Neuroblastoma: Ornithine Decarboxylase and Polyamines are Novel Targets for Therapeutic Intervention

Bachmann-Bupp syndrome and treatment

Bachmann-Bupp syndrome (BABS) is a neurodevelopmental disorder characterized by developmental delay, hypotonia, and varying forms of non-congenital alopecia. The condition is caused by 3'-end mutations of the ornithine decarboxylase 1 (ODC1) gene, which produce carboxy (C)-terminally truncated variants of ODC, a pyridoxal 5'-phosphate-dependent enzyme. C-terminal truncation of ODC prevents its ubiquitin-independent proteasomal degradation and leads to cellular accumulation of ODC enzyme that remains catalytically active. ODC is the first rate-limiting enzyme that converts ornithine to putrescine in the polyamine pathway. Polyamines (putrescine, spermidine, spermine) are aliphatic molecules found in all forms of life and are important during embryogenesis, organogenesis, and tumorigenesis. BABS is an ultra-rare condition with few reported cases, but it serves as a convincing example for drug repurposing therapy. α-Difluoromethylornithine (DFMO, also known as eflornithine) is an ODC inhibitor with a strong safety profile in pediatric use for neuroblastoma and other cancers as well as West African sleeping sickness (trypanosomiasis). Patients with BABS have been treated with DFMO and have shown improvement in hair growth, muscle tone, and development.

Pharmacological targeting of polyamine and hypusine biosynthesis reduces tumor activity of endometrial cancer

Endometrial cancer (EC) is a common and deadly cancer in women and novel therapeutic approaches are urgently needed. Polyamines (putrescine, spermidine, spermine) are critical for mammalian cell proliferation and MYC coordinately regulates polyamine metabolism through ornithine decarboxylase (ODC). ODC is a MYC target gene and rate-limiting enzyme of polyamine biosynthesis and the FDA-approved anti-protozoan drug α-difluoromethylornithine (DFMO) inhibits ODC activity and induces polyamine depletion that leads to tumor growth arrest. Spermidine is required for the hypusine-dependent activation of eukaryotic translation initiation factors 5A1 (eIF5A1) and 5A2 (eIF5A2) and connects the MYC/ODC-induced deregulation of spermidine to eIF5A1/2 protein translation, which is increased during cancer cell proliferation. We show that the eIF5A1 is significantly upregulated in EC cells compared to control cells (p = 0.000038) and that combined pharmacological targeting of ODC and eIF5A hypusination with cytostatic drugs DFMO and N1-guanyl-1,7-diaminoheptane (GC7), respectively, reduces eIF5A1 activation and synergistically induces apoptosis in EC cells. In vivo, DFMO/GC7 suppressed xenografted EC tumor growth in mice more potently than each drug alone compared to control (p = 0.002) and decreased putrescine (p = 0.045) and spermidine levels in tumor tissues. Our data suggest DFMO and GC7 combination therapy may be useful in the treatment or prevention of EC.

Probenecid increases renal retention and antitumor activity of DFMO in neuroblastoma

BACKGROUND

Neuroblastoma (NB) is the most common extracranial solid tumor in children. Interference with the polyamine biosynthesis pathway by inhibition of MYCN-activated ornithine decarboxylase (ODC) is a validated approach. The ODC inhibitor α-difluoromethylornithine (DFMO, or Eflornithine) has been FDA-approved for the treatment of trypanosomiasis and hirsutism and has advanced to clinical cancer trials including NB as well as cancer-unrelated human diseases. One key challenge of DFMO is its rapid renal clearance and the need for high and frequent drug dosing during treatment.

METHODS

We performed in vivo pharmacokinetic (PK), antitumorigenic, and molecular studies with DFMO/probenecid using NB patient-derived xenografts (PDX) in mice. We used LC-MS/MS, HPLC, and immunoblotting to analyze blood, brain tissue, and PDX tumor tissue samples collected from mice.

RESULTS

The organic anion transport 1/3 (OAT 1/3) inhibitor probenecid reduces the renal clearance of DFMO and significantly increases the antitumor activity of DFMO in PDX of NB (P < 0.02). Excised tumors revealed that DFMO/probenecid treatment decreases polyamines putrescine and spermidine, reduces MYCN protein levels and dephosphorylates retinoblastoma (Rb) protein (p-Rb^Ser795), suggesting DFMO/probenecid-induced cell cycle arrest.

