Novel Biodistribution of PSMA Radiotracer in the Uvula of Patients Undergoing PSMA PET/CT

ABSTRACT

Prostate-specific membrane antigen (PSMA) PET/CT is an imaging technique that detects primary and metastatic prostate cancer and evaluates treatment effectiveness. The radioligands for PSMA PET/CT are known to have physiological off-target uptake in various tissues. These include the well-known off-target major and minor salivary glands. We report that, in addition to this location, radioligand uptake can be seen in the uvula, which we suggest is from salivary tissue in this location. PSMA uptake in the uvula is not reported in the literature and is a rare, but normal location for tracer biodistribution in some patients.

Abstract 2081: Poly ADP-ribose polymerase inhibition enhances T cell cytotoxicity and anti-tumor function by altering NAD+ levels

Poly ADP-ribose polymerase (PARP) is an NAD-dependent DNA repair enzyme. Inhibition of PARP in BRCA-mutant cancers has been a clinically approved therapy for several years. While the PARP inhibitor, olaparib, has been approved for the treatment of BRCA-mutant breast and ovarian cancers, it has not been used in other cancers because its mechanism of action relies on synthetic lethality. We have found that olaparib may also have use in other cancers due to its ability to modulate the immune system directly. Specifically, we have found that olaparib is able to increase the proliferation, activation, and cytotoxicity of T cells in vitro and reduce tumor growth in vivo at low doses. Recent papers have proposed that olaparib is able to enhance T cell function through the stimulator of interferon genes (STING) pathway by enhancing antigen presenting cell activity and resulting in more granzyme B expression and enhanced tumor killing. Using purified T cells from both wild type and STING knockout mice, we have shown that olaparib can also cause an increase in granzyme B and perforin expression, an increase in T cell proliferation, and enhanced tumor cell killing in a STING-independent manner. Because PARP is an NAD-consuming enzyme, we examined the role of NAD+ in the activation T cells and the production of granzyme B and perforin. By using an inhibitor of NAD+ recycling, FK866, we show that T cells have reduced NAD+ levels, reduced granzyme B and perforin expression, and even fail to activate in the presence of CD3 and CD28 when NAD+ is limited; however, adding olaparib to these cells rescues the effects of FK866 and allows proliferation and granzyme B production. In vivo we can see that even at olaparib doses that are significantly below those which induce synthetic lethality of tumor cells, olaparib is able to inhibit tumor growth in models of both colorectal and breast cancer. This suggests that olaparib can modulate the activity of T cells directly and has implications for enhancing the anti-tumor activity of these cells both alone and in combination with other forms of immunotherapy.

Citation Format: Matthew J. Dean, Liqin Zheng, Phaethon Philbrook, Augusto C. Ochoa, A. Hamid Boulares. Poly ADP-ribose polymerase inhibition enhances T cell cytotoxicity and anti-tumor function by altering NAD+ levels [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2081.

Severe COVID-19 Is Characterized by an Impaired Type I Interferon Response and Elevated Levels of Arginase Producing Granulocytic Myeloid Derived Suppressor Cells

COVID-19 ranges from asymptomatic in 35% of cases to severe in 20% of patients. Differences in the type and degree of inflammation appear to determine the severity of the disease. Recent reports show an increase in circulating monocytic-myeloid-derived suppressor cells (M-MDSC) in severe COVID 19 that deplete arginine but are not associated with respiratory complications. Our data shows that differences in the type, function and transcriptome of granulocytic-MDSC (G-MDSC) may in part explain the severity COVID-19, in particular the association with pulmonary complications. Large infiltrates by Arginase 1+ G-MDSC (Arg+G-MDSC), expressing NOX-1 and NOX-2 (important for production of reactive oxygen species) were found in the lungs of patients who died from COVID-19 complications. Increased circulating Arg+G-MDSC depleted arginine, which impaired T cell receptor and endothelial cell function. Transcriptomic signatures of G-MDSC from patients with different stages of COVID-19, revealed that asymptomatic patients had increased expression of pathways and genes associated with type I interferon (IFN), while patients with severe COVID-19 had increased expression of genes associated with arginase production, and granulocyte degranulation and function. These results suggest that asymptomatic patients develop a protective type I IFN response, while patients with severe COVID-19 have an increased inflammatory response that depletes arginine, impairs T cell and endothelial cell function, and causes extensive pulmonary damage. Therefore, inhibition of arginase-1 and/or replenishment of arginine may be important in preventing/treating severe COVID-19.

