Rate-Tunable, Metal-Mediated Amide Bond Cleavage for the Controlled Release of Pharmaceuticals

That the incorporation of <i>N</i>-methyl amino acids adjacent to a hydrolytic, azamacrocyclic metal complex results in rate-tunable, metal-mediated amide bond cleavage (TMAC) under physiological conditions. Spectroscopic and crystallographic data provide unprecedented mechanistic insight: the Ga³⁺ complex of (7-amido-1,4,7-triazonane-1,4-diyl)diacetic acid polarizes the amide bond proximal to canonical and noncanonical amino acids, forming two coordination isomers with different cleavage rates, N₃O₃ (fast) and N₄O₂ (slow) in aqueous solution. Both were characterized by NMR spectroscopy and identified by single-crystal X-ray diffraction. Subsequent hydrolysis of the amide bond occurs by exogenous nucleophilic attack, as demonstrated by ¹⁸O-isotope labeling experiments and proceeds with a variable rate, depending on the nature of the amino acid side chain and amide-methylation status. The in vivo applicability of TMAC was subsequently demonstrated by pharmacokinetic modulation of a cancer targeted, ⁶⁸Ga-labeled radiopharmaceutical. Specifically, 6 serum-albumin binding chelates, linked to a peptide targeting the prostate specific membrane antigen (PSMA) were constructed. Variable amino-acid-chelate linkers allow tuning of the rate of release and clearance of the radioactive isotope. Indeed, diagnostic positron emission tomography (PET) imaging, metabolite and biodistribution analysis indicate that rate tunable cleavage and release of the ⁶⁸Ga-chelate minimize tracer accumulation in blood and liver compartments while maximizing tumor uptake. In contrast, a [⁶⁸Ga]Ga-chelate incorporating a noncleavable glycine linker, exhibited elevated blood and liver uptake with moderate tumor localization. Taken together, TMAC provides remarkable control over the in vivo behavior of targeted pharmaceuticals.