Chromone-based PDE2 inhibitors as potential prostate cancer therapeutics

Prostate cancer is currently the second largest cause of male cancer deaths. Due to differing levels of cAMP and cGMP in cancer cells compared to normal cells, it is implied that cyclic nucleotides play an important role and may prove novel targets for prostate cancer therapy. A proposed treatment is to inhibit hydrolysis of cGMP via PDE inhibition, triggering downstream apoptotic mechanisms in tumour cells. Recently, PDE inhibitors Exisulind and papaverine, have demonstrated the ability to promote apoptosis in the LnCAP prostate cancer cell line, whilst allowing survival of normal prostate cells. With the major PDE isoforms (2, 4 and 5) identified in the virulent LnCAP cells, this thesis aims to explore PDE2 inhibition as an attractive prostate cancer target. The first part of this thesis investigates the interactions between PDE2 and its substrate/inhibitors. The docking studies of known PDE2 inhibitors, compared a PDE2 homology model and a PDE2 X-ray crystal structure, exploring the major differences between them and comparing the way they interacted with known inhibitors. This allowed for a better understanding of the PDE2 active site highlighting PDE2 specific interactions which may prove important for inhibition and selectivity. These ligand protein relationships were incorporated into the ligand design with an in silico screen highlighting key compounds to channel synthetic effort towards. The second part of this thesis investigates synthetic methodology to synthesise key compounds. Previously described synthetic methodology was adapted, allowing for the readily amenable generation of numerous 2-(4-pyridinyl) and 2-morpholinochromones. Methods were established for the synthesis of 8-amino-6-methoxy chromones bearing either 2-morphonyl or 2-(4-pyridinyl) substituents. The use of an acetylated amino group offers advantages over nitro functions in the key chromone synthesis steps, and yields an amino product amenable to generation of multiple analogues. Two of these analogues, along with 12 morpholinochromone and 27 4-pyridinylchromone analogues viii were synthesised as a small representation, targeted based upon molecular modelling, of potential analogues. In the third section of this thesis, the chromone analogues were evaluated against PDE2 and the prostate cancer cell lines, PC3 and LNCaP. Screening of the chromones against recombinant PDE2, utilising both RP-HPLC and SPA analysis, further refined the structure-activity relationship between chromone compounds and PDE2. Although all synthesised analogues were screened at the prostate cancer cell lines, 4 compounds were highlighted for further analysis, with one found to be potent at the LNCaP prostate cancer cell line, acting via an apoptotic mechanism. Not an extremely potent PDE2 enzyme inhibitor, the significant inhibition at the prostate cancer cell lines needed further exploration. Analysis of the PDEs in prostate cancer cell lines showed that selective PDE2 and PDES inhibitors halted growth in the LNCaP cell line, which suggests this compound may be acting via dual inhibition of PDE2 and PDES. The studies which have been detailed in this thesis have advanced the understanding of PDE2 inhibition, in particular by 2-(4-pyridinyl)chromones. Furthermore, the identification of and proven ability to inhibit, in particular the PDE2 isoform, in prostate cancer cell lines has provided new opportunities for drug development.