Tetrahydroisoquinoline Sulfamates as Potent Microtubule Disruptors: Synthesis, Antiproliferative and Antitubulin Activity of Dichlorobenzyl-Based Derivatives, and a Tubulin Cocrystal Structure.

Wolfgang Dohle,Andrea E Prota,Ernest Hamel,Grégory Menchon,Barry V L Potter,Michel O Steinmetz

doi:10.1021/acsomega.8b02879

Abstract

Tetrahydroisoquinoline (THIQ) 6-O-sulfamate-based anticancer agents, inspired by the endogenous steroid 2-methoxyestradiol and its sulfamate derivatives, are further explored for antiproliferative and microtubule disruptor activity. Based on recently designed C3-methyl C7-methoxy-substituted THIQ derivatives, compounds with mono- and dichloro-substitutions on the pendant N-benzyl ring were synthesized and evaluated. Although improved antiproliferative activity was observed, for example, 4a versus 4b and 4b versus 8c, it was relatively modest. Compound 8c, a 2′,5′-dichlorobenzyl derivative was, however, identified as a promising antiproliferative agent with in vitro activities exceeding that of the parent steroid (e.g., GI50 90 nM in DU-145 cells) and was highly potent against a range of tumor cell lines (e.g., GI50 26 nM for OVCAR-3). 8c inhibited the polymerization of tubulin in vitro with an IC50 only twofold less potent than combretastatin A-4 and inhibited colchicine binding to tubulin. Tubulin polymerization assays showed the parent THIQ 4a to be only a very weak inhibitor, but a striking potency difference was seen between compounds with C2′ methoxy and chloro substituents, whereas this was much smaller when these substituents were positioned at C5′. To confirm the target in atomic detail and because 8c is a racemic mixture, an achiral parent THIQ 6-O-sulfamate derivative 10 was successfully cocrystallized with the αβ-tubulin heterodimer. The derivative 10 binds at the colchicine site on tubulin, the first example of this compound class investigated in such detail, with its sulfamate group interacting with residues beyond the reach of colchicine itself, similar to a recently reported quinazolinone sulfamate derivative, 6a. The structure also suggests that for racemic C3-methyl-substituted THIQ derivatives, such as 8c, the (S)-enantiomer is likely to be preferentially accommodated within the colchicine site for steric reasons. The results further confirm the potential of nonsteroidal THIQ sulfamate derivatives for oncology and suggest that the mechanism of microtubule destabilization for the THIQ compound class is to prevent the curved-to-straight conformational transition of tubulin required for polymerization.

Highlights

We described N-benzyl-substituted tetrahydroisoquinoline (THIQ) derivatives as novel microtubule disruptors with potential therapeutic application for the treatment of cancer.[1−5] These compounds were designed to mimic the 2-substituted estratriene class of microtubule disruptors derived from the endogenous steroid 2-methoxyestradiol (2ME) and, in particular, its sulfamate derivatives (e.g., 2a (STX140), Figure 1).[6−10] Incorporation of a phenolic 3-O-sulfamate group is generally observed to be highly beneficial for both activity and oral bioavailability, and STX140 is an optimized anticancer agent with both 3-O- and 17-O-sulfamate groups.[11]
The candidate THIQs were synthesized in two steps starting from (±)-6-hydroxy-7-methoxy-3-methyl-1,2,3,4THIQ 9.3 N-Benzylation using dichlorobenzyl halides, diisopropylethylamine (DIPEA) in N,N-dimethylformamide (DMF) at 140 °C for 18 h gave phenols 7a−c in moderate yields
All six compounds were evaluated at the US National Cancer Institute (NCI) in the full 60-cell-line assay that allows activity across a wide range of cancer types to be assessed (Tables 1 and 2)

Summary

■ INTRODUCTION

We described N-benzyl-substituted tetrahydroisoquinoline (THIQ) derivatives as novel microtubule disruptors with potential therapeutic application for the treatment of cancer.[1−5] These compounds were designed to mimic the 2-substituted estratriene class of microtubule disruptors derived from the endogenous steroid 2-methoxyestradiol (2ME) and, in particular, its sulfamate derivatives (e.g., 2a (STX140), Figure 1).[6−10] Incorporation of a phenolic 3-O-sulfamate group is generally observed to be highly beneficial for both activity and oral bioavailability, and STX140 is an optimized anticancer agent with both 3-O- and 17-O-sulfamate groups.[11]. STX140 was developed primarily for hormone-independent cancer applications, and the nonsteroidal THIQ core was used as a mimic of the steroidal AB-ring system from which steroidomimetics were constructed Such compounds have markedly better physicochemical properties than 2ME and even STX140 but can maintain in vivo potency. THIQ derivatives likely have more developmental potential, their charged nature in particular affording better solubility via salt formation at the N2 position They have been shown to be orally active in vivo when dosed in an aqueous buffer, with some being as potent as STX140.4 It was of interest to explore the potential for further optimization of such derivatives via new substitutions in the pendant aryl ring and, if this THIQ class could be cocrystallized with the αβ-tubulin heterodimer, such that atomic level information could be obtained, for further structure−activity relationship (SAR) development.

H Cl H H H Cl

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES