Induction of hyperintense signal on T2-weighted MR images correlates with infusion distribution from intracerebral convection-enhanced delivery of a tumor-targeted cytotoxin

Evaluating drug distribution in children and young adults with diffuse midline glioma of the pons (DIPG) treated with convection-enhanced drug delivery

AIMS

To determine an imaging protocol that can be used to assess the distribution of infusate in children with DIPG treated with CED.

METHODS

13 children diagnosed with DIPG received between 3.8 and 5.7 ml of infusate, through two pairs of catheters to encompass tumor volume on day 1 of cycle one of treatment. Volumetric T2-weighted (T2W) and diffusion-weighted MRI imaging (DWI) were performed before and after day 1 of CED. Apparent diffusion coefficient (ADC) maps were calculated. The tumor volume pre and post CED was automatically segmented on T2W and ADC on the basis of signal intensity. The ADC maps pre and post infusion were aligned and subtracted to visualize the infusate distribution.

RESULTS

There was a significant increase (p < 0.001) in mean ADC and T2W signal intensity (SI) ratio and a significant (p < 0.001) increase in mean tumor volume defined by ADC and T2W SI post infusion (mean ADC volume pre: 19.8 ml, post: 24.4 ml; mean T2W volume pre: 19.4 ml, post: 23.4 ml). A significant correlation (p < 0.001) between infusate volume and difference in ADC/T2W SI defined tumor volume was observed (ADC, r = 0.76; T2W, r = 0.70). Finally, pixel-by-pixel subtraction of the ADC maps pre and post infusion demonstrated a volume of high signal intensity, presumed infusate distribution.

CONCLUSIONS

ADC and T2W MRI are proposed as a combined parameter method for evaluation of CED infusate distribution in brainstem tumors in future clinical trials.

Evaluation of a patient-specific algorithm for predicting distribution for convection-enhanced drug delivery into the brainstem of patients with diffuse intrinsic pontine glioma

OBJECTIVE

With increasing use of convection-enhanced delivery (CED) of drugs, the need for software that can predict infusion distribution has grown. In the context of a phase I clinical trial for pediatric diffuse intrinsic pontine glioma (DIPG), CED was used to administer an anti-B7H3 radiolabeled monoclonal antibody, iodine-124-labeled omburtamab. In this study, the authors retrospectively evaluated a software algorithm (iPlan Flow) for the estimation of infusate distribution based on the planned catheter trajectory, infusion parameters, and patient-specific MRI. The actual infusate distribution, as determined on MRI and PET imaging, was compared to the distribution estimated by the software algorithm. Similarity metrics were used to quantify the agreement between predicted and actual distributions.

METHODS

Ten pediatric patients treated at the same dose level in the NCT01502917 trial conducted at Memorial Sloan Kettering Cancer Center were considered for this retrospective analysis. T2-weighted MRI in combination with PET imaging was used to determine the distribution of infusate in this study. The software algorithm was applied for the generation of estimated fluid distribution maps. Similarity measures included object volumes, intersection volume, union volume, Dice coefficient, volume difference, and the center and average surface distances. Acceptable similarity was defined as a simulated distribution volume (Vd Sim) object that had a Dice coefficient higher than or equal to 0.7, a false-negative rate (FNR) lower than 50%, and a positive predictive value (PPV) higher than 50% compared to the actual Vd (Vd PET).

RESULTS

Data for 10 patients with a mean infusion volume of 4.29 ml (range 3.84-4.48 ml) were available for software evaluation. The mean Vd Sim found to be covered by the actual PET distribution (PPV) was 77% ± 8%. The mean percentage of PET volume found to be outside the simulated volume (FNR) was 34% ± 10%. The mean Dice coefficient was 0.7 ± 0.05. In 8 out of 10 patients, the simulation algorithm fulfilled the combined acceptance criteria for similarity.

CONCLUSIONS

iPlan Flow software can be useful to support planning of trajectories that produce intraparenchymal convection. The simulation algorithm is able to model the likely infusate distribution for a CED treatment in DIPG patients. The combination of trajectory planning guidelines and infusion simulation in the software can be used prospectively to optimize personalized CED treatment.

Infusion Mechanisms in Brain White Matter and Their Dependence on Microstructure: An Experimental Study of Hydraulic Permeability

OBJECTIVE

Hydraulic permeability is a topic of deep interest in biological materials because of its important role in a range of drug delivery-based therapies. The strong dependence of permeability on the geometry and topology of pore structure and the lack of detailed knowledge of these parameters in the case of brain tissue makes the study more challenging. Although theoretical models have been developed for hydraulic permeability, there is limited consensus on the validity of existing experimental evidence to complement these models. In the present study, we measure the permeability of white matter (WM) of fresh ovine brain tissue considering the localised heterogeneities in the medium using an infusion-based experimental set up, iPerfusion. We measure the flow across different parts of the WM in response to applied pressures for a sample of specific dimensions and calculate the permeability from directly measured parameters. Furthermore, we directly probe the effect of anisotropy of the tissue on permeability by considering the directionality of tissue on the obtained values. Additionally, we investigate whether WM hydraulic permeability changes with post-mortem time. To our knowledge, this is the first report of experimental measurements of the localised WM permeability, also demonstrating the effect of axon directionality on permeability. This work provides a significant contribution to the successful development of intra-tumoural infusion-based technologies, such as convection-enhanced delivery (CED), which are based on the delivery of drugs directly by injection under positive pressure into the brain.

Repeat convection-enhanced delivery for diffuse intrinsic pontine glioma

OBJECTIVE

While the safety and efficacy of convection-enhanced delivery (CED) have been studied in patients receiving single-dose drug infusions, agents for oncological therapy may require repeated or chronic infusions to maintain therapeutic drug concentrations. Repeat and chronic CED infusions have rarely been described for oncological purposes. Currently available CED devices are not approved for extended indwelling use, and the only potential at this time is for sequential treatments through multiple procedures. The authors report on the safety and experience in a group of pediatric patients who received sequential CED into the brainstem for the treatment of diffuse intrinsic pontine glioma.

METHODS

Patients in this study were enrolled in a phase I single-center clinical trial using 124I-8H9 monoclonal antibody (124I-omburtamab) administered by CED (clinicaltrials.gov identifier NCT01502917). A retrospective chart and imaging review were used to assess demographic data, CED infusion data, and postoperative neurological and surgical outcomes. MRI scans were analyzed using iPlan Flow software for volumetric measurements. Target and catheter coordinates as well as radial, depth, and absolute error in MRI space were calculated with the ClearPoint imaging software.

RESULTS

Seven patients underwent 2 or more sequential CED infusions. No patients experienced Clinical Terminology Criteria for Adverse Events grade 3 or greater deficits. One patient had a persistent grade 2 cranial nerve deficit after a second infusion. No patient experienced hemorrhage or stroke postoperatively. There was a statistically significant decrease in radial error (p = 0.005) and absolute tip error (p = 0.008) for the second infusion compared with the initial infusion. Sequential infusions did not result in significantly different distribution capacities between the first and second infusions (volume of distribution determined by the PET signal/volume of infusion ratio [mean ± SD]: 2.66 ± 0.35 vs 2.42 ± 0.75; p = 0.45).

