Molecular chemotherapy and chemotherapy: a new front against late-stage hormone-refractory prostate cancer

Perspectives and challenges in developing small molecules targeting purine nucleoside phosphorylase

As a cytosolic enzyme involved in the purine salvage pathway metabolism, purine nucleoside phosphorylase (PNP) plays an important role in a variety of cellular functions but also in immune system, including cell growth, apoptosis and cancer development and progression. Based on its T-cell targeting profile, PNP is a potential target for the treatment of some malignant T-cell proliferative cancers including lymphoma and leukemia, and some specific immunological diseases. Numerous small-molecule PNP inhibitors have been developed so far. However, only Peldesine, Forodesine and Ulodesine have entered clinical trials and exhibited some potential for the treatment of T-cell leukemia and gout. The most recent direction in PNP inhibitor development has been focused on PNP small-molecule inhibitors with better potency, selectivity, and pharmacokinetic property. In this perspective, considering the structure, biological functions, and disease relevance of PNP, we highlight the recent research progress in PNP small-molecule inhibitor development and discuss prospective strategies for designing additional PNP therapeutic agents.

Prostate cancer addiction to oxidative stress defines sensitivity to anti-tumor neutrophils

Bone metastatic prostate cancer (BM-PCa) remains one of the most difficult cancers to treat due to the complex interactions of cancer and stromal cells. We previously showed that bone marrow neutrophils elicit an anti-tumor immune response against BM-PCa. Further, we demonstrated that BM-PCa induces neutrophil oxidative burst, which has previously been identified to promote primary tumor growth of other cancers, and a goal of this study was to define the importance of neutrophil oxidative burst in BM-PCa. To do this, we first examined the impact of depletion of reactive oxygen species (ROS), via systemic deletion of the main source of ROS in phagocytes, NADPH oxidase (Nox)2, which we found to suppress prostate tumor growth in bone. Further, using pharmacologic ROS inhibitors and Nox2-null neutrophils, we found that ROS depletion specifically suppresses growth of androgen-insensitive prostate cancer cells. Upon closer examination using bulk RNA sequencing analysis, we identified that metastatic prostate cancer induces neutrophil transcriptomic changes that activates pathways associated with response to oxidative stress. In tandem, prostate cancer cells resist neutrophil anti-tumor response via extracellular (i.e., regulation of neutrophils) and intracellular alterations of glutathione synthesis, the most potent cellular antioxidant. These findings demonstrate that BM-PCa thrive under oxidative stress conditions and such that regulation of ROS and glutathione programming could be leveraged for targeting of BM-PCa progression.

Purine-Metabolising Enzymes and Apoptosis in Cancer

The enzymes of both de novo and salvage pathways for purine nucleotide synthesis are regulated to meet the demand of nucleic acid precursors during proliferation. Among them, the salvage pathway enzymes seem to play the key role in replenishing the purine pool in dividing and tumour cells that require a greater amount of nucleotides. An imbalance in the purine pools is fundamental not only for preventing cell proliferation, but also, in many cases, to promote apoptosis. It is known that tumour cells harbour several mutations that might lead to defective apoptosis-inducing pathways, and this is probably at the basis of the initial expansion of the population of neoplastic cells. Therefore, knowledge of the molecular mechanisms that lead to apoptosis of tumoural cells is key to predicting the possible success of a drug treatment and planning more effective and focused therapies. In this review, we describe how the modulation of enzymes involved in purine metabolism in tumour cells may affect the apoptotic programme. The enzymes discussed are: ectosolic and cytosolic 5'-nucleotidases, purine nucleoside phosphorylase, adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase, and inosine-5'-monophosphate dehydrogenase, as well as recently described enzymes particularly expressed in tumour cells, such as deoxynucleoside triphosphate triphosphohydrolase and 7,8-dihydro-8-oxoguanine triphosphatase.

Use of E. coli Purine Nucleoside Phosphorylase in the Treatment of Solid Tumors

BACKGROUND

The selective expression of non-human genes in tumor tissue to activate non-toxic compounds (Gene Directed Prodrug Enzyme Therapy, GDEPT) is a novel strategy designed for killing tumor cells in patients with little or no systemic toxicity. Numerous non-human genes have been evaluated, but none have yet been successful in the clinic.

METHODS

Unlike human purine nucleoside phosphorylase (PNP), E. coli PNP accepts adenine containing nucleosides as substrates, and is therefore able to selectively activate non-toxic purine analogs in tumor tissue. Various in vitro and in vivo assays have been utilized to evaluate E. coli PNP as a potential activating enzyme.

