Initial testing of the hypoxia-activated prodrug PR-104 by the pediatric preclinical testing program

Expression of Long Non-Coding RNA H19 in Acute Lymphoblastic Leukemia

OObjective: Long non-coding RNA (lncRNA) H19 has essential roles in growth, migration, invasion, and metastasis of most cancers. H19 dysregulation is present in a large number of solid tumors and leukemia. However, the expression level of H19 in acute lymphoblastic leukemia (ALL) has not been elucidated yet. The current study aimed to explore H19 expression in ALL patients and cell lines.

MATERIALS AND METHODS

This experimental study was conducted in bone marrow (BM) samples collected from 25 patients with newly diagnosed ALL. In addition, we cultured the RPMI-8402, Jurkat, Ramos, and Daudi cell lines and assessed the effects of internal (hypoxia) and external (chemotherapy medications L-asparaginase [ASP] and vincristine [VCR]) factors on h19 expression. The expressions of H19, P53, c-Myc, HIF-1α and β-actin were performed using quantitative real-time polymerase chain reaction (qRT-PCR) method.

RESULTS

There was significantly increased H19 expression in the B-cell ALL (B-ALL, P<0.05), T-cell ALL (T-ALL, P<0.01) patients and the cell lines. This upregulation was governed by the P53, HIF-1α, and c-Myc transcription factors. We observed that increased c-Myc expression induced H19 expression; however, P53 adversely affected H19 expression. In addition, the results indicated that chemotherapy changed the gene expression pattern. There was a considerable decrease in H19 expression after exposure to chemotherapy medications; nonetheless, hypoxia induced H19 expression through P53 downregulation.

CONCLUSION

Our findings suggest that H19 may have an important role in pathogenesis in ALL and may act as a promising and potential therapeutic target.

In Vivo Identification of Adducts from the New Hypoxia-Activated Prodrug CP-506 Using DNA Adductomics

Many chemotherapeutic drugs exert their cytotoxicity through the formation of DNA modifications (adducts), which interfere with DNA replication, an overactive process in rapidly dividing cancer cells. Side effects from the therapy are common, however, because these drugs also affect rapidly dividing noncancerous cells. Hypoxia-activated prodrugs (HAPs) have been developed to reduce these side effects as they preferentially activate in hypoxic environments, a hallmark of solid tumors. CP-506 is a newly developed DNA-alkylating HAP designed to exert strong activity under hypoxia. The resulting CP-506-DNA adducts can be used to elucidate the cellular and molecular effects of CP-506 and its selectivity toward hypoxic conditions. In this study, we characterize the profile of adducts resulting from the reaction of CP-506 and its metabolites CP-506H and CP-506M with DNA. A total of 39 putative DNA adducts were detected in vitro using our high-resolution/accurate-mass (HRAM) liquid chromatography tandem mass spectrometry (LC-MS³) adductomics approach. Validation of these results was achieved using a novel strategy involving ¹⁵N-labeled DNA. A targeted MS/MS approach was then developed for the detection of the 39 DNA adducts in five cancer cell lines treated with CP-506 under normoxic and hypoxic conditions to evaluate the selectivity toward hypoxia. Out of the 39 DNA adducts initially identified, 15 were detected, with more adducts observed from the two reactive metabolites and in cancer cells treated under hypoxia. The presence of these adducts was then monitored in xenograft mouse models bearing MDA-MB-231, BT-474, or DMS114 tumors treated with CP-506, and a relative quantitation strategy was used to compare the adduct levels across samples. Eight adducts were detected in all xenograft models, and MDA-MB-231 showed the highest adduct levels. These results suggest that CP-506-DNA adducts can be used to better understand the mechanism of action and monitor the efficacy of CP-506 in vivo, as well as highlight a new role of DNA adductomics in supporting the clinical development of DNA-alkylating drugs.

