Potency enhancement of the κ-opioid receptor antagonist probe ML140 through sulfonamide constraint utilizing a tetrahydroisoquinoline motif

https://doi.org/10.1016/j.bmc.2014.12.033Get rights and content

Abstract

Optimization of the sulfonamide-based kappa opioid receptor (KOR) antagonist probe molecule ML140 through constraint of the sulfonamide nitrogen within a tetrahydroisoquinoline moiety afforded a marked increase in potency. This strategy, when combined with additional structure–activity relationship exploration, has led to a compound only six-fold less potent than norBNI, a widely utilized KOR antagonist tool compound, but significantly more synthetically accessible. The new optimized probe is suitably potent for use as an in vivo tool to investigate the therapeutic potential of KOR antagonists.

Introduction

Activation of the kappa opioid receptor (KOR) by endogenous neuropeptides, primarily dynorphin, initiates complex signaling cascades. The downstream effects of KOR agonism vary greatly and include antinociception, dysphoria and anxiety, though the details of the pharmacological pathways are still being elucidated.1, 2 In contrast, KOR antagonists have been investigated as potential therapeutic treatments for addiction, depression, post traumatic stress disorder, eating disorders and other conditions related to anxiety or aversion-reward responses.3, 4, 5 Many canonical KOR antagonists (Fig. 1) are derived from, or bear a structural element of morphinan opioids, such as the widely-utilized tool compounds norBNI,6 5′-GNTI7 and JDTic.8 Some KOR antagonists that are widely used, such as norBNI, have a remarkable long duration of action in animal models (weeks), which could introduce difficulties in interpreting the effect of chronic antagonist exposure, or some other effects of the drug that are not fully understood.9, 10, 11, 12 Accordingly, one goal of our laboratories is to discover and evaluate structurally-distinct KOR antagonists with novel pharmacological properties, including a focus on KOR antagonists that are more rapidly cleared in vivo. We recently disclosed a new sulfonamide series of KOR antagonists, exemplified by the Molecular Libraries probe molecule ML140 (Fig. 1).13

Although ML140 only exhibited modest potency in the DiscoveRx βarrestin2 PathHunter™ assay (403 nM), it was selective against the μ and δ receptors (IC50 values of >24 and >32 μM, respectively). In contrast to the better-established chemotypes noted, ML140 is highly modular, possesses no stereogenic centers and bears little structural similarity to known opioid ligands. Together, these properties inspired us to further investigate the structure–activity relationship (SAR) of this chemotype. In our original report,13 we evaluated the effect on biological potency through ca. forty structural changes focused primarily on the ‘eastern’ portion of the molecule (Fig. 2). We investigated both replacing the substituents of the basic nitrogen as well as introducing constraint into the eastern region of the molecule. Perhaps most interestingly, we found that replacing the N-isopropyl with N-tert-butyl can increase the potency by as much as 14-fold. Less successful were our attempts to enhance antagonist potency through constraint of the nitrogen within various ring systems as illustrated by the structures in Figure 2.

Each of these changes eradicated essentially all KOR activity, suggesting that the constraints examined placed this portion of the molecule into a conformation inconsistent with receptor activity, introduced unpalatable steric clashes, deprived the compound of an important hydrogen bond, or some combination of these effects. Herein we describe a more fruitful approach to conformational constraint by focusing on the modification of the sulfonamide portion of the molecule while retaining the promising substitution patterns around the basic nitrogen. This process afforded a series of antagonists with single-digit nanomolar potency in G protein coupling and consistent SAR trends across the series.

Section snippets

Results and discussion

In this investigation we measured the potency of all test compounds by three complementary methods for assessing KOR antagonist potency:14 [35S]GTPγS binding,15 ERK activation,16 and βarrestin2 recruitment.17 Since our compounds, like norBNI, are found to be most potent in the [35S]GTPγS assay over the other two, we chose this assay for primary SAR determination. The analogues presented here were prepared in the manner of those previously reported,11 which is summarized in Scheme 1. Briefly,

