Pharmacokinetics of Direct Brain Infusion: Distribution of therapeutic agents in the central nervous system (CNS) with currently available delivery techniques is problematic. Systemic delivery is limited by the blood-brain barrier, non-targeted distribution, and systemic toxicity. Diffusion-dependent methods that delivery substances by push- pull catheters, intrathecal injection, mini-osmotic pumps, and drug impregnated polymers, can result in non-targeted distribution and a volume of distribution (Vd) limited by molecular weight and infusate diffusivity. An approach that we and our colleagues developed to overcome the obstacles associated with current CNS drug delivery techniques is convective delivery, which uses bulk flow to enhance distribution. Our studies have demonstrated that convection-enhanced delivery to the brain, spinal cord, and peripheral nerve in large and small animals can be used to distribute macromolecules in a homogenous, targeted, and safe manner with clinically effective Vd. Recent efforts have focused on determining the factors that optimize convection-enhanced delivery into the brain, spinal cord, and peripheral nerve. To further our understanding of the variables which affect convective delivery we are examining the effect of particle size on distribution in brain, spinal cord, and peripheral nerves. High-flow interstitial infusion is being used to deliver various agents, such as immunotoxins, genetic vectors, and chemotherapeutic molecules in the investigation of treatment of various disorders of the central nervous system. Currently, no technique is available to monitor infusions of macromolecular agents. We have developed two new imaging contrast agents for noninvasively monitoring infusion volume of distribution and concentration: 1) iodopanoic acid (telepaque) covalently linked to bovine serum albumin (MW 98 kDa) for CT and (2) gadolinium- DTPA (Magnevist) covalently linked to human serum albumin (MW 77 Kda) for MRI. Neurotoxicity testing of these imaging agents revealed no evidence of neuronal degeneration up to three months after infusion in rat brains. Thus, we have demonstrated the utility of these imaging agents for monitoring the distribution of treatment during infusion of macromolecular drugs. Finally, the successful use of convection to distribute glucocerebrosidase, the defective enzyme in Gaucher?s disease, and neuronal uptake of the recombinant enzyme after distribution was demonstrated in the brains of rats. Clinical investigations are planned.Nitric oxide donors for enhancement of drug delivery across the blood - tumor barrier for treatment of CNS tumors: New approaches to enhance drug delivery across the blood-tumor barrier are being investigated. We have demonsterated that the short-acting nitric oxide (NO) donor Proli/NO selectively opens the blood-tumor barrier in rat brain tumors, but not in normal brain, to various-sized radiotracers (up to 70 kD) and to an adenoviral vector after intracarotid infusion and does so without significant systemic effects. Proli-NO has been evaluated for its ability to selectively increase the permeability of the blood-tumor barrier to molecules of a wide range of molecular weights over a wide range of doses and infusion regimens. Further studies have been focused on 1) the mechanism of selective action of nitric oxide in tumor endothelial cells (the reason for its selectivity for tumor blood vessels) and 2) the determination of the active compound(s) which induce the permeability changes. Our group is also studying the NO effect on other vascular physiology, such as tumor blood flow. Furthermore, we plan to study the pathophysiology of the capillary permeability differences between tumor vasculature and normal brain vasculature as part of an effort to develop approaches that enhance microvessel permeability to increase delivery and therapeutic efficacy of drug therapy. Treatment studies using NO donors in combination with water-soluble drugs and other therapeutic agents, such as viral vectors, in rodents have been completed. The results clearly indicate that this approach might be an effective new way to opening the BBB in patients with brain tumors to enhance delivery of biologically-active macromolecules, including chemotherapuetic agents, genetic vectors etc., to malignant brain tumors for therapy. A clinical study using PET with a small group of patients with malignant brain tumors is planned. It should determine whether the tumor-selective blood-brain barrier opening to tracer molecules which has been seen in the rodent model can be reproduced in humans and provide pharmacokinetic information that can be used to predict its usefulness in patients with tumors. Finally, recent in vitro experiments suggest that NO can directly destroy tumor cells, and increase radiosensitivity of tumor cells, as well as increase the sensitivity of tumor cells to chemotherapeutic agents. These encouraging in vitro results, prompted us to examine, in ongoing studies, intracarotid and local infusion of proliNO in a rat model of intracerebral glioma (9L cells) to assess these hypotheses in vivo.Gene therapy of disorders of the central nervous system: Limited gene transfer into tumor cells occurs with the current approaches for delivery and distribution of genetic vectors into the tumor of patients with brain tumors. We need improved methods of vector delivery and distribution in solid tumors. Therefore, we are currently investigating techniques to enhance the delivery of genetic vectors to the CNS and to CNS tumors via intracarotid infusion after selectively opening the blood-tumor barrier using a short-acting nitric oxide donor. The results indicate that this approach might be an effective and safe new way of opening the blood-brain barrier in patients with brain tumors to enhance delivery and distribution of genetic vectors. In addition, we are studying the distribution of an adenoviral vector in normal brain and tumors with convection-enhanced delivery. We are also developing methods to quantify gene delivery to the brain and to tumors. Using a radiolabeled adenoviral vector, we are studying the distribution of the viral vector in normal brain tissue and tumors using quantitative autoradiography (QAR) and modern image analysis techniques. With this approach, it might be possible to develop a method to monitor the distribution of genetic material in patients with brain tumors using positron-emission tomography (PET). Improving the efficacy of gene therapy for CNS malignancies also requires the design of novel, more effective genetic vectors. In contrast to replication-deficient viral vectors whose distribution is limited to a small diameter around the injection site, replication-competent vectors are more likely to be distributed in a larger portion of the tumor. We are currently investigating the efficacy of replication-permissive adenoviral vector carrying the TK gene which preferably replicates in p53 mutant cells in a nude rat glioma model.
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