Antitumor alkylating agents: in vitro cross-resistance and collateral sensitivity studies

Strategies to improve outcomes of autologous hematopoietic cell transplant in lymphoma

High-dose chemotherapy and autologous hematopoietic cell transplantation (HDT-AHCT) remains an effective therapy in lymphoma. Over the past several decades, HDT with BEAM (carmustine, etoposide, cytarabine, and melphalan) and CBV (cyclophosphamide, carmustine, and etoposide) have been the most frequently used preparatory regimens for AHCT in Hodgkin (HL) and non-Hodgkin lymphoma (NHL). This article reviews alternative combination conditioning regimens, as well as novel transplant strategies that have been developed, to reduce transplant-related toxicity while maintaining or improving efficacy. These data demonstrate that incorporation of maintenance therapy posttransplant might be the best way to improve outcomes.

Deletion lengthening at chromosomes 6q and 16q targets multiple tumor suppressor genes and is associated with an increasingly poor prognosis in prostate cancer

Prostate cancer is characterized by recurrent deletions that can considerably vary in size. We hypothesized that large deletions develop from small deletions and that this “deletion lengthening” might have a “per se” carcinogenic role through a combinatorial effect of multiple down regulated genes. In vitro knockdown of 37 genes located inside the 6q12-q22 deletion region identified 4 genes with additive tumor suppressive effects, further supporting a role of the deletion size for cancer aggressiveness. Employing fluorescence in-situ hybridization analysis on prostate cancer tissue microarrays, we determined the deletion size at 6q and 16q in more than 3,000 tumors. 16q and 6q deletion length was strongly linked to poor clinical outcome and this effect was even stronger if the length of both deletions was combined. To study deletion lengthening in cancer progression we eventually analyzed the entire cancers from 317 patients for 6q and 16q deletion length heterogeneity and found that the deletion expanded within 50-60% of 6q and 16q deleted cancers. Taken together, these data suggest continuous “deletion lengthening” as a key mechanism for prostate cancer progression leading to parallel down regulation of genes with tumor suppressive properties, some of which act cooperatively.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Insights into oxazaphosphorine resistance and possible approaches to its circumvention

The oxazaphosphorines cyclophosphamide, ifosfamide and trofosfamide remain a clinically useful class of anticancer drugs with substantial antitumour activity against a variety of solid tumors and hematological malignancies. A major limitation to their use is tumour resistance, which is due to multiple mechanisms that include increased DNA repair, increased cellular thiol levels, glutathione S-transferase and aldehyde dehydrogenase activities, and altered cell-death response to DNA damage. These mechanisms have been recently re-examined with the aid of sensitive analytical techniques, high-throughput proteomic and genomic approaches, and powerful pharmacogenetic tools. Oxazaphosphorine resistance, together with dose-limiting toxicity (mainly neutropenia and neurotoxicity), significantly hinders chemotherapy in patients, and hence, there is compelling need to find ways to overcome it. Four major approaches are currently being explored in preclinical models, some also in patients: combination with agents that modulate cellular response and disposition of oxazaphosphorines; antisense oligonucleotides directed against specific target genes; introduction of an activating gene (CYP3A4) into tumor tissue; and modification of dosing regimens. Of these approaches, antisense oligonucleotides and gene therapy are perhaps more speculative, requiring detailed safety and efficacy studies in preclinical models and in patients. A fifth approach is the design of novel oxazaphosphorines that have favourable pharmacokinetic and pharmacodynamic properties and are less vulnerable to resistance. Oxazaphosphorines not requiring hepatic CYP-mediated activation (for example, NSC 613060 and mafosfamide) or having additional targets (for example, glufosfamide that also targets glucose transport) have been synthesized and are being evaluated for safety and efficacy. Characterization of the molecular targets associated with oxazaphosphorine resistance may lead to a deeper understanding of the factors critical to the optimal use of these agents in chemotherapy and may allow the development of strategies to overcome resistance.

