Phase-I human clinical-trial of onconase(r) (p-30 protein) administered intravenously on a weekly schedule in cancer-patients with solid tumors

Role of the Ribonuclease ONCONASE in miRNA Biogenesis and tRNA Processing: Focus on Cancer and Viral Infections

The majority of transcribed RNAs do not codify for proteins, nevertheless they display crucial regulatory functions by affecting the cellular protein expression profile. MicroRNAs (miRNAs) and transfer RNA-derived small RNAs (tsRNAs) are effectors of interfering mechanisms, so that their biogenesis is a tightly regulated process. Onconase (ONC) is an amphibian ribonuclease known for cytotoxicity against tumors and antiviral activity. Additionally, ONC administration in patients resulted in clinical effectiveness and in a well-tolerated feature, at least for lung carcinoma and malignant mesothelioma. Moreover, the ONC therapeutic effects are actually potentiated by cotreatment with many conventional antitumor drugs. This review not only aims to describe the ONC activity occurring either in different tumors or in viral infections but also to analyze the molecular mechanisms underlying ONC pleiotropic and cellular-specific effects. In cancer, data suggest that ONC affects malignant phenotypes by generating tRNA fragments and miRNAs able to downregulate oncogenes expression and upregulate tumor-suppressor proteins. In cells infected by viruses, ONC hampers viral spread by digesting the primer tRNAs necessary for viral DNA replication. In this scenario, new therapeutic tools might be developed by exploiting the action of ONC-elicited RNA derivatives.

Strengths and Challenges of Secretory Ribonucleases as AntiTumor Agents

Approaches to develop effective drugs to kill cancer cells are mainly focused either on the improvement of the currently used chemotherapeutics or on the development of targeted therapies aimed at the selective destruction of cancer cells by steering specific molecules and/or enhancing the immune response. The former strategy is limited by its genotoxicity and severe side effects, while the second one is not always effective due to tumor cell heterogeneity and variability of targets in cancer cells. Between these two strategies, several approaches target different types of RNA in tumor cells. RNA degradation alters gene expression at different levels inducing cell death. However, unlike DNA targeting, it is a pleotropic but a non-genotoxic process. Among the ways to destroy RNA, we find the use of ribonucleases with antitumor properties. In the last few years, there has been a significant progress in the understanding of the mechanism by which these enzymes kill cancer cells and in the development of more effective variants. All the approaches seek to maintain the requirements of the ribonucleases to be specifically cytotoxic for tumor cells. These requirements start with the competence of the enzymes to interact with the cell membrane, a process that is critical for their internalization and selectivity for tumor cells and continue with the downstream effects mainly relying on changes in the RNA molecular profile, which are not only due to the ribonucleolytic activity of these enzymes. Although the great improvements achieved in the antitumor activity by designing new ribonuclease variants, some drawbacks still need to be addressed. In the present review, we will focus on the known mechanisms used by ribonucleases to kill cancer cells and on recent strategies to solve the shortcomings that they show as antitumor agents, mainly their pharmacokinetics.

Targeted delivery of immuno-RNase may improve cancer therapy

Background

Immunotoxins are typical therapeutic drugs that can target cancer cells. They exploit the affinity of specific monoclonal antibodies or ligands to cancer cells to deliver a conjugated protein toxin to target sites, thus, attacking the cancer cells.

Methods

The immuno-RNase, Onc-V3, showed the stability of Onc-V3 in the blood stream. Flow cytometry showed that apoptosis occurred in the HO-8910PM cells when treated with Onc-V3. Under the confocal microscope, the green fluorescent, FITC-Onc-V3, were located in the cytoplasm, suggesting that Onc-V3 had a function in the cytoplasm of cancer cells. Moreover, after staining by DAPI, the blue fluorescent nuclei showed shrinkage and grainy. Wound healing assay showed that high concentrations of Onc-V3 inhibited cell migration and the transwell invasion assay showed that Onc-V3 could inhibit cell invasion to the basement membrane. Western blot results showed significantly decreased PARP, procaspase-9, and procaspase-3 in Onc-V3-induced apoptosis.

Results

These results of the experiments in vitro had shown that the Onc-V3 could be delivered to the cancer cells accurately and it had strong cytotoxicity on high metastatic cancer cells.

