Spero Therapeutics, Inc. (SPRO) stock intrinsic value fair value estimate & owner earnings analysis

Spero Therapeutics has one asset that matters today: it licensed its oral antibiotic tebipenem to GlaxoSmithKline (GSK) in 2022 and collects milestone payments from that deal. GSK paid $66.0 million upfront and $9.0 million for Spero shares, and later paid a $30.0 million milestone in 2023 and a $95.0 million milestone in four instalments from 2024 to 2025. In May 2025 the Phase 3 trial (PIVOT-PO) was stopped early because it met its goal, and Spero received a further $25.0 million in February 2026 when the FDA filing was made. GSK announced FDA approval of the drug, branded Utebzi, on June 17, 2026, for complicated urinary tract infections in adults, with US availability expected by the end of 2026. Spero has no marketed product of its own and its earlier antibiotic programs (SPR206 and SPR720) have been stopped. What the business earns: on our measure of owner earnings (operating cash flow with working-capital swings, stock compensation and capital spending removed), Spero lost about $0.45 a share in a typical year since the GSK deal (the middle of the four years 2022 to 2025, which were -$1.05, +$0.53, -$1.27 and +$0.15). Earlier years, 2018 to 2021, were all losses of $2.70 to $3.50 a share, when the company spent heavily on drug development. We measure from 2022 because the GSK license changed the business. Owner earnings rose from -$1.27 in 2024 to +$0.15 in 2025, but 2025 included $66.8 million of revenue from GSK milestones, which will not repeat at that scale, so we do not extend it forward. In the most recent quarter (June 2026) revenue was zero and the net loss was $9.6 million. The main change since the drug was approved is financing. In July 2026 Spero raised $105 million from HealthCare Royalty (a KKR business) by selling rights to a portion of the payments GSK owes it on Utebzi. The loan carries 10% interest and a nine-year term, and is generally repayable only from GSK proceeds. Spero keeps a 35% interest in GSK proceeds left after the required repayments. The GSK payments that are contracted and tied to the launch are $51.0 million at first US commercial sale and $25.0 million on the second anniversary of that sale, together $76.0 million, which is less than the $105 million borrowed plus interest. We therefore credit no further GSK income in any case beyond the cash already received. Royalties are only 1% of annual sales up to $750 million, and sales milestones begin only above $200 million in annual sales; we have no evidence yet of sales at those levels, so none are counted. We could not confirm from the filings how the July 2026 deal divides each milestone; those terms come from the company's announcement. Fees on the financing are not known and are not deducted. How we value it: with no earnings we can name and evidence, the shares are worth the cash the company holds less what it will spend before that cash is gone. Cash at June 30, 2026 was $50.8 million; adding the $105 million raised gives about $155.8 million, or roughly $2.69 a share on 58.0 million shares. Because the business consumes cash and we credit nothing to its new early-stage antibody program (an anti-CD40L medicine the company has said the financing will support), we subtract the present value of the spending at a 6% discount rate. We count the cash directly because the company spends more than it earns, so an earnings-based figure would miss the cash it already holds. Bear case, $0.19 a share: spending of about $62 million a year, so the cash is used up in roughly two and a half years, with no recovery from the new program and no GSK income to Spero beyond the near-term payments already sold. Base case, $0.24 a share: spending of about $48 million a year, which matches management's statement that cash lasts into the second half of 2029; the cash is used up in about three and a quarter years and nothing is credited after that. Bull case, $0.31 a share: spending stays near the current quarterly pace of roughly $36 million a year (Q2 2026 costs of $9.9 million, less about $0.8 million of stock compensation, times four), making the cash last about four and a quarter years. We do not include Utebzi sales, sales milestones, additional royalties or any value from the antibody program because none has evidence behind it yet. What would make this wrong: management could hold or return cash rather than spend it, Utebzi sales could exceed the $200 million a year at which sales milestones start, or the antibody program could succeed. None of these can be named and evidenced today, so they are upside we have not counted. The other direction is possible too: costs could rise as the antibody program advances, and the company would need new funding after 2029. Because the shares are those of a small company whose value rests on cash and a drug it does not control, the margin-of-safety price sits well below our base value.