Phase I Trial of Tirapazamine and Cyclophosphamide in Children With Refractory Solid Tumors: A Pediatric Oncology Group Study

Targeting Hypoxia: Revival of Old Remedies

Tumour hypoxia is significantly correlated with patient survival and treatment outcomes. At the molecular level, hypoxia is a major driving factor for tumour progression and aggressiveness. Despite the accumulative scientific and clinical efforts to target hypoxia, there is still a need to find specific treatments for tumour hypoxia. In this review, we discuss a variety of approaches to alter the low oxygen tumour microenvironment or hypoxia pathways including carbogen breathing, hyperthermia, hypoxia-activated prodrugs, tumour metabolism and hypoxia-inducible factor (HIF) inhibitors. The recent advances in technology and biological understanding reveal the importance of revisiting old therapeutic regimens and repurposing their uses clinically.

Targeting Hypoxia: Hypoxia-Activated Prodrugs in Cancer Therapy

Hypoxia is an important characteristic of most solid malignancies, and is closely related to tumor prognosis and therapeutic resistance. Hypoxia is one of the most important factors associated with resistance to conventional radiotherapy and chemotherapy. Therapies targeting tumor hypoxia have attracted considerable attention. Hypoxia-activated prodrugs (HAPs) are bioreductive drugs that are selectively activated under hypoxic conditions and that can accurately target the hypoxic regions of solid tumors. Both single-agent and combined use with other drugs have shown promising antitumor effects. In this review, we discuss the mechanism of action and the current preclinical and clinical progress of several of the most widely used HAPs, summarize their existing problems and shortcomings, and discuss future research prospects.

Hypoxia and its therapeutic possibilities in paediatric cancers

In tumours, hypoxia-a condition in which the demand for oxygen is higher than its availability-is well known to be associated with reduced sensitivity to radiotherapy and chemotherapy, and with immunosuppression. The consequences of hypoxia on tumour biology and patient outcomes have therefore led to the investigation of strategies that can alleviate hypoxia in cancer cells, with the aim of sensitising cells to treatments. An alternative therapeutic approach involves the design of prodrugs that are activated by hypoxic cells. Increasing evidence indicates that hypoxia is not just clinically significant in adult cancers but also in paediatric cancers. We evaluate relevant methods to assess the levels and extent of hypoxia in childhood cancers, including novel imaging strategies such as oxygen-enhanced magnetic resonance imaging (MRI). Preclinical and clinical evidence largely supports the use of hypoxia-targeting drugs in children, and we describe the critical need to identify robust predictive biomarkers for the use of such drugs in future paediatric clinical trials. Ultimately, a more personalised approach to treatment that includes targeting hypoxic tumour cells might improve outcomes in subgroups of paediatric cancer patients.

Risk-based treatment for patients with first relapse or progression of rhabdomyosarcoma: A report from the Children's Oncology Group

BACKGROUND

The purpose of this study was to evaluate risk and response-based multi-agent therapy for patients with rhabdomyosarcoma (RMS) at first relapse.

METHODS

Patients with RMS and measurable disease at first relapse with unfavorable-risk (UR) features were randomized to a 6-week phase 2 window with 1 of 2 treatment schedules of irinotecan with vincristine (VI) (previously reported). Those with at least a partial response to VI continued to receive 44 weeks of multi-agent chemotherapy including the assigned VI regimen. UR patients who did not have measurable disease at study entry, did not have a radiographic response after the VI window, or declined VI window therapy received 31 weeks of multi-agent chemotherapy including tirapazamine (TPZ) at weeks 1, 4, 10, 19, and 28. Favorable-risk (FR) patients received 31 weeks of the same multi-agent chemotherapy without VI and TPZ.

RESULTS

One hundred thirty-six eligible patients were enrolled. For 61 patients not responding to VI, the 3-year failure-free survival (FFS) and overall survival (OS) rates were 17% (95% confidence interval [CI], 8%-29%) and 24% (13%-37%), respectively. For 30 UR patients not treated with VI, the 3-year FFS and OS rates were 21% (8%-37%) and 39% (20%-57%), respectively. FR patients had 3-year FFS and OS rates of 79% (47%-93%) and 84% (50%-96%), respectively. There were no unexpected toxicities.

