Distribution of hematopoietic stem cells in the bone marrow according to regional hypoxia

The interaction of stem cells with their bone marrow microenvironment is a critical process in maintaining normal hematopoiesis. We applied an approach to resolve the spatial organization that underlies these interactions by evaluating the distribution of hematopoietic cell subsets along an in vivo Hoechst 33342 (Ho) dye perfusion gradient. Cells isolated from different bone marrow regions according to Ho fluorescence intensity contained the highest concentration of hematopoietic stem cell (HSC) activity in the lowest end of the Ho gradient (i.e., in the regions reflecting diminished perfusion). Consistent with the ability of Ho perfusion to simulate the level of oxygenation, bone marrow fractions separately enriched for HSCs were found to be the most positive for the binding of the hypoxic marker pimonidazole. Moreover, the in vivo administration of the hypoxic cytotoxic agent tirapazamine exhibited selective toxicity to the primitive stem cell subset. These data collectively indicate that HSCs and the supporting cells of the stem cell niche are predominantly located at the lowest end of an oxygen gradient in the bone marrow with the implication that regionally defined hypoxia plays a fundamental role in regulating stem cell function.

Prognostic significance of [18F]-misonidazole positron emission tomography-detected tumor hypoxia in patients with advanced head and neck cancer randomly assigned to chemoradiation with or without tirapazamine: a substudy of Trans-Tasman Radiation Oncology Group Study 98.02

PURPOSE

To determine the association between tumor hypoxia, treatment regimen, and locoregional failure (LRF) in patients with stage III or IV squamous cell carcinoma of the head and neck randomly assigned to radiotherapy (70 Gy in 35 fractions over 7 weeks) plus either tirapazamine and cisplatin in weeks 1, 4, and 7 and tirapazamine alone in weeks 2 and 3 (TPZ/CIS) or cisplatin and infusional fluorouracil during weeks 6 and 7 (chemoboost).

PATIENTS AND METHODS

Forty-five patients were enrolled onto a hypoxic imaging substudy of a larger randomized trial. Pretreatment and midtreatment [18F]-fluoromisonidazole positron emission tomography scans (FMISO-PET) were performed 2 hours after tracer administration, with qualitative scoring of uptake in both primary tumors and nodes.

RESULTS

Thirty-two patients (71%) had detectable hypoxia in either or both primary and nodal disease. In patients who received chemoboost, one of 10 patients without hypoxia had LRF compared with eight of 13 patients with hypoxia; the risk of LRF was significantly higher in hypoxic patients (exact log-rank, P = .038; hazard ratio [HR] = 7.1). By contrast, in patients who received the TPZ/CIS regimen, only one of 19 patients with hypoxic tumors had LRF; risk of LRF was significantly higher in chemoboost patients (P = .001; HR = 15). Similarly, looking at the primary site alone, in patients with hypoxic primaries, zero of eight patients treated with TPZ/CIS experienced failure locally compared with six of nine patients treated with chemoboost (P = .011; HR = 0).

CONCLUSION

Hypoxia on FMISO-PET imaging, in patients receiving a nontirapazamine-containing chemoradiotherapy regimen, is associated with a high risk of LRF. Our data provide the first clinical evidence to support the experimental observation that tirapazamine acts by specifically targeting hypoxic tumor cells.

Tumour hypoxia, chemotherapeutic resistance and hypoxia-related therapies

Tissue hypoxia occurs where there is an imbalance between oxygen supply and consumption. Hypoxia occurs in solid tumours as a result of an inadequate supply of oxygen, due to exponential cellular proliferation and an inefficient vascular supply. It is an adverse prognostic indicator in cancer as it is associated with tumour progression and resistance to therapy. The expression of several genes controlling tumour cell survival are regulated by hypoxia, e.g., growth factors governing the formation of new blood vessels, and hypoxia-responsive transcription factors modulating the expression of genes, which promote tumour cell survival. This review outlines some of the pathways by which tumour hypoxia leads to chemotherapeutic resistance, directly due to lack of oxygen availability, and indirectly due to alterations in the proteome/genome, angiogenesis and pH changes. Some innovative therapies are also detailed which may potentially minimise or eliminate these problems associated with targeting solid tumours.

