On January 8, 2025 Vir Biotechnology, Inc. (Nasdaq: VIR) reported initial Phase 1 data from two of its dual-masked T-cell engagers (TCEs): VIR-5818, targeting a variety of HER2-expressing solid tumors; and VIR-5500, targeting PSMA in metastatic castration-resistant prostate cancer (mCRPC) (Press release, Vir Biotechnology, JAN 8, 2025, View Source [SID1234649526]). Data show encouraging preliminary safety and efficacy profiles with no dose-limiting cytokine release syndrome (CRS), maximum tolerated dose (MTD) not yet reached as dose escalation continues, and early clinical response signals observed in heavily pretreated participants. These initial results provide clinical support for Vir Biotechnology’s in-licensed PRO-XTEN masking technology, which is designed to enable the selective activation of TCEs in the tumor microenvironment, mitigating damage to healthy cells and reducing toxicity.
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"Overcoming the toxicity-driven limitations of traditional T-cell engagers could address an important unmet medical need in cancer care," said Marianne De Backer, M.Sc., Ph.D., MBA, Chief Executive Officer, Vir Biotechnology. "Preliminary safety and efficacy data for our dual-masked T-cell engagers VIR-5818 and VIR-5500 are compelling, and we will continue dose escalation with an opportunity to expand the therapeutic window. We are encouraged that our candidates may enable efficacious and well-tolerated treatment regimens, potentially improving outcomes for people living with a range of solid tumors."
VIR-5818: PRO-XTEN Initial Proof-of-Concept and Potential First-in-Class HER2 Immunotherapy
Despite availability of HER2-targeting therapies, there remains a significant unmet need for treatments with novel mechanisms of action to improve tolerability and extend survival. Currently, no HER2-directed immunotherapies are approved for solid tumors. The preliminary safety and efficacy data of VIR-5818 support the tumor-specific activation of PRO-XTEN dual-masked TCEs and the potential of this technology to broaden the therapeutic index of TCEs.
VIR-5818 is being evaluated in a Phase 1 clinical trial (NCT05356741) designed to study its safety and pharmacokinetics alone, and in combination with pembrolizumab, in participants with a variety of HER2-expressing cancers, including breast and colorectal cancer (CRC). The study has enrolled 79 heterogeneous and heavily pretreated participants in monotherapy cohorts.
Early efficacy data indicate that 50% (10/20) of participants receiving VIR-5818 doses ≥400 µg/kg experienced dose-dependent tumor shrinkage across multiple HER2-positive tumor types. This includes participants who had received up to 9 prior lines of therapy. Strong anti-tumor activity was observed in a subset of participants with HER2-positive CRC who have exhausted standard of care. In this subset, confirmed partial responses (cPRs) were seen in 33% (2/6) of participants at early doses, and one patient continued in cPR for more than 18 months as of the data cut-off.
Preliminary safety data demonstrate that VIR-5818 is generally well-tolerated, with minimal grade 1 or 2 CRS (20% and 10%, respectively) and no grade 3 or greater CRS observed in any of the 79 participants across doses up to 1000 µg/kg. Most treatment-emergent adverse events (TEAEs) were low grade, reversible and manageable. The MTD has not yet been reached. Preliminary pharmacokinetics and safety data indicate low systemic unmasking of the TCE, suggesting tumor-specific activation. Dual masking results in a half-life of approximately 6 days, which may enable a less frequent dosing regimen. As a result, Vir Biotechnology is currently evaluating a Q3W dosing regimen.
VIR-5500: First Dual-Masked PSMA-Targeting TCE
Prostate cancer is the second most diagnosed cancer in men1. Despite advancements, there is still a significant unmet need for efficacious, well-tolerated treatments that can extend survival and improve quality of life.
VIR-5500 is being evaluated in a Phase 1 clinical trial (NCT05997615) designed to assess its safety, pharmacokinetics, and preliminary efficacy in participants with mCRPC. The study has enrolled 18 participants with significant disease burden who have received 3 to 6 prior lines of therapy.
Early efficacy data show encouraging signs of prostate-specific antigen (PSA) responses, and PSA reductions were observed in 100% (12/12) of participants after an initial dose ≥120 µg/kg. PSA50 response was confirmed in 58% (7/12) of participants receiving a first dose ≥120 µg/kg.
Preliminary data show a promising safety profile, with no dose-limiting toxicities observed up to 1000 µg/kg without prophylactic corticosteroids. Safety findings showed minimal grade 1 or 2 CRS (17% and 11%, respectively) and no grade 3 or greater CRS at any dose. Most TEAEs were low grade. No hearing loss has been reported, suggesting safety benefits of dual masking in preventing on-target, off-tumor toxicities. Dose escalation is ongoing, and the MTD for VIR-5500 has not yet been reached. Preliminary pharmacokinetics and safety data indicate tumor-specific activation with minimal unmasking outside the tumor. The dual-masked TCE shows a desirable half-life of 8-10 days, which is enabling Vir Biotechnology to evaluate a Q3W dosing regimen. The safety and tolerability profile observed for VIR-5500 in ongoing dose escalation, together with the observed signs of early anti-tumor activity at low doses, may enable a wide therapeutic index.
About VIR-5818, VIR-5500, VIR-5525
VIR-5818, VIR-5500, VIR-5525 are investigational, clinical candidates currently being evaluated for the treatment of solid tumors. These assets leverage the PRO-XTEN masking technology with three different T-cell engagers (TCEs) targeting HER2, PSMA, and EGFR, respectively.
TCEs are powerful anti-tumor agents that can direct the immune system, specifically T-cells, to destroy cancer cells. The PRO-XTEN masking technology is designed to keep the TCEs inactive (or masked) until they reach the tumor microenvironment, where tumor-specific proteases cleave off the mask and activate the TCEs, leading to killing of cancer cells. By driving the activity exclusively to the tumor microenvironment, we aim to circumvent the traditionally high toxicity associated with TCEs and increase their efficacy and tolerability. Additionally, the mask is designed to help drug candidates stay in the bloodstream longer in their inactive form, allowing them to better reach the site of action and potentially allowing less frequent dosing regimens for patients and clinicians.