Theseus Pharmaceuticals has terminated its pan-variant tyrosine kinase receptor (KIT) inhibitor (THE-630) drug development programme and discontinued enrolment in the corresponding Phase I/II trial to treat patients with gastrointestinal stromal tumours (GIST).

The programme termination announcement was preceded by sizable increase in share volume (3.3m increase in share volume from the previous day), with a 4% decline in Theseus’ prices at market close on 13 July, compared to the start of the day.

The company has reported cash reserves totalling approximately $234m as of 30 June 2023, which is expected to support its operations into 2026.

Theseus CEO Tim Clackson said in a press release: “We are disappointed that we will not be able to achieve the target exposure for pan-variant inhibition with THE-630, as we continue to believe a therapy with potent activity against all major classes of activating and resistance mutations in KIT has the potential to confer significant clinical benefit, given the unmet need in GIST,

“We remain committed to helping GIST patients with plans to nominate a new, highly selective pan-variant KIT inhibitor candidate for GIST in the first half of 2024. Moving forward, we are excited to have THE-349 as our next near-term clinical programme, with its potential best-in-class profile as a fourth-generation epidermal growth factor receptor (EGFR) inhibitor appropriate for both monotherapy and combination approaches.”  

In the open-label, non-randomised Phase I/II trial (NCT05160168), two out of six patients experienced dose-related toxicity (DRT). These patients were in the cohort that received a dose above 27mg, which Theseus believes is the minimum dose required to achieve efficacy.

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Overall, nine out of the 32 patients have reported a hand-foot skin reaction (HFSR), which is commonly associated with multikinase inhibitors such as THE-630. Six of these patients were in the 27mg cohort.

Although the company has discontinued trial enrolment, current trial participants would still receive treatment until a treatment discontinuation criterion is met such as DRT.

Theseus also released a list of the company’s strategic priorities, including plans for an investigational new drug application (IND) application for THE-349 with the US Food and Drug Administration (FDA) in the last quarter of 2023. THE-349 is an EGFR TKI inhibitor for the treatment of EGFR mutant non-small cell lung cancer.