RESULTS

We and others have demonstrated excellent in vitro and in vivo anti-tumor activity with various GDEPT strategies utilizing E. coli PNP to activate purine nucleoside analogs. A phase I clinical trial utilizing recombinant adenoviral vector for delivery of E. coli PNP to solid tumors followed by systemic administration of fludarabine phosphate (NCT01310179; IND# 14271) has recently been completed. In this trial, significant anti-tumor activity was demonstrated with negligible toxicity related to the therapy. The mechanism of cell kill (inhibition of RNA and protein synthesis) is distinct from all currently used anticancer drugs and all experimental compounds under development. The approach has demonstrated excellent ability to kill neighboring tumor cells that do not express E. coli PNP, is active against non-proliferating and proliferating tumors cells (as well as tumor stem cells, stroma), and is therefore very effective against solid tumors with a low growth fraction.

CONCLUSION

The unique attributes distinguish this approach from other GDEPT strategies and are precisely those required to mediate significant improvements in antitumor therapy.

Targeted prodrug approaches for hormone refractory prostate cancer

Due to the propensity of relapse and resistance with prolonged androgen deprivation therapy (ADT), there is a growing interest in developing non-hormonal therapeutic approaches as alternative treatment modalities for hormone refractory prostate cancer (HRPC). Although the standard treatment for HRPC consists of a combination of ADT with taxanes and anthracyclines, the clinical use of chemotherapeutics is limited by systemic toxicity stemming from nondiscriminatory drug exposure to normal tissues. In order to improve the tumor selectivity of chemotherapeutics, various targeted prodrug approaches have been explored. Antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT) strategies leverage tumor-specific antigens and transcription factors for the specific delivery of cytotoxic anticancer agents using various prodrug-activating enzymes. In prostate cancer, overexpression of tumor-specific proteases such as prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) is being exploited for selective activation of anticancer prodrugs designed to be activated through proteolysis by these prostate cancer-specific enzymes. PSMA- and PSA-activated prodrugs typically comprise an engineered high-specificity protease peptide substrate coupled to a potent cytotoxic agent via a linker for rapid release of cytotoxic species in the vicinity of prostate cancer cells following proteolytic cleavage. Over the past two decades, various such prodrugs have been developed and they were effective at inhibiting prostate tumor growth in rodent models; several of these prodrug approaches have been advanced to clinical trials and may be developed into effective therapies for HRPC.

Targeted enzyme prodrug therapy for metastatic prostate cancer - a comparative study of L-methioninase, purine nucleoside phosphorylase, and cytosine deaminase

BACKGROUND

Enzyme prodrug therapy shows promise for the treatment of solid tumors, but current approaches lack effective/safe delivery strategies. To address this, we previously developed three enzyme-containing fusion proteins targeted via annexin V to phosphatidylserine exposed on the tumor vasculature and tumor cells, using the enzymes L-methioninase, purine nucleoside phosphorylase, or cytosine deaminase. In enzyme prodrug therapy, the fusion protein is allowed to bind to the tumor before a nontoxic drug precursor, a prodrug, is introduced. Upon interaction of the prodrug with the bound enzyme, an anticancer compound is formed, but only in the direct vicinity of the tumor, thereby mitigating the risk of side effects while creating high intratumoral drug concentrations. The applicability of these enzyme prodrug systems to treating prostate cancer has remained unexplored. Additionally, target availability may increase with the addition of low dose docetaxel treatment to the enzyme prodrug treatment, but this effect has not been previously investigated. To this end, we examined the binding strength and the cytotoxic efficacy (with and without docetaxel treatment) of these enzyme prodrug systems on the human prostate cancer cell line PC-3.

RESULTS

All three fusion proteins exhibited strong binding; dissociation constants were 0.572 nM for L-methioninase-annexin V (MT-AV), 0.406 nM for purine nucleoside phosphorylase-annexin V (PNP-AV), and 0.061 nM for cytosine deaminase-annexin V (CD-AV). MT-AV produced up to 99% cell death (p < 0.001) with limited cytotoxicity of the prodrug alone. PNP-AV with docetaxel created up to 78% cell death (p < 0.001) with no cytotoxicity of the prodrug alone. CD-AV with docetaxel displayed up to 60% cell death (p < 0.001) with no cytotoxicity of the prodrug alone. Docetaxel treatment created significant increases in cytotoxicity for PNP-AV and CD-AV.

CONCLUSIONS

Strong binding of fusion proteins to the prostate cancer cells and effective cell killing suggest that the enzyme prodrug systems with MT-AV and PNP-AV may be effective treatment options. Additionally, low-dose docetaxel treatment was found to increase the cytotoxic effect of the annexin V-targeted therapeutics for the PNP-AV and CD-AV systems.

