Synthesis and structure–activity relationships of guanine analogues as phosphodiesterase 7 (PDE7) inhibitors
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Introduction
The secondary messengers cAMP and cGMP are responsible for the regulation of numerous intracellular processes. They are regulated by phosphodiesterases (PDEs) which hydrolyze them to the corresponding inactive 5′-monophosphate nucleotides. Eleven PDE gene families have been identified to date, varying in substrate specificity, inhibitor sensitivity and regulatory characteristics.1 PDE7 is a low Km (0.2 μM) cAMP specific enzyme which is insensitive to the standard PDE4 inhibitor, rolipram. Two splice variants, PDE7A1 and PDE7A2 were originally identified (>90% homology).2 A further PDE7 gene has recently been identified,3 which has been designated PDE7B as it possesses 70% homology with PDE7A in the catalytic domain. PDE7 mRNA has been found to be widely distributed, although active protein has been identified predominantly in T-cells.4 The proposed role of PDE7 in T-cell activation5 implies that selective inhibitors could have benefits in T-cell mediated diseases. Additionally, the presence of PDE7 in airway epithelial cells6 suggests that inhibitors could be beneficial in diseases of the airway.
To date only two series of synthetic PDE7 inhibitors have been described,7 generally having low micromolar IC50 values and no significant selectivity over the PDE4 and PDE3 isozymes. As inhibition of the other PDE isoenzymes may result in some side effects such as emesis and cardiotoxicity, selectivity for PDE7 over the other members of the phosphodiesterase family of enzymes is important. In this communication, we exemplify a series of guanine analogues which possesses PDE7 inhibitory activity in vitro and demonstrates some evidence of selectivity over the PDE4 isoenzyme.
To find a lead, screening of internal and external databases was performed, and an initial guanine based hit (1) was identified.
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Chemistry
Synthesis of these guanine analogues was initially attempted by alkylation of either 2-amino-6-chloropurine or 6-chloropurine (2). Intermediates (3) were generated using potassium carbonate as the base and a suitable alkylating agent. Displacement of the chlorine was then achieved using sodium hydroxide to yield the guanines (4).8 Bromination9 of the 8-position gave the desired analogues (5) (Scheme 1).
It was found that this route was not generally applicable, and so the synthesis of the
Structure–activity relationships
Initial results for the inhibiton of PDE7 using our guanine analogues (Table 1) showed removal of the bromine reduced activity (compare 1 and 5a) as did the removal of the amino group (5a and 5b).
Several analogues were synthesised starting with the replacement of the saturated six membered ring with a five membered (5c) and a seven membered ring (9a) (Table 2). Neither of these alterations was found to enhance either the activity or the selectivity. However when the tetralin ring was
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