Conclusion

The specific toxicity of Onc-V3 on highly metastatic cancer cells can make it a promising anti-cancer drug by using V3 to target delivery of Onconase.

Updates in the Development of ImmunoRNases for the Selective Killing of Tumor Cells

Targeted cancer therapy includes, amongst others, antibody-based delivery of toxic payloads to selectively eliminate tumor cells. This payload can be either a synthetic small molecule drug composing an antibody-drug conjugate (ADC) or a cytotoxic protein composing an immunotoxin (IT). Non-human cytotoxic proteins, while potent, have limited clinical efficacy due to their immunogenicity and potential off-target toxicity. Humanization of the cytotoxic payload is essential and requires harnessing of potent apoptosis-inducing human proteins with conditional activity, which rely on targeted delivery to contact their substrate. Ribonucleases are attractive candidates, due to their ability to induce apoptosis by abrogating protein biosynthesis via tRNA degradation. In fact, several RNases of the pancreatic RNase A superfamily have shown potential as anti-cancer agents. Coupling of a human RNase to a humanized antibody or antibody derivative putatively eliminates the immunogenicity of an IT (now known as a human cytolytic fusion protein, hCFP). However, RNases are tightly regulated in vivo by endogenous inhibitors, controlling the ribonucleolytic balance subject to the cell’s metabolic requirements. Endogenous inhibition limits the efficacy with which RNase-based hCFPs induce apoptosis. However, abrogating the natural interaction with the natural inhibitors by mutation has been shown to significantly enhance RNase activity, paving the way toward achieving cytolytic potency comparable to that of bacterial immunotoxins. Here, we review the immunoRNases that have undergone preclinical studies as anti-cancer therapeutic agents.

A Humanized Anti-CD22-Onconase Antibody-Drug Conjugate Mediates Highly Potent Destruction of Targeted Tumor Cells

Antibody-drug conjugates (ADCs) have evolved as a new class of potent cancer therapeutics. We here report on the development of ADCs with specificity for the B-cell lineage specific (surface) antigen CD22 being expressed in the majority of hematological malignancies. As targeting moiety a previously generated humanized anti-CD22 single-chain variable fragment (scFv) derivative from the monoclonal antibody RFB4 was reengineered into a humanized IgG1 antibody format (huRFB4). Onconase (ranpirnase), a clinically active pancreatic-type ribonuclease, was employed as cytotoxic payload moiety. Chemical conjugation via thiol-cleavable disulfide linkage retained full enzymatic activity and full binding affinity of the ADC. Development of sophisticated purification procedures using size exclusion and ion exchange chromatography allowed the separation of immunoconjugate species with stoichiometrically defined number of Onconase cargos. A minimum of two Onconase molecules per IgG was required for achieving significant in vitro cytotoxicity towards lymphoma and leukemia cell lines. Antibody-drug conjugates with an Onconase to antibody ratio of 3 : 1 exhibited an IC₅₀ of 0.08 nM, corresponding to more than 18,400-fold increased cytotoxicity of the ADC when compared with unconjugated Onconase. These results justify further development of this ADC as a promising first-in-class compound for the treatment of CD22-positive malignancies.

Trop-2-targeting tetrakis-ranpirnase has potent antitumor activity against triple-negative breast cancer

Background

Ranpirnase (Rap) is an amphibian ribonuclease with reported antitumor activity, minimal toxicity, and negligible immunogenicity in clinical studies, but the unfavorable pharmacokinetics and suboptimal efficacy hampered its further clinical development. To improve the potential of Rap-based therapeutics, we have used the DOCK-AND-LOCK™ (DNL™) method to construct a class of novel IgG-Rap immunoRNases. In the present study, a pair of these constructs, (Rap)₂-E1-(Rap)₂ and (Rap)₂-E1*-(Rap)₂, comprising four copies of Rap linked to the C_H3 and C_K termini of hRS7 (humanized anti-Trop-2), respectively, were evaluated as potential therapeutics for triple-negative breast cancer (TNBC).

Methods

The DNL-based immunoRNases, (Rap)₂-E1-(Rap)₂ and (Rap)₂-E1*-(Rap)₂, were characterized and tested for biological activities in vitro on a panel of breast cancer cell lines and in vivo in a MDA-MB-468 xenograft model.