CONCLUSIONS

This series demonstrates the ability to safely perform sequential CED infusions into the pediatric brainstem. Past treatments did not negatively influence the procedural workflow, technical application of the targeting interface, or distribution capacity. This limited experience provides a foundation for using repeat CED for oncological purposes.

Three-dimensional nonlinear finite element model to estimate backflow during flow-controlled infusions into the brain

Convection-enhanced delivery is a technique to bypass the blood–brain barrier and deliver therapeutic drugs into the brain tissue. However, animal investigations and preliminary clinical trials have reported reduced efficacy to transport the infused drug in specific zones, attributed mainly to backflow, in which an annular gap is formed outside the catheter and the fluid preferentially flows toward the surface of the brain rather than through the tissue in front of the cannula tip. In this study, a three-dimensional human brain finite element model of backflow was developed to study the influence of anatomical structures during flow-controlled infusions. Predictions of backflow length were compared under the influence of ventricular pressure and the distance between the cannula and the ventricles. Simulations with zero relative ventricle pressure displayed similar backflow length predictions for larger cannula-ventricle distances. In addition, infusions near the ventricles revealed smaller backflow length and the liquid was observed to escape to the longitudinal fissure and ventricular cavities. Simulations with larger cannula-ventricle distances and nonzero relative ventricular pressure showed an increase of fluid flow through the tissue and away from the ventricles. These results reveal the importance of considering both the subject-specific anatomical details and the nonlinear effects in models focused on analyzing current and potential treatment options associated with convection-enhanced delivery optimization for future clinical trials.

Longitudinal Monitoring of Gd-DTPA Following Convection Enhanced Delivery in the Brainstem

BACKGROUND

Convection-enhanced delivery (CED) has been introduced into contemporary therapeutic strategies for incurable brain neoplasms as diffuse intrinsic pontine glioma. Therapeutic benefit in part is predictably dependent on drug distribution within tumors. However, therapeutics can rarely be detected through conventional imaging techniques. Coinfusion of the tracer gadolinium-diethylenetriaminepentacetate (Gd-DTPA) has been advocated to monitor drug distributive features including volume, tumor coverage, and efflux during and after administration. The kinetics of Gd-DTPA are unclear as longitudinal magnetic resonance imaging is rarely performed. Understanding these changes would have important implications related to the timing of diagnostic imaging and reliance on tracers as surrogates of pharmacokinetic drug monitoring.

CASE DESCRIPTION

The behavior of Gd-DTPA as a surrogate is presented in a time-dependent fashion as measured by repeated magnetic resonance imaging based on the case of a child with recurrent diffuse intrinsic pontine glioma treated with an oncolytic virus (ICOVIR-5) delivered by CED with coinfused Gd-DTPA (1 mM, for a volume of 2000 μL). Initial Vd/Vi was 1.46. Gd-DTPA was observed up to 18 hours post CED but not thereafter.

CONCLUSIONS

This longitudinal imaging assessment provides a rare opportunity to better characterize the kinetics of surrogate tracers delivered by CED to the brainstem, highlighting the importance of immediate and longitudinal monitoring.

Delivering therapy to target: improving the odds for successful drug development

The direct delivery of drugs and other agents into tissue (in contrast to systemic administration) has been used in clinical trials for brain cancer, neurodegenerative diseases and peripheral tumors. However, continuing evidence suggests that clinical efficacy depends on adequate delivery to a target. Inadequate delivery may have doomed otherwise effective drugs, through failure to distinguish drug inefficacy from poor distribution at the target. Conventional pretreatment clinical images of the patient fail to reveal the complexity and diversity of drug transport pathways in tissue. We discuss the richness of these pathways and argue that development and patient treatment can be sped up and improved by: using quantitative as well as 'real-time' imaging; customized simulations using data from that imaging; and device designs that optimize the drug-device combination.

Imaging of Convective Drug Delivery in the Nervous System

Convection-enhanced delivery permits the direct homogeneous delivery of small- and large-molecular-weight putative therapeutics to the nervous system in a manner that bypasses the blood-nervous system barrier. The development of co-infused surrogate imaging tracers (for computed tomography and MRI) allows for the real-time, noninvasive monitoring of infusate distribution during convective delivery. Real-time image monitoring of convective distribution of therapeutic agents insures that targeted structures/nervous system regions are adequately perfused, enhances safety, informs efficacy (or lack thereof) of putative agents, and provides critical information regarding the properties of convection-enhanced delivery in normal and various pathologic tissue states.

Antibody-drug conjugates in glioblastoma therapy: the right drugs to the right cells

Glioblastomas are high-grade brain tumours with a poor prognosis and, currently, few available therapeutic options. This lack of effective treatments has been linked to diverse factors, including target selection, tumour heterogeneity and poor penetrance of therapeutic agents through the blood-brain barrier and into tumours. Therapies using monoclonal antibodies, alone or linked to cytotoxic payloads, have proved beneficial for patients with different solid tumours; these approaches are currently being explored in patients with glioblastoma. In this Review, we summarise clinical data regarding antibody-drug conjugates (ADCs) against a variety of targets in glioblastoma, and compare the efficacy and toxicity of targeting EGFR with ADCs versus naked antibodies in order to illustrate key aspects of the use of ADCs in this malignancy. Finally, we discuss the complex challenges related to the biology and mutational changes of glioblastoma that can affect the use of ADC-based therapies in patients with this disease, and highlight potential strategies to improve efficacy.

Convection-Enhanced Delivery

Convection-enhanced delivery (CED) is a promising technique that generates a pressure gradient at the tip of an infusion catheter to deliver therapeutics directly through the interstitial spaces of the central nervous system. It addresses and offers solutions to many limitations of conventional techniques, allowing for delivery past the blood-brain barrier in a targeted and safe manner that can achieve therapeutic drug concentrations. CED is a broadly applicable technique that can be used to deliver a variety of therapeutic compounds for a diversity of diseases, including malignant gliomas, Parkinson's disease, and Alzheimer's disease. While a number of technological advances have been made since its development in the early 1990s, clinical trials with CED have been largely unsuccessful, and have illuminated a number of parameters that still need to be addressed for successful clinical application. This review addresses the physical principles behind CED, limitations in the technique, as well as means to overcome these limitations, clinical trials that have been performed, and future developments.

