Prostate-specific Antigen Cleavage Research Articles

Synthesis and evaluation of new peptide-linked doxorubicin conjugates as prodrugs activated by prostate-specific antigen

The targeted delivery of cytotoxic agents to prostate cancer cells via selective activation of peptide-linked prodrugs by prostate-specific antigen (PSA) has been previously demonstrated. In our continued efforts to design prodrugs with improved prostate tumor specificity, we developed GABA ← mGly-Ala-Ser-Chg-Gln and glutaryl-Ser-Ala-Ser-Chg-Gln as promoieties with enhanced PSA specificity starting from the known substrate sequence glutaryl-Hyp-Ala-Ser-Chg-Gln. Based on their PSA cleavage rates and resistance to non-PSA-mediated hydrolysis in plasma, GABA ← mGly-Ala-Ser-Chg-Gln and glutaryl-Ser-Ala-Ser-Chg-Gln were selected as optimal promoieties and coupled to doxorubicin (Dox) as PSA-targeted prodrugs, using Ser-Leu linkers. Following 72-h incubations with Dox prodrugs, there was insignificant cytotoxicity in non-PSA-producing DU145 cells. The Dox prodrugs, glutaryl-Hyp-Ala-Ser-Chg-Gln-Ser-Leu-Dox (I), glutaryl-Ser-Ala-Ser-Chg-Gln-Ser-Leu-Dox (II) and GABA ← mGly-Ala-Ser-Chg-Gln-Ser-Leu-Dox (III) demonstrated comparable PSA cleavage rates (t1/2 values <23 min), and were cytotoxic against PSA-producing LNCaP cells with IC50 values of 0.18, 0.27 and 0.082 μM, respectively. To mitigate neprilysin-mediated hydrolysis of the Ser-Leu linker in prodrugs I–III and further improve PSA specificity, 3-aminooxypropionate was incorporated between the promoiety and Dox (prodrug IV). Despite its slower PSA cleavage rate (t1/2 value of 67 min), GABA ← mGly-Ala-Ser-Chg-Gln-NH-O-CH2-C(Me)2C(O)-14-O-Dox (IV) was equipotent (IC50 value of 0.19 μM) to prodrug I at killing PSA-producing LNCaP cells due to its ability to release free Dox through a cyclization activation mechanism. Further metabolism and PK/PD studies will be conducted to evaluate the tumor specificity of the novel Dox prodrugs (II–IV) reported herein.

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

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.

Synthesis and Characterization of a Novel Prostate Cancer-Targeted Phosphatidylinositol-3-kinase Inhibitor Prodrug

The phosphatidylinositol-3-kinase/Akt (PI3K/Akt) pathway is constitutively activated in a substantial proportion of prostate tumors and is considered a key mechanism supporting progression toward an androgen-independent status, for which no effective therapy is available. Therefore, PI3K inhibitors, alone or in combination with other cytotoxic drugs, could potentially be used to treat cancer with a constitutive activated PI3K/Akt pathway. To selectively target advanced prostate tumors with a constitutive activated PI3K/Akt pathway, a prostate cancer-specific PI3K inhibitor was generated by coupling the chemically modified form of the quercetin analogue LY294002 (HO-CH(2)-LY294002, compound 8) with the peptide Mu-LEHSSKLQL, in which the internal sequence HSSKLQ is a substrate for the prostate-specific antigen (PSA) protease. The result is a water-soluble and latent PI3K inhibitor prodrug (compound 11), its activation being dependent on PSA cleavage. Once activated, the L-O-CH(2)-LY294002 (compound 10) can specifically inhibit PI3K in PSA-secreting prostate cancer cells and induce apoptosis with a potency comparable to that of the original LY294002 compound.

Abstract LB-38: Evaluation of a PSA cleavable doxorubicin prodrug in the in vitro and in vivo LNCaP LN model

