Characterization of Novel Salvinorin A Analogues as Active State Probes of the κ-Opioid Receptor †

Salvinorin A, the most potent naturally occurring hallucinogen, has attracted an increasing amount of attention since the κ-opioid receptor (KOR) was identified as its principal molecular target by us [Roth, B. L., et al. (2002)Proc. Natl. Acad. Sci. U.S.A. 99, 11934-11939]. Here we report the design, synthesis, and biochemical characterization of novel, irreversible, salvinorin A-derived ligands suitable as active state probes of the KOR. On the basis of prior substituted cysteine accessibility and molecular modeling studies, C315 was chosen as a potential anchoring point for covalent labeling of salvinorin A-derived ligands. Automated docking of a series of potential covalently bound ligands suggested that either a haloacetate moiety or other similar electrophilic groups could irreversibly bind with C315. 22-Thiocyanatosalvinorin A (RB-64) and 22-chlorosalvinorin A (RB-48) were both found to be extraordinarily potent and selective KOR agonists in vitro and in vivo. As predicted on the basis of molecular modeling studies, RB-64 induced wash-resistant inhibition of binding with a strict requirement for a free cysteine in or near the binding pocket. Mass spectrometry (MS) studies utilizing synthetic KOR peptides and RB-64 supported the hypothesis that the anchoring residue was C315 and suggested one biochemical mechanism for covalent binding. These studies provide direct evidence of the presence of a free cysteine in the agonist-bound state of the KOR and provide novel insights into the mechanism by which salvinorin A binds to and activates the KOR. Salvinorin A, the active ingredient of the hallucinogenic plant Salvia divinorum, is the most potent known naturally occurring hallucinogen (1, 2). In 2002, we discovered that the κ-opioid receptor (KOR) was the molecular target for the actions of salvinorin A in vitro (3). Studies with KOR knockout mice (4) unequivocally demonstrated that the KOR was also the site of action of salvinorin A in vivo, a finding which has been widely replicated (see refs 5 and 6 for reviews). Subsequently, salvinorin A emerged as an attractive lead compound for drug discovery, and over the past few years, hundreds of salvinorin A derivates have been synthesized (6). Some of these analogues present interesting pharmacological profiles, from full KOR agonist to partial δ-opioid receptor (DOR) or μ-opioid receptor (MOR) agonists and antagonists (7-11). However, most of the hundreds of analogues displayed decreased affinity (or even no affinity) for the KOR. The challenge now is to use the knowledge about salvinorin A-KOR interactions (12, 13) to design unique salvinorin A derivatives with novel pharmacological profiles and therapeutic potential. In recent years, covalently bound ligands emerged as a new class of receptor ligands with unique pharmacological properties. The successful design of the covalently bound ligands includes an allosteric modulator for the GABAA receptor (14), fluorescently tagged inhibitors for calcium-bound protein arginine deiminase 4 (PAD4) (15), selective kinase inhibitors (16, 17), estradiols for the estrogen receptor (18, 19), and antagonists for the N-methyl-D-aspartate (NMDA) receptor (a ligand-gated cation channel) (20). This covalent labeling method takes advantage of the chemically reactive amino acids inside or close to the ligand binding site, and the reactive group is frequently a nucleophile. Because the recognition This research was supported in part by National Institutes of Health (NIH) Grant R01DA017204 (to B.L.R.) and the NIMH Psychoactive Drug Screening Program. J.A.A. was supported by a Neurodevelopmental Training Program Fellowship Grant from the NIH. *To whom correspondence should be addressed: Department of Pharmacology, 4072 Genetics Medicine Building, Medical School, University of North Carolina, Chapel Hill, NC 27599. Phone: (919) 966-7535. Fax: (919) 843-5788. E-mail: bryan_roth@med.unc.edu. Abbreviations: GPCR, G protein-coupled receptor; KOR, κ-opioid receptor; hKOR, human KOR; GR, G protein R subunit; TM, transmembrane domain; CHCA, R-cyano-4-hydroxycinnamic acid; CID, collision-induced dissociation; EL, extracellular loop; IL, intracellular loop; MALDI, matrix-assisted laser desorption ionization; MS, mass spectrometry;MS/MS, tandemmass spectrometry;m/z, mass-to-charge ratio; S/N, signal-to-noise ratio; TOF, time-of-flight; bn and yn, Biemanmodified Roepstorff and Fohlman peptide ion nomenclature. D ow nl oa de d by U O F C A L IF O R N IA L O S A N G E L E S on A ug us t 1 3, 2 00 9 Pu bl is he d on J un e 25 , 2 00 9 on h ttp :// pu bs .a cs .o rg | do i: 10 .1 02 1/ bi 90 06 05 n Article Biochemistry, Vol. 48, No. 29, 2009 6899 between the ligand and receptor is specific, proximity-accelerated covalent labeling is expected to be highly selective. Introduction of a proper electrophilic group into the ligand structure is the key condition for promoting covalent labeling between