LETd Optimization Verification With an SOI Microdosimeter

PURPOSE

A first of its kind experimental verification of dose-averaged linear energy transfer (LETd) optimized treatment plans for proton therapy has been carried out using a silicon-on-insulator microdosimeter at the Massachusetts General Hospital (MGH), Boston, USA.

METHODS AND MATERIALS

Three clinical treatment plans of a typical ependymoma structure set were designed using the standard clinical approach, the proposed protocol approach, and a one-field approach. The plans were then reoptimized to reduce the LETd-weighted dose in the brain stem. All six plans were delivered in a solid water phantom and the experimental yD‾ measured.

RESULTS

After LETd optimization, a reduction in yD‾ was found within the brain stem by an average of 12%, 19%, and 4% for the clinical, protocol, and one-field plans, respectively, while maintaining adequate coverage of the tumor structure. The experimental LETd-weighted doses were in agreement with the treatment planning system calculations and Monte Carlo simulations and reinforced the improvement of the optimization.

CONCLUSIONS

This work demonstrates the first experimental verification of the clinical implementation of LETd optimization for patient treatment with proton therapy.

Impact of spot size variations on dose in scanned proton beam therapy

BACKGROUND

In scanned proton beam therapy systematic deviations in spot size at iso-center can occur as a result of changes in the beam-line optics. There is currently no general guideline of the spot size accuracy required clinically. In this work we quantify treatment plan robustness to systematic spot size variations as a function of spot size and spot spacing, and we suggest guidelines for tolerance levels for spot size variations.

METHODS

Through perturbation of spot size in treatment plans for 7 patients and a phantom, we evaluated the dose impact of systematic spot size variations of 5% up to 50%. We investigated the dependence on nominal spot size by studying scenarios with small, medium and large spot sizes for various inter-spot spacings. To come to tolerance levels, we used the Γ passing rate and dose-volume-histograms.

RESULTS

Limits on spot size accuracy were extracted for 8 sites, 3 different spot sizes and 3 different inter-spot spacings. While the allowable spot size variation strongly depends on the spot size, the inter-spot spacing turned out to be only of limited influence.

CONCLUSIONS

Plan robustness to spot size variations strongly depend on spot size, with small spot plans being much more robust than larger spots plans. Inter-spot spacing did not influence plan robustness. Combining our results with existing literature, we propose limits of ±25%, ±20% and ±10% of the spot width σ, for spots with σ of 2.5, 5.0 and 10 mm in proton therapy spot scanning facilities, respectively.

Fludarabine with pharmacokinetically guided IV busulfan is superior to fixed-dose delivery in pretransplant conditioning of AML/MDS patients

We hypothesized that IV Busulfan (Bu) dosing could be safely intensified through pharmacokinetic (PK-) dose guidance to minimize the inter-patient variability in systemic exposure (SE) associated with body-sized dosing, and this should improve outcome of AML/MDS patients undergoing allogeneic stem cell transplantation (allo-HSCT). To test this hypothesis, we treated 218 patients (median age 50.7 years, male/female 50/50%) with fludarabine (Flu) 40 mg/m² once daily ×4, each dose followed by IV Bu, randomized to 130 mg/m² (N=107) or PK-guided to average daily SE, AUC of 6,000 µM-min (N=111), stratified for remission-status, and allo-grafting from HLA-matched donors. Toxicity and graft vs. host disease (GvHD) rates in the groups were similar; the risk of relapse or treatment-related mortality remained higher in the fixed-dose group throughout the 80-month observation period. Further, PK-guidance yielded safer disease-control, leading to improved overall and progression-free survival, most prominently in MDS-patients and in AML-patients not in remission at allo-HSCT. We conclude that AML/MDS patients receiving pretransplant conditioning treatment with our 4-day regimen may benefit significantly from PK-guided Bu-dosing. This could be considered an alternative to fixed dose delivery since it provides the benefit of precise dose delivery to a predetermined SE without increasing risk(s) of serious toxicity and/or GvHD.

