Nektar's Rezpegaldesleukin: Aiming To Restore Peripheral Immune Balance

Disclosure: The author holds a beneficial long position in Nektar Therapeutics, Inc. (NASDAQ: NKTR). The author holds no position, long or short, in Eli Lilly and Company (NYSE: LLY). This article is for informational and educational purposes only and is not financial advice. Although the author is a physician, this content is commentary on the underlying science and does not constitute medical advice, diagnosis, or treatment recommendation. The author receives no compensation for this article and has no business relationship with the companies mentioned. See the full Legal Information and Disclosures section below.

Atopic dermatitis (AD) affects roughly 20% of children and 7% to 14% of adults in developed countries and can substantially impair quality of life. For decades, the only systemic options were broad immunosuppressants: corticosteroids, later cyclosporine and methotrexate used off-label. Targeted therapy did not arrive until 2017, when dupilumab, which blocks IL-4 and IL-13 signaling through IL-4Rα, became the first approved biologic. Tralokinumab and lebrikizumab, both hitting IL-13 directly, followed. Later oral JAK inhibitors, upadacitinib and abrocitinib for moderate-to-severe AD and baricitinib, ritlecitinib, and deuruxolitinib for severe alopecia areata (AA), act inside the cell and carry boxed warnings for major adverse cardiovascular events, malignancy, thromboembolism, and death.

Both AD and AA reflect an imbalance between the effector and regulatory arms of peripheral immunity. In AA, the regulatory T cells (Tregs) that guard the hair follicle are reduced and functionally impaired. In AD, the dominant type 2 response itself impairs Treg function. None of the approved drugs act on that imbalance. Rezpegaldesleukin (REZPEG, previously NKTR-358) is a pegylated IL-2 receptor agonist aiming to do exactly that. It expands the regulatory T cell (Treg) compartment selectively to restore peripheral immune balance.

At low doses, IL-2 expands Tregs selectively, but only within a narrow window. It binds two receptors that differ by a single subunit, CD25: a high-affinity trimer that includes it (Kd around 10 pM) and a lower-affinity dimer that does not (around 1 nM). Only Tregs express CD25 constitutively, so only they carry the high-affinity trimer and respond to IL-2 at roughly 100-fold lower concentrations than effector T cells and NK cells, which depend on the dimer. Low doses therefore reach Tregs and little else. Higher doses engage the dimer, switching on effector and NK cells and bringing dose-limiting toxicity (vascular leak, fever, cytokine release).

At high doses, aldesleukin (recombinant IL-2), approved in 1992 for renal cell carcinoma, activates the cytotoxic effector and NK cells that attack the tumor. At low doses, IL-2 has been tested in autoimmunity since 2011, when Saadoun et al. reported improvement in HCV-associated vasculitis and Koreth et al. reported the same in chronic graft-versus-host disease. Two problems have held the autoimmune use back. IL-2's half-life is only minutes, and the dose window is so narrow it is hard to expand Tregs without also switching on effector cells.

Rezpegaldesleukin addresses both by attaching stable PEG chains to aldesleukin. The PEG lowers IL-2's affinity for every receptor subunit, but Tregs stay responsive through their constitutive high-affinity trimer and dense surface CD25, while effector and NK cells, dependent on the dimer alone, respond far less. That widens the selectivity margin, and the half-life stretches to roughly 7 to 13 days against minutes for native IL-2, supporting subcutaneous induction every two weeks. However, Phase 2b clinical data suggest that maintenance can be as infrequent as every twelve weeks, since the Treg expansion and the clinical response outlast drug clearance by months.

The scientific rationale for expanding Tregs lies in evolutionary biology. For most of human evolution, chronic helminth infection was common, and the immune system had to manage parasites it could not otherwise clear. A worm is too large to phagocytose, and a Th1 response, the body's usual attack on intracellular microbes, would damage host tissue more than the parasite. The Th2 arm evolved as the counter, deploying IL-4, IL-5, and IL-13 to drive the mucus secretion, eosinophil recruitment, smooth-muscle contraction, and IgE-armed mast cells that make the gut and skin inhospitable to a large invader.

Helminths evolved countermeasures to persist. The mouse parasite Heligmosomoides polygyrus secretes Hp-TGM, a functional mimic of TGF-β that engages mammalian TGF-β receptors and drives FOXP3 expression in host CD4+ T cells. Other species push host dendritic cells toward a tolerogenic state and induce IL-10-producing regulatory B cells. Chronically infected people carry elevated circulating CD4+CD25+FOXP3+ Tregs, respond weakly to bystander antigens, and class-switch toward the non-inflammatory IgG4 isotype. Over evolutionary time this settled into an equilibrium, with the Th2 arm engaged but held in check by parasite-driven regulatory tone. Developed-world populations now live with almost no helminth exposure, and immune-mediated disease has grown more common over the same period.

