Enhancing the Drug-Like Profile of a Potent Peptide PACE4 Inhibitor by the Formation of a Host-Guest Inclusion Complex with ß-Cyclodextrin.

The enzyme PACE4 has been validated as a promising therapeutic target to expand the range of prostate cancer (PCa) treatments. In recent years, we have developed a potent peptidomimetic inhibitor, namely, compound C23 (Ac-(DLeu)LLLRVK-4-amidinobenzylamide). Like many peptides, C23 suffers from an unfavorable drug-like profile which, despite our efforts, has not yet benefited from the usual SAR studies. Hence, we turned our attention toward a novel formulation strategy, i.e., the use of cyclodextrins (CDs). CDs can benefit compounds through the formation of "host-guest" complexes, shielding the guest from degradation and enhancing biological survival. In this study, a series of ßCD-C23 complexes have been generated and their properties evaluated, including potency toward the enzyme in vitro, a cell-based proliferation assay, and stability in plasma. As a result, a new ßCD-formulated lead compound has been identified, which, in addition to being more soluble and more potent, also showed an improved stability profile.

Efficacy of PACE4 pharmacotherapy in JHU-LNCaP-SM preclinical model of androgen independent prostate cancer.

Prostate cancer (PCa) is a complex disease progressing from in situ to invasive or metastatic tumors while also being capable of modulating its androgen dependence. Understanding how novel therapies are working across the different stages of the disease is critical for their proper positioning in the spectrum of PCa treatments. The targeting of proprotein convertase PACE4 (Paired basic Amino Acid-Cleaving Enzyme 4) has been proposed as a novel approach to treat PCa. Animal studies performed on LNCaP xenografts, an androgen-dependent model, already yielded positive results. In this study, we tested PACE4 inhibition on JHU-LNCaP-SM, a newly described androgen-independent model, in cell-based and xenograft assays. Like LNCaP, JHU-LNCaP-SM cells express PACE4 and its oncogenic isoform PACE4-altCT. Using isoform-specific siRNAs, downregulation of PACE4-altCT resulted in JHU-LNCaP-SM growth inhibition. Furthermore, JHU-LNCaP-SM responded to the PACE4 pharmacological inhibitor known as C23 in cell-based assays as well as in athymic nude mice xenografts. These data support the efficacy of PACE4 inhibitors against androgen independent PCa thereby demonstrating that PACE4 is a key target in PCa. The JHU-LNCaP-SM cell line represents a model featuring important aspects of androgen-independent PCa, but it also represents a very convenient model as opposed to LNCaP cells for in vivo studies, as it allows rapid screening due to its high implantation rate and growth characteristics as xenografts.

Upregulation of PACE4 in prostate cancer is not dependent on E2F transcription factors.

Recent studies in prostate cancer have identified PACE4, a proprotein convertase enzyme, as an emerging therapeutic target. Inhibition of PACE4-altCT, an oncogenic isoform of PACE4, using molecular or pharmacological approaches results in decreased cell proliferation and tumor progression in xenograft models. Although several validations have confirmed PACE4-altCT as a novel therapeutic target, the transcriptional regulation of PACE4 isoforms and mechanism of action remain a challenge. Previously, it has been reported that the human PACE4 promoter possesses potential binding sites for the E2F family of transcription factors, all of which are involved in cell cycle regulation and synthesis of DNA in mammalian cells. Therefore, we attempted to conduct in-depth evaluation of E2Fs on PACE4 and PACE4 isoform expression in prostate cancer. We conducted in vitro molecular silencing studies in various prostate cancer cell lines and determined the change in PACE4 expression levels. The results clearly show that the E2Fs alone do not alter PACE4 expression.

Enhanced anti-tumor activity of the Multi-Leu peptide PACE4 inhibitor transformed into an albumin-bound tumor-targeting prodrug.