ABCB5-mediated doxorubicin transport and chemoresistance in human malignant melanoma

Enhanced drug efflux mediated by ABCB1 P-glycoprotein and related ATP-binding cassette transporters is one of several mechanisms of multidrug resistance thought to impair chemotherapeutic success in human cancers. In malignant melanoma, its potential contribution to chemoresistance is uncertain. Here, we show that ABCB5, which functions as a determinant of membrane potential and regulator of cell fusion in physiologic skin progenitor cells, is expressed in clinical malignant melanoma tumors and preferentially marks a subset of hyperpolarized, CD133+ stem cell phenotype-expressing tumor cells in malignant melanoma cultures and clinical melanomas. We found that ABCB5 blockade significantly reversed resistance of G3361 melanoma cells to doxorubicin, an agent to which clinical melanomas have been found refractory, resulting in a 43% reduction in the LD50 from 4 to 2.3 micromol/L doxorubicin (P < 0.05). Our results identified ABCB5-mediated doxorubicin efflux transport as the underlying mechanism of resistance, because ABCB5 blockade significantly enhanced intracellular drug accumulation. Consistent with this novel ABCB5 function and mechanism in doxorubicin resistance, gene expression levels of the transporter across a panel of human cancer cell lines used by the National Cancer Institute for drug screening correlated significantly with tumor resistance to doxorubicin (r = 0.44; P = 0.016). Our results identify ABCB5 as a novel drug transporter and chemoresistance mediator in human malignant melanoma. Moreover, our findings show that ABCB5 is a novel molecular marker for a distinct subset of chemoresistant, stem cell phenotype-expressing tumor cells among melanoma bulk populations and indicate that these chemoresistant cells can be specifically targeted via ABCB5 to enhance cytotoxic efficacy.

Therapeutic options for patients with Hodgkin's disease and non-Hodgkin's lymphoma who relapse after autologous transplant

Although autologous stem cell transplantation for Hodgkin's disease and non-Hodgkin's lymphoma has become a safe and effective therapy, relapses after transplant are common. Emerging data indicate that an increasing number of patients can be re-induced into durable complete remission. Conventional dose- salvage chemotherapy and single-agent monoclonal antibody treatment provided limited success, but combination chemotherapy-monoclonal antibody treatments, second autografts, and reduced-intensity conditioning allografts provide encouraging results. For some patients, the best strategy may consist of participation in phase I to II studies of novel agents. New strategies designed to prevent relapse after autograft include cytokine therapy such as interleukin-2 in combination with monoclonal antibodies and the use of autologous antilymphoma vaccines.

Regulation of progenitor cell fusion by ABCB5 P-glycoprotein, a novel human ATP-binding cassette transporter

Cell fusion involving progenitor cells is a newly recognized phenomenon thought to contribute to tissue differentiation. The molecular mechanisms governing cell fusion are unknown. P-glycoprotein and related ATP-binding cassette transporters are expressed by progenitor cells, but their physiological role in these cell types has not been defined. Here, we have cloned ABCB5, a rhodamine efflux transporter and novel member of the human P-glycoprotein family, which marks CD133-expressing progenitor cells among human epidermal melanocytes and determines as a regulator of membrane potential the propensity of this subpopulation to undergo cell fusion. Our findings show that polyploid ABCB5+ cells are generated by cell fusion and that this process is specifically enhanced by ABCB5 P-glycoprotein blockade. Remarkably, multinucleated cell hybrids gave rise to mononucleated progeny, demonstrating that fusion contributes to culture growth and differentiation. Thus, our findings define a molecular mechanism for cell fusion involving progenitor cells and show that fusion and resultant growth and differentiation are not merely spontaneous events, but phenomena regulated by ABCB5 P-glycoprotein.

Allogeneic and autologous transplantation for chronic lymphocytic leukemia

Autologous and allogeneic transplantation are increasingly used in the management of patients with chronic lymphocytic leukemia. Many questions regarding patient selection, efficacy and outcome are unresolved, hence a review of the literature through Medline search. Autologous transplantation for CLL has been used mainly in selected patients under the age of 60. Conditioning typically involves total body irradiation (TBI). Bone marrow and more recently peripheral blood stem cells are used. Treatment-related mortality in most series is less than 10%. Molecular remissions after autologous transplantation are common, and clinical remissions can be prolonged in some patients. Randomized studies are needed to establish whether autologous transplantation confers a survival benefit over standard chemotherapy approaches. Allogeneic transplantation has a considerable treatment-related mortality, but durable remissions sometimes occur in patients with advanced disease. The use of non-myeloablative 'mini-transplants' has been investigated as a method to reduce treatment-related mortality, but prolonged follow-up will be required to establish the cure rate obtained with this procedure. Autologous and allogeneic transplantation are promising treatment modalities. Further refinements of transplant techniques and properly designed prospective studies are necessary to establish the role of stem cell transplantation in the overall management of CLL.

