Synthesis and biochemical evaluation of potential steroidal and non-steroidal inhibitors of 17[beta]-hydroxysteroid dehydrogenase (17[beta]-HSD) in the treatment of hormone-dependent cancers
Enzymes such as aromatase, 17[beta]-hydroxysteroid dehydrogenase [types 1 (17[beta]-HSD1) and 3 (17[beta]-HSD3)] and estrone sulfatase (ES) are all involved in the biosynthesis of steroids via the steroidal cascade. The inhibition of these enzymes may lead to a reduction in the levels of steroids present, thereby leading to a decrease in the stimulation of hormone-dependent tissues, in particular, hormone-dependent breast and prostate cancers. This approach has proved to be successful in postmenopausal women where the use of aromatase inhibitors has led to the decrease in tumour yield and has thus led to the treatment of the disease. Within the current study, the synthesis and biochemical evaluation of a number of compounds of varying structural features has been undertaken, in particular, the synthesis of sulfonate derivatives of 4-hydroxyphenyl ketone - the parent compound having already been shown to be a potent inhibitor of 17[beta]-HSD3 (with good specificity towards 17[beta]-HSD3) and the synthesis of a range of alkyl and cycloalkyl esters of steroids [in particular, testosterone (T) dehydroepiandrosterone (DHEA) and estrone (E10] as probes of the active sites of the HSD family of enzymes. The results show that the sulfonate (methanesulfonate and trifluromethanesulfonate) derivatives of 4-hydroxyphenyl ketone-based compounds were found to possess weak inhibitory activity against all three HSD enzymes considered (namely, 17[beta]-HSD1, 17[beta]-HSD3 and 3[beta]-HSD) in comparison to the parent 4-hydroxyphenyl ketone-based compounds. For example, within the methanesulfonate derivatives, methane sulfonic acid (4-cyclohexane carbonyl)-phenyl ester (164) was found to be the most potent inhibitor against 17[beta]-HSD3, however, it possessed ~30% inhibitory against this enzyme at an inhibitor concentration of 100[mu]M. Against 17[beta]-HSD1, the most potent compound within the same range was also compound 164 which pssessed ~45% inhibitory activity under similar conditions. Within the trifluromethane sulfonate derivatives, the most potent compounds proved to be extremely weak inhibitors of 17[beta]-HSD3, however, against 17[beta]-HSD1, the most potent compound was trifluromethane sulfonic acid 4-benzoyl-phenyl ester (180) which possess 43% inhibitory activity. The molecular modeling of these compounds within representations of the active sites of 17[beta]-HSD1 and 17[beta]-HSD3 shows that the lack of inhibitory activity is due to steric hindrance, in particular, the sulfonate moeity undergoes steric hindrance with groups at the active site which is close to the C(17) area of the natural substrate. The synthesis of the esters of T, DHEA and E1 and the subsequent biochemical evaluation of these compounds resulted in an interesting structure-activity relationship. In general, the compounds based on DHEA were found to be potent inhibitors of 17[beta]-HSD3 with weak inhibitory activity against 17[beta]-HSD1 and 3[beta]-HSD. For example, DHEA acetate (196) was found to possess an IC[sub]50 value of 0.74[mu]M in comparison to the most potent standard, namely 1-(4hydroxy-phenyl)-nonan-1-one (139) which was found to possess an IC[sub]50 value of 12.32[mu]M - this compound was found to possess good selectivity as it possessed ~40% and ~25% inhibitory activity against 17[beta]-HSD1 and 3[beta]-HSD respectively at an inhibitor concentration of 100[mu]M. The esters of E1 and T proved to be weaker inhibitors in comparison to the esters based on DHEA, however, the E1-based esters also showed some selectivity towards 17[beta]-HSD3. For example, E1 hexanoate (216) possessed an IC[sub]50 value of 37.28[mu]M and possessed 45% and 35% inhibitory activity against 17[beta]-HSD1 and 3[beta]-HSD respectively at an inhibitor concentration of 100[mu]M. The modelling of these compounds (using representations of the active sites of 17[beta]-HSD1 and 17[beta]-HSD3) showed that the lack of inhibitory activity was due to steric interactions between the inhibitors and groups within the active site. As such, these compounds proved to be extremely useful probes of the active sites of 17[beta]-HSD1 and 17[beta]-HSD3 and have further enhanced the models used in the design of these compounds.