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. 2016 Apr 4;142(9):2013–2022. doi: 10.1007/s00432-016-2144-1

Hematologic malignancies: newer strategies to counter the BCL-2 protein

Abdul Shukkur Ebrahim 1, Hussam Sabbagh 1, Allison Liddane 1, Ali Raufi 1, Mustapha Kandouz 2, Ayad Al-Katib 1,✉
PMCID: PMC11819263  PMID: 27043233

Abstract

Introduction

BCL-2 is the founding member of the BCL-2 family of apoptosis regulatory proteins that either induce (pro-apoptotic) or inhibit (anti-apoptotic) apoptosis. The anti-apoptotic BCL-2 is classified as an oncogene, as damage to the BCL-2 gene has been shown to cause a number of cancers, including lymphoma. Ongoing research has demonstrated that disruption of BCL-2 leads to cell death. BCL-2 is also known to be involved in the development of resistance to chemotherapeutic agents, further underscoring the importance of targeting the BCL-2 gene in cancer therapeutics. Thus, numerous approaches have been developed to block or modulate the production of BCL-2 at the RNA level using antisense oligonucleotides or at the protein level with BCL-2 inhibitors, such as the novel ABT737.

Methods

In this article, we briefly review previous strategies to target the BCL-2 gene and focus on a new approach to silence DNA, DNA interference (DNAi).

Results and conclusion

DNA interference is aimed at blocking BCL-2 gene transcription. Evaluations of this technology in preclinical and early clinical studies are very encouraging and strongly support further development of DNAi as cancer therapeutics. A pilot phase II clinical trial in patients with relapsed or refractory non-Hodgkin lymphoma, PNT2258 demonstrated clinical benefit in 11 of 13 patients with notable responses in diffuse large B cell lymphoma and follicular lymphoma. By targeting the DNA directly, the DNAi technology promises to be more effective compared with other gene-interference strategies that target the RNA or protein but leaves the dysregulated DNA functional.

Keywords: BCL-2, Non-Hodgkin lymphoma, DNA interference, Antisense oligonucleotides, Small-molecule inhibitors

Introduction

BCL-2 is a member of the BCL-2 family of protein regulators of cell apoptosis. This family modulates apoptosis by either inducing (pro-apoptotic) or inhibiting (anti-apoptotic) apoptotic factors (Tsujimoto et al. 1984; Cleary et al. 1986). Damage to BCL-2, classified as an oncogene, has been identified as a cause for the onset of a myriad of cancerous manifestations including lymphoma (Reed 2008). BCL-2 disruption can lead to cell death and has been reported to play a role in the development of resistance to chemotherapeutic agents (Minn et al. 1995; Thomas et al. 2013; Yang et al. 2002; Sagawa et al. 2011; Tabuchi et al. 2009). Therefore, numerous approaches have been developed to block or modulate the production of BCL-2 at the RNA level using drugs such as antisense oligonucleotides (Tsujimoto 1998) or at the protein level with inhibitors, such as ABT-263 (Wendt 2008). Current therapy for BCL-2 biologically “relevant” tumors, such as non-Hodgkin lymphomas (NHLs), is inadequate. Treatment traditionally consists of chemotherapy administered in combination to maximize cell killing while minimizing overlapping toxicity. These therapeutic regimens (e.g., R-CHOP, EPOCH) utilize drugs that are non-specific and cause DNA intercalation (Maxwell and Mousavi-Fard 2013) and DNA cross-linking (Dunleavy et al. 2013), which often causes significant off-target toxicity to normal tissue. A better understanding of tumor biology has identified BCL-2 as being causative to the genesis of lymphomas, critical for the survival of cancer cells, and promoting chemo-resistance mechanisms that enhance tumor survival (Abdullah and Chow 2013; Banerjee et al. 2012). The successful development of a BCL-2 targeted drug has the potential to enhance cell killing when used alone or in combination with traditional cytotoxic agents, and may reduce the need for currently used drug schedules and doses. Importantly, this may lead to greater efficacy and reduce toxicity, creating a profound shift in current practice paradigms.

In this overview, we present an update of these strategies and focus on a new approach to target the BCL-2 gene, called DNA interference (DNAi). DNAi therapeutic drugs, such as PNT2258, are a class of nucleic acid-based drugs that contain sequences designed against noncoding, non-transcribed regions of genomic DNA upstream of gene transcription initiation sites, thus effectively blocking transcription.

