Highly Efficient Technetium-99m Labeling Procedure Based on the Conjugation of N-[N-(3-Diphenylphosphinopropionyl)glycyl] cysteine Ligand with Poly(ethylene glycol)

The PN2S N-(N-(3-diphenylphosphinopropionyl)glycyl)cysteine ligand was conjugated to methoxy-poly(ethylene glycol)amino (mPEG-NH2) 5 and 20 kDa to yield PN2S(Trt)-PEG5000 1 and PN2S(Trt)-PEG20000 2, and then detritylated to PN2S-PEG5000 4 and PN2S-PEG20000 5. When an acidic solution of 99mTcO4- is added to 4 or 5 in solid form, a quantitative yield in a single labeled species, 99mTc-labeled PN2S-PEG5000 9 and 99mTc-labeled PN2S-PEG20000 10, respectively, is obtained. The reaction occurs in less than 15 min at room temperature for 4 and 35 (degree)C for 5. This labeling procedure avoids the use of an external reducing agent, and it is based on the amphiphilic properties of PN2S-PEGs. Once in water, 4 and 5 self-assemble in micelles, which catalyze the metal reduction by means of an electron pair transfer from the phosphorus to technetium. The [99mTcO]^3+ species is then coordinated, and at micelle level, both the (P)ON2S and the PN2S coordinations are possible, as demonstrated by reacting 99mTc-gluconate and ReOCl3(PPh3)2 with 4 and 5 and with the oxidized analogous (P)ON2S-PEG5000 6. Compounds 9 and 10 exhibited a high stability both in vitro and in vivo. Biodistribution studies in mice also indicated that PN2S linking and 99mTc labeling do not modify PEG behavior in water and in vivo since the polymer dictates the fate of the conjugate.