwhile, Burlingame, CA–based Tallac Therapeutics has TAC-001, a TLR9 agonist ADC in a Phase I/II trial for solid tumors. While most ADCs carry small molecule payloads, Tallac’s platform can conjugate an oligonucleotide to an antibody, allowing modalities beyond cytotoxicity, Hong Wan, CEO of Tallac, told BioSpace. Because TLR9 is part of the innate immune system and expressed near-exclusively on immune cells, it has potential for a durable response via “immune memory,” in addition to increased safety and tolerability, Wan said. Wan pointed to the high toxicity of many current ADCs and the very high target protein expression required in the tumor to elicit the cytotoxic effect, which can restrict the efficacy to a limited patient population. In contrast, iADCs don’t require very high target expression, she said, so they could potentially benefit a broader patient population. Though data from a Phase I/II dose escalation trial have not yet been disclosed, Wan indicated the company has seen a positive response to TAC-001 in melanoma. The candidate has also been studied in the preclinical setting alongside a cancer vaccine, with positive results shared in April. And it has been combined with multiple chemotherapies preclinically. Next, Tallac is planning to test the candidate in combination with a PD1 checkpoint inhibitor. In a twist on the combination approach, South San Francisco’s Sutro—in partnership with Astellas—is taking a dual conjugation approach with its iADC. “When we put both elements, the chemotherapy and the PD1 immune activator, on an antibody . . . we reduce the tumor volume and activate the immune system to get rid of the remaining tumor cells,” said Hans-Peter Gerber, chief scientific officer at Sutro. Balancing ratios between the two elements can be precarious and the technology is critical, Gerber told BioSpace, adding that he believes Sutro stands alone in being able to dial in the correct balance. While the Astellas-partnered program does not yet have a disclosed target, Gerber said the obvious strategy would be to start in indications where immune checkpoint inhibitors already work, such as melanoma and lung and kidney cancers, then expand to populations that cannot benefit from a checkpoint inhibitor. Colorectal cancer is a “bold goal” for the company, he added. A Difficult Path The path has not been a smooth one for iADCs, though. Thanks to some highly visible clinical failures, “the field has a bad rap,” Tumey said. In 2022, an iADC combining an anti-HER2 MAB with a TLR8 agonist failed. Its developer, Silverback Therapeutics, ceased operations and became a shell company for a reverse merger with a different biotech. Two years later, Bolt Therapeutics had essentially the same results for a similar treatment. A HER2-targeting iSAC developed by Novartis also failed a Phase I trial for non-breast malignancies in 2021. “This remains a challenging area for drug discovery with considerable technical risk, as the common targets such as TLR-7, -8, -9 and STING have not yet been successfully drugged with any type of therapy,” Dan Chancellor, vice president of thought leadership at pharma solutions provider Norstella, told BioSpace in an email. He added that the iADC technology is in the beginning stages of development. However, the aforementioned three failures really amount to one, Tumey said. The companies were doing essentially the same thing—using non-cleavable linkers and effector function ADCs targeting HER2—so if one failed, “of course they’re all going to fail.” Additionally, “I think maybe a mistake that the field made early on was trying to go after breast cancer,” he said. While it seemed like the lowest hanging fruit, it’s difficult because it is an immunologically cold tumor. Like Gerber, Tumey said he believes iADCs have the highest potential in areas checkpoint inhibitors have been most successful, like non-small cell lung cancer or bladder cancer. Thanks to the success on the cytotoxic side of the ADC space, Tumey said companies appear to be more willing to take risks, potentially opening the field for meaningful success. iADCs in the Limelight In a notoriously tough biopharma funding climate, iADCs are one area drawing investment from companies. Mersana, for example, has partnerships with both GSK and Merck KGaA. The GSK deal, worth up to $1.4 billion, involves a HER2-targeted iADC currently in a Phase I dose-escalation trial. In 2022, Merck KGaA dropped $30 million up front with another $800 million on the line to conjugate Mersana’s preclinical STING-agonist to Merck KgaA’s proprietary antibodies for up to two targets. Additionally, Astellas Pharma has invested $90 million up front with Sutro to develop iADCs for three targets. Sutro could receive an additional $422.5 million in milestone payments, plus royalties. Lowinger shared the story of a study Mersana conducted treating tumors in mice with the company’s STING iADC: their tumors disappeared. Mersana then rechallenged the mice, and in mice treated with an iADC, tumors would not regrow. “This allows for the hope and dream that there’s a chance to cure patients,” Lowinger said. “I think that’s the big differentiator and why there’s interest in this approach.” Kate Goodwin is a freelance life science writer based in Des Moines, Iowa. She can be reached at kate.goodwin@biospace.com and on LinkedIn.

