GSK and Spero Therapeutics have shared phase 3 results on their antibiotic candidate, revealing that the oral therapy closely matched the intravenous incumbent in a trial that could support filings for approval.
The partners stopped the study in hospitalized adults with complicated urinary tract infections (cUTIs) early for efficacy in May. At an interim analysis, GSK and Spero’s oral tebipenem HBr was non-inferior to intravenous imipenem-cilastatin on a primary endpoint that looked at clinical cure and microbiological eradication.
GSK and Spero used ID Week 2025 in Atlanta to share the data behind the primary endpoint success. The partners saw a 58.5% overall success rate in patients who took 600 mg of tebipenem HBr, compared to 60.2% in a cohort that received 500 mg of imipenem-cilastatin. The difference was within the trial’s 10% margin for non-inferiority, achieving the primary endpoint of the study.
Data on secondary endpoints add to the evidence that tebipenem HBr is as effective as the intravenous treatment. The partners said 93.5% of patients on tebipenem HBr were symptom-free, versus 95.2% of their counterparts on imipenem-cilastatin. Microbiological response rates were 60.3% in the tebipenem HBr group and 61.3% in the imipenem-cilastatin cohort.
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GSK and Spero said clinical and microbiological response rates in patients with antimicrobial-resistant Enterobacterales were consistent with data from the broader primary analysis population. The efficacy of tebipenem HBr against ESBL-producing Enterobacterales—which break down some antibiotics—and pathogens that have limited susceptibility to levofloxacin could influence the use of the drug candidate.
Under the terms of a deal that rescued the once-rejected tebipenem HBr, GSK will file for approval of the oral antibiotic. Back in 2022, after the FDA requested another phase 3 trial and denied Spero’s approval application, the Big Pharma scooped up rights to the drug for $66 million upfront.
Now, GSK’s regulatory submission is expected this year. One question is how the FDA, which has talked up the importance of including data from U.S. patients in filings, will respond to a study that relied heavily on sites in Eastern Europe. Spero’s earlier phase 3 also featured many sites in Eastern Europe.
If approved, tebipenem HBr could enable physicians to treat more cUTIs outside of hospitals. The current reliance on intravenous therapies means some patients are hospitalized for treatment. An effective oral option could free healthcare providers from the costs of treating patients in hospitals.
GSK is betting there is untapped demand for an oral option. Basilea Pharmaceutica has reached a similar conclusion, licensing an oral beta-lactam/beta-lactamase inhibitor for up to $325 million in August. The deal with Venatorx Pharmaceuticals gave the Swiss biotech rights to a phase 3-ready drug candidate. While GSK and Spero have a head start, Basilea sees potential advantages to its molecule.
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On an earnings call in August, Marc Engelhardt, M.D., chief medical officer at Basilea, named physicians’ concerns about the resistance potential of carbapenems and the pill burden of tebipenem HBr as areas where the biotech can differentiate its asset. Basilea’s candidate is given two to three times a day, versus four times a day for tebipenem HBr.
While GSK is handling the next steps for tebipenem HBr, the program’s fate still has implications for Spero. The biotech could receive up to $351 million in milestones tied to the oral antibiotic, starting with a $25 million payment when GSK files for approval in the U.S. Spero has forecast that milestones could fund its operations into 2028.
