Abstract A08: Distribution, cutoff, and functional significance of a potential biomarker for Imprime PGG, an experimental cancer immunotherapeutic, in a healthy subject population
Authors: McMurray Diane , Harrison Ben , Ertelt Katie , Walsh Richard , Wurst Lindsay , Leonardo Steven , Ottoson Nadine , Jonas Adria Bykowski , Qiu Xiaohong , Bose Nandita , Maimonis Peter 
Published: October 2015

Imprime PGG (Imprime) is a yeast-derived beta-1,3/1,6 glucan that binds complement receptor 3 (CR3) on innate immune cells and enables these cells to exert anti-tumor activity against tumor cells that have been opsonized by iC3b following targeting by anti-tumor antibodies. Following incubation with whole blood (WB) from healthy subjects, Imprime was shown to bind to neutrophils and monocytes via CR3 and modulate complement receptor expression, activation marker expression, and interleukin-8 (IL-8) production. These effects, however, differed widely among subjects. It was subsequently observed that Imprime-induced functional activities occurred predominantly in individuals with higher levels of endogenous immunoglobulin G (IgG) or IgM anti-beta-glucan antibodies (ABA) and that ABA are a prerequisite for opsonizing Imprime to allow its binding to CR3. A small pilot study (n=32) with healthy subjects using prototype enzyme-linked immunosorbent assays (ELISAs) to measure IgG and IgM ABA suggested correlations between ABA levels, Imprime binding to neutrophils in WB, and consequent induction of functional responses. The objectives of this study were to further optimize and qualify the prototype ELISAs and, in a larger cohort of healthy subjects (n=143), to confirm the significance of elevated ABA levels with respect to binding of Imprime to neutrophils and monocytes and Imprime–induced functional changes. Qualification of the IgG and IgM ABA ELISAs yielded good inter-assay precision (coefficients of variation &lt;10%), linearity (recovery 97-112%), and dynamic range (2.5 logs). Incubation of WB samples with Imprime resulted in high binding to neutrophils (&gt;5% of cells) in samples from 52% of the subjects. Samples from these same subjects also exhibited high Imprime binding to monocytes. ABA levels in the high binding individuals were significantly greater than those in the low binding individuals (p&lt;0.0001 for IgG and p=0.01 for IgM). IgG ABA levels were more highly correlated with Imprime binding to neutrophils and monocytes (r=0.64 and r=0.61, respectively) than IgM ABA levels (r=0.22 and r=0.33, respectively). There was no correlation of ABA levels with age, gender, or total serum immunoglobulin concentration. Receiver operator characteristic (ROC) curve analysis comparing sensitivity and specificity of ABA levels vs. Imprime binding to neutrophils yielded an area under the curve (AUC) of 0.89 for IgG and 0.70 for IgM and generated several options for cutoffs. At one proposed ABA cutoff set at 95% specificity for binding, 36% of subjects had IgG ABA levels ≥ the cutoff, 20% had IgM ABA levels ≥ the cutoff, and 46% had levels ≥ either cutoff. The sensitivity to detect high Imprime binding to neutrophils was 64% for IgG and 33% for IgM, with a combined sensitivity of 80%. Similar correlations were seen with Imprime binding to monocytes. In subsets of the healthy population in which functional evaluations were also performed following in vitro exposure to Imprime, IgG ABA levels at or above vs. below the cutoff correlated with higher C4a (n=32; p=0.0003), C5a (n=32; p&lt;0.0001), and SC5b-9 (n=32; p&lt;0.0001) production, increased production of IL-8 (n=129; p&lt;0.0001), and increased neutrophil and monocyte CR3 expression (n=40; p=0.006 and n=37; p=0.009 respectively). Data presented here, using the further optimized ELISAs for measurement of IgG and IgM ABA in WB samples from a larger healthy population, confirm that serum ABA levels correlate with in vitro neutrophil as well as monocyte binding and response to Imprime. Furthermore, ABA cutoffs at high specificity could identify the majority of healthy individuals whose cells demonstrated this response. The use of ABA as a clinical biomarker for Imprime response in cancer patients is under investigation.

Citation Format: Diane McMurray, Ben Harrison, Katie Ertelt, Richard Walsh, Lindsay Wurst, Steven Leonardo, Nadine Ottoson, Adria Bykowski Jonas, Xiaohong Qiu, Nandita Bose, Peter Maimonis. Distribution, cutoff, and functional significance of a potential biomarker for Imprime PGG, an experimental cancer immunotherapeutic, in a healthy subject population. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr A08.