Pegenzileukin

Pegenzileukin (SAR444245) is Sanofi's pegylated 'non-alpha' IL-2, the lead asset from its $2.5B 2020 acquisition of Synthorx [1]. The drug was engineered to bypass the immunosuppressive arm of IL-2 signaling and stimulate cancer-killing T cells without the toxicity that has limited first-generation IL-2 therapy. The most recent disclosed data is a Phase 1/2 evaluation combined with Regeneron's cemiplimab in advanced unresectable or metastatic skin cancers, published in J Immunother Cancer in 2026 [2]. The readout matters as a class verdict: every meaningful non-alpha IL-2 program in the past five years has underperformed, including Nektar's bempegaldesleukin, which Bristol-Myers Squibb paid $1.85B upfront for in 2021 and then walked away from after multiple Phase 3 failures across melanoma, renal cell, lung, and bladder [3]. Sanofi has not publicly committed to a pegenzileukin Phase 3 program; in October 2022 Sanofi took an approximately $1.6B impairment against the Synthorx acquisition after Phase 2 efficacy came in below internal projections [8]. With ~$47B in 2024 revenue (€44.3B) [4], Sanofi can sustain quiet development indefinitely, but the asset has slipped well behind Sarclisa (isatuximab) and externally partnered ADC programs in the company's oncology priority stack.

Novel compound, never approved. The asset traveled from THOR-707 (Synthorx pre-acquisition) to SAR444245 (Sanofi internal) to pegenzileukin - its official international nonproprietary name (INN/USAN). RxNorm assigns it CUI 2629296. Synthorx was acquired by Sanofi in January 2020 for approximately $2.5 billion, with pegenzileukin as the centerpiece of the deal rationale [1]. The Pegathor program ran multiple Phase 2 cohorts across solid tumors, melanoma, and gastrointestinal cancers - Pegathor Gastrointestinal 203 (NCT05104567) enrolled 138 patients and is listed as completed, with objective response rate as the primary endpoint [5]. Sanofi has been notably quiet on the asset since 2023; the drug has not been featured as a near-term value driver on earnings calls or in R&D day decks, and Sanofi recorded an approximately $1.6B impairment against the Synthorx acquisition in Q3 2022 after disappointing Phase 2 efficacy [8]. The 2026 J Immunother Cancer publication of Phase 1/2 data in skin cancers paired with cemiplimab is the most recent disclosed clinical effort [2]. No FDA designations - no breakthrough therapy, no fast track, no orphan, no priority review. No publicly committed key readout. Patent and market exclusivity terms have not been independently disclosed in public Sanofi filings beyond standard biologic protection. Note: the source database tagged this node with NCT04643002, which is actually Sanofi's isatuximab master protocol in relapsed multiple myeloma, not a pegenzileukin trial [6]. The relevant identifier is NCT05104567.

IL-2 is a cytokine, a signaling protein the immune system uses to wake up T cells. Aldesleukin, the original recombinant IL-2 from the 1990s, can shrink metastatic melanoma and kidney cancer but is brutally toxic because it activates three different IL-2 receptor configurations. The high-affinity receptor (which includes the 'alpha' subunit, IL-2Rα/CD25) sits on regulatory T cells, the immune system's own brake pedal, so wild-type IL-2 ends up activating the very cells that suppress anti-tumor immunity, while also causing vascular leak syndrome that lands patients in ICUs. Pegenzileukin uses Synthorx's expanded genetic alphabet technology to incorporate a synthetic amino acid into IL-2 at a position that prevents binding to the alpha subunit, while pegylation extends the drug's half-life enough to allow dosing every few weeks instead of multiple times per day [1]. In theory, the drug preferentially activates cytotoxic CD8 T cells and NK cells via the beta-gamma receptor pair found on cancer-killing immune subsets. The mechanism is biologically clean and the in vivo preclinical data was strong enough to attract a $2.5B acquisition. But the validating clinical evidence has been weak - every drug in this class has underperformed against the hypothesis that selective β/γ engagement plus checkpoint blockade produces durable, deep responses.

