Fentanyl-Removal Drug CS-1103 Meets All Endpoints in Phase 2 Trial

A Cambridge, Massachusetts, biotechnology company reported Thursday that an experimental injectable drug pulled fentanyl out of the human body roughly 25 times faster than it would otherwise be cleared, an early-stage result that could eventually give emergency physicians a second instrument for overdoses that outlast the standard antidote.

Clear Scientific, Inc. said its Phase 2 study of CS-1103 met all endpoints. The compound is a small-molecule sequestrant — a drug designed to bind fentanyl in the bloodstream and inactivate it, then carry it into urine rather than merely displacing it from opioid receptors. The company said the trial showed a favorable safety profile alongside the accelerated clearance, and that results were consistent with earlier non-clinical work in which the drug rapidly reversed synthetic-opioid respiratory depression.

What the trial actually measured

The study was a single-site, double-blind, randomized, inpatient pharmacokinetic and clearance trial that enrolled 16 healthy adults, the company said. Participants received fentanyl and naloxone — reproducing, in sequence, what happens in a real overdose and its rescue — and were then given either CS-1103 or a placebo. A single intravenous dose was well tolerated, no serious adverse events were reported, and all 16 volunteers completed the study.

ClinicalTrials.gov lists the protocol as NCT07459166, describing a Phase 2 design that evaluates safety, tolerability, pharmacokinetics and efficacy of a single intravenous dose of CS-1103 following a fentanyl challenge under naloxone blockade.

Removal, not just displacement

Naloxone, the overdose-reversal drug carried by first responders, stocked in pharmacies and available over the counter, works by competing with opioids at the receptor. It does not remove fentanyl from the body, a mismatch that has become more consequential as illicit supplies shifted toward the synthetic opioid. Clear Scientific's announcement points to three limitations that follow from that: re-narcotization, where the fentanyl poisoning outlasts the naloxone blockade and breathing stops again; polydrug exposure, where naloxone is less effective when methamphetamine, xylazine or cocaine are present; and precipitated withdrawal, the abrupt and severe onset of symptoms when an antagonist displaces an opioid faster than the body would have cleared it.

That last effect is not merely uncomfortable. Precipitated withdrawal makes the transition into medication-assisted treatment harder and riskier, precisely at the moment a patient is most reachable.

"Fentanyl in the body makes rescue and transition to treatment significantly more challenging," said Shekar Shetty, chief executive of Clear Scientific. "By removing fentanyl from the plasma and brain rather than replacing it, as with receptor antagonists, CS-1103 could help prevent potentially fatal re-narcotization and reduce or eliminate precipitated withdrawal enabling faster, safer transitions to addiction treatment, such as buprenorphine and methadone."

An early result, not a rescue-ready product

The caveats matter. Sixteen healthy volunteers are not patients in overdose, and clearance kinetics in a controlled inpatient setting are not the same as resuscitation on a sidewalk. The figures released this week are top-line and company-reported; they have not appeared in a peer-reviewed journal. CS-1103 is investigational and has not been cleared by any regulator, and the company says it plans to begin Phase 3 testing in the first half of 2027.

The program is supported by the National Institute on Drug Abuse under award number UG3DA059286-01. Clear Scientific was co-founded in 2019 by George M. Whitesides of Harvard University, with technology licensed from Massachusetts General Hospital and the University of Maryland. The company is initially aiming at fentanyl, methamphetamine and cocaine, which it says appear in close to 95 percent of U.S. drug-related fatalities.

The numbers behind the urgency

The company cited 2024 estimates that synthetic opioids were associated with approximately 1 million emergency department visits and more than 55,000 deaths, and that related hospital costs exceed $30 billion a year. It also pointed to the White House's estimate that the fentanyl-fueled crisis cost the U.S. economy nearly $2.7 trillion in 2023.

Interventions that move patients from an emergency room into durable care have become a central focus of the field, from hospital-based initiation of buprenorphine to post-overdose peer programs. For now, naloxone remains the only widely distributed reversal agent, and it stays effective in the majority of fentanyl overdoses when given promptly and in sufficient doses — which is why public health agencies continue to press for broader distribution even as researchers look for something that works longer.

Whether a sequestrant becomes that second instrument depends on trials that have not yet been run. The Phase 2 data released this week are, for now, a proof of mechanism: evidence that the molecule does in the body what its inventors designed it to do.