Synthesis and evaluation of new 4-peptidamido-2-fluorobenzyl phosphoramide mustard conjugates as prodrugs activated by prostate-specific antigen

Abstract

In our continued efforts to develop targeted prodrugs activated by prostate-specific antigen (PSA), we designed and synthesized novel phosphoramide mustard peptide conjugates using previously optimized PSA substrates. Initial Nu/Nu mouse PK studies indicated that prodrug I (glutaryl-Hyp-Ala-Ser-Chg-Gln-NH-2-F-Bn-phosphoramide mustard) exhibits high clearance with significant extrahepatic metabolism in vivo. Substrate optimization studies were thus carried out to further improve PSA specificity and enable the design of prodrugs with reduced in vivo clearance and enhanced tumor selectivity. To assess the utility of the newly optimized sequences as promoieties, they were coupled to phosphoramide mustard using a 4-amino-2-fluorobenzyl alcohol linker akin to prodrug I. In the presence of human PSA, prodrug I was rapidly cleaved with a half-life (t1/2) of 35 min. Prodrugs II (glutaryl-Ser-Ala-Ser-Chg-Gln-NH-2-F-Bn-phosphoramide mustard) and III (GABA ← mGly-Ala-Ser-Chg-Gln-NH-2-F-Bn-phosphoramide mustard) were hydrolyzed at slower rates with t1/2 values of 80 and 107 min, respectively. These results we observed here are different from our previously reported data but may be explained by the fact that PSA-activated release of phosphoramide mustard and reactive quinonimine methides resulted in mechanism-based inhibition of PSA, thereby preventing further hydrolysis of prodrugs I–III. Prodrug I was cytotoxic to PSA-producing LNCaP cells with an IC50 value of 7.3 μM and demonstrated 14-fold selectivity over the non-PSA-producing DU145. Despite its poor in vitro antiproliferative activity (IC50 = 30 µM), prodrug III was found to be more stable against non-PSA-mediated hydrolysis compared with prodrug I as revealed by metabolite profiling studies, which was in agreement with its improved stability in human hepatocyte cultures. These results suggested that a combination of the peptide sequence GABA ← mGly-Ala-Ser-Chg-Gln with optimal linkers and/or other cytotoxic agents can help achieve an adequate balance between PSA cleavage rate and enhanced resistance to non-PSA-mediated hydrolysis.