CONCLUSION

Addition of probenecid as an adjuvant to DFMO therapy may be suitable to decrease overall dose and improve drug efficacy in vivo.

Allicin, a Potent New Ornithine Decarboxylase Inhibitor in Neuroblastoma Cells

The natural product allicin is a reactive sulfur species (RSS) from garlic (Allium sativum L.). Neuroblastoma (NB) is an early childhood cancer arising from the developing peripheral nervous system. Ornithine decarboxylase (ODC) is a rate-limiting enzyme in the biosynthesis of polyamines, which are oncometabolites that contribute to cell proliferation in NB and other c-MYC/MYCN-driven cancers. Both c-MYC and MYCN directly transactivate the E-box gene ODC1, a validated anticancer drug target. We identified allicin as a potent ODC inhibitor in a specific radioactive in vitro assay using purified human ODC. Allicin was ∼23 000-fold more potent (IC₅₀ = 11 nM) than DFMO (IC₅₀ = 252 μM), under identical in vitro assay conditions. ODC is a homodimer with 12 cysteines per monomer, and allicin reversibly S-thioallylates cysteines. In actively proliferating human NB cells allicin inhibited ODC enzyme activity, reduced cellular polyamine levels, inhibited cell proliferation (IC₅₀ 9-19 μM), and induced apoptosis. The natural product allicin is a new ODC inhibitor and could be developed for use in conjunction with other anticancer treatments, the latter perhaps at a lower than usual dosage, to achieve drug synergism with good prognosis and reduced adverse effects.

Polyamine synthesis as a target of MYC oncogenes

This paper is in recognition of the 100th birthday of Dr. Herbert Tabor, a true pioneer in the polyamine field for over 70 years, who served as the editor-in-chief of the Journal of Biological Chemistry from 1971 to 2010. We review current knowledge of MYC proteins (c-MYC, MYCN, and MYCL) and focus on ornithine decarboxylase 1 (ODC1), an important bona fide gene target of MYC, which encodes the sentinel, rate-limiting enzyme in polyamine biosynthesis. Although notable advances have been made in designing inhibitors against the "undruggable" MYCs, their downstream targets and pathways are currently the main avenue for therapeutic anticancer interventions. To this end, the MYC-ODC axis presents an attractive target for managing cancers such as neuroblastoma, a pediatric malignancy in which MYCN gene amplification correlates with poor prognosis and high-risk disease. ODC and polyamine levels are often up-regulated and contribute to tumor hyperproliferation, especially of MYC-driven cancers. We therefore had proposed to repurpose α-difluoromethylornithine (DFMO), an FDA-approved, orally available ODC inhibitor, for management of neuroblastoma, and this intervention is now being pursued in several clinical trials. We discuss the regulation of ODC and polyamines, which besides their well-known interactions with DNA and tRNA/rRNA, are involved in regulating RNA transcription and translation, ribosome function, proteasomal degradation, the circadian clock, and immunity, events that are also controlled by MYC proteins.

Polyamine Biosynthetic Pathway as a Drug Target for Osteosarcoma Therapy

Osteosarcoma (OS) is the most common bone tumor in children. Polyamines (PAs) are ubiquitous cations involved in many cell processes including tumor development, invasion and metastasis. In other pediatric cancer models, inhibition of the PA biosynthesis pathway with ornithine decarboxylase (ODC) inhibitor alpha-difluoromethylornithine (DFMO) results in decreased cell proliferation and differentiation. In OS, the PA pathway has not been evaluated. DFMO is an attractive, orally administered drug, is well tolerated, can be given for prolonged periods, and is already used in pediatric patients. Three OS cell lines were used to study the cellular effects of PA inhibition with DFMO: MG-63, U-2 OS and Saos-2. Effects on proliferation were analyzed by cell count, flow cytometry-based cell cycle analysis and RealTime-Glo™ MT Cell Viability assays. Intracellular PA levels were measured with high-performance liquid chromatography (HPLC). Western blot analysis was used to evaluate cell differentiation. DFMO exposure resulted in significantly decreased cell proliferation in all cell lines. After treatment, intracellular spermidine levels were drastically decreased. Cell cycle arrest at G₂/M was observed in U-2 OS and Saos-2. Cell differentiation was most prominent in MG-63 and U-2 OS as determined by increases in the terminal differentiation markers osteopontin and collagen 1a1. Cell proliferation continued to be suppressed for several days after removal of DFMO. Based on our findings, DFMO is a promising new adjunct to current osteosarcoma therapy in patients at high risk of relapse, such as those with poor necrosis at resection or those with metastatic or recurrent osteosarcoma. It is a well-tolerated oral drug that is currently in phase II clinical trials in pediatric neuroblastoma patients as a maintenance therapy. The same type of regimen may also improve outcomes in osteosarcoma patients in whom there have been essentially no medical advances in the last 30 years.