The proprotein convertase PACE4 has been validated as a potential target to develop new therapeutic interventions in prostate cancer (PCa). So far, the most effective compound blocking the activity of this enzyme has been designed based on the structure of a small peptide Ac-LLLLRVKR-NH2 known as the Multi-Leu (ML) peptide. Optimization of this scaffold led to the synthesis of compound C23 (Ac-[DLeu]LLLRVK-amidinobenzylamide) with a potent in vivo inhibitory effect on the tumor growth. However, further developments of PACE4 inhibitors may require additional improvements to counter their rapid renal clearance and to increase their tumor targeting efficiency. Herein, we explored the transformation of the ML-peptide into an albumin-binding prodrug containing a tumor specific release mechanism based on the prostate-specific antigen. Our data confirms that intravenous treatment using the ML-peptide alone has little effect on tumor growth, whereas by using the ML-prodrug in LNCaP xenograft-bearing mice it was significantly reduced. Additionally, excellent in vivo stability and tumor-targeting efficiency was demonstrated using a radiolabelled version of this compound. Taken together, these results provide a solid foundation for further development of targeted PACE4 inhibition in PCa.

Improving the Selectivity of PACE4 Inhibitors through Modifications of the P1 Residue.

Paired basic amino acid cleaving enzyme 4 (PACE4), a serine endoprotease of the proprotein convertases family, has been recognized as a promising target for prostate cancer. We previously reported a selective and potent peptide-based inhibitor for PACE4, named the multi-Leu peptide (Ac-LLLLRVKR-NH2 sequence), which was then modified into a more potent and stable compound named C23 with the following structure: Ac-dLeu-LLLRVK-Amba (Amba: 4-amidinobenzylamide). Despite improvements in both in vitro and in vivo profiles of C23, its selectivity for PACE4 over furin was significantly reduced. We examined other Arg-mimetics instead of Amba to regain the lost selectivity. Our results indicated that the replacement of Amba with 5-(aminomethyl)picolinimidamide increased affinity for PACE4 and restored selectivity. Our results also provide a better insight on how structural differences between S1 pockets of PACE4 and furin could be employed in the rational design of selective inhibitors.

Increasing C-Terminal Hydrophobicity Improves the Cell Permeability and Antiproliferative Activity of PACE4 Inhibitors against Prostate Cancer Cell Lines.

The serine protease, PACE4, is a proprotein convertase that plays a substantial role in malignancy of prostate cancer. Our initial selective PACE4 inhibitor (Ac-LLLLRVKR-NH2) has evolved to the current lead compound C23 (Ac-dLeu-LLLRVK-Amba), which is active both in vitro and in vivo. By screening natural residues, except Cys, in C-terminal P1' position, it was established that increasing hydrophobicity was improving cell permeability, which was directly translated into PCa cells antiproliferative activity. This cell antiproliferation enhancement seems independent from effect of P1' residue on PACE4 affinity. Replacement of P1-Amba of C23 by Acpa (( S)-2-amino-3-(4-carbamimidoylphenyl)propanoic acid) followed by addition of tryptamine in P1' resulted in compound 32 exhibiting superior PCa cells antiproliferative activity over the reference compound C23 (3-fold). This study sheds light on key factors that improve cell penetrating property and antiproliferative activity of PACE4 inhibitors.

PACE4 Undergoes an Oncogenic Alternative Splicing Switch in Cancer.

Inhibition of PACE4, a proprotein convertase that is overexpressed in prostate cancer, has been shown to block cancer progression in an androgen-independent manner. However, the basis for its overexpression and its growth-inhibitory effects are mitigated and uncertain. Here, we report that PACE4 pre-mRNA undergoes DNA methylation-sensitive alternative splicing of its terminal exon 3' untranslated region, generating an oncogenic, C-terminally modified isoform (PACE4-altCT). We found this isoform to be strongly expressed in prostate cancer cells, where it displayed an enhanced autoactivating process and a distinct intracellular routing that prevented its extracellular secretion. Together, these events led to a dramatic increase in processing of the progrowth differentiation factor pro-GDF15 as the first PACE4 substrate to be identified in prostate cancer. We detected robust expression of PACE4-altCT in other cancer types, suggesting that an oncogenic switch for this proenzyme may offer a therapeutic target not only in advanced prostate cancer but perhaps also more broadly in human cancer. Cancer Res; 77(24); 6863-79. ©2017 AACR.

Rational Design of a Highly Potent and Selective Peptide Inhibitor of PACE4 by Salt Bridge Interaction with D160 at Position P3.

