A novel ternary ligand system useful for preparation of cationic (99m)Tc-diazenido complexes and (99m)Tc-labeling of small biomolecules

Abstract

This report describes a novel ternary ligand system composed of a phenylhydrazine, a crown ether-containing dithiocarbamate (DTC), and a PNP-type bisphosphine (PNP). The combination of three different ligands with (99m)Tc results in cationic (99m)Tc-diazenido complexes, [(99m)Tc(NNAr)(DTC)(PNP)]+, with potential radiopharmaceuticals for heart imaging. Synthesis of cationic (99m)Tc-diazenido complexes can be accomplished in two steps. For example, the reaction of phenylhydrazine with (99m)TcO4- at 100 degrees C in the presence of excess stannous chloride and 1,2-diaminopropane-N,N,N',N'-tetraacetic acid (PDTA) results in the [(99m)Tc(NNPh)(PDTA)n] intermediate, which then reacts with sodium N-(dithiocarbamato)-2-aminomethyl-15-Crown-5 (L4) and N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]ethoxyethylamine (PNP6) at 100 degrees C for 15 min to give the complex, [(99m)Tc(NNPh)(L4)(PNP6)]+ in high yield (>90%). Cationic complexes [(99m)Tc(NNPh)(DTC)(PNP)]+ are stable for > or = 6 h. Their composition was determined to be 1:1:1:1 for Tc:NNPh:DTC:PNP using the mixed-ligand experiments on the tracer ((99m)Tc) level and was further confirmed by the ESI-MS spectral data of a model compound [Re(NNPh)(L4)(L6)]+. It was found that both DTCs and bisphosphines have a significant impact on the lipophilicity of their cationic (99m)Tc-diazenido complexes. Results from a (99m)Tc-labeling efficiency experiment showed that 4-hydrazinobenzoic acid (HYBA) might be useful as a bifunctional coupling agent for (99m)Tc-labeling of small biomolecules. However, the (99m)Tc-labeling efficiency of HYBA is much lower than that of 6-hydrazinonicotinic acid (HYNIC) with tricine and trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS) as coligands.

Figure 1

Binary and ternary ligand ^99mTc complexes of HYNIC-conjugated biomolecules (HYNIC-BM). The combination of HYNIC, tricine, and a phosphine or pyridine analog results in a versatile ternary ligand system that forms ternary ligand ^99mTc complexes [^99mTc(HYNIC-BM)(tricine)(TPPTS)] (BM = biomolecule) with extremely high solution stability.

Figure 2

Structures of crown ether-containing dithiocarbamates (crowned DTCs), bisphosphine coligands, ^99mTcN-DBODC5 and cationic ^99mTc-diazenido complexes.

Figure 3

Typical radio-HPLC chromatograms (Method 1) for cationic complexes [^99mTc(NNPh)(L2)(PNP6)]⁺ and [^99mTc(NNPh)(L4)(PNP6)]⁺. Small radioimpurity peaks at ~12.5 min are most likely caused by the partial oxidation of the bisphosphine during preparation.

Figure 4

A typical radio-HPLC chromatogram (Method 2) of the reaction mixture containing cationic complexes [^99mTc(NNPh)(L4)(PNP5)]⁺ and [^99mTc(NNPh)(DBODC)(PNP5)]⁺.

Figure 5

Radio-HPLC chromatogram (Method 1) of the reaction mixture containing cationic complexes [^99mTc(NNPh)(L4)(PNP5)]⁺ (left) and [^99mTc(NNPh)(L4)(PNP6)]⁺.

Figure 6

Typical radio-HPLC chromatograms (Method 1) for cationic complexes [^99mTc(NNPh)(L2)(L6)]⁺ (solid line) and [^99mTc(NNPh-2,5-Me₂)(L4)(L6)]⁺ (dashed line). Small radioimpurity peaks at ~12.5 min are most likely caused by the partial oxidation of the bisphosphine during preparation.

Figure 7

The HPLC concordance (Method 1) for the HPLC-purified [Re(NNPh)(L4)(L6)]⁺ (dashed line) and [^99mTc(NNPh)(L4)(L6)]⁺ (solid line).

Figure 8

ESI mass spectrum (top) and the proposed fragmentation pattern (bottom) for the cationic complex [Re(NNPh)(L4)(L6)]⁺.

Figure 9

The ³¹P NMR spectrum of [Re(NNPh)(L4)(L6)]⁺.

Scheme I

Synthesis of Crowned DTCs (L1 – L5).

Scheme II

Synthesis of the Bisphosphines (PNP5, PNP6 and L6).

Scheme III

Synthesis of Cationic Complexes [^99mTc(NNPh)(DTC)(PNP)]⁺

Scheme IV

Synthesis of [Re(NNPh)(L4)(L6)]⁺