Phase I study of O6-benzylguanine and temozolomide administered daily for 5 days to pediatric patients with solid tumors

Genotoxic therapy and resistance mechanism in gliomas

Glioma is one of the most common and lethal brain tumors. Surgical resection followed by radiotherapy plus chemotherapy is the current standard of care for patients with glioma. The existence of resistance to genotoxic therapy, as well as the nature of tumor heterogeneity greatly limits the efficacy of glioma therapy. DNA damage repair pathways play essential roles in many aspects of glioma biology such as cancer progression, therapy resistance, and tumor relapse. O6-methylguanine-DNA methyltransferase (MGMT) repairs the cytotoxic DNA lesion generated by temozolomide (TMZ), considered as the main mechanism of drug resistance. In addition, mismatch repair, base excision repair, and homologous recombination DNA repair also play pivotal roles in treatment resistance as well. Furthermore, cellular mechanisms, such as cancer stem cells, evasion from apoptosis, and metabolic reprogramming, also contribute to TMZ resistance in gliomas. Investigations over the past two decades have revealed comprehensive mechanisms of glioma therapy resistance, which has led to the development of novel therapeutic strategies and targeting molecules.

Clinical Trial of MGMT(P140K) Gene Therapy in the Treatment of Pediatric Patients with Brain Tumors

Gene transfer targeting hematopoietic stem cells (HSC) in children has shown sustained therapeutic benefit in the treatment of genetic diseases affecting the immune system, most notably in severe combined immunodeficiencies affecting T-cell function. The HSC compartment has also been successfully targeted using gene transfer in children with genetic diseases affecting the central nervous system, such as metachromatic leukodystrophy and adrenoleukodystrophy. HSCs are also a target for genetic modification in strategies aiming to confer drug resistance to chemotherapy agents so as to reduce off-target toxicity, and to allow for chemotherapy dose escalation with the possibility of enhanced therapeutic benefit. In a trial of this strategy in adult glioma patients, significant engraftment of gene-modified HSCs expressing a mutant of the DNA repair protein O6-methyl-guanine-methyl-transferase (MGMT(P140K)) showed potential in conferring drug resistance against the combined effect of O6-benzylguanine (O6BG)/temozolomide (TMZ) chemotherapy. The aim was to test the safety and feasibility of this approach in children with poor prognosis brain tumors. In this Phase I trial, seven patients received gene-modified HSC following myelo-suppressive conditioning, but with only transient low-level engraftment of MGMT(P140K) gene-modified cells detectable in four patients. All patients received O6BG/TMZ chemotherapy following infusion of gene-modified cells, with five patients eligible for chemotherapy dose escalation, though in the absence of demonstrable transgene-mediated chemoprotection. Since all gene-modified cell products met the criteria for release and assays for engraftment potential met expected outcome measures, inadequate cell dose, conditioning chemotherapy, and/or underlying bone-marrow function may have contributed to the lack of sustained engraftment of gene-modified cells. We were able to demonstrate safe conduct of a technically complex Phase I study encompassing manufacture of the gene therapy vector, genetically modified cells, and a drug product specifically for the trial in compliance with both local and national regulatory requirements.

A comparison of safety and efficacy of cytotoxic versus molecularly targeted drugs in pediatric phase I solid tumor oncology trials

BACKGROUND

Prior reviews of phase I pediatric oncology trials involving primarily cytotoxic agents have reported objective response rates (ORRs) and toxic death rates of 7.9-9.6% and 0.5%, respectively. These data may not reflect safety and efficacy in phase I trials of molecularly targeted (targeted) drugs.

METHODS

A systematic review of pediatric phase I solid tumor trials published in 1990-2013 was performed. The published reports were evaluated for patient characteristics, toxicity information, and response numbers.

RESULTS

A total of 143 phase I pediatric clinical trials enrolling 3,896 children involving 53 targeted and 48 cytotoxic drugs were identified. A meta-analysis demonstrated that the ORR is 2.1-fold higher with cytotoxic drugs (0.066 vs. 0.031 per subject; P = 0.007). By contrast, the pooled estimate of the stable disease rate (SDR) is similar for cytotoxic and targeted drugs (0.2 vs. 0.23 per subject; P = 0.27).  The pooled estimate of the dose-limiting toxicity rate is 1.8-fold larger with cytotoxic drugs (0.24 vs. 0.13 per subject; P = 0.0003). The hematologic grade 3-4 (G3/4) toxicity rate is 3.6-fold larger with cytotoxic drugs (0.43 vs. 0.12 per treatment course; P = 0.0001); however, the nonhematologic G3/4 toxicities and toxic deaths occur at similar rates for cytotoxic and targeted drugs.

CONCLUSIONS

In phase I pediatric solid tumor trials, ORRs were significantly higher for cytotoxic versus targeted agents. SDRs were similar in targeted and cytotoxic drug trials. Patients treated with cytotoxic agents were more likely to experience hematologic G3/4 toxicities than those patients receiving targeted drugs.

Antineoplastic effect of decoy oligonucleotide derived from MGMT enhancer

Silencing of O(6)-methylguanine-DNA-methyltransferase (MGMT) in tumors, mainly through promoter methylation, correlates with a better therapeutic response and with increased survival. Therefore, it is conceivable to consider MGMT as a potential therapeutic target for the treatment of cancers. Our previous results demonstrated the pivotal role of NF-kappaB in MGMT expression, mediated mainly through p65/NF-kappaB homodimers. Here we show that the non-canonical NF-KappaB motif (MGMT-kappaB1) within MGMT enhancer is probably the major inducer of MGMT expression following NF-kappaB activation. Thus, in an attempt to attenuate the transcription activity of MGMT in tumors we designed locked nucleic acids (LNA) modified decoy oligonucleotides corresponding to the specific sequence of MGMT-kappaB1 (MGMT-kB1-LODN). Following confirmation of the ability of MGMT-kB1-LODN to interfere with the binding of p65/NF-kappaB to the NF-KappaB motif within MGMT enhancer, the efficacy of the decoy was studied in-vitro and in-vivo. The results of these experiments show that the decoy MGMT-kB1-LODN have a substantial antineoplastic effect when used either in combination with temozolomide or as monotherapy. Our results suggest that MGMT-kB1-LODN may provide a novel strategy for cancer therapy.