Convection-Enhanced Delivery for Diffuse Intrinsic Pontine Glioma Treatment

Convection-enhanced delivery (CED) is a technique designed to deliver drugs directly into the brain or tumors. Its ability to bypass the blood-brain barrier (BBB), one of the major hurdles in delivering drugs to the brain, has made it a promising drug delivery method for the treatment of primary brain tumors. A number of clinical trials utilizing CED of various therapeutic agents have been conducted to treat patients with supratentorial high-grade gliomas. Significant responses have been observed in certain patients in all of these trials. However, the insufficient ability to monitor drug distribution and pharmacokinetics hampers CED from achieving its potentials on a larger scale. Brainstem CED for diffuse intrinsic pontine glioma (DIPG) treatment is appealing because this tumor is compact and has no definitive treatment. The safety of brainstem CED has been established in small and large animals, and recently in early stage clinical trials. There are a few current clinical trials of brainstem CED in treating DIPG patients using targeted macromolecules such as antibodies and immunotoxins. Future advances for CED in DIPG treatment will come from several directions including: choosing the right agents for infusion; developing better agents and regimen for DIPG infusion; improving instruments and technique for easier and accurate surgical targeting and for allowing multisession or prolonged infusion to implement optimal time sequence; and better understanding and control of drug distribution, clearance and time sequence. CED-based therapies for DIPG will continue to evolve with new understanding of the technique and the disease.

Convection-enhanced delivery in glioblastoma: a review of preclinical and clinical studies

Glioblastoma is the most common malignant brain tumor, and it carries an extremely poor prognosis. Attempts to develop targeted therapies have been hindered because the blood-brain barrier prevents many drugs from reaching tumors cells. Furthermore, systemic toxicity of drugs often limits their therapeutic potential. A number of alternative methods of delivery have been developed, one of which is convection-enhanced delivery (CED), the focus of this review. The authors describe CED as a therapeutic measure and review preclinical studies and the most prominent clinical trials of CED in the treatment of glioblastoma. The utilization of this technique for the delivery of a variety of agents is covered, and its shortcomings and challenges are discussed in detail.

Convection-enhanced delivery to the central nervous system

Convection-enhanced delivery (CED) is a bulk flow–driven process. Its properties permit direct, homogeneous, targeted perfusion of CNS regions with putative therapeutics while bypassing the blood-brain barrier. Development of surrogate imaging tracers that are co-infused during drug delivery now permit accurate, noninvasive real-time tracking of convective infusate flow in nervous system tissues. The potential advantages of CED in the CNS over other currently available drug delivery techniques, including systemic delivery, intrathecal and/or intraventricular distribution, and polymer implantation, have led to its application in research studies and clinical trials. The authors review the biophysical principles of convective flow and the technology, properties, and clinical applications of convective delivery in the CNS.

Convection-enhanced drug delivery for gliomas

In spite of aggressive multi-modality treatments, patients diagnosed with anaplastic astrocytoma and glioblastoma continue to display poor median survival. The success of our current conventional and targeted chemotherapies are largely hindered by systemic- and neurotoxicity, as well as poor central nervous system (CNS) penetration. Interstitial drug administration via convection-enhanced delivery (CED) is an alternative that potentially overcomes systemic toxicities and CNS delivery issues by directly bypassing the blood–brain barrier (BBB). This novel approach not only allows for directed administration, but also allows for newer, tumor-selective agents, which would normally be excluded from the CNS due to molecular size alone. To date, randomized trials of CED therapy have yet to definitely show survival advantage as compared with today's standard of care, however, early studies appear to have been limited by “first generation” delivery techniques. Taking into consideration lessons learned from early trials along with decades of research, newer CED technologies and therapeutic agents are emerging, which are reviewed herein.

Interstitial chemotherapy for malignant glioma: Future prospects in the era of multimodal therapy

The advent of interstitial chemotherapy has significantly increased therapeutic options for patients with malignant glioma. Interstitial chemotherapy can deliver high concentrations of chemotherapeutic agents, directly at the site of the brain tumor while bypassing systemic toxicities. Gliadel, a locally implanted polymer that releases the alkylating agent carmustine, given alone and in combination with various other antitumor and resistance modifying therapies, has significantly increased the median survival for patients with malignant glioma. Convection enhanced delivery, a technique used to directly infuse drugs into brain tissue, has shown promise for the delivery of immunotoxins, monoclonal antibodies, and chemotherapeutic agents. Preclinical studies include delivery of chemotherapeutic and immunomodulating agents by polymer and microchips. Interstitial chemotherapy was shown to maximize local efficacy and is an important strategy for the efficacy of any multimodal approach.

Metalloporphyrins as therapeutic catalytic oxidoreductants in central nervous system disorders

SIGNIFICANCE

Metalloporphyrins, characterized by a redox-active transitional metal (Mn or Fe) coordinated to a cyclic porphyrin core ligand, mitigate oxidative/nitrosative stress in biological systems. Side-chain substitutions tune redox properties of metalloporphyrins to act as potent superoxide dismutase mimics, peroxynitrite decomposition catalysts, and redox regulators of transcription factor function. With oxidative/nitrosative stress central to pathogenesis of CNS injury, metalloporphyrins offer unique pharmacologic activity to improve the course of disease.

RECENT ADVANCES

Metalloporphyrins are efficacious in models of amyotrophic lateral sclerosis, Alzheimer's disease, epilepsy, neuropathic pain, opioid tolerance, Parkinson's disease, spinal cord injury, and stroke and have proved to be useful tools in defining roles of superoxide, nitric oxide, and peroxynitrite in disease progression. The most substantive recent advance has been the synthesis of lipophilic metalloporphyrins offering improved blood-brain barrier penetration to allow intravenous, subcutaneous, or oral treatment.

CRITICAL ISSUES

Insufficient preclinical data have accumulated to enable clinical development of metalloporphyrins for any single indication. An improved definition of mechanisms of action will facilitate preclinical modeling to define and validate optimal dosing strategies to enable appropriate clinical trial design. Due to previous failures of "antioxidants" in clinical trials, with most having markedly less biologic activity and bioavailability than current-generation metalloporphyrins, a stigma against antioxidants has discouraged the development of metalloporphyrins as CNS therapeutics, despite the consistent definition of efficacy in a wide array of CNS disorders.

FUTURE DIRECTIONS

Further definition of the metalloporphyrin mechanism of action, side-by-side comparison with "failed" antioxidants, and intense effort to optimize therapeutic dosing strategies are required to inform and encourage clinical trial design.

A novel implantable catheter system with transcutaneous port for intermittent convection-enhanced delivery of carboplatin for recurrent glioblastoma

CONTEXT

Inadequate penetration of the blood-brain barrier (BBB) by systemically administered chemotherapies including carboplatin is implicated in their failure to improve prognosis for patients with glioblastoma. Convection-enhanced delivery (CED) of carboplatin has the potential to improve outcomes by facilitating bypass of the BBB.

OBJECTIVE

We report the first use of an implantable CED system incorporating a novel transcutaneous bone-anchored port (TBAP) for intermittent CED of carboplatin in a patient with recurrent glioblastoma.