However, the link from this mechanism to allergic disease epidemiology is weaker than the underlying cell biology would suggest. A 2022 meta-analysis of 80 studies and roughly 100,000 participants found no overall association between helminth infection and allergic disease, with wide heterogeneity by species and outcome. Ascaris lumbricoides actually raises the risk of bronchial hyperreactivity in children and of atopy in adults, while earlier work suggested an inverse association for hookworm. Treg induction by helminth signals is well established, but helminth infection carries many effects beyond it, so the population-level picture is confounded. The argument for REZPEG is therefore mechanistic, not epidemiological. Human immunity was calibrated under constant regulatory stimulation, and restoring that tone where it is deficient is a defensible target.

The 2025 Nobel Prize in Physiology or Medicine went to Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi for their discoveries concerning peripheral immune tolerance. Sakaguchi had identified and named regulatory T cells, the CD4+CD25+ subset whose depletion caused systemic autoimmunity in mice and whose transfer prevented it. Brunkow and Ramsdell traced the murine scurfy phenotype and human IPEX syndrome to mutations in FOXP3. The two lines met in three 2003 papers that fixed CD4+CD25+FOXP3+ cells as the enforcers of peripheral tolerance.

Two randomized, double-blind, placebo-controlled Phase 1b trials, in moderate-to-severe AD and chronic plaque psoriasis, were published together in October 2024. Eczema severity is scored by the Eczema Area and Severity Index (EASI), which combines how much skin is involved with how severe the lesions are, and a 75% improvement (EASI-75) is the usual benchmark for a strong response. In the AD trial, dosed every two weeks for 12 weeks, the higher dose lowered EASI by 83% (LS mean percent change from baseline) at week 12, with 41% of patients reaching EASI-75. Tregs rose with dose and stayed elevated long after the drug cleared. The responses outlasted the drug, with 71% of week-12 responders still at EASI-75 thirty-six weeks after the final dose. Approved AD biologics like dupilumab block cytokine signaling without moving the regulatory set-point, so they require continuous dosing. A response that persists nine months off drug is therefore mechanistically very informative.

The Phase 2b REZOLVE-AD trial enrolled 393 biologic- and JAK-naive adults with moderate-to-severe AD, randomized to one of three rezpegaldesleukin regimens (24 μg/kg every 2 weeks, 18 μg/kg every 2 weeks, or a low-frequency arm of 24 μg/kg every 4 weeks) or placebo for a 16-week induction. All three arms beat placebo on the primary endpoint, with mean EASI improvement at week 16 of 61%, 58%, and 53% versus 31% (p < 0.001 for each). All three also reached EASI-75, and the two every-2-week arms separated further on vIGA-AD 0/1 and itch. In placebo patients who later crossed to the high dose, responses kept deepening, in data presented at EADV, where mean EASI rose from 68% to 75% and EASI-75 reached 62% by week 24, the basis for a 24-week induction in Phase 3.

The 36-week blinded maintenance phase, read out in February 2026, is where the profile separates from approved biologics. EASI-50 responders were re-randomized 1:1 to stay on their induction dose either monthly (Q4W) or quarterly (Q12W) through week 52. Both held. At the high dose, 71% maintained EASI-75 monthly and 83% quarterly, with vIGA-AD 0/1 maintenance of 85% and 63%. Responses also deepened, with complete clearance (EASI-100) rising from 4% to 22% on Q4W and from 9% to 18% on Q12W. AE-related discontinuation was 3.5%, with injection site reactions the most common event. Holding efficacy at quarterly dosing is unusual in AD, and it is the central claim Phase 3 needs to reproduce.

The second Phase 2b study, REZOLVE-AA, enrolled 92 biologic- and JAK-naive adults with severe-to-very-severe alopecia areata, randomized 3:3:2 to 24 µg/kg, 18 µg/kg, or placebo every 2 weeks for 36 weeks. The primary endpoint was the mean percent reduction in SALT (the Severity of Alopecia Tool, which scores how much of the scalp has lost hair) at week 36. Topline results reported in December 2025 showed reductions of 28.2% and 30.3% versus 11.2% on placebo. The primary modified-intent-to-treat (mITT) analysis narrowly missed statistical significance (p = 0.186 and 0.121). Review then identified four of the 92 randomized patients with major baseline eligibility violations that should have barred them from the trial. In an exploratory post-hoc analysis excluding those four patients, both active arms reached roughly 30% reduction versus 6% on placebo (29.6% and 30.4% versus 5.7%, both p < 0.05). The efficacy figures for the drug arms themselves barely changed between the two analyses; the move into significance came almost entirely from the placebo response falling from 11.2% to 5.7%, which Nektar, on its December 16, 2025 conference call, attributed to a single ineligible placebo patient who had been diagnosed with alopecia areata less than six months before enrollment, when the disease is typically unstable and prone to spontaneous regrowth. The violations have drawn a putative securities class action, and the FDA still has to decide whether the post-hoc analysis can anchor Phase 3 dose selection. Whether rezpegaldesleukin works in alopecia areata is a separate question from how the violations were disclosed.