Management of high-grade gliomas in the pediatric patient: Past, present, and future

High-grade gliomas (HGGs) constitute ∼15% of all primary brain tumors in children and adolescents. Routine histopathological diagnosis is based on tissue obtained from biopsy or, preferably, from the resected tumor itself. The majority of pediatric HGGs are clinically and biologically distinct from histologically similar adult malignant gliomas; these differences may explain the disparate responses to therapy and clinical outcomes when comparing children and adults with HGG. The recently proposed integrated genomic classification identifies 6 distinct biological subgroups of glioblastoma (GBM) throughout the age spectrum. Driver mutations in genes affecting histone H3.3 (K27M and G34R/V) coupled with mutations involving specific proteins (TP53, ATRX, DAXX, SETD2, ACVR1, FGFR1, NTRK) induce defects in chromatin remodeling and may play a central role in the genesis of many pediatric HGGs. Current clinical practice in pediatric HGGs includes surgical resection followed by radiation therapy (in children aged > 3 years) with concurrent and adjuvant chemotherapy with temozolomide. However, these multimodality treatment strategies have had a minimal impact on improving survival. Ongoing clinical trials are investigating new molecular targets, chemoradiation sensitization strategies, and immunotherapy. Future clinical trials of pediatric HGG will incorporate the distinction between GBM molecular subgroups and stratify patients using group-specific biomarkers.

DNA repair by reversal of DNA damage

Endogenous and exogenous factors constantly challenge cellular DNA, generating cytotoxic and/or mutagenic DNA adducts. As a result, organisms have evolved different mechanisms to defend against the deleterious effects of DNA damage. Among these diverse repair pathways, direct DNA-repair systems provide cells with simple yet efficient solutions to reverse covalent DNA adducts. In this review, we focus on recent advances in the field of direct DNA repair, namely, photolyase-, alkyltransferase-, and dioxygenase-mediated repair processes. We present specific examples to describe new findings of known enzymes and appealing discoveries of new proteins. At the end of this article, we also briefly discuss the influence of direct DNA repair on other fields of biology and its implication on the discovery of new biology.

Differences in molecular genetics between pediatric and adult malignant astrocytomas: age matters

The microscope - the classical tool for the investigation of cells and tissues - remains the basis for the classification of tumors throughout the body. Nowhere has this been more true than in the grading of astrocytomas. In spite of the fact that our parents warned us not to judge a book by its cover, we have continued to assume that adult and pediatric malignant gliomas that look the same, will have the same mutations, and thus respond to the same therapy. Rapid advances in molecular biology have permitted us the opportunity to go inside the cell and characterize the genetic events that underlie the true molecular heterogeneity of adult and pediatric brain tumors. In this paper, we will discuss some of the important clinical differences between pediatric and adult gliomas, with a focus on the molecular analysis of these different age groups.

Combined modality approaches in the management of adult glioblastoma

Over the past two decades, management of newly diagnosed glioblastoma has undergone significant evolution. While surgery has long been a mainstay of management for this disease, and while radiotherapy has a proven survival role, initial efforts at radiotherapy dose escalation, use of radiosurgery, brachytherapy, and altered fractionation did not improve patient survival. Recently, multiple modality therapy integrating maximal safe resection, postoperative radiation, and new systemic therapies have resulted in improved patient outcomes compared with older regimens utilizing surgery and postoperative radiation alone. Numerous trials are currently underway investigating the combination of surgery, radiation, and systemic therapy with targeted agents to find ways to further improve outcomes for adults with glioblastoma.

A phase II study of O6-benzylguanine and temozolomide in pediatric patients with recurrent or progressive high-grade gliomas and brainstem gliomas: a Pediatric Brain Tumor Consortium study

To estimate the sustained (≥8 weeks) objective response rate in pediatric patients with recurrent or progressive high-grade gliomas (HGG, Stratum A) or brainstem gliomas (BSG, Stratum B) treated with the combination of O6-benzylguanine (O6BG) and temozolomide(®) (TMZ). Patients received O6BG 120 mg/m(2)/d IV followed by TMZ 75 mg/m(2)/d orally daily for 5 consecutive days of each 28-day course. The target objective response rate to consider the combination active was 17%. A two-stage design was employed. Forty-three patients were enrolled; 41 were evaluable for response, including 25 patients with HGG and 16 patients with BSG. The combination of O6BG and TMZ was tolerable, and the primary toxicities were myelosuppression and gastrointestinal symptoms. One sustained (≥8 weeks) partial response was observed in the HGG cohort; no sustained objective responses were observed in the BSG cohort. Long-term (≥6 courses) stable disease (SD) was observed in 4 patients in Stratum A and 1 patient in Stratum B. Of the 5 patients with objective response or long-term SD, 3 underwent central review with 2 reclassified as low-grade gliomas. The combination of O6BG and TMZ did not achieve the target response rate for activity in pediatric patients with recurrent or progressive HGG and BSG.

Inhibition of DNA repair with MGMT pseudosubstrates: phase I study of lomeguatrib in combination with dacarbazine in patients with advanced melanoma and other solid tumours

Background:

The DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT) reverses the O⁶-methylguanine (O⁶-meG) lesion induced by dacarbazine. Depletion of MGMT can be achieved using O⁶-meG pseudosubstrates. Herein, we report the first phase I experience of the novel O⁶-meG pseudosubstrate lomeguatrib, combined with dacarbazine.

