Synthesis and receptor binding properties of fluoro- and iodo-substituted high affinity sigma receptor ligands: identification of potential PET and SPECT sigma receptor imaging agents.

Unlabeled fluoro- and iodo-substituted ligands exhibiting very high affinity and selectivity for sigma receptors were synthesized based on three different structural classes of sigma receptor ligands. These compounds were evaluated for sigma receptor affinity and specificity in order to assess their potential as PET/SPECT imaging agents. Thus, (+)- and (-)-N-(5-fluoro-1-pentyl)normetazocines (+)- and (-)-4] based on the (+)-benzomorphan class of sigma ligands were synthesized via N-alkylation of optically pure (+)- and (-)-normetazocine with 5-(methylsulfonyl)oxy]-1-pentyl fluoride (11). (+)- and (-)-4 displaced 3H](+)-3-PPP with Ki values of 0.29 and 73.6 nM and 3H](+)-pentazocine with Ki values of 10.5 and 38.9 nM, respectively. The second class of PET/SPECT ligands was based upon the N-(arylethyl)-N-alkyl-2-(1-pyrrolidinyl)ethylamine class of sigma ligands; N-2-(3,4-dichlorophenyl)-1-ethyl]-N-(3-fluoro-1-propyl)-2-(1- pyrrolidinyl)ethylamine (5) was obtained via N-alkylation of N-2-(3,4-dichlorophenyl)-1-ethyl]-2-(1-pyrrolidinyl)ethylamine (14) with 3-fluoropropyl p-toluenesulfonate. 5 exhibited Ki values of 4.22 and 5.07 nM for displacement of 3H](+)-3-PPP and 3H](+)-pentazocine, respectively, comparable with the parent N-propyl compound. Attempts to synthesize N-2-(3,4-dichlorophenyl)-1-ethyl]-N-3- (methylsulfonyl)oxy]-1-propyl]-2-(1-pyrrolidinyl)ethylamine (26), a precursor to 5 that could conceivably be converted to 18F]-5 by treatment with 18F-, proved unsuccessful. The sequence of regioselective nitration, catalytic hydrogenation, and diazotization followed by NaI quench of N-2-(3,4-dichlorophenyl)-1-ethyl]-N-methyl-2-(1- pyrrolidinyl)ethylamine (2) afforded the iodinated ethylenediamine N-2-(2-iodo-4,5-dichlorophenyl)-1-ethyl]-N-methyl-2-(1- pyrrolidinyl)ethylamine (8), a potential SPECT ligand for sigma receptors. This compound showed an affinity of 0.54 nM (3H](+)-3-PPP) comparable with the parent compound 2 (Ki = 0.34 nM, 3H](+)-3-PPP). Ligand 8 exhibited a similar potency against 3H](+)-pentazocine. The third class of high-affinity sigma receptor ligands was rationalized based on rearrangement of the bonds in ethylenediamine 2 to give 1-2-(3,4-dichlorophenyl)-1-ethyl]-4-(1-propyl)piperazine (3). This compound exhibited very high affinity (Ki = 0.31 nM, 3H](+)-3-PPP) and selectivity for sigma receptors.(ABSTRACT TRUNCATED AT 400 WORDS)