The efficacy signal is meaningful under either analysis. At week 36, the stringent SALT ≤ 10 endpoint (≥ 90% scalp coverage) was reached by 11.5% of high-dose patients versus 0.7% on placebo, and placebo-adjusted ClinRO response rates in that arm, an exploratory endpoint, were 15% for eyebrow and 18% for eyelash regrowth, outcomes that weigh heavily on quality of life. 52-week extension data in April 2026 showed continued gains. Among patients not yet at SALT ≤ 20 by week 36, 29% on low dose and 31% on high dose reached it by week 52. Across the full population at week 52, SALT50 responses hit roughly 38% in both arms versus 14% on placebo. At the high dose, 35% reached at least 70% scalp coverage (SALT ≤ 30) versus 8% (p = 0.023) and nearly 28% reached at least 80% coverage (SALT ≤ 20) versus 7% (p = 0.049; per April 2026 company presentation). The deepening between months 9 and 12, seen in both trials, fits a mechanism that slowly grows a regulatory population and damps effector activity. It is suggestive rather than proof, but it tracks with the Phase 1b durability and the close Q4W and Q12W maintenance numbers in REZOLVE-AD.

Safety has held across the program. The most common events are injection site reactions and mild transient eosinophilia, with the reactions nearly all mild or moderate and rarely a cause for discontinuation. Nothing has surfaced so far resembling the serious infections, malignancy, or thromboembolism that constrain JAK inhibitors in dermatology, and there has been no vascular leak or other high-dose IL-2 toxicity, consistent with pegylation-driven Treg selectivity at the doses tested. The longest published continuous exposure is about a year. Rare and long-term effects remain Phase 3 questions.

Lilly, which co-developed rezpegaldesleukin with Nektar from 2017 until April 2023, ran the two Phase 1b trials and first presented the AD data at EADV in 2022, reporting a 66% mean EASI improvement at the high dose that missed significance. After the partnership ended and Nektar regained the raw data, Nektar alleged in its complaint that an audited reanalysis found the original calculation had not used the validated 72-point EASI scale and had excluded some available patient data. On the corrected, full-scale analysis, the high-dose mean EASI improvement rose to 83% (p = 0.002), with EASI-75 of 41% against the 29% Lilly had reported. Both sides agree the original figures were wrong, but they dispute who was responsible, and the matter remains in active litigation.

Available treatments for AD and AA work by dampening the inflammatory signal. Rezpegaldesleukin tests a different approach, one in which strengthening regulatory control can hold the disease down even after the drug clears. The Phase 2b data make that credible without yet proving it. The responses are large and keep deepening over a year, holding under quarterly dosing and for months after dosing stops, so far without the malignancy or thrombosis signals that constrain the JAK inhibitors. What they have not shown is whether that durability survives Phase 3 scale and several years, the real test of a disease-modifying claim. The Phase 3 ZENITH-AD program aims to take that on, with dosing planned to start in July 2026 and a BLA targeted for 2029.

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Accuracy, Third-Party Data, and Errors: Clinical trial data, press releases, regulatory filings, court filings, and published literature referenced in this article are sourced from ClinicalTrials.gov, Nektar Therapeutics investor relations, SEC filings, the CourtListener docket, peer-reviewed journals, and conference abstracts. Statements describing the Nektar v. Eli Lilly litigation reflect allegations and positions taken in court filings, which remain contested and unproven. While the author believes these sources to be reliable, the completeness, timeliness, or correctness of this data cannot be guaranteed. Furthermore, analyzing and synthesizing complex medical, statistical, and legal information involves an inherent risk of human error. The author makes no representations or warranties regarding the absolute accuracy of this commentary, assumes no obligation to update or correct any misstatements, and accepts no liability for any errors or omissions within this article.

Disclosure of Interest: The author holds a beneficial long position in Nektar Therapeutics, Inc. (NASDAQ: NKTR). The author holds no position, long or short, in Eli Lilly and Company (NYSE: LLY). The author reserves the right to buy or sell these securities at any time without further notice and undertakes no obligation to update this article. The author receives no direct compensation for the production of this content and maintains no business relationship with either company. The author has no access to non-public trial data, unblinded interim results, sealed court filings, or internal company information. Readers should weigh this article with awareness that the author's financial interest in Nektar creates a risk of confirmation bias, which the author has tried to mitigate by presenting evidence and limitations evenly.

Forward-Looking Statements & Risk: This article contains forward-looking statements regarding regulatory outcomes, clinical results, market potential, and the resolution of pending litigation. These statements are predictions based on current expectations and are subject to significant risks and uncertainties. Actual results may differ materially. The REZOLVE-AA modified intent-to-treat primary analysis did not reach statistical significance, and regulators have not yet ruled on whether the post hoc analysis excluding four ineligible patients is acceptable as the basis for Phase 3 advancement. The Nektar v. Eli Lilly litigation remains pending, and any settlement or verdict could differ substantially from the most likely outcomes discussed here. Securities class actions related to the REZOLVE-AA disclosure are at an early procedural stage. Investing in biotechnology and pharmaceutical securities involves a high degree of risk, including the potential for total loss of principal. Past performance is not indicative of future results.

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