Boundless Bio has announced a collaboration with Eli Lilly for its first-in-human Phase I/II trial investigating BBI-355 in solid tumours.

Under the clinical trial collaboration and supply agreement, Lilly will provide its CDK4/6 inhibitor Verzenio (abemaciclib) as a clinical drug supply at no cost for the trial.

This marks Boundless Bio's second collaboration announced this week after the US-based company made a similar deal with Taiho Oncology for its pan-FGFR inhibitor Lytgobi (futibatinib).

BBI-335 is an orally administered selective checkpoint kinase 1 (CHK1) inhibitor. According to Boundless Bio, this is the first extrachromosomal DNA (ecDNA)-directed therapy in development for oncogene-amplified cancers.

Earlier this year, the company raised $100m in an oversubscribed series C financing round. The funds will be used to fund the initial clinical development of BBI-355.

Phase I/II trial design

The Phase I/II trial (NCT05827614) will investigate BBI-355 as a single agent and in combination with certain selected therapies such as Verzenio and Lytgobi in patients with specific oncogene-amplified tumours.

The Verzenio combo therapy will be administered to participants with CDK4 or CDK6 tumour amplifications whereas patients with FGFR application will receive combo treatment with Lytgobi.

The trial consists of three parts: dose escalation and expansion of BBI-355 as a single agent, dose escalation of BBI-355 in combination with other therapies, and combo therapy dose expansion with specific oncogene amplification on ecDNA. The third part will implement an ecDNA diagnostic clinical trial assay to determine the presence of ecDNA in tumour samples.

The trial’s primary endpoints will measure the frequency and severity of treatment-emergent adverse events and determine the maximum tolerated dose and/or recommended Phase II dose.

According to the ClinicalTrials.gov registry, the open-label and non-randomised trial plans to enrol 47 patients whose disease has progressed despite all standard therapies or who have no other treatment options.

Boundless Bio has listed several oncology indications in the trial registry, including non-small cell lung cancer and adenocarcinoma, oesophageal, gastric, breast and ovarian cancers.

In May, Boundless Bio dosed the first patient in the trial. The trial’s estimated completion day is January 2026, according to ClinicalTrials.gov.