Chiral Aromatase and Dual Aromatase−Steroid Sulfatase Inhibitors from the Letrozole Template: Synthesis, Absolute Configuration, and In Vitro Activity

Abstract

To explore aromatase inhibition and to broaden the structural diversity of dual aromatase-sulfatase inhibitors (DASIs), we introduced the steroid sulfatase (STS) inhibitory pharmacophore to letrozole. Letrozole derivatives were prepared bearing bis-sulfamates or mono-sulfamates with or without adjacent substituents. The most potent of the achiral and racemic aromatase inhibitor was 40 (IC 50 = 3.0 nM). Its phenolic precursor 39 was separated by chiral HPLC, and the absolute configuration of each enantiomer was determined using vibrational and electronic circular dichroism in tandem with calculations of the predicted spectra. Of the two enantiomers, ( R)-phenol ( 39a) was the most potent aromatase inhibitor (IC 50 = 0.6 nM, comparable to letrozole), whereas the ( S)-sulfamate, ( 40b) inhibited STS most potently (IC 50 = 553 nM). These results suggest that a new structural class of DASI for potential treatment of hormone-dependent breast cancer has been identified, and this is the first report of STS inhibition by an enantiopure nonsteroidal compound.

Determination of the absolute configuration of aromatase and dual aromatase‐sulfatase inhibitors by vibrational and electronic circular dichroism spectra analysis

“…A chiral HPLC study of the two enantiomers of II following sulfamoylation confirmed that no racemization had taken place during this step. 5 We can thus safely conclude that the sulfamate retains the AC of its corresponding phenol, as confirmed by the observation that the ECD spectra of the two compounds are quite similar. In other cases presented in the literature, the AC of a compound has been assessed by the preparation of a convenient derivative as long as the synthesis of the derivative leaves the stereogenic carbons unchanged (see for example 18 ).…”

“…2) reveals that with the first basis set, already used in Ref. 5, two negative bands are calculated and not observed at 1370 cm 21 and at 1435 cm…”

“…Herein we reconsider the work that was previously presented in Ref. 5 with the use of more accurate calculations and further experiments.…”

“…The two compounds I and II were prepared and the racemic mixture of compound (I) was separated by semi-prep chiral HPLC with a Chiralpak 1 AD-H column as previously described. 5 The optical rotation (OR) measured in ethanolic solutions for ( Each phenolic enantiomer was tested against aromatase and in addition to this each enantiomer of the corresponding sulfamate was tested against both aromatase and STS. The enantiomers were found to have differing activities against each enzyme and (1) enantiomer of the phenol was similar in potency against aromatase to letrozole.…”

“…The enantiomers were found to have differing activities against each enzyme and (1) enantiomer of the phenol was similar in potency against aromatase to letrozole. 5 After the HPLC separation, it was necessary to determine the AC of each enantiomer of I. Unlike the racemic mixture, the separated enantiomers could not be crystallized successfully, as slow evaporation of the solvent from a solution of either enantiomer gave an oil.…”