NHS-MAS(3): a bifunctional chelator alternative to NHS-MAG(3)

This laboratory uses an N-hydroxysuccinimide derivative of S-acetylmercapto acetyltriglycine (NHS-MAG(3)) to conjugate amines for subsequent labeling w ith Tc-99m. However, the synthesis from triglycerine is general and not res tricted to this tripeptide. We had earlier selected a small number of alter native tripeptides and synthesized the corresponding NHS derivatives. Each was then evaluated in a search for bifunctional chelators with properties s uperior to NHS-MAG(3), such as lower serum protein binding or improved stab ility to cysteine challenge. Based on these preliminary results, NHS-S-acet ylmercaptoacetyltriserine (NHS-MAS(3)) was selected for further investigati on. We have now conjugated this bifunctional chelator to biocytin and to an amine-derivatized peptide nucleic acid (PNA). Both carriers were also conj ugated with NHS-MAG(3) under identical conditions and all were labeled with Tc-99m at neutral pH and at boiling temperature while the conjugated PNAs were radiolabelled at neutral pH and at room temperature. Regardless of the chelator, reverse phase HPLC radiochromatograms of the labeled biotins and PNAs after purification showed a single peak. However, by size exclusion H PLC, the radiochromatograms always showed several peaks even after purifica tion, but the MAS(3) radiochromatograms were less complicated. For biotin a nd PNA both, radiolabeling via MAS(3) showed improved Tc-99m stability in 3 7 degrees C serum and in cysteine solution. The four preparations were admi nistered to mice implanted in one thigh with avidin beads (biotins) or comp lementary PNA beads (PNAs). At 5h post-administration, no significant diffe rences were observed in the targeting of PNA beads between the two chelator s, however the target thigh/normal thigh ratio was significantly higher for MAS(3)-biotin compared to MAG(3)-biotin. We conclude that labeling biocyti n and amine-derivatized PNA with NHS-MAS(3) compared to NHS-MAG(3) provides simpler radiochromatographic profiles, improved stability of the label in serum and cysteine solution and can improve in vivo targeting. (C) 1999 Els evier Science Ltd. All rights reserved.