SR-4233: a new bioreductive agent with high selective toxicity for hypoxic mammalian cells

We have examined the effects of the benzotriazine di-N-oxide SR-4233 (3-amino-1,2,4-benzotriazine-1,4 dioxide) on a variety of aerobic and hypoxic cells in culture, and on tumors in mice. The cell lines used were Chinese hamster ovary (HA-1), mouse 10T1/2, RIF-1, and SCC VII cells, and the human cell lines HCT-8, AG1522, and A549. The effect of SR 4233 in combination with irradiation was also examined in the SCC VII tumor growing in the flank of C3H mice using clonogenic assay (tumors excised 24 hr after irradiation). We found SR-4233 to be a potent and selective killer of hypoxic cells. Cell killing as a function of time for the various cell lines was exponential, with no shoulder. Drug concentrations producing equivalent levels of cell killing were 75-200 fold lower in hypoxic than in aerobic cells for the mouse and hamster lines, and 15-50 fold lower for the human cells. In vivo experiments showed that the non-toxic dose of 0.3 mmole/kg of SR-4233 enhanced radiation-induced tumor cell kill when the drug was given between 1 hr before and 2 hr after the radiation dose. We have also shown that the drug metabolizes more rapidly under hypoxic than aerobic conditions, both in vitro and in vivo. The toxic product(s) is unknown, but could be the 1-electron reduction product, the radical anion, because the mono N-oxide (the 2-electron reduction product) did not display cytotoxicity or selective killing under hypoxic conditions. This compound could therefore be a useful tool in tumor biology, as well as being a new lead in the development of bioreductive cytotoxic agents for cancer therapy.

SR 4233 (tirapazamine): a new anticancer drug exploiting hypoxia in solid tumours

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide, WIN 59075, tirapazamine) is the lead compound in a new class of bioreductive anticancer drugs, the benzotriazine di-N-oxides. It is currently undergoing Phase I clinical testing. The preferential tumour cell killing of SR 4233 is a result of its high specific toxicity to cells at low oxygen tensions. Such hypoxic cells are a common feature of solid tumours, but not normal tissues, and are resistant to cancer therapies including radiation and some anticancer drugs. The killing of these tumour cells by SR 4233, particularly when given on multiple occasions, can increase total tumour cell killing by fractionated irradiation by several orders of magnitude without increasing toxicity to surrounding normal tissues. Topics covered in this review include the rationale for developing a hypoxic cytotoxic agent, the cytotoxicity of SR 4233 as a function of oxygen concentration, the mechanism of action of the drug and its intracellular target and the in vivo evidence that the drug may be useful as an adjunct both to radiotherapy and chemotherapy. Finally, the major unanswered questions on the drug are outlined.

Exploiting the hypoxic cancer cell: mechanisms and therapeutic strategies

Human solid tumours are considerably less well oxygenated than normal tissues. This leads to resistance to radiotherapy and anticancer chemotherapy, as well as predisposing to increased tumour metastases. However, tumour hypoxia can be exploited in cancer treatment. One such strategy is to use drugs that are toxic only under hypoxic conditions, and the first drug of this class to enter clinical testing, tirapazamine, is showing considerable promise. The second way to exploit hypoxia is to take advantage of the selective induction of the transcription factor hypoxia-inducible factor 1 (HIF-1) under hypoxic conditions; gene therapy strategies based on this are in development.

Direct measurement of pO2 distribution and bioreductive enzymes in human malignant brain tumors

PURPOSE

To measure the oxygen status of human malignant brain tumors in vivo and to determine the activities and expression of bioreductive enzymes in these same human brain tumor samples, as a means of assessing their suitability as targets for bioreductive drug therapy.

METHODS AND MATERIALS

A polarographic oxygen electrode was used to measure the intratumoral oxygen tension in twenty patients with malignant brain tumors during open brain surgery, performed under standard anaesthetic conditions. Six different tracks, each with a path length of 22 mm, were recorded per patient representing 192 readings. Following pO2 measurements the tumors were resected and stored in liquid N2 for subsequent bioreductive enzyme analysis. Eight human malignant brain tumors were assessed, by enzyme activity and western blot expression, for the presence of various bioreductive enzymes. These enzymes included DT-diaphorase, NADH cytochrome b5 reductase, and NADPH cytochrome P-450 reductase. Of these eight gliomas analyzed six samples were incubated with the bioreductive drug tirapazamine, in the presence of cofactor(s), to establish whether human brain tumors could metabolize this compound.

RESULTS

Both the high grade intrinsic and metastatic brain tumors showed significant regions of hypoxia. All the tumors subjected to enzyme profiling contained the bioreductive enzymes, DT-diaphorase, NADH cytochrome b5 reductase and NADPH cytochrome P-450 reductase. Also all six of the brain tumors investigated could metabolize tirapazamine to the two-electron reduction product.