Restoring Tumour Selectivity of the Bioreductive Prodrug PR-104 by Developing an Analogue Resistant to Aerobic Metabolism by Human Aldo-Keto Reductase 1C3

PR-104 is a phosphate ester pre-prodrug that is converted in vivo to its cognate alcohol, PR-104A, a latent alkylator which forms potent cytotoxins upon bioreduction. Hypoxia selectivity results from one-electron nitro reduction of PR-104A, in which cytochrome P450 oxidoreductase (POR) plays an important role. However, PR-104A also undergoes ‘off-target’ two-electron reduction by human aldo-keto reductase 1C3 (AKR1C3), resulting in activation in oxygenated tissues. AKR1C3 expression in human myeloid progenitor cells probably accounts for the dose-limiting myelotoxicity of PR-104 documented in clinical trials, resulting in human PR-104A plasma exposure levels 3.4- to 9.6-fold lower than can be achieved in murine models. Structure-based design to eliminate AKR1C3 activation thus represents a strategy for restoring the therapeutic window of this class of agent in humans. Here, we identified SN29176, a PR-104A analogue resistant to human AKR1C3 activation. SN29176 retains hypoxia selectivity in vitro with aerobic/hypoxic IC50 ratios of 9 to 145, remains a substrate for POR and triggers γH2AX induction and cell cycle arrest in a comparable manner to PR-104A. SN35141, the soluble phosphate pre-prodrug of SN29176, exhibited superior hypoxic tumour log cell kill (>4.0) to PR-104 (2.5–3.7) in vivo at doses predicted to be achievable in humans. Orthologues of human AKR1C3 from mouse, rat and dog were incapable of reducing PR-104A, thus identifying an underlying cause for the discrepancy in PR-104 tolerance in pre-clinical models versus humans. In contrast, the macaque AKR1C3 gene orthologue was able to metabolise PR-104A, indicating that this species may be suitable for evaluating the toxicokinetics of PR-104 analogues for clinical development. We confirmed that SN29176 was not a substrate for AKR1C3 orthologues across all four pre-clinical species, demonstrating that this prodrug analogue class is suitable for further development. Based on these findings, a prodrug candidate was subsequently identified for clinical trials.

OT-82, a novel anticancer drug candidate that targets the strong dependence of hematological malignancies on NAD biosynthesis

Effective treatment of some types of cancer can be achieved by modulating cell lineage-specific rather than tumor-specific targets. We conducted a systematic search for novel agents selectively toxic to cells of hematopoietic origin. Chemical library screenings followed by hit-to-lead optimization identified OT-82, a small molecule with strong efficacy against hematopoietic malignancies including acute myeloblastic and lymphoblastic adult and pediatric leukemias, erythroleukemia, multiple myeloma, and Burkitt's lymphoma in vitro and in mouse xenograft models. OT-82 was also more toxic towards patients-derived leukemic cells versus healthy bone marrow-derived hematopoietic precursors. OT-82 was shown to induce cell death by inhibiting nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in the salvage pathway of NAD synthesis. In mice, optimization of OT-82 dosing and dietary niacin further expanded the compound's therapeutic index. In toxicological studies conducted in mice and nonhuman primates, OT-82 showed no cardiac, neurological or retinal toxicities observed with other NAMPT inhibitors and had no effect on mouse aging or longevity. Hematopoietic and lymphoid organs were identified as the primary targets for dose limiting toxicity of OT-82 in both species. These results reveal strong dependence of neoplastic cells of hematopoietic origin on NAMPT and introduce OT-82 as a promising candidate for the treatment of hematological malignancies.

Challenges and Opportunities for Childhood Cancer Drug Development

Cancer in children is rare with approximately 15,700 new cases diagnosed in the United States annually. Through use of multimodality therapy (surgery, radiation therapy, and aggressive chemotherapy), 70% of patients will be "cured" of their disease, and 5-year event-free survival exceeds 80%. However, for patients surviving their malignancy, therapy-related long-term adverse effects are severe, with an estimated 50% having chronic life-threatening toxicities related to therapy in their fourth or fifth decade of life. While overall intensive therapy with cytotoxic agents continues to reduce cancer-related mortality, new understanding of the molecular etiology of many childhood cancers offers an opportunity to redirect efforts to develop effective, less genotoxic therapeutic options, including agents that target oncogenic drivers directly, and the potential for use of agents that target the tumor microenvironment and immune-directed therapies. However, for many high-risk cancers, significant challenges remain.