Methods:

This is a phase I dose-escalation study to determine the maximum tolerated dose and recommended phase II dose (RP2D) of lomeguatrib combined with a single dose of dacarbazine on a 21-day schedule.

Results:

The vast majority of the 41 patients enrolled had metastatic melanoma (36/41) and most had no previous chemotherapy (30/41). The most frequent non-hematological adverse events (AEs) were nausea (52%), and fatigue (42%). The most frequent AEs of grade 3–4 severity were neutropaenia (42%), leukopaenia (17%), and thrombocytopaenia (12%). Only 1 patient had a partial response and 10 patients had stable disease.

Conclusion:

The RP2D of lomeguatrib was 40 mg orally twice daily for 10 days combined with 400 mg m⁻² of dacarbazine IV on day 2. Oral administration of lomeguatrib substantially increases the haematological toxicity of dacarbazine consistent with experience with other O⁶-meG pseudosubstrates.

The use and effectiveness of temozolomide in children with central nervous system tumours: a survey from the Canadian Paediatric Brain Tumour Consortium

OBJECTIVE

To describe the use of temozolomide (tmz) in Canadian children treated for brain tumours and to evaluate survival and predictors of survival for children treated with this agent.

METHODS

A survey was conducted within the Canadian Paediatric Brain Tumour Consortium (cpbtc), a group of tertiary care centres in pediatric neuro-oncology (n = 16) in Canada that are involved in the treatment of children with central nervous system tumours.

RESULTS

In 10 of the 16 participating pediatric oncology centres of the cpbtc, 137 children with brain tumours were treated with tmz between January 2000 and March 2006. Although 33% of the children were enrolled into a clinical trial, 67% were treated outside open studies. Most patients (72%) received tmz treatment on recurrence of their brain tumour (first or subsequent). The most commonly administered regimen was single-agent tmz 150-200 mg/m(2) administered on 5 consecutive days every 28 days. The median duration of tmz treatment was 141 days (range: 4-1102 days). Response data were provided for 127 of the 137 patients, of whom 6 showed a complete response. Sixteen patients experienced a minor or partial response, 53 had stable disease, and 52 had progressive disease. Of 32 patients alive at last follow-up, 19 had a diagnosis of low-grade glioma.

CONCLUSIONS

Temozolomide is used in a variety of pediatric brain tumours, often at the time of recurrence. The lack of insight into clear indications for this agent in pediatric brain tumours-used either alone or in combination therapy-may be a result of suboptimal design of phase i and ii studies and a lack of phase iii trials in the pediatric brain tumour population.

Targeting O⁶-methylguanine-DNA methyltransferase with specific inhibitors as a strategy in cancer therapy

O (6)-methylguanine-DNA methyltransferase (MGMT) repairs the cancer chemotherapy-relevant DNA adducts, O (6)-methylguanine and O (6)-chloroethylguanine, induced by methylating and chloroethylating anticancer drugs, respectively. These adducts are cytotoxic, and given the overwhelming evidence that MGMT is a key factor in resistance, strategies for inactivating MGMT have been pursued. A number of drugs have been shown to inactivate MGMT in cells, human tumour models and cancer patients, and O (6)-benzylguanine and O (6)-[4-bromothenyl]guanine have been used in clinical trials. While these agents show no side effects per se, they also inactivate MGMT in normal tissues and hence exacerbate the toxic side effects of the alkylating drugs, requiring dose reduction. This might explain why, in any of the reported trials, the outcome has not been improved by their inclusion. It is, however, anticipated that, with the availability of tumour targeting strategies and hematopoetic stem cell protection, MGMT inactivators hold promise for enhancing the effectiveness of alkylating agent chemotherapy.

Plasma and CNS pharmacokinetics of O4-benzylfolic acid (O4BF) and metabolite in a non-human primate model

PURPOSE

O(6)-alkylguanine-DNA alkyltransferase (AGT) repairs DNA damage from alkylating agents by transferring the alkyl adducts from the O(6)-position of guanine in DNA to AGT. The folate analog O(4)-benzylfolic acid (O(4)BF) is an inhibitor of AGT with reported selectivity of the alpha-folate receptor in tumors. We studied plasma and cerebrospinal fluid (CSF) pharmacokinetics and CSF penetration of O(4)BF in a non-human primate model.

METHODS

Rhesus monkeys (Macaca mulatta) received O(4)BF (10-50 mg/kg) intravenously, and serial blood and CSF samples were obtained. Analyte concentrations in plasma were measured by HPLC/photo diode array, and an HPLC/MS/MS assay was used for CSF samples.

RESULTS

A putative metabolite of O(4)BF was detected in plasma and CSF. O(4)BF and the metabolite inactivated purified AGT with ED(50) of 0.04 mcM. The median clearance of O(4)BF was 8 ml/min/kg and half-life was 1.1 h. The metabolite had a substantially longer half-life (>20 h) and greater AUC than O(4)BF. The AUC of the metabolite increased disproportionately to the dose of O(4)BF, suggesting saturable elimination. CSF penetration of O(4)BF and its metabolite was < 1%. At the 50 mg/kg dose level, the C(max) in CSF for O(4)BF was less than 0.09 mcM and for the metabolite the C(max) ranged from 0.02 to 0.04 mcM (O(4)BF equivalents).

CONCLUSIONS

Concentrations of O(4)BF and the metabolite in CSF exceeded the ED(50) of AGT; however, recently reported lack of receptor specificity and pharmacokinetic data suggesting saturable elimination of both O(4)BF and its metabolite may limit dose-escalation and future clinical development of this agent.