Abstract The goal of this work was to investigate the biological activity of a doxorubicin prodrug that is efficiently cleaved by prostate specific antigen (PSA) rapidly liberating doxorubicin as the final cleavage product. For this purpose we have developed an albumin-binding prodrug of doxorubicin, EMC-Arg-Ser-Ser-Tyr-Tyr-Ser-Leu-PABC-DOXO [EMC: ε-maleimidocaproyl; PABC: p-aminobenzyloxycarbonyl; DOXO: doxorubicin]. The incorporation of the PABC self-eliminating linker between the peptide sequence and doxorubicin enhanced the enzymatic cleavage rate and improved the cleavage profile. PSA cleavage produced the metabolite H-Ser-Leu-PABC-DOXO which was further degraded to release doxorubicin as a final cleavage product within a few hours in prostate tumor tissue homogenates as well as in PSA-positive LNCaP LN cell lysates. In the cell culture experiments, our prodrug exhibited antiproliferative activity in the low micromolar range with an IC50 value of ∼1 μM against a PSA-expressing prostate cancer cell line with a stably integrated Luciferase-Neomycin (LN) resistance fusion gene cassette (LNCaP LN). Moreover, H-Ser-Leu-PABC-DOXO showed comparable in vitro activity to doxorubicin (∼0.2 μM). Orientating toxicity studies in mice showed that the maximum tolerated dose of our novel prodrug was twice that of conventional doxorubicin. Consequently, the in vivo efficacy of the prodrug (6 mg/kg doxorubicin equivalents) is currently being compared to free doxorubicin (3 mg/kg) in a mouse model of human prostate cancer using luciferase transduced LNCaP cells orthotopically implanted in SCID mice. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-38.

A prostate-specific antigen–activated N-(2-hydroxypropyl) methacrylamide copolymer prodrug as dual-targeted therapy for prostate cancer

Prostate cancer targeted peptide prodrugs that are activated by the serine protease activity of prostate-specific antigen (PSA) are under development in our laboratory. To enhance delivery and solubility of these prodrugs, macromolecular carriers consisting of N-(2-hydroxypropyl) methacrylamide (HPMA)-based copolymers were covalently coupled to a PSA-activated peptide prodrug. HPMA copolymers are water-soluble, nonimmunogenic synthetic carriers that exhibit promise for drug delivery applications. These macromolecular copolymers enter the interstitium of solid tumors by the enhanced permeability and retention effect. The PSA-activated peptide substrate imparts selectivity because it is specifically hydrolyzed to release a cytotoxin at the site of prostate tumor. Enzymatically active PSA is present in high amounts in the extracellular fluid of a tumor, but PSA is inactivated in blood by binding to serum protease inhibitors. As an initial proof of concept, the HPMA copolymer was synthesized with a peptide substrate (HSSKLQ) bound to a fluorophore, 7-amino-4-methylcoumarin (AMC). PSA cleavage of the HPMA-HSSKLQ-AMC copolymer was observed, which led to the synthesis of an HPMA-based copolymer with the prodrug SSKYQ-L12ADT [HPMA-morpholinocarbonyl-Ser-Ser-Lys-Tyr-Gln-Leu-12-aminododecanoyl thapsigargin (JHPD)]. L12ADT is a potent analogue of the highly cytotoxic natural product thapsigargin. HPMA-JHPD was hydrolyzed by PSA in vitro and was toxic to prostate cancer cells in the presence of active PSA. The HPMA-JHPD produced no systemic toxicity when given at a 500 micromol/L L12ADT equivalent dose. Analysis of tumor tissue from mice treated with a single or multiple dose of the HPMA-JHPD copolymer showed release and accumulation of the L12ADT toxin within the tumor tissue.

Engineering of Human Coagulation Factor X Variants Activated by Prostate-Specific Antigen

In this study, we present a novel approach for the induction of tumor vessel thrombosis using genetically modified coagulation factor X. Human factor X was engineered in its activation peptide in a way that it can be specifically activated by prostate-specific antigen (PSA), a tumor-specific proteinase secreted into the bloodstream by prostate cancer cells. For this purpose we inserted different sequences of known PSA cleavage sites from the natural substrate of PSA, semenogelin I, into the activation peptide of factor X. One FX variant (FX-V4) was further optimized by site-directed mutagenesis of the P2 position and the P5 position (FX-V4-P2YP5R). After preincubation with PSA, FX-V4-P2YP5R was able to efficiently induce coagulation in vitro. These FX variants should be useful for site-specific induction of blood coagulation in the tumor vasculature.

In vitro assay for site-specific proteases using bead-attached GFP substrate.