Conclusion

We have improved the antagonist potency of the sulfonamide-based KOR antagonist probe molecule ML140 from 138 nM to 1.6 nM as measured by [35S]GTPγS binding to afford a new molecular tool appropriate to investigate the physiological role of the KOR and the therapeutic potential of KOR antagonists. The constrained sulfonamide analogue 1l is almost 100-fold more potent than ML140 and permits for the first time the potential use of this chemotype in behavioral or in vivo efficacy models. These

General methods

All reagents and materials were purchased from commercial vendors (Sigma, Alfa Aesar, Oakwood or ASW Medchem) and used as received. Ethyl ether, toluene, THF, MeCN and CH2Cl2 were degassed with nitrogen and passed through two columns of basic alumina on an Innovative Technology solvent purification system. 1H and 13C NMR spectra were recorded on a Bruker AM 400 spectrometer (operating at 400 and 100 MHz, respectively) in CDCl3 with 0.03% TMS as an internal standard, unless otherwise specified.

Acknowledgments

We gratefully acknowledge financial support from the National Institute on Drug Abuse (Grant R01 DA031927 to L.M.B. and J.A.). We thank Ben Neuenswander for performing HPLC compound purification and high resolution mass determinations. Ki determinations were generously provided by the National Institute of Mental Health’s Psychoactive Drug Screening Program, Contract # HHSN-271-2013-00017-C (NIMH PDSP). The NIMH PDSP is directed by Bryan L. Roth MD, PhD at the University of North Carolina at

References and notes (18)

  • W.A. Carlezon et al.

    Pharmacol. Ther.

    (2009)
  • M.R. Bruchas et al.

    J. Biol. Chem.

    (2007)
  • C.L. Schmid et al.

    J. Biol. Chem.

    (2013)
  • R.I.W. Osmond et al.

    J. Biomol. Screen.

    (2005)
  • X. Zhao et al.

    J. Biomol. Screen.

    (2008)
  • L. Zhou et al.

    J. Biol. Chem.

    (2013)
  • A.-H. Ream et al.

    Anesthesiology

    (2011)
  • J.T. Lamberts et al.

    Curr. Pharm. Design

    (2013)
  • M.D. Metcalf et al.

    AAPS J.

    (2005)
There are more references available in the full text version of this article.

Cited by (7)

  • 7β-Methyl substituent is a structural locus associated with activity cliff for nepenthone analogues

    2018, Bioorganic and Medicinal Chemistry
    Citation Excerpt :

    The antidepressant-like effects of KOR antagonists were observed in both non-clinical and clinical studies.9–13 Early identified selective non-peptide KOR antagonists, e.g., nor-BNI14,15 and GNTI16(Fig. 1), have demonstrated potent KOR antagonistic effects in rodent or non-human primate models, but their unfavorable physiochemical and pharmacokinetic properties are the main challenges to overcome before they could be considered as potential drug candidates. JDTic17, a structurally unique (3R, 4R)-dimethyl-4-(3-hydroxyphenyl)-piperidine, was identified as a potent, orally-active and selective KOR antagonist.

  • Endogenous Opiates and Behavior: 2015

    2017, Peptides
    Citation Excerpt :

    C7beta-methyl analogues of the orvinols were identified as KOR antagonists with ORL-1 partial agonism and no MOR efficacy [274]. The KOR antagonist probe ML 140 was enhanced through sulfonamide constraint utilizing a tetrahydroisoquinoline motif [422]. 2-alkyl-2-methoxymethyl-salvinorin ethers were identified as selective KOR agonists [750].

  • Characterization of kappa opioid receptor mediated, dynorphin-stimulated [<sup>35</sup>S]GTPγS binding in mouse striatum for the evaluation of selective KOR ligands in an endogenous setting

    2015, Neuropharmacology
    Citation Excerpt :

    These data, summarized in Table 2, indicate that both KOR and MOR are major components of the dynorphin-induced responses in mouse striatum and that norBNI and 5′-GNTI are not selective for KOR. We have previously reported on a novel series of KOR antagonists based on a sulfonamide scaffold (Frankowski et al., 2014). Compound 1 (Fig. 7 A) has higher affinity for KOR over DOR and MOR when evaluated in CHO cells overexpressing the human MOR, DOR or KOR (Fig. 7B).

View all citing articles on Scopus
View full text