BackgroundIL-7 receptor activation is essential for the proper development and homeostasis of T-cell subpopulations, and maintenance of the TCR clonal repertoire. Emerging evidence indicates potential clinical utility of IL-7 for immunotherapy of lymphopenia, oncology, and other indications. Here we report the discovery of MDK1319, a small novel peptidyl agonist of IL-7R. This peptide is structurally unrelated to IL-7, with a MW less than 5000D. To improve in vivo properties, we fused MDK1319 to an IgG-Fc-domain to construct MDK-701, which exhibits biological properties similar to those of IL-7 in vitro, and when administered to non-human primates.MethodsPeptides were selected from peptide libraries by screens designed to identify molecules binding simultaneously to the Rα and γc subunits of the human IL-7 receptor. Synthetic peptides, and peptides fused to IgG Fc-domains were evaluated for efficacy, potency, and quality of signaling in IL-7-responsive cell lines and human lymphocytes. PK/PD properties in non-human primates were also determined.ResultsMDK1319 and MDK-701 activate the major IL-7R signaling pathways, JAK-STAT (pSTAT5), and PI3K (pAKT), and induce proliferation in human PBMCs, exhibiting lymphocyte subpopulation selectivity, kinetics, efficacy, and potency similar to those of IL-7. Agonism is attributable to direct activation of IL-7R, as shown by dependence on the presence of the IL-7Rα subunit for response in test cells, and insensitivity to IL-7 neutralizing antibodies. MDK1319 and MDK-701 do not activate nor inhibit any other (off target) Rγc family receptors at concentrations 100-fold greater than required for maximal IL-7R activation. MDK-701 administered to cynomologous macaques (single dose, IV at 1 mg/kg) exhibits a circulating terminal half life of ~32 hr; and induces peripheral lymphocyte profiles similar to IL-7 treatment, including initial reduction (tissue migration), followed by sustained elevation of peripheral lymphocytes remaining above baseline for 29 days, with no observed adverse effects.ConclusionsIn addition to the utility of Fc-fusion MDK-701 for monotherapy, the small peptidyl nature of the active peptide MDK1319, fusable to recombinant protein partners, offers opportunities for incorporation into bispecific molecules, linking IL-7 activity to a variety of useful functions. These include synergistic cytokine activities, checkpoint blockade, and tissue targeting. Cells engineered to secrete MDK1319 display autocrine stimulation potentially useful in T-cell therapeutics. The structural novelty of MDK1319 substantially decreases risk of cross reactivity of any anti-drug immune response with endogenous IL-7, and may provide a safer alternative to modified forms of IL-7 reported to produce significant anti-IL-7 immunogenicity.Ethics ApprovalAnimal studies were performed by Envol Biomedical or Charles Rivers Laboratories, as approved by the Institution Ethics Boards with the following study and approval numbers:Envol Biomedical 7037-20 MDK2002; 7037-20: PK/PD Cynomolgus monkeysCharles Rivers Laboratories 20200121001K; US19001: PK Mouse The use of human PBMC in this study was authorized under Minimal Risk Research Related Activities at Stanford Blood Center (SQL 79075)