Site-specific proteases, which catalyze cleavage of peptide bonds in specific amino acid sequences of target proteins, play important roles in various biological events of many living organisms. In humans, disruption in regulation of these site-specific proteases can lead to pathological consequences. Here, we report a simple in vitro assay for enzymatic activities of site-specific proteases. This assay system employs a protein substrate molecule that is comprised of (i) His-tag binding module, (ii) cleavage sites, and (iii) green fluorescent protein (GFP) detection module. In this study, prostate-specific antigen (PSA) and Thrombin-specific cleavage sites were introduced into the substrate molecules. The overexpressed GFP substrate protein was purified with the aid of Ni++-charged magnetic beads. On cleavage by either PSA or Thrombin, GFP was released from the bound magnetic beads, enabling a direct measurement of the cleaved product by fluorescence. Detection sensitivity, as well as the kinetics of reaction of PSA cleavage with the GFP substrate, was similar or better than commercially available PSA fluorogenic peptide substrate. This bead-attached GFP substrate was also used for an inhibition assay using a competitive inhibitor of Thrombin. In conclusion, this assay offers a simple fluorescent method for monitoring the activity of the site-specific proteases. Furthermore, this system provides flexible means of incorporating varying sizes of flanking sequences adjacent to the cleavage site, which can be essential for studying the regulatory macromolecular interactions between proteases and their substrates.

Prostate Specific Antigen Cleaves Parathyroid Hormone-Related Protein in the PTH-like Domain: Inactivation of PTHrP-Stimulated cAMP Accumulation in Mouse Osteoblasts

Purpose To determine whether parathyroid hormone-related protein (PTHrP) is a substrate of prostate-specific antigen (PSA) and how the biological activity of PTHrP may be altered by cleavage with PSA. Materials and Methods Prostate-specific antigen cleavage of recombinant human PTHrP 1-141 was conducted in vitro at 37C and analyzed by SDS-PAGE. Five rounds of automated amino-terminal amino acid sequence analysis were performed on blotted PSA-cleaved PTHrP peptide fragments to determine the PSA cleavage sites. The mouse osteoblast cell line MC3T3-E1 was used to test whether PSA cleavage of PTHrP 1-141 altered its ability to stimulate cAMP production. Results Prostate-specific antigen was found to specifically cleave PTHrP 1-141 in a time- and dose-dependent manner. Cleavage of PTHrP 1-141 by PSA generated fragments on Coomassiestained acrylamide gels that migrated with mobilities that corresponded to 19.5, 17, 15 and less than 7 kd. The preferred PSA cleavage site of PTHrP 1-141 was determined to be at the carboxyl-terminus of phenylalanine 23, consistent with chymotryptic-like enzymatic activity of PSA. Cleavage of PTHrP by PSA completely abolished the ability of PTHrP to stimulate cAMP production. Conclusions Cleavage of PTHrP 1-141 by PSA carboxyl-terminal to phenylalanine 23 represents a unique pattern of PTHrP processing that may be specific to the prostate. Prostate-specific antigen inactivation of the cAMP-inducing activity of PTHrP 1-141 demonstrates that PSA cleavage regulates the biological activity of PTHrP. These results have implications for the role of PTHrP in prostate cancer metastasis to bone and its subsequent regulation of bone remodeling Study of the biological activities of the PSA-generated PTHrP peptides identified in this study merits further investigation.

Prostate Specific Antigen Cleaves Parathyroid Hormone-Related Protein in the PTH-like Domain

To determine whether parathyroid hormone-related protein (PTHrP) is a substrate of prostate-specific antigen (PSA) and how the biological activity of PTHrP may be altered by cleavage with PSA. Prostate-specific antigen cleavage of recombinant human PTHrP 1-141 was conducted in vitro at 37C and analyzed by SDS-PAGE. Five rounds of automated amino-terminal amino acid sequence analysis were performed on blotted PSA-cleaved PTHrP peptide fragments to determine the PSA cleavage sites. The mouse osteoblast cell line MC3T3-E1 was used to test whether PSA cleavage of PTHrP 1-141 altered its ability to stimulate cAMP production. Prostate-specific antigen was found to specifically cleave PTHrP 1-141 in a time- and dose-dependent manner. Cleavage of PTHrP 1-141 by PSA generated fragments on Coomassie-stained acrylamide gels that migrated with mobilities that corresponded to 19.5, 17, 15 and < 7 kd. The preferred PSA cleavage site of PTHrP 1-141 was determined to be at the carboxyl-terminus of phenylalanine 23, consistent with chymotryptic-like enzymatic activity of PSA. Cleavage of PTHrP by PSA completely abolished the ability of PTHrP to stimulate cAMP production. Cleavage of PTHrP 1-141 by PSA carboxyl-terminal to phenylalanine 23 represents a unique pattern of PTHrP processing that may be specific to the prostate. Prostate-specific antigen inactivation of the cAMP-inducing activity of PTHrP 1-141 demonstrates that PSA cleavage regulates the biological activity of PTHrP. These results have implications for the role of PTHrP in prostate cancer metastasis to bone and its subsequent regulation of bone remodeling. Study of the biological activities of the PSA-generated PTHrP peptides identified in this study merits further investigation.