DTX3L and ARTD9 inhibit IRF1 expression and mediate in cooperation with ARTD8 survival and proliferation of metastatic prostate cancer cells

Background

Prostate cancer (PCa) is one of the leading causes of cancer-related mortality and morbidity in the aging male population and represents the most frequently diagnosed malignancy in men around the world. The Deltex (DTX)-3-like E3 ubiquitin ligase (DTX3L), also known as B-lymphoma and BAL-associated protein (BBAP), was originally identified as a binding partner of the diphtheria-toxin-like macrodomain containing ADP-ribosyltransferase-9 (ARTD9), also known as BAL1 and PARP9. We have previously demonstrated that ARTD9 acts as a novel oncogenic survival factor in high-risk, chemo-resistant, diffuse large B cell lymphoma (DLBCL). The mono-ADP-ribosyltransferase ARTD8, also known as PARP14 functions as a STAT6-specific co-regulator of IL4-mediated proliferation and survival in B cells.

Methods

Co-expression of DTX3L, ARTD8, ARTD9 and STAT1 was analyzed in the metastatic PCa (mPCa) cell lines PC3, DU145, LNCaP and in the normal prostate luminal epithelial cell lines HPE and RWPE1. Effects on cell proliferation, survival and cell migration were determined in PC3, DU145 and/or LNCaP cells depleted of DTX3L, ARTD8, ARTD9, STAT1 and/or IRF1 compared to their proficient control cells, respectively. In further experiments, real-time RT-PCR, Western blot, immunofluorescence and co-immunoprecipitations were conducted to evaluate the physical and functional interactions between DTX3L, ARTD8 and ARTD9.

Results

Here we could identify DTX3L, ARTD9 and ARTD8 as novel oncogenic survival factors in mPCa cells. Our studies revealed that DTX3L forms a complex with ARTD8 and mediates together with ARTD8 and ARTD9 proliferation, chemo-resistance and survival of mPCa cells. In addition, DTX3L, ARTD8 and ARTD9 form complexes with each other. Our study provides first evidence that the enzymatic activity of ARTD8 is required for survival of mPCa cells. DTX3L and ARTD9 act together as repressors of the tumor suppressor IRF1 in mPCa cells. Furthermore, the present study shows that DTX3L together with STAT1 and STAT3 is implicated in cell migration of mPCa cells.

Conclusions

Our data strongly indicate that a crosstalk between STAT1, DTX3L and ARTD-like mono-ADP-ribosyltransferases mediates proliferation and survival of mPCa cells. The present study further suggests that the combined targeted inhibition of STAT1, ARTD8, ARTD9 and/or DTX3L could increase the efficacy of chemotherapy or radiation treatment in prostate and other high-risk tumor types with an increased STAT1 signaling.

Liposomal co-delivery of omacetaxine mepesuccinate and doxorubicin for synergistic potentiation of antitumor activity

PURPOSE

Anticancer chemotherapy usually involves the administration of several anticancer drugs that differ in their action mechanisms. Here, we aimed to test whether the combination of omacetaxine mepesuccinate (OMT) and doxorubicin (DOX) could show synergism, and whether the liposomal co-delivery of these two drugs could enhance their antitumor effects in cervical carcinoma model.

METHOD

OMT-loaded liposomes (OL) were prepared by loading the drug in the lipid bilayers. OL were then electrostatically complexed with DOX, yielding double-loaded liposomes (DOL). DOX-loaded liposomes (DL) were formulated by electrostatic interaction with negatively charged empty liposomes (EL). The combination index (CI) values were calculated to evaluate the synergism of two drugs. In vitro antitumor effects against HeLa cells were measured using CCK-8, calcein staining, and crystal violet staining. In vivo antitumor effects of various liposomes were tested using HeLa cell-bearing mice.

RESULTS

Combination of DOX and OMT had ratio-dependent synergistic activities, with very strong synergism observed at a molar ratio of 4:1 (DOX:OMT). The sizes of EL, DL, OL, and DOL did not significantly differ, but the zeta potentials of DL and DOL were slightly higher than those of OL and EL. In vitro, DOL showed higher antitumor activity than OL, DL or EL in cervical carcinoma HeLa cells. In vivo, unlike other liposomes, DOL reduced the tumor growths by 98.6% and 97.3% relative to the untreated control on day 15 and 25 after the cessation of treatment, respectively.

CONCLUSIONS

These results suggest that liposomal co-delivery of DOX and OMT could synergistically potentiate antitumor effects.