Nitrogen Mustards as Anticancer Chemotherapies: Historic Perspective, Current Developments and Future Trends

Nitrogen mustards, a family of DNA alkylating agents, marked the start of cancer pharmacotherapy. While traditionally characterized by their dose-limiting toxic effects, nitrogen mustards have been the subject of intense research efforts, which have led to safer and more effective agents. Even though the alkylating prodrug mustards were first developed decades ago, active research on ways to improve their selectivity and cytotoxic efficacy is a currently active topic of research. This review addresses the historical development of the nitrogen mustards, outlining their mechanism of action, and discussing the improvements on their therapeutic profile made through rational structure modifications. A special emphasis is made on discussing the nitrogen mustard prodrug category, with Cyclophosphamide (CPA) serving as the main highlight. Selected insights on the latest developments on nitrogen mustards are then provided, limiting such information to agents that preserve the original nitrogen mustard mechanism as their primary mode of action. Additionally, future trends that might follow in the quest to optimize these invaluable chemotherapeutic medications are succinctly suggested.

OBI-3424, a Novel AKR1C3-Activated Prodrug, Exhibits Potent Efficacy against Preclinical Models of T-ALL

PURPOSE

OBI-3424 is a highly selective prodrug that is converted by aldo-keto reductase family 1 member C3 (AKR1C3) to a potent DNA-alkylating agent. OBI-3424 has entered clinical testing for hepatocellular carcinoma and castrate-resistant prostate cancer, and it represents a potentially novel treatment for acute lymphoblastic leukemia (ALL).

EXPERIMENTAL DESIGN

We assessed AKR1C3 expression by RNA-Seq and immunoblotting, and evaluated the in vitro cytotoxicity of OBI-3424. We investigated the pharmacokinetics of OBI-3424 in mice and nonhuman primates, and assessed the in vivo efficacy of OBI-3424 against a large panel of patient-derived xenografts (PDX).

RESULTS

AKR1C3 mRNA expression was significantly higher in primary T-lineage ALL (T-ALL; n = 264) than B-lineage ALL (B-ALL; n = 1,740; P < 0.0001), and OBI-3424 exerted potent cytotoxicity against T-ALL cell lines and PDXs. In vivo, OBI-3424 significantly prolonged the event-free survival (EFS) of nine of nine ALL PDXs by 17.1-77.8 days (treated/control values 2.5-14.0), and disease regression was observed in eight of nine PDXs. A significant reduction (P < 0.0001) in bone marrow infiltration at day 28 was observed in four of six evaluable T-ALL PDXs. The importance of AKR1C3 in the in vivo response to OBI-3424 was verified using a B-ALL PDX that had been lentivirally transduced to stably overexpress AKR1C3. OBI-3424 combined with nelarabine resulted in prolongation of mouse EFS compared with each single agent alone in two T-ALL PDXs.

CONCLUSIONS

OBI-3424 exerted profound in vivo efficacy against T-ALL PDXs derived predominantly from aggressive and fatal disease, and therefore may represent a novel treatment for aggressive and chemoresistant T-ALL in an AKR1C3 biomarker-driven clinical trial.

Overcoming Radioresistance: Small Molecule Radiosensitisers and Hypoxia-activated Prodrugs

The role of hypoxia in radiation resistance is well established and many approaches to overcome hypoxia in tumours have been explored, with variable success. Two small molecule strategies for targeting hypoxia have dominated preclinical and clinical efforts. One approach has been the use of electron-affinic nitroheterocycles as oxygen-mimetic sensitisers. These agents are best exemplified by the 5-nitroimidazole nimorazole, which has limited use in conjunction with radiotherapy in head and neck squamous cell carcinoma. The second approach seeks to leverage tumour hypoxia as a tumour-specific address for hypoxia-activated prodrugs. These prodrugs are selectively activated by reductases under hypoxia to release cytotoxins, which in some instances may diffuse to kill surrounding oxic tumour tissue. A number of these hypoxia-activated prodrugs have been examined in clinical trial and the merits and shortcomings of recent examples are discussed. There has been an evolution from delivering DNA-interactive cytotoxins to molecularly targeted agents. Efforts to implement these strategies clinically continue today, but success has been elusive. Several issues have been identified that compromised these clinical campaigns. A failure to consider the extravascular transport and the micropharmacokinetic properties of the prodrugs has reduced efficacy. One key element for these 'targeted' approaches is the need to co-develop biomarkers to identify appropriate patients. Hypoxia-activated prodrugs require biomarkers for hypoxia, but also for appropriate activating reductases in tumours, as well as markers of intrinsic sensitivity to the released drug. The field is still evolving and changes in radiation delivery and the impact of immune-oncology will provide fertile ground for future innovation.

