Preparation and Properties of 99mTc(CO)3+-Labeled N,N-Bis(2-pyridylmethyl)-4-aminobutyric Acid

Labeling biomolecules with 99mTc(CO)3+ (99mTc tricarbonyl) is attracting increasing attention. Although histidine is often considered an ideal bifunctional chelator for 99mTc (or 188Re) tricarbonyl, the family of dipicolylamine carboxylate chelators may be a useful alternative because of the expected ease of synthesis and because the structure provides a pendent carboxylate for potential conjugation to biomolecules. The dipicolylamine chelator N,N-bis(2-pyridylmethyl)-4-aminobutyric acid (BPABA) was synthesized using 4-aminobutyric acid in place of glycine or aminopropionic acid in the literature, to avoid possible involvement of the carboxylate in the complex formation process by forming five- or six-membered chelation rings. Using a commercial tricarbonyl kit (Mallinckrodt), the complex formation properties of both BPABA and commercial histidine with 99mTc tricarbonyl were investigated, and the in vitro complex stabilities in saline and in serum were compared. Stability in vivo was also examined following i.v. administration to normal mice. BPABA was synthesized simply and quantitatively by reacting picolyl chloride with aminobutyric acid in one step. On RP HPLC, the product eluted essentially in one peak and the structure was confirmed by ESI-MS. After labeling, both BPABA and histidine were shown by RP HPLC to form tricarbonyl complexes. In both cases, after incubation at 100 (degree)C for 20 min, only one predominant peak of 99mTc(CO)3+-histidine or 99mTc(CO)3+-BPABA was apparent, and both complexes were stable at room temperature in saline for at least 24 h. After incubation for 24 h in 37 (degree)C serum, by SE HPLC, 20% of the 99mTc(CO)3+-histidine was bound to serum protein compared to less than 10% for 99mTc(CO)3+-BPABA. A 5000 molar excess of histidine at 100 (degree)C for 6 h was unable to dissociate 99mTc(CO)3+-BPABA. By contrast, BPABA easily dissociated 99mTc(CO)3+histidine under the same conditions. Both complexes were stable in vivo in mice, and 99mTc(CO)3+-BPABA showed rapid and specific hepatobiliary clearance while 99mTc(CO)3+-histidine was cleared through the kidneys. In conclusion, BPABA was easily synthesized and was shown to possess properties comparable to histidine for labeling of biomolecules with 99mTc tricarbonyl. However, it was found that the chelator concentration required for quantitative 99mTc tricarbonyl labeling with both BPABA and histidine were 2 orders higher than that required with more conventional labeling using MAG3. Finally, the complex 99mTc(CO)3+-BPABA itself was found to clear exclusively via the hepatobiliary pathway and may have value as a potential hepatobiliary imaging agent.