CONCLUSIONS

Patients with UR RMS at first relapse or disease progression have a poor prognosis when they are treated with this multi-agent therapy, whereas FR patients have a higher chance of being cured with second-line therapy.

The cellular adaptations to hypoxia as novel therapeutic targets in childhood cancer

Exposure of tumour cells to reduced levels of oxygen (hypoxia) is a common finding in adult tumours. Hypoxia induces a myriad of adaptive changes within tumour cells which result in increased anaerobic glycolysis, new blood vessel formation, genetic instability and a decreased responsiveness to both radio and chemotherapy. Hypoxia correlates with disease stage and outcome in adult epithelial tumours and increasingly it is becoming apparent that hypoxia is also important in paediatric tumours. Despite its adverse effects upon tumour response to treatment hypoxia offers several avenues for new drug development. Bioreductive agents already exist, which are preferentially activated in areas of hypoxia, and thus have less toxicity for normal tissue. Additionally the adaptive cellular response to hypoxia offers several novel targets, including vascular endothelial growth factor (VEGF), carbonic anhydrase, and the central regulator of the cellular response to hypoxia, hypoxia inducible factor-1 (HIF-1). Novel agents have emerged against all of these targets and are at various stages of clinical and pre-clinical development. Hypoxia offers an exciting opportunity for new drug development that can include paediatric tumours at an early stage.

Advantage of combining NLCQ-1 (NSC 709257) with radiation in treatment of human head and neck xenografts

NLCQ-1 (NSC 709257), a hypoxia-selective cytotoxin that targets DNA through weak intercalation, was investigated for efficacy in combination with single or fractionated radiotherapy of human head and neck xenografts. A staged tumor experiment was performed in tumor-bearing female athymic nude mice that were locally irradiated with or without NLCQ-1. Tumor hypoxia was assessed by immunohistochemistry for pimonidazole adducts in tumors of varying weight. Fractionated radiation, depending on the dose, was administered either once daily for 4 days or once daily for 4 days followed by a 7-day rest and repeat. NLCQ-1 was administered i.p. at 15 mg/kg alone or 45 min before each radiation dose. Hypoxia (1-52%) was detected in all tumors and was positively correlated with tumor size. NLCQ-1 alone resulted in about 10 days of tumor growth delay, measured at sixfold the tumor's original size, without causing toxicity. All combination treatments with NLCQ-1 were more effective than treatments with radiation alone. Radiation at 1 Gy given once daily for 4 days on days 20 and 30 caused 3.5 days of tumor growth delay, whereas in combination with NLCQ-1 it caused 14.5 days of growth delay. Radiation at 5 Gy given in two doses 10 days apart resulted in 3.5 days of tumor growth delay, whereas more than 20 additional days of delay were observed in combination with NLCQ-1. Radiation given as a single dose of 10 Gy resulted in about 7 days of tumor growth delay, whereas in combination with NLCQ-1 about 30 additional days of delay were seen. These results suggest a significant advantage in combining radiation with NLCQ-1 in treatment of human head and neck tumors, which are known to have hypoxic areas.

Tirapazamine plus cisplatin and irradiation in a mouse model: improved tumor control at the cost of increased toxicity

PURPOSE

Tirapazamine (TPZ) reportedly enhances the tumor cell killing effect of cisplatin up to fivefold and it is an attractive drug for combination with radiotherapy. We evaluated the toxicity of a fractionated combined treatment.

METHODS

Murine RIF-1 fibrosarcomas growing on the right hind foot of C3-H mice were used. Within 2 weeks, animals were treated with six i.p. injections of TPZ (43.2-172.8 mg/kg total), and/or cisplatin (24 mg/kg total) and ten fractions of 2 Gy to the tumor. All treatments were carried out under anesthesia. Maximum follow-up was 35 days. The local tumor control was determined by calculating the tumor doubling time t (2vo). In addition to standard toxicity assessment, the major inner organs were examined histologically.