PACE4, a member of the proprotein convertases (PCs) family of serine proteases, is a validated target for prostate cancer. Our group has developed a potent and selective PACE4 inhibitor: Ac-LLLLRVKR-NH2 . In seeking for modifications to increase the selectivity of this ligand toward PACE4, we replaced one of its P3 Val methyl groups with a basic group capable of forming a salt bridge with D160 of PACE4. The resulting inhibitor is eight times more potent than the P3 Val parent inhibitor and two times more selective over furin, because the equivalent salt bridge with furin E257 is not optimal. Moreover, the ß-branched nature of the new P3 residue favors the extended ß-sheet conformation usually associated with substrates of proteases. This work provides new insight for better understanding of ß-sheet backbone-backbone interactions between serine proteases and their peptidic ligands.

Thrombin activation of protein C requires prior processing by a liver proprotein convertase.

Protein C, a secretory vitamin K-dependent anticoagulant serine protease, inactivates factors Va/VIIIa. It is exclusively synthesized in liver hepatocytes as an inactive zymogen (proprotein C). In humans, thrombin cleavage of the propeptide at PR221↓ results in activated protein C (APC; residues 222-461). However, the propeptide is also cleaved by a furin-like proprotein convertase(s) (PCs) at KKRSHLKR199↓ (underlined basic residues critical for the recognition by PCs), but the order of cleavage is unknown. Herein, we present evidence that at the surface of COS-1 cells, mouse proprotein C is first cleaved by the convertases furin, PC5/6A, and PACE4. In mice, this cleavage occurs at the equivalent site, KKRKILKR198↓, and requires the presence of Arg198 at P1 and a combination of two other basic residues at either P2 (Lys197), P6 (Arg193), or P8 (Lys191) positions. Notably, the thrombin-resistant R221A mutant is still cleaved by these PCs, revealing that convertase cleavage can precede thrombin activation. This conclusion was supported by the fact that the APC-specific activity in the medium of COS-1 cells is exclusively dependent on prior cleavage by the convertases, because both R198A and R221A lack protein C activity. Primary cultures of hepatocytes derived from wild-type or hepatocyte-specific furin, PC5/6, or complete PACE4 knock-out mice suggested that the cleavage of overexpressed proprotein C is predominantly performed by furin intracellularly and by all three proprotein convertases at the cell surface. Indeed, plasma analyses of single-proprotein convertase-knock-out mice showed that loss of the convertase furin or PC5/6 in hepatocytes results in a ∼30% decrease in APC levels, with no significant contribution from PACE4. We conclude that prior convertase cleavage of protein C in hepatocytes is critical for its thrombin activation.

Macrocyclization of a potent PACE4 inhibitor: Benefits and limitations.

PACE4, one of the seven members of the proprotein convertase family, plays an important role in the progression of prostate cancer. Therefore, its inhibition has become an attractive target to develop new therapies against this disease. Recently, we have developed a highly potent and selective PACE4 inhibitor, known as the multi-Leu peptide with the following sequence Ac-LLLLRVKR-NH2. Herein, with the aim of improving the stability profile of this inhibitor for potential in vivo application, we investigated the impact of different cyclization strategies. The inhibitory activity of new peptides was tested and compared to their linear counterparts. The potent analogues were further selected for stability evaluation. Our results showed that the cyclization involving a C-terminal carboxylic acid (head-to-tail or side chain-to-tail) led to compounds with significantly diminished inhibitory potency towards PACE4, indicating that an appropriate balance between rigidity and flexibility of the structure is necessary to allow the optimal binding with the enzyme. On the other hand, the modification within a multi-Leu core in combination with the incorporation of a C-terminal 4-amidinobenzylamide (Amba) residue yielded potent cyclic analogues. The best compound derived from this group, (&)[Mpa]LLLC(&)RVK[Amba] (where & indicates cyclization, Mpa - 3-mercaptopropionic acid), exhibited promising overall profile comprising of potent inhibitory effect against PACE4 and prostate cancer cell lines as well as improved stability. We believe that this cyclic framework could be further used to design even more potent and stable PACE4 inhibitors.

PACE4 is an important driver of ZR-75-1 estrogen receptor-positive breast cancer proliferation and tumor progression.