CONCLUSION

These findings would favor primary brain tumors as suitable targets for bioreductive therapy.

Tirapazamine plus cisplatin versus cisplatin in advanced non-small-cell lung cancer: A report of the international CATAPULT I study group. Cisplatin and Tirapazamine in Subjects with Advanced Previously Untreated Non-Small-Cell Lung Tumors

PURPOSE

A phase III trial, Cisplatin and Tirapazamine in Subjects with Advanced Previously Untreated Non-Small-Cell Lung Tumors (CATAPULT I), was designed to determine the efficacy and safety of tirapazamine plus cisplatin for the treatment of non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

Patients with previously untreated NSCLC were randomized to receive either tirapazamine (390 mg/m(2) infused over 2 hours) followed 1 hour later by cisplatin (75 mg/m(2) over 1 hour) or 75 mg/m(2) of cisplatin alone, every 3 weeks for a maximum of eight cycles.

RESULTS

A total of 446 patients with NSCLC (17% with stage IIIB disease and pleural effusions; 83% with stage IV disease) were entered onto the study. Karnofsky performance status (KPS) was >/= 60 for all patients (for 10%, KPS = 60; for 90%, KPS = 70 to 100). Sixty patients (14%) had clinically stable brain metastases. The median survival was significantly longer (34.6 v 27. 7 weeks; P =.0078) and the response rate was significantly greater (27.5% v 13.7%; P <.001) for patients who received tirapazamine plus cisplatin (n = 218) than for those who received cisplatin alone (n = 219). The tirapazamine-plus-cisplatin regimen was associated with mild to moderate adverse events, including acute, reversible hearing loss, reversible, intermittent muscle cramping, diarrhea, skin rash, nausea, and vomiting. There were no incremental increases in myelosuppression, peripheral neuropathy, or renal, hepatic, or cardiac toxicity and no deaths related to tirapazamine.

CONCLUSION

The CATAPULT I study shows that tirapazamine enhances the activity of cisplatin in patients with advanced NSCLC and confirms that hypoxia is an exploitable therapeutic target in human malignancies.

Tirapazamine, Cisplatin, and Radiation versus Fluorouracil, Cisplatin, and Radiation in patients with locally advanced head and neck cancer: a randomized phase II trial of the Trans-Tasman Radiation Oncology Group (TROG 98.02)

PURPOSE

To select one of two chemoradiotherapy regimens for locally advanced squamous cell carcinoma (SCC) of the head and neck as the experimental arm for the next Trans-Tasman Radiation Oncology Group phase III trial.

PATIENTS AND METHODS

One hundred twenty-two previously untreated patients with stage III/IV SCC of the head and neck were randomized to receive definitive radiotherapy (70 Gy in 7 weeks) concurrently with either cisplatin (75 mg/m(2)) plus tirapazamine (290 mg/m(2)/d) on day 2 of weeks 1, 4, and 7, and tirapazamine alone (160 mg/m(2)/d) on days 1, 3, and 5 of weeks 2 and 3 (TPZ/CIS), or cisplatin (50 mg/m(2)) on day 1 and infusional fluorouracil (360 mg/m(2)/d) on days 1 through 5 of weeks 6 and 7 (chemoboost).

RESULTS

Three-year failure-free survival rates were 55% with TPZ/CIS (95% CI, 39% to 70%) and 44% with chemoboost (95% CI, 30% to 60%; log-rank P = .16). Three-year locoregional failure-free rates were 84% in the TPZ/CIS arm (95% CI, 71% to 92%) and 66% in the chemoboost arm (95% CI, 51% to 79%; P = .069). More febrile neutropenia and grade 3 or 4 late mucous membrane toxicity were observed with TPZ/CIS, while acute skin radiation reaction was more severe and prolonged with chemoboost. Compliance with protocol treatment was satisfactory on both arms.

CONCLUSION

Both regimens are feasible and are associated with significant but acceptable toxicity profiles in the cooperative group setting. Based on the promising efficacy seen in this trial, TPZ/CIS is being evaluated in a large phase III trial.

