Ambros Therapeutics adds interim analysis to Phase 3 CRPS trial

Ambros Therapeutics has reached agreement with the US Food and Drug Administration on incorporating a formal interim analysis into its ongoing pivotal Phase 3 CRPS-RISE trial, which is evaluating neridronate in patients with complex regional pain syndrome type 1 (CRPS-1). The interim look will be conducted once half of the trial's approximately 270 participants have completed the study, and could allow the trial to stop early if the prespecified statistical threshold is met.

The company intends to amend the CRPS-RISE protocol to reflect the group sequential design, and will submit a revised statistical analysis plan and a data monitoring committee charter for FDA review. An independent data monitoring committee will govern the process.

Trial design and regulatory context

CRPS-RISE is a randomised, triple-blind, placebo-controlled study enrolling patients with warm CRPS-1 of six months' duration or less, with a confirmed diagnosis under the Budapest Clinical Criteria. Participants receive four intravenous infusions over ten days of either 100 mg neridronate (400 mg total) or placebo, with the primary endpoint measuring change in pain intensity from baseline to week 12 on an 11-point numerical rating scale.

Chief executive Jay Hagan said the interim analysis "gives CRPS-RISE the potential to reach a decision earlier than the full trial would otherwise require," citing the estimated 65,000 newly diagnosed US patients each year and the absence of any FDA-approved pharmacological therapy for the condition. Chief medical officer Gail Cawkwell added that the change builds on a body of earlier Italian and global studies and supports the company's planned FDA filing.

Neridronate is a bisphosphonate developed by Italian firm Abiogen Pharma and already approved in Italy for CRPS, osteogenesis imperfecta, and Paget's disease, with a reported patient exposure of around 600,000 across all indications. The FDA has granted the compound Breakthrough Therapy, Fast Track, and Orphan Drug designations for CRPS.

Market landscape and competitive picture

CRPS-1 sits in a niche that has long resisted pharmacological intervention. Current management relies on a patchwork of off-label agents, physiotherapy, and interventional pain procedures, none of which carry FDA approval for the indication. The orphan designation and the absence of approved competitors are meaningful commercial tailwinds for Ambros, though the condition's heterogeneity and the subjectivity of pain endpoints have historically complicated late-stage development in chronic pain.

Interim analyses with group sequential designs are increasingly common in rare-disease trials, where enrolment is slow and the ethical case for early stopping is strong. The FDA's willingness to engage on this amendment is consistent with the agency's broader accommodation of adaptive trial features under Breakthrough Therapy designation, which carries intensive regulatory guidance. Investors will now focus on the pace of enrolment and the projected timing of the interim readout, as well as on whether Ambros can secure the additional capital a clinical-stage company without approved products will require to complete the programme.

The CRPS-RISE registry and the planned open-label extension for completers with active disease are practical features that should support long-term outcomes data and a post-approval commitment package, both of which FDA typically requests for orphan indications entering the market without a comparator arm.