Prodrugs: a challenge for the drug development

It is estimated that about 10% of the drugs approved worldwide can be classified as prodrugs. Prodrugs, which have no or poor biological activity, are chemically modified versions of a pharmacologically active agent, which must undergo transformation in vivo to release the active drug. They are designed in order to improve the physicochemical, biopharmaceutical and/or pharmacokinetic properties of pharmacologically potent compounds. This article describes the basic functional groups that are amenable to prodrug design, and highlights the major applications of the prodrug strategy, including the ability to improve oral absorption and aqueous solubility, increase lipophilicity, enhance active transport, as well as achieve site-selective delivery. Special emphasis is given to the role of the prodrug concept in the design of new anticancer therapies, including antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT).

Antitumor activity of IL-32β through the activation of lymphocytes, and the inactivation of NF-κB and STAT3 signals

Cytokine and activation of lymphocytes are critical for tumor growth. We investigated whether interleukin (IL)-32β overexpression changes other cytokine levels and activates cytotoxic lymphocyte, and thus modify tumor growth. Herein, IL-32β inhibited B16 melanoma growth in IL-32β-overexpressing transgenic mice (IL-32β mice), and downregulated the expressions of anti-apoptotic proteins (bcl-2, IAP, and XIAP) and cell growth regulatory proteins (Ki-67 antigen (Ki-67) and proliferating cell nuclear antigen (PCNA)), but upregulated the expressions of pro-apoptotic proteins (bax, cleaved caspase-3, and cleaved caspase-9). IL-32β also inhibited colon and prostate tumor growth in athymic nude mice inoculated with IL-32β-transfected SW620 colon or PC3 prostate cancer cells. The forced expression of IL-32β also inhibited cell growth in cultured colon and prostate cancer cells, and these inhibitory effects were abolished by IL-32 small interfering RNA (siRNA). IL-10 levels were elevated, but IL-1β, IL-6, and tumor necrosis factor-alpha (TNF-α) levels were reduced in the tumor tissues and spleens of IL-32β mice, and athymic nude mice. The number of cytotoxic T (CD8(+)) and natural killer (NK) cells in tumor tissues, spleen, and blood was significantly elevated in IL-32β mice and athymic nude mice inoculated with IL-32β-transfected cancer cells. Constituted activated NF-κB and STAT3 levels were reduced in the tumor tissues of IL-32β mice and athymic nude mice, as well as in IL-32β-transfected cultured cancer cells. These findings suggest that IL-32β inhibits tumor growth by increasing cytotoxic lymphocyte numbers, and by inactivating the NF-κB and STAT3 pathways through changing of cytokine levels in tumor tissues.

The role of hypoxia-inducible factor 1α in determining the properties of castrate-resistant prostate cancers

BACKGROUND

Castrate-resistant prostate cancer (CRPC) is a lethal condition in patients receiving androgen deprivation therapy for prostate cancer (PC). Despite numerous studies showing the expression of HIF1α protein under normoxia in PC cell lines, the role of this normoxic HIF1α expression in chemo-resistance and migration has not been investigated previously. As no method is currently available to determine which tumors will progress to CRPC, the role of HIF1α in PC and its potential for predicting the development of CRPC was also investigated.

METHODS

The effect of HIF1α protein knockdown on chemo-resistance and migration of PC3 cells was assessed by cell counting and Transwell assays, respectively. Translation efficiency of HIF1α mRNA was determined in PC cells using a HIF1α 5'UTR-luciferase construct. Clinical outcomes were correlated following the staining of 100 prostate tumors for HIF1α expression.

RESULTS

The CRPC-like cell lines (PC3 and DU145) expressed more HIF1α protein than an androgen sensitive cell line (LNCaP). Migration rate and chemo-resistance were higher in the PC3 cells and both were decreased when HIF1α expression was reduced. Increased translation of HIF1α mRNA may be responsible for HIF1α overexpression in PC3 cells. Patients whose tumors expressed HIF1α had significantly decreased metastasis-free survival and the patients who were on androgen-deprivation therapy had decreased CRPC-free survival on Kaplan-Meier analysis. On multivariate analysis HIF1α was an independent risk factor for progression to metastatic PC (Hazard ratio (HR) 9.8, p = 0.017) and development of CRPC (HR 10.0, p = 0.021) in patients on androgen-deprivation therapy. Notably the tumors which did not express HIF1α did not metastasize or develop CRPC.

CONCLUSIONS

HIF1α is likely to contribute to metastasis and chemo-resistance of CRPC and targeted reduction of HIF1α may increase the responsiveness of CRPCs to chemotherapy. Expression of HIF1α may be a useful screening tool for development of CRPC.