Results

(Rap)₂-E1-(Rap)₂ was highly purified (>95%), exhibited specific cell binding and rapid internalization in MDA-MB-468, a Trop-2-expressing TNBC line, and displayed potent in vitro cytotoxicity (EC₅₀ ≤ 1 nM) against diverse breast cancer cell lines with moderate to high expression of Trop-2, including MDA-MB-468, BT-20, HCC1806, SKBR-3, and MCF-7. In comparison, structural counterparts of (Rap)₂-E1-(Rap)₂, generated by substituting hRS7 with selective non-Trop-2-binding antibodies, such as epratuzumab (anti-CD22), were at least 50-fold less potent than (Rap)₂-E1-(Rap)₂ in MDA-MB-468 and BT-20 cells, both lacking the expression of the cognate antigen. Moreover, (Rap)₂-E1-(Rap)₂ was less effective (EC₅₀ > 50 nM) in MDA-MB-231 (low Trop-2) or HCC1395 (no Trop-2), and did not show any toxicity to human peripheral blood mononuclear cells. In a mouse TNBC model, a significant survival benefit was achieved with (Rap)₂-E1*-(Rap)₂ when given the maximal tolerated dose.

Conclusions

A new class of immunoRNases was generated with enhanced potency for targeted therapy of cancer. The promising results from (Rap)₂-E1-(Rap)₂ and (Rap)₂-E1*-(Rap)₂ support their further investigation as a potential treatment option for TNBC and other Trop-2-expressing cancers.

Ranpirnase and its potential for the treatment of unresectable malignant mesothelioma

Ribonucleases are a superfamily of enzymes which operate at the crossroads of transcription and translation, catalyzing the degradation of RNA; they can be cytotoxic because the cleavage of RNA renders indecipherable its information. Ranpirnase is a novel ribonuclease which preferentially degrades tRNA, thus leading to inhibition of protein synthesis and, ultimately, to cytostasis and cytotoxicity. Ranpirnase has demonstrated antitumor activity both in vitro and in vivo in several tumor models. The maximum tolerated dose emerging from phase I studies was 960 g/m², with renal toxicity as the main dose-limiting toxicity. A large phase II trial showed that ranpirnase has disease-modifying activity against malignant mesothelioma. Ranpirnase proved to be superior to doxorubicin in a phase III trial, while preliminary results of another large, phase III trial, suggest that the combination of ranpirnase and doxorubicin could be more effective than doxorubicin alone. In all the above studies, ranpirnase seems to act mainly as a cytostatic rather than a cytotoxic drug, stabilizing progressive disease and potentially prolonging patients’ survival. Ranpirnase may thus find its niche in combination with doxorubicin for mesothelioma as a second-line therapy, where no standard of care presently exists.

Onconase responsive genes in human mesothelioma cells: implications for an RNA damaging therapeutic agent

Background

Onconase represents a new class of RNA-damaging drugs. Mechanistically, Onconase is thought to internalize, where it degrades intracellular RNAs such as tRNA and double-stranded RNA, and thereby suppresses protein synthesis. However, there may be additional or alternative mechanism(s) of action.

Methods

In this study, microarray analysis was used to compare gene expression profiles in untreated human malignant mesothelioma (MM) cell lines and cells exposed to 5 μg/ml Onconase for 24 h. A total of 155 genes were found to be regulated by Onconase that were common to both epithelial and biphasic MM cell lines. Some of these genes are known to significantly affect apoptosis (IL-24, TNFAIP3), transcription (ATF3, DDIT3, MAFF, HDAC9, SNAPC1) or inflammation and the immune response (IL-6, COX-2). RT-PCR analysis of selected up- or down-regulated genes treated with varying doses and times of Onconase generally confirmed the expression array findings in four MM cell lines.

Results

Onconase treatment consistently resulted in up-regulation of IL-24, previously shown to have tumor suppressive activity, as well as ATF3 and IL-6. Induction of ATF3 and the pro-apoptotic factor IL-24 by Onconase was highest in the two most responsive MM cell lines, as defined by DNA fragmentation analysis. In addition to apoptosis, gene ontology analysis indicated that pathways impacted by Onconase include MAPK signaling, cytokine-cytokine-receptor interactions, and Jak-STAT signaling.

