Synergistic Suppression of NF1 Malignant Peripheral Nerve Sheath Tumor Cell Growth in Culture and Orthotopic Xenografts by Combinational Treatment with Statin and Prodrug Farnesyltransferase Inhibitor PAMAM G4 Dendrimers.

Neurofibromatosis type 1 (NF1) is a disorder in which RAS is constitutively activated due to the loss of the Ras-GTPase-activating activity of neurofibromin. RAS must be prenylated (i.e., farnesylated or geranylgeranylated) to traffic and function properly. Previous studies showed that the anti-growth properties of farnesyl monophosphate prodrug farnesyltransferase inhibitors (FTIs) on human NF1 malignant peripheral nerve sheath tumor (MPNST) cells are potentiated by co-treatment with lovastatin. Unfortunately, such prodrug FTIs have poor aqueous solubility. In this study, we synthesized a series of prodrug FTI polyamidoamine generation 4 (PAMAM G4) dendrimers that compete with farnesyl pyrophosphate for farnesyltransferase (Ftase) and assessed their effects on human NF1 MPNST S462TY cells. The prodrug 3-tert-butylfarnesyl monophosphate FTI-dendrimer (i.e., IG 2) exhibited improved aqueous solubility. Concentrations of IG 2 and lovastatin (as low as 0.1 µM) having little to no effect when used singularly synergistically suppressed cell proliferation, colony formation, and induced N-RAS, RAP1A, and RAB5A deprenylation when used in combination. Combinational treatment had no additive or synergistic effects on the proliferation/viability of immortalized normal rat Schwann cells, primary rat hepatocytes, or normal human mammary epithelial MCF10A cells. Combinational, but not singular, in vivo treatment markedly suppressed the growth of S462TY xenografts established in the sciatic nerves of immune-deficient mice. Hence, prodrug farnesyl monophosphate FTIs can be rendered water-soluble by conjugation to PAMAM G4 dendrimers and exhibit potent anti-tumor activity when combined with clinically achievable statin concentrations.

Intracellular targets for a phosphotyrosine peptidomimetic include the mitotic kinesin, MCAK.

SH2 domains are attractive targets for chemotherapeutic agents due to their involvement in the formation of protein-protein interactions critical to many signal transduction cascades. Little is known, however, about how synthetic SH2 domain ligands would influence the growth properties of tumor cells or with which intracellular proteins they would interact due to their highly charged nature and enzymatic lability. In this study, a prodrug delivery strategy was used to introduce an enzymatically stable, phosphotyrosine peptidomimetic into tumor cells. When tested in a human tumor cell panel, the prodrug exhibited a preference for inhibiting the growth of leukemia and lymphoma cells. In these cells, it was largely cytostatic and induced endoreduplication and the appearance of midbodies. Proteomic analyses identified multiple targets that included mitotic centromere-associated kinesin (MCAK). Molecular modeling studies suggested the ATP-binding site on MCAK as the likely site of drug interaction. Consistent with this, ATP inhibited the drug-MCAK interaction and the drug inhibited MCAK ATPase activity. Accordingly, the effects of the prodrug on the assembly of the mitotic spindle and alignment of chromosomes were consistent with the identification of MCAK as an important intracellular target.

Control of antigen-binding to aluminum adjuvants and the immune response with a novel phosphonate linker.

The strongest mechanism for adsorption of antigens to aluminum adjuvants is ligand exchange, which involves the replacement of a surface hydroxyl on the adjuvant by a terminal phosphate group of the antigen. A novel phosphonate linker was developed that allows the addition of phosphonate (C-PO3) groups to proteins under controlled and chemically mild conditions. Increasing the number of linkers per protein molecule progressively increased the adsorption strength to aluminum hydroxide adjuvant (AH) as measured by elution in serum. The effect of phosphonate conjugation on the antibody response was determined with hen egg lysozyme (HEL), a protein that has the same charge as AH at neutral pH and does not adsorb to AH. The phosphonylated form of HEL (HEL-P) adsorbed to AH, indicating that the ligand exchange interaction could overcome the electrostatic repulsion. Mice injected with HEL-P/AH had a higher antibody titer to HEL than mice injected with HEL/AH, especially at lower antigen doses, suggesting that adsorption of antigen has a dose-sparing effect. Conjugation of CRM197, an antigen that adsorbs electrostatically to AH, with phosphonate linkers did not enhance the antibody response, indicating that adsorption by either electrostatic or ligand exchange to AH is sufficient to enhance the antibody response.