Tumor microenvironment and the response to anticancer therapy

Human solid tumors are invariably less well oxygenated than normal tissues. This leads to resistance to radiotherapy and anticancer chemotherapy, as well as predisposing for increased tumor metastases. Prolonged hypoxia of the tumor tissue also leads to necrosis, and necrotic regions are also characteristic of solid tumors. These two characteristics-hypoxia and necrosis-represent clear differences between tumors and normal tissues and are potentially exploitable in cancer treatment. This review focuses on the phenomenon of tumor hypoxia and how hypoxia and its accompanying necrosis can be exploited in therapy. One such strategy is to use drugs that are toxic only under hypoxic conditions, and the first drug of this class to enter clinical testing, tirapazamine, is showing considerable promise. The second way to exploit hypoxia is to take advantage of the selective induction under hypoxia of the transcription factor HIF-1 (hypoxia-inducible factor 1). Gene therapy strategies based on this are in development. Finally, tumor hypoxia can be exploited using live obligate anaerobes that have been genetically engineered to express enzymes that can activate non-toxic prodrugs into toxic chemotherapeutic agents.

Phase I trial of concurrent tirapazamine, cisplatin, and radiotherapy in patients with advanced head and neck cancer

PURPOSE

To determine the maximum-tolerated dose of tirapazamine when combined with cisplatin and radiation in patients with T3/4 and/or N2/3 squamous cell carcinoma of the head and neck.

PATIENTS AND METHODS

The starting schedule was conventionally fractionated radiotherapy (70 Gy in 7 weeks) with concomitant cisplatin 75 mg/m2 and tirapazamine 290 mg/m2 (before cisplatin) in weeks 1, 4, and 7 and tirapazamine alone 160 mg/m2 three times a week in weeks 2, 3, 5, and 6. Positron emission tomography scans for tumor hypoxia (18F misonidazole) were performed before and during radiotherapy.

RESULTS

We treated 16 patients with predominantly oropharyngeal primary tumors, including 10 patients with T4 or N3 disease. Febrile neutropenia occurred toward the end of radiotherapy in three out of six patients treated on the initial dose level. Two of these patients also developed grade 4 acute radiation reactions. Another 10 patients were treated with the same doses, but the week 5 and week 6 tirapazamine doses were omitted. This resulted in less neutropenia and only one dose-limiting toxicity (DLT) (febrile neutropenia), and eight out of 10 patients completed treatment without any dose omissions. In these 10 patients, the acute radiation toxicities were not obviously enhanced compared with chemoradiotherapy regimens using concurrent platinum and fluorouracil. 18F misonidazole scans detected hypoxia in 14 of 15 patients at baseline, with only one patient having detectable hypoxia at the end of treatment. With a median follow-up of 2.7 years, the 3-year failure-free survival rate was 69% (SE, 12%), the 3-year local progression-free rate was 88% (SE, 8%), and the 3-year overall survival rate was 69% (SE, 12%).

CONCLUSION

DLT was due unexpectedly to febrile neutropenia, which could be overcome by omitting tirapazamine in weeks 5 and 6. The combination of tirapazamine, cisplatin, and radiotherapy resulted in remarkably good and durable clinical responses in patients with very advanced head and neck cancers. It warrants further investigation.

Bioreductive prodrugs as cancer therapeutics: targeting tumor hypoxia

Hypoxia, a state of low oxygen, is a common feature of solid tumors and is associated with disease progression as well as resistance to radiotherapy and certain chemotherapeutic drugs. Hypoxic regions in tumors, therefore, represent attractive targets for cancer therapy. To date, five distinct classes of bioreactive prodrugs have been developed to target hypoxic cells in solid tumors. These hypoxia-activated prodrugs, including nitro compounds, N-oxides, quinones, and metal complexes, generally share a common mechanism of activation whereby they are reduced by intracellular oxidoreductases in an oxygen-sensitive manner to form cytotoxins. Several examples including PR-104, TH-302, and EO9 are currently undergoing phase II and phase III clinical evaluation. In this review, we discuss the nature of tumor hypoxia as a therapeutic target, focusing on the development of bioreductive prodrugs. We also describe the current knowledge of how each prodrug class is activated and detail the clinical progress of leading examples.

On-line coupling of in vivo microdialysis sampling with capillary electrophoresis

Microdialysis sampling has become an important means of continuously monitoring reactions in vivo. This sampling technique places a constraint on the analysis method because of the very small sample volume provided. On the other hand, microdialysis provides the advantage of clean samples that do not require cleanup prior to analysis. An on-line coupling of microdialysis sampling to capillary electrophoretic (CE) analysis is described that uses the advantages of microcolumn separations to overcome the small volume limitation. An interface was designed which converts the continuous microdialysis sample stream into discrete 60-nL sample plugs and then injects a portion of this plug into the CE system. The on-line interface provided precision of 2.6% with minimal band broadening or peak height loss relative to off-line sampling. Using a high-speed micellar electrokinetic chromatography (MEKC) separation, resolution of the investigational antineoplastic SR 4233 from its main metabolite SR 4317 was achieved in less than 60 s. This allowed the on-line system to achieve a 90-s temporal resolution for determining the pharmacokinetics of SR 4233 in vivo.