Breast cancer is the most frequent and deadly malignancy in women worldwide. Despite national screening programs combined with new treatments relapse rate remain high and new therapies are needed. From previous work, we identified PACE4, a member of the proprotein convertase (PCs) family of endoproteases, as a novel therapeutic target in prostate cancer. In the present study we asked the question if PACE4 could also be a potential target in breast cancer. In clinical samples of breast adenocarcinoma, we observed a specific overexpression of PACE4 in the estrogen-receptor (ER) positive subtype. We therefore looked for a breast cancer cell line model which would be representative and thus focused on the ZR-75-1 since it both expresses PACE4 and is estrogen-receptor positive. We compared stable knockdowns of furin, PACE4 and PC7 in the estrogen-receptor-positive cell line ZR-75-1 to evaluate their respective contribution to cell growth and tumor progression. PACE4 was the only PC displaying an impact on cell growth. A PACE4 peptide-based inhibitor (C23) was tested and shown to decrease proliferation of ZR-75-1 cells in cell based assays. C23 also had potent effects of tumor progression in vivo on xenografts of the ZR-75-1 cell line in athymic nude mice. Thus, PACE4-silencing and systemic administration of a PACE4 inhibitor resulted in hindered tumor progression with reduction in proliferative indices and increased cell quiescence assessed with biomarkers. Our results suggest that PACE4 is a promising target for estrogen-receptor-positive breast cancer.

Positional Scanning Identifies the Molecular Determinants of a High Affinity Multi-Leucine Inhibitor for Furin and PACE4.

The proprotein convertase family of enzymes includes seven endoproteases with significant redundancy in their cleavage activity. We previously described the peptide Ac-LLLLRVK-Amba that displays potent inhibitory effects on both PACE4 and prostate cancer cell lines proliferation. Herein, the molecular determinants for PACE4 and furin inhibition were investigated by positional scanning using peptide libraries that substituted its leucine core with each natural amino acid. We determined that the incorporation of basic amino acids led to analogues with improved inhibitory potency toward both enzymes, whereas negatively charged residues significantly reduced it. All the remaining amino acids were in general well tolerated, with the exemption of the P6 position. However, not all of the potent PACE4 inhibitors displayed antiproliferative activity. The best analogues were obtained by the incorporation of the Ile residue at the P5 and P6 positions. These substitutions led to inhibitors with increased PACE4 selectivity and potent antiproliferative effects.

An Unbiased Mass Spectrometry Approach Identifies Glypican-3 as an Interactor of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) and Low Density Lipoprotein Receptor (LDLR) in Hepatocellular Carcinoma Cells.

The mechanism of LDL receptor (LDLR) degradation mediated by the proprotein convertase subtilisin/kexin type 9 (PCSK9) has been extensively studied; however, many steps within this process remain unclear and still require characterization. Recent studies have shown that PCSK9 lacking its Cys/His-rich domain can still promote LDLR internalization, but the complex does not reach the lysosome suggesting the presence of an additional interaction partner(s). In this study we carried out an unbiased screening approach to identify PCSK9-interacting proteins in the HepG2 cells' secretome using co-immunoprecipitation combined with mass spectrometry analyses. Several interacting proteins were identified, including glypican-3 (GPC3), phospholipid transfer protein, matrilin-3, tissue factor pathway inhibitor, fibrinogen-like 1, and plasminogen activator inhibitor-1. We then validated these interactions by co-immunoprecipitation and Western blotting. Furthermore, functional validation was examined by silencing each candidate protein in HepG2 cells using short hairpin RNAs to determine their effect on LDL uptake and LDLR levels. Only GPC3 and phospholipid transfer protein silencing in HepG2 cells significantly increased LDL uptake in these cells and displayed higher total LDLR protein levels compared with control cells. Moreover, our study provides the first evidence that GPC3 can modulate the PCSK9 extracellular activity as a competitive binding partner to the LDLR in HepG2 cells.

Novel Insights into Structure-Activity Relationships of N-Terminally Modified PACE4 Inhibitors.

PACE4 plays important roles in prostate cancer cell proliferation. The inhibition of this enzyme has been shown to slow prostate cancer progression and is emerging as a promising therapeutic strategy. In previous work, we developed a highly potent and selective PACE4 inhibitor, the multi-Leu (ML) peptide, an octapeptide with the sequence Ac-LLLLRVKR-NH2 . Here, with the objective of developing a useful compound for in vivo administration, we investigate the effect of N-terminal modifications. The inhibitory activity, toxicity, stability, and cell penetration properties of the resulting analogues were studied and compared to the unmodified inhibitor. Our results show that the incorporation of a polyethylene glycol (PEG) moiety leads to a loss of antiproliferative activity, whereas the attachment of a lipid chain preserves or improves it. However, the lipidated peptides are significantly more toxic when compared with their unmodified counterparts. Therefore, the best results were achieved not by the N-terminal extension but by the protection of both ends with the d-Leu residue and 4-amidinobenzylamide, which yielded the most stable inhibitor, with an excellent activity and toxicity profile.