A short course of induction chemotherapy followed by two cycles of high-dose chemotherapy with stem cell rescue for chemotherapy naive metastatic breast cancer: sequential phase I/II studies

Two cycles of high-dose chemotherapy with stem cell support (HDC) may increase the total dose delivered and dose intensity. A brief induction phase and different non-cross-resistant agents for each HDC cycle were used to avoid drug resistance. Twenty-six women with metastatic BC had induction and stem cell mobilization with two cycles of doxorubicin/G-CSF given every 14 days. Patients with stable disease or better after induction received HD CTCb followed by HD melphalan and dose-escalated paclitaxel. At 475 mg/m(2) of paclitaxel by 24-h infusion, dose-limiting transient peripheral sensory neuropathy was encountered. No toxic deaths occurred. Complete and near complete response after completion of therapy was achieved in 22 (85%) of 26 patients. The median EFS was 38 months. The median OS has not yet been reached. At a median follow-up of 33 (25-43) months, actuarial EFS and OS were 54% (95% confidence interval (CI), 39-69%) and 69% (95% CI, 56-79%), respectively. This double transplant approach lasts only 14 weeks and is feasible, safe, and tolerable. Whilst selection biases may in part contribute to favorable EFS and OS, a randomized comparison of standard therapy vs double transplant in both metastatic and locally advanced breast cancer is warranted.

Stem-cell transplant preparative regimens

Owing to the relatively high probability of recurrent disease in patients receiving hematopoietic stem-cell transplantation (HSCT) for malignancies, further development of new preparative regimens is warranted. Based on the data presented, one can predict that it will continue to be difficult to identify HSCT regimens that are more effective. Incremental improvements are expected to be small, difficult to measure, and will require inclusion of very large numbers of patients. Controlled trials to evaluate the effectiveness of specific treatment regimens for specific groups of patients will require only centers with large numbers of patients or co-operative groups to successfully conduct these studies. Development of HSCT regimens with low mortality and a minimum of morbidity without compromising efficacy are needed. This may be accomplished through the use of agents to protect normal, non-hematopoietic tissues from regimen-related toxicity (RRT), further exploration and expansion of applications of targeted radiolabeled antibody approaches and mixed chimerism approaches. The future holds much work, but great promise, in the development of new HSCT regimens.

A preclinical model for sequential high-dose chemotherapy

Dose-intensive chemotherapy regimens have entered clinical trial based on the notion that log-linear tumor-cell killing, especially with antitumor alkylating agents, is maintained at higher drug doses. Several clinical trials employing two intensifications are underway. Using the tumor-cell survival assay, animals bearing the FSaII fibrosarcoma were treated with single doses of various chemotherapeutic agents once or twice with a 3- or 7-day interval between the drugs. Isobologram methodology was used to determine if the sequential treatment regimens resulted in subadditive, additive or greater-than-additive tumor-cell killing. When melphalan was followed 3 or 7 days later by a second dose of melphalan there was evidence of resistance to the second dose of melphalan as indicated by subadditive tumor-cell killing. Melphalan followed 3 days later by cyclophosphamide (300 mg/kg) produced greater-than-additive tumor-cell killing, however, when the interval was 7 days the resulting tumor-cell killing was subadditive. Melphalan followed 3 or 7 days later by thiotepa or carboplatin produced subadditive-to-additive tumor-cell killing. Adriamycin followed 3 days later by melphalan, cyclophosphamide, thiotepa, or carboplatin resulted in subadditive-to-additive tumor-cell killing by the combinations. These results indicate that sequential drug-intensive treatments may not optimize tumor-cell killing in vivo.

