Combination of the PI3K Inhibitor ZSTK474 with a PSMA-Targeted Immunotoxin Accelerates Apoptosis and Regression of Prostate Cancer

Abstract

The phosphoinositide 3-kinase (PI3K) pathway is activated in most advanced prostate cancers, yet so far treatments with PI3K inhibitors have been at best tumorostatic in preclinical cancer models and do not show significant antitumor efficacy in clinical trials. Results from tissue culture experiments in prostate cancer cells suggest that PI3K inhibitors should be combined with other cytotoxic agents; however, the general toxicity of such combinations prevents translating these experimental data into preclinical and clinical models. We investigated the emerging concept of tumor-targeted synthetic lethality in prostate cancer cells by using the pan-PI3K inhibitor ZSTK474 and the immunotoxin J591PE, a protein chimera between the single-chain variable fragment of the monoclonal antibody J591 against the prostate-specific membrane antigen (PSMA) and the truncated form of the Pseudomonas aeruginosa exotoxin A (PE38QQR). The combination of ZSTK474 and J591PE increased apoptosis within 6 hours and cell death (monitored at 24-48 hours) in the PSMA-expressing cells LNCaP, C4-2, and C4-2Luc but not in control cells that do not express PSMA (PC3 and BT549 cells). Mechanistic analysis suggested that induction of apoptosis requires Bcl-2-associated death promoter (BAD) dephosphorylation and decreased expression of myeloid leukemia cell differentiation protein 1 (MCL-1). A single injection of ZSTK474 and J591PE into engrafted prostate cancer C4-2Luc cells led to consistent and stable reduction of luminescence within 6 days. These results suggest that the combination of a PI3K inhibitor and a PSMA-targeted protein synthesis inhibitor toxin represents a promising novel strategy for advanced prostate cancer therapy that should be further investigated.

Personalized prostate cancer therapy based on systems analysis of the apoptosis regulatory network

“… 41 The second approach is based on the generation of chimera toxins with antibodies that recognize prostate-specific membrane antigen (PSMA). 42 43 46 Combinations of PSMA-targeted bacterial toxins that inhibit protein synthesis, and latent PSA-activated kinase inhibitors could accomplish simultaneous inhibition of Mcl-1 expression and BAD phosphorylation in a prostate-selective fashion ( Figure 2 ).…”

“…Combined loss of BAD phosphorylation and Mcl-1 expression induces rapid apoptosis in PTEN-deficient prostate cancer cells. 27 43 …”

Targeting prostate cancer: Prostate‐specific membrane antigen based diagnosis and therapy

“…Baiz et al combined a J591-based immunotoxin comprising the truncated Pseudomonas aeruginosa exotoxin A with a phosphoinositide 3-kinase (PI3K) inhibitor. 76 They were able to show specific in vitro cytotoxicity in PSMA-positive cells within 6 hr after treatment. In subsequent mechanistical analyses, they were able to track down participating pathways and in vivo application proved tumor targeting.…”

Enhancement of PSMA-Directed CAR Adoptive Immunotherapy by PD-1/PD-L1 Blockade

“…PSMA is a glycosylated type-II membrane protein that is upregulated during malignant transformation in more aggressive prostate cancer, resulting in abnormally high levels of it on the cell surface 23 . We24, 25 and others21, 26, 27, 28 (Slovin et al., 2012, J. Clin. Oncol, abstract) have participated in developing imaging approaches to visualize anti-hPSMA CAR T cells and optimization of the structure of CARs to achieve more profound effects in the targeting of tumors bearing the corresponding antigen (hPSMA).…”