the ligand and receptor. However, the structural variety of receptors and ligands creates uncertainty in choosing the optimal reactive group. Among the common reactive groups, halomethylketones, haloacetamides, isothiocyanates,Michael acceptors, aldol esters, nitrogenmustards, and other electrophilic moieties have been used for covalently bound ligands because of their relatively high reactivity under physiological conditions (e.g., pH 7.4). Our prior molecular modeling studies predicted that a chemically reactive cysteine (C315) would reside in or near salvinorin A’s binding site in the agonist-bound state of theKOR (12, 13, 21). Indeed, our prior studies showed there was a direct interaction between the acetoxy group of salvinorinA andY313, a critical residue inside the binding pocket (12) . Nearby, awater-accessible C315 is highly reactive tomethanethiosulfonate (MTS) reagent (Figure 1A), as reported in the substituted cysteine accessibility method (SCAM) studies byXu et al. (22, 23) and us (21). Because the C315S mutation did not significantly affect salvinorin A’s binding affinity, we predicted that C315 exists as a free cysteine rather than as a contributor to disulfide bonds or the global structure of the KOR. Theoretically, C315 could provide an anchoring point for covalent labeling. In this paper, we describe molecular modeling studies which predict that 22-substituted salvinorin A derivatives will interact with C315. On the basis of these predictions, 22-thiocyanatosalvinorin A (RB-64) and 22-chlorosalvinorin A (RB-48) were synthesized and found to be extraordinarily potent, selective, and apparently irreversible KOR agonists in vitro. In vivo studies revealed RB-64 to be 20-fold more efficacious than salvinorin A. Mutagenesis experiments and biochemical studies with purified KOR peptides in vitro confirmed the mechanism of irreversible binding to be nucleophilic substitution of C315. These studies provide convincing evidence for a free cysteine in or near the agonist-bound active state of the KOR. EXPERIMENTAL PROCEDURES Materials. Standard reagents were purchased from SigmaAldrich (St. Louis, MO). Some of the salvinorin A used as a standard in these studies was kindly provided by T. Prisinzano (University of Kansas, Lawrence, KS). Syntheses and Characterization of 22-Chlorosalvinorin A (RB-48) and 22-Thiocyanatosalvinorin A (RB-64). Salvinorin B (25 mg, 64 μmol) and catalytic (dimethylamino)pyridine (DMAP) were dissolved in 5 mL of dichloromethane (DCM). Chloroacetyl chloride (10 mg, 90 μmol) was added, and the reaction mixture was stirred at room temperature for 2 h. Then the solvents were evaporated in vacuo, and the residue was chromatographed on a silica gel (3:1 hexane/ethyl acetate) to yield 22-chlorosalvinorin A (22.7 mg, 76% yield): white solid; mp 216-218 C; RD -27.5 (c 0.08, MeCN); HRESIMS m/z [MþH]þ 467.1457 (calcd for C23H27ClO8 466.1394); C NMR (100 MHz, CDCl3) δ 15.2, 16.4, 18.1, 30.5, 35.4, 38.1, 40.6, 42.1, 43.0, 51.2, 52.0, 53.3, 63.7, 71.9, 76.5, 108.5, 125.2, 139.6, 143.7, 166.6, 171.1, 171,4, 201.2; H NMR (400 MHz, CDCl3) δ 1.08 (s, 3H), 1.39 (s, 3H), 1.52-1.65 (m, 3H), 1.75 (m, 1H), 2.04-2.15 (m, 2H), 2.25-2.34 (m, 3H), 2.39 (dd, J=5, 13 Hz, 1H), 2.76 FIGURE 1: Molecularmodeling reveals potential sites of adduct formation in theKOR. (A) TheC-2 position of salvinorinA is in the proximity of Y313 andC315 in thewild-type (WT)KOR.Ribbons (white) indicate the position of the backbone, and aConnolly channel surface (green) describes the regions of steric accessibilitywithin the activated receptor.Anenlarged regionof accessibility in the intracellular portion of the helical bundle is indicative of an activated GPCR. (B) The RB-64 thiocyanate group is in the proximity of C315. (C) The mechanisms for covalent labeling of cysteine involve nucleophilic substitution at C-22 or the adjacent S atom. The molecular weight change for the modified peptides is 431 (i) or 463 (ii) depending on the site of substitution. D ow nl oa de d by U O F C A L IF O R N IA L O S A N G E L E S on A ug us t 1 3, 2 00 9 Pu bl is he d on J un e 25 , 2 00 9 on h ttp :// pu bs .a cs .o rg | do i: 10 .1 02 1/ bi 90 06 05 n 6900 Biochemistry, Vol. 48, No. 29, 2009 Yan et al. (m, 1H), 3.70 (s, 3H), 4.18 (ABq, J=15 Hz, 2H), 5.20 (m, 1H), 5.43 (dd, J=5, 12 Hz, 1H), 6.36 (s, 1H), 7.37 (br s, 1H), 7.39 (s, 1H). 22-Chlorosalvinorin A (20mg, 43 μmol) was dissolved in anhydrous ethanol (2 mL) with potassium thiocyanate (5.4 mg, 56 μmol). The reaction mixture was refluxed for 5 h, allowed to reach room temperature, and concentrated in vacuo.Water (2mL) was then added, and the reaction mixture was extracted with chloroform (three 5 mL portions). Solvents were removed in vacuo, and the mixture was separated by HPLC [C18 column, MeCN/H2O (1:1), detection at 210 nm] to yield 22-thiocyanatosalvinorin A (16.6 mg, 79% yield): semisolid; RD 20 -32.5 (c 0.08, MeCN);HRESIMSm/z [MþH]þ 490.1519 (calcd forC24H27NO8S 489.1452); IR (neat) 2160 cm (SCN); C NMR (100 MHz, CDCl3) δ 15.2, 16.4