On the applicability of [¹⁸F]FBPA to predict L-BPA concentration after amino acid preloading in HuH-7 liver tumor model and the implication for liver boron neutron capture therapy

Abstract

Introduction

In recent years extra-corporal application of boron neutron capture therapy (BNCT) was evaluated for liver primary tumors or liver metastases. A prerequisite for such a high-risk procedure is proof of preferential delivery and high uptake of a ¹⁰B-pharmaceutical in liver malignancies. In this work we evaluated in a preclinical tumor model if [¹⁸F]FBPA tissue distribution measured with PET is able to predict the tissue distribution of [¹⁰B]L-BPA.

Methods

Tumor bearing mice (hepatocellular carcinoma cell line, HuH-7) were either subject of a [¹⁸F]FBPA-PET scan with subsequent measurement of radioactivity content in extracted organs using a gamma counter or injected with [¹⁰B]L-BPA with tissue samples analyzed by prompt gamma activation analysis (PGAA) or quantitative neutron capture radiography (QNCR). The impact of L-tyrosine, L-DOPA and L-BPA preloading on the tissue distribution of [¹⁸F]FBPA and [¹⁰B]L-BPA was evaluated and the pharmacokinetics of [¹⁸F]FBPA investigated by compartment modeling.

Results

We found a significant correlation between [¹⁸F]FBPA and [¹⁰B]L-BPA uptake in tumors and various organs as well as high accumulation levels in pancreas and kidneys as reported in previous studies. Tumor-to-liver ratios of [¹⁸F]FBPA ranged from 1.2 to 1.5. Preloading did not increase the uptake of [¹⁸F]FBPA or [¹⁰B]L-BPA in any organ and compartment modeling showed no statistically significant differences in [¹⁸F]FBPA tumor kinetics.

Conclusions

[¹⁸F]FBPA-PET predicts [¹⁰B]L-BPA concentration after amino acid preloading in HuH-7 hepatocellular carcinoma models. Preloading had no effect on tumor uptake of [¹⁸F]FBPA.

Advances in knowledge

Despite differences in chemical structure and administered dose [¹⁸F]FBPA and [¹⁰B]L-BPA demonstrate an equivalent biodistribution in a preclinical tumor model.

Implications for patient care

[¹⁸F]FBPA-PET is suitable for treatment planning and dose calculations in BNCT applications for liver malignancies. However, alternative tracers with more favorable tumor-to-liver ratios should be investigated.

Introduction

Boron neutron capture therapy (BNCT) is a targeted radiotherapy, in which thermal neutrons are captured with high probability by the isotope ¹⁰B leading to the nuclear reaction ¹⁰B(n,α,γ)⁷Li. This reaction leads to the production of high linear energy transfer (LET) particles, namely an alpha-particle (⁴He) and a recoiling lithium-7 (⁷Li) ion. Additionally the emission of 478 keV prompt gamma rays with 93.9% incidence occurs. The range of the high LET particles in tissue is 5–9 μm limiting their effects to the diameter of the boron containing cell. The success of BNCT depends on the concentration of ¹⁰B in tumor cells, which should be higher than 20 μg/g or 10⁹ atoms/cell [1]. A frequently used ¹⁰B carrier is a derivative of the neutral amino acid L-phenylalanine 4-borono-L-phenylalanine (L-BPA), which has been evaluated in several clinical trials mostly focusing on glioma and other brain tumors as well as melanoma [2], [3], [4], [5], [6]. More recently BNCT was evaluated for primary liver tumors or liver metastases from colorectal cancer [7], [8], [9], [10], [11], [12], [13]. In 2001 the first extra-corporal application of BNCT in isolated livers of two patients was performed in Pavia, Italy [7], [8]. The performance of BNCT on explanted livers is especially attractive for several reasons [9]:

• By explanting the liver no other organs are irradiated.

• In principle, macroscopic and microscopic metastases disseminated within healthy liver can be selectively treated.

• By perfusion of the organ with Wisconsin solution (for organ preservation), ¹⁰B-containing blood is washed out, reducing the radiation dose to the vessels and decreasing the risk of radiation-induced liver disease.

• The limitations of liver transplantation in oncological patients due to the necessity of immune-suppression are not applicable in allo-transplantations. The surgical technique is similar to the technique used in living donors.

• The irradiation of the explanted liver takes place within the thermal column of a research reactor where the scattering of the neutrons causes a much more isotropic field and therefore enables a smaller depth dose gradient in tissue [14].

However, such a high-risk procedure is only justified, if it has the potential of cure. Thus, a prerequisite to any future research is the proof of preferential delivery and high uptake of a ¹⁰B-pharmaceutical in liver metastases.

To examine the distribution of L-BPA in vivo in brain tumors Ishiwata et al. [15], [16] proposed a positron-labeled boronophenylalanine analogue, 4-borono-2-[¹⁸F]fluoro-phenylalanine ([¹⁸F]FBPA). Clinical and preclinical studies using [¹⁸F]FBPA-PET and L-BPA have suggested analogous uptake of both substances [17], [18], [19], [20], [21], however none of these studies investigated liver malignancies and/or the effect of preloading. The benefit of using [¹⁸F]FBPA-PET in (pre)clinical studies in BNCT is to provide key data on L-BPA tumor accumulation versus normal tissue and to envisage the efficacy of the treatment based on individual patient analysis [19]. However, there are several potential limitations of [¹⁸F]FBPA-PET to predict L-BPA uptake in tumor and healthy tissues. Firstly, the chemical structures are different. Secondly, [¹⁸F]FBPA-PET is performed using a tracer dose, while in BNCT therapeutic doses of [¹⁰B]L-BPA or [¹⁰B]L-BPA-fr (L-boronophenylalanine labeled with ¹⁰B and conjugated with fructose) are administered. Thirdly, in PET the tracer is usually administered by a single i.v. bolus injection (~1 min) whereas [¹⁰B]L-BPA or [¹⁰B]L-BPA-fr is administered by slow i.v. bolus injection followed by drip infusion during neutron irradiation [21]. Thus a direct comparison between [¹⁸F]FBPA and [¹⁰B]L-BPA using the same administration protocols in the same animal model is of high interest.

To increase intracellular [¹⁰B]L-BPA concentration, it was suggested that preloading with molecules targeting the L-type amino acid transport system enhances the accumulation of BPA in tumor cells [22], [23], [24]. In particular, Papaspyrou et al. [23] showed that BPA uptake can be significantly increased in mouse melanoma cells by preloading with L-tyrosine in vitro. Similarly, L-DOPA has been described as an enhancer of BPA accumulation in malignant glioma and melanoma [24].

Therefore, in order to further validate [¹⁸F]FBPA-PET as a diagnostic tool in BNCT, the present study directly compares the pharmacokinetics and tissue distribution of [¹⁸F]FBPA and [¹⁰B]L-BPA in mice bearing tumors derived from a hepatocellular carcinoma cell line. In addition, we evaluated the impact of L-tyrosine, L-DOPA and L-BPA preloading on tumor uptake of [¹⁸F]FBPA and [¹⁰B]L-BPA.