PT101: A Treg selective agonist IL-2 mutein therapy for autoimmunity

Abstract Regulatory T cells (Treg) play a critical role in immune homeostasis and are dysfunctional in many autoimmune diseases. Interleukin 2 (IL-2), via the heterotrimeric IL-2 receptor (CD25/CD122/CD132), drives the proliferation and function of Treg. CD25 loss-of-function in mice is associated with Treg deficiency and widespread autoimmunity. Low dose IL-2 expands Tregs and is being evaluated therapeutically, however it has a narrow selectivity window over conventional T cells and Natural Killer cells which is dose limiting. To enhance IL-2 selectivity for activating and expanding Treg, mutations can be introduced that create a dependency on CD25 binding for signaling through CD122/CD132 upon IL-2 facilitated CD25/CD122/CD132 trimer formation. We created PT101, a mutant IL-2 Fc fusion, containing a N88D mutation that significantly decreases CD122 binding affinity in addition to other mutations that increase CD25 binding affinity and introduce improved molecular stability and drug-like properties. We found that PT101 selectively induces STAT5 phosphorylation in human and cynomolgus monkey Tregs in vitro. In humanized NOD-scid IL-2Rg-null (NSG) mice and cynomolgus monkeys, single or multiple dose administration of PT101 dose-dependently and selectively expands Treg without significant effects on other immune cell types, and without eliciting proinflammatory cytokine production. A 1-month toxicity study in cynomolgus monkeys indicated that PT101 was safe and well-tolerated at 300× the minimum efficacious dose whilst maintaining its Treg selectivity. PT101 is being evaluated in a randomized, placebo-controlled, double-blind, single ascending dose-escalation trial in healthy volunteers.