Multi-Leu PACE4 Inhibitor Retention within Cells Is PACE4 Dependent and a Prerequisite for Antiproliferative Activity.

The overexpression as well as the critical implication of the proprotein convertase PACE4 in prostate cancer progression has been previously reported and supported the development of peptide inhibitors. The multi-Leu peptide, a PACE4-specific inhibitor, was further generated and its capability to be uptaken by tumor xenograft was demonstrated with regard to its PACE4 expression status. To investigate whether the uptake of this inhibitor was directly dependent of PACE4 levels, uptake and efflux from cancer cells were evaluated and correlations were established with PACE4 contents on both wild type and PACE4-knockdown cell lines. PACE4-knockdown associated growth deficiencies were established on the knockdown HepG2, Huh7, and HT1080 cells as well as the antiproliferative effects of the multi-Leu peptide supporting the growth capabilities of PACE4 in cancer cells.

Chymase inhibitor-sensitive synthesis of endothelin-1 (1-31) by recombinant mouse mast cell protease 4 and human chymase.

Important structural differences imply that human and mouse mast cell chymases may differ with respect to their enzymatic properties. We compared in this study the catalytic efficiencies of recombinant human chymase (rCMA1) and its functional murine homologue recombinant mouse mast cell protease-4 (rmMCP-4) toward a fluorogenic chymase substrate (Suc-Ala-Ala-Pro-Phe-7-amino-4-methylcoumarin (AMC) and by their ability to convert Big-endothelin (ET)-1 into ET-1 (1-31) using a LC/MS/MS system. Activities toward a fluorogenic substrate (Suc-Leu-Leu-Val-Tyr-AMC) and Big ET-1 were also measured in extracts from mouse peritoneal mast cells, LUVA human mast cell-like cells and human aortas. The specificity of these activities was assessed with the chymase inhibitor TY-51469 (2-[4-(5-fluoro-3-methylbenzo[b]thiophen-2-yl)sulfonamido-3-methanesulfonyl-phenyl]thiazole-4-carboxylic acid). For similar affinities, rmMCP-4 showed a higher activity toward the fluorogenic substrate and a higher ability to process Big ET-1 as compared to recombinant CMA1 (chymase activity (kcat/KM in µM(-1)s(-1)): 2.29 × 10(-4)vs. 6.41 × 10(-6); ET-1 (1-31) production: 2.19 × 10(-3)vs. 6.57 × 10(-5)), and both of these activities of mouse and human chymase were sensitive to TY-51469. Furthermore, extracts from mouse peritoneal mast cells, LUVA cells and human aorta homogenates contained processing activities toward the fluorogenic chymase substrate as well as Big ET-1, all of which were sensitive to TY-51469. Finally, the pressor responses to Big ET-1 but not to ET-1 were significantly reduced in conscious and free moving mMCP-4 KO mice when compared to wild type congeners. Our results suggest that both mouse and human chymases have potent ET-1 (1-31)-producing abilities, with the murine isoform being more efficient.

PACE4 inhibitors and their peptidomimetic analogs block prostate cancer tumor progression through quiescence induction, increased apoptosis and impaired neovascularisation.

Prostate cancer is the leading cancer in North American men. Current pharmacological treatments are limited to anti-androgen strategies and the development of new therapeutic approaches remains a challenge. As a fundamentally new approach, we propose the inhibition of PACE4, a member of the proprotein convertases family of enzymes, as a therapeutic target in prostate cancer. We developed an inhibitor named the Multi-Leu peptide, with potent in vitro anti-proliferative effects. However, the Multi-Leu peptide has not been tested under in vivo conditions and its potency under such conditions is most likely limited, due to the labile characteristics of peptides in general. Using a peptidomimetic approach, we modified the initial scaffold, generating the analog Ac-[DLeu]LLLRVK-Amba, which demonstrates increased inhibitory potency and stability. The systemic administration of this peptidomimetic significantly inhibits tumor progression in the LNCaP xenograft model of prostate cancer by inducing tumor cell quiescence, increased apoptosis and neovascularization impairment. Pharmacokinetic and biodistribution profiles of this inhibitor confirm adequate tumor delivery properties of the compound. We conclude that PACE4 peptidomimetic inhibitors could result in stable and potent drugs for a novel therapeutic strategy for prostate cancer.