The BCL-2 family of proteins

BCL-2 was first discovered by virtue of its involvement in the t (14;18) chromosomal translocations commonly found in NHL (Reed et al. 1996). Increased BCL-2 expression has been shown to play an important role in neoplastic expansion. BCL-2 increases the survival kinetics of the cell by specifically blocking apoptosis, preventing the cell from going into suicidal activities that usually require ATP, new RNA, and protein synthesis, and inducing a variety of cellular ultra-structural changes such as cell shrinkage, nuclear fragmentation, and DNA degradation.

The 25 members of the BCL-2 family of proteins identified to date can be divided into three subfamilies based on structural and functional features (Fig. 1) (Reed and Pellecchia 2005). The anti-apoptotic subfamily members, BCL-2 and BCL-XL, contain all four BCL-2 homology (BH) domains, designated BH1-4. MCL-1 is another BCL-2-related survival protein, but is somewhat structurally distinct and probably lacks a “classical” BH4 domain. Pro-apoptotic proteins such as BAX, BAK, and BOK share BH1-3 domains and are termed “multidomains,” whereas other pro-apoptotic proteins, such as BIM, BAD, and BID, contain only the BH3 domain and are known as “BH3-only.” Some BCL-2 family proteins also contain a carboxy-terminal transmembrane domain (Packham and Stevenson 2005).

Fig. 1.

Fig. 1

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Pro- and anti-apoptotic members of the BCL-2 family

The common explanation of how BCL-2 (and its anti-apoptotic homologues) inhibits apoptosis focuses on preservation of mitochondrial membrane integrity. BCL-2 prevents BAX/BAK oligomerization that would otherwise lead to the release of several apoptogenic molecules from the mitochondrion by an as yet poorly understood mechanism. Like BAX and BAK, BCL-2 is inserted in the outer mitochondrial membrane but whether it directly binds to these molecules under physiological conditions remains largely unresolved. Interaction of a BH3-only protein with BCL-2 allows BAX and BAK to fulfill their death-inducing functions (Kirkin et al. 2004; Chipuk and Green 2008; Gillies and Kuwana 2014).

Regulating the activation of several initiator caspases like caspase-2 may be one of the functions of BCL-2. Currently unknown cell death protein (CED)-4-like proteins, other than Apaf-1, might serve to oligomerize these caspases. As the CED3/CED4 complex in C. elegans is inhibited by CED9 binding, it was presumed that BCL-2 might directly interact with such adaptor proteins to prevent caspase activation. Initially, BCL-2 and BCL-xL were believed to bind to Apaf-1 but later publications clearly dismiss these interactions (Kirkin et al. 2004; Cory and Adams 2002). BCL-2 localizes to the endoplasmic reticulum (ER) and nuclear membrane as well as to the mitochondrial membrane (Thomenius et al. 2003). BCL-2 may influence apoptosis by affecting calcium storage in the ER and thus intracellular calcium levels. ER-associated BCL-2 may also protect cells from various types of apoptosis by scavenging pro-apoptotic BCL-2 family members, thereby preventing their translocation to the mitochondria (Kirkin et al. 2004; Foyouzi-Youssefi et al. 2000).

Understanding the BCL-2 family and the complex interaction between its members is clinically relevant since some cancers, notably non-Hodgkin lymphoma, are dependent on BCL-2 for survival and resistance to chemotherapy. Since apoptosis is a form of regulated cell death (RCD), it is subject to modulation through pharmacologic or genetic interventions (Galluzzi et al. 2015). The impact of interfering with BCL-2 expression or function on cancer cell survival is testable in the clinical and preclinical settings. Recent advances in BH3 profiling now allow identification of tumors that are BCL-2-dependent which are likely to respond to BCL-2-disrupting strategy (Moore and Letai 2013). This approach, however, was developed in the context of BH-3 mimetics as small-molecule inhibitors of apoptosis, like ABT-737, but may be applicable to other anti-BCL-2 strategies like the DNAi.

