Brain Interstitium Temozolomide Concentration Pre and Post Regadenoson Administration

Blood brain barrier:

The blood-brain barrier (BBB) is an intricate barricade composed of three distinct cell types: brain endothelial cells, pericytes, and astrocyte foot processes. While molecules that are small and lipophilic may easily traverse the BBB, large (> 180 Daltons) and/or hydrophilic particles require active transport, or receptor mediation. Specifically, the BBB integrity and degree of permeability is regulated by the capillary endothelial cells in response to astrocytic signals. The strength of intercellular junctions (ex. tight junctions, adherins) amongst endothelial cells also plays a major role in permeability. Therefore, modulation of all these paracellular properties may serve a large part in decreasing BBB integrity and thus improving drug penetration to the tumor bed.

Rationale for Regadenoson:

Adenosine may be a plausible means to transiently disrupt the BBB. Adenosine function is regulated by its four structurally related G-protein coupled receptors: A1, A2A, A2B and A3. Specifically, A1 and A2A have high expression levels within the brain. In 2011, Carman et al demonstrated, with the selective A2A receptor agonist, Regadenoson, increased BBB permeability to 70kD Dextran in both mice and rat brains. The large molecule of Dextran was seen in the brain for up to 180 minutes following a single injection in both mice and rats. Studies performed at Johns Hopkins have also demonstrated a transient disruption of the BBB from Regadenoson in rodents by measuring temozolomide concentrations. The investigators found that brain/plasma concentrations in rats post Regadenoson resulted in a 1.6 fold increase of temozolomide compared to temozolomide given alone in normal rat brains. Future animal studies will evaluate varied chemotherapy agents within the brain and brain tumor with and without Regadenoson.

Currently, Regadenoson is approved by the US Food and Drug Administration as a coronary vasodilator, for myocardial perfusion imaging in patients with suspected myocardial coronary artery disease. Single-photon emission computed tomography (SPECT) imaging performed with 99mTc-sestamibi radiotracer is used to measure coronary blood flow both at rest and stress conditions with Regadenoson. While the adverse event profile is similar to adenosine, the most common side effects of Regadenoson are flushing, chest pain, dyspnea and headaches.

Intracerebral Microdialysis:

Plasma drug concentration measurements are an unreliable method to assess delivery of drugs across the BBB. In contrast, intracerebral microdialysis monitoring allows for approximate measurements within extracellular fluid (ECF) sampling of the brain. Microdialysis placement within the brain is not a novel technique and has been utilized routinely in the intensive care unit setting to measure brain metabolism by sampling of ECF of traumatic brain injury patients. Microdialysis catheters are now FDA approved and are being placed routinely with intracranial pressure monitors. This method allows for continuous measurement of ECF within a tumor or normal tissue. The dialysis probe has a semipermeable membrane which is less than 1 mm in diameter into which two sections of microcatheter are fused. Previous studies have demonstrated the feasibility of keeping the catheters in place of critically injured patients for up to 2 weeks. When placed at the time of surgical resection, the microcatheters are sterotactically implanted, placing the probe within the desired brain and/or tumor region. Externally, the catheter is connected to a syringe pump, which delivers a low flow rate (µl/min) of continuous perfusion fluid (Lactated Ringers or artificial CSF) and dialysate is collected in a microvial from the outlet tube. This sterile, single use catheter is minimally invasive and developed to achieve optimal diffusing characteristics similar to passive diffusion of a capillary blood vessel. Just as in the function of brain capillary vessel, water, inorganic ions and small organic molecules freely diffuse across the membrane of the probe, whereas proteins and protein bound compounds are impermeable. Additionally, lipophilic compounds are poorly recovered.

Use of Intracerebral Microdialysis to Quantitate temozolomide Concentrations Temozolomide is a FDA approved oral alkylating agent used in upfront and relapsed therapy of adult and pediatric patients with high grade glioma. While temozolomide (TMZ) with radiotherapy has assisted with improving overall survival rates of high grade gliomas, previous studies have proven approximately only 20% of the drug crosses the BBB after peripheral administration. The peak concentration of TMZ in the brain has been shown to be approximately 1-2 hours after ingestion. Once ingested, TMZ undergoes degradation from its prodrug form to the highly reactive alkylating agent, methyl-triazenyl imidazole carboxamide (MTIC). Previous studies have utilized brain microdialysis catheters to measure TMZ brain interstitium levels post primary or metastatic brain resection/biopsy. Both serum and dialysates were collected at 30 minute time intervals and analyzed to quantitate TMZ drug delivery to the tumor bed and surrounding tissue yet due to drug stability issues, MTIC concentrations were not feasible for measurement in the microdialysis setting. Thus, the investigators propose to measure TMZ concentrations pre and post Regadenoson administration with the hypothesis that transient BBB disruption will result in improved drug efficacy to the brain interstitium and tumor bed. If gross total resection cannot be achieved two microdialysis catheters will be placed after resection. One microdialysis catheter will be placed in the MRI enhancing tumor area and one catheter will be placed in a non-enhancing area, allowing for later assessment of the degree of BBB permeability both in the tumor and normal brain. These studies will be beneficial to evaluate the role of increasing temozolomide delivery in an effort to further increase tumor cytotoxicity and progression-free survival in such an aggressive tumor.

Use of Intracerebral Microdialysis to Quantitate Markers of Neuroinflammation Previous studies have demonstrated that patients with increased inflammatory cytokines from medical illnesses exhibit increased rates of depression. Both chronic and acutely elevated levels of inflammatory cytokines can result in the development of neuropsychiatric dysfunction, including cognitive slowing, inattention, or depression. Specifically, Dowlati et al concluded from a meta-analytic study that a small proportion of patients with idiopathic major depressive disorder exhibited increased serum cytokine levels. Thus, the investigators propose to quantitate markers of neuroinflammation and oxidative stress in collected dialysate, with the hypothesis that variance in these levels may correlate with affective and/or cognitive dysfunction.

Risks of Microdialysis:

These studies will be a novel investigation in the use of brain microdialysis to measure TMZ concentration in the post-operative setting with combinational administration of Regadenoson and temozolomide. To minimize risks post-surgical resection or biopsy, patients will only receive temozolomide 150mg/m2 twice, which is less than half the standard 5 day course administered during maintenance therapy for malignant glioma post radiation therapy. In addition, therapy will be given for only two days in the ICU setting, with minimal tumor toxicity anticipated with a suboptimal treatment dose over a short duration.[ The procedures involved with the insertion and removal of the dialysis probe can be performed with minimal injury at the time of surgery.[18,19] Although insertion of the microdialysis probe could conceivably disrupt the BBB, this has not proven to be problematic, as demonstrated by rapid normalization of the BBB after placement, allowing for equilibration and minimization of artifact.[20-22] Although it can be safely performed, microdialysis is nevertheless an invasive surgical procedure with a small risk of bleeding at the site of insertion and discomfort for the patient. The previous study which evaluated brain interstitium concentrations of TMZ evaluated dialysate samples from seven patients. All of the patients tolerated placement of the catheters and collection of the dialysate samples well, yet 1 patient developed several grade 3 and 4 neurologic adverse events whose etiologies remain unclear. All of the neurologic deficits improved without completely resolving.