Bioreductive drugs: from concept to clinic

One of the key issues for radiobiologists is the importance of hypoxia to the radiotherapy response. This review addresses the reasons for this and primarily focuses on one aspect, the development of bioreductive drugs that are specifically designed to target hypoxic tumour cells. Four classes of compound have been developed since this concept was first proposed: quinones, nitroaromatics, aliphatic and heteroaromatic N-oxides. All share two characteristics: (1) they require hypoxia for activation and (2) this activation is dependent on the presence of specific reductases. The most effective compounds have shown the ability to enhance the anti-tumour efficacy of agents that kill better-oxygenated cells, i.e. radiation and standard cytotoxic chemotherapy agents such as cisplatin and cyclophosphamide. Tirapazamine (TPZ) is the most widely studied of the lead compounds. After successful pre-clinical in vivo combination studies it entered clinical trial; over 20 trials have now been reported. Although TPZ has enhanced some standard regimens, the results are variable and in some combinations toxicity was enhanced. Banoxantrone (AQ4N) is another agent that is showing promise in early phase I/II clinical trials; the drug is well tolerated, is known to locate in the tumour and can be given in high doses without major toxicities. Mitomycin C (MMC), which shows some bioreductive activation in vitro, has been tested in combination trials. However, it is difficult to assign the enhancement of its effects to targeting of the hypoxic cells because of the significant level of its hypoxia-independent toxicity. More specific analogues of MMC, e.g. porfiromycin and apaziquone (EO9), have had variable success in the clinic. Other new drugs that have good pre-clinical profiles are PR 104 and NLCQ-1; data on their clinical safety/efficacy are not yet available. This paper reviews the pre-clinical data and discusses the clinical studies that have been reported.

Utility of FMISO PET in advanced head and neck cancer treated with chemoradiation incorporating a hypoxia-targeting chemotherapy agent

PURPOSE

The purpose of the study was to evaluate [(18)F]fluoromisonidazole (FMISO) PET in advanced head and neck cancer during hypoxia-targeting therapy.

METHODS

Fifteen of 16 patients in a phase I trial of chemoradiation plus tirapazamine (specific cytotoxin for hypoxic cells) in advanced (T3/4 and/or N2/3) head and neck cancer underwent serial [(18)F]fluorodeoxyglucose (FDG) and FMISO PET. We have previously reported excellent early clinical outcome of these patients and now review FMISO PET results in the context of longer follow-up of this patient cohort.

RESULTS

Based on blinded qualitative scoring by two readers, FMISO PET was positive in 13/15 patients at baseline: 12/15 of primary sites and 8/13 neck nodes were scored as positive. All sites of corresponding FDG and FMISO abnormality at baseline showed marked qualitative reduction of uptake within 4 weeks of commencing therapy, consistent with effective hypoxia-targeted therapy. With a median follow-up of 6.9 years, there have been only four locoregional failures, while three other patients have died of metachronous lung cancer. The 5-year overall survival was 50% (95% CI 27-73%), the 5-year failure-free survival was 44% (95% CI 22-68%) and the 5-year freedom from locoregional failure was 68% (95% CI 38-88%).

CONCLUSION

The high prevalence of hypoxia demonstrated on FMISO PET imaging is consistent with the advanced disease stage of these patients and would be expected to predict an adverse prognosis. Evidence of the early resolution of FMISO abnormality during treatment, associated with excellent locoregional control in this patient cohort, supports further investigation of hypoxia-targeting agents in advanced head and neck cancer.

Molecular mechanisms for the hypoxia-dependent activation of 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233)

The reduction of the hypoxic cell toxin 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) was investigated using pulse radiolysis, radiation chemical reduction, and xanthine oxidase. Evidence was found that the one-electron reduction product of the parent compound is an oxidizing radical that caused single- and double-strand breaks in plasmid DNA and that produced a malondialdehyde-like thiobarbituric acid adduct from 2-deoxy-D-ribose. Possible forms of the reactive radical, either carbon- or nitrogen-centered, are suggested. The "natural" lifetime of the radical was sufficiently long that it could diffuse over significant distances within hypoxic cells and thus inflict oxidative damage on cellular targets. The radical reacted with O2 at a rate comparable to those of the nitroimidazoles misonidazole and metronidazole. Thus, the selectivity for hypoxic cells is probably due to the elimination of "futile" reduction when the cellular oxygen concentration is sufficiently low.

