Key Highlights

  • SPRO shares rose 15.05% to $2.43 in premarket trading after closing at $2.11.
  • The FDA approved Utebzi as the first oral carbapenem antibiotic available in the US.
  • Commercial success will depend on GSK’s launch execution, physician adoption and milestone economics for Spero.

FDA Approval Drives SPRO Stock Higher

Shares of Spero Therapeutics, Inc. (NASDAQ: SPRO) rose 15.05% to $2.43 in premarket trading today, June 18, 2026, after the US Food and Drug Administration approved Utebzi for certain adults with complicated urinary tract infections.

The stock had closed the previous session at $2.11, down 23.83%, after trading between $1.95 and $3.05 on volume of 11 million shares. The premarket rebound reflects renewed investor focus on the commercial value of Utebzi and Spero’s partnership with GSK.

The approval is a significant regulatory milestone for a company with a market capitalisation of approximately $122 million.

Utebzi Opens a New Oral Treatment Market

Utebzi, also known as tebipenem pivoxil, is the first and only oral carbapenem antibiotic approved in the US. It is intended for adults with complicated urinary tract infections, including pyelonephritis, who have limited or no alternative oral treatment options.

Carbapenems are commonly used against severe or drug-resistant infections, but they have historically required intravenous administration. An oral option could reduce reliance on hospital-based treatment and support wider outpatient use.

More than 3 million complicated urinary tract infections are treated annually in the US, while resistant infections affect roughly one-third of patients. This gives Utebzi a meaningful addressable market, although actual adoption will depend on prescribing patterns, reimbursement and competition.

Phase 3 Data Supported Approval

The approval was based on the PIVOT-PO Phase 3 trial, which showed that oral tebipenem pivoxil was non-inferior to intravenous imipenem-cilastatin.

Utebzi achieved an overall response rate of 58.5%, compared with 60.2% for the intravenous treatment. Its safety profile was broadly similar to other carbapenem antibiotics, with diarrhoea and headache among the most commonly reported adverse events.

The drug is expected to become available to US patients by the end of 2026.

GSK Partnership Shapes the Financial Upside

GSK holds exclusive global development and commercialisation rights outside selected Asian markets. Spero previously disclosed a partnership structure that includes potential development, regulatory and commercial milestone payments, alongside tiered royalties on future sales.

This limits Spero’s direct commercial burden but also means the company will depend heavily on GSK’s launch strategy and market execution.

Valuation and Risk Considerations

Spero reported trailing earnings per share of $0.27, giving the stock a displayed price-to-earnings ratio of about 7.8 at the previous close.

The main risks include slower-than-expected physician adoption, reimbursement constraints, antibiotic stewardship policies, manufacturing dependence and uncertainty around the timing of milestone and royalty income.

Conclusion

Spero Therapeutics stock is rising after a major FDA approval transformed Utebzi from a development-stage asset into a potential commercial product. The approval strengthens the company’s valuation case, but the durability of the rally will depend on GSK’s launch execution and the drug’s ability to gain market share.