Characterization of FlipIDAM as a SERT-selective SPECT imaging agent.

INTRODUCTION: Biological evaluation of ([(125)I]4), a new single-photon emission computed tomography (SPECT) radioligand for imaging the serotonin transporter (SERT) which displayed improved in vivo kinetics for mapping SERT binding sites in the brain. METHODS: In vitro binding studies of [(125)I]4 were performed with membrane homogenates of LLC-PK1 cells stably transfected and overexpressing one of the monoamine transporter (SERT, DAT or NET) and rat cortical homogenates. Biodistribution and ex vivo autoradiography studies were carried out in rats. In vivo competition experiments were evaluated to determine the SERT selectivity of [(125)I]4 vs. ([(125)I]1). RESULTS: In vitro binding studies of 4 showed excellent binding affinity (Ki,SERT=0.90 ± 0.05 nM) and excellent selectivity over the other monoamine transporters (100 fold and >4000 fold for NET and DAT respectively). Scatchard analysis of saturation binding of [(125)I]4 to rat cortical homogenates gave a Kd value of 0.5 ± 0.09 nM and a Bmax value of 801.4 ± 58.08 fmol/mg protein. The biodistribution study showed rapid high brain uptake (3.09 ± 0.11% dose/organ at 2 min) and a good target to non-target ratio (hypothalamus to cerebellum) at 30 min (2.62) compared to [(125)I]1 (2.19). Ex vivo autoradiography showed that FlipIDAM localizes in accordance with SERT distribution patterns in the brain. In vivo and ex vivo competition experiments with specific and non-specific SERT compounds also showed that [(125)I]4 binds specifically to SERT rich regions. CONCLUSIONS: The biological evaluation of [(125)I]4 demonstrates that [(123)I]4 would be a good candidate for SPECT imaging of SERT.

A new single-photon emission computed tomography (SPECT) imaging agent for serotonin transporters: [(125)I]Flip-IDAM, (2-((2-((dimethylamino)methyl)-4-iodophenyl)thio)phenyl)methanol.

New ligands for in vivo brain imaging of serotonin transporter (SERT) with single photon emission tomography (SPECT) were prepared and evaluated. An efficient synthesis and radiolabeling of a biphenylthiol, FLIP-IDAM, 4, was accomplished. The affinity of FLIP-IDAM was evaluated by an in vitro inhibitory binding assay using [(125)I]-IDAM as radioligand in rat brain tissue homogenates (K(i) = 0.03 nM). New [(125)I]Flip-IDAM exhibited excellent binding affinity to SERT binding sites with a high hypothalamus to cerebellum ratio of 4 at 30 min post iv injection. The faster in vivo kinetics for brain uptake and a rapid washout from non-specific regions provide excellent signal to noise ratio. This new agent, when labeled with (123)I, may be a useful imaging agent for mapping SERT binding sites in the human brain.

Synthesis and biological evaluation of 3-alkyl-dihydrotetrabenazine derivatives as vesicular monoamine transporter-2 (VMAT2) ligands.

In the search of new probes for in vivo brain imaging of vesicular monoamine transporter type 2 (VMAT2), we have developed an efficient synthesis of a novel series of 3-alkyl-dihydrotetrabenazine (DTBZ) derivatives. The affinity of VMAT2 was evaluated by an in vitro inhibitory binding assay using [(125)I]-iodovinyl-TBZ or [(18)F](+)-FP-DTBZ as radioligands in rat striatal tissue homogenates. New DTBZ derivatives exhibited moderate to good binding affinity to VMAT2. Among these new ligands, compound 4b showed the best affinity for VMAT2 (K(i)=5.98 nM) and may be a useful lead compound for future structure-activity studies.

Late-stage diversification of chiral N-heterocyclic-carbene precatalysts for enantioselective homoenolate additions.

A library of chiral triazolium salts has been prepared by late-state diversification of a triazolium amine salt. By utilizing a primary amine as a functional handle, a single triazolium salt can be transformed into a variety of chiral N-heterocyclic carbene precatalysts. This approach makes the preparation of chiral N-heterocyclic carbenes possible by a single-step modification of a triazolium salt, rather than the usual need for multistep organic synthesis and challenging heterocycle formation for each member of a catalyst library. We have screened these catalysts for control of diastereo- and enantioselectivity in a γ-lactam-forming reaction between α,ß-unsaturated aldehydes and cyclic ketimines.

An enantioselective Claisen rearrangement catalyzed by N-heterocyclic carbenes.

In the presence of a chiral azolium salt (10 mol %), enols and ynals undergo a highly enantioselective annulation reaction to form enantiomerically enriched dihydropyranones via an N-heterocyclic carbene catalyzed variant of the Claisen rearrangement. Unlike other azolium-catalyzed reactions, this process requires no added base to generate the putative NHC-catalyst, and our investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species. Detailed kinetic studies eliminate a potential 1,4-addition as the mechanistic pathway; the observed rate law and activation parameters are consistent with a Claisen rearrangement as the rate-limiting step. This catalytic system was applied to the synthesis of enantioenriched kojic acid derivatives, a reaction of demonstrated synthetic utility for which other methods for catalytic enantioselective Claisen rearrangements have not provided a satisfactory solution.

A stereospecific synthesis of chiral cyclic sulfinamides.

Treatment of racemic or enantiomerically pure 2,1-benzothiazines (cyclic sulfoximines) with lithium triethylborohydride results in clean loss of the S-aryl group with complete retention of configuration at sulfur to produce diastereomerically and/or enantiomerically pure cyclic sulfinamides in excellent yield.

Expedient synthesis of sulfinamides from sulfonyl chlorides.

Sulfinamides were synthesized from sulfonyl chlorides using a procedure involving in situ reduction of sulfonyl chlorides. The reaction is broad in scope and easy to perform.

New synthesis of benzothiazines and benzoisothiazoles containing a sulfoximine functional group.

The reaction of S-2-bromophenyl-S-methylsulfoximine with terminal alkynes in the presence of a palladium catalyst resulted in the formation of both 1,2-benzothiazines and 1,2-benzoisothiazoles. A preference for the former was seen with alkylalkynes, while the latter were preferentially formed with alkynylarenes.