Enhanced Delivery of Boronophenylalanine for Neutron Capture Therapy by Means of Intracarotid Injection and Blood-Brain Barrier Disruption

Abstract

There has been increasing interest in the possible use of boronophenylalanine as a capture agent for boron neutron capture therapy of brain tumors. The purpose of the present study was to determine whether the uptake of boronophenylalanine in F98 glioma-bearing rats could be enhanced by means of intracarotid (i.c.) injection with or without blood-brain barrier disruption (BBB-D). Glioma cells (10(5)) were stereotactically implanted into the right cerebral hemisphere of Fischer rats, and 12 days later, BBB-D was performed by infusing 25% mannitol (1.373 mOsmol/ml) into the right carotid artery and then immediately injecting L-boronophenylalanine (300 mg/kg of body weight) intracarotidly. The animals were killed 0.5, 1, 2.5, and 4 hours later, and the brains were removed for boron determination by direct current plasma atomic emission spectroscopy. BBB-D was assessed by the intravenous injection of Evans blue or horseradish peroxidase, and the barrier-disrupted hemispheres and tumors showed intense staining with each. The mean tumor boron concentration after i.c. injection and BBB-D was 34.8 +/- 6.8 micrograms/g at 2.5 hours compared with 20.3 +/- 6.2 micrograms/g after i.c. injection without BBB-D and 10.7 +/- 0.7 micrograms/g after intravenous injection. No significant differences in boron concentration in muscle, skin, and eye were observed among the different groups. Boron concentrations in the ipsilateral, disrupted hemisphere increased transiently but rapidly returned to background levels by 2.5 hours after BBB-D. The tumor:brain and tumor:blood ratios were 5.2 and 5.6, respectively, compared to 3.2 and 2.1 for intravenous injection groups at 2.5 hours. The present study is the first to show that BBB-D combined with i.c. injection can enhance the tumor uptake of boron compounds for boron neutron capture therapy.

Determination of Boronophenylalanine in Biological Samples Using Precolumn o-Phthalaldehyde Derivatization and Reversed-Phase High-Performance Liquid Chromatography

“…A cochromatogram effected with the addition of BPA with known concentration as an internal standard and retention factor comparison with a chromatographic run of standard BPA allowed us to attribute the peak with kЈ ϭ 23.90 to BPA in blood and cerebral tissue samples. Cerebral BPA concentrations in rats sacrificed 120 min after the intracarotid administration of 300 mg/kg of body weight BPA were 528.70 nmol/g wet weight (SD ϭ 275.95), which, expressed in terms of boron concentration, gave 5.71 g/g wet weight (SD ϭ 2.98), i.e., a BPA value very close to that previously obtained under similar experimental conditions and by determining BPA using direct-current plasma atomic emission spectroscopy (3). To assess the recovery of this HPLC method for BPA determination, 16 additional anesthetized rats were sacrificed and brains (divided into four groups) were homogenized with 1.…”

“…After OPA derivatization, the standard mixture was diluted to 1 ml with the starting eluent (Buffer A) and 100 l was then used for the analysis. The HPLC column was previously equilibrated with Buffer A having the following composition: 50 mM CH 3 COONa, pH 7.4, 50 mM Na 2 HPO 4 , pH 7.4, CH 3 OH, tetrahydrofuran (48:48:2:2; v:v:v:v). Separation was obtained by using a step gradient with a second buffer (Buffer B) consisting of CH 3 OH:H 2 O (65: 35; v:v).…”

“…Brain tumors that cannot surgically be removed are complicated to treat pharmacologically since most anticancer drugs do not easily permeate the bloodbrain barrier. In this light, a therapeutic approach characterized by boronophenylalanine (BPA) 3 administration has recently been set up in animal models of induced cerebral tumors (1)(2)(3) and it is currently under evaluation in various clinical trials (4 -6). The method is based on the principle that this modified amino acid is able to cross the blood-brain barrier and to accumulate preferentially within tumoral cells.…”

“…The subsequent step is to submit BPA-enriched cancerous tissues to low-energy neutron bombardment, which, through a nuclear scission, should cause the selective disruption and death of malignant cells (the so-called boron neutron capture therapy, BNCT). To obtain positive results, it is critical that the level of BPA inside the tumor be much higher than that present not only in healthy cerebral cells but also in other tissues and in the bloodstream (3,6).…”

“…The concentration of BPA in different tissues has been determined up to now by measuring boron levels mainly by direct-current plasma atomic emission spectroscopy (3,7). Even though this spectroscopic method is highly selective, it is not equally sensitive, it has a high instrument cost (another main disadvantage), and thus a relatively low number of laboratories are equipped with such an instrument.…”