MATERIALS AND METHODS

The CED catheter system was implanted using a robot-assisted surgical method. Catheter targeting accuracy was verified by performing intra-operative O-arm imaging. The TBAP was implanted using a skin-flap dermatome technique modeled on bone-anchored hearing aid surgery. Repeated infusions were performed by attaching a needle administration set to the TBAP. Drug distribution was monitored with serial real-time T2-weighted magnetic resonance imaging (MRI).

RESULTS

All catheters were implanted to within 1.5 mm of their planned target. Intermittent infusions of carboplatin were performed on three consecutive days and repeated after one month without the need for further surgical intervention. Infused volumes of 27.9 ml per day were well tolerated, with the exception of a single seizure episode. Follow-up MRI at eight weeks demonstrated a significant reduction in the volume of tumor enhancement from 42.6 ml to 24.6 ml, and was associated with stability of the patient's clinical condition.

CONCLUSION

Reduction in the volume of tumor enhancement indicates that intermittent CED of carboplatin has the potential to improve outcomes in glioblastoma. The novel technology described in this report make intermittent CED infusion regimes an achievable treatment strategy.

Convection-enhanced drug delivery to the brain: therapeutic potential and neuropathological considerations

Convection-enhanced delivery (CED) describes a direct method of drug delivery to the brain through intraparenchymal microcatheters. By establishing a pressure gradient at the tip of the infusion catheter in order to exploit bulk flow through the interstitial spaces of the brain, CED offers a number of advantages over conventional drug delivery methods-bypass of the blood-brain barrier, targeted distribution through large brain volumes and minimization of systemic side effects. Despite showing early promise, CED is yet to fulfill its potential as a mainstream strategy for the treatment of neurological disease. Substantial research effort has been dedicated to optimize the technology for CED and identify the parameters, which govern successful drug distribution. It seems likely that successful clinical translation of CED will depend on suitable catheter technology being used in combination with drugs with optimal physicochemical characteristics, and on neuropathological analysis in appropriate preclinical models. In this review, we consider the factors most likely to influence the success or failure of CED, and review its application to the treatment of high-grade glioma, Parkinson's disease (PD) and Alzheimer's disease (AD).

Robot-guided convection-enhanced delivery of carboplatin for advanced brainstem glioma

BACKGROUND

Patients with diffuse intrinsic pontine glioma (DIPG) have a poor prognosis with median survival reported as 9 months. The failure of systemic chemotherapy to improve prognosis may be due to inadequate penetration of the blood-brain barrier (BBB). Convection-enhanced delivery (CED) has the potential to improve outcomes by facilitating bypass of the BBB. We describe the first use of carboplatin for the treatment of advanced DIPG using a robot-guided catheter implantation technique.

METHODS

A 5-year-old boy presented with a pontine mass lesion. The tumor continued to progress despite radiotherapy. Using an in-house modification to neuroinspire stereotactic planning software (Renishaw Plc., Gloucestershire, UK), the tumor volume was calculated as 43.6 ml. A transfrontal trajectory for catheter implantation was planned facilitating the in-house manufacture of a recessed-step catheter. The catheter was implanted using a neuromate robot (Renishaw Plc., Gloucestershire, UK). The initial infusion of carboplatin (0.09 mg/ml) was commenced with real-time T2-weighted MRI, facilitating estimation of the volume of infusate distribution. Infusions were repeated on a total of 5 days.

RESULTS

The catheter implantation and infusions were well tolerated. A total volume of 49.8 ml was delivered over 5 days. T2-weighted MRI on completion of the final infusion demonstrated signal change through a total volume of 35.1 ml, representing 95 % of the targeted tumor volume. Follow-up at 4 weeks revealed clinical signs of improvement and increased T2 signal change throughout the volume of distribution. However, there was tumor progression in the regions outside the volume of distribution.

CONCLUSIONS

This case demonstrates the feasibility of accurately and safely delivering small-diameter catheters to the brainstem using a robot-guided implantation procedure, and real-time MRI tracking of infusate distribution.

Automated segmentation tool for brain infusions

This study presents a computational tool for auto-segmenting the distribution of brain infusions observed by magnetic resonance imaging. Clinical usage of direct infusion is increasing as physicians recognize the need to attain high drug concentrations in the target structure with minimal off-target exposure. By co-infusing a Gadolinium-based contrast agent and visualizing the distribution using real-time using magnetic resonance imaging, physicians can make informed decisions about when to stop or adjust the infusion. However, manual segmentation of the images is tedious and affected by subjective preferences for window levels, image interpolation and personal biases about where to delineate the edge of the sloped shoulder of the infusion. This study presents a computational technique that uses a Gaussian Mixture Model to efficiently classify pixels as belonging to either the high-intensity infusate or low-intensity background. The algorithm was implemented as a distributable plug-in for the widely used imaging platform OsiriX®. Four independent operators segmented fourteen anonymized datasets to validate the tool’s performance. The datasets were intra-operative magnetic resonance images of infusions into the thalamus or putamen of non-human primates. The tool effectively reproduced the manual segmentation volumes, while significantly reducing intra-operator variability by 67±18%. The tool will be used to increase efficiency and reduce variability in upcoming clinical trials in neuro-oncology and gene therapy.

Convection enhanced delivery of macromolecules for brain tumors

The blood brain barrier (BBB) poses a significant challenge for drug delivery of macromolecules into the brain. Convection-enhanced delivery (CED) circumvents the BBB through direct intracerebral infusion using a hydrostatic pressure gradient to transfer therapeutic compounds. The efficacy of CED is dependent on the distribution of the therapeutic agent to the targeted region. Here we present a review of convection enhanced delivery of macromolecules, emphasizing the role of tracers in enabling effective delivery anddiscuss current challenges in the field.

Diphtheria toxin-based targeted toxin therapy for brain tumors

Targeted toxins (TT) are molecules that bind cell surface antigens or receptors such as the transferrin or interleukin-13 receptor that are overexpressed in cancer. After internalization, the toxin component kills the cell. These recombinant proteins consist of an antibody or carrier ligand coupled to a modified plant or bacterial toxin such as diphtheria toxin (DT). These fusion proteins are very effective against brain cancer cells that are resistant to radiation therapy and chemotherapy. TT have shown an acceptable profile for toxicity and safety in animal studies and early clinical trials have demonstrated a therapeutic response. This review summarizes the characteristics of DT-based TT, the animal studies in malignant brain tumors and early clinical trial results. Obstacles to the successful treatment of brain tumors include poor penetration into tumor, the immune response to DT and cancer heterogeneity.