Role of cellular glutathione and glutathione S-transferase in the expression of alkylating agent cytotoxicity in human breast cancer cells

Glutathione (GSH) and glutathione S-transferases (GSTs) play an important role in the protection of cells against toxic effects of many electrophilic drugs and chemicals. Modulation of cellular GSH and/or GST activity levels provides a potentially useful approach to sensitizing tumor cells to electrophilic anti-cancer drugs. In this study, we describe the interactions of four representative alkylating agents (AAs), melphalan, 4-hydroperoxy-cyclophosphamide (4HC), an an activated form of cyclophosphamide, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and cisplatin, with GSH and GST in the human breast cancer cell line MCF-7. Depletion of cellular GSH pools by approximately 80% by treatment of the cells with the GSH synthesis inhibitor buthionine sulfoximine (BSO) sensitized the tumor cells to each AA to a different extent, with dose-modifying factors of 2.39, 2.21, 1.64, and 1.27 observed for melphalan, 4HC, cisplatin, and BCNU, respectively. Treatment of the cells with the GST inhibitor ethacrynic acid (EA) failed to show any significant effects on the cytotoxicity of these AAs. However, EA did potentiate the cytotoxicity of melphalan when given in combination with BSO, an effect that may be due to a more complete depletion of cellular GSH levels by the combined modulator treatment. Following a 1-hr exposure to cytotoxic-equivalent concentrations of these AAs, GSH levels decreased substantially in the case of 4HC and BCNU, but increased by 30-50% in the case of cisplatin and melphalan. BSO pretreatment largely blocked this effect of cisplatin and melphalan on cellular GSH, while it further enhanced the GSH-depleting activity of both 4HC and BCNU. On the basis of these results, it is concluded that (a) GSH affects the cytotoxicity of different AAs to different extents, (b) basal GST expression in MCF-7 cells does not play a major role in AA metabolism, (c) EA can potentiate the enhancing effect of BSO on melphalan cytotoxicity in MCF-7 cells, and (d) depletion of cellular GSH by pretreatment with BCNU or cyclophosphamide may correspond to a useful strategy for enhancing the anti-tumor activity of other AAs given in a sequential combination.

Cyclooxygenase and lipoxygenase inhibitors as modulators of cancer therapies

Like many clinical non-small-cell lung cancers, the Lewis lung carcinoma produces prostaglandins. The Lewis lung carcinoma was used as a model of both primary and metastatic disease to assess the ability of cyclooxygenase inhibitors (mefenamic acid, diflunisal, sulindac, and indomethacin), the collagenase inhibitor minocycline, and the lipoxygenase inhibitor phenidone to act as modulators of cytotoxic cancer therapies. Although none of the single modulators given i.p. daily on days 4-18 altered tumor growth or the number of metastases found on day 20, modulator combinations consisting of minocycline/a cyclooxygenase inhibitor and, especially, of phenidone/a cyclooxygenase inhibitor resulted in modest tumor growth delay and a decreased number of lung metastases on day 20. The most effective modulators of cisplatin (CDDP) were phenidone/sulindac and phenidone/indomethacin, which led to 2.4- to 2.5-fold increases in the tumor growth delay produced by CDDP. The most effective modulations of cyclophosphamide resulted from administration of minocycline, minocycline/sulindac, or phenidone/sulindac and led to 2.0- to 2.1-fold increases in tumor growth delay by cyclophosphamide. The most effective modulators of melphalan produced 4.5- to 4.7-fold increases in tumor growth delay by the drug and were minocycline/sulindac, minocycline/mefenamic acid, and phenidone/sulindac. The most effective modulation of carmustine (BCNU) was obtained with minocycline/sulindac and minocycline/diflunisal leading to 2.8- to 3.1-fold increases in tumor growth delay by BCNU. Finally, the most effective modulation of radiation was obtained with minocycline/sulindac and phenidone/sulindac and resulted in 2.8- to 3.3-fold increases in tumor growth delay by radiation. The modulator combination that along with the cytotoxic therapies was most effective against metastatic disease was phenidone/mefenamic acid. There was no clear relationship between effective modulation of the cancer therapies and the degree of reduction in serum levels of prostaglandin E2 and leukotriene B4 by the agents in Lewis lung tumor bearing mice.