Antisense oligonucleotides (ASOs)

Antisense technology involves the use of a sequence that is complementary to a specific mRNA which inhibits its expression and subsequently induces a blockade in the transfer of genetic information from DNA to protein (Fig. 2) (Dias and Stein 2002). Considerable effort has been made toward the use of this approach to treat various diseases (Evers et al. 2015; Farooqi et al. 2014; Gerard et al. 2016; Castanotto and Stein 2014). There are two types of ASOs with different mechanisms of action (Bennett and Swayze 2010; Vickers and Crooke 2014). The major type is the RNase H-dependent oligonucleotide that inhibits protein expression by degrading the mRNA (Wu et al. 2004). The second type is a steric blocker that inhibits initiation of protein translation by targeting either the promoter region or a region around the initiation codon (Dias and Stein 2002).

Fig. 2.

Fig. 2

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Mechanisms of action of ASOs

Antisense agents are complementary strands of small portions of “sense” messenger RNA (mRNA). When the antisense oligonucleotide hybridizes with its cognate mRNA target and forms a hybrid, the unnatural structure leads to the destruction of mRNA by RNase H. Thus, translation of the protein encoded by the target mRNA is inhibited. In the steric blocking process, however, interruption of protein production does not necessitate mRNA degradation. Both approaches have been used to inhibit BCL-2 expression. Peptide nucleic acids (PNAs) consist of an antisense nucleic acid conjugated to a membrane-permeating peptide to facilitate intracellular delivery (Nielsen 2004, 2010). The earliest PNA blockade was attempted over a decade ago (Mologni et al. 1999; Gallazzi et al. 2003; Liu et al. 2015).

Oblimersen sodium (G3139, Genasense)

Oblimersen sodium (G3139, Genasense) is an 18-base antisense phosphorothioate oligonucleotide compound designed to specifically target the first six codons of the human BCL-2 mRNA sequence, resulting in degradation of BCL-2 mRNA and a subsequent decrease in BCL-2 protein translation and intracellular concentration (Profile 2002).

Oblimersen was the first effective antisense oligonucleotides in the treatment of human cancer to demonstrate inhibition of the BCL-2 protein (Herbst and Frankel 2004). The apoptotic effect of oblimersen is the result of an increase in BAX and poly ADP ribose polymerase (PARP) and cytochrome C release. Caspases are then activated ultimately releasing second mitochondria-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low pI (Smac/DIABLO) to either antagonize inhibitors of apoptosis proteins or release apoptosis-inducing factors from the mitochondria, thus inducing DNA fragmentation. Inhibition of apoptosis proteins activates caspase-3 and caspase-9, which initiates apoptosis; release of apoptosis-inducing factors induces cellular necrosis (Kang and Reynolds 2009). Oblimersen was shown to be effective in BCL-2-overexpressing human mantle cell lymphoma cell lines in vitro and in vivo, by decreasing proliferation and apoptosis regulatory proteins (Tucker et al. 2008). Preclinical studies with oblimersen showed that treatment with the drug alone resulted in some long-term disease-free survival, while combination therapy with cyclophosphamide resulted in long-term disease-free survival with no histological or molecular evidence of lymphoma. Of note, treatment with antisense oligonucleotides lowers the concentration of other chemotherapies required for treatment, decreasing side effects and toxicity. Studies carried out in SCID mice overexpressing BCL-2 and mice deficient in natural killer cells showed the same suppression of tumor growth, due to the fact that oligonucleotides can induce a tumor response by stimulating innate immunity. Combining ASOs therapy with rituximab also exhibited activity in preclinical models of indolent B cell malignancies and prolonged survival in tumor-bearing mice (Klasa et al. 2000).

In a phase II study, oblimersen in combination with rituximab was given to 42 patients who had relapsed/refractory B cell NHL. Ten individuals (42 %) showed a complete response (CR) and eight partial responses (PR); twelve (28 %) patients achieved a minimal response or stable disease. Among the 20 patients with follicular lymphoma, the overall response rate (ORR) was 60 % (eight CR, four PR). Three of the responders were refractory to prior treatment with rituximab, and two of the responses occurred in patients who had failed an autologous stem cell transplant (Pro et al. 2008).