Use of three-dimensional tissue cultures to model extravascular transport and predict in vivo activity of hypoxia-targeted anticancer drugs

BACKGROUND

Because of the inefficient vasculature of solid tumors, anticancer drugs must penetrate relatively long distances through the extravascular compartment. The requirement for such diffusion may limit their activity, especially that of hypoxia-targeted drugs. We tested whether a three-dimensional pharmacokinetic/pharmacodynamic (PK/PD) model based on a representative mapped tumor microvascular network could predict the therapeutic activity of anticancer drugs in mouse xenograft tumors.

METHODS

Diffusion coefficients of the hypoxia-activated anticancer drug tirapazamine (TPZ) and of 15 TPZ analogs were estimated by measuring their transport through HT29 colon cancer multicellular layers (MCLs). Anoxic cytotoxic potency (by clonogenic assay) and metabolism of the TPZ analogs were measured in HT29 cell suspensions, and their plasma pharmacokinetics was measured in CD-1 nude mice. This information was used to create a spatially resolved PK/PD model for the tumor microvascular network. Model predictions were compared with actual hypoxic cell kill as measured by clonogenic assays on HT29 xenograft tumors 18 hours after treatment with each TPZ analog.

RESULTS

Modeling TPZ transport in the tumor microvascular network showed substantial drug depletion in the most hypoxic regions, with predicted maximum cell kill of only 3 logs, compared with more than 10 logs if there were no transport impediment. A large range of tissue diffusion coefficients (0.027 x 10(-6)-1.87 x 10(-6) cm2/s) was observed for the TPZ analogs. There was a strong correlation between model-predicted and measured hypoxic cell kill (R2 = 0.89) but a poor correlation when the model did not include extravascular transport (R2 = 0.32).

CONCLUSIONS

Extravascular transport in tumors, and its consequences for tumor cell killing, can be predicted by measuring drug penetration through MCLs in vitro and modeling pharmacokinetics at each position in three-dimensional microvascular networks.

The experimental development of bioreductive drugs and their role in cancer therapy

Bioreductive drugs undergo metabolic reduction to generate cytotoxic metabolites. This process is facilitated by bioreductive enzymes and the lower oxygen conditions present in solid tumours compared to normal tissues. Because of this specificity, bioreductive drugs have enormous potential to contribute to modern cancer therapy. Examples undergoing clinical trials include N-oxides such as tirapazamine, aziridinylnitroimidazoles RSU 1069/RBU 6145 and quinones such as indoloquinone EO9. Other novel structures are also under study. Here we review the experimental development of bioreductive drugs and their role in cancer therapy.

Electron transfer and oxidative stress as key factors in the design of drugs selectively active in hypoxia

Hypoxia is a feature of some regions of many tumours, ischaemic events, and arthritis. Drugs activated in hypoxia have wide potential application, particularly in overcoming the resistance of hypoxic tumour cells to radiotherapy. Key features of such drugs include redox properties appropriate for activation by reductase enzymes (typically flavoproteins), and oxygen-sensitive reduction chemistry such that normal levels of oxygen inhibit or reverse reduction. In many cases this selectivity is achieved by a fast, free-radical reaction in which the drug radical (often an obligate intermediate in drug reduction) reduces oxygen to form superoxide radicals and thus 'futile cycles' the drug in normoxic tissues. However, this enhances cellular oxidative stress, which may be linked to normal tissue toxicity. Appropriate redox properties are found with nitroarene, quinone, or aromatic N-oxide moieties. A particularly promising and versatile exploitation of bioreductive activation is for reduction of such 'triggers' to activate release of an 'effector', an agent that can obviously be active against diverse conditions associated with hypoxia. The same approach can also be used in diagnosis of hypoxia. Much information concerning the reactions of intermediates in drug action and the quantitative prediction of redox properties of analogues has been accrued. Drug design can be mechanism-led, with the wealth of literature quantifying redox properties of drug candidates a rich source of potential new leads. There is a clear appreciation of the kinetic factors that limit drug efficacy or selectivity. Thus the potential for rapid expansion of these concepts to diverse diseases is considerable.

