Repare Therapeutics has reported monotherapy dose selection and safety results from its Phase I/II TRESR clinical trial of RP-3500 for its potential to treat solid tumours. 

RP-3500 is a potent and selective oral small molecule Ataxia-Telangiectasia and Rad3-related protein kinase (ATR) inhibitor.

It is being developed to treat solid tumours that have certain synthetic-lethal genomic alterations, including those in the Ataxia-Telangiectasia mutated kinase (ATM) gene.

The first-in-human, open-label, multicentre, dose-escalation and expansion trial has been designed to analyse safety and pharmacokinetics as well as to detect the initial anti-tumour activity of RP-3500, administered as a monotherapy and along with talazoparib.

The recommended Phase II dose (RP2D) and schedule of the therapy will be chosen based on this trial.

Furthermore, the trial assessed biomarker responses and their associations with antitumor activity of RP-3500.

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The safety results comprise a complete safety assessment from three monotherapy dosing schedules and demonstrated that RP-3500 is well tolerated.

This safety assessment validated the acceptable tolerability of RP2D 160mg RP-3500 given for three days and after a gap of four days.

The most commonly observed toxicity was anaemia with under a quarter of the subjects having Grade-3 toxicities.

To alleviate the on-target toxicity of anaemia and keep subjects on an RP-3500 dosing schedule that acts on antitumor activity, a plan to modify doses with two different dosing schedules was created.

Furthermore, a nomogram, based on toxicities evaluated in cycle 1, is being investigated for identifying subjects who are at greater anaemia risk and suggest intervention at an early stage.

Repare Therapeutics chief medical officer Maria Koehler said: “The dose optimisation approach used in the trial provides robust evidence to support the recommended Phase II dose and schedule in our ongoing trials. 

“We believe these data confirm a well-differentiated and likely best-in-class profile of RP-3500. 

“We look forward to presenting updated clinical data from 120 patients enrolled in Phase I/II TRESR trial in the first half of this year.”

Apart from RP-3500, the company’s pipeline comprises a CCNE1-SL inhibitor, RP-6306, and a Polθ inhibitor programme.