Treeline opens Phase I trial of TLN-499 plus venetoclax in BCL-2-dependent solid tumors

Massachusetts-based biotech Treeline Biosciences, Inc. has registered a first-in-human Phase I clinical trial of TLN-499, an oral BCL-XL protein degrader, in combination with AbbVie's venetoclax (Venclexta) for patients with relapsed or refractory solid tumors, including small cell lung cancer (SCLC), neuroendocrine cancer, malignant pleural mesothelioma, and synovial sarcoma. The study (NCT07713732) is scheduled to begin enrolling in Q3 2026 across three Australian sites. The trial registration follows Treeline's June announcement that TLN-499 would enter the clinic in 2026 as part of its pipeline of protein degraders and targeted oncology therapies.

The open-label, multicenter, single-arm trial is structured in two parts: a dose-escalation phase followed by a dose-expansion phase, enrolling up to 120 adults with histologically confirmed relapsed or refractory disease and ECOG performance status of 0–1. The protocol lists overall response rate (ORR) alongside safety as co-primary endpoints—an uncommon feature for a Phase I study that suggests Treeline is seeking an early efficacy signal across multiple tumor types. Secondary endpoints include duration of response, progression-free survival, overall survival, and pharmacokinetic parameters. Primary completion is projected for August 2030.

TLN-499 selectively degrades BCL-XL, an anti-apoptotic protein that enables many solid tumors to evade programmed cell death. According to Treeline, selective degradation of BCL-XL is intended to enhance the activity of chemotherapy and targeted therapies while avoiding some of the liabilities associated with broader inhibition of the apoptotic pathway. The combination with venetoclax provides a mechanistic rationale by simultaneously targeting two complementary pro-survival proteins, BCL-XL and BCL-2, which are frequently co-expressed in tumors and can mediate resistance when either pathway is inhibited alone.

The selected basket of SCLC, neuroendocrine cancers, mesothelioma, and synovial sarcoma is consistent with this biology. These malignancies have been reported to depend on BCL-2 family proteins for survival and generally have limited treatment options after progression on standard therapies. Venetoclax has shown only modest activity as monotherapy in solid tumors, and dual targeting of BCL-2 and BCL-XL has been explored as a strategy to restore apoptotic sensitivity in resistant disease.

Competition in this area remains early. While BeOne Medicines' sonrotoclax (BGB-11417) is advancing as a next-generation BCL-2 inhibitor, its primary development remains in hematologic malignancies. Treeline's approach differs by combining venetoclax with selective degradation of BCL-XL rather than developing another direct BCL-2 inhibitor, potentially addressing a broader spectrum of apoptotic resistance mechanisms in solid tumors.

TLN-499 forms part of Treeline's expanding oncology pipeline, which also includes the Phase I BCL6 degrader TLN-121, the pan-KRAS inhibitor TLN-372, and the EZH2 inhibitor TLN-254. The company is currently progressing its proposed all-stock merger with Standard BioTools Inc., which is expected to provide the combined company with more than USD 900 million in pro forma cash to support pipeline development into 2029.

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