RNAi technology

RNA interference (RNAi) was first described by Fire et al. (1998) in C. elegans. RNAi refers to a double-stranded RNA (dsRNA) that specifically targets a gene’s product resulting in the destruction of its mRNA (Fire et al. 1998). This occurs through a two-step mechanism in which the first step uses RNase III-like activity to degrade the dsRNA into a 21–25 nucleotides of small interfering RNAs. The second step involves combining the siRNAs with RNase complex, RISC (RNA-induced silencing complex), which acts on the cognate mRNA and degrades it (Agrawal et al. 2003). Inhibition of overexpressed oncogenes, blocking cell division by interfering with cyclin E and related genes, and promoting apoptosis by suppressing anti-apoptotic genes are all mechanisms whereby RNAi technology has been used in cancer treatment. RNAi against multidrug resistance genes or chemoresistance targets may also provide useful cancer treatments (Izquierdo 2005; Sioud 2015).

Survivin is an example of a gene that serves as a potential target for RNAi that is highly expressed in NHL tissues and the Burkitt lymphoma cell line, Daudi. A vector-based short hairpin RNA (shRNA) was used to investigate the effect of RNAi-mediated Survivin gene silencing on apoptosis and proliferation of Daudi cells. Transfection of recombinant plasmid survivin-shRNA significantly reduced levels of survivin mRNA and protein levels and increased apoptosis in Daudi cells (Congmin et al. 2006). The use of the RNAi approach in general and in targeting BCL-2 in particular is still in its infancy. However, the effectiveness of RNAi-mediated BCL-2 silencing at inhibiting tumor growth was shown in vitro in BCR-ABL-positive cells acute lymphoblastic leukemia (ALL) as well as in vivo using a xenograft model of patient-derived leukemic blasts (Scherr et al. 2014). Enthusiasm for RNAi as therapeutic modality for cancer has fluctuated over the years. There was great interest in the technology following its initial description in 1998 with many pharmaceutical companies investing in its development. However, early clinical trials failed to meet expectations because of either lack of patient benefit, immunogenicity, or both. As a result, clinical trials were halted and several companies abandoned their RNAi programs. However, enthusiasm for this technology is resurfacing again following refinement in the products and in delivery systems with a number of new products under development (Watts and Corey 2012).

BCL-2 protein inhibitors

Small-molecule inhibitors

Small-molecule inhibitors (SMIs) are a group of drugs designed to mimic BH3-only proteins in order to inhibit the action of anti-apoptotic BCL-2 proteins (Table 1). They are assumed to compete with pro-apoptotic BCL-2s to occupy BH3 docking grooves on the surfaces of anti-apoptotic family members, thus functioning as valuable anti-neoplastic drugs.

Table 1.

BCL-2 family SMIs evaluated in hematologic malignancies

Agent Molecular target References
Obatoclax BCL-2, BCL-XL, BCL-w, BCL-B, BCL-1, MCL-1 Goard and Schimmer (2013)
Gossypol BCL-2 family members and other non-specific targets Mohammad et al. (2005)
ABT-199 BCL-2 Souers et al. (2013)
TW-37 BCLw, BCLXL, A1, MCL-1, BCL-2 Wang et al. (2006)
ABT-263 BCLXL, BCL-2, BCLw Tse et al. (2008)
ABT-737 BCL-2, BCLXL Zhai et al. (2006)
ApoG2 BCL-2, BCL-XL, and MCL-1 Arnold et al. (2008)
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One such drug is obatoclax (GX15-070) D, a new experimental pan inhibitor of BCL-2 family proteins, which is currently undergoing phase I and II trials. Obatoclax binds to BCL-2, BCL-XL, BCL-w, BCL-B, BFL-1, and more importantly, to MCL-1, as many hematologic malignancies depend on this protein for survival. Obatoclax can induce oligomerization of BAK in the mitochondria, disrupt mitochondrial function, and activate caspases leading to cell death and cell cycle arrest. Obatoclax was shown to overcome drug resistance and potentiate the efficacy of traditional chemotherapeutic agents. In preclinical studies using mouse models, obatoclax showed single-agent efficacy in hematologic malignancies but no reports or no efficacy in non-hematologic tumors (Goard and Schimmer 2013). Phase I and II clinical trials of obatoclax in patients with advanced hematologic tumors have been reported. In a phase II clinical trial on 12 refractory and pretreated patients with Hodgkin lymphoma, several patients maintained stable disease, but no partial remission or complete remission was reported (Goard and Schimmer 2013; Oki et al. 2012). However, obatoclax likely will undergo a redesignation due to the fact that a recent study carried out in mice indicated that the drug failed to reach effective blood concentrations and that raising drug doses resulted in neurotoxic effects (Wendt 2008).