Extended O6-methylguanine methyltransferase promoter hypermethylation following n-butylidenephthalide combined with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) on inhibition of human hepatocellular carcinoma cell growth

Epigenetic alteration of DNA methylation plays an important role in the regulation of gene expression associated with chemosensitivity of human hepatocellular (HCC) carcinoma cells. With the aim of improving the chemotherapeutic efficacy for HCC, the effect of the naturally occurring compound n-butylidenephthalide (BP), which is isolated from a chloroform extract of Angelica sinensis, was investigated. In both HepG2 and J5 HCC cell lines, a synergistic antiproliferative effect was observed when a low dosage of BP was combined with the chemotherapeutic drug 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU is an alkylating agent, and it prompts us to examine one of DNA repair genes, O(6)-methylguanine methyltransferase (MGMT). It was evident from methylation-specific polymerase chain reaction (PCR) analysis that BP/BCNU combined treatment caused a time- and concentration-dependent enhancement of MGMT promoter methylation. Overexpression of MGMT could abolish BP-induced growth inhibition in the J5 tumor cell line as measured by colony formation assay. When BP was combined with BCNU and administered, it showed significant antitumor effects in both HepG2 and J5 xenograft tumors as compared with the use of only one of these drugs. The BCNU-induced apoptosis and inhibited MGMT protein expression in HCC cells, both in vitro and in vivo, resulting from the combination treatment of BP and BCNU suggest a potential clinical use of this compound for improving the prognosis for HCCs.

Critical appraisal of temozolomide formulations in the treatment of primary brain tumors: patient considerations

Chemotherapy is assuming an increasingly important role in the treatment of malignant gliomas, of which temozolomide (TMZ) is a key part. TMZ belongs to a class of second-generation imidazotetrazinone prodrugs that exhibit linear pharmacokinetics and do not require hepatic metabolism for activation to the active metabolite. New intravenous (iv) TMZ formulations have recently been approved based on studies of bioequivalence between iv and oral TMZ. The efficacy of TMZ was initially evaluated in patients with recurrent disease but phase II and III trials in newly diagnosed gliomas are available. The results of a large phase III trial that compared RT alone vs RT concomitant with oral TMZ created a new standard of adjuvant treatment. Efficacy data for iv TMZ on which its approval was based are those extrapolated from clinical trials with oral TMZ. No comparative data are available on the differences in tolerability and patient satisfaction between oral and iv formulations of TMZ, or for quality of life. New oral formulations could encourage the adherence of patients to treatment. Although patients presumably would prefer oral treatment, iv formulations may be an alternative in noncompliant patients or patients for whom good adherence could not be expected.

Salvage chemotherapy for metastatic and recurrent ependymoma of childhood

INTRODUCTION

Chemotherapy has limited role in the up-front management of ependymoma. At the time of recurrence, the role of chemotherapy is also ill defined and the choice of chemotherapeutic agents is often arbitrary, based on anecdotal data and personal experience.

METHODS

The purpose of this review is to describe and critically analyze the published literature on chemotherapy in patients with recurrent and metastatic ependymoma.

DISCUSSION

The disappointing response rate with single agents (12.9%) and combinations (17.4%) emphasizes the need to re-evaluate the current chemotherapeutic approach of intracranial ependymoma, and biological studies are needed to identify targets that may be considered for clinical trials.

Poly(ADP-ribose) polymerase inhibitor ABT-888 potentiates the cytotoxic activity of temozolomide in leukemia cells: influence of mismatch repair status and O6-methylguanine-DNA methyltransferase activity

The poly(ADP-ribose) polymerase (PARP) inhibitor ABT-888 potentiates the antitumor activity of temozolomide (TMZ). TMZ resistance results from increased O(6)-methylguanine-DNA methyltransferase (MGMT) activity and from mismatch repair (MMR) system mutations. We evaluated the relative importance of MGMT activity, MMR deficiency, nonhomologous end joining (NHEJ), and PARP activity in ABT-888 potentiation of TMZ. MMR-proficient and MMR-deficient leukemia cells with varying MGMT activity, as well as primary leukemia samples, were used to determine TMZ IC(50) alone and with ABT-888. ABT-888 effectively inhibited PARP activity and enhanced TMZ growth inhibition in most leukemia cells. ABT-888 potentiation was most effective in MMR-deficient cells with low MGMT activity [potentiation factor (PF) = 21]. ABT-888 also potentiated TMZ activity in MMR-deficient cells with elevated MGMT activity. Unexpectedly, ABT-888 also enhanced TMZ activity in MMR-proficient cells (PF = 3-7). ABT-888 potentiation was unrelated to NHEJ activity. ABT-888 potentiated TMZ (PF = 2-5) in two of four acute myeloid leukemia patient samples but showed little potentiation in primary acute lymphoblastic leukemia. In conclusion, although ABT-888 potentiation of TMZ was most pronounced in MMR-deficient cells with low MGMT activity, neither MMR proficiency nor MGMT overexpression completely abrogated ABT-888 potentiation of TMZ.

Pharmacokinetics of temozolomide administered in combination with O6-benzylguanine in children and adolescents with refractory solid tumors

PURPOSE

Temozolomide pharmacokinetics were evaluated in children receiving concurrent O(6)-benzylguanine (O(6)BG), which enhanced the hematological toxicity of temozolomide.

METHODS

Temozolomide was administered orally, daily for 5 days starting at 28 mg/m(2) per day with escalations to 40, 55, 75 and 100 mg/m(2) per day with O(6)BG intravenously daily for 5 days at doses of 60, 90 or 120 mg/m(2) per day. Plasma samples were drawn over 48 h after the day 5 dose. Temozolomide was quantified with a validated HPLC/tandem mass spectroscopic assay.

RESULTS

Temozolomide was rapidly absorbed (mean T (max), 2.1 h). The mean apparent clearance (CL/F) (96 mL/min/m(2)) was similar to the CL/F for temozolomide alone and was not age- or gender-dependent. There was minimal inter-patient variability.