Synthesis of a phosphoserine mimetic prodrug with potent 14-3-3 protein inhibitory activity.

Many protein-protein interactions in cells are mediated by functional domains that recognize and bind to motifs containing phosphorylated serine and threonine residues. To create small molecules that inhibit such interactions, we developed methodology for the synthesis of a prodrug that generates a phosphoserine peptidomimetic in cells. For this study, we synthesized a small molecule inhibitor of 14-3-3 proteins that incorporates a nonhydrolyzable difluoromethylenephosphoserine prodrug moiety. The prodrug is cytotoxic at low micromolar concentrations when applied to cancer cells and induces caspase activation resulting in apoptosis. The prodrug reverses the 14-3-3-mediated inhibition of FOXO3a resulting from its phosphorylation by Akt1 in a concentration-dependent manner that correlates well with its ability to inhibit cell growth. This methodology can be applied to target a variety of proteins containing phosphoserine and other phosphoamino acid binding domains.

Design and synthesis of a potential SH2 domain inhibitor bearing a stereodiversified 1,4-cis-enediol scaffold.

Synthesis of a potential Src family SH2 domain inhibitor incorporating a 1,4-cis-enediol scaffold is reported. The synthetic route offers straightforward and highly selective access to the enediol and its associated chiral centers. Key steps include stereocontrolled syn-aldol coupling, amide alkynylation, and asymmetric ketone reduction.

Functional analysis of novel analogues of E3330 that block the redox signaling activity of the multifunctional AP endonuclease/redox signaling enzyme APE1/Ref-1.

APE1 is a multifunctional protein possessing DNA repair and redox activation of transcription factors. Blocking these functions leads to apoptosis, antiangiogenesis, cell-growth inhibition, and other effects, depending on which function is blocked. Because a selective inhibitor of the APE redox function has potential as a novel anticancer therapeutic, new analogues of E3330 were synthesized. Mass spectrometry was used to characterize the interactions of the analogues (RN8-51, 10-52, and 7-60) with APE1. RN10-52 and RN7-60 were found to react rapidly with APE1, forming covalent adducts, whereas RN8-51 reacted reversibly. Median inhibitory concentration (IC(50) values of all three compounds were significantly lower than that of E3330. EMSA, transactivation assays, and endothelial tube growth-inhibition analysis demonstrated the specificity of E3330 and its analogues in blocking the APE1 redox function and demonstrated that the analogues had up to a sixfold greater effect than did E3330. Studies using cancer cell lines demonstrated that E3330 and one analogue, RN8-51, decreased the cell line growth with little apoptosis, whereas the third, RN7-60, caused a dramatic effect. RN8-51 shows particular promise for further anticancer therapeutic development. This progress in synthesizing and isolating biologically active novel E3330 analogues that effectively inhibit the APE1 redox function validates the utility of further translational anticancer therapeutic development.

A novel geranylgeranyl transferase inhibitor in combination with lovastatin inhibits proliferation and induces autophagy in STS-26T MPNST cells.

Prenylation inhibitors have gained increasing attention as potential therapeutics for cancer. Initial work focused on inhibitors of farnesylation, but more recently geranylgeranyl transferase inhibitors (GGTIs) have begun to be evaluated for their potential antitumor activity in vitro and in vivo. In this study, we have developed a nonpeptidomimetic GGTI, termed GGTI-2Z [(5-nitrofuran-2-yl)methyl-(2Z,6E,10E)-3,7,11,15-tetramethylhexadeca-2,6,10,14-tetraenyl 4-chlorobutyl(methyl)phosphoramidate], which in combination with lovastatin inhibits geranylgeranyl transferase I (GGTase I) and GGTase II/RabGGTase, without affecting farnesylation. The combination treatment results in a G(0)/G(1) arrest and synergistic inhibition of proliferation of cultured STS-26T malignant peripheral nerve sheath tumor cells. We also show that the antiproliferative activity of drugs in combination occurs in the context of autophagy. The combination treatment also induces autophagy in the MCF10.DCIS model of human breast ductal carcinoma in situ and in 1c1c7 murine hepatoma cells, where it also reduces proliferation. At the same time, there is no detectable toxicity in normal immortalized Schwann cells. These studies establish GGTI-2Z as a novel geranylgeranyl pyrophosphate derivative that may work through a new mechanism involving the induction of autophagy and, in combination with lovastatin, may serve as a valuable paradigm for developing more effective strategies in this class of antitumor therapeutics.