Treatment of autoimmune diseases like lupus has long relied on steroids to knock down the immune system, but more targeted therapies are currently undergoing clinical trials. To make sure these therapies get to the patients who will benefit, work is needed to identify the specific mutations behind each patient's disease. Researchers now report several dozen mutations associated with oversensitive toll-like receptors -- a major cause of autoimmune disease -- and linked two mutations to patients. Lupus is a lifelong, often painful and occasionally lethal autoimmune disease. Few treatments exist today beyond powerful steroids to knock down a patient's immune system -- a therapy that has its own serious risks. The good news is that new and promising treatments are in clinical trials. But the term lupus belies the fact that the disease has a variety of causes, which means that treatments will have to be highly personalized to guarantee that each patient is given the drug that targets the specific genetic mutation responsible for their variety of lupus. Researchers are just now beginning to link specific genetic mutations to subsets of lupus patients, allowing physicians to target therapies to those who will benefit most. In the latest advance, researchers at the University of California, Berkeley, report in a new paper the discovery of two sets of patients with genetic mutations that are nearly identical to mutations that the researchers had earlier pinpointed in mouse and cell lines as linked to autoimmune disease. These two genetic links are among several dozen mutations that the UC Berkeley team recently discovered and linked to lupus, all in one gene that regulates a prime suspect in a subset of lupus patients -- proteins called toll-like receptors (TLR), which enable immune cells to recognize foreign DNA and RNA. According to study leader Gregory Barton, UC Berkeley professor of molecular and cell biology, identifying these mutations could help doctors deliver a personalized treatment to patients with oversensitive TLRs and, in particular, oversensitive TLR7 receptors. "We basically have a map now," said Barton, who is also an investigator in the Howard Hughes Medical Institute. "It's not like everybody that has lupus has a mutation in the gene that causes overactivation of TLRs and TLR7. But there are drugs coming online that very specifically inhibit TLR7. As we sequence more and more people, it will become easier to identify those patients and put them on those drugs. That's a lot better than the current course of therapy for lupus, which is brutal." "This is exciting because the drug will be orally available and is in clinical trials now," said Victoria Rael, a UC Berkeley graduate student who, with fellow graduate student Julian Yano, is a co-first author of the paper. The results of the genetic screens and details of the patients' mutations were published today (May 23) in the Journal of Experimental Medicine. A problem recognizing 'self' Autoimmune diseases, which range from rheumatoid arthritis and Crohn's disease to scleroderma and numerous thyroid conditions, stem from attacks by the immune system on the body's own cells that destroy normal, healthy tissue. Many studies have linked at least two types of autoimmune disease, lupus and psoriasis, to TLRs, which are part of the innate immune system that initially detects foreign invaders, such as viruses and bacteria, and stimulates a first line of attack. Normally, TLRs are delicately tuned to react only to foreign DNA and RNA, but if that tuning is off, they can react to a body's own nucleic acids and proteins associated with nucleic acids, which look much like those of pathogens. What makes this autoimmune reaction so deadly is that the TLRs also activate the body's second-line defense, the more powerful adaptive immune response, mobilizing T and B cells, macrophages, and other cells. These cells then mount a sustained attack that destroys the body's healthy tissue and causes chronic inflammation. The most common form, systemic lupus erythematosus (SLE), for example, is characterized initially by skin rashes -- in particular, a butterfly-shaped rash on the face -- but later by damage to joints, muscles, organs and skin, causing pain and fatigue. It's most commonly seen in females, often starting during the teen years. Lupus, in general, is two to three times more prevalent among women from many ethnic and racial minority groups than among white women. "We think the way the system works is that if nucleic acids find these receptors, most likely they're going to be from a virus," Barton said. "But in some people, the receptor is more responsive, so now levels of self-nucleic acids that otherwise wouldn't stimulate the receptor in a normal person activate the receptor. We think that one of the ways that these mutations are working is that they're making levels of self-nucleic acids that normally wouldn't be stimulatory, stimulatory." Barton and his lab colleagues have been investigating the role of TLRs that are misregulated in lupus, and in particular, one of the main proteins that regulates them: UNC93B1, or UNC for short. Several years ago, a team of postdoctoral fellows and graduate students in his lab screened in cell culture more than 100 genetic mutations in the UNC gene to see which ones overstimulated TLRs and would be good targets for further study. While they published some details in earlier papers, they didn't publish the complete list because there seemed to be little point -- almost no data was available on the genome sequences of lupus patients to compare with the mutations that overstimulated TLRs. But that has changed in recent years, thanks to a plunge in the cost of genome sequencing. That's how the mother of a young girl with severe autoimmune