Targeted toxins for glioblastoma multiforme: pre-clinical studies and clinical implementation

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults. GBM is very aggressive due to its poor cellular differentiation and invasiveness, which makes complete surgical resection virtually impossible. Therefore, GBM's invasive nature as well as its intrinsic resistance to current treatment modalities makes it a unique therapeutic challenge. Extensive examination of human GBM specimens has uncovered that these tumors overexpress a variety of receptors that are virtually absent in the surrounding non-neoplastic brain. Human GBMs overexpress receptors for cytokines, growth factors, ephrins, urokinase-type plasminogen activator (uPA), and transferrin, which can be targeted with high specificity by linking their ligands with highly cytotoxic molecules, such as Diptheria toxin and Pseudomonas exotoxin A. We review the preclinical development and clinical translation of targeted toxins for GBM. In view of the clinical experience, we conclude that although these are very promising therapeutic modalities for GBM patients, efforts should be focused on improving the delivery systems utilized in order to achieve better distribution of the immuno-toxins in the tumor/resection cavity. Delivery of targeted toxins using viral vectors would also benefit enormously from improved strategies for local delivery.

A novel ligand delivery system to non-invasively visualize and therapeutically exploit the IL13Rα2 tumor-restricted biomarker

Our objective was to exploit a novel ligand-based delivery system for targeting diagnostic and therapeutic agents to cancers that express interleukin 13 receptor alpha 2 (IL13Rα2), a tumor-restricted plasma membrane receptor overexpressed in glioblastoma multiforme (GBM), meningiomas, peripheral nerve sheath tumors, and other peripheral tumors. On the basis of our prior work, we designed a novel IL13Rα2-targeted quadruple mutant of IL13 (TQM13) to selectively bind the tumor-restricted IL13Rα2 with high affinity but not significantly interact with the physiologically abundant IL13Rα1/IL4Rα heterodimer that is also expressed in normal brain. We then assessed the in vitro binding profile of TQM13 and its potential to deliver diagnostic and therapeutic radioactivity in vivo. Surface plasmon resonance (SPR; Biacore) binding experiments demonstrated that TQM13 bound strongly to recombinant IL13Rα2 (Kd∼5 nM). In addition, radiolabeled TQM13 specifically bound IL13Rα2-expressing GBM cells and specimens but not normal brain. Of importance, TQM13 did not functionally activate IL13Rα1/IL4Rα in cells or bind to it in SPR binding assays, in contrast to wtIL13. Furthermore, in vivo targeting of systemically delivered radiolabeled TQM13 to IL13Rα2-expressing subcutaneous tumors was demonstrated and confirmed non-invasively for the first time with 124I-TQM13 positron emission tomography imaging. In addition, 131I-TQM13 demonstrated in vivo efficacy against subcutaneous IL13Rα2-expressing GBM tumors and in an orthotopic synergeic IL13Rα2-positive murine glioma model, as evidenced by statistically significant survival advantage. Our results demonstrate that we have successfully generated an optimized biomarker-targeted scaffolding that exhibited specific binding activity toward the tumor-associated IL13Rα2 in vitro and potential to deliver diagnostic and therapeutic payloads in vivo.

Evaluation of pressure-driven brain infusions in nonhuman primates by intra-operative 7 Tesla MRI

PURPOSE

To characterize the effects of pressure-driven brain infusions using high field intra-operative MRI. Understanding these effects is critical for upcoming neurodegeneration and oncology trials using convection-enhanced delivery (CED) to achieve large drug distributions with minimal off-target exposure.

MATERIALS AND METHODS

High-resolution T2-weighted and diffusion-tensor images were acquired serially on a 7 Tesla MRI scanner during six CED infusions in nonhuman primates. The images were used to evaluate the size, distribution, diffusivity, and temporal dynamics of the infusions.

RESULTS

The infusion distribution had high contrast in the T2-weighted images. Diffusion tensor images showed the infusion increased diffusivity, reduced tortuosity, and reduced anisotropy. These results suggested CED caused an increase in the extracellular space.

CONCLUSION

High-field intra-operative MRI can be used to monitor the distribution of infusate and changes in the geometry of the brain's porous matrix. These techniques could be used to optimize the effectiveness of pressure-driven drug delivery to the brain.

Colocalization of gadolinium-diethylene triamine pentaacetic acid with high-molecular-weight molecules after intracerebral convection-enhanced delivery in humans

BACKGROUND

Convection-enhanced delivery (CED) permits site-specific therapeutic drug delivery within interstitial spaces at increased dosages through circumvention of the blood-brain barrier. CED is currently limited by suboptimal methodologies for monitoring the delivery of therapeutic agents that would permit technical optimization and enhanced therapeutic efficacy.

OBJECTIVE

To determine whether a readily available small-molecule MRI contrast agent, gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA), could effectively track the distribution of larger therapeutic agents.

METHODS

Gd-DTPA was coinfused with the larger molecular tracer, I-labeled human serum albumin (I-HSA), during CED of an EGFRvIII-specific immunotoxin as part of treatment for a patient with glioblastoma.

RESULTS

Infusion of both tracers was safe in this patient. Analysis of both Gd-DTPA and I-HSA during and after infusion revealed a high degree of anatomical and volumetric overlap.

CONCLUSION

Gd-DTPA may be able to accurately demonstrate the anatomic and volumetric distribution of large molecules used for antitumor therapy with high resolution and in combination with fluid-attenuated inversion recovery (FLAIR) imaging, and provide additional information about leaks into cerebrospinal fluid spaces and resection cavities. Similar studies should be performed in additional patients to validate our findings and help refine the methodologies we used.

IL-13Rα2-Targeted Therapy Escapees: Biologic and Therapeutic Implications

Glioblastoma multiforme (GBM) overexpresses interleukin 13 receptor α2 (IL-13Rα2), a tumor-restricted receptor that is not present in normal brain. We and others have created targeted therapies that specifically eradicate tumors expressing this promising tumor-restricted biomarker. As these therapies head toward clinical implementation, it is critical to explore mechanisms of potential resistance. We therefore used a potent IL-13Rα2-targeted bacterial cytotoxin to select for naturally occurring "escapee" cells from three different IL-13Rα2-expressing GBM cell lines. We found that these side populations of escapee cells had significantly decreased IL-13Rα2 expression. We examined clinically relevant biologic characteristics of escapee cell lines compared to their parental cell lines and found that they had similar proliferation rates and equal sensitivity to temozolomide and radiation, the standard therapies given to GBM patients. In contrast, our escapee cell lines were less likely to form colonies in culture and migrated more slowly in wound healing assays. Furthermore, we found that escapee cells formed significantly less neurospheres in vitro, suggesting that IL-13Rα2-targeted therapy preferentially targeted the "stem-like" cell population and possibly indicating decreased tumorigenicity in vivo. We therefore tested escapee cells for in vivo tumorigenicity and found that they were significantly less tumorigenic in both subcutaneous and intracranial mouse models compared to matching parental cells. These data, for the first time, establish and characterize the clinically relevant biologic properties of IL-13Rα2-targeted therapy escapees and suggest that these cells may have less malignant characteristics than parental tumors.