Intravenous BU plus Mel: an effective, chemotherapy-only transplant conditioning regimen in patients with ALL

We investigated the administration of i.v. BU combined with melphalan (Mel) in patients with ALL undergoing allogeneic hematopoietic SCT. Forty-seven patients with a median age of 33 years (range 20-61) received a matched sibling (n=27) or matched unrelated donor transplant (n=20) for ALL in first CR (n=26), second CR (n=13), or with more advanced disease (n=8). BU was infused daily for 4 days, either at a fixed dose of 130 mg/m² (5 patients) or using pharmacokinetic (PK) dose adjustment (42 patients), to target an average daily area-under-the-curve (AUC) of 5000 μmol/min, determined by a test dose of i.v. BU at 32 mg/m². This was followed by a rest day, then two daily doses of Mel at 70 mg/m². Stem cells were infused on the following day. The 2-year OS, PFS and non-relapse mortality (NRM) rates were 35% (95% confidence interval (CI), 23-51%), 31% (95% CI, 21-48%) and 37% (95% CI, 23-50%), respectively. Acute NRM at 100 days was favorable at 12% (95% CI, 5-24%); however, the 2-year NRM was significantly higher for patients older than 40 years, 58% vs 20%, mainly due to GVHD.

Firmware lower-level discrimination and compression applied to streaming x-ray photon correlation spectroscopy area-detector data

We present a data acquisition system to perform on-the-fly background subtraction and lower-level discrimination compression of streaming x-ray photon correlation spectroscopy data from a fast charge-coupled device (CCD) area detector. The system is built using a commercial frame grabber with an on-board field-programmable gate array. The system is capable of continuously processing at least 60 CCD frames per second each consisting of 1024 × 1024 16-bit pixels with ≲ 15,000 photon hits per frame at a maximum compression factor of ≈95%.

Final results of a phase I trial with the novel anthracycline derivative RTA 744 in patients with primary brain tumors

2062

Background: RTA 744 is an anthracycline derivative that was shown preclinically to cross the blood-brain barrier, not be a substrate for P-gp or MRP, and improve survival in an orthotopic murine model of glioblastoma. A trial of RTA 744 was conducted in patients with progressive high-grade gliomas. Methods: RTA 744 was administered as a 2-hour intravenous infusion on each of the first 3 days of a 21-day cycle. Five dose levels were tested until an MTD was reached. Pharmacokinetic samples were taken at multiple time points on days 1–5 of cycle 1. Tumor activity was assessed according to the MacDonald criteria. Results: As of December 2006, RTA 744 has been administered to 20 patients at doses ranging from 1.2 to 9.6 mg/m2/day. The MTD was determined as 7.5 mg/m2/day, and the DLTs were neutropenia and thrombocytopenia. No neurotoxicity, cardiotoxicity, alopecia, or drug-related nausea and vomiting were reported. The pharmacokinetic profile indicates dose proportionality, with a mean plasma half-life of 35 hours. Clear evidence of anti-tumor activity has been reported in 7 of 20 patients. A Complete Response was observed in a patient with GBM after seven cycles at 2.4 mg/m2/day, the patient continues greater than 5 months post-RTA744 treatment. Additionally, a Partial Response (81% tumor reduction) has been observed in a patient with AO after two cycles at 7.5 mg/m2/day who continues on treatment at this time. A Minor Response was observed at 2.4 mg/m2/day, and several patients have experienced Stable Disease of several months duration. Two patients with stable disease have shown radiographic evidence of necrosis. Median time to tumor progression was 6 weeks (range 2 to >50). Median number of cycles was 2 (range 1 to 7). The study is now enrolling additional patients at the MTD to further profile the drug’s safety and activity. Additionally, a once-weekly dosing schedule is being tested in a satellite cohort. Conclusions: RTA 744 is well tolerated when administered at doses up to and including 7.5 mg/m2/day for the first three days of a 21-day cycle. Based on clear evidence of drug activity, pivotal trials of RTA 744 in high-grade gliomas are planned as well as trials in patients with brain metastases.