Annexin A2 reduces PCSK9 protein levels via a translational mechanism and interacts with the M1 and M2 domains of PCSK9.

Annexin A2 (AnxA2) was reported to be an extracellular endogenous inhibitor of proprotein convertase subtilisin kexin type 9 (PCSK9) activity on cell-surface LDL receptor degradation. In this study, we investigated the effect of silencing the expression of AnxA2 and PCSK9 in HepG2 and Huh7 cells to better define the role of AnxA2 in PCSK9 regulation. AnxA2 knockdown in Huh7 cells significantly increased PCSK9 protein levels as opposed to AnxA2 knockdown in HepG2 cells. However, HepG2 cells overexpressing AnxA2 had lower levels of PCSK9 protein. Overall, our data revealed a plausible new role of AnxA2 in the reduction of PCSK9 protein levels via a translational mechanism. Moreover, the C-terminal Cys/His-rich domain of PCSK9 is crucial in the regulation of PCSK9 activity, and we demonstrated by far-Western blot assay that the M1 and M2 domains are necessary for the specific interaction of PCSK9's C-terminal Cys/His-rich domain and AnxA2. Finally, we produced and purified recombinant PCSK9 from humans and mice, which was characterized and used to perform 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate LDL cell-based assays on the stable knockdown HepG2 and Huh7 cells. We also demonstrated for the first time the equipotency of human and mouse PCSK9 R218S on human cells.

Implications of Proprotein Convertases in Ovarian Cancer Cell Proliferation and Tumor Progression: Insights for PACE4 as a Therapeutic Target.

Proprotein convertases are a family of kexin-like serine proteases that process proteins at single and multiple basic residues. Among the predicted and identified PC substrates, an increasing number of proteins having functions in cancer progression indicate that PCs may be potential targets for antineoplastic drugs. In support of this notion, we identified PACE4 as a vital PC involved in prostate cancer proliferation and progression, contrasting with the other co-expressed PCs. The aim of the present study was to test the importance of PCs in ovarian cancer cell proliferation and tumor progression. Based on tissue-expression profiles, furin, PACE4, PC5/6 and PC7 all displayed increased expression in primary tumor, ascites cells and metastases. These PCs were also expressed in variable levels in three model ovarian cell lines tested, namely SKOV3, CAOV3 and OVCAR3 cells. Since SKOV3 cells closely represented the PC expression profile of ovarian cancer cells, we chose them to test the effects of PC silencing using stable gene-silencing shRNA strategy to generate knockdown SKOV3 cells for each expressed PC. In vitro and in vivo assays confirmed the role of PACE4 in the sustainment of SKOV3 cell proliferation, which was not observed with the other three PCs. We also tested PACE4 peptide inhibitors on all three cell lines and observed consequent reduced cell proliferation which was correlated with PACE4 expression. Overall, these data support a role of PACE4 in promoting cell proliferation in ovarian cancer and provides further evidence for PACE4 as a potential therapeutic target.

Design, synthesis, and structure-activity relationship studies of a potent PACE4 inhibitor.

PACE4 plays an important role in the progression of prostate cancer and is an attractive target for the development of novel inhibitor-based tumor therapies. We previously reported the design and synthesis of a novel, potent, and relatively selective PACE4 inhibitor known as a Multi-Leu (ML) peptide. In the present work, we examined the ML peptide through detailed structure-activity relationship studies. A variety of ML-peptide analogues modified at the P8-P5 positions with leucine isomers (Nle, DLeu, and DNle) or substituted at the P1 position with arginine mimetics were tested for their inhibitory activity, specificity, stability, and antiproliferative effect. By incorporating d isomers at the P8 position or a decarboxylated arginine mimetic, we obtained analogues with an improved stability profile and excellent antiproliferative properties. The DLeu or DNle residue also has improved specificity toward PACE4, whereas specificity was reduced for a peptide modified with the arginine mimetic, such as 4-amidinobenzylamide.