Sage Therapeutics has successfully resolved a rare seizure disorder among the majority of patients in its small but closely watched Phase I/II study, adding more promising outcomes to the early evidence that helped inspire one of this year's most successful biotech IPOs. In the data update out Monday morning, investigators say the 73% overall response rate in their study is also being mirrored among a small group of patients who are receiving the drug on an emergency-use basis.

The Cambridge, MA-based biotech's ($SAGE) lead drug is SAGE-547, an intravenous treatment designed to halt "super-refractory" status epilepticus (SRSE), a persistent brain seizure that--on the extreme end--currently may require a medically induced coma to stop its course. SAGE-547 targets GABA A receptors in the brain to prevent the seizures, and the biotech says that 11 evaluable patients were "successfully weaned off their anesthetic agents while SAGE-547 was being administered, and eight patients were successfully weaned off SAGE-547 without recurrence of SRSE. The mean duration of status epilepticus prior to treatment with SAGE-547 was 11 days. With an overall response rate of 73%, SAGE-547 was generally well tolerated and no drug-related serious adverse events" were reported.

Sage picked up fast-track status for the drug in July, just after completing an IPO that raised $103 million. The plan now is to adapt their trial to expand the patient population, including kids who currently have no treatment options. In a group of 7 emergency-use cases involving patients with a mean age of 12.5, 5 had their SRSE conditions resolved after treatment, a 71% response rate.

Sage CEO Jeff Jonas

"We plan to keep on enrolling these patients," Sage CEO Jeffrey Jonas tells FierceBiotech. Jonas has an end-of-Phase II meeting scheduled with the FDA and Phase III will begin next year, he adds, possibly by continuing to add more patients aged two and older to the ongoing study.

Sage wants to keep the U.S. rights to the drug, aiming for an approval to launch by 2018--a conservative estimate that could be improved on if the FDA helps accelerate the development effort. The CEO estimates that the U.S. market of treatment-resistant patients is probably around 25,000. There are ongoing discussions about licensing ex-U.S. rights, adds Jonas, but no firm plans on that score as of yet.

The upbeat news today did little for the company's stock price, which ticked up 3% in morning trading.

Sage emerged from Third Rock's stable of biotech upstarts, successfully pursuing a rare disease strategy that has characterized a big group of the latest generation of biotech launches to come along. Rare diseases offer developers a chance to mount small studies that can deliver solid efficacy results with just a handful of patients. And adaptive designs like this Phase I/II, which help blur the lines between different stages of development, allow biotechs to go after efficacy endpoints while establishing the safety of a new therapy.

The biotech also has a pair of preclinical drugs in development, which it calls second generation neurosteroid pills, which could make dosing much easier for patients. In addition to GABA A ,Sage has been working on new NMDA regulators as well, another big target in the CNS field.

"We are pleased that we were able to complete this portion of our development plan ahead of our projected timelines and would like to thank all of our investigators, patients and their families involved in this trial," commented Dr. Steve Kanes, chief medical officer of SAGE. "We are also pleased that the approved protocol amendment to our Phase I/II trial will enable us to explore the potential of SAGE-547 in a broader population, particularly in very young children affected with this disorder that have no other treatment options."

- here's the release