Novel glutamate conjugates for radiotargeting PSMA

Babich, John

Abstract

? The development of novel approaches for radioimmunoimaging and radioimmunotherapy is driven by the high incidence and mortality rate associated with prostate cancer in the United States. One strategy to diagnostic and prognostic markers exploits prostate-specific membrane antigen (PSMA), an integral transmembrane glycoprotein, associated with the prostate epithelium, prostatic tumor cells and the neovasculature of other tumor types, which is also highly homologous to NAALDase (N-acetylated a-Iinked acidic dipeptidase) which releases the neurotransmitter glutamate. A range of PSMA inhibitors have been reported, based on two fundamental structures, phosphonate and phosphinate containing transition state analogues and novel dipeptides connected through a simple urea linkage. One objective of the research is to prepare derivatives of the urea-based inhibitors modified at the terminus not involved with binding to the specificity pocket with an amino acid containing a head group suitable for coordination to the {M(CO)3} *1 core (M = Tc and Re). ? ? Our approach relies on our recently developed a novel approach to 99mTc-radiolabeling of peptides based on single amino acid analogues (SAAC) modified to provide three potential donor groups for facial chelation to the {M(CO)3} *1 core (M = Tc or Re). We have demonstrated the rapid and simple introduction of SAAC into bioactive peptides via standard solid phase peptide synthesis protocols and subsequent labeling with 99mTc and Re28 The innovation of this proposal is to incorporate single amino acid chelators into the existing medicinal chemistry knowledge base of inhibitors of PSMA, so as to allow subsequent labeling with 99mTc or 186/1888Re radioisotopes for imaging or therapeutic applications, respectively. ? ? The urea-based analogues will be comprised of a glutamate coupled to a single amino acid chelate by a urea linkage through the a-NH2 groups. Representative SAACs include e-derivatives of lysine. The SAACs will be derivatized at the amino terminus with donor groups based on pyridine, imidazole, carboxylate, thiolate and thiazole. Altering the donor group substituents will yield changes in overall charge, hydrophobicity and steric influence. A series of SAAC glutamate conjugates will be prepared as potential NAAG inhibitors allowing the assessment of the influence of factors such as donor group identity, tether length and geometry, steric influences and charge on enzyme inhibition and binding. ? ? The {Re(CO)3} *1 complexes of the conjugates will be prepared as models for the {Tc(CO)3} *1 complexes and will be used for biological assays. The 99mTc(CO)3(H20) 3}?1core is readily prepared using the Isolink TM kit, which has been supplied by Tyco-Mallinckrodt, Inc. The SAAC-urea-glutamate series of complexes will be evaluated for binding and inhibitory activity. Cell uptake studies will be performed using PSMA-positive and PSMA-negative cell lines. Upon evaluation of these results, compounds that demonstrate specific binding in PSMA-positive cells in vitro will be studied further in mice containing both PSMA-positive and PSMA-negative human prostate tumor models. ? ?