Gossypol is a natural product derived from cotton seeds and roots and has been studied as an anticancer agent, both in vitro and in vivo, since the 1980s (Jones 1979). Natural gossypol is a racemic mixture of two enantiomers, (−)-gossypol and (+)-gossypol, that have a similar binding affinity for BCL-XL and BCL-2 in competitive binding assays. Levo gossypol (AT-101, Ascenta) phase II clinical trials are ongoing in chronic lymphocytic leukemia (CLL) (in combination with rituximab) and in hormone refractory prostate cancer (in combination with docetaxel). In studies performed in SCID mice in our laboratory at WSU, (−)-gossypol decreased tumor weight when used alone. However, when 60 mg/kg (−)-gossypol was administered in conjunction with cyclophosphamide, hydroxydaunorubicin, vincristine, and prednisone (CHOP), it achieved a significantly longer tumor growth delay (P = 0.01) compared with either CHOP or (−)-gossypol alone (Mohammad et al. 2005). ABT-199 (ventoclax) is a newly developed BCL-2 selective BH3 mimetic. In a mouse model, ABT-199 demonstrated in vivo efficacy against aggressive lymphoma without provoking thrombocytopenia (Vandenberg and Cory 2013). More recently, ABT-199 was found to induce remissions in patients with relapsed/refractory CLL and small lymphocytic lymphoma (SLL). According to recently published data, the ORR among all currently evaluated patients was 84 %, including a 23 % CR rate. The ORR among patients with deletion (17p) was 82 %, and among those with fludarabine-refractory disease, it was 89 %. A limiting factor of the use of ABT-199 is its increased risk of tumor lysis syndrome, and modification of the dose is currently being investigated (Davids et al. 2013). In May 2015, the US FDA granted breakthrough therapy designation for ABT-199 in 17p deletion relapsed-refractory chronic lymphocytic leukemia.

ABT-737 is a BH3 mimetic that induces apoptosis by binding and neutralizing BCL-2, BCL-xL, and BCL-w (Oltersdorf et al. 2005). This agent is effective against leukemias, lymphomas, and small cell lung cancer (SCLC). However, some resistance to the drug, ascribed to the overexpression of MCL-1, was noticed (Mazumder et al. 2012). In a study in preleukemic Eμ-myc transgenic mice, treatment with ABT-737 resulted in augmented apoptosis of preneoplastic B lymphoid cells, reducing their numbers and greatly prolonging lymphoma-free survival (Kelly et al. 2013).

ABT-263 (navitoclax) is an oral small molecular inhibitor of the BCL-2 family of proteins, specifically BCL-2, BCL-xL, and BCL-w. Several studies have demonstrated significant activity of ABT-263 as a single agent in the treatment of hematologic malignancies (Roberts et al. 2008; Ackler et al. 2008). In vitro, a study of ABT-263 and rapamycin demonstrated a dose-dependent effect on cell death as a single agent in diffuse large B cell lymphoma (DLBCL) and mantle cell lymphoma. DLBCL cells proved to be most sensitive. The effect of ABT-263 was increased drastically in combination with rapamycin. An in vivo study also showed a single agent effect against DLBCL and modest activity against mantle cell lymphoma. Again, a combination of ABT-263 and rapamycin led to a significant tumor response with 100 % ORR and more than 90 % tumor inhibition, suggesting that there is a benefit of ABT-263 and rapamycin as a combination therapy in the treatment of lymphoma (Ackler et al. 2008). ABT-263 is currently undergoing clinical trials in adults with small cell lung cancer (Gandhi et al. 2011). However, a side effect of this agent in clinical trials was thrombocytopenia attributed to BCL-XL inhibition. The agent was reengineered to create a BCL-2-selective inhibitor, ABT-199. This agent was shown to inhibit the growth of BCL-2-dependent tumors in vivo and spares human platelets (Souers et al. 2013).

