Platinum-resistant ovarian cancer (PROC) is associated with poor prognosis and represents a significant unmet clinical need, as patients often progress on standard therapies and have few effective options. According to the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) Program, it was estimated that 20,890 women in the United States would be diagnosed with ovarian cancer and 12,730 would die from the disease. The five-year relative survival rate is estimated to be 51.6%. [1]
Claudin-6 (CLDN6), a tight junction protein minimally expressed in normal adult tissues but upregulated in several solid tumors—including ovarian cancer—has emerged as a promising therapeutic target.
Patients treated with QLS5132, a novel antibody-drug conjugate (ADC) developed by Qilu Pharmaceutical Co. experienced clinical benefit. This conclusion is based on results from a phase 1 clinical study presented at the annual meeting of the American Association for Cancer Research, held April 17 – 22, 2026, in San Diego, CA.[2]
QLS5132 consists of a humanized anti-CLDN6 IgG1 monoclonal antibody conjugated to a proprietary topoisomerase-1 inhibitor (QLS6916) via a hydrophilic, cleavable linker (LK1b) at a drug-to-antibody ratio of 8:1.
In a first-in-human, open-label, single-arm phase I study (NCT06932094) funded by Qilu Pharmaceutical Co., 28 patients with advanced PROC, who had progressed on or were ineligible for standard therapy, were enrolled. Patients received QLS5132 intravenously every three weeks at escalating doses (1.6 mg/kg, 3.2 mg/kg, 4.8 mg/kg, 5.6 mg/kg, and 6.4 mg/kg). The primary endpoints were safety, tolerability, and pharmacokinetics; secondary endpoints included preliminary efficacy measures such as objective response rate (ORR) and disease control rate (DCR). Dose escalation followed an accelerated titration design with a Bayesian optimal interval method.
“The primary purpose of this first-in-human study was to evaluate the safety, tolerability, and pharmacokinetic profile of QLS5132 in patients with platinum-resistant ovarian cancer and determine the recommended phase 2 dose (RP2D) for future clinical development,” explained Tao Zhu, MD, chief physician and vice president of Zhejiang Cancer Hospital in China, who presented the study results.
“Additionally, we aimed to assess preliminary antitumor activity to establish an early signal of clinical benefit in this heavily pretreated population with limited options,” he added.
Study Results
The median age of patients was 57.5 years; most had received ≥5 prior lines of therapy, and the majority had prior exposure to bevacizumab and PARP inhibitors. Treatment-related adverse events (TRAEs) were common in 26 patients (92.9%), with the most frequent being nausea (85.7%), anorexia (60.7%), anemia (53.6%), and weakness (53.6%). Grade ≥3 TRAEs occurred in 32.1% of patients—mainly hematologic toxicities—but there were no TRAEs leading to treatment discontinuation or death, and no cases of interstitial lung disease, ocular toxicity, or febrile neutropenia. A single case of grade 4 thrombocytopenia was observed at the highest dose.
After a median follow-up of 2.2 months, 9 patients achieved partial responses across various dose levels, including 2 patients without detectable CLDN6 expression. Among 18 efficacy-evaluable patients, the ORR and DCR were 50.0% and 94.4%, respectively. In patients receiving ≥3.2 mg/kg, the ORR and DCR were 52.9% and 100%, respectively. Declines in CA125 tumor marker levels were also observed. Importantly, responses were seen regardless of baseline CLDN6 expression, suggesting a possible bystander effect or tumor heterogeneity.
Clinical benefit
QLS5132 demonstrated a manageable safety profile and compelling antitumor activity in heavily pretreated PROC, with ORR exceeding 50% at recommended phase 2 dose levels. The lack of severe or unexpected toxicities is notable, as is the absence of treatment-limiting interstitial lung disease or ocular toxicity. The observation of clinical responses in patients with undetectable CLDN6 expression warrants further mechanistic exploration but may be explained by tumor heterogeneity and bystander killing.
“The most encouraging finding from our study was that QLS5132 demonstrated compelling antitumor
activity in patients with platinum-resistant ovarian cancer, with an objective response rate exceeding
50%,” Zhu said.
“Equally important, at the potential recommended phase II dose, we observed a
favorable safety profile with no reported cases of interstitial lung disease, ocular toxicity, oral mucositis, or febrile neutropenia,” he added.
Zhu also noted that, though more research would be needed to confirm, preliminary data indicated antitumor activity from QLS5132 regardless of CLDN6 expression levels—which, he said, could expand its potential as a treatment option for a broad cohort of patients with platinum-resistant ovarian cancer
Preclinical studies have supported the rationale for CLDN6 as an ADC target, with QLS5132 showing superior efficacy and tolerability compared to earlier CLDN6-targeted ADCs, such as TORL-1-23, and demonstrating potential synergy with PARP inhibitors. The current results support further clinical development of QLS5132, both as monotherapy and in combination regimens for patients with CLDN6-positive or heterogeneous PROC.[3]
Based on preliminary phase 1 study results, QLS5132 is a promising new ADC for advanced platinum-resistant ovarian cancer, demonstrating a favorable safety profile and encouraging efficacy in a patient population with limited treatment options. The results from this phase 1 study justify phase 2/3 development, with the potential to expand the benefit of CLDN6-targeted therapy to a broader cohort of ovarian cancer patients.
Study limitations
Given the small sample size and an exploratory single-arm design, Zhu acknowledged that further research would be needed to fully understand why QLS5132 can have anticancer effects in patients with undetectable CLDN6 tumoral expression. But he suggested that this phenomenon may have several explanations, including tumor heterogeneity and a potent bystander effect, resulting in antitumor efficacy even in cells with low or no CLDN6 expression.
“These findings support the advancement of QLS5132 into phase 3 studies, with the goal of providing a much-needed new treatment option for these patients,” Zhu concluded.
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Clinical trials
Phase 1 Clinical Study of QLS5132 Monotherapy in Advanced Solid Tumors – ClinicalTrials.gov ID NCT06932094
QLS5132 Combination Therapy in Advanced Solid Tumors – ClinicalTrials.gov ID NCT07453394
References
[1] Cancer Stat Facts: Ovarian Cancer. Epidemiology, and End Results (SEER) Program, National Cancer Institute. Online. Last accessed on April 19, 2026.
[2] Xiang Y, Song Z, Li J, Chen L, Sun Y, Zou D, Li Y, Zhang S, Ding Y, Gu Y, Liu X, Zhang T, Kang X, Tao W, Zhu T. A first-in-human phase 1 trial of a novel claudin 6 (CLDN6)-targeting antibody drug conjugate (ADC) QLS5132 in patients (pts) with platinum-resistant ovarian cancer (PROC). In: Proceedings of the 117th Annual Meeting of the American Association for Cancer Research; 2026 April 17-22; San Diego, CA.: AACR; 2026. Abstract nr CT037
[3] Huang Y, Sun Z, Zhao S, Wang X, Zhang L, Yang Y, Ying H, Tao W. QLS5132, a highly selective anti-CLDN6 ADC with broader therapeutic window [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6750.
Featured image © 2016 – 2026 AACR. Used with permission.
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