DNA Adducts from Anticancer Drugs as Candidate Predictive Markers for Precision Medicine

Biomarker-driven drug selection plays a central role in cancer drug discovery and development, and in diagnostic strategies to improve the use of traditional chemotherapeutic drugs. DNA-modifying anticancer drugs are still used as first line medication, but drawbacks such as resistance and side effects remain an issue. Monitoring the formation and level of DNA modifications induced by anticancer drugs is a potential strategy for stratifying patients and predicting drug efficacy. In this perspective, preclinical and clinical data concerning the relationship between drug-induced DNA adducts and biological response for platinum drugs and combination therapies, nitrogen mustards and half-mustards, hypoxia-activated drugs, reductase-activated drugs, and minor groove binding agents are presented and discussed. Aspects including measurement strategies, identification of adducts, and biological factors that influence the predictive relationship between DNA modification and biological response are addressed. A positive correlation between DNA adduct levels and response was observed for the majority of the studies, demonstrating the high potential of using DNA adducts from anticancer drugs as mechanism-based biomarkers of susceptibility, especially as bioanalysis approaches with higher sensitivity and throughput emerge.

Hypoxia promotes chemoresistance in acute lymphoblastic leukemia cell lines by modulating death signaling pathways

BACKGROUND

Several studies show that bone marrow (BM) microenvironment and hypoxia condition can promote the survival of leukemic cells and induce resistance to anti-leukemic drugs. However, the molecular mechanism for chemoresistance by hypoxia is not fully understood.

METHODS

In the present study, we investigated the effect of hypoxia on resistance to two therapies, methotrexate (MTX) and prednisolone (PRD), in two cell models for acute lymphoblastic leukemia (ALL). To look for an implication of hypoxia in chemoresistance, cell viability, total cell density and cell proliferation were analyzed. Survival and death signaling pathways were also screened by "reverse phase protein array" (RPPA) and western blotting experiments conducted on selected proteins to confirm the results.

RESULTS

We found that hypoxia promotes chemoresistance in both ALL cell lines. The induction of drug-resistance by hypoxia was not associated with an increase in total cell density nor an increase in cell proliferation. Using RPPA, we show that chemoresistance induced by hypoxia was mediated through an alteration of cell death signaling pathways. This protective effect of hypoxia seems to occur via a decrease in pro-apoptotic proteins and an increase in anti-apoptotic proteins. The results were confirmed by immunoblotting. Indeed, hypoxia is able to modulate the expression of anti-apoptotic proteins independently of chemotherapy while a pro-apoptotic signal induced by a chemotherapy is not modulated by hypoxia.

CONCLUSIONS

Hypoxia is a factor in leukemia cell resistance and for two conventional chemotherapies modulates cell death signaling pathways without affecting total cell density or cell proliferation.

Biology and Treatment of Rhabdoid Tumor

Rhabdoid tumor is a rare, highly aggressive malignancy that primarily affects infants and young children. These tumors typically arise in the brain and kidney, although extrarenal, non-central nervous system tumors in almost all soft-tissue sites have been described. SMARCB1 is a member of the SWI/SNF chromatin-remodeling complex and functions as a tumor suppressor in the vast majority of rhabdoid tumors. Patients with germline mutations or deletions affecting SMARCB1 are predisposed to the development of rhabdoid tumors, as well as the genetic disorder schwannomatosis. The current hypothesis is that rhabdoid tumors are driven by epigenetic dysregulation, as opposed to the alteration of a specific biologic pathway. The strategies for novel therapeutic approaches based on what is currently known about rhabdoid tumor biology are presented.

