Therapeutic advantage from combining 5-fluorouracil with the hypoxia-selective cytotoxin NLCQ-1 in vivo; comparison with tirapazamine

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

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.

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.

Optimization of the Auxiliary Ligand Shell of Cobalt(III)(8-hydroxyquinoline) Complexes as Model Hypoxia-Selective Radiation-Activated Prodrugs

A potential approach for activating prodrugs in hypoxic regions of tumors is to use ionizing radiation, rather than bioreductive enzymes, to effect reduction. This study investigates radiolytic release of 8-hydroxyquinoline (8-HQ), as a model for hydroxyaza-chloromethylbenzindoline DNA minor groove alkylators, from Co(III) complexes under hypoxia. 8-HQ release, measured by HPLC, showed higher efficiency (one-electron stoichiometry) when the auxiliary ligand was a tetraazamacrocycle [e.g. 1,4,7,10-tetraazacyclododecane (cyclen)] rather than a triazamacrocycle [1,4,7-triazacyclononane (TACN)]. These complexes differ from the bioreductive cobalt complex SN 24771 in that their reduction provides stable cobalt-containing products rather than free (aquated) Co(2+). Radiolytic release of 8-HQ from Co(cyclen)(8-HQ) and Co(TACN)(CN)(8-HQ) was also demonstrated in deoxygenated human plasma, selectively in the absence of oxygen, again with higher efficiency for the cyclen system. The cobalt complexes were >1000-fold less potent than free 8-HQ as inhibitors of cell proliferation and were metabolically stable in aerobic and hypoxic cell cultures. Investigation of cell uptake of total cobalt, by inductively coupled plasma mass spectrometry, showed that these complexes enter cells but do not accumulate to the high concentrations seen with SN 24771. The results demonstrate the feasibility of masking the cytotoxicity of hydroxyquinoline-based cytotoxins as Co(III) complexes and demonstrate the utility of cyclen-based auxiliary ligands for optimizing radiolytic activation of these novel prodrugs under hypoxia.

Single-dose pharmacokinetics of the DNA-binding bioreductive agent NLCQ-1 (NSC 709257) in CD2F1 mice

NLCQ-1 (NSC 709257) is a weak DNA-binding bioreductive antiproliferative agent, with potent in vitro antiproliferative activity against rodent and human tumor cell lines under aerobic and anaerobic conditions. Interest in this quinoline analog is based in part on its in vivo synergistic antitumor effect with radiotherapy or chemotherapy against mouse tumors and human xenografts. A sensitive, specific HPLC method was developed to measure NLCQ-1 in biological fluids. Calibration curves were linear in the range 10.4-667 ng/ml and the lower limit of quantitation was 10.4 ng/ml in plasma. NLCQ-1 was stable in organic solvents, buffered solutions and human plasma for 24 h at 37 degrees C. NLCQ-1 was unstable in rodent and dog plasma when incubated for longer than 10 h. NLCQ-1 human plasma protein binding was high (about 99%), and included binding to both alpha(1)-acid glycoprotein and serum albumin. The plasma elimination of NLCQ-1 in mice after a 10-mg/kg intravenous bolus dose was described by a two-compartment open model with t(1/2beta), V(ss), and Cl(TB) values of 41.3 min, 2.04 l/kg and 69.9 ml/min per kg, respectively. NLCQ-1 had high (85%) intraperitoneal and modest (28%) oral relative bioavailability. Little of the administered NLCQ-1 dose (6.4%) was excreted in 24-h urine. The mouse pharmacokinetic data suggested that oral administration may achieve plasma concentration and systemic exposure similar to those observed after intravenous administration of NLCQ-1.

Current understanding of stem cell mobilization: the roles of chemokines, proteolytic enzymes, adhesion molecules, cytokines, and stromal cells

Mobilization of hematopoietic stem and progenitor cells from the bone marrow into the circulation by repetitive, daily stimulations with G-CSF alone, or in combination with cyclophosphamide, is increasingly used clinically; however, the mechanism is not fully understood. Moreover, following mobilization stem cells also home back to the bone marrow, suggesting that stem cell release/mobilization and homing are sequential events with physiological roles. Previously, a role for cytokines such as G-CSF and SCF, and adhesion molecules such as VLA-4 and P/E selectins, was determined for stem cell mobilization. Recent results using experimental animal models and samples from clinical mobilization protocols demonstrate major involvement of chemokines such as stromal derived factor-1 (SDF-1) and IL-8, as well as proteolytic enzymes such as elastase, cathepsin G, and various MMPs in the mobilization process. These results will be reviewed together with the central roles of SDF-1 and CXCR4 interactions in G-CSF or G-CSF in combination with cyclophosphamide-induced mobilization. Furthermore, the central role of this chemokine in stem cell homing to the bone marrow as well as retention of undifferentiated cells within this tissue will also be discussed.