Conclusions

These results provide a broad picture of gene activity after treatment with a drug that targets small non-coding RNAs and contribute to our overall understanding of MM cell response to Onconase as a therapeutic strategy. The findings provide insights regarding mechanisms that may contribute to the efficacy of this novel drug in clinical trials of MM patients who have failed first line chemotherapy or radiation treatment.

Ribonucleases as novel chemotherapeutics : the ranpirnase example

Ranpirnase, a cytotoxic ribonuclease from the frog Rana pipiens, is the archetype of a novel class of cancer chemotherapeutic agents based on homologs and variants of bovine pancreatic ribonuclease (RNase A). Ranpirnase in combination with doxorubicin is in clinical trials for the treatment of unresectable malignant mesothelioma and other cancers. The putative mechanism for ranpirnase-mediated cytotoxicity involves binding to anionic components of the extracellular membrane, cytosolic internalization, and degradation of transfer RNA leading to apoptosis. The maintenance of ribonucleolytic activity in the presence of the cytosolic ribonuclease inhibitor protein is a key aspect of the cytotoxic activity of ranpirnase. The basis for its specific toxicity for cancer cells is not known. This review describes the development of ranpirnase as a cancer chemotherapeutic agent.

Effective therapy of human lymphoma xenografts with a novel recombinant ribonuclease/anti-CD74 humanized IgG4 antibody immunotoxin

Ranpirnase (Rap) is a cytotoxic ribonuclease (RNase) isolated from frog oocytes. Here we describe high antitumor activity of a novel immunotoxin, 2L-Rap-hLL1-gamma4P, composed of 2 Rap molecules, each fused to the N terminus of the light chain of hLL1, an internalizing anti-CD74 humanized antibody. To reduce unwanted side effects, the constant region of hLL1 was changed from gamma1 to gamma4 and further to gamma4P by replacing serine228 to proline to prevent the formation of a half immunoglobulin G (IgG) common for IgG4. In vitro, 2L-Rap-hLL1-gamma4P retained RNase activity, specific binding to CD74, and was significantly more potent against CD74+ cell lines (Daudi, Raji, and MC/CAR) than naked hLL1. In vivo, the pharmacokinetic profile of 2L-Rap-hLL1-gamma4P was similar to that of naked hLL1. The maximum tolerated dose of 2L-Rap-hLL1-gamma4P in severe combined immunodeficient mice (SCID) or BALB/c mice was 50 microg per mouse. In Raji and Daudi Burkitt lymphoma xenograft models, treatment with a single 5 to 50 microg dose of 2L-Rap-hLL1-gamma4P, given as early or delayed treatment, resulted in cures of most animals. Treatment with 2L-Rap-hLL1-gamma4P was significantly better than all controls, including saline, naked hLL1, and nonspecific immunotoxin. In conclusion, 2L-Rap-hLL1-gamma4P demonstrated excellent in vitro and in vivo efficacy and thus merits further consideration as a therapeutic for CD74+ tumors.

A phase II trial of weekly intravenous ranpirnase (Onconase), a novel ribonuclease in patients with metastatic kidney cancer

Ranpirnase (Onconase) is the first ribonuclease to enter cancer clinical trials. In prior phase II trials, responses were seen in mesothelioma and other solid tumors. This phase II trial tested ranpirnase (480 microg/m2/w) in 14 patients with refractory advanced renal cell cancer. The median performance status was zero and the median age was 55. All patients had prior immunotherapy and three had prior chemotherapy. No responses were seen in 14 patients. The median survival from on study was 16 months (range two to 28 months). At this dose and schedule ranpirnase has minimal activity in metastatic renal cell cancer.

Phase II trial of a single weekly intravenous dose of ranpirnase in patients with unresectable malignant mesothelioma

PURPOSE

A multicenter phase II trial of ranpirnase (Onconase; Alfacell Corp, Bloomfield, NJ) as a single agent was conducted to further assess the safety and clinical efficacy of this novel antitumor ribonuclease. Patients with unresectable and histologically confirmed malignant mesothelioma (MM) were eligible.