RESULTS

The administration of low TPZ doses to the cisplatin/radiotherapy treatment caused only little changes in tumor doubling time (t (2vo)) and led to a lethality rate of 15-30%. Higher TPZ doses caused an increase in t (2vo), but also a further increase in lethality and toxicity in particular to the heart, liver, kidney and stomach. Cisplatin/radiotherapy treatment without TPZ produced no severe toxicity.

CONCLUSIONS

This is a detailed study of both the acute and delayed toxicities of combined TPZ treatment in a mouse model. In our study the addition of TPZ to the cisplatin/radiotherapy treatment caused a significant increase in toxicity with only moderate effect on the tumor.

Effects of fluctuating oxygenation on tirapazamine efficacy: Theoretical predictions

PURPOSE

To examine the effects of fluctuating oxygen levels on the hypoxic cytotoxin tirapazamine (TPZ) using theoretical predictions.

METHODS AND MATERIALS

Tirapazamine's pharmacokinetic and pharmacodynamic oxygen dependence has previously been characterized in vitro. Here, a one-dimensional theoretical model was used to examine the effects of fluctuating hypoxia on metabolized TPZ concentration, assuming sinusoidally fluctuating oxygen levels. TPZ concentration is changing according to published experimental data. Simulations of experimentally observed time-courses of perivascular pO2 were also conducted.

RESULTS

The predicted pharmacodynamic effect of TPZ was increased with fluctuating (vs. constant) hypoxia at all frequencies (1-30 min period) and all amplitudes (1-15 mm Hg). Additionally, fluctuating oxygen resulted in more metabolized TPZ near the oxygen source as compared with the steady-state condition of the same overall average pO2.

CONCLUSIONS

Fluctuating pO2 reduced the concentration of metabolized TPZ at distances farther from the source, thereby limiting its ability to reach and kill the most hypoxic cells. These results suggest that the kinetics of fluctuating oxygenation should be taken into account when considering drug designs that involve oxygen-sensitive agents.

New drugs for the treatment of metastatic or refractory soft tissue sarcomas in children

Children with relapsed, recurrent or metastatic sarcomas represent a therapeutic challenge for the pediatric oncologist. Strategies for the development of newer therapies for children with these sarcomas have, in the past, been histology-specific. For example, drug development in rhabdomyosarcoma has relied upon the preclinical xenograft model, whereas therapies for pediatric nonrhabdomyosarcomatous soft tissue sarcomas have mostly been derived from adult trials. The progress to date and the tools used in the treatment of advanced pediatric sarcomas will be summarized in this review.

The Japanese experiences with hypoxia-targeting pharmacoradiotherapy: from hypoxic cell sensitisers to radiation-activated prodrugs

Tumour hypoxia is a negative factor in cancer radiotherapy. In order to overcome the problem, various pharmacotherapies have been investigated as an adjunct to radiotherapy. The use of hypoxic cell sensitisers is a classical strategy, and many new compounds have been developed and investigated. Development of more efficient compounds than those currently available seems difficult and clinical studies to prove the efficacy of the existing compounds are encouraged, especially in combination with radiosurgery, intraoperative radiotherapy, and interstitial irradiation, in which a single high dose of radiation is used. Following the advent of hypoxic cell sensitisers, hypoxic cytotoxins have become available. Among them, tirapazamine has already gained success when combined with cisplatin in non-small cell lung cancer. The beneficial effect of tirapazamine when combined with radiation needs to be determined. As a third-generation compound in this field, antitumour prodrugs that are activated by irradiation under hypoxic conditions via one-electron reduction have been proposed. Prodrugs of 5-fluorouracil and 5-fluoro-2'-deoxyuridine have shown in vivo as well as in vitro activity. Although clinical evaluation of the compounds is not warranted due to a relatively low in vivo effect, this strategy appears promising if the prodrug design can be applied to more potent agents that shall be developed in future.