Transcriptome and Functions of Granulocytic Myeloid-Derived Suppressor Cells Determine their Association with Disease Severity of COVID-19

COVID-19 ranges from asymptomatic in 35% of cases to severe in 20% of patients. Differences in the type and degree of inflammation appear to determine the severity of the disease. Recent reports show an increase in circulating monocytic-myeloid-derived suppressor cells (M-MDSC) in severe COVID 19, that deplete arginine but are not associated with respiratory complications. Our data shows that differences in the type, function and transcriptome of Granulocytic-MDSC (G-MDSC) may in part explain the severity COVID-19, in particular the association with pulmonary complications. Large infiltrates by Arginase 1 ⁺ G-MDSC (Arg ⁺ G-MDSC), expressing NOX-1 and NOX-2 (important for production of reactive oxygen species) were found in the lungs of patients who died from COVID-19 complications. Increased circulating Arg ⁺ G-MDSC depleted arginine, which impaired T cell receptor and endothelial cell function. Transcriptomic signatures of G-MDSC from patients with different stages of COVID-19, revealed that asymptomatic patients had increased expression of pathways and genes associated with type I interferon (IFN), while patients with severe COVID-19 had increased expression of genes associated with arginase production, and granulocyte degranulation and function. These results suggest that asymptomatic patients develop a protective type I IFN response, while patients with severe COVID-19 have an increased inflammatory response that depletes arginine, impairs T cell and endothelial cell function, and causes extensive pulmonary damage. Therefore, inhibition of arginase-1 and/or replenishment of arginine may be important in preventing/treating severe COVID-19.

Targeted Elimination of Immunodominant B Cells Drives the Germinal Center Reaction toward Subdominant Epitopes

Rapidly evolving pathogens such as HIV or influenza can quickly mutate their antigenic profiles, reducing the efficacy of conventional vaccines. Despite this challenge, functionally required epitopes are highly conserved among heterologous viral strains and represent a key vulnerability that could be targeted during vaccine development. As the antigenicity of these conserved epitopes is frequently subdominant, there is a critical need for innovative vaccination strategies designed to target these neutralizing epitopes. Here, we immunized mice with antigens containing discrete immunodominant and subdominant moieties and show that treatment with soluble heterologous antigen bearing only the immunodominant epitope selectively suppresses these germinal center (GC) B cells. By exploiting this intrinsic tolerance mechanism, we promote the expansion of subdominant B cells in the GC and the subsequent long-lived components of the humoral response. We propose that this strategy may be applied to elicit preferential expansion of subdominant B cells that recognize weakly immunogenic epitopes on microbial pathogens.

The GS Protein-coupled A2a Adenosine Receptor Controls T Cell Help in the Germinal Center

T follicular helper (T_FH) cells have been shown to be critically required for the germinal center (GC) reaction where B cells undergo class switch recombination and clonal selection to generate high affinity neutralizing antibodies. However, detailed knowledge of the physiological cues within the GC microenvironment that regulate T cell help is limited. The cAMP-elevating, G_s protein-coupled A2a adenosine receptor (A2aR) is an evolutionarily conserved receptor that limits and redirects cellular immunity. However, the role of A2aR in humoral immunity and B cell differentiation is unknown. We hypothesized that the hypoxic microenvironment within the GC facilitates an extracellular adenosine-rich milieu, which serves to limit T_FH frequency and function, and also promotes immunosuppressive T follicular regulatory cells (T_FR). In support of this hypothesis, we found that following immunization, mice lacking A2aR (A2aRKO) exhibited a significant expansion of T follicular cells, as well as increases in T_FH to T_FR ratio, GC T cell frequency, GC B cell frequency, and class switching of GC B cells to IgG1. Transfer of CD4 T cells from A2aRKO or wild type donors into T cell-deficient hosts revealed that these increases were largely T cell-intrinsic. Finally, injection of A2aR agonist, CGS21680, following immunization suppressed T follicular differentiation, GC B cell frequency, and class switching of GC B cells to IgG1. Taken together, these observations point to a previously unappreciated role of G_S protein-coupled A2aR in regulating humoral immunity, which may be pharmacologically targeted during vaccination or pathological states in which GC-derived autoantibodies contribute to the pathology.

Germinal Center Hypoxia Potentiates Immunoglobulin Class Switch Recombination

Germinal centers (GCs) are anatomic sites where B cells undergo secondary diversification to produce high-affinity, class-switched Abs. We hypothesized that proliferating B cells in GCs create a hypoxic microenvironment that governs their further differentiation. Using molecular markers, we found GCs to be predominantly hypoxic. Compared to normoxia (21% O₂), hypoxic culture conditions (1% O₂) in vitro accelerated class switching and plasma cell formation and enhanced expression of GL-7 on B and CD4⁺ T cells. Reversal of GC hypoxia in vivo by breathing 60% O₂ during immunization resulted in reduced frequencies of GC B cells, T follicular helper cells, and plasmacytes, as well as lower expression of ICOS on T follicular helper cells. Importantly, this reversal of GC hypoxia decreased Ag-specific serum IgG1 and reduced the frequency of IgG1⁺ B cells within the Ag-specific GC. Taken together, these observations reveal a critical role for hypoxia in GC B cell differentiation.

