Library Screening and Preliminary Characterization of Synthetic Cannabinoids Against Prostate and Pancreatic Cancer Cell Lines

Background: Our previous screening efforts with colorectal cancer cell lines suggested potential cannabinoid therapeutic leads for other solid cancers.

Objectives: The aim of this study was to identify cannabinoid lead compounds that have cytostatic and cytocidal activities against prostate and pancreatic cancer cell lines and profile cellular responses and molecular pathways of select leads.

Materials and Methods: A library of 369 synthetic cannabinoids was screened against 4 prostate and 2 pancreatic cancer cell lines with 48 h of exposure at 10 μM in medium with 10% fetal bovine serum using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) viability assay. Concentration titration of the top 6 hits was carried out to identify their concentration-response patterns and calculate IC₅₀ values. Three select leads were examined for cell cycle, apoptosis, and autophagy responses. The role of cannabinoid receptors (CB₁ and CB₂) and noncanonical receptors in apoptosis signaling was examined with selective antagonists.

Results: Two independent screening experiments in each cell line detected growth inhibitory activities against all six or a majority of cancer cell lines for HU-331 (a known cannabinoid topoisomerase II inhibitor), (±)5-epi-CP55,940, and PTI-2, each previously identified in our colorectal cancer study. 5-Fluoro NPB-22, FUB-NPB-22, and LY2183240 were novel hits. Morphologically and biochemically, (±)5-epi-CP55,940 elicited caspase-mediated apoptosis of PC-3-luc2 (a PC-3 subline with luciferase) prostate cancer and Panc-1 pancreatic cancer cell lines, each the most aggressive of the respective organ site. The apoptosis induced by (±)5-epi-CP55,940 was abolished by the CB₂ antagonist, SR144528, but not modulated by the CB₁ antagonist, rimonabant, and GPR55 antagonist, ML-193, nor TRPV₁ antagonist, SB-705498. In contrast, 5-fluoro NPB-22 and FUB-NPB-22 did not cause substantial apoptosis in either cell line, but resulted in cytosolic vacuoles and increased LC3-II formation (suggestive of autophagy) and S and G₂/M cell cycle arrests. Combining each fluoro compound with an autophagy inhibitor, hydroxychloroquine, enhanced the apoptosis.

Conclusions: 5-Fluoro NPB-22, FUB-NPB-22, and LY2183240 represent new leads against prostate and pancreatic cancer cells in addition to the previously reported compounds, HU-331, (±)5-epi-CP55,940, and PTI-2. Mechanistically, the two fluoro compounds and (±)5-epi-CP55,940 differed regarding their structures, CB receptor involvement, and death/fate responses and signaling. Safety and antitumor efficacy studies in animal models are warranted to guide further R&D.