[Table: see text]

Phase I clinical pharmacokinetics of RTA 744: A blood brain barrier penetrating anthracycline active against high-grade glioma

2045

Background: Preclinical studies demonstrated that RTA 744, a 4’-O benzyl anthracycline designed to circumvent P-gp and MRP1-mediated efflux, effectively crosses the BBB, is retained in brain & brain tumor tissue for >24 hrs, and has demonstrated in vivo activity against glioblastoma multiforme (GBM) in an orthotopic model. Methods: We designed a multicenter, phase I dose- escalation study of RTA 744 administered as a short intravenous infusion for 3 consecutive days, every 3 weeks. Patients enrolled in the study were adult patients with recurrent or refractory GBM, anaplastic astrocytoma, or other primary brain tumors. Peripheral blood samples for PK analysis were collected prior to and at selected timepoints from 5 min to 96 hrs after drug administration, and quantified by LC/MS/MS. PK parameters describing RTA 744 disposition were determined by fitting compartmental models to individual patient plasma concentration-time data. Results: Twenty patients have been enrolled at daily doses ranging from 1.2 to 9.6 mg/m2. Mean (range) population terminal half-life was 34.6 (20.5–89.2) hrs, plasma drug clearance was 45.0 (27.4–86.9) L/hr/m2, and Vss was 1942 (684.9–4721.7) L/m2; population PK values reported for doxorubicin are 20–32 hrs, 24–35 L/hr/m2, and 700–1100 L/m2, respectively. Regimen related toxicity has been minimal with the most common adverse event being myelosuppression. Percentage of unchanged parent drug renally eliminated was 2.4% (0.42–6.03%). Several partial responses and one complete response have been noted, even at dose levels below the observed MTD of 7.5 mg/m2/day. Conclusions: These results are similar to what we have observed in our preclinical studies, demonstrating increased lipophillicity and enhanced biodistribution of RTA 744 when compared to doxorubicin. Direct confirmation of drug penetration into the CNS has not been determined in this study, however this is the focus of an ongoing trial in patients with leptomenigeal malignancies.

No significant financial relationships to disclose.

A novel inhibitor of the STAT3 pathway induces apoptosis in malignant glioma cells both in vitro and in vivo

Signal transducer and activator of transcription-3 (STAT3) is constitutively activated in a variety of cancer types, including malignant gliomas. STAT3 is activated by phosphorylation of a tyrosine residue, after which it dimerizes and translocates into the nucleus. There it regulates the expression of several genes responsible for proliferation and survival at the transcriptional level. A selective inhibitor of STAT3 phosphorylation, AG490, has been shown to inhibit growth and induce apoptosis in some cancer cell types. However, although AG490 routinely shows in vitro anticancer activity, it has not consistently demonstrated an in vivo anticancer effect in animal models. Here, we have tested WP1066, a novel inhibitor structurally related to AG490 but significantly more potent and active, against human malignant glioma U87-MG and U373-MG cells in vitro and in vivo. IC(50) values for WP1066 were 5.6 muM in U87-MG cells and 3.7 muM in U373-MG cells, which represents 18-fold and eightfold increases in potency, respectively, over that of AG490. WP1066 activated Bax, suppressed the expression of c-myc, Bcl-X(L) and Mcl-1, and induced apoptosis. Systemic intraperitoneal administration of WP1066 in mice significantly (P<0.001) inhibited the growth of subcutaneous malignant glioma xenografts during the 30-day follow-up period. Immunohistochemical analysis of the excised tumors revealed that phosphorylated STAT3 levels in the WP1066 treatment group remained inhibited at 3 weeks after the final WP1066 injection, whereas tumors from the control group expressed high levels of phosphorylated STAT3. We conclude that WP1066 holds promise as a therapeutic agent against malignant gliomas.