Design and synthesis of novel quinone inhibitors targeted to the redox function of apurinic/apyrimidinic endonuclease 1/redox enhancing factor-1 (Ape1/ref-1).

The multifunctional enzyme apurinic endonuclease 1/redox enhancing factor 1 (Ape1/ref-1) maintains genetic fidelity through the repair of apurinic sites and regulates transcription through redox-dependent activation of transcription factors. Ape1 can therefore serve as a therapeutic target in either a DNA repair or transcriptional context. Inhibitors of the redox function can be used as either therapeutics or novel tools for separating the two functions for in vitro study. Presently there exist only a few compounds that have been reported to inhibit Ape1 redox activity; here we describe a series of quinones that exhibit micromolar inhibition of the redox function of Ape1. Benzoquinone and naphthoquinone analogues of the Ape1-inhibitor E3330 were designed and synthesized to explore structural effects on redox function and inhibition of cell growth. Most of the naphthoquinones were low micromolar inhibitors of Ape1 redox activity, and the most potent analogues inhibited tumor cell growth with IC(50) values in the 10-20 microM range.

Role of the multifunctional DNA repair and redox signaling protein Ape1/Ref-1 in cancer and endothelial cells: small-molecule inhibition of the redox function of Ape1.

The DNA base excision-repair pathway is responsible for the repair of DNA damage caused by oxidation/alkylation and protects cells against the effects of endogenous and exogenous agents. Removal of the damaged base creates a baseless (AP) site. AP endonuclease1 (Ape1) acts on this site to continue the BER-pathway repair. Failure to repair baseless sites leads to DNA strand breaks and cytotoxicity. In addition to the repair role of Ape1, it also functions as a major redox-signaling factor to reduce and activate transcription factors such as AP1, p53, HIF-1alpha, and others that control the expression of genes important for cell survival and cancer promotion and progression. Thus, the Ape1 protein interacts with proteins involved in DNA repair, growth-signaling pathways, and pathways involved in tumor promotion and progression. Although knockdown studies with siRNA have been informative in studying the role of Ape1 in both normal and cancer cells, knocking down Ape1 does not reveal the individual role of the redox or repair functions of Ape1. The identification of small-molecule inhibitors of specific Ape1 functions is critical for mechanistic studies and translational applications. Here we discuss small-molecule inhibition of Ape1 redox and its effect on both cancer and endothelial cells.

Induction of apoptosis in neurofibromatosis type 1 malignant peripheral nerve sheath tumor cell lines by a combination of novel farnesyl transferase inhibitors and lovastatin.

Neurofibromatosis type 1 (NF1) is a genetic disorder that is driven by the loss of neurofibromin (Nf) protein function. Nf contains a Ras-GTPase-activating protein domain, which directly regulates Ras signaling. Numerous clinical manifestations are associated with the loss of Nf and increased Ras activity. Ras proteins must be prenylated to traffic and functionally localize with target membranes. Hence, Ras is a potential therapeutic target for treating NF1. We have tested the efficacy of two novel farnesyl transferase inhibitors (FTIs), 1 and 2, alone or in combination with lovastatin, on two NF1 malignant peripheral nerve sheath tumor (MPNST) cell lines, NF90-8 and ST88-14. Single treatments of 1, 2, or lovastatin had no effect on Ras prenylation or MPNST cell proliferation. However, low micromolar combinations of 1 or 2 with lovastatin (FTI/lovastatin) reduced Ras prenylation in both MPNST cell lines. Furthermore, this FTI/lovastatin combination treatment reduced cell proliferation and induced an apoptotic response as shown by morphological analysis, procaspase-3/-7 activation, loss of mitochondrial membrane potential, and accumulation of cells with sub-G(1) DNA content. Little to no detectable toxicity was observed in normal rat Schwann cells following FTI/lovastatin combination treatment. These data support the hypothesis that combination FTI plus lovastatin therapy may be a potential treatment for NF1 MPNSTs.

Synthesis and biological activity of a gemcitabine phosphoramidate prodrug.