CONCLUSIONS

The enhanced hematologic toxicity resulting from combining O(6)BG with temozolomide does not appear to be the result of a pharmacokinetic interaction between the agents.

Substitution of aminomethyl at the meta-position enhances the inactivation of O6-alkylguanine-DNA alkyltransferase by O6-benzylguanine

O(6)-Benzylguanine is an irreversible inactivator of O(6)-alkylguanine-DNA alkyltransferase currently in clinical trials to overcome alkyltransferase-mediated resistance to certain cancer chemotherapeutic alkylating agents. In order to produce more soluble alkyltransferase inhibitors, we have synthesized three aminomethyl-substituted O(6)-benzylguanines and the three methyl analogs and found that the substitution of aminomethyl at the meta-position greatly enhances inactivation of alkyltransferase, whereas para-substitution has little effect and ortho-substitution virtually eliminates activity. Molecular modeling of their interactions with alkyltransferase provided a molecular explanation for these results. The square of the correlation coefficient (R(2)) obtained between E-model scores (obtained from GLIDE XP/QPLD docking calculations) vs log(ED(50)) values via a linear regression analysis was 0.96. The models indicate that the ortho-substitution causes a steric clash interfering with binding, whereas the meta-aminomethyl substitution allows an interaction of the amino group to generate an additional hydrogen bond with the protein.

Phase II trial of temozolomide plus o6-benzylguanine in adults with recurrent, temozolomide-resistant malignant glioma

PURPOSE

This phase II trial was designed to define the role of O(6)-benzylguanine (O(6)-BG) in restoring temozolomide sensitivity in patients with recurrent or progressive, temozolomide-resistant malignant glioma and to evaluate the safety of administering O(6)-BG in combination with temozolomide.

PATIENTS AND METHODS

Patients were accrued into two independent strata on the basis of histology: glioblastoma multiforme (GBM) and anaplastic glioma. Both temozolomide and O(6)-BG were administered on day 1 of a 28-day treatment cycle. Patients were administered a 1-hour O(6)-BG infusion at a dose of 120 mg/m(2) followed immediately by a 48-hour infusion at a dose of 30 mg/m(2)/d. Temozolomide was administered orally within 60 minutes of the end of the 1-hour O(6)-BG infusion at a dose of 472 mg/m(2). The primary end point was objective response rate. Secondary end points included progression-free survival, overall survival, and safety.

RESULTS

Sixty-six of 67 patients who enrolled were treated with temozolomide and O(6)-BG. One of 34 patients (3%) with GBM (95% CI, 0.1% to 15%) and five of 32 assessable patients (16%) with anaplastic glioma (95% CI, 5% to 33%) were responders. The most commonly reported adverse events were grade 4 hematologic events experienced in 48% of the patients.

CONCLUSION

O(6)-BG when added to a 1-day dosing regimen of temozolomide was able to restore temozolomide sensitivity in patients with temozolomide-resistant anaplastic glioma, but there seemed to be no significant restoration of temozolomide sensitivity in patients with temozolomide-resistant GBM.

Characteristics and outcome of pediatric patients enrolled in phase I oncology trials

PURPOSE

To describe the characteristics of pediatric subjects who enroll in phase I trials, to determine the associations between pre-enrollment characteristics and the risk for toxicity, and to analyze response and survival outcomes.

EXPERIMENTAL DESIGN

Pre-enrollment characteristics and study outcomes were retrospectively analyzed for children with refractory solid tumors treated in one of 16 phase I trials with similar eligibility criteria at the National Cancer Institute between 1992 and 2005.

RESULTS

The 262 subjects analyzed had received a median of two (range, 0-9) prior chemotherapy regimens, and were on one (range, 0-12) concomitant medication. The Eastern Cooperative Oncology Group performance status scores for subjects were 0 (29%), 1 (48%), and 2 (19%); 19% had received a prior stem cell transplantation and 73% had received prior radiation. Approximately 90% of subjects were evaluable for the primary trial endpoints (toxicity and pharmacokinetics). Seventeen percent of subjects experienced a dose-limiting toxicity (DLT), 5% discontinued the study drug because of toxicity, and a drug-related death occurred in one subject (0.4%). Variables associated with a higher risk for developing a DLT, by multiple logistic regression analysis, were drug dose and prior radiation, for myelosuppressive agents, and drug dose and performance status, for nonmyelosuppressive agents. The complete and partial response rate was 4%; however, 17% of subjects had stable disease (received three or more cycles). The median overall survival time from the time of enrollment was five months.

CONCLUSIONS

Primary trial objectives are achieved in approximately 90% of subjects with the standard phase I trial design and eligibility criteria despite the intensification of frontline and salvage therapies in pediatric subjects with cancer.

Phase 1 trial of O6-benzylguanine and BCNU in children with CNS tumors: a Children's Oncology Group study

BACKGROUND

Efficacy of nitrosoureas is limited by host repair of drug-induced alkylation. O(6)-benzylguanine (O(6)-BG), an inhibitor of host alkylation repair, and BCNU were studied in children with refractory/untreatable central nervous system tumors to determine dose-limiting toxicities (DLTs) and maximum tolerated dose (MTD) of BCNU administered following O(6)-BG.

PROCEDURE

O(6)-BG (120 mg/m(2) IV over 1 hr) was followed by BCNU (IV over 1 hr). Cohorts of three to six patients were treated with escalating doses of BCNU. Courses were repeated every 6 weeks. Patients in Stage 1 were accrued irrespective of prior treatment. Once the MTD was exceeded, Stage II accrual was limited to less heavily pretreated patients (</= two prior chemotherapy regimens, no prior central axis radiation, no prior bone marrow transplant, and no bone marrow involvement).