The most informative disclosed dataset is the Phase 1/2 evaluation of pegenzileukin combined with cemiplimab in advanced unresectable or metastatic skin cancers, published in J Immunother Cancer in 2026 by Rojas, Mortier, Park and colleagues [2]. Cemiplimab is Regeneron's anti-PD-1 antibody, approved for cutaneous squamous cell carcinoma and basal cell carcinoma - pairing it with an IL-2 boost tests whether you can amplify or rescue responses in PD-1-experienced patients. The benchmark to beat: cemiplimab monotherapy achieves approximately 44-47% ORR in advanced CSCC across EMPOWER-CSCC-1 cohorts (47.2% in groups 1-3 at long-term follow-up; 44.8% in fixed-dose group 6) [9]. A meaningful pegenzileukin contribution would require combination ORR clearly exceeding this baseline in a PD-1-experienced population - anything within the monotherapy confidence interval is uninterpretable as evidence of cytokine-driven benefit. The Pegathor Gastrointestinal 203 trial (NCT05104567) was a Phase 2 master protocol that enrolled 138 patients across GI tumors, with cohort-level ORR as the primary endpoint; the trial is listed as completed but Sanofi has not headlined topline results [5]. No Phase 3 trial has been initiated against any tumor type. The trial designs themselves are reasonable for early-stage IL-2 combination work - single-arm, biomarker-light, checkpoint-paired - but they were not powered to support direct accelerated-approval submissions, and the choice of indications (skin cancer combo with cemiplimab, GI cohorts) suggests Sanofi was hunting for a signal rather than betting on a specific tumor type.

Our model estimates a 15% chance this drug is eventually approved. It starts from the historical base rate for Phase 2 drugs in this area (about 21%), then adjusts using ten facts about the trial and sponsor. What moves the number most: it is helped by more secondary endpoints than usual, its light or open-label blinding, and larger-than-typical enrollment for this phase; it is held back by weak or limited earlier-phase results. The other facts land near average for this stage, so they leave the estimate roughly where the base rate put it.

Efficacy risk dominates. Bempegaldesleukin was the most clinically validated non-alpha IL-2, and it failed across five Phase 3 trials. The basic biology rationale, preferential CD8 stimulation via β/γ receptor signaling, did not translate into meaningful added response on top of checkpoint inhibitors [3]. Pegenzileukin has no obvious differentiator from bempegaldesleukin at the receptor-binding level - both are engineered IL-2 variants (muteins - proteins with altered amino acids designed to change binding behavior) built for the same selectivity profile. On-target toxicity remains real: pyrexia, hypotension, and capillary leak persist even with alpha-sparing designs because IL-2 signaling itself drives inflammatory cytokine release regardless of which receptor catches the ligand. Combination with anti-PD-1 is the standard approach, but if the IL-2 doesn't add measurable response on top of pembrolizumab or cemiplimab monotherapy, the regimen has no commercial niche - payers will not cover added cost and toxicity for a marginal response bump. Execution risk includes Sanofi's apparent deprioritization: the company has shifted oncology investment toward Sarclisa expansion and externally-sourced ADCs (Adagene, Innovent collaborations). Sanofi has already booked an approximately $1.6B impairment against the Synthorx acquisition [8], so further conviction in pegenzileukin would require either a clear Phase 1/2 signal in the skin cancer combination or a willing external partner to share Phase 3 development cost.

Watch but don't chase. Pegenzileukin matters as a class verdict, not as a standalone bet - if the cemiplimab combination skin cancer data shows an ORR meaningfully above the ~44-47% cemiplimab monotherapy benchmark in PD-1-experienced patients, it would be the first credible signal for non-alpha IL-2 in years and could revive a class that biotech has effectively buried. Without that, it becomes the third major program after bempegaldesleukin and nemvaleukin telling us the elegant receptor-selectivity biology doesn't translate into clinical responses patients and payers care about. Still-active IL-2 engineering programs worth tracking include Synthekine's STK-009, an orthogonal pegylated IL-2 paired with engineered-receptor CAR-T cells (SYNCAR-001), which avoids endogenous IL-2 receptor activation entirely and has produced complete responses in CD19+ non-Hodgkin lymphoma in early Phase 1 dose escalation [10]. The orthogonal design is mechanistically differentiated from pegenzileukin and bempegaldesleukin: it requires a paired engineered receptor rather than relying on selectivity against the native alpha subunit, which sidesteps the on-target toxicity ceiling that has limited the muteins. The specific signals to watch for pegenzileukin: a public Sanofi commitment to a Phase 3 trial, a partnering or out-licensing deal, or inclusion in next-generation R&D day pipeline slides - any of those would indicate Sanofi internally believes the data is real. Absence of those signals through 2026 is itself a data point. Check back at Sanofi's next R&D day and on Q3/Q4 2026 earnings calls for any forward commentary triggered by the J Immunother Cancer publication.