TW-37, a non-peptide SMI of BCL-2 (Al-Katib et al. 2009), binds with high affinity to the hydrophobic groove found in the multidomain anti-apoptotic BCL-2 family proteins. Drug binding is thought to block the anti-apoptotic proteins from heterodimerizing with the pro-apoptotic members of the BCL-2 family (Bad, Bid, Bim) although it may produce conformational changes that disable the anti-apoptotic members (Al-Katib et al. 2009). In a study done in our laboratory, we examined the activity of TW-37 against established human B cell tumor lines and fresh patient samples representing the spectrum of B cell tumors. Nanomolar concentrations of TW-37 induced apoptosis in both fresh samples and established cell lines with an IC50 of 165–320 nM. Apoptosis was independent of proliferative status or pathological classification of B cell tumor. TW-37 was able to block BIM-BCL-XL and BIM-MCL-1 heterodimerization and induced apoptosis via activation of caspase-9, caspase-3, PARP, and DNA fragmentation. TW-37 administered to tumor-bearing SCID mice led to significant tumor growth inhibition, tumor growth delay, and Log10-kill when used at its maximum tolerated dose (40 mg/kg × 3 days) via tail vein. Our study suggests that the use of small-molecule inhibitors of pan BCL-2 is an effective way of inducing apoptosis in a wide range of B cell tumors in humans as well as diffuse large B cell lymphoma (WSU-DLCL2) bearing SCID mice (Al-Katib et al. 2009).

ApoG2 is a very potent non-peptidic SMI that was designed to overcome the side effects of gossypol (Zhang et al. 2007; Arnold et al. 2008). Binding affinity of ApoG2 for BCL-2 is greater than eightfold that of its predecessors, TW-37 and (−)-gossypol, but the exact mechanism of action of ApoG2 is unclear. It is likely that ApoG2 binds to BCL-2 (or MCL-1, BCL-XL, A1, BCL-w) and prevents its association with BH3-only pro-apoptotic proteins, thus unleashing the pro-apoptotic proteins to participate in the apoptotic response (Arnold et al. 2008). Our laboratory was the first to study the effect of ApoG2 on follicular lymphoma.

ApoG2 significantly inhibited the growth of follicular small cleaved cell lymphoma WSU-FSCCL cell line, with an IC50 of 109 nM and decreased the number of fresh lymphoma cells. ApoG2 activated caspase-9, caspase-3, and caspase-8, PARP cleavage, and apoptosis-inducing factor (AIF). In the WSU-FSCCL-SCID xenograft model, ApoG2 showed a significant anti-lymphoma effect, with percent increase in host life span of 84 % in the intravenously and 63 % in intraperitoneally treated mice; indicating that ApoG2 can be an effective therapeutic agent against follicular lymphoma (Arnold et al. 2008).

The BH3 natural peptides and their mimetics represent one strategy of peptide interference, i.e., interfering with heterodimerization between pro-apoptotic and anti-apoptotic proteins of the BCL-2 family. Another peptide interference strategy is to block corepressor recruitment. Such a strategy may not be applicable to BCL-2 but has been applied to BCL-6 where a BCL-6 peptide inhibitor (BPI) was generated using natural peptide mimicry approach. This peptide successfully blocked BCL-6-mediated recruitment of SMART (silencing mediator of retinoid and thyroid receptors) and reactivated target gene expression (Polo et al. 2004).

DNAi technology

DNAi represents nucleic acid-based drugs that target non-transcribed regions of specific disease genes involved in complex genetic diseases, such as cancer, and neurodegenerative diseases. DNAi constructs contain single-stranded sequences of unmodified phosphodiester DNA. These sequences are designed to be complementary to noncoding regions of genomic DNA upstream of gene transcription start sites. Binding of the DNA interference oligonucleotide to its target region results in gene modulation with phenotypic changes and a modulation of mRNA and protein levels (Sheikhnejad 2009). Sheikhnejad (2009) have suggested two possible mechanisms of action for DNAi: (1) binding of DNAi oligonucleotide to noncoding regions of the target gene to prevent transcription or (2) DNAi binds to transcription factors directly, preventing transcription (Fig. 3).

Fig. 3.

Fig. 3

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Proposed models for the mechanism of action of DNAi (adapted from Sheikhnejad 2009)

The ProNAi Company has developed a new nucleic acid-based therapeutic approach to treat oncogene-overexpressing cancers. The first drug candidate, PNT2258, is an intravenously administered, BCL-2-targeting agent consists of 24-bases of DNA encapsulated in a protective liposome. Rather than disrupting its interactions with other BCL-family proteins, PNT2258 reduces BCL-2 levels, triggering programmed cell death and offering a more direct approach to kill the culprit protein that drives tumor genesis and survival.

