Utilizing electrostatic interactions to facilitate F-18 radiolabeling of poly(amido)amine (PAMAM) dendrimers

The development of methods for the facile conjugation and radiolabeling of poly(amido) amine (PAMAM) dendrimers would be of great benefit in evaluating biomedical applications of these intriguing molecularly defined polymers. Two anionic N-hydroxysuccinimide (NHS) esters (7 and 10) were developed and radiolabeled with fluorine-18 using Cu(I)-catalyzed click reactions. The radiolabeling of a primary amine-terminated PAMAM generation-6 (G6) dendrimer with [F-18]7 or [F-18]10 was complete in water or methanol within 5 min at room temperature. This highly efficient conjugation reaction benefits from a high, localized concentration of these NHS esters on the surface of PAMAM dendrimers, due to the electrostatic attraction between the anionic NHS esters and the positively-charged PAMAM dendrimers. The large medium effect (pH, salt, solvent) observed for these conjugation reactions is consistent with this mechanism. This novel strategy of utilizing electrostatic interactions provides a novel, facile, and efficient method for the conjugation and radiolabeling of PAMAM dendrimers that also has potential for radiolabeling other appropriate nanoparticles.