RESULTS

Twelve patients in Stage I and 13 in Stage II (less heavily pretreated patients only) were evaluable for toxicity. The MTD of BCNU administered with O(6)-BG (120 mg/m(2) IV) was 58 mg/m(2) in less-heavily pretreated patients. Myelosuppression, which was cumulative in some patients receiving multiple cycles of therapy, was the predominate DLT. Twenty-four patients were evaluable for response: after two courses of therapy, 6 had stable disease, 17 had progressive disease, and 1 patient had a minor response but progressed after four courses of therapy.

CONCLUSIONS

Based on lack of activity of this combination in adult phase II studies, no further testing of O(6)-BG plus BCNU in children is planned. Strategies to decrease hematopoeitic toxicity of BCNU plus O(6)-BG are required.

Phase I trial of single-dose temozolomide and continuous administration of o6-benzylguanine in children with brain tumors: a pediatric brain tumor consortium report

PURPOSE

To estimate the maximum tolerated dose (MTD) and dose-limiting toxicity (DLT) of escalating doses of temozolomide combined with O(6)-benzylguanine in patients < or =21 years with recurrent brain tumors.

EXPERIMENTAL DESIGN

Treatment strata consisted of patients who had previously received no or local radiotherapy (Str1) and patients who had undergone craniospinal radiotherapy or myeloablative chemotherapy (Str2). One-hour i.v. administration of O(6)-benzylguanine at 120 mg/m(2) was followed by 48-h continuous infusion at 30 mg/m(2)/day. Single-dose temozolomide at five dosage levels (267, 355, 472, 628, and 835 mg/m(2)) was given at least 6 h after completion of O(6)-benzylguanine bolus. Treatment was repeated after recovery from toxicities at least 4 weeks apart for a maximum of 12 courses. Dose escalation followed the modified continual reassessment method. Pharmacokinetic analyses of temozolomide and 5-triazeno imidazole carboxamide (MTIC) were done in 28 patients.

RESULTS

A total of 44 and 26 eligible patients were enrolled on Str1 and Str2, respectively. Median age at study entry in each stratum was 8.6 and 11.3 years, respectively. Predominant diagnoses were high-grade/brainstem glioma in Str1 and medulloblastoma in Str2. Whereas the estimated MTDs of temozolomide for Str1 and Str2 were 562 and 407 mg/m(2), respectively, the doses recommended for phase II investigations are 472 and 355 mg/m(2), respectively. DLTs were predominantly neutropenia and thrombocytopenia. Three patients with gliomas experienced centrally confirmed partial responses to therapy. Four patients completed all planned therapy. Temozolomide and MTIC exposures were statistically associated with temozolomide dosage.

CONCLUSIONS

The current schedule of temozolomide and O(6)-benzylguanine is safe and showed modest activity against recurrent brain tumors in children.

Targeting methylguanine-DNA methyltransferase in the treatment of neuroblastoma

PURPOSE

The combination of temozolomide and irinotecan has preclinical schedule-dependent synergy against neuroblastoma but is not curative for relapsed high-risk patients. We hypothesized that the DNA repair protein methylguanine-DNA methyltransferase (MGMT) is an important resistance factor, and that inactivation of MGMT would sensitize neuroblastoma cells to these agents.

EXPERIMENTAL DESIGN

MGMT protein expression was assessed in 74 primary neuroblastoma tumors. Growth inhibition assays were done to determine the IC(50) and the extent of synergy observed with various concentrations of temozolomide, irinotecan, and the MGMT-inactivating agent O(6)-benzylguanine, using cultured syngeneic neuroblastoma cells with either low or high levels of MGMT expression. We then assessed efficacy in a mouse xenograft model of metastatic neuroblastoma.

RESULTS

MGMT was expressed by all 74 tumors evaluated. Pretreatment of neuroblastoma cells with O(6)-benzylguanine reduced the IC(50) of temozolomide by 10-fold regardless of level of MGMT expression, and pretreatment with BG followed by temozolomide + irinotecan further reduced the IC(50) in cells with high MGMT expression another 10-fold, to well below clinically achievable concentrations. The combination index was 0.27 to 0.30 for all three drugs in both cell lines, indicating strong synergy. Survival at 100 days for mice with metastatic neuroblastoma was 56% with three-drug treatment, compared with untreated controls (0%, P < 0.001) or temozolomide + irinotecan (10%, P = 0.081).

CONCLUSIONS

MGMT is widely expressed in primary neuroblastoma tumors, and is a relevant therapeutic target. Both in vitro and in vivo studies suggest inactivation of MGMT with O(6)-benzylguanine may increase the activity of temozolomide and irinotecan against neuroblastoma.

Phase 2 study of temozolomide in children and adolescents with recurrent central nervous system tumors: a report from the Children's Oncology Group

BACKGROUND

Effective chemotherapy is lacking for most types of central nervous system (CNS) tumors in children. Temozolomide, an agent with activity against adult brain tumors, was investigated in children and adolescents with recurrent CNS tumors.

METHODS

Temozolomide was administered orally as monthly 5-day courses at doses of 200 mg/m(2)/d (patients with no prior craniospinal irradiation [CSI]) or 180 mg/m(2)/d (prior CSI). Patients with a complete (CR) or partial (PR) response or stable disease (SD) could continue temozolomide for up to 12 cycles.

RESULTS

The cohort comprised 122 patients, including 113 with CNS tumors. Median age was 11 years (range, 1-23 years). Among 104 evaluable patients with CNS tumors, 5 PRs and 1 CR were observed. PRs occurred in 1 of 23 evaluable patients with high-grade astrocytoma, 1 of 21 with low-grade astrocytoma, and 3 of 25 with medulloblastoma/primitive neuroectodermal tumor (PNET). The CR occurred in an additional patient with medulloblastoma/PNET. No responses were observed in patients with ependymoma, brain-stem glioma, or other CNS tumors. Notably, 41% of patients with low-grade astrocytoma had SD through 12 courses. The most frequent toxicities were grade 3 or 4 neutropenia (19%) and thrombocytopenia (25%); nonhematologic toxicity was infrequent.