Pharmacokinetic/pharmacodynamic modeling identifies SN30000 and SN29751 as tirapazamine analogues with improved tissue penetration and hypoxic cell killing in tumors

PURPOSE

Tirapazamine (TPZ) has attractive features for targeting hypoxic cells in tumors but has limited clinical activity, in part because of poor extravascular penetration. Here, we identify improved TPZ analogues by using a spatially resolved pharmacokinetic/pharmacodynamic (SR-PKPD) model that considers tissue penetration explicitly during lead optimization.

EXPERIMENTAL DESIGN

The SR-PKPD model was used to guide the progression of 281 TPZ analogues through a hierarchical screen. For compounds exceeding hypoxic selectivity thresholds in single-cell cultures, SR-PKPD model parameters (kinetics of bioreductive metabolism, clonogenic cell killing potency, diffusion coefficients in multicellular layers, and plasma pharmacokinetics at well tolerated doses in mice) were measured to prioritize testing in xenograft models in combination with radiation.

RESULTS

SR-PKPD-guided lead optimization identified SN29751 and SN30000 as the most promising hypoxic cytotoxins from two different structural subseries. Both were reduced to the corresponding 1-oxide selectively under hypoxia by HT29 cells, with an oxygen dependence quantitatively similar to that of TPZ. SN30000, in particular, showed higher hypoxic potency and selectivity than TPZ in tumor cell cultures and faster diffusion through HT29 and SiHa multicellular layers. Both compounds also provided superior plasma PK in mice and rats at equivalent toxicity. In agreement with SR-PKPD predictions, both were more active than TPZ with single dose or fractionated radiation against multiple human tumor xenografts.

CONCLUSIONS

SN30000 and SN29751 are improved TPZ analogues with potential for targeting tumor hypoxia in humans. Novel SR-PKPD modeling approaches can be used for lead optimization during anticancer drug development.

Phase III trial of paclitaxel plus carboplatin with or without tirapazamine in advanced non-small-cell lung cancer: Southwest Oncology Group Trial S0003

PURPOSE

Tumor hypoxia confers chemotherapy resistance. Tirapazamine is a cytotoxin that selectively targets hypoxic cells. We conducted a phase III clinical trial to determine whether the addition of tirapazamine to paclitaxel and carboplatin offered a survival advantage when used in the treatment of patients with advanced non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

Of 396 patients registered, 367 eligible patients were randomly assigned to either arm 1 (n = 181), which consisted of treatment every 21 days with paclitaxel 225 mg/m2/3 h, carboplatin (area under the curve = 6), and tirapazamine 260 mg/m2 in cycle 1 (which was escalated, if tolerable, to 330 mg/m(2) in cycle 2), or arm 2 (n = 186), which consisted of paclitaxel and carboplatin as in arm 1 with no tirapazamine.

RESULTS

Patient characteristics were similar between the two arms. There were no statistically significant differences in response rates, progression-free survival, or overall survival. Patients on arm 1 had significantly (P < .05) more abdominal cramps, fatigue, transient hearing loss, febrile neutropenia, hypotension, myalgias, and skin rash and were removed from treatment more often as a result of toxicity than patients in arm 2 (26% v 13%, respectively; P = .003). More than 40% of patients did not have the tirapazamine dose escalated, primarily because of toxicity. The trial was closed early after an interim analysis demonstrated that the projected 37.5% improvement in survival (8 v 11 months median survival) in arm 1 was unachievable (P = .003).

CONCLUSION

The addition of tirapazamine to paclitaxel and carboplatin does not result in improved survival in advanced NSCLC compared with paclitaxel and carboplatin alone but substantially increases toxicity.

Generation of oxygen gradients in microfluidic devices for cell culture using spatially confined chemical reactions

This paper reports a microfluidic device capable of generating oxygen gradients for cell culture using spatially confined chemical reactions with minimal chemical consumption. The microfluidic cell culture device is constructed by single-layer polydimethylsiloxane (PDMS) microfluidic channels, in which the cells can be easily observed by microscopes. The device can control the oxygen gradients without the utilization of bulky pressurized gas cylinders, direct addition of oxygen scavenging agents, or tedious gas interconnections and sophisticated flow control. In addition, due to the efficient transportation of oxygen within the device using the spatially confined chemical reactions, the microfluidic cell culture device can be directly used in conventional cell incubators without altering their gaseous compositions. The oxygen gradients generated in the device are numerically simulated and experimentally characterized using an oxygen-sensitive fluorescence dye. In this paper, carcinomic human alveolar basal epithelial (A549) cells have been cultured in the microfluidic device with a growth medium and an anti-cancer drug (Tirapazamine, TPZ) under various oxygen gradients. The cell experiment results successfully demonstrate the hyperoxia-induced cell death and hypoxia-induced cytotoxicity of TPZ. In addition, the results confirm the great cell compatibility and stable oxygen gradient generation of the developed device. Consequently, the microfluidic cell culture device developed in this paper is promising to be exploited in biological labs with minimal instrumentation to study cellular responses under various oxygen gradients.