Synergistic drug combination GC7/DFMO suppresses hypusine/spermidine-dependent eIF5A activation and induces apoptotic cell death in neuroblastoma

The eukaryotic initiation factor 5A (eIF5A), which contributes to several crucial processes during protein translation, is the only protein that requires activation by a unique post-translational hypusine modification. eIF5A hypusination controls cell proliferation and has been linked to cancer. eIF5A hypusination requires the enzymes deoxyhypusine synthase (DHPS) and deoxyhypusine hydroxylase and uniquely depends on the polyamine (PA) spermidine as the sole substrate. Ornithine decarboxylase (ODC) is the rate-limiting enzyme in PA biosynthesis. Both ODC and PAs control cell proliferation and are frequently dysregulated in cancer. Since only spermidine can activate eIF5A, we chose the hypusine-PA nexus as a rational target to identify new drug combinations with synergistic antiproliferative effects. We show that elevated mRNA levels of the two target enzymes DHPS and ODC correlate with poor prognosis in a large cohort of neuroblastoma (NB) tumors. The DHPS inhibitor GC7 (N1-guanyl-1,7-diaminoheptane) and the ODC inhibitor α-difluoromethylornithine (DFMO) are target-specific and in combination induced synergistic effects in NB at concentrations that were not individually cytotoxic. Strikingly, while each drug alone at higher concentrations is known to induce p21/Rb- or p27/Rb-mediated G1 cell cycle arrest, we found that the drug combination induced caspase 3/7/9, but not caspase 8-mediated apoptosis, in NB cells. Hypusinated eIF5A levels and intracellular spermidine levels correlated directly with drug treatments, signifying specific drug targeting effects. This two-pronged GC7/DFMO combination approach specifically inhibits both spermidine biosynthesis and post-translational, spermidine-dependent hypusine-eIF5A activation, offering an exciting clue for improved NB drug therapy.

Effect of sulfasalazine on human neuroblastoma: analysis of sepiapterin reductase (SPR) as a new therapeutic target

Background

Neuroblastoma (NB) is an aggressive childhood malignancy in children up to 5 years of age. High-stage tumors frequently relapse even after aggressive multimodal treatment, and then show therapy resistance, typically resulting in patient death. New molecular-targeted compounds that effectively suppress tumor growth and prevent relapse with more efficacy are urgently needed. We and others previously showed that polyamines (PA) like spermidine and spermine are essential for NB tumorigenesis and that DFMO, an inhibitor of the key PA synthesis gene product ODC, is effective both in vitro and in vivo, securing its evaluation in NB clinical trials. To find additional compounds interfering with PA biosynthesis, we tested sulfasalazine (SSZ), an FDA-approved salicylate-based anti-inflammatory and immune-modulatory drug, recently identified to inhibit sepiapterin reductase (SPR). We earlier presented evidence for a physical interaction between ODC and SPR and we showed that RNAi-mediated knockdown of SPR expression significantly reduced native ODC enzyme activity and impeded NB cell proliferation.

Methods

Human NB mRNA expression datasets in the public domain were analyzed using the R2 platform. Cell viability, isobologram, and combination index analyses as a result of SSZ treatment with our without DFMO were carried out in NB cell cultures. Molecular protein-ligand docking was achieved using the GRAMM algorithm. Statistical analyses were performed with the Kruskal-Wallis test, 2log Pearson test, and Student’s t test.

Results

In this study, we show the clinical relevance of SPR in human NB tumors. We found that high SPR expression is significantly correlated to unfavorable NB characteristics like high age at diagnosis, MYCN amplification, and high INSS stage. SSZ inhibits the growth of NB cells in vitro, presumably due to the inhibition of SPR as predicted by computational docking of SSZ into SPR. Importantly, the combination of SSZ with DFMO produces synergistic antiproliferative effects in vitro.