A gemcitabine (2',2'-difluorodeoxycytidine, dFdC) phosphoramidate prodrug designed for the intracellular delivery of gemcitabine 5'-monophosphate was synthesized. The prodrug was about an order of magnitude less active than gemcitabine against wild-type cells, and the nucleoside transport inhibitor dipyridamole reduced prodrug activity. The prodrug was more active than gemcitabine against two deoxycytidine kinase-deficient cell lines. The results suggest that the prodrug is a potent growth inhibitor that can bypass dCK deficiency at higher drug concentrations.

Synthesis, biochemical, and cellular evaluation of farnesyl monophosphate prodrugs as farnesyltransferase inhibitors.

Certain farnesyl diphosphate (FPP) analogs are potent inhibitors of the potential anticancer drug target protein farnesyltransferase (FTase), but these compounds are not suitable as drug candidates. Thus, phosphoramidate prodrug derivatives of the monophosphate precursors of FPP-based FTase inhibitors have been synthesized. The monophosphates themselves were significantly more potent inhibitors of FTase than the corresponding FPP analogs. The effects of the prodrug 5b (a derivative of 3-allylfarnesyl monophosphate) have been evaluated on prenylation of RhoB and on the cell cycle in a human malignant schwannoma cell line (STS-26T). In combination treatments, 1-3 microM 5b plus 1 microM lovastatin induced a significant inhibition of RhoB prenylation, and a combination of these drugs at 1 microM each also resulted in significant cell cycle arrest in G1. Indeed, combinations as low as 50 nM lovastatin + 1 microM 5c or 250 nM lovastatin + 50 nM 5c were highly cytostatic in STS-26T cell culture.

Highly efficient synthesis of enantiomerically enriched 2-hydroxymethylaziridines by enzymatic desymmetrization.

Both enantiomers of protected and unprotected 2-hydroxymethylaziridines are efficiently and enantiospecifically synthesized by using a combination of enzymatic and synthetic methods. PPL was used for lipase-catalyzed desymmetrization of N-protected serinol. [reaction: see text].

Synthesis and cell-based activity of a potent and selective protein tyrosine phosphatase 1B inhibitor prodrug.

Our laboratory recently reported the development of novel prodrug chemistry for the intracellular delivery of phosphotyrosine mimetics. This chemistry has now been adapted for the synthesis of a prodrug that delivers the nonhydrolyzable difluoromethylphosphonate moiety intracellularly. Activation of the prodrug generates a difluoromethylphosphonamidate anion that undergoes subsequent cyclization and hydrolysis with a t1/2 = 44 min. A highly potent and selective inhibitor of protein tyrosine phosphatase 1B (PTP1B) with a nanomolar Ki has been reported, but this bis(difluoromethylphosphonate) lacks potential utility due to its exceedingly low membrane permeability at physiological pH. A prodrug of this inhibitor has been synthesized and evaluated in a cell-based assay. The prodrug exhibits nanomolar PTP1B inhibitory activity in this assay, confirming the efficacy of intracellular phosphonate delivery using this prodrug approach.

Synthesis and biological activity of N-2,3-dihydroxypropyl-N-4-chlorobutyl nucleoside phosphoramidate prodrugs.

N-2,3-dihydroxypropyl-N-4-chlorobutyl phosphoramidate prodrugs of thymidine and 5-fluoro-2'-deoxyuridine (5-FdU) 2-4 were synthesized as analogues of the known prodrugs 1a,b to assess the effects of the dihydroxypropyl moiety on prodrug activation, log P, and growth inhibitory activity in vitro. The thymidine analogues 2 and 3 were prepared as model compounds for kinetics and log P studies. 31P NMR kinetics following hydrogenolysis of 2 showed that the thymidine N-dihydroxypropyl phosphoramidate released thymidine monophosphate with a half-life of 212 min under model physiologic conditions compared to approximately 2 min for the corresponding N-methyl phosphoramidate reported previously. The measured log P for compound 3 was 1.1 log units lower than that of the analogous 1b, confirming that the dihydroxypropyl group significantly decreased prodrug lipophilicity. The dihydroxypropyl prodrug 4 showed cell growth inhibition activity in the NCI 60 cell line panel similar to that of the N-methyl analogue 1a previously reported.

Molecular targets for emerging anti-tumor therapies for neurofibromatosis type 1.

