An Open Label Study of Alpn-101, a First-in-Class Dual CD28/ICOS Antagonist, in Subjects with Steroid-Resistant or Steroid-Refractory Acute Graft Versus Host Disease (aGVHD)

Background: Cluster of differentiation 28 (CD28) and inducible T cell costimulator (ICOS) provide costimulatory signals required for optimal T cell activation when bound to their respective ligands, CD80 (B7-1) and CD86 (B7-2), and ICOS ligand (ICOSL). CD28 is involved in initiation of the pathogenic process in GVHD and recent studies suggest therapeutic utility of CD28 pathway inhibitors for prophylaxis and treatment. However, CD28 pathway inhibition alone appears insufficient to control established disease in most patients. In this context, it is recognized that following initial activation, CD28 is often down-regulated while ICOS, its most closely related family member, is upregulated, providing additional T cell costimulation that may sustain GVHD including gastrointestinal manifestations (Adom D et al. Blood. 2018; 132:355). In murine models of aGVHD, combined blockade of CD28 and ICOS was significantly superior to isolated blockade of the CD28 or ICOS pathways alone.

ALPN-101 is an Fc fusion protein of a human ICOSL variant immunoglobulin domain (vIgD^TM) designed to inhibit both the CD28 and ICOS costimulatory pathways. Nonclinical studies of ALPN-101 demonstrate high affinity binding to CD28 and ICOS, potent inhibition of T cell activation, and suppression of disease activity in a human xenogeneic model of GVHD in mice, after only a single dose (Dillon SR et al. Blood. 2018; 132:2037).

ALPN-101 is in development as a novel and potentially transformative approach for GVHD. A first-in-human study of ALPN-101 in healthy subjects is ongoing. This clinical study will assess the safety, efficacy, pharmacokinetics (PK), and pharmacodynamics (PD) of ALPN-101 in subjects with aGVHD.

Study Design and Methods: Adults with Grade II-IV aGVHD per Mount Sinai Acute GVHD international Consortium (MAGIC) criteria (Harris et al. 2016) that is resistant or refractory to glucocorticoids will receive a single intravenous dose of ALPN-101. This study will be conducted in two parts including dose escalation and dose expansion. Dose escalation will proceed by using an accelerated titration design. Thereafter, a selected dose level(s) will be evaluated in an expansion cohort of 10 subjects; if ≥ 25% (n=3) subjects achieve response, 15 additional subjects will be enrolled (Simon two-stage design). Background Therapy:At the investigator's discretion, subjects may continue therapies administered for prophylaxis, continue or increase glucocorticoids, and/or add another salvage therapy. Responders will be considered for glucocorticoid taper. Endpoints: Safety will be assessed based on the incidence, severity, and seriousness of adverse events. Efficacy endpoints include the objective response rate, duration of response, failure-free survival, event-free survival, non-relapse mortality, malignancy relapse/progression, overall survival, and glucocorticoid use. The incidence and titer of anti-drug antibodies will be assessed. Serum concentrations of ALPN-101 will be measured and PK parameters will be estimated. PD endpoints include target saturation and immunophenotyping of circulating leucocytes, and may include quantification of circulating cytokines, immunoglobulins, acute phase reactants, and soluble forms of the targeted pathway receptor, evaluation of changes in mRNA expression in circulating leucocytes, evaluation of risk alleles, and correlates of response.