Ultragenyx Pharmaceutical has announced that the US Food and Drug Administration (FDA) reviewed and agreed to amend the protocol of its Phase I/II trial of GTX-102 to treat paediatric patients with Angelman syndrome.

The amended protocol allows Ultragenyx to match dose ranges in the US with those being used in the expansion cohorts outside of the US.

Excluding the US, the trial has been adding and dosing patients in the expansion cohorts to confirm the GTX-102 dose and treatment regimen for the Phase III programme.

Ex-US portion of the trial covers sites in Europe, Canada and Australia.

The trial will include around 40 patients in the expansion cohorts. The US patients who began the trial on a 2mg dose or in the comparator group will start dosing as per the amended protocol.

The Phase I/II open-label, dose-escalating trial is examining GTX-102’s safety and tolerability in paediatric patients with Angelman syndrome with a genetically proven diagnosis of full maternal UBE3A gene removal.

It will also identify the clinical response as measured by a panel of efficacy evaluations for the functional domains affected in Angelman syndrome.

Ultragenyx Pharmaceutical senior vice-president and neurology clinical development chief medical officer Scott Stromatt said: “Agreement on the protocol amendment enables comparable dose ranges across all geographies and allows us to move forward rapidly to complete the study.

“We have begun working urgently to activate multiple study sites in the US and plan to begin enrolment as quickly as possible.”