PATIENTS AND METHODS

One hundred five patients with Eastern Cooperative Oncology Group performance status 0 to 2 were enrolled onto the study. Thirty-seven percent of patients had not responded to prior chemotherapy. The primary end point of the study was survival. Tumor responses and time to progression were also assessed. The Cancer and Leukemia Group B (CALGB) prognostic group criteria were used to define a treatment target group (TTG). Both the intent-to-treat (ITT) and the TTG populations were analyzed for survival.

RESULTS

Median survival times of 6 months for the ITT and 8.3 months for the TTG populations were observed. The 1- and 2-year survival rates were 34.3% and 21.6% for ITT, respectively, and 42% and 26.8% for TTG, respectively. Among the 81 patients assessable for tumor response, four had partial responses, two had minor regressions, and thirty-five experienced stabilization of previously progressive disease. Patients with responses and stable disease demonstrated markedly prolonged survival. Ranpirnase was well tolerated in the majority of patients, and there were no drug-related deaths.

CONCLUSION

Ranpirnase demonstrated activity and a tolerable toxicity profile in patients with unresectable MM. The prognostic value of the CALGB groups was confirmed.

A gender-specific mRNA encoding a cytotoxic ribonuclease contains a 3' UTR of unusual length and structure

A cDNA (2855 nt) encoding a putative cytotoxic ribonuclease (rapLR1) related to the antitumor protein onconase was cloned from a library derived from the liver of gravid female amphibian Rana pipiens. The cDNA was mainly comprised (83%) of 3' untranslated region (UTR). Secondary structure analysis predicted two unusual folding regions (UFRs) in the RNA 3' UTR. Two of these regions (711-1442 and 1877-2130 nt) contained remarkable, stalk-like, stem-loop structures greater than 38 and 12 standard deviations more stable than by chance, respectively. Secondary structure modeling demonstrated similar structures in the 3' UTRs of other species at low frequencies (0.01-0.3%). The size of the rapLR1 cDNA corresponded to the major hybridizing RNA cross-reactive with a genomic clone encoding onconase (3.6 kb). The transcript was found only in liver mRNA from female frogs. In contrast, immunoreactive onconase protein was detected only in oocytes. Deletion of the 3' UTR facilitated the in vitro translation of the rapLR1 cDNA. Taken together these results suggest that these unusual UFRs may affect mRNA metabolism and/or translation.

A dimeric mutant of human pancreatic ribonuclease with selective cytotoxicity toward malignant cells

Monomeric human pancreatic RNase, devoid of any biological activity other than its RNA degrading ability, was engineered into a dimeric protein with a cytotoxic action on mouse and human tumor cells, but lacking any appreciable toxicity on mouse and human normal cells. This dimeric variant of human pancreas RNase selectively sensitizes to apoptotic death cells derived from a human thyroid tumor. Because of its selectivity for tumor cells, and because of its human origin, this protein represents a potentially very attractive, novel tool for anticancer therapy.

Ribonuclease A variants with potent cytotoxic activity

Select members of the bovine pancreatic ribonuclease A (RNase A) superfamily are potent cytotoxins. These cytotoxic ribonucleases enter the cytosol, where they degrade cellular RNA and cause cell death. Ribonuclease inhibitor (RI), a cytosolic protein, binds to members of the RNase A superfamily with inhibition constants that span 10 orders of magnitude. Here, we show that the affinity of a ribonuclease for RI plays an integral role in defining the potency of a cytotoxic ribonuclease. RNase A is not cytotoxic and binds RI with high affinity. Onconase, a cytotoxic RNase A homolog, binds RI with low affinity. To disrupt the RI-RNase A interaction, three RNase A residues (Asp-38, Gly-88, and Ala-109) that form multiple contacts with RI were replaced with arginine. Replacing Asp-38 and Ala-109 with an arginine residue has no effect on the RI-RNase interaction. In addition, these variants are not cytotoxic. In contrast, replacing Gly-88 with an arginine residue yields a ribonuclease (G88R RNase A) that retains catalytic activity in the presence of RI and is cytotoxic to a transformed cell line. Replacing Gly-88 with aspartate also yields a ribonuclease (G88D RNase A) with a decreased affinity for RI and cytotoxic activity. The cytotoxic potency of onconase, G88R RNase A, and G88D RNase A correlate with RI evasion. We conclude that ribonucleases that retain catalytic activity in the presence of RI are cytotoxins. This finding portends the development of a class of chemotherapeutic agents based on pancreatic ribonucleases.