Conclusions

The results suggest the use of SSZ in combination with DFMO for further experiments, and possible prioritization as a novel therapy for the treatment of NB patients.

Deoxyhypusine synthase (DHPS) inhibitor GC7 induces p21/Rb-mediated inhibition of tumor cell growth and DHPS expression correlates with poor prognosis in neuroblastoma patients

PURPOSE

Neuroblastoma (NB) is an aggressive pediatric malignancy that typically occurs in infants and children under the age of 5 years. High-stage tumors relapse frequently even after aggressive multimodal treatment, resulting in therapy resistance and eventually in patient death. Clearly, new biologically-targeted drugs are needed that more efficiently suppress tumor growth and prevent relapse. We and others previously showed that polyamines such as spermidine play an essential role in NB tumorigenesis and that DFMO, an inhibitor of the central polyamine synthesis gene ODC, is effective in vitro and in vivo, prompting its evaluation in NB clinical trials. However, the specific molecular actions of polyamines remain poorly defined. Spermidine and deoxyhypusine synthase (DHPS) are essential components in the hypusination-driven post-translational activation of eukaryotic initiation factor 5A (eIF5A).

METHODS

We assessed the role of DHPS in NB and the impact of its inhibition by N(1)-guanyl-1,7-diaminoheptane (GC7) on tumor cell growth using cell proliferation assays, Western blot, immunofluorescence microscopy, and Affymetrix micro-array mRNA expression analyses in NB tumor samples.

RESULTS

We found that GC7 inhibits NB cell proliferation in a dose-dependent manner, through induction of the cell cycle inhibitor p21 and reduction of total and phosphorylated Rb proteins. Strikingly, high DHPS mRNA expression correlated significantly with unfavorable clinical parameters, including poor patient survival, in a cohort of 88 NB tumors (all P < 0.04).

CONCLUSIONS

These results suggest that spermidine and DHPS are key contributing factors in NB tumor proliferation through regulation of the p21/Rb signaling axis.

AMXT-1501, a novel polyamine transport inhibitor, synergizes with DFMO in inhibiting neuroblastoma cell proliferation by targeting both ornithine decarboxylase and polyamine transport

Neuroblastoma (NB) is associated with MYCN oncogene amplification occurring in approximately 30% of NBs and is associated with poor prognosis. MYCN is linked to a number of genes including ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. ODC expression is elevated in many forms of cancer including NB. Alpha-difluoromethylornithine (DFMO), an ODC inhibitor, is currently being used in a Phase I clinical trial for treatment of NB. However, cancer cells treated with DFMO may overcome their polyamine depletion by the uptake of polyamines from extracellular sources. A novel polyamine transport inhibitor, AMXT-1501, has not yet been tested in NB. We propose that inhibiting ODC with DFMO, coupled with polyamine transport inhibition by AMXT-1501 will result in enhanced NB growth inhibition. Single and combination drug treatments were conducted on three NB cell lines. DFMO IC50 values ranged from 20.76 to 33.3 mM, and AMXT-1501 IC50 values ranged from 14.13 to 17.72 µM in NB. The combination treatment resulted in hypophosphorylation of retinoblastoma protein (Rb), suggesting growth inhibition via G1 cell cycle arrest. Increased expression of cleaved PARP and cleaved caspase 3 in combination-treated cells starting at 48 hr suggested apoptosis. The combination treatment depleted intracellular polyamine pools and decreased intracellular ATP, further verifying growth inhibition. Given the current lack of effective therapies for patients with relapsed/refractory NB and the preclinical effectiveness of DFMO with AMXT-1501, this combination treatment provides promising preclinical results. DFMO and AMXT-1501 may be a potential new therapy for children with NB.

Difluoromethylornithine: the proof is in the polyamines

In this issue (beginning on page 1368), Kreul and colleagues report a retrospective review of long-term efficacy and toxicity for subjects participating in a phase III study of difluoromethylornithine (DFMO) for prevention of nonmelanoma skin cancer (NMSC). They conclude that those treated with DFMO had a nonsignificant, persistent decrease in NMSC after completion of treatment and that treatment with DFMO did not result in late toxicity after the discontinuation of treatment. We review the data on DFMO as a chemopreventive agent for skin and other cancers, discuss the necessary qualities of a cancer chemopreventive agent, and reflect on the requirements for a well-conducted cancer chemoprevention study, including the rationale for long-term follow-up in cancer prevention studies.