Neurofibromatosis type 1 (NF1) is the most common cancer predisposition syndrome. NF1 patients present with a constellation of clinical manifestations and have an increased risk of developing certain benign and malignant tumors. This disease results from mutation within the gene encoding neurofibromin, a GTPase activating protein (GAP) for Ras. Functional loss of this protein compromises Ras inactivation, which leads to the aberrant growth and proliferation of neural crest-derived cells and, ultimately, tumor formation. Current management of NF1-associated malignancy involves radiation, surgical excision, and cytotoxic drugs. The limited success of these strategies has fueled researchers to further elucidate the molecular changes that drive tumor formation and progression. This discussion will highlight how intracellular signaling molecules, cell-surface receptors, and the tumor microenvironment constitute potential therapeutic targets, which may be relevant not only to NF1-related malignancy but also to other human cancers.

Design and synthesis of phosphotyrosine peptidomimetic prodrugs.

A novel approach to the intracellular delivery of aryl phosphates has been developed that utilizes a phosphoramidate-based prodrug approach. The prodrugs contain an ester group that undergoes reductive activation intracellularly with concomitant expulsion of a phosphoramidate anion. This anion undergoes intramolecular cyclization and hydrolysis to generate aryl phosphate exclusively with a t(1/2) = approximately 20 min. Phosphoramidate prodrugs (8-10) of phosphate-containing peptidomimetics that target the SH2 domain were synthesized. Evaluation of these peptidomimetic prodrugs in a growth inhibition assay and in a cell-based transcriptional assay demonstrated that the prodrugs had IC50 values in the low micromolar range. Synthesis of phosphorodiamidate analogues containing a P-NH-Ar linker (16-18) was also carried out in the hope that the phosphoramidates released might be phosphatase-resistant. Comparable activation rates and cell-based activities were observed for these prodrugs, but the intermediate phosphoramidate dianion underwent spontaneous hydrolysis with a t(1/2) = approximately 30 min.

A novel method for the preparation of nucleoside triphosphates from activated nucleoside phosphoramidates.

[reaction: see text] A novel method for the preparation of nucleoside triphosphates has been developed. The strategy employs a highly reactive pyrrolidinium phosphoramidate zwitterion intermediate that undergoes efficient coupling with tris(tetra-n-butylammonium) hydrogen pyrophosphate to generate nucleoside triphosphate.

Synthesis and biological evaluation of a cytarabine phosphoramidate prodrug.

Recently, we reported a novel approach for the intracellular delivery of the anti-cancer nucleotide 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) using phosphoramidate-based prodrugs. These phosphoramidate prodrugs contain an ester group that undergoes intracellular activation, liberating phosphoramidate anion, which in turn undergoes spontaneous cyclization and P-N bond cleavage to yield the nucleoside monophosphate quantitatively. This approach has now been extended to cytarabine [1-beta-D-arabinofuranosylcytosine (Ara-C)], an anti-cancer nucleoside that is limited in its utility because of poor intracellular transport characteristics and weak activity as a substrate for tumor cell kinases. The cytarabine phosphoramidate prodrug 1 has been synthesized and evaluated in comparison with cytarabine for growth inhibitory activity against wild-type, nucleoside transport-deficient, and nucleoside kinase-deficient CEM leukemia cell lines. The prodrug was comparable in growth inhibitory activity (IC50 = 32 nM) to cytarabine (IC50 = 16 nM) in wild-type CCRF-CEM cells following drug treatment for 72 h. The nucleoside transport-deficient CEM/AraC8C exhibited a high level of resistance (6400-fold) to cytarabine but was more sensitive (210-fold resistant vs CCRF-CEM cells) to prodrug 1. Similarly, the deoxycytidine kinase-deficient cell line (CEM/dCK-) was highly resistant to cytarabine (13900-fold) but more sensitive (106-fold resistant vs CCRF-CEM cells) to prodrug 1. These results indicate that prodrug 1 is significantly more potent than cytarabine against transport- and kinase-deficient cell lines and are consistent with a mechanism involving intracellular delivery of cytarabine 5'-monophosphate.

Novel method for the immobilization of nucleotides.

[reaction: see text] A novel method for the immobilization of nucleotides has been developed. The strategy employs a highly reactive pyrrolidinium phosphoramidate zwitterion intermediate that undergoes nucleophilic attack by long-chain alkylamine-controlled pore glass (LCAA-CPG) to generate an immobilized nucleotide. Quantification of nucleotide loading was accomplished by acidic hydrolysis of the P-N bond and subsequent HPLC analysis of TMP in the presence of an internal standard. Typical nucleotide loadings of 51-59 micromol/g of support were observed.