CONCLUSIONS

Although overall objective responses were limited, further exploration of temozolomide may be warranted in children with medulloblastoma and other PNETs, or in patients with low-grade astrocytoma, perhaps in a setting of less pretreatment than the patients in the current study, or in the context of multiagent therapy.

Differential inactivation of polymorphic variants of human O6-alkylguanine-DNA alkyltransferase

The human DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (hAGT) is an important source of resistance to some therapeutic alkylating agents and attempts to circumvent this resistance by the use of hAGT inhibitors have reached clinical trials. Several human polymorphisms in the MGMT gene that encodes hAGT have been described including L84F and the linked double alteration I143V/K178R. We have investigated the inactivation of these variants and the much rarer variant W65C by O(6)-benzylguanine, which is currently in clinical trials, and a number of other second generation hAGT inhibitors that contain folate derivatives (O(4)-benzylfolic acid, the 3' and 5' folate esters of O(6)-benzyl-2'-deoxyguanosine and the folic acid gamma ester of O(6)-(p-hydroxymethyl)benzylguanine). The I143V/K178R variant was resistant to all of these compounds. The resistance was due solely to the I143V change. These results suggest that the frequency of the I143V/K178R variant among patients in the clinical trials with hAGT inhibitors and the correlation with response should be considered.

Inactivation of O(6)-alkylguanine-DNA alkyltransferase by folate esters of O(6)-benzyl-2'-deoxyguanosine and of O(6)-[4-(hydroxymethyl)benzyl]guanine

O6-Alkylguanine-DNA alkyltransferase (alkyltransferase) provides an important source of resistance to some cancer chemotherapeutic alkylating agents. Folate ester derivatives of O6-benzyl-2'-deoxyguanosine and of O6-[4-(hydroxymethyl)benzyl]guanine were synthesized and tested for their ability to inactivate human alkyltransferase. Inactivation of alkyltransferase by the gamma-folate ester of O6-[4-(hydroxymethyl)benzyl]guanine was similar to that of the parent base. The gamma-folate esters of O6-benzyl-2'-deoxyguanosine were more potent alkyltransferase inactivators than the parent nucleoside. The 3'-ester was considerably more potent than the 5'-ester and was more than an order of magnitude more active than O6-benzylguanine, which is currently in clinical trials to enhance therapy with alkylating agents. They were also able to sensitize human tumor cells to killing by 1,3-bis(2-chloroethyl)-1-nitrosourea, with O6-benzyl-3'-O-(gamma-folyl)-2'-deoxyguanosine being most active. These compounds provide a new class of highly water-soluble alkyltransferase inactivators and form the basis to construct more tumor-specific and potent compounds targeting this DNA repair protein.

Live and let die: in vivo selection of gene-modified hematopoietic stem cells via MGMT-mediated chemoprotection

Gene transfer into hematopoietic stem cells (HSC) provides a potential means of correcting monogenic defects and altering drug sensitivity of normal bone marrow to cytotoxic agents. These applications have significant therapeutic potential but the translation of successful murine studies into human therapies has been hindered by low gene transfer in large animals (including humans), and recent serious side effects in a human immunodeficiency trial related to insertional mutagenesis. The latter trial, along with other subsequent trials, while bringing into focus the potential risks of integrating vector systems, also clearly demonstrate the potential usefulness of in vivo selection as it relates to inefficient stem cell transduction. Developing from initial studies by our group and other investigators in which drug resistance was utilized to demonstrate the feasibility of using gene transfer to effect protection from myelotoxicity of chemotherapeutic agents, expression of mutant forms of O(6)-methyguanine-DNA-methytransferase (MGMT) coupled with the simultaneous use of pharmacologic inhibitors and chemotherapeutic agents has been shown to provide a powerful method to select HSC in vivo. While stem and progenitor cell protection and resulting selection in vivo has potential applications for the treatment of selected cancers (allowing dose escalation) and for correction of monogenic disease (allowing an iatrogenic survival advantage of transduced cells in vivo), such an in vivo selection may have untoward effects on stem cell behavior. These deleterious effects may include stem cell exhaustion; lineage skewing; accumulation of genotoxic lesions; and clonal dominance driven towards a pro-leukemic phenotype. Knowledge of the likelihood of such deleterious events occurring as well as their potential implications will be critical to future clinical applications and may also enhance our understanding of both normal stem cell behavior and the evolution of hematopoietic malignancies.

DNA binding, nucleotide flipping, and the helix-turn-helix motif in base repair by O6-alkylguanine-DNA alkyltransferase and its implications for cancer chemotherapy

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a crucial target both for the prevention of cancer and for chemotherapy, since it repairs mutagenic lesions in DNA, and it limits the effectiveness of alkylating chemotherapies. AGT catalyzes the unique, single-step, direct damage reversal repair of O(6)-alkylguanines by selectively transferring the O(6)-alkyl adduct to an internal cysteine residue. Recent crystal structures of human AGT alone and in complex with substrate DNA reveal a two-domain alpha/beta fold and a bound zinc ion. AGT uses its helix-turn-helix motif to bind substrate DNA via the minor groove. The alkylated guanine is then flipped out from the base stack into the AGT active site for repair by covalent transfer of the alkyl adduct to Cys145. An asparagine hinge (Asn137) couples the helix-turn-helix DNA binding and active site motifs. An arginine finger (Arg128) stabilizes the extrahelical DNA conformation. With this newly improved structural understanding of AGT and its interactions with biologically relevant substrates, we can now begin to unravel the role it plays in preserving genetic integrity and discover how it promotes resistance to anticancer therapies.