disease found Barton. Her daughter's DNA had been sequenced and showed a mutation in a region of UNC that Barton's team had noted in an earlier paper. Lupus in the family Rael and undergraduate Madeleine Weiss used the same cell culture screening technique to test the novel mutation from the young girl and found that it had an overstimulating effect, similar to the effect of other mutations in that area of the UNC gene. Surprisingly, the patient had the genetic mutation on only one of the two UNC alleles, meaning that she had one normal UNC gene, yet she still suffered severe autoimmune symptoms. Barton and his team also connected with a family of five afflicted with lupus. All had mutations on one UNC allele in another area of the UNC protein that Barton's team had previously identified. That mutation, when screened in cell lines, also produced overactive TLRs. "We were skeptical that just one copy of a gene would be sufficient to cause a disease," Rael said. "It wasn't until we put the patients' mutations into cell lines and saw that they led to very convincing TLR hyper-responsiveness that we realized they had the possibility of being sufficient to be disease-causing." Rael and Yano then repeated the screening work previously performed in the lab and confirmed that 32 distinct mutations in the UNC gene -- about one-third of the mutations tested -- increased the sensitivity of TLR7 to nucleic acids at least twofold. About another five mutants increased TLR7 sensitivity but to a lesser degree. Before these screens, only two mutations in the UNC protein had been linked to increased sensitivity of TLR7 in mice, though three additional human mutations were reported within the last two months. Barton is hopeful that by publishing the complete list of TLR hypersensitivity mutations, doctors can identify other lupus patients who could benefit from the anti-TLR drugs now in clinical trials. One drug, M5049, or Enpatoran, appears to work by latching onto two human receptors, TLR7 and TLR8, and preventing them from binding nucleic acids. Rael, Yano and other members of Barton's lab are investigating further how these unique UNC mutations affect the way a patient manifests the disease. They have recreated these patients' mutations in mice so that they can model human lupus. "With mouse models, you can start thinking about how, even though the mutations are in the same protein, the different mechanisms of TLR regulation break down, which immune cells get activated as a result, and how this can lead to differences in the symptoms patients suffer from," Rael said. The lab also is trying to understand how UNC tunes TLRs, which may be by regulating the number and arrangement of TLRs on immune cells. More TLRs may make a person more sensitive to the small number of self-nucleic acids circulating in the body. "UNC93B1 is important for getting the receptors to the place where they can function, but it also is important for regulating them when they get there," Barton said. "The protein is a very baroque way of trying to make decisions about whether the nucleic acid that you just bound to a TLR is from a virus or from one of your own cells." He hopes that physicians add this gene to the list of lupus-associated genes, "so if they see a mutation like these, even a heterozygous mutation, they will investigate further." Other senior authors of the paper are Bo Liu of the Chinese Academy of Sciences in Shanghai and Olivia Majer of the Max Planck Institute for Infection Biology in Berlin, Germany. The co-authors also include physicians from UC San Francisco, Stanford University, and hospitals in Missouri, North Carolina and Washington. The work was funded in part by the Lupus Research Institute, the Lupus Research Alliance and the National Institutes of Health (R01AI072429).

San Francisco-based Bolt Biotherapeutics on Tuesday said it is ending development of its lead cancer drug, laying off half its staff and changing up its leadership.Alongside announcing first quarter earnings, the biotechnology company revealed it will no longer advance its antibody-drug conjugate trastuzumab imbotolimod, or BDC-1001, after concluding the program was unlikely to meet its high bar for advancement.Instead, the company will focus on two earlier-stage candidates: BDC-3042, an agonist antibody now in Phase 1 testing, and BDC-4182, currently in preclinical evaluation. The latter is a product of Bolts immune-stimulating antibody conjugate, or ISAC, platform, which is meant to trigger anti-tumor immune responses. Its aimed at a protein called Claudin 18.2 thats becoming an increasingly popular target among cancer drug developers.The companys ISAC research will continue to be supported by collaborations with Genmab and Toray. The former is funding up to three ISAC candidates through early-stage development, while the latter is funding one ISAC candidate through Phase 1 testing.In a client note downgrading her rating on Bolts stock, Leerink Partners analyst Daina Graybosch described the two newly prioritized programs as at the high end of risk, given novelty and early nature of development. While Graybosch expressed confidence in the promise of Bolts platform, she expects multiple surprises and restarts to come.Bolt is laying off 50% of its workforce, or approximately 50 people, as part of its pipeline reprioritization. The company expects the cuts to extend its cash runway into the second half of 2026.Additionally, Willie Quinn, who was previously Bolts chief financial officer, has replaced Randal Schatzman as CEO. Chief Business Officer Grant Yonehiro will be promoted to chief operating officer, and Chief Medical Offier Edith Perez will move into an advisory role.Shares for Bolt fell around 34% in Wednesday morning trading. The company is now worth about $33 million. '