A phase I trial of carboplatin administered by convection-enhanced delivery to patients with recurrent/progressive glioblastoma multiforme

Glioblastoma multiforme (GBM) is the commonest primary malignant brain tumour in adults. Standard treatment comprises surgery, radiotherapy and chemotherapy; however this condition remains incurable as these tumours are highly invasive and involve critical areas of the brain making it impossible to remove them surgically or cure them with radiotherapy. In the majority of cases the tumour recurs within 2 to 3 cm of the original site of tumour resection. Furthermore, the blood-brain barrier profoundly limits the access of many systemically administered chemotherapeutics to the tumour. Convection-enhanced delivery (CED) is a promising technique of direct intracranial drug delivery involving the implantation of microcatheters into the brain. Carboplatin represents an ideal chemotherapy to administer using this technique as glioblastoma cells are highly sensitive to carboplatin in vitro at concentrations that are not toxic to normal brain in vivo. This protocol describes a single-centre phase I dose-escalation study of carboplatin administered by CED to patients with recurrent or progressive GBM despite full standard treatment. This trial will incorporate 6 cohorts of 3 patients each. Cohorts will be treated in a sequential manner with increasing doses of carboplatin, subject to dose-limiting toxicity not being observed. This protocol should facilitate the identification of the maximum-tolerated infused concentration of carboplatin by CED into the supratentorial brain. This should facilitate the safe application of this technique in a phase II trial, treating patients with GBM, as well as for the treatment of other forms of malignant brain tumours, including metastases.

Implications of transvascular fluid exchange in nonlinear, biphasic analyses of flow-controlled infusion in brain

A nonlinear, coupled biphasic-mass transport model that includes transvascular fluid exchange is proposed for flow-controlled infusions in brain tissue. The model accounts for geometric and material nonlinearities, a hydraulic conductivity dependent on deformation, and transvascular fluid exchange according to Starling's law. The governing equations were implemented in a custom-written code assuming spherical symmetry and using an updated Lagrangian finite-element algorithm. Results of the model indicate that, using normal physiological values of vascular permeability, transvascular fluid exchange has negligible effects on tissue deformation, fluid pressure, and transport of the infused agent. As vascular permeability may be increased artificially through methods such as administering nitric oxide, a parametric study was conducted to determine how increased vascular permeability affects flow-controlled infusion. Increased vascular permeability reduced both tissue deformation and fluid pressure, possibly reducing damage to tissue adjacent to the infusion catheter. Furthermore, the loss of fluid to the vasculature resulted in a significantly increased interstitial fluid concentration but a modestly increased tissue concentration. From a clinical point of view, this increase in concentration could be beneficial if limited to levels below which toxicity would not occur. However, the modestly increased tissue concentration may make the increase in interstitial fluid concentration difficult to assess in vivo using co-infused radiolabeled agents.

Tracking accuracy of T2- and diffusion-weighted magnetic resonance imaging for infusate distribution by convection-enhanced delivery

OBJECT

Because convection-enhanced delivery relies on bulk flow of fluid in the interstitial spaces, MR imaging techniques that detect extracellular fluid and fluid movement may be useful for tracking convective drug distribution. To determine the tracking accuracy of T2-weighted and diffusion-weighted MR imaging sequences, the authors followed convective distribution of radiolabeled compounds using these imaging sequences in nonhuman primates.

METHODS

Three nonhuman primates underwent thalamic convective infusions (5 infusions) with (14)C-sucrose (MW 342 D) or (14)C-dextran (MW 70,000 D) during serial MR imaging (T2- and diffusion-weighted imaging). Imaging, histological, and autoradiographic findings were analyzed.

RESULTS

Real-time T2- and diffusion-weighted imaging clearly demonstrated the region of infusion, and serial images revealed progressive filling of the bilateral thalami during infusion. Imaging analysis for T2- and diffusion-weighted sequences revealed that the tissue volume of distribution (Vd) increased linearly with volume of infusion (Vi; R(2) = 0.94, R(2) = 0.91). Magnetic resonance imaging analysis demonstrated that the mean ± SD Vd/Vi ratios for T2-weighted (3.6 ± 0.5) and diffusion-weighted (3.3 ± 0.4) imaging were similar (p = 0.5). While (14)C-sucrose and (14)C-dextran were homogeneously distributed over the infused region, autoradiographic analysis revealed that T2-weighted and diffusion-weighted imaging significantly underestimated the Vd of both (14)C-sucrose (mean differences 51.3% and 52.3%, respectively; p = 0.02) and (14)C-dextran (mean differences 49.3% and 59.6%; respectively, p = 0.001).

CONCLUSIONS

Real-time T2- and diffusion-weighted MR imaging significantly underestimate tissue Vd during convection-enhanced delivery over a wide range of molecular sizes. Application of these imaging modalities may lead to inaccurate estimation of convective drug distribution.

Novel platform for MRI-guided convection-enhanced delivery of therapeutics: preclinical validation in nonhuman primate brain

BACKGROUND/AIMS

A skull-mounted aiming device and integrated software platform has been developed for MRI-guided neurological interventions. In anticipation of upcoming gene therapy clinical trials, we adapted this device for real-time convection-enhanced delivery of therapeutics via a custom-designed infusion cannula. The targeting accuracy of this delivery system and the performance of the infusion cannula were validated in nonhuman primates.

METHODS

Infusions of gadoteridol were delivered to multiple brain targets and the targeting error was determined for each cannula placement. Cannula performance was assessed by analyzing gadoteridol distributions and by histological analysis of tissue damage.

RESULTS

The average targeting error for all targets (n = 11) was 0.8 mm (95% CI = 0.14). For clinically relevant volumes, the distribution volume of gadoteridol increased as a linear function (R(2) = 0.97) of the infusion volume (average slope = 3.30, 95% CI = 0.2). No infusions in any target produced occlusion, cannula reflux or leakage from adjacent tracts, and no signs of unexpected tissue damage were observed.

CONCLUSIONS

This integrated delivery platform allows real-time convection-enhanced delivery to be performed with a high level of precision, predictability and safety. This approach may improve the success rate for clinical trials involving intracerebral drug delivery by direct infusion.

Imaging of convection enhanced delivery of toxins in humans

Drug delivery of immunotoxins to brain tumors circumventing the blood brain barrier is a significant challenge. Convection-enhanced delivery (CED) circumvents the blood brain barrier through direct intracerebral application using a hydrostatic pressure gradient to percolate therapeutic compounds throughout the interstitial spaces of infiltrated brain and tumors. The efficacy of CED is determined through the distribution of the therapeutic agent to the targeted region. The vast majority of patients fail to receive a significant amount of coverage of the area at risk for tumor recurrence. Understanding this challenge, it is surprising that so little work has been done to monitor the delivery of therapeutic agents using this novel approach. Here we present a review of imaging in convection enhanced delivery monitoring of toxins in humans, and discuss future challenges in the field.