Intravenous (i.v.) busulfan (Bu) plus melphalan (Mel) is a well-tolerated preparative regimen for stem cell transplantation (SCT) in patients (pts) with advanced lymphoid malignancies

6548

Background: High dose chemotherapy and SCT is an accepted treatment option for pts with relapsed lymphoid malignancies. However, relapse remains a significant problem. A double alkylating regimen of Bu and Mel has been suggested as an effective and myeloablative pre-transplant conditioning regimen. Historically, oral Bu was used and the combination resulted in considerable mucositis and VOD. An i.v. formulation of Bu has been developed that has less pharmacokinetic variability. We are investigating i.v. Bu-Mel in pts with lymphoid malignancies undergoing auto- or allo-SCT. Patients and Methods: The conditioning regimen consists of i.v. Bu 130 mg/m2 over 3 hr daily for 4 days, either as a fixed dose per BSA, or to target an average daily AUC of 5,000 μMol-min ± 12% determined by a test dose of i.v. Bu at 32 mg/m2 given 48 hours prior to the high dose regimen. After the 4 daily Bu doses, there is a rest day, followed by 2 daily doses of Mel at 70mg/m2. Stem cells are infused the following day. Dilantin is given for seizure prophylaxis. GVHD prophylaxis is tacrolimus and methotrexate for pts receiving allo-SCT. Results: Patient characteristics, engraftment, and regimen-related toxicities are listed below: Grade I or II mucositis was the most common regimen-related toxicity. There were no cases of VOD; reversible hyperbilirubinemia was observed in 3 pts receiving allo-SCT. No grade IV toxicity was noted, and there were no regimen-related deaths with longest follow-up of 10 months. All allo-SCT pts had 100% donor chimerism by day 30. 27 pts had i.v. Bu delivered per test dose guidance; 6 pts received fixed dose Bu at 130 mg/m2. The median daily systemic Bu exposure was 4867 μMol-min. Conclusion: Intravenous Bu-Mel is well tolerated, and enables prompt neutrophil and platelet engraftment. Individualized PK-directed dosing of i.v. Bu is feasible, and likely contributes to the low toxicity profile of this regimen. Longer time is needed to assess disease control.

[Table: see text]

No significant financial relationships to disclose.

Survival study of RTA 744 (currently a single agent phase I study) alone and in combination with temozolomide in orthotopic model of glioma

1577

Background: Treatment for high-grade gliomas represents an area of significant unmet medical need, largely because few agents cross the blood-brain barrier (BBB). RTA 744744 (WP744, 4’-O-benzyldoxorubicin hydrochloride) is a potent topoisomerase II poison and novel anthracycline screened for its ability to circumvent P-gp and MRP1allowing it to cross the BBB. To assess the drug’s potential in gliomas, RTA 744 was tested alone or in combination with temozolomide in an intracranial xenograft model of glioblastoma multiforme (GBM). It was hypothesized that a DNA repair inhibitor such as RTA 744 would enhance the effects of temozolomide. In vitro cell based assays confirmed this hypothesis and led to in vivo testing of this novel combination. Methods: U87MG cells were implanted into the right basal ganglia of nu/nu mice via an implantable screw system. In all studies, groups of 5 or 6 mice received PBS, temozolomide, RTA 744, or the combination of RTA 744 and temozolomide administered IP. PBS and temozolomide were given on a QDx5 schedule, and several schedules were tested for RTA 744. The primary endpoint in these studies was survival. Results: In the single agent study, by day 33, all 5 of the control animals had died compared with just one of the RTA 744-treated animals. Median survival for control animals was 28 days versus 37 for RTA 744-treated animals (T/C=132%). In the combination study, median survival was 30 days for controls, 40 days for temozolomide alone (T/C=133%), and 48 days for animals receiving the optimal combination of RTA 744 and temozolomide (T/C=160%). One animal in the combination group survived to day 78, whereas no control or Temodar treated animals survived past days 32 and 45, respectively. Conclusions: RTA 744 demonstrated potent single agent activity in a rigorous preclinical model of GBM. The combination of RTA 744 and temozolomide produced survival results significantly better than those from any chemotherapeutics tested in this model. This data supports the current Phase 1 trial of RTA 744 in patients with brain tumors as well as future trials testing the combination of RTA 744 and temozolomide.

[Table: see text]