We recently reported that PNT2258 shows anti-tumor activity against BCL-2-driven WSU-DLCL2 lymphoma, PC-3 prostate tumor, A375 melanoma, and Daudi Burkitt lymphoma xenografts (Rodrigueza et al. 2014). We hypothesized that PNT2258 would have differential activity in NHL subtypes with different molecular drivers of cell growth and tested this hypothesis in our well-defined lymphoma cell lines representing a wide spectrum of B cell lineage tumors. WSU-FSCCL, WSU-DLCL2, and WSU-WM (Waldenstrom macroglobulinemia) cells were used as targets to investigate the effect of PNT2258 in vitro at different concentrations (2.5, 5.0, and 10 µM) on cell growth and survival. A significant decrease in cell viability was observed after 48 h in all of the cell lines exposed to PNT2258 at the lowest concentration (2.5 µM). The magnitude of response to PNT2258 varied among the three cell lines: The WSU-FSCCL line was more sensitive at 72–96 h when compared to the WSU-DLCL2 and WSU-WM cells (unpublished data), supporting our hypothesis that PNT2258 would be more effective in cells with highest levels of BCL-2 overexpression. Additional investigation is ongoing at our laboratory to elucidate the exact mechanism of action of BCL-2 DNAi.

The first in class BCL-2 DNAi (PNT2258) has entered clinical trials in human. In a phase I study, PNT2258 was well tolerated at doses up to 150 mg/m2 (equivalent to ~4 mg/kg). At this dose, there was a clinically manageable decrease in lymphocyte and platelet counts in patients with malignant solid tumors (Tolcher et al. 2014). In a pilot phase II clinical trial in patients with relapsed/refractory non-Hodgkin lymphoma (NHL), PNT2258 demonstrated clinical activity (tumor shrinkage or tumor stabilization) in 11 of 13 patients (Harb et al. 2014). There were notable responses in patients with diffuse large B cell lymphoma (DLBCL) where 3 of 4 patients achieved complete response (CR) and the fourth patient achieved partial response (PR). Two of five patients with follicular lymphoma (FL) responded (1 CR, 1 PR). The drug is now undergoing additional phase II clinical trials in DLBCL (ProNAi Therapeutics, Inc., NCT02226965).

The molecular biology and pathogenesis of NHL is very complex. One or both hallmarks of cancer, i.e., increased cell proliferation and resistance to apoptosis can be found in different types of NHL. The first principle is exemplified in Burkitt lymphoma (BL) where the t(8;14) translocation results in c-myc deregulation leading to increased proliferation and rapid progression of disease. This type, however, is not associated with resistance to apoptosis which may explain its curable nature despite the aggressive growth behavior. In contrast, follicular lymphoma is characterized by the t(14;18) translocation and overexpression of BCL-2. This translocation explains the low-grade (indolent) nature of this lymphoma and its incurable nature. While these are examples of single deregulated molecular pathway, some NHL types, like DLBCL, are heterogeneous or involve multiple pathways. As new therapeutic options that target specific molecular node become available, it is important to correlate response to therapy with specific genetic aberrations of NHL.

Conclusions and future perspectives

Gene-interference strategies targeting protein or RNA have a number of limitations impacting their success in the clinic. The fundamental limitation stems from the fact that such strategies aim at the product of a target gene rather than the DNA which remains active. As a result, large doses and continuous exposure to the drug are necessary for effective ongoing suppression of gene function. Side effects and host toxicity may occur as a result of such a therapeutic strategy, limiting the delivery of effective doses. These limitations can be overcome if one can target a gene of interest at the DNA level, preventing production of RNA and protein, as opposed to inhibiting function (in the case of protein) or neutralizing RNA. DNAi can interact with genomic DNA leading to an apoptotic cell death cascade by gene silencing. It could unravel the efficacy of nucleic acid therapeutics and show its significant potential as an alternative tool in parallel with the highly trafficked RNAi road to drug development. Both technologies, DNAi and RNAi, are applicable to cancer therapy.

Acknowledgments

This study was funded by the St. John Hospital and Medical Center Foundation and by Michigan Corporate Relations Network’s (MCRN) Small Company Innovation Program (SCIP). The authors also wish to thank Dr. Mary Walsh for assistance in editing the manuscript.

Compliance with ethical standards

Conflict of interest

The authors report no conflicts of interest in this work.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

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