Interventional MRI-guided putaminal delivery of AAV2-GDNF for a planned clinical trial in Parkinson's disease

Clinical trials involving direct infusion of neurotrophic therapies for Parkinson's disease (PD) have suffered from poor coverage of the putamen. The planned use of a novel interventional-magnetic resonance imaging (iMRI) targeting system for achieving precise, real-time convection-enhanced delivery in a planned clinical trial of adeno-associated virus serotype 2 (AAV2)-glial-derived neurotrophic factor (GDNF) in PD patients was modeled in nonhuman primates (NHP). NHP received bilateral coinfusions of gadoteridol (Gd)/AAV2-GDNF into two sites in each putamen, and three NHP received larger infusion volumes in the thalamus. The average targeting error for cannula tip placement in the putamen was <1 mm, and adjacent putamenal infusions were distributed in a uniform manner. GDNF expression patterns in the putamen were highly correlated with areas of Gd distribution seen on MRI. The distribution volume to infusion volume ratio in the putamen was similar to that in the thalamus, where larger infusions were achieved. Modeling the placement of adjacent 150 and 300 µl thalamic infusions into the three-dimensional space of the human putamen demonstrated coverage of the postcommissural putamen, containment within the striatum and expected anterograde transport to globus pallidus and substantia nigra pars reticulata. The results elucidate the necessary parameters for achieving widespread GDNF expression in the putamenal motor area and afferent substantia nigra of PD patients.

Applications of nanotechnology to imaging and therapy of brain tumors

In the past decade, numerous advances in the understanding of brain tumor physiology, tumor imaging, and tumor therapy have been attained. In some cases, these advances have resulted from refinements of pre-existing technologies (eg, improvements of contrast-enhanced magnetic resonance imaging). In other instances, advances have resulted from development of novel technologies. The development of nanomedicine (ie, applications of nanotechnology to the field of medicine) is an example of the latter. In this review, the authors explain the principles that underlay nanoparticle design and function as well as the means by which nanoparticles can be used for imaging and therapy of brain tumors.

A nonlinear biphasic model of flow-controlled infusions in brain: mass transport analyses

A biphasic nonlinear mathematical model is proposed for the mass transport that occurs during constant flow-rate infusions into brain tissue. The model takes into account geometric and material nonlinearities and a hydraulic conductivity dependent upon strain. The biphasic and convective-diffusive transport equations were implemented in a custom-written code assuming spherical symmetry and using an updated Lagrangian finite element algorithm. Results of the model indicate that the inclusion of these nonlinearities produced modest changes in the interstitial concentration but important variations in drug penetration and bulk concentration. Increased penetration of the drug but smaller bulk concentrations were obtained at smaller strains caused by combination of parameters such as increased Young's modulus and initial hydraulic conductivity. This indicates that simulations of constant flow-rate infusions under the assumption of infinitesimal deformations or rigidity of the tissue may yield lower bulk concentrations near the infusion cavity and over-predictions of the penetration of the infused agent. The analyses also showed that decrease in the infusion flow rate of a fixed amount of drug results in increased penetration of the infused agent. From the clinical point-of-view, this may promote a safer infusion that delivers the therapeutic range over the desired volume while avoiding damage to the tissue by minimizing deformation and strain.

Poor drug distribution as a possible explanation for the results of the PRECISE trial

Object

Convection-enhanced delivery (CED) is a novel intracerebral drug delivery technique with considerable promise for delivering therapeutic agents throughout the CNS. Despite this promise, Phase III clinical trials employing CED have failed to meet clinical end points. Although this may be due to inactive agents or a failure to rigorously validate drug targets, the authors have previously demonstrated that catheter positioning plays a major role in drug distribution using this technique. The purpose of the present work was to retrospectively analyze the expected drug distribution based on catheter positioning data available from the CED arm of the PRECISE trial.

Methods

Data on catheter positioning from all patients randomized to the CED arm of the PRECISE trial were available for analyses. BrainLAB iPlan Flow software was used to estimate the expected drug distribution.

Results

Only 49.8% of catheters met all positioning criteria. Still, catheter positioning score (hazard ratio 0.93, p = 0.043) and the number of optimally positioned catheters (hazard ratio 0.72, p = 0.038) had a significant effect on progression-free survival. Estimated coverage of relevant target volumes was low, however, with only 20.1% of the 2-cm penumbra surrounding the resection cavity covered on average. Although tumor location and resection cavity volume had no effect on coverage volume, estimations of drug delivery to relevant target volumes did correlate well with catheter score (p < 0.003), and optimally positioned catheters had larger coverage volumes (p < 0.002). Only overall survival (p = 0.006) was higher for investigators considered experienced after adjusting for patient age and Karnofsky Performance Scale score.

Conclusions

The potential efficacy of drugs delivered by CED may be severely constrained by ineffective delivery in many patients. Routine use of software algorithms and alternative catheter designs and infusion parameters may improve the efficacy of drugs delivered by CED.

Gene therapy for late infantile neuronal ceroid lipofuscinosis: neurosurgical considerations

OBJECT

The authors conducted a phase I study of late infantile neuronal ceroid lipofuscinosis using an adenoassociated virus serotype 2 (AAV2) vector containing the deficient CLN2 gene (AAV2(CU)hCLN2). The operative technique, radiographic changes, and surgical complications are presented.

METHODS

Ten patients with late infantile neuronal ceroid lipofuscinosis disease each underwent infusion of AAV2(CU)hCLN2 (3 x 10(12) particle units) into 12 distinct cerebral locations (2 depths/bur hole, 75 minutes/infusion, and 2 microl/minute). Innovative surgical techniques were developed to overcome several obstacles for which little or no established techniques were available. Successful infusion relied on preoperative stereotactic planning to optimize a parenchymal target and diffuse administration. Six entry sites, each having 2 depths of injections, were used to reduce operative time and enhance distribution. A low-profile rigid fixation system with 6 integrated holding arms was utilized to perform simultaneous infusions within a practical time frame. Dural sealant with generous irrigation was used to avoid CSF egress with possible subdural hemorrhage or altered stereotactic registration.

RESULTS

Radiographically demonstrated changes were seen in 39 (65%) of 60 injection sites, confirming localization and infusion. There were no radiographically or clinically defined complications.

CONCLUSIONS

The neurosurgical considerations and results of this study are presented to offer guidance and a basis for the design of future gene therapy or other clinical trials in children that utilize direct therapeutic delivery.