Endogenous Markers of Tumor Hypoxia

BACKGROUND

Eppendorf electrode measurements of tumor oxygenation have defined an adverse effect of tumor hypoxia on prognosis after radiotherapy and other treatment modalities, in particular in head and neck and cervix carcinomas as well as soft tissue sarcomas. Recently, the immunohistochemical detection of proteins involved in the "hypoxic response" of tumor cells has been discussed as a method to estimate hypoxia in clinical tumor specimens.

MATERIAL AND METHODS

This review focuses on clinical and experimental data, regarding prognostic impact and comparability with other methods of hypoxia detection, for three proteins suggested as endogenous markers of tumor hypoxia: hypoxia-inducible factor-1alpha(HIF-1alpha), carbonic anhydrase 9 (CA 9), and glucose transporter 1 (GLUT1).

RESULTS

None of the three potential hypoxia markers is exclusively hypoxia-specific, and in each case protein can be detected under normoxic conditions in vitro. HIF-1alpha responds rapidly to hypoxia but also to reoxygenation, making this marker quite unstable in the context of clinical sample collection. The perinecrotic labeling pattern typical of chronic hypoxia and a reasonable agreement with injectable hypoxia markers such as pimonidazole have most consistently been described for CA 9. All three markers showed correlation with Eppendorf electrode measurements of tumor oxygenation in carcinoma of the cervix. In nine of 13 reports, among them all three that refer to curative radiotherapy for head and neck cancer, HIF-1alpha overexpression was associated with poor outcome. CA 9 was an adverse prognostic factor in cervix, head and neck and lung cancer, but not in two other head and neck cancer reports. GLUT1 predicted for poor survival in colorectal, cervix and lung cancer.

CONCLUSION

Endogenous markers have the potential to indicate therapeutically relevant levels of hypoxia within tumors. Clinical trials assessing a marker's ability to predict a benefit from specific hypoxia-directed treatment (e. g., tirapazamine, "ARCON" concept) are necessary to define the potential of individual markers.

Lower osteopontin plasma levels are associated with superior outcomes in advanced non-small-cell lung cancer patients receiving platinum-based chemotherapy: SWOG Study S0003

PURPOSE

S0003 was a phase III trial of carboplatin/paclitaxel with or without the hypoxic cytotoxin tirapazamine in patients with advanced or metastatic non-small-cell lung cancer (NSCLC). We investigated the relationship between clinical outcomes and plasma levels of the hypoxia-associated protein osteopontin (OPN) in patients on this protocol.

PATIENTS AND METHODS

Baseline plasma was obtained from 172 patients. In 56 patients, sequential plasma was obtained after one or two cycles. Concentrations of OPN, as well as plasminogen activator inhibitor-1 (PAI-1) and vascular endothelial growth factor (VEGF), were measured using enzyme-linked immunosorbent assay. Tumor expression of OPN was assessed by immunohistochemistry in 61 matched archival specimens.

RESULTS

Patients with lower OPN levels (below the median) had a significantly superior overall survival compared with patients with higher levels, regardless of treatment arm (hazard ratio [HR] = 0.60, P = .002). A similar correlation was observed for progression-free survival (HR = 0.69, P = .02). When examined as a continuous variable, OPN maintained its significant association with both progression-free (HR = 1.05, P = .01) and overall survival (HR = 1.09, P < .0001). Patients with lower plasma OPN levels were significantly more likely to have tumor response (P = .03). No differences were observed between treatment arms. Tumor OPN levels did not correlate with patient outcomes or with plasma levels. No associations were observed between patient outcomes and VEGF or PAI-1 levels; however, plasma concentrations of these markers were significantly interrelated (P < .0001) and significantly decreased after treatment (P = .0002 and P = .03, respectively).

CONCLUSION

Pretreatment plasma levels of OPN are significantly associated with patient response, progression-free survival, and overall survival in chemotherapy-treated patients with advanced NSCLC.