Imaging the L-Type Amino Acid Transporter-1 (LAT1) with Zr-89 ImmunoPET

Abstract

The L-type amino acid transporter-1 (LAT1, SLC7A5) is upregulated in a wide range of human cancers, positively correlated with the biological aggressiveness of tumors, and a promising target for both imaging and therapy. Radiolabeled amino acids such as O-(2-[18F]fluoroethyl)-L-tyrosine (FET) that are transport substrates for system L amino acid transporters including LAT1 have met limited success for oncologic imaging outside of the brain, and thus new strategies are needed for imaging LAT1 in systemic cancers. Here, we describe the development and biological evaluation of a novel zirconium-89 labeled antibody, [89Zr]DFO-Ab2, targeting the extracellular domain of LAT1 in a preclinical model of colorectal cancer. This tracer demonstrated specificity for LAT1 in vitro and in vivo with excellent tumor imaging properties in mice with xenograft tumors. PET imaging studies showed high tumor uptake, with optimal tumor-to-non target contrast achieved at 7 days post administration. Biodistribution studies demonstrated tumor uptake of 10.5 ± 1.8 percent injected dose per gram (%ID/g) at 7 days with a tumor to muscle ratio of 13 to 1. In contrast, the peak tumor uptake of the radiolabeled amino acid [18F]FET was 4.4 ± 0.5 %ID/g at 30 min after injection with a tumor to muscle ratio of 1.4 to 1. Blocking studies with unlabeled anti-LAT1 antibody demonstrated a 55% reduction of [89Zr]DFO-Ab2 accumulation in the tumor at 7 days. These results are the first report of direct PET imaging of LAT1 and demonstrate the potential of immunoPET agents for imaging specific amino acid transporters.

Glypican-3–Targeted 89Zr PET Imaging of Hepatocellular Carcinoma: Where Antibody Imaging Dares to Tread

“…However, the remarkable accumulation of 89 Zr-aGPC3 in HCC tumorsgreater than 800 %ID/g-overshadows liver uptake, yielding tumor-to-liver ratios greater than 30 in the larger (3.8-mm-diameter) tumor model. This ratio is substantially higher than that achieved with most other antibody probes, which have tumor-to-liver ratios ranging from 1.5 to 4 (10,16,(18)(19)(20) at their optimal time points after injection of 89 Zrlabeled antibody. These values are consistent with 89 Zr-aGPC3 uptake in smaller (,1-mmdiameter) tumors of the liver with a tumor-toliver ratio of 1.49-1.57 at 3-6 d after injection.…”

“…Immuno-PET imaging of HCC is a daunting task because the liver is the main clearance organ for most antibodies. Typically, liver uptake values range from 5 to 10 percentage injected dose (%ID)/g (11,16,17), which is persistent over several days, resulting in low tumor-toliver ratios. However, the remarkable accumulation of 89 Zr-aGPC3 in HCC tumorsgreater than 800 %ID/g-overshadows liver uptake, yielding tumor-to-liver ratios greater than 30 in the larger (3.8-mm-diameter) tumor model.…”

PET for Therapy Response Assessment in Glioblastoma

“…Labeled amino acid tracers developed so far for PET imaging are divided into two categories: tracers actively incorporated into the proteins, such as 11 C-Methionine ( 11 C-MET), potentially allowing investigating protein synthesis, and tracers not integrated into proteins, such as 18 F-fluoroethyltyrosine ( 18 F-FET) and 3,4-dihydroxy-6-18 F-fluoro-l-phenylalanine ( 18 F-FDOPA), which are valuable tools to evaluate amino acid transport (59). The increased uptake of 18 F-FET and 18 F-FDOPA by cerebral glioma tissue appears to be caused mainly by increased transport via sodium-independent amino acid transport system L for large neutral amino acids (LATs) and Na + -dependent general amino acid transporters B 0,+ and B 0 , with a disruption of the BBB not being a prerequisite for intratumoral accumulation (20,(60)(61)(62). Most PET studies of cerebral gliomas have been performed with 11 C-MET, although the short half-life of 11 C (20 min) limits the use of this tracer to the few centers that are equipped with an on-site cyclotron facility.…”

Evaluation of Prostate Cancer with Radiolabeled Amino Acid Analogs

“…patients were successfully visualized (70). Finally, although not specifically yet studied with prostate cancer, immuno-PET imaging with radiotracers such as 89 Zr-DFO-Ab2, targeting the extracellular domain of LAT1, enable direct interrogation of the amino acid transport complex itself (8).…”

“…Therefore, the molecular imaging of amino acid activity is an attractive target, especially in situations in which 18 F-FDG PET has limitations, such as in the detection of prostate cancer (5-7). Amino acid-based radiotracer cancer imaging with both naturally occurring and synthetic radiolabeled amino acids chiefly reflects substrate transport across the tumor cell membrane (1,2,8).…”