Defining regional infusion treatment strategies for extremity melanoma: comparative analysis of melphalan and temozolomide as regional chemotherapeutic agents

Five different human melanoma xenografts were used in a xenograft model of extremity melanoma to evaluate the variability of tumor response to regionally administered melphalan or temozolomide and to determine if various components of pertinent drug resistance pathways for melphalan [glutathione S-transferase (GST)/glutathione] and temozolomide [O(6)-alkylguanine DNA alkyltranferase (AGT)/mismatch repair (MMR)] could be predictive of tumor response. Xenograft-bearing rats underwent regional isolated limb infusion with either melphalan (90 mg/kg) or temozolomide (2,000 mg/kg). The levels of AGT activity, GST activity, glutathione level, and GST/AGT expression were examined in this group of xenografts and found to be quite heterogeneous. No correlation was identified between melphalan sensitivity and the GST/glutathione cellular detoxification pathway. In contrast, a strong correlation between the levels of AGT activity and percentage increase in tumor volume on day 30 (r = 0.88) was noted for tumors treated with temozolomide. Regional therapy with temozolomide was more effective when compared with melphalan for the xenograft with the lowest AGT activity, whereas melphalan was more effective than temozolomide in another xenograft that had the highest AGT activity. In three other xenografts, there was no significant difference in response between the two chemotherapy agents. This study shows that AGT activity may be useful in predicting the utility of temozolomide-based regional therapy for advanced extremity melanoma tumors. Our observations also point out the limited ability of analysis of the GST/glutathione pathway to predict response to chemotherapies like melphalan whose resistance is primarily mediated through a complex mechanism of detoxification.

Temozolomide in children with progressive low-grade glioma

We conducted a phase II study to assess the efficacy of oral temozolomide (TMZ) in children with progressive low-grade glioma. Thirty eligible patients were enrolled on this study. Median age at enrollment was 10 years (range, 4-18 years). Eligible patients received TMZ (200 mg/m(2) per day) by mouth for five days every four weeks. Patients received a median of nine cycles (range, 2-12 cycles) of treatment. Best responses in the 26 patients (86%) with optic pathway glioma (OPG)/pilocytic astrocytoma (PA) included partial response in 3 patients (11%), minor response in 1 (4%), stable disease in 10 (38%), and progressive disease in 12 (46%). Only one of four patients with fibrillary astrocytoma had stable disease for 29 months after TMZ. The overall disease stabilization rate in patients with OPG/PA was 54%, and disease control was maintained for a median interval of 34 months. Seventeen of 26 patients had progressive disease either on or off therapy, and three have died of disease. The two-year progression-free and overall survivals in patients with OPG/PA were 49% (95% CI, 30%-67%) and 96% (95% CI, 89%-100%), respectively. Worst toxicity related to TMZ in all 30 patients included grade 2-4 thrombocytopenia in seven patients, grade 2-4 neutropenia in seven, grade 2 skin rash in one, and intratumor hemorrhage in one. TMZ given in this schedule was successful in stabilizing disease in a significant proportion of the patients with OPG/PA, with manageable toxicity.

Irinotecan Plus Temozolomide for Relapsed or Refractory Neuroblastoma

Purpose

To report on an irinotecan and temozolomide regimen for neuroblastoma (NB). Quality of life and minimizing toxicity were major considerations.

Patients and Methods

The plan stipulated 5-day courses of irinotecan 50 mg/m2 (1-hour infusion) and temozolomide 150 mg/m2 (oral) every 3 to 4 weeks, with a pretreatment platelet count more than 30,000/μL. Granulocyte colony-stimulating factor was used when the absolute neutrophil count was less than 1,000/μL.

Results

Forty-nine NB patients received 1 to 15 courses (median, 5). Gastrointestinal and myelosuppressive toxicities were readily managed. Lymphocyte responses to phytohemagglutinin after 2 to 10 courses (median, 3.5) were normal in 10 of 10 patients treated after nonimmunosuppressive therapy, and normalized in five of seven patients first treated less than 2 months after high-dose alkylators. Of 19 patients treated for refractory NB and assessable for response, nine showed evidence of disease regression, including two complete responses and seven objective responses. Of 17 patients treated for progressive disease, three showed evidence of disease regression, including one partial response and two objective responses. Multiple courses entailed no cumulative toxicity and controlled disease for prolonged periods in many patients, including some who were unable to complete prior treatments because of hematologic, infectious, cardiac, or renal problems.

Conclusion

This regimen has anti-NB activity, spares vital organs, is feasible with poor bone marrow reserve, causes limited immunosuppression, and allows good quality of life.

Inactivation of O6-alkylguanine DNA alkyltransferase as a means to enhance chemotherapy

DNA adducts at the O6-position of guanine are a result of the carcinogenic, mutagenic and cytotoxic actions of methylating and chloroethylating agents. The presence of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) renders cells resistant to the biological effects induced by agents that attack at this position. O6-Benzylguanine (O6-BG) is a low molecular weight substrate of AGT and therefore, results in sensitizing cells and tumors to alkylating agent-induced cytotoxicity and antitumor activity. Presently, chemotherapy regimens of O6-BG in combination with BCNU, temozolomide and Gliadel are in clinical development. Other ongoing clinical trials include expression of mutant AGT proteins that confer resistance to O6-BG in bone marrow stem cells, in an effort to reduce the potential enhanced toxicity and mutagenicity of alkylating agents in the bone marrow. O6-BG has also been found to enhance the cytotoxicity of agents that do not form adducts at the O6-position of DNA, including platinating agents. O6-BG's mechanism of action with these agents is not fully understood; however, it is independent of AGT activity or AGT inactivation. A better understanding of the effects of this agent will contribute to its clinical usefulness and the design of better analogs to further improve cancer chemotherapy.