Immunological mechanisms of the antitumor effects of supplemental oxygenation

Antitumor T cells either avoid or are inhibited in hypoxic and extracellular adenosine-rich tumor microenvironments (TMEs) by A2A adenosine receptors. This may limit further advances in cancer immunotherapy. There is a need for readily available and safe treatments that weaken the hypoxia-A2-adenosinergic immunosuppression in the TME. Recently, we reported that respiratory hyperoxia decreases intratumoral hypoxia and concentrations of extracellular adenosine. We show that it also reverses the hypoxia-adenosinergic immunosuppression in the TME. This, in turn, stimulates (i) enhanced intratumoral infiltration and reduced inhibition of endogenously developed or adoptively transfered tumor-reactive CD8 T cells, (ii) increased proinflammatory cytokines and decreased immunosuppressive molecules, such as transforming growth factor-β (TGF-β), (iii) weakened immunosuppression by regulatory T cells, and (iv) improved lung tumor regression and long-term survival in mice. Respiratory hyperoxia also promoted the regression of spontaneous metastasis from orthotopically grown breast tumors. These effects are entirely T cell- and natural killer cell-dependent, thereby justifying the testing of supplemental oxygen as an immunological coadjuvant to combine with existing immunotherapies for cancer.

Abstract 5080: Inhibiting the ALK1/BMP9 signaling pathway with dalantercept as an antiangiogenic therapy

Activin receptor-like kinase 1 (ALK1) is an endothelial cell-specific receptor for bone morphogenetic proteins (BMP) 9 and 10, and is a key regulator of angiogenesis and vascular morphogenesis. Preclinical data suggest that the BMP9/ALK1 pathway is operating in the resolution and maturation stage of angiogenesis, which is distinct from the VEGF/VEGFR pathway which drives the initiation stage of angiogenesis. Dalantercept is an ALK1 extracellular domain-Fc fusion protein that inhibits the BMP9/ALK1 pathway. Thus, dalantercept may prove useful either as a monotherapy to inhibit tumor angiogenesis or in combination with VEGFR inhibitors to target different phases of vascular network formation and potentially to overcome acquired resistance to anti-VEGF therapy.

To test this hypothesis, the A498 renal cell carcinoma xenograft model was used to test dalantercept efficacy either alone or in combination with sunitinib. In this model, tumor progression was defined as a 2 mm increase in tumor length. In the placebo group, mean (± SD) tumor progression was 5.8 ±0.6 days. Treatment with dalantercept delayed tumor progression to 10.8 ± 1.7 days (p<0.01 vs placebo) and sunitinib delayed tumor progression to 13.8 ± 2.5 days (p<0.01 vs placebo). The combination treatment of dalantercept plus sunitinib had a greater beneficial effect with a delay in tumor progression to 35 ± 12.6 days (p=0.01 vs sunitinib).

We used immunohistochemistry (IHC) analysis to evaluate effects of dalantercept on tumor vasculature and to look for potential biomarkers of response to treatment in A498 tumors. Id1 is a transcription factor expressed in endothelial cells and is known to be regulated via the BMP9/ALK1/SMAD pathway. We observed a decrease in Id1 protein expression in endothelial cells of A498 xenografts treated with dalantercept, consistent with our previous experiments. Because BMP9 is one of the main target ligands of dalantercept, IHC analysis of archived human tumor samples was performed to determine BMP9 expression levels in various tumor types. In squamous cell carcinoma of the head and neck it was shown that out of 28 tumor samples 79% had high or medium levels of BMP9 staining, with little to no staining in normal tissues. Data from BMP9 expression in renal cell carcinoma and hepatocellular carcinoma will also be presented.

In a Phase 1 study in patients with advanced, refractory solid tumors, dalantercept was generally well tolerated and exhibited signs of clinical activity including patients with objective response and prolonged stable disease. Dalantercept is currently being tested in several Phase 2 oncology studies, including a study of dalantercept in combination with the anti-VEGFR TKI axitinib in second-line advanced renal cell carcinoma.

Citation Format: Marat Alimzhanov, Nicolas Solban, Michael Lee, Phaethon Philbrook, Xiaoen Wang, Lin Wei, Jiaxi Song, Sabina Signoretti, R. Scott Pearsall, Matthew L. Sherman, Ravindra Kumar, Rupal S. Bhatt. Inhibiting the ALK1/BMP9 signaling pathway with dalantercept as an antiangiogenic therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5080. doi:10.1158/1538-7445.AM2013-5080