Effect of imaging and catheter characteristics on clinical outcome for patients in the PRECISE study

The PRECISE study used convection enhanced delivery (CED) to infuse IL13-PE38QQR in patients with recurrent glioblastoma multiforme (GBM) and compared survival to Gliadel Wafers (GW). The objectives of this retrospective evaluation were to assess: (1) catheter positioning in relation to imaging features and (2) to examine the potential impact of catheter positioning, overall catheter placement and imaging features on long term clinical outcome in the PRECISE study. Catheter positioning and overall catheter placement were scored and used as a surrogate of adequate placement. Imaging studies obtained on day 43 and day 71 after resection were each retrospectively reviewed. Catheter positioning scores, catheter overall placement scores, local tumor control and imaging change scores were reviewed and correlated using Generalized Linear Mixed Models. Cox PH regression analysis was used to examine whether these imaging based variables predicted overall survival (OS) and progression free survival (PFS) after adjusting for age and KPS. Of 180 patients in the CED group, 20 patients did not undergo gross total resection. Of the remaining 160 patients only 53% of patients had fully conforming catheters in respect to overall placement and 51% had adequate catheter positioning scores. Better catheter positioning scores were not correlated with local tumor control (P = 0.61) or imaging change score (P = 0.86). OS and PFS were not correlated with catheter positioning score (OS: P = 0.53; PFS: P = 0.72 respectively), overall placement score (OS: P = 0.55; PFS: P = 0.35) or imaging changes on day 43 MRI (P = 0.88). Catheter positioning scores and overall catheter placement scores were not associated with clinical outcome in this large prospective trial.

Future of convection-enhanced delivery in the treatment of brain tumors

Gliomas are one of the most lethal forms of cancer. The poor prognosis associated with these malignant primary brain tumors treated with surgery, radiotherapy and chemotherapy has led researchers to develop new strategies for cure. Interstitial drug delivery has been the most appealing method for the treatment of primary brain tumors because it provides the most direct method of overcoming the barriers to tumor drug delivery. By administering therapeutic agents directly to the brain interstitium and, more specifically, to tumor-infiltrated parenchyma, one can overcome the elevated interstitial pressure produced by brain tumors. Convection-enhanced delivery (CED) has emerged as a leading investigational delivery technique for the treatment of brain tumors. Clinical trials utilizing these methods have been completed, with mixed results, and several more are being initiated. However, the potential efficacy of these drugs may be limited by ineffective tissue distribution. The development of computer models/algorithms to predict drug distribution, new catheter designs, and utilization of tracer models and nanocarriers have all laid the groundwork for the advancement of CED. In this review, we summarize the recent past of the clinical trials utilizing CED and discuss emerging technologies that will shape future CED trials.

Long-term safety of combined intracerebral delivery of free gadolinium and targeted chemotherapeutic agent PRX321

OBJECTIVES

While convection enhanced delivery (CED) is an effective delivery method that bypasses the blood-brain barrier, its utility is limited by infusate leakage due to catheter misplacement. Therefore, it is critical to evaluate drug distribution during CED infusion. Gadolinium conjugated to diethylenetriamine penta-acetic acid (Gd-DTPA) is a common, readily available MRI contrast agent, which may be able to predict and actively monitor drug distribution. In this study, we assess the long-term safety and toxicity of intracerebrally infused Gd-DTPA along with an experimental targeted agent PRX321.

METHODS

Fifty-four immunocompetent rats were implanted with intracerebral cannulas linked to subcutaneously placed osmotic pumps. After pump implantation, the rats were randomized into six groups of nine rats each in order to assess the toxicities of six different concentrations of human serum albumin (HSA) with and without Gd-DTPA and PRX321. The rats were monitored clinically for 6 weeks before they were autopsied and assessed for histological toxicity to their central nervous system (CNS).

RESULTS

There was one unexplained death in a group infusing low concentration HSA, Gd-DTPA and PRX321. Upon microscopic examination of the CNS in that animal, no unexpected histological toxicity was found. Additionally, there were no signs of clinical or histological toxicity in any of the remaining rats, which all survived until the end of the 6 week observation period.

DISCUSSION

Free Gd-DTPA can be safely infused via CED in a pre-clinical animal model. Future studies should include its use in predicting and actively monitoring CED drug infusions in early phase human clinical trials.

Convection-enhanced delivery of free gadolinium with the recombinant immunotoxin MR1-1

A major obstacle in glioblastoma (GBM) therapy is the restrictive nature of the blood-brain barrier (BBB). Convection-enhanced delivery (CED) is a novel method of drug administration which allows direct parenchymal infusion of therapeutics, bypassing the BBB. MR1-1 is a novel recombinant immunotoxin that targets the GBM tumor-specific antigen EGFRvIII and can be delivered via CED infusion. However, drug distribution via CED varies dramatically, which necessitates active monitoring. Gadolinium conjugated to diethylenetriamine penta-acetic acid (Gd-DTPA) is a commonly used MRI contrast agent which can be co-infused with therapies using CED and may be useful in monitoring infusion leak and early distribution. Forty immunocompetent rats were implanted with intracerebral cannulas that were connected to osmotic pumps and subsequently randomized into four groups that each received 0.2% human serum albumin (HSA) mixed with a different experimental infusion: (1) 25 ng/ml MR1-1; (2) 0.1 micromol/ml Gd-DTPA; (3) 25 ng/ml MR1-1 and 0.1 micromol/ml Gd-DTPA; (4) 250 ng/ml MR1-1 and 0.1 micromol/ml Gd-DTPA. The rats were monitored clinically for 6 weeks then necropsied and histologically assessed for CNS toxicity. All rats survived the entirety of the study without clinical or histological toxicity attributable to the study drugs. There was no statistically significant difference in weight change over time among groups (P > 0.999). MR1-1 co-infused with Gd-DTPA via CED is safe in the long-term setting in a pre-clinical animal model. Our data supports the use of Gd-DTPA, as a surrogate tracer, co-infused with MR1-1 for drug distribution monitoring in patients with GBM.

A nonlinear biphasic model of flow-controlled infusion in brain: fluid transport and tissue deformation analyses

A biphasic nonlinear mathematical model is proposed for the concomitant fluid transport and tissue deformation that occurs during constant flow rate infusions into brain tissue. The model takes into account material and geometrical nonlinearities, a hydraulic conductivity dependent on strain, and nonlinear boundary conditions at the infusion cavity. The biphasic equations were implemented in a custom written code assuming spherical symmetry and using an updated Lagrangian finite element algorithm. Results of the model showed that both, geometric and material nonlinearities play an important role in the physics of infusions, yielding important differences from infinitesimal analyses. Geometrical nonlinearities were mainly due to the significant enlargement of the infusion cavity, while variations of the parameters that describe the degree of nonlinearity of the stress-strain curve yielded significant differences in all distributions. For example, a parameter set showing stiffening under tension yielded maximum values of radial displacement and porosity not localized at the infusion cavity. On the other hand, a parameter set showing softening under tension yielded a slight decrease in the fluid velocity for a three-fold increase in the flow rate, which can be explained by the substantial increase of the infusion cavity, not considered in linear analyses. This study strongly suggests that significant enlargement of the infusion cavity is a real phenomenon during infusions that may produce collateral damage to brain tissue. Our results indicate that more experimental tests have to be undertaken in order to determine material nonlinearities of brain tissue over a range of strains. With better understanding of these nonlinear effects, clinicians may be able to develop protocols that can minimize the damage to surrounding tissue.