Identifying novel therapeutic agents using xenograft models of pediatric cancer

In the USA, the overall cure rate for all childhood cancers is seventy percent, and in many patients that ultimately fail curative therapy, initial responses to current multimodality treatments (surgery, radiation therapy and chemotherapy) is good, with overall 5-year event-free survival approaching 80 %. However, current approaches to curative therapy result in significant morbidity and long-term sequelae, including cardiac dysfunction and cognitive impairment. Furthermore, dose-intensive chemotherapy with conventional agents has not significantly improved outcomes for patients that present with advanced or metastatic disease. Classical cytotoxic agents remain the backbone for curative therapy of both hematologic and solid tumors of childhood. While 'molecularly' targeted agents have shown some clinical activity, responses are often modest and of short duration; hence, there is a need to identify new classes of cytotoxic agent that are effective in patients at relapse and that have reduced or different toxicity profiles to normal tissues. Here we review the pediatric preclinical testing program experience of testing novel agents, and the value and limitations of preclinical xenograft models and genetically engineered mouse models for developing novel agents for treatment of childhood cancer.

Hypoxia-activated prodrugs: paths forward in the era of personalised medicine

Tumour hypoxia has been pursued as a cancer drug target for over 30 years, most notably using bioreductive (hypoxia-activated) prodrugs that target antineoplastic agents to low-oxygen tumour compartments. Despite compelling evidence linking hypoxia with treatment resistance and adverse prognosis, a number of such prodrugs have recently failed to demonstrate efficacy in pivotal clinical trials; an outcome that demands reflection on the discovery and development of these compounds. In this review, we discuss a clear disconnect between the pathobiology of tumour hypoxia, the pharmacology of hypoxia-activated prodrugs and the manner in which they have been taken into clinical development. Hypoxia-activated prodrugs have been evaluated in the manner of broad-spectrum cytotoxic agents, yet a growing body of evidence suggests that their activity is likely to be dependent on the coincidence of tumour hypoxia, expression of specific prodrug-activating reductases and intrinsic sensitivity of malignant clones to the cytotoxic effector. Hypoxia itself is highly variable between and within individual tumours and is not treatment-limiting in all cancer subtypes. Defining predictive biomarkers for hypoxia-activated prodrugs and overcoming the technical challenges of assaying them in clinical settings will be essential to deploying these agents in the era of personalised cancer medicine.

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

The presence of a microenvironment within most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implications. Tumour hypoxia is known to promote the development of an aggressive phenotype, resistance to both chemotherapy and radiotherapy and is strongly associated with poor clinical outcome. Paradoxically, it is recognised as a high-priority target and one of the therapeutic strategies designed to eradicate hypoxic cells in tumours is a group of compounds known collectively as hypoxia-activated prodrugs (HAPs) or bioreductive drugs. These drugs are inactive prodrugs that require enzymatic activation (typically by 1 or 2 electron oxidoreductases) to generate cytotoxic species with selectivity for hypoxic cells being determined by (1) the ability of oxygen to either reverse or inhibit the activation process and (2) the presence of elevated expression of oxidoreductases in tumours. The concepts underpinning HAP development were established over 40 years ago and have been refined over the years to produce a new generation of HAPs that are under preclinical and clinical development. The purpose of this article is to describe current progress in the development of HAPs focusing on the mechanisms of action, preclinical properties and clinical progress of leading examples.

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

The presence of a microenvironment within most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implications. Tumour hypoxia is known to promote the development of an aggressive phenotype, resistance to both chemotherapy and radiotherapy and is strongly associated with poor clinical outcome. Paradoxically, it is recognised as a high-priority target and one of the therapeutic strategies designed to eradicate hypoxic cells in tumours is a group of compounds known collectively as hypoxia-activated prodrugs (HAPs) or bioreductive drugs. These drugs are inactive prodrugs that require enzymatic activation (typically by 1 or 2 electron oxidoreductases) to generate cytotoxic species with selectivity for hypoxic cells being determined by (1) the ability of oxygen to either reverse or inhibit the activation process and (2) the presence of elevated expression of oxidoreductases in tumours. The concepts underpinning HAP development were established over 40 years ago and have been refined over the years to produce a new generation of HAPs that are under preclinical and clinical development. The purpose of this article is to describe current progress in the development of HAPs focusing on the mechanisms of action, preclinical properties and clinical progress of leading examples.

Preclinical Childhood Sarcoma Models: Drug Efficacy Biomarker Identification and Validation

Over the past 35 years, cure rates for pediatric cancers have increased dramatically. However, it is clear that further dose intensification using cytotoxic agents or radiation therapy is not possible without enhancing morbidity and long-term effects. Consequently, novel, less genotoxic, agents are being sought to complement existing treatments. Here, we discuss preclinical human tumor xenograft models of pediatric cancers that may be used practically to identify novel agents for soft tissue and bone sarcomas, and "omics" approaches to identifying biomarkers that may identify sensitive and resistant tumors to these agents.

AKR1C3 is a biomarker of sensitivity to PR-104 in preclinical models of T-cell acute lymphoblastic leukemia

PR-104, a phosphate ester of the nitrogen mustard prodrug PR-104A, has shown evidence of efficacy in adult leukemia clinical trials. Originally designed to target hypoxic cells, PR-104A is independently activated by aldo-keto-reductase 1C3 (AKR1C3). The aim of this study was to test whether AKR1C3 is a predictive biomarker of in vivo PR-104 sensitivity. In a panel of 7 patient-derived pediatric acute lymphoblastic leukemia (ALL) xenografts, PR-104 showed significantly greater efficacy against T-lineage ALL (T-ALL) than B-cell-precursor ALL (BCP-ALL) xenografts. Single-agent PR-104 was more efficacious against T-ALL xenografts compared with a combination regimen of vincristine, dexamethasone, and l-asparaginase. Expression of AKR1C3 was significantly higher in T-ALL xenografts compared with BCP-ALL, and correlated with PR-104/PR-104A sensitivity in vivo and in vitro. Overexpression of AKR1C3 in a resistant BCP-ALL xenograft resulted in dramatic sensitization to PR-104 in vivo. Testing leukemic blasts from 11 patients confirmed that T-ALL cells were more sensitive than BCP-ALL to PR-104A in vitro, and that sensitivity correlated with AKR1C3 expression. Collectively, these results indicate that PR-104 shows promise as a novel therapy for relapsed/refractory T-ALL, and that AKR1C3 expression could be used as a biomarker to select patients most likely to benefit from such treatment in prospective clinical trials.

Selective AKR1C3 inhibitors do not recapitulate the anti-leukaemic activities of the pan-AKR1C inhibitor medroxyprogesterone acetate

Background:

We and others have identified the aldo-keto reductase AKR1C3 as a potential drug target in prostate cancer, breast cancer and leukaemia. As a consequence, significant effort is being invested in the development of AKR1C3-selective inhibitors.

Methods:

We report the screening of an in-house drug library to identify known drugs that selectively inhibit AKR1C3 over the closely related isoforms AKR1C1, 1C2 and 1C4. This screen initially identified tetracycline as a potential AKR1C3-selective inhibitor. However, mass spectrometry and nuclear magnetic resonance studies identified that the active agent was a novel breakdown product (4-methyl(de-dimethylamine)-tetracycline (4-MDDT)).

Results:

We demonstrate that, although 4-MDDT enters AML cells and inhibits their AKR1C3 activity, it does not recapitulate the anti-leukaemic actions of the pan-AKR1C inhibitor medroxyprogesterone acetate (MPA). Screens of the NCI diversity set and an independently curated small-molecule library identified several additional AKR1C3-selective inhibitors, none of which had the expected anti-leukaemic activity. However, a pan AKR1C, also identified in the NCI diversity set faithfully recapitulated the actions of MPA.

Conclusions:

In summary, we have identified a novel tetracycline-derived product that provides an excellent lead structure with proven drug-like qualities for the development of AKR1C3 inhibitors. However, our findings suggest that, at least in leukaemia, selective inhibition of AKR1C3 is insufficient to elicit an anticancer effect and that multiple AKR1C inhibition may be required.

Design of optimized hypoxia-activated prodrugs using pharmacokinetic/pharmacodynamic modeling

Hypoxia contributes to resistance of tumors to some cytotoxic drugs and to radiotherapy, but can in principle be exploited with hypoxia-activated prodrugs (HAP). HAP in clinical development fall into two broad groups. Class I HAP (like the benzotriazine N-oxides tirapazamine and SN30000), are activated under relatively mild hypoxia. In contrast, Class II HAP (such as the nitro compounds PR-104A or TH-302) are maximally activated only under extreme hypoxia, but their active metabolites (effectors) diffuse to cells at intermediate O2 and thus also eliminate moderately hypoxic cells. Here, we use a spatially resolved pharmacokinetic/pharmacodynamic (SR-PK/PD) model to compare these two strategies and to identify the features required in an optimal Class II HAP. The model uses a Green's function approach to calculate spatial and longitudinal gradients of O2, prodrug, and effector concentrations, and resulting killing in a digitized 3D tumor microregion to estimate activity as monotherapy and in combination with radiotherapy. An analogous model for a normal tissue with mild hypoxia and short intervessel distances (based on a cremaster muscle microvessel network) was used to estimate tumor selectivity of cell killing. This showed that Class II HAP offer advantages over Class I including higher tumor selectivity and greater freedom to vary prodrug diffusibility and rate of metabolic activation. The model suggests that the largest gains in class II HAP antitumor activity could be realized by optimizing effector stability and prodrug activation rates. We also use the model to show that diffusion of effector into blood vessels is unlikely to materially increase systemic exposure for realistic tumor burdens and effector clearances. However, we show that the tumor selectivity achievable by hypoxia-dependent prodrug activation alone is limited if dose-limiting normal tissues are even mildly hypoxic.

The Role of Bystander Effects in the Antitumor Activity of the Hypoxia-Activated Prodrug PR-104

Activation of prodrugs in tumors (e.g., by bioreduction in hypoxic zones) has the potential to generate active metabolites that can diffuse within the tumor microenvironment. Such “bystander effects” may offset spatial heterogeneity in prodrug activation but the relative importance of this effect is not understood. Here, we quantify the contribution of bystander effects to antitumor activity for the first time, by developing a spatially resolved pharmacokinetic/pharmacodynamic (SR-PK/PD) model for PR-104, a phosphate ester pre-prodrug that is converted systemically to the hypoxia-activated prodrug PR-104A. Using Green’s function methods we calculated concentrations of oxygen, PR-104A and its active metabolites, and resultant cell killing, at each point of a mapped three-dimensional tumor microregion. Model parameters were determined in vitro, using single cell suspensions to determine relationships between PR-104A metabolism and clonogenic cell killing, and multicellular layer (MCL) cultures to measure tissue diffusion coefficients. LC-MS/MS detection of active metabolites in the extracellular medium following exposure of anoxic single cell suspensions and MCLs to PR-104A confirmed that metabolites can diffuse out of cells and through a tissue-like environment. The SR-PK/PD model estimated that bystander effects contribute 30 and 50% of PR-104 activity in SiHa and HCT116 tumors, respectively. Testing the model by modulating PR-104A-activating reductases and hypoxia in tumor xenografts showed overall clonogenic killing broadly consistent with model predictions. Overall, our data suggest that bystander effects are important in PR-104 antitumor activity, although their reach may be limited by macroregional heterogeneity in hypoxia and reductase expression in tumors. The reported computational and experimental techniques are broadly applicable to all targeted anticancer prodrugs and could be used to identify strategies for rational prodrug optimization.

Activity of the hypoxia-activated prodrug, TH-302, in preclinical human acute myeloid leukemia models

PURPOSE

Acute myeloid leukemia (AML) is an aggressive hematologic neoplasm. Recent evidence has shown the bone marrow microenvironment in patients with AML to be intrinsically hypoxic. Adaptive cellular responses by leukemia cells to survive under low oxygenation also confer chemoresistance. We therefore asked whether therapeutic exploitation of marrow hypoxia via the hypoxia-activated nitrogen mustard prodrug, TH-302, could effectively inhibit AML growth.

EXPERIMENTAL DESIGN

We assessed the effects of hypoxia and TH-302 on human AML cells, primary samples, and systemic xenograft models.

RESULTS

We observed that human AML cells and primary AML colonies cultured under chronic hypoxia (1% O2, 72 hours) exhibited reduced sensitivity to cytarabine-induced apoptosis as compared with normoxic controls. TH-302 treatment resulted in dose- and hypoxia-dependent apoptosis and cell death in diverse AML cells. TH-302 preferentially decreased proliferation, reduced HIF-1α expression, induced cell-cycle arrest, and enhanced double-stranded DNA breaks in hypoxic AML cells. Hypoxia-induced reactive oxygen species by AML cells were also diminished. In systemic human AML xenografts (HEL, HL60), TH-302 [50 mg/kg intraperitoneally (i.p.) 5 times per week] inhibited disease progression and prolonged overall survival. TH-302 treatment reduced the number of hypoxic cells within leukemic bone marrows and was not associated with hematologic toxicities in nonleukemic or leukemic mice. Later initiation of TH-302 treatment in advanced AML disease was as effective as earlier TH-302 treatment in xenograft models.

CONCLUSIONS

Our results establish the preclinical activity of TH-302 in AML and provide the rationale for further clinical studies of this and other hypoxia-activated agents for leukemia therapy.

Inhibition of Bcl-2 improves effect of LCL161, a SMAC mimetic, in hepatocellular carcinoma cells

In this study, we investigated the effect of LCL161, a SMAC mimetic, in hepatocellular carcinoma (HCC). LCL161 showed differential effects on apoptosis in four HCC cell lines, and the endogenous level of Bcl-2 determined the sensitivity of HCC cells to LCL161. Cytotoxicity and apoptosis were observed in sensitive PLC5 and Hep3B cells that express lower levels of Bcl-2, but not in resistant Huh-7 and SK-Hep1 cells with higher Bcl-2 expression. Down regulation of Bcl-2 by small interference RNA overcame the resistance to LCL161 in Huh-7, and the apoptotic effect was rescued in Bcl-2-expressing Hep3B. To test the hypothesis that Bcl-2 determines the sensitivity of HCC cells to LCL161, we assayed the biological effect of SC-2001, a novel Bcl-2 inhibitor derived from obatoclax, in LCL161-resistant cell lines. Huh-7 cells co-treated with LCL161 and SC-2001 showed a significant dose-dependent apoptotic effect demonstrated by sub-G1 assay and cleavage of PARP. Furthermore, the combination index (CI) of LCL161 and SC-2001 showed a convincing synergism in resistant Huh-7. In addition, the combinational therapy showed significant growth inhibition in Huh-7-bearing xenograft tumors. Notably, down regulation of Bcl-2 was observed in a tumor sample treated with LCL161 and SC-2001. In conclusion, targeting Bcl-2 with SC-2001 overcomes drug resistance to LCL161 in HCC cells thus suggesting a new anti-IAP combinational therapy for HCC.

Pronounced hypoxia in models of murine and human leukemia: high efficacy of hypoxia-activated prodrug PR-104

Recent studies indicate that interactions between leukemia cells and the bone marrow (BM) microenvironment promote leukemia cell survival and confer resistance to anti-leukemic drugs. There is evidence that BM microenvironment contains hypoxic areas that confer survival advantage to hematopoietic cells. In the present study we investigated whether hypoxia in leukemic BM contributes to the protective role of the BM microenvironment. We observed a marked expansion of hypoxic BM areas in immunodeficient mice engrafted with acute lymphoblastic leukemia (ALL) cells. Consistent with this finding, we found that hypoxia promotes chemoresistance in various ALL derived cell lines. These findings suggest to employ hypoxia-activated prodrugs to eliminate leukemia cells within hypoxic niches. Using several xenograft models, we demonstrated that administration of the hypoxia-activated dinitrobenzamide mustard, PR-104 prolonged survival and decreased leukemia burden of immune-deficient mice injected with primary acute lymphoblastic leukemia cells. Together, these findings strongly suggest